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Establishing a clinical cardiac MRI service
Clinical Radiology (2006) 61, 211–224
REVIEW
Establishing a clinical cardiac MRI service D.P. O’Regan*, S.A. Schmitz Imaging Sciences Department, MRC Clinical Sciences Centre, Hammersmith Hospital Campus, Imperial College, London, UK Received 6 July 2005; received in revised form 27 October 2005; accepted 29 October 2005
After several years of research development cardiovascular MRI has evolved into a widely accepted clinical tool. It offers important diagnostic and prognostic information for a variety of clinical indications, which include ischaemic heart disease, cardiomyopathies, valvular dysfunction and congenital heart disorders. It is a safe non-invasive technique that employs a variety of imaging sequences optimized for temporal or spatial resolution, tissue-specific contrast, flow quantification or angiography. Cardiac MRI offers specific advantages over conventional imaging techniques for a significant number of patients. The demand for cardiac MRI studies from cardiothoracic surgeons, cardiologists and other referrers is likely to continue to rise with pressure for more widespread local service provision. Setting up a cardiac MRI service requires careful consideration regarding funding issues and how it will be integrated with existing service provision. The purchase of cardiac phased array coils, monitoring equipment and software upgrades must also be considered, as well as the training needs of those involved. The choice of appropriate imaging protocols will be guided by operator experience, clinical indication and equipment capability, and is likely to evolve as the service develops. Post-processing and offline analysis form a significant part of the time taken to report studies and an efficient method of providing quantitative reports is an important requirement. Collaboration between radiologists and cardiologists is needed to develop a successful service and multi-disciplinary meetings are key component of this. This review will explore these issues from our perspective of a new clinical cardiac MRI service operating over its first year in a teaching hospital imaging department. Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction Over the last decade cardiac magnetic resonance imaging (CMR) has evolved from a research tool into a sophisticated and robust method for investigating cardiovascular pathology in a clinical setting. Improvements in gradient performance, coil design and imaging sequences have considerably improved acquisition times and image quality, establishing CMR as a reliable non-invasive method of assessing cardiac morphology and function. It combines excellent spatial and temporal resolution with high contrast between the blood pool and the myocardium. This allows highly reproducible and * Guarantor and correspondent: D.P. O’Regan, Imaging Sciences Department, Clinical Sciences Centre, Faculty of Medicine, Imperial College, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK. Tel.: C44 20 83831510; fax: C44 20 83833038. E-mail address: [email protected] (D.P. O’Regan).
accurate1–3 measurements to be made of global ventricular function and mass, making it the ideal method for assessing responses to treatment.4 It also offers accurate quantification of the severity of valvular disease,5,6 including the safe evaluation of prosthetic valve replacements.7 A key role for CMR is in the investigation of ischaemic heart disease where regions of myocardial ischaemia and infarction may be identified, and tissue viability assessed before revascularization. It also has clinical value in the investigation of aortic disease, cardiomyopathy and myocarditis, assessing pericardial pathology, cardiac tumours and in the diagnosis and follow-up of congenital heart disease.8 Applications such as coronary artery imaging and plaque characterization are also becoming more widely available. CMR now has a mainstream role in the investigation of cardiovascular disease and increasing demand is likely to lead to the development of services outside supra-regional academic centres. Both radiologists and cardiologists may be expected
0009-9260/$ - see front matter Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2005.10.015
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to take the lead in developing new cardiac imaging services. However, the initiation of a CMR facility requires careful consideration regarding issues such as funding, equipment, training and the effective use of specialized imaging sequences and analysis software. CMR benefits from a multidisciplinary approach and clinico-radiological conferences involving radiologists, radiographers, cardiologists and cardiothoracic surgeons are an important component of this. This review will explore these issues from our perspective of setting up a new clinical cardiac MRI service operating over its first year in a teaching hospital imaging department.
The place of CMR in cardiac imaging CMR has emerged as a reliable and valid tool for the investigation of heart disease. A key advantage of CMR is that it provides a detailed assessment of cardiac morphology, function and tissue characterization in a single examination. A standard study can readily be performed within an hour including patient preparation. Its accuracy and reproducibility make it in ideal technique for assessing disease severity and response to treatment. There is intrinsically high contrast between the myocardium and blood pool, which allows cine images and coronary angiograms to be acquired without contrast. The high spatial resolution of CMR and tissue contrast allow quantification of ischaemic subendocardial scarring that surpasses other techniques. Its three-dimensional capabilities allow complete flexibility for imaging not only the heart, but the great vessels and pulmonary vasculature as well. Despite the restrictions of the MR hardware more invasive procedures such as stress-testing can be performed safely. There are a number of other well-validated imaging techniques for investigating cardiac disease, which have strengths and weaknesses compared with CMR. Echocardiography currently provides the mainstay of non-invasive cardiac imaging. Transthoracic echocardiography is a rapid and inexpensive imaging technique, which will remain a first-line modality for assessing cardiac function and morphology. However, it is an operator-dependent technique that requires an adequate acoustic window to obtain images and may suffer from near-field artefacts. In comparative studies CMR has demonstrated better interobserver4 and inter-study1 variability of functional and structural indices than echocardiography. Radionuclide techniques allow assessment of myocardial metabolism and provide low-resolution
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morphological imaging. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) have an established role in identifying hibernating myocardium and assessing tissue viability. However, the specificity of SPECT imaging without attenuation correction is significantly reduced by artefacts due to photon absorption9 especially in obese patients. The high spatial resolution of CMR allows detection of subendocardial infarcts that are missed by SPECT,10 especially those involving the inferior wall.11 Catheter angiography remains the gold standard for luminal coronary imaging. However, it has a small but recognized morbidity and mortality, requires iodinated contrast medium and uses ionizing radiation. The development of non-invasive techniques for coronary angiography has a huge potential for cost reduction and patient safety. Both MR and contrast-enhanced computed tomography (CT) have made substantial progress in improving visualization of the coronary vessels in recent years. Comparisons between these techniques are hampered by the profusion of MR sequence designs and the continuing development of CT detector systems. In a multicentre trial threedimensional coronary MR angiography (MRA) with navigator respiratory gating achieved an accuracy of 72% at detecting clinically significant lesions compared with catheter angiography,12 and has demonstrated equivalent accuracy to 4 and 16-row multidetector CT.13,14 MR has a negative predictive value for any coronary artery disease of 81%, and 100% for left main stem or triple vessel disease.12 However, newer 64 detector row CT systems providing an isotropic voxel size of 0.4 mm and a temporal resolution of up to 83 ms may improve on these standards.15 Although CT angiography is rapid and requires minimal planning, MRA has the advantages of avoiding the need for contrast medium, ionizing radiation and premedication with b-blockers. CT16,17 and MR18 also have an important use in assessing the patency of bypass grafts, which are subject to less cardiac movement and may be difficult to demonstrate with catheter angiography. Luminal angiography masks the severity of early atherosclerosis,19 and both MRI20 and CT15 have a promising role in characterizing and quantifying atherosclerotic plaque. The evaluation of congenital cardiac defects in both infants and adults is a strength of CMR due to its multiplanar imaging of the heart and great vessels. It allows a detailed assessment of complex cardiovascular anatomy especially after surgical correction. It has the advantage over transthoracic echocardiography of being able to image the
Establishing a clinical cardiac MRI service
anatomy of the great vessels and other extracardiac abnormalities,21 and may also reduce the need for invasive catheter studies.22 Although expert supervision is usually needed to plan these studies, the development of new three-dimensional volume acquisitions allows offline reconstructions to be made in any plane.23 Despite the appeal of CMR for a number of clinical indications it has unproven cost-effectiveness, has limited availability, and requires trained personnel.24 The cost of a resting CMR is estimated as five times that of echocardiography, but is only a quarter of the cost of invasive catheterization.25 The oncogenic potential of imaging techniques using ionizing radiation is not insignificant25 and should be an important consideration for the investigation of low-risk groups and those needing repeated examinations. However, some patients may have contraindications to MRI or may not tolerate the study due to claustrophobia. Image quality may also be adversely affected by inadequate breath-holds or cardiac arrhythmias. The role of CMR amongst the armamentarium of conventional imaging techniques is still evolving and may continue to do so as new applications are developed.
The business case If there is a demand for a service development such as CMR then a business case may need to be made to demonstrate that the project is financially viable, will benefit patients and is supported by purchasers.26 Issues such as how to provide an effective and efficient service, the amount of capital spending required and ensuring value for money need to be addressed. Potential cost savings in reducing referrals elsewhere or lessening the need for other investigations may offset some of the expenditure. Provision needs to be made for both the capital costs of acquiring additional MR equipment and a post-processing workstation, as well as the recruitment and training of staff. Depending on workload, at least one radiographer should be fully trained in cardiac MR who can then train other members of the department. Experienced hands-on supervision is required to assist in the planning of studies and the choice of sequences during the training period. The additional pressure on existing MRI workload also needs to be considered and providing an extended hours service may be one way of increasing capacity. Close liaison with clinicians to ensure an efficient referral base is essential.
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Hardware Cardiac imaging capability should be considered during the tendering process for new MR installations if there is a likelihood that this service may be provided in the future. However, many conventional 1.5 T whole-body MRI systems may be adapted or upgraded to cardiac capability. Regional units and research institutes may be able to establish a dedicated CMR facility within their departments. There are high gradient performance requirements for CMR in order to minimize repetition times and improve temporal resolution. As a result radiofrequency specific absorption rates (SAR) may approach their maximum permissible levels in some sequences particularly at higher field strengths. Most experience in CMR has been gained at 1.5 T, however there is increasing interest in the use of 3 T systems, which have already shown promise in other body MR applications.27 Imaging at 3 T has the immediate advantage of a potential doubling of signal-to-noise ratio compared with conventional 1.5 T platforms.28–30 However, greater susceptibility effects, field inhomogeneity and SAR limits have posed difficulties for ultra-high field cardiac systems29,31 and they are likely to remain a research tool for CMR at present. Dedicated phased array cardiac surface coils are used which offer favourable signal to noise characteristics and allow the use of parallel imaging techniques. These coils typically comprise at least four elements and are positioned over the anterior and posterior chest walls (Fig. 1). Satisfactory positioning of the coil over the heart may be confirmed on the initial scout images and this is essential to minimize off-resonance artefacts.32
Figure 1 The anterior elements of a six-channel phased array cardiac coil. Accurate positioning over the heart is needed to optimize image quality and reduce offresonance artefacts.
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CMR studies are gated to the cardiac cycle and this is usually achieved with a modified ECG. Due to the magnetohydrodynamic effect of flowing blood in a magnetic field the morphology of the ECG waveform is altered which interferes with conventional triggering methods. To overcome this manufacturers provide a vector-ECG system to improve the reliability of R-wave detection33 (Fig. 2). Most sequences employ retrospective cardiac gating in which the operator predetermines the number of cardiac phases required and data are acquired continuously throughout the cardiac cycle. If the R–R interval lies outside a predetermined arrhythmia rejection window then the scanner will ignore the data and continue acquiring phase-encoding steps. This may mean that patients with irregular heart rates have longer breath-holds to perform. Parallel imaging can be used to substantially reduce acquisition time but this is at the expense of signalto-noise ratio and may lead to reconstruction artefacts over the centre of the image.34 For delayed enhancement studies a hand injection of contrast medium via an intravenous cannula is sufficient, while perfusion studies are ideally performed with a dual-headed MR compatible pump. If stress CMR is undertaken attention needs to be paid to the potential difficulties involved in monitoring and treating patients in an MR unit. Patients with cardiac pacemakers or implantable defibrillators are not imaged with MR, although there is evidence of their potential safety in certain circumstances.35,36 Valve prostheses,7 epicardial pacing wires37 and coronary stents38 are not considered contraindications to MRI at 1.5 T.
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However, metallic devices cause a local susceptibility effect, which may impair image quality.
Cardiac imaging sequences CMR makes use of specialized pulse sequence designs that are tailored to the demands of cardiac imaging (Table 1). Knowledge of the characteristics of each sequence is necessary for their appropriate use and interpretation. Each sequence uses some form of cardiac gating and is performed either as a breath-hold or using respiratory gating. The imaging planes that are chosen are analogous to that used by echocardiography, and are orientated around the short and long axes of the left ventricle.39 A basic protocol would include cine gradient echo sequences of the two and four-chamber views, the left ventricular outflow tract and a stack of parallel slices through the left ventricular short axis (Fig. 3). Additional axial, coronal and sagittal planes may be planned to produce an overview of the heart and great vessels using either cine gradient echo sequences or spin-echo black blood sequences. Free-breathing real-time MR sequences40 are an ideal way to interactively plan the cardiac imaging planes, and they may also have a role in investigating septal motion during respiration.41 Dedicated sequences are used to assess perfusion, delayed enhancement, flow quantification and coronary angiography according to the clinical indication.
Morphology and function
Figure 2 A vector electrocardiograph system uses four electrodes placed over the chest wall. Software processing allows identification of the R-wave whilst the patient undergoes scanning.
The mainstay of functional imaging is the balanced steady-state free procession (b-SSFP) sequence, which is known by various manufacturer’s acronyms (Philips: b-TFE, Siemens: TrueFisp, General Electric: Fiesta). This is a bright-blood rapid gradient echo sequence that is well suited to CMR owing to its inherent high signal to noise ratio per unit time and excellent contrast between the lumen and myocardium. Its contrast properties depend on the ratio of T2* to T1 relaxation times, and is therefore relatively insensitive to gadolinium enhancement. Dephasing of spins by turbulent blood flow, such as valvular regurgitation, is visible as signal loss. During a breath-hold a user-defined number of frames per RR interval may be acquired over six to 10 heart beats to produce cine images of the cardiac cycle. A minimum of 11 cardiac phases is required in order to accurately identify the endsystolic phase,42 but higher temporal resolutions
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Table 1
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Summary of principal imaging protocols for clinical cardiac magnetic resonance.
Purpose
Sequence
Imaging planes
Cardiac gating
Respiratory gating
Intravenous contrast
Function
Cine b-SSFP
VLA, HLA, LVOT, LV short axis
Retrospective: 20–40 cardiac phases
Breath-hold
None
Flow quantification
Phase contrast gradient echo
Perpendicular to a vessel
Triggered: 10–20 cardiac phases
Breath-hold/ navigator
None
High resolution anatomical imaging
Double IR TSEG anterior saturation slab
Transaxial, coronal or sagittal
Triggered: end diastole
Breath-hold
None
Perfusion
Gradient echo, EPI or b-SSFP
LV short axis
Triggered: end diastole
Breath-hold/free breath
First pass bolus of Gd-DTPA
Viability/late enhancement
Two or threedimensional IR gradient echo
LV short axis, HLA, VLA, LVOT
Triggered: end diastole
Breath-hold
15 min postinjection Gd-DTPA
Coronary MRA
Three-dimensional b-SSFP with frequency selective fat suppression and T2 preparation prepulse
Targeted volume of a coronary artery or whole heart
Triggered to diastasis (latediastole)
Breath-hold or navigator echo gating
None
b-SSFP, balanced steady-state free precession; VLA, vertical long axis; HLA, horizontal long axis; LVOT, left ventricular outflow tract; LV, left ventricle; IR, inversion recovery; EPI, echo planar imaging; TSE, turbo spin echo.
are readily achieved and are particularly useful for assessing valve motion. Black blood spin-echo sequences have a complementary role to play in conjunction with b-SSFP cine images in providing detailed anatomical imaging of the heart and great vessels. Fast spinecho techniques obtain excellent suppression of flowing blood by using a double-inversion pulse technique. The sequences are usually triggered to end-diastole and are respiratory gated. These are useful for assessing cardiac anatomy in congenital heart disease and evaluating the aorta for dissection or coarctation. T1-weighted sequences may be supplemented by fat-suppressed images for confirming the presence of intramyocardial fat in arrhythmogenic right ventricular dysplasia (ARVD).43
Perfusion imaging Coronary vasodilators, such as adenosine, are used to induce a perceptible disparity in first-pass perfusion between normal and hypoperfused myocardium supplied by a diseased artery.44 Perfusion imaging demonstrates delayed contrast wash-in in myocardium supplied by a stenosed artery compared with normally perfused tissue. If rest and stress perfusion studies are performed together
this has the advantage of being able to derive myocardial perfusion reserve indices. The use of intravascular contrast agents in perfusion studies45 and dual-bolus techniques46 may offer the potential to determine absolute myocardial blood flow in the future. Perfusion imaging after a bolus of a contrast agent has demanding imaging requirements. High temporal resolution is required to image the first pass of contrast, and high spatial resolution is needed to distinguish the sub-endocardial and subepicardial layers of the myocardium. In addition good contrast is needed between normal and ischaemic myocardium, as well as a near-linear relationship between contrast dose and signal intensity.47 A spatial presaturation pulse to suppress background myocardial signal is generally employed, and data readout can be achieved with gradient echo, echo planar imaging (EPI), or steadystate free precession techniques.
Delayed hyperenhancement A strength of CMR is its ability to provide myocardial tissue characterization at a resolution that cannot be matched by other imaging techniques. Hyperenhancement occurs when the concentration of gadolinium–diethylenetriamine penta-acetic acid
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Figure 3 b-SSFP sequences of the principal cardiac planes (top left: two chamber, top right: four chamber, bottom left: left ventricular outflow tract, bottom right: short axis views).
(Gd-DTPA) in infarcted tissue is greater than its concentration in non-infarcted tissue. The concentration difference depends on both the volume of distribution of the contrast medium48 and to differential wash-in and wash-out rates.49 The GdDTPA is usually given in a double dose of 0.2 mmol/ kg and approximately 15 min later there is optimum contrast between normal and infarcted tissue to accurately assess subendocardial enhancement. An inversion recovery rapid gradient echo technique is used to acquire the delayed enhancement images (Fig. 4). In order to achieve maximal
contrast between normal and infarcted myocardium the inversion time is carefully chosen to null the signal from normal tissue, which may vary between patients due to contrast kinetics and tissue characteristics. This may be achieved by performing a range of inversion times on a single mid-ventricular section and choosing the image with optimal myocardial suppression. Alternative approaches such as phase-sensitive inversion recovery50 or the Look Locker technique51 are also available. A short axis stack through the left ventricle in end-diastole is then performed either
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myocardial contractile reserve. Stress testing may be performed to exceed this reserve and reveal regions of inducible ischaemia. For instance the b-agonist dobutamine increases myocardial oxygen demand beyond that which can be met by a stenosed artery. This induces a wall motion abnormality in heart muscle that may have contracted normally at rest. Another question that arises is whether hypokinetic myocardium seen during a rest study is viable; will its function improve after revascularization? Hibernating myocardium shows a biphasic response to increasing doses of dobutamine. Initially, there is a transient improvement in contractility at lower doses of dobutamine, which then declines at higher doses as the contractile reserve is exceeded. Short and long-axis b-SSFP cine sequences of the left ventricle are acquired at rest and then during each incremental increase of dobutamine until predefined termination criteria are reached.
Figure 4 Inversion recovery gradient echo sequence 15 min after Gd-DTA contrast medium injection. This demonstrates transmural enhancement and thinning in the left anterior descending artery territory. This indicates non-viability of these segments.
as a two or three-dimensional acquisition. This is supplemented by end-diastolic images in the other conventional cardiac planes after increasing the inversion time incrementally to maintain optimum contrast. Regional myocardial contrast enhancement is associated with histological evidence of irreversible ischaemic injury.52 In clinical trials there is an inverse relationship between the mural thickness of hyperenhancement and functional recovery after revascularization.53 This technique provides invaluable prognostic information on the viability of dysfunctional myocardium and its subsequent response to bypass grafting. The same sequence may be used for the investigation of dilated cardiomyopathy,54 amyloidosis,39 sarcoidosis55 and myocarditis,56 which show non-specific enhancement that does not conform to the typical subendocardial pattern seen in ischaemic disease.57 Post-contrast studies are also of particular value in characterizing intra-cardiac thrombus58 and tumours.59
Stress imaging The ischaemic effects of coronary artery disease may not be apparent at rest if there is sufficient
Delayed enhancement versus stress imaging to assess viability How do the two techniques of delayed-enhancement and dobutamine-stress MRI compare in terms of assessing myocardial viability? For dysfunctional regions with R50% scar on delayed-enhancement MR the negative predictive value for functional response to revascularization is around 90%53— which offers equal or higher confidence than dobutamine stress MR.60 However, for less extensive subendocardial scars (25–50%) dobutamine stress MR has shown that the group with a positive contractile reserve show a significant improvement in the rate of functional improvement after revascularization. 60 However, the functional response to revascularization may not be the only useful end-point in such patients with an intermediate thickness of viable myocardium. For instance, failure to improve global left ventricular function after coronary artery bypass grafting for ischaemic cardiomyopathy is not always associated with a poorer outcome.61 A possible explanation might be that effective revascularization of viable tissue protects against future infarction and arrhythmias. For subendocardial scars of !25% mural thickness dobutamine stress MR may offer a higher positive predictive value than delayedenhancement MR.60 However, these patients have a significant potential for a response to revascularization that might be denied to them if the dobutamine stress MR produced a false-negative result.62 In practical terms, delayed-enhancement
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MR is relatively safe to implement and straightforward to analyse. After suitable training these studies can be readily performed by unsupervised radiographers as part of a routine cardiac protocol. Stress testing may also be performed safely,63 but requires careful patient selection and monitoring, and experienced supervision throughout the study.
Coronary MRA Stress-MR and perfusion imaging provide information about the haemodynamics of coronary artery disease. However, a method of non-invasively assessing the coronary artery lumen and vessel wall would have advantages for surgical planning and risk stratification. Unfortunately, imaging the coronary arteries is challenging because of their small diameter and considerable cardio-respiratory motion. A conventional approach to performing a coronary MRA would begin with identifying the quiescent period of cardiac motion on a four-chamber cine sequence. A stack of axial images through the heart with the appropriate trigger delay and acquisition window for diastasis are then acquired using the navigator echo for respiratory gating. These images are then used to plan suitable imaging planes for each coronary artery using a three-point planning technique. A typical MRA protocol would use an ECGtriggered three-dimensional b-SSFP sequence, with fat suppression and a T2 prepulse, using a navigator echo for motion correction64 (Fig. 7). An increasingly used alternative is to acquire a threedimensional acquisition of the whole heart and subsequently reformat the images to demonstrate the coronary arteries. This approach obviates the need for targeted planning and can be achieved in under 15 min.65 A yet more efficient strategy uses image data extracted from a free-breathing acquisition to allow for retrospective motion correction.66 The increasing use of coronary stents also poses problems in creating a local susceptibility artefact, although prototype artefact-free stents have been developed.67 In clinical use, the value of coronary " Figure 5 Apico-septal hypertrophic cardiomyopathy. (a) A b-SSFP image shows asymmetrical thickening of the septum. (b) Myocardial tagging demonstrates reduced contractile deformation in the hypertrophied septum. (c) An inversion recovery gradient echo sequence shows patchy focal enhancement (arrows) that spares the subendocardium.
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MRA sequences is generally limited to evaluating the patency of the proximal arteries and identifying coronary artery anatomical variants (Fig. 5).
Flow quantification Cine phase contrast techniques are used to provide quantitative assessments of flow through a vascular lumen. These sequences use a bipolar gradient to create a linear relationship between the velocity of the blood and the phase of the transverse component of the MR signal. A velocity-encoding (venc) parameter is chosen by the operator at which the maximum phase shift will occur. However, if the flow velocity exceeds this value then the phase shift will become aliased in the opposite direction. The choice of venc parameter is a compromise between avoiding aliasing and reducing sensitivity to slower velocities and this is usually assessed with a trial breath-hold acquisition. Grey-scale images are derived which show stationary blood and surrounding tissues as midgrey, with through-plane flow in one direction as mid-grey to white and in the opposite direction as mid-grey to black. The phase shift is directly proportional to velocity within each voxel. Flow versus time curves may be derived with the area under the curve indicating the stroke volume during the cardiac cycle (Fig. 6). In order to accurately perform this method a slice perpendicular to the vessel being examined should be obtained by planning on two orthogonal views. The aortic root and right ventricular outflow tract are most commonly examined to obtain pulmonary to systemic flow ratios (QP:QS) as well as cardiac output, peak systolic velocities and regurgitant fractions. Measurements across the mitral and tricuspid valves are complicated by the through plane movement of the base of the heart during the cardiac cycle, although attempts have been made to correct for this.68 Another approach is to use velocity-encoding sequences to measure pulmonary and systemic cardiac outputs, with the difference indicating the regurgitant volume across an atrioventricular valve. A similar logic can be used to estimate the severity of an intracardiac shunt if there is no co-existing valvular regurgitation. Attempts have also been made to measure flow characteristics of aortic coartation69 and of internal mammary coronary grafts70 with phase contrast techniques.
Figure 6 The top image shows blood flowing through a tricuspid aortic valve using a velocity encoded sequence. The bottom image demonstrates that a flow versus time curve may be derived showing a significant regurgitant fraction.
Contrast-enhanced MRA (CE-MRA) Conventional CE-MRA has a complementary role to play with cardiac MRI for certain conditions. Threedimensional CE-MRA is useful in evaluating congenital heart disease especially when there is involvement of the great vessels or suspected anomalous pulmonary venous drainage. CE-MRA is also indicated for imaging aortic dissection and coarctation. In patients with pulmonary hypertension referred for an MR pulmonary angiogram a concurrent cardiac MR can provide reliable measures of right ventricular function3 and its accuracy is useful for assessing response to treatment and prognosis. A delayed enhancement study may be readily combined with CE-MRA in cases where myocardial viability is also being investigated.
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Quantified tissue characterization Patients with thalassaemia major are at risk of cardiac haemosiderosis which may impair cardiac function. A multi-echo sequence may be used to measure the T2* decay of the myocardial septum, which shows an inverse correlation with iron deposition.71 This may have value in assessing response to treatment using chelation therapy. Myocardial tagging is a technique that allows the direct visualization of the contractility of the left ventricle. A gradient echo cine sequence is acquired, which is preceded by a set of spatially selective pulses that form a two-dimensional grid across the heart. These tag lines decay with T1 recovery but persist long enough during the cardiac cycle to demonstrate the deformation of the contracting myocardial tissue. The images may be reviewed qualitatively to assess focal regions of reduced contraction, such as in hypertrophic cardiomyopathy72 (Fig. 5). However, quantitative strain analysis requires access to advanced image analysis software. Research into MR spectroscopy has delivered useful insights into cardiac energetics73 and the measurement of intramyocardial lipid.74 However, these techniques require considerable expertise and physics support and are not currently relevant to clinical practice (Fig. 7).
Post-processing A workstation to review the images offline is an essential requirement. Dedicated cardiac analysis
Figure 7 A three-dimensional b-SSFP sequence of the right coronary artery using navigator respiratory gating.
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software is required for quantitative analysis. The time taken to analyse and report a study will vary considerably according to the complexity of a case, but even a routine CMR protocol may require 30 min to complete. Cine images may be replayed to assess cardiac morphology and function. The most accurate technique for determining left ventricular volumes and mass is to trace the endo- and epicardial borders on the short axis images. A quicker method, which uses geometrical assumptions, involves tracing the endocardial border on the systolic and diastolic frames of just the two chamber view. Semi-automated techniques are available with edge-detection tools,75 but these are prone to inaccuracy. Papillary muscle should be separately demarcated and included in the mass measurement (Fig. 8). Care needs to be taken in identifying the basal section as this is a potential source of error.76 Normal ranges for ventricular function, mass and chamber size will depend on gender, age and constitution as well as the imaging planes chosen and the type of imaging sequence performed.77 However, there are limited data on physiological ranges for CMR using standardized data acquisition and analysis. 39 Perfusion sequences and wall motion abnormalities are generally appreciated qualitatively, but radial maps, analogous to those produced by radionuclide techniques, may also be generated. Valvular disease may be assessed qualitatively on the cine sequences that are sensitive to turbulent flow. Planimetry of the aortic valve area is also readily achieved and offers an alternative to transvalvular pressure gradient measurements.78
Figure 8 Edge-detection tools are employed on a workstation to identify endocardial and epicardial borders. The papillary muscle is separately defined.
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Regions of interest are drawn around the vessel wall on velocity-encoded images to derive cardiac output, peak systolic velocity and regurgitant fraction. The wealth of numerical and image data produced by a CMR study creates a potential difficulty in storing the information and imparting the salient information to clinicians. Creating reports on text-based radiology information systems may be time consuming and inefficient. Some manufacturer’s software allows the creation of structured report cards that include selected images and radial maps. Certainly an effective method of exporting images, graphs and maps to the picture archiving and communication system (PACS), as well as the written report, is a key advantage.
Running an efficient service A practised CMR team will aim to perform a myocardial viability protocol in less than an hour. However, during the initial phases two or three patients should be scheduled for a session to allow for training. A set of illustrated protocols to assist with planning image planes and administering contrast media is helpful, as is a well-organized set of sequences on the MR console. There is a wide range of indications for CMR,8 but the patients referred will depend on local service provision and clinicians’ preferences. Initially viability imaging for patients with ischaemic heart disease will probably form the majority of the referrals in a general adult centre. CMR may also be used in cases where other investigations have been equivocal or to evaluate a specific aspect of cardiac pathology. More complex cases will occasionally be referred, which may require the advice of an experienced CMR centre for interpretation. A core protocol of cine b-SSFP sequences is performed for every patient and these may be supplemented by pre-defined sets of additional sequences depending on the indication. Once the service is established a protocol for imaging under pharmacological stress may be considered. Experience from stress testing in the nuclear medicine or echocardiography departments will be invaluable, and careful consideration needs to be given to patient safety. There are a limited number of formal CMR training opportunities in the UK. For instance, a 1-week residential course is offered at Leeds General Infirmary and a 3-month attachment at The Royal Brompton Hospital in London. Specialized courses and fellowships are more widely available
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in continental Europe and North America, and CMR conferences offer opportunities for continuing medical education. The American College of Radiologists has made recommendations on the minimum training and experience required for radiologists and non-radiologists who perform CMR.79 They also recommend advanced life support accreditation for those supervising stress testing. A UK equivalent of these guidelines for sub-specialty training has not yet been made. CMR is a developing specialty and should be considered as part of a multi-technique approach to cardiac imaging. Clinico-radiological meetings are a useful way of discussing cases amongst the referring clinicians. Collaboration with cardiologists and cardiothoracic surgeons is essential in building a consistent and reliable service. Comparing CMR findings with clinical outcomes and other imaging techniques is also an important audit exercise. There are many research opportunities in this expanding field, and CMR is also likely to take a key role in measuring outcome variables of interventional studies.
Conclusion CMR has evolved from a research tool into a sophisticated and accurate method for imaging and quantifying cardiovascular disease. In particular, it is likely to play an increasingly important role in the assessment of myocardial ischaemia and viability. However, it has a wide role in both adult and paediatric cardiac diagnostics and offers a number of specific advantages over other imaging methods. Increasing demand for CMR will lead to its expansion into greater numbers of centres that provide medical or surgical cardiac services. This will have significant implications for resources and time management in imaging departments. However, with appropriate planning and training an efficient CMR service can be readily established and make an important contribution to patient care.
Acknowledgements The authors thank the patients and staff of the Imaging Department, Hammersmith Hospitals NHS Trust, where the clinical service operates. The authors also thank Julie Fitzpatrick for commenting on a draft of the manuscript. DPO’R is funded by a grant from Schering Healthcare Ltd.
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References 1. Grothues F, Smith GC, Moon JCC, et al. Comparison of interstudy reproducibility of cardiovascular magnetic resonance with two-dimensional echocardiography in normal subjects and in patients with heart failure or left ventricular hypertrophy. Am J Cardiol 2002;90:29—34. 2. Fieno DS, Jaffe WC, Simonetti OP, Judd RM, Finn JP. TrueFISP: assessment of accuracy for measurement of left ventricular mass in an animal model. J Magn Reson Imaging 2002;15:526—31. 3. Beygui F, Furber A, Delepine S, et al. Routine breath-hold gradient echo MRI-derived right ventricular mass, volumes and function: accuracy, reproducibility and coherence study. Int J Cardiovasc Imaging 2004;20:509—16. 4. Bellenger NG, Davies LC, Francis JM, Coats AJ, Pennell DJ. Reduction in sample size for studies of remodeling in heart failure by the use of cardiovascular magnetic resonance. J Cardiovasc Magn Reson 2000;2:271—8. 5. Higgins CB, Wagner S, Kondo C, Suzuki J, Caputo GR. Evaluation of valvular heart disease with cine gradient echo magnetic resonance imaging. Circulation 1991;84: I198—I207. 6. Eichenberger AC, Jenni R, von Schulthess GK. Aortic valve pressure gradients in patients with aortic valve stenosis: quantification with velocity-encoded cine MR imaging. AJR Am J Roentgenol 1993;160:971—7. 7. Pruefer D, Kalden P, Schreiber W, et al. In vitro investigation of prosthetic heart valves in magnetic resonance imaging: evaluation of potential hazards. J Heart Valve Dis 2001;10: 410—4. 8. Pennell DJ, Sechtem UP, Higgins CB, et al. Clinical indications for cardiovascular magnetic resonance (CMR): consensus panel report. Eur Heart J 2004;25:1940—65. 9. Fricke E, Fricke H, Weise R, et al. Attenuation correction of myocardial SPECT perfusion images with low-dose CT: evaluation of the method by comparison with perfusion PET. J Nucl Med 2005;46:736—44. 10. Wagner A, Mahrholdt H, Holly TA, et al. Contrast-enhanced MRI and routine single photon emission computed tomography (SPECT) perfusion imaging for detection of subendocardial myocardial infarcts: an imaging study. Lancet 2003; 361:374—9. 11. Lund GK, Stork A, Saeed M, et al. Acute myocardial infarction: evaluation with first-pass enhancement and delayed enhancement MR imaging compared with 201Tl SPECT Imaging. Radiology 2004;232:49—57. 12. Kim WY, Danias PG, Stuber M, et al. Coronary magnetic resonance angiography for the detection of coronary stenoses. N Engl J Med 2001;345:1863—9. 13. Gerber BL, Coche E, Pasquet A, et al. Coronary artery stenosis: direct comparison of four-section multi-detector row CT and 3D navigator MR imaging for detection—initial results. Radiology 2005;234:98—108. 14. Kefer J, Coche E, Legros G, et al. Head-to-head comparison of three-dimensional navigator-gated magnetic resonance imaging and 16-slice computed tomography to detect coronary artery stenosis in patients. J Am Coll Cardiol 2005;46:92—100. 15. Nikolaou K, Flohr T, Knez A, et al. Advances in cardiac CT imaging: 64-slice scanner. Int J Cardiovasc Imaging 2004;20: 535—40. 16. Chiurlia E, Menozzi M, Ratti C, Romagnoli R, Modena MG. Follow-up of coronary artery bypass graft patency by multislice computed tomography. Am J Cardiol 2005;95: 1094—7.
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17. Schlosser T, Konorza T, Hunold P, Kuhl H, Schmermund A, Barkhausen J. Noninvasive visualization of coronary artery bypass grafts using 16-detector row computed tomography. J Am Coll Cardiol 2004;44:1224—9. 18. Langerak SE, Vliegen HW, de Roos A, et al. Detection of vein graft disease using high-resolution magnetic resonance angiography. Circulation 2002;105:328—33. 19. Pasterkamp G, Galis ZS, de Kleijn DPV. Expansive arterial remodeling: location, location, location. Arterioscler Thromb Vasc Biol 2004;24:650—7. 20. Kim WY, Stuber M, Bornert P, Kissinger KV, Manning WJ, Botnar RM. Three-dimensional black-blood cardiac magnetic resonance coronary vessel wall imaging detects positive arterial remodeling in patients with nonsignificant coronary artery disease. Circulation 2002;106:296—9. 21. Hoppe UC, Dederichs B, Deutsch HJ, Theissen P, Schicha H, Sechtem U. Congenital heart disease in adults and adolescents: comparative value of transthoracic and transesophageal echocardiography and MR imaging. Radiology 1996;199: 669—77. 22. Geva T, Vick III GW, Wendt RE, Rokey R. Role of spin echo and cine magnetic resonance imaging in presurgical planning of heterotaxy syndrome. Comparison with echocardiography and catheterization. Circulation 1994;90:348—56. 23. Razavi RS, Hill DL, Muthurangu V, et al. Three-dimensional magnetic resonance imaging of congenital cardiac anomalies. Cardiol Young 2003;13:461—5. 24. Soman P, Bokor D, Lahiri A. Why cardiac magnetic resonance imaging will not make it. J Comput Assist Tomogr 1999; 23(Suppl 1):S143—S9. 25. Picano E. Economic and biological costs of cardiac imaging. Cardiovasc Ultrasound 2005;3:13. 26. Board of the Faculty of Clinical Radiology, The Royal College of Radiologists. Clinical radiology—writing a good business case. London: Royal College of Radiologists; 1996. 27. Schick F. Whole-body MRI at high field: technical limits and clinical potential. Eur Radiol 2005;15:946—59. 28. Wen H, Denison TJ, Singerman RW, Balaban RS. The intrinsic signal-to-noise ratio in human cardiac imaging at 1.5, 3, and 4 T. J Magn Reson 1997;125:65—71. 29. Hinton DP, Wald LL, Pitts J, Schmitt F. Comparison of cardiac MRI on 1.5 and 3.0 Tesla clinical whole body systems. Invest Radiol 2003;38:436—42. 30. Gutberlet M, Spors B, Grothoff M, et al. Comparison of different cardiac MRI sequences at 1.5 T/3.0 T with respect to signal-to-noise and contrast-to-noise ratios—initial experience. Rofo 2004;176:801—8. 31. Schar M, Kozerke S, Fischer SE, Boesiger P. Cardiac SSFP imaging at 3 Tesla. Magn Reson Med 2004;51:799—806. 32. Scheffler K, Lehnhardt S. Principles and applications of balanced SSFP techniques. Eur Radiol 2003;13:2409—18. 33. Fischer SE, Wickline SA, Lorenz CH. Novel real-time R-wave detection algorithm based on the vectorcardiogram for accurate gated magnetic resonance acquisitions. Magn Reson Med 1999;42:361—70. 34. Hunold P, Maderwald S, Ladd ME, Jellus V, Barkhausen J. Parallel acquisition techniques in cardiac cine magnetic resonance imaging using TrueFISP sequences: comparison of image quality and artifacts. J Magn Reson Imaging 2004;20: 506—11. 35. Martin ET, Coman JA, Shellock FG, Pulling CC, Fair R, Jenkins K. Magnetic resonance imaging and cardiac pacemaker safety at 1.5-Tesla. J Am Coll Cardiol 2004;43:1315—24. 36. Roguin A, Donahue JK, Bomma CS, Bluemke DA, Halperin HR. Cardiac magnetic resonance imaging in a patient with implantable cardioverter-defibrillator. Pacing Clin Electrophysiol 2005;28:336—8.
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37. Hartnell GG, Spence L, Hughes LA, Cohen MC, Saouaf R, Buff B. Safety of MR imaging in patients who have retained metallic materials after cardiac surgery. AJR Am J Roentgenol 1997;168:1157—9. 38. Schroeder AP, Houlind K, Pedersen EM, Thuesen L, Nielsen TT, Egeblad H. Magnetic resonance imaging seems safe in patients with intracoronary stents. J Cardiovasc Magn Reson 2000;2:43—9. 39. Bogaert J, Dymarkowski S, Taylor AM. Clinical cardiac MRI. Berlin: Springer; 2005. 40. Nagel E, Schneider U, Schalla S, et al. Magnetic resonance real-time imaging for the evaluation of left ventricular function. J Cardiovasc Magn Reson 2000;2:7—14. 41. Francone M, Dymarkowski S, Kalantzi M, Bogaert J. Realtime cine MRI of ventricular septal motion: a novel approach to assess ventricular coupling. J Magn Reson Imaging 2005; 21:305—9. 42. Roussakis A, Baras P, Seimenis I, Andreou J, Danias PG. Relationship of number of phases per cardiac cycle and accuracy of measurement of left ventricular volumes, ejection fraction, and mass. J Cardiovasc Magn Reson 2004;6:837—44. 43. Abbara S, Migrino RQ, Sosnovik DE, Leichter JA, Brady TJ, Holmvang G. Value of fat suppression in the MRI evaluation of suspected arrhythmogenic right ventricular dysplasia. AJR Am J Roentgenol 2004;182:587—91. 44. Pennell DJ. Cardiovascular magnetic resonance and the role of adenosine pharmacologic stress. Am J Cardiol 2004;94: 26—31. 45. Reimer P, Bremer C, Allkemper T, et al. Myocardial perfusion and MR angiography of chest with SH U 555 C: results of placebo-controlled clinical phase I study. Radiology 2004; 231:474—81. 46. Christian TF, Rettmann DW, Aletras AH, et al. Absolute myocardial perfusion in canines measured by using dualbolus first-pass MR imaging. Radiology 2004;232:677—84. 47. Barkhausen J, Hunold P, Jochims M, Debatin JF. Imaging of myocardial perfusion with magnetic resonance. J Magn Reson Imaging 2004;19:750—7. 48. Pereira RS, Prato FS, Wisenberg G, Sykes J, Yvorchuk KJ. The use of Gd-DTPA as a marker of myocardial viability in reperfused acute myocardial infarction. Int J Cardiovasc Imaging 2001;17:395—404. 49. Kim RJ, Chen E-L, Lima JAC, Judd RM. Myocardial Gd-DTPA kinetics determine MRI contrast enhancement and reflect the extent and severity of myocardial injury after acute reperfused infarction. Circulation 1996;94: 3318—26. 50. Kellman P, Arai AE, McVeigh ER, Aletras AH. Phase-sensitive inversion recovery for detecting myocardial infarction using gadolinium-delayed hyperenhancement. Magn Reson Med 2002;47:372—83. 51. Look DC, Locker DR. Time saving in measurement of NMR and EPR relaxation times. Rev Sci Instrum 1970;41:250—1. 52. Rehwald WG, Fieno DS, Chen E-L, Kim RJ, Judd RM. Myocardial magnetic resonance imaging contrast agent concentrations after reversible and irreversible ischemic injury. Circulation 2002;105:224—9. 53. Kim RJ, Wu E, Rafael A, et al. The use of contrast-enhanced magnetic resonance imaging to identify reversible myocardial dysfunction. N Engl J Med 2000;343:1445—53. 54. McCrohon JA, Moon JCC, Prasad SK, et al. Differentiation of heart failure related to dilated cardiomyopathy and coronary artery disease using gadolinium-enhanced cardiovascular magnetic resonance. Circulation 2003;108:54—9. 55. Vignaux O. Cardiac sarcoidosis: spectrum of MRI features. AJR Am J Roentgenol 2005;184:249—54.
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56. Mahrholdt H, Goedecke C, Wagner A, et al. Cardiovascular magnetic resonance assessment of human myocarditis: a comparison to histology and molecular pathology. Circulation 2004;109:1250—8. 57. Hunold P, Schlosser T, Vogt FM, et al. Myocardial late enhancement in contrast-enhanced cardiac MRI: distinction between infarction scar and non-infarction-related disease. AJR Am J Roentgenol 2005;184:1420—6. 58. Selvanayagam JB, Spyrou N, Francis JM, Neubauer S. Resolution of ventricular thrombus identified by contrast enhanced cardiac MRI. Int J Cardiovasc Imaging 2004;20: 369—70. 59. Luna A, Ribes R, Caro P, Vida J, Erasmus JJ. Evaluation of cardiac tumors with magnetic resonance imaging. Eur Radiol 2005;15:1446—55. 60. Wellnhofer E, Olariu A, Klein C, et al. Magnetic resonance low-dose dobutamine test is superior to scar quantification for the prediction of functional recovery. Circulation 2004; 109:2172—4. 61. Samady H, Elefteriades JA, Abbott BG, Mattera JA, McPherson CA, Wackers FJT. Failure to improve left ventricular function after coronary revascularization for ischemic cardiomyopathy is not associated with worse outcome. Circulation 1999;100:1298—304. 62. Kim RJ, Manning WJ. Viability assessment by delayed enhancement cardiovascular magnetic resonance: will low-dose dobutamine dull the shine? Circulation 2004;109:2476—9. 63. Nagel E, Lorenz C, Baer F, et al. Stress cardiovascular magnetic resonance: consensus panel report. J Cardiovasc Magn Reson 2001;3:267—81. 64. So NMC, Lam WWM, Li D, Chan AKY, Sanderson JE, Metreweli C. Magnetic resonance coronary angiography with 3D TrueFISP: breath-hold versus respiratory gated imaging. Br J Radiol 2005;78:116—21. 65. Oliver M, Weber AJM, Higgins CB. Whole-heart steady-state free precession coronary artery magnetic resonance angiography. Magn Reson Med 2003;50:1223—8. 66. Stehning C, Bornert P, Nehrke K, Eggers H, Stuber M. Freebreathing whole-heart coronary MRA with 3D radial SSFP and self-navigated image reconstruction. Magn Reson Med 2005; 54:476—80. 67. Spuentrup E, Ruebben A, Mahnken A, et al. Artifact-free coronary magnetic resonance angiography and coronary vessel wall imaging in the presence of a new, metallic, coronary magnetic resonance imaging stent. Circulation 2005;111:1019—26. 68. Westenberg JJ, Danilouchkine MG, Doornbos J, et al. Accurate and reproducible mitral valvular blood flow measurement with three-directional velocity-encoded magnetic resonance imaging. J Cardiovasc Magn Reson 2004;6: 767—76. 69. Mohiaddin RH, Kilner PJ, Rees S, Longmore DB. Magnetic resonance volume flow and jet velocity mapping in aortic coarctation. J Am Coll Cardiol 1993;22:1515—21. 70. Stauder NI, Scheule AM, Hahn U, et al. Perioperative monitoring of flow and patency in native and grafted internal mammary arteries using a combined MR protocol. Br J Radiol 2005;78:292—8. 71. Westwood M, Anderson LJ, Firmin DN, et al. A single breathhold multiecho T2* cardiovascular magnetic resonance technique for diagnosis of myocardial iron overload. J Magn Reson Imaging 2003;18:33—9. 72. Young AA, Kramer CM, Ferrari VA, Axel L, Reichek N. Threedimensional left ventricular deformation in hypertrophic cardiomyopathy. Circulation 1994;90:854—67. 73. Beer M, Seyfarth T, Sandstede J, et al. Absolute concentrations of high-energy phosphate metabolites in normal,
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hypertrophied, and failing human myocardium measured noninvasively with 31P-SLOOP magnetic resonance spectroscopy. J Am Coll Cardiol 2002;40:1267—74. 74. Szczepaniak LS, Dobbins RL, Metzger GJ, et al. Myocardial triglycerides and systolic function in humans: in vivo evaluation by localized proton spectroscopy and cardiac imaging. Magn Reson Med 2003;49:417—23. 75. Latson LA, Powell KA, Sturm B, Schvartzman PR, White RD. Clinical validation of an automated boundary tracking algorithm on cardiac MR images. Int J Cardiovasc Imaging 2001;17:279—86. 76. Marcus JT, Gotte MJ, DeWaal LK, et al. The influence of through-plane motion on left ventricular volumes measured by magnetic resonance imaging: implications for image acquisition and analysis. J Cardiovasc Magn Reson 1999;1:1—6.
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77. Alfakih K, Plein S, Thiele H, Jones T, Ridgway JP, Sivananthan MU. Normal human left and right ventricular dimensions for MRI as assessed by turbo gradient echo and steady-state free precession imaging sequences. J Magn Reson Imaging 2003;17:323—9. 78. Haimerl J, Freitag-Krikovic A, Rauch A, Sauer E. Quantification of aortic valve area and left ventricular muscle mass in healthy subjects and patients with symptomatic aortic valve stenosis by MRI. Z Kardiol 2005;94:173—81. 79. American College of Radiology. ACR practice guideline for the perfomance of cardiovascular magnetic resonance imaging. In: Practice guidelines and technical standards. Reston, VA; 2002. p. 157–70.
REVIEW
Establishing a clinical cardiac MRI service D.P. O’Regan*, S.A. Schmitz Imaging Sciences Department, MRC Clinical Sciences Centre, Hammersmith Hospital Campus, Imperial College, London, UK Received 6 July 2005; received in revised form 27 October 2005; accepted 29 October 2005
After several years of research development cardiovascular MRI has evolved into a widely accepted clinical tool. It offers important diagnostic and prognostic information for a variety of clinical indications, which include ischaemic heart disease, cardiomyopathies, valvular dysfunction and congenital heart disorders. It is a safe non-invasive technique that employs a variety of imaging sequences optimized for temporal or spatial resolution, tissue-specific contrast, flow quantification or angiography. Cardiac MRI offers specific advantages over conventional imaging techniques for a significant number of patients. The demand for cardiac MRI studies from cardiothoracic surgeons, cardiologists and other referrers is likely to continue to rise with pressure for more widespread local service provision. Setting up a cardiac MRI service requires careful consideration regarding funding issues and how it will be integrated with existing service provision. The purchase of cardiac phased array coils, monitoring equipment and software upgrades must also be considered, as well as the training needs of those involved. The choice of appropriate imaging protocols will be guided by operator experience, clinical indication and equipment capability, and is likely to evolve as the service develops. Post-processing and offline analysis form a significant part of the time taken to report studies and an efficient method of providing quantitative reports is an important requirement. Collaboration between radiologists and cardiologists is needed to develop a successful service and multi-disciplinary meetings are key component of this. This review will explore these issues from our perspective of a new clinical cardiac MRI service operating over its first year in a teaching hospital imaging department. Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction Over the last decade cardiac magnetic resonance imaging (CMR) has evolved from a research tool into a sophisticated and robust method for investigating cardiovascular pathology in a clinical setting. Improvements in gradient performance, coil design and imaging sequences have considerably improved acquisition times and image quality, establishing CMR as a reliable non-invasive method of assessing cardiac morphology and function. It combines excellent spatial and temporal resolution with high contrast between the blood pool and the myocardium. This allows highly reproducible and * Guarantor and correspondent: D.P. O’Regan, Imaging Sciences Department, Clinical Sciences Centre, Faculty of Medicine, Imperial College, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK. Tel.: C44 20 83831510; fax: C44 20 83833038. E-mail address: [email protected] (D.P. O’Regan).
accurate1–3 measurements to be made of global ventricular function and mass, making it the ideal method for assessing responses to treatment.4 It also offers accurate quantification of the severity of valvular disease,5,6 including the safe evaluation of prosthetic valve replacements.7 A key role for CMR is in the investigation of ischaemic heart disease where regions of myocardial ischaemia and infarction may be identified, and tissue viability assessed before revascularization. It also has clinical value in the investigation of aortic disease, cardiomyopathy and myocarditis, assessing pericardial pathology, cardiac tumours and in the diagnosis and follow-up of congenital heart disease.8 Applications such as coronary artery imaging and plaque characterization are also becoming more widely available. CMR now has a mainstream role in the investigation of cardiovascular disease and increasing demand is likely to lead to the development of services outside supra-regional academic centres. Both radiologists and cardiologists may be expected
0009-9260/$ - see front matter Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2005.10.015
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to take the lead in developing new cardiac imaging services. However, the initiation of a CMR facility requires careful consideration regarding issues such as funding, equipment, training and the effective use of specialized imaging sequences and analysis software. CMR benefits from a multidisciplinary approach and clinico-radiological conferences involving radiologists, radiographers, cardiologists and cardiothoracic surgeons are an important component of this. This review will explore these issues from our perspective of setting up a new clinical cardiac MRI service operating over its first year in a teaching hospital imaging department.
The place of CMR in cardiac imaging CMR has emerged as a reliable and valid tool for the investigation of heart disease. A key advantage of CMR is that it provides a detailed assessment of cardiac morphology, function and tissue characterization in a single examination. A standard study can readily be performed within an hour including patient preparation. Its accuracy and reproducibility make it in ideal technique for assessing disease severity and response to treatment. There is intrinsically high contrast between the myocardium and blood pool, which allows cine images and coronary angiograms to be acquired without contrast. The high spatial resolution of CMR and tissue contrast allow quantification of ischaemic subendocardial scarring that surpasses other techniques. Its three-dimensional capabilities allow complete flexibility for imaging not only the heart, but the great vessels and pulmonary vasculature as well. Despite the restrictions of the MR hardware more invasive procedures such as stress-testing can be performed safely. There are a number of other well-validated imaging techniques for investigating cardiac disease, which have strengths and weaknesses compared with CMR. Echocardiography currently provides the mainstay of non-invasive cardiac imaging. Transthoracic echocardiography is a rapid and inexpensive imaging technique, which will remain a first-line modality for assessing cardiac function and morphology. However, it is an operator-dependent technique that requires an adequate acoustic window to obtain images and may suffer from near-field artefacts. In comparative studies CMR has demonstrated better interobserver4 and inter-study1 variability of functional and structural indices than echocardiography. Radionuclide techniques allow assessment of myocardial metabolism and provide low-resolution
D.P. O’Regan, S.A. Schmitz
morphological imaging. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) have an established role in identifying hibernating myocardium and assessing tissue viability. However, the specificity of SPECT imaging without attenuation correction is significantly reduced by artefacts due to photon absorption9 especially in obese patients. The high spatial resolution of CMR allows detection of subendocardial infarcts that are missed by SPECT,10 especially those involving the inferior wall.11 Catheter angiography remains the gold standard for luminal coronary imaging. However, it has a small but recognized morbidity and mortality, requires iodinated contrast medium and uses ionizing radiation. The development of non-invasive techniques for coronary angiography has a huge potential for cost reduction and patient safety. Both MR and contrast-enhanced computed tomography (CT) have made substantial progress in improving visualization of the coronary vessels in recent years. Comparisons between these techniques are hampered by the profusion of MR sequence designs and the continuing development of CT detector systems. In a multicentre trial threedimensional coronary MR angiography (MRA) with navigator respiratory gating achieved an accuracy of 72% at detecting clinically significant lesions compared with catheter angiography,12 and has demonstrated equivalent accuracy to 4 and 16-row multidetector CT.13,14 MR has a negative predictive value for any coronary artery disease of 81%, and 100% for left main stem or triple vessel disease.12 However, newer 64 detector row CT systems providing an isotropic voxel size of 0.4 mm and a temporal resolution of up to 83 ms may improve on these standards.15 Although CT angiography is rapid and requires minimal planning, MRA has the advantages of avoiding the need for contrast medium, ionizing radiation and premedication with b-blockers. CT16,17 and MR18 also have an important use in assessing the patency of bypass grafts, which are subject to less cardiac movement and may be difficult to demonstrate with catheter angiography. Luminal angiography masks the severity of early atherosclerosis,19 and both MRI20 and CT15 have a promising role in characterizing and quantifying atherosclerotic plaque. The evaluation of congenital cardiac defects in both infants and adults is a strength of CMR due to its multiplanar imaging of the heart and great vessels. It allows a detailed assessment of complex cardiovascular anatomy especially after surgical correction. It has the advantage over transthoracic echocardiography of being able to image the
Establishing a clinical cardiac MRI service
anatomy of the great vessels and other extracardiac abnormalities,21 and may also reduce the need for invasive catheter studies.22 Although expert supervision is usually needed to plan these studies, the development of new three-dimensional volume acquisitions allows offline reconstructions to be made in any plane.23 Despite the appeal of CMR for a number of clinical indications it has unproven cost-effectiveness, has limited availability, and requires trained personnel.24 The cost of a resting CMR is estimated as five times that of echocardiography, but is only a quarter of the cost of invasive catheterization.25 The oncogenic potential of imaging techniques using ionizing radiation is not insignificant25 and should be an important consideration for the investigation of low-risk groups and those needing repeated examinations. However, some patients may have contraindications to MRI or may not tolerate the study due to claustrophobia. Image quality may also be adversely affected by inadequate breath-holds or cardiac arrhythmias. The role of CMR amongst the armamentarium of conventional imaging techniques is still evolving and may continue to do so as new applications are developed.
The business case If there is a demand for a service development such as CMR then a business case may need to be made to demonstrate that the project is financially viable, will benefit patients and is supported by purchasers.26 Issues such as how to provide an effective and efficient service, the amount of capital spending required and ensuring value for money need to be addressed. Potential cost savings in reducing referrals elsewhere or lessening the need for other investigations may offset some of the expenditure. Provision needs to be made for both the capital costs of acquiring additional MR equipment and a post-processing workstation, as well as the recruitment and training of staff. Depending on workload, at least one radiographer should be fully trained in cardiac MR who can then train other members of the department. Experienced hands-on supervision is required to assist in the planning of studies and the choice of sequences during the training period. The additional pressure on existing MRI workload also needs to be considered and providing an extended hours service may be one way of increasing capacity. Close liaison with clinicians to ensure an efficient referral base is essential.
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Hardware Cardiac imaging capability should be considered during the tendering process for new MR installations if there is a likelihood that this service may be provided in the future. However, many conventional 1.5 T whole-body MRI systems may be adapted or upgraded to cardiac capability. Regional units and research institutes may be able to establish a dedicated CMR facility within their departments. There are high gradient performance requirements for CMR in order to minimize repetition times and improve temporal resolution. As a result radiofrequency specific absorption rates (SAR) may approach their maximum permissible levels in some sequences particularly at higher field strengths. Most experience in CMR has been gained at 1.5 T, however there is increasing interest in the use of 3 T systems, which have already shown promise in other body MR applications.27 Imaging at 3 T has the immediate advantage of a potential doubling of signal-to-noise ratio compared with conventional 1.5 T platforms.28–30 However, greater susceptibility effects, field inhomogeneity and SAR limits have posed difficulties for ultra-high field cardiac systems29,31 and they are likely to remain a research tool for CMR at present. Dedicated phased array cardiac surface coils are used which offer favourable signal to noise characteristics and allow the use of parallel imaging techniques. These coils typically comprise at least four elements and are positioned over the anterior and posterior chest walls (Fig. 1). Satisfactory positioning of the coil over the heart may be confirmed on the initial scout images and this is essential to minimize off-resonance artefacts.32
Figure 1 The anterior elements of a six-channel phased array cardiac coil. Accurate positioning over the heart is needed to optimize image quality and reduce offresonance artefacts.
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CMR studies are gated to the cardiac cycle and this is usually achieved with a modified ECG. Due to the magnetohydrodynamic effect of flowing blood in a magnetic field the morphology of the ECG waveform is altered which interferes with conventional triggering methods. To overcome this manufacturers provide a vector-ECG system to improve the reliability of R-wave detection33 (Fig. 2). Most sequences employ retrospective cardiac gating in which the operator predetermines the number of cardiac phases required and data are acquired continuously throughout the cardiac cycle. If the R–R interval lies outside a predetermined arrhythmia rejection window then the scanner will ignore the data and continue acquiring phase-encoding steps. This may mean that patients with irregular heart rates have longer breath-holds to perform. Parallel imaging can be used to substantially reduce acquisition time but this is at the expense of signalto-noise ratio and may lead to reconstruction artefacts over the centre of the image.34 For delayed enhancement studies a hand injection of contrast medium via an intravenous cannula is sufficient, while perfusion studies are ideally performed with a dual-headed MR compatible pump. If stress CMR is undertaken attention needs to be paid to the potential difficulties involved in monitoring and treating patients in an MR unit. Patients with cardiac pacemakers or implantable defibrillators are not imaged with MR, although there is evidence of their potential safety in certain circumstances.35,36 Valve prostheses,7 epicardial pacing wires37 and coronary stents38 are not considered contraindications to MRI at 1.5 T.
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However, metallic devices cause a local susceptibility effect, which may impair image quality.
Cardiac imaging sequences CMR makes use of specialized pulse sequence designs that are tailored to the demands of cardiac imaging (Table 1). Knowledge of the characteristics of each sequence is necessary for their appropriate use and interpretation. Each sequence uses some form of cardiac gating and is performed either as a breath-hold or using respiratory gating. The imaging planes that are chosen are analogous to that used by echocardiography, and are orientated around the short and long axes of the left ventricle.39 A basic protocol would include cine gradient echo sequences of the two and four-chamber views, the left ventricular outflow tract and a stack of parallel slices through the left ventricular short axis (Fig. 3). Additional axial, coronal and sagittal planes may be planned to produce an overview of the heart and great vessels using either cine gradient echo sequences or spin-echo black blood sequences. Free-breathing real-time MR sequences40 are an ideal way to interactively plan the cardiac imaging planes, and they may also have a role in investigating septal motion during respiration.41 Dedicated sequences are used to assess perfusion, delayed enhancement, flow quantification and coronary angiography according to the clinical indication.
Morphology and function
Figure 2 A vector electrocardiograph system uses four electrodes placed over the chest wall. Software processing allows identification of the R-wave whilst the patient undergoes scanning.
The mainstay of functional imaging is the balanced steady-state free procession (b-SSFP) sequence, which is known by various manufacturer’s acronyms (Philips: b-TFE, Siemens: TrueFisp, General Electric: Fiesta). This is a bright-blood rapid gradient echo sequence that is well suited to CMR owing to its inherent high signal to noise ratio per unit time and excellent contrast between the lumen and myocardium. Its contrast properties depend on the ratio of T2* to T1 relaxation times, and is therefore relatively insensitive to gadolinium enhancement. Dephasing of spins by turbulent blood flow, such as valvular regurgitation, is visible as signal loss. During a breath-hold a user-defined number of frames per RR interval may be acquired over six to 10 heart beats to produce cine images of the cardiac cycle. A minimum of 11 cardiac phases is required in order to accurately identify the endsystolic phase,42 but higher temporal resolutions
Establishing a clinical cardiac MRI service
Table 1
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Summary of principal imaging protocols for clinical cardiac magnetic resonance.
Purpose
Sequence
Imaging planes
Cardiac gating
Respiratory gating
Intravenous contrast
Function
Cine b-SSFP
VLA, HLA, LVOT, LV short axis
Retrospective: 20–40 cardiac phases
Breath-hold
None
Flow quantification
Phase contrast gradient echo
Perpendicular to a vessel
Triggered: 10–20 cardiac phases
Breath-hold/ navigator
None
High resolution anatomical imaging
Double IR TSEG anterior saturation slab
Transaxial, coronal or sagittal
Triggered: end diastole
Breath-hold
None
Perfusion
Gradient echo, EPI or b-SSFP
LV short axis
Triggered: end diastole
Breath-hold/free breath
First pass bolus of Gd-DTPA
Viability/late enhancement
Two or threedimensional IR gradient echo
LV short axis, HLA, VLA, LVOT
Triggered: end diastole
Breath-hold
15 min postinjection Gd-DTPA
Coronary MRA
Three-dimensional b-SSFP with frequency selective fat suppression and T2 preparation prepulse
Targeted volume of a coronary artery or whole heart
Triggered to diastasis (latediastole)
Breath-hold or navigator echo gating
None
b-SSFP, balanced steady-state free precession; VLA, vertical long axis; HLA, horizontal long axis; LVOT, left ventricular outflow tract; LV, left ventricle; IR, inversion recovery; EPI, echo planar imaging; TSE, turbo spin echo.
are readily achieved and are particularly useful for assessing valve motion. Black blood spin-echo sequences have a complementary role to play in conjunction with b-SSFP cine images in providing detailed anatomical imaging of the heart and great vessels. Fast spinecho techniques obtain excellent suppression of flowing blood by using a double-inversion pulse technique. The sequences are usually triggered to end-diastole and are respiratory gated. These are useful for assessing cardiac anatomy in congenital heart disease and evaluating the aorta for dissection or coarctation. T1-weighted sequences may be supplemented by fat-suppressed images for confirming the presence of intramyocardial fat in arrhythmogenic right ventricular dysplasia (ARVD).43
Perfusion imaging Coronary vasodilators, such as adenosine, are used to induce a perceptible disparity in first-pass perfusion between normal and hypoperfused myocardium supplied by a diseased artery.44 Perfusion imaging demonstrates delayed contrast wash-in in myocardium supplied by a stenosed artery compared with normally perfused tissue. If rest and stress perfusion studies are performed together
this has the advantage of being able to derive myocardial perfusion reserve indices. The use of intravascular contrast agents in perfusion studies45 and dual-bolus techniques46 may offer the potential to determine absolute myocardial blood flow in the future. Perfusion imaging after a bolus of a contrast agent has demanding imaging requirements. High temporal resolution is required to image the first pass of contrast, and high spatial resolution is needed to distinguish the sub-endocardial and subepicardial layers of the myocardium. In addition good contrast is needed between normal and ischaemic myocardium, as well as a near-linear relationship between contrast dose and signal intensity.47 A spatial presaturation pulse to suppress background myocardial signal is generally employed, and data readout can be achieved with gradient echo, echo planar imaging (EPI), or steadystate free precession techniques.
Delayed hyperenhancement A strength of CMR is its ability to provide myocardial tissue characterization at a resolution that cannot be matched by other imaging techniques. Hyperenhancement occurs when the concentration of gadolinium–diethylenetriamine penta-acetic acid
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Figure 3 b-SSFP sequences of the principal cardiac planes (top left: two chamber, top right: four chamber, bottom left: left ventricular outflow tract, bottom right: short axis views).
(Gd-DTPA) in infarcted tissue is greater than its concentration in non-infarcted tissue. The concentration difference depends on both the volume of distribution of the contrast medium48 and to differential wash-in and wash-out rates.49 The GdDTPA is usually given in a double dose of 0.2 mmol/ kg and approximately 15 min later there is optimum contrast between normal and infarcted tissue to accurately assess subendocardial enhancement. An inversion recovery rapid gradient echo technique is used to acquire the delayed enhancement images (Fig. 4). In order to achieve maximal
contrast between normal and infarcted myocardium the inversion time is carefully chosen to null the signal from normal tissue, which may vary between patients due to contrast kinetics and tissue characteristics. This may be achieved by performing a range of inversion times on a single mid-ventricular section and choosing the image with optimal myocardial suppression. Alternative approaches such as phase-sensitive inversion recovery50 or the Look Locker technique51 are also available. A short axis stack through the left ventricle in end-diastole is then performed either
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myocardial contractile reserve. Stress testing may be performed to exceed this reserve and reveal regions of inducible ischaemia. For instance the b-agonist dobutamine increases myocardial oxygen demand beyond that which can be met by a stenosed artery. This induces a wall motion abnormality in heart muscle that may have contracted normally at rest. Another question that arises is whether hypokinetic myocardium seen during a rest study is viable; will its function improve after revascularization? Hibernating myocardium shows a biphasic response to increasing doses of dobutamine. Initially, there is a transient improvement in contractility at lower doses of dobutamine, which then declines at higher doses as the contractile reserve is exceeded. Short and long-axis b-SSFP cine sequences of the left ventricle are acquired at rest and then during each incremental increase of dobutamine until predefined termination criteria are reached.
Figure 4 Inversion recovery gradient echo sequence 15 min after Gd-DTA contrast medium injection. This demonstrates transmural enhancement and thinning in the left anterior descending artery territory. This indicates non-viability of these segments.
as a two or three-dimensional acquisition. This is supplemented by end-diastolic images in the other conventional cardiac planes after increasing the inversion time incrementally to maintain optimum contrast. Regional myocardial contrast enhancement is associated with histological evidence of irreversible ischaemic injury.52 In clinical trials there is an inverse relationship between the mural thickness of hyperenhancement and functional recovery after revascularization.53 This technique provides invaluable prognostic information on the viability of dysfunctional myocardium and its subsequent response to bypass grafting. The same sequence may be used for the investigation of dilated cardiomyopathy,54 amyloidosis,39 sarcoidosis55 and myocarditis,56 which show non-specific enhancement that does not conform to the typical subendocardial pattern seen in ischaemic disease.57 Post-contrast studies are also of particular value in characterizing intra-cardiac thrombus58 and tumours.59
Stress imaging The ischaemic effects of coronary artery disease may not be apparent at rest if there is sufficient
Delayed enhancement versus stress imaging to assess viability How do the two techniques of delayed-enhancement and dobutamine-stress MRI compare in terms of assessing myocardial viability? For dysfunctional regions with R50% scar on delayed-enhancement MR the negative predictive value for functional response to revascularization is around 90%53— which offers equal or higher confidence than dobutamine stress MR.60 However, for less extensive subendocardial scars (25–50%) dobutamine stress MR has shown that the group with a positive contractile reserve show a significant improvement in the rate of functional improvement after revascularization. 60 However, the functional response to revascularization may not be the only useful end-point in such patients with an intermediate thickness of viable myocardium. For instance, failure to improve global left ventricular function after coronary artery bypass grafting for ischaemic cardiomyopathy is not always associated with a poorer outcome.61 A possible explanation might be that effective revascularization of viable tissue protects against future infarction and arrhythmias. For subendocardial scars of !25% mural thickness dobutamine stress MR may offer a higher positive predictive value than delayedenhancement MR.60 However, these patients have a significant potential for a response to revascularization that might be denied to them if the dobutamine stress MR produced a false-negative result.62 In practical terms, delayed-enhancement
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MR is relatively safe to implement and straightforward to analyse. After suitable training these studies can be readily performed by unsupervised radiographers as part of a routine cardiac protocol. Stress testing may also be performed safely,63 but requires careful patient selection and monitoring, and experienced supervision throughout the study.
Coronary MRA Stress-MR and perfusion imaging provide information about the haemodynamics of coronary artery disease. However, a method of non-invasively assessing the coronary artery lumen and vessel wall would have advantages for surgical planning and risk stratification. Unfortunately, imaging the coronary arteries is challenging because of their small diameter and considerable cardio-respiratory motion. A conventional approach to performing a coronary MRA would begin with identifying the quiescent period of cardiac motion on a four-chamber cine sequence. A stack of axial images through the heart with the appropriate trigger delay and acquisition window for diastasis are then acquired using the navigator echo for respiratory gating. These images are then used to plan suitable imaging planes for each coronary artery using a three-point planning technique. A typical MRA protocol would use an ECGtriggered three-dimensional b-SSFP sequence, with fat suppression and a T2 prepulse, using a navigator echo for motion correction64 (Fig. 7). An increasingly used alternative is to acquire a threedimensional acquisition of the whole heart and subsequently reformat the images to demonstrate the coronary arteries. This approach obviates the need for targeted planning and can be achieved in under 15 min.65 A yet more efficient strategy uses image data extracted from a free-breathing acquisition to allow for retrospective motion correction.66 The increasing use of coronary stents also poses problems in creating a local susceptibility artefact, although prototype artefact-free stents have been developed.67 In clinical use, the value of coronary " Figure 5 Apico-septal hypertrophic cardiomyopathy. (a) A b-SSFP image shows asymmetrical thickening of the septum. (b) Myocardial tagging demonstrates reduced contractile deformation in the hypertrophied septum. (c) An inversion recovery gradient echo sequence shows patchy focal enhancement (arrows) that spares the subendocardium.
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MRA sequences is generally limited to evaluating the patency of the proximal arteries and identifying coronary artery anatomical variants (Fig. 5).
Flow quantification Cine phase contrast techniques are used to provide quantitative assessments of flow through a vascular lumen. These sequences use a bipolar gradient to create a linear relationship between the velocity of the blood and the phase of the transverse component of the MR signal. A velocity-encoding (venc) parameter is chosen by the operator at which the maximum phase shift will occur. However, if the flow velocity exceeds this value then the phase shift will become aliased in the opposite direction. The choice of venc parameter is a compromise between avoiding aliasing and reducing sensitivity to slower velocities and this is usually assessed with a trial breath-hold acquisition. Grey-scale images are derived which show stationary blood and surrounding tissues as midgrey, with through-plane flow in one direction as mid-grey to white and in the opposite direction as mid-grey to black. The phase shift is directly proportional to velocity within each voxel. Flow versus time curves may be derived with the area under the curve indicating the stroke volume during the cardiac cycle (Fig. 6). In order to accurately perform this method a slice perpendicular to the vessel being examined should be obtained by planning on two orthogonal views. The aortic root and right ventricular outflow tract are most commonly examined to obtain pulmonary to systemic flow ratios (QP:QS) as well as cardiac output, peak systolic velocities and regurgitant fractions. Measurements across the mitral and tricuspid valves are complicated by the through plane movement of the base of the heart during the cardiac cycle, although attempts have been made to correct for this.68 Another approach is to use velocity-encoding sequences to measure pulmonary and systemic cardiac outputs, with the difference indicating the regurgitant volume across an atrioventricular valve. A similar logic can be used to estimate the severity of an intracardiac shunt if there is no co-existing valvular regurgitation. Attempts have also been made to measure flow characteristics of aortic coartation69 and of internal mammary coronary grafts70 with phase contrast techniques.
Figure 6 The top image shows blood flowing through a tricuspid aortic valve using a velocity encoded sequence. The bottom image demonstrates that a flow versus time curve may be derived showing a significant regurgitant fraction.
Contrast-enhanced MRA (CE-MRA) Conventional CE-MRA has a complementary role to play with cardiac MRI for certain conditions. Threedimensional CE-MRA is useful in evaluating congenital heart disease especially when there is involvement of the great vessels or suspected anomalous pulmonary venous drainage. CE-MRA is also indicated for imaging aortic dissection and coarctation. In patients with pulmonary hypertension referred for an MR pulmonary angiogram a concurrent cardiac MR can provide reliable measures of right ventricular function3 and its accuracy is useful for assessing response to treatment and prognosis. A delayed enhancement study may be readily combined with CE-MRA in cases where myocardial viability is also being investigated.
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Quantified tissue characterization Patients with thalassaemia major are at risk of cardiac haemosiderosis which may impair cardiac function. A multi-echo sequence may be used to measure the T2* decay of the myocardial septum, which shows an inverse correlation with iron deposition.71 This may have value in assessing response to treatment using chelation therapy. Myocardial tagging is a technique that allows the direct visualization of the contractility of the left ventricle. A gradient echo cine sequence is acquired, which is preceded by a set of spatially selective pulses that form a two-dimensional grid across the heart. These tag lines decay with T1 recovery but persist long enough during the cardiac cycle to demonstrate the deformation of the contracting myocardial tissue. The images may be reviewed qualitatively to assess focal regions of reduced contraction, such as in hypertrophic cardiomyopathy72 (Fig. 5). However, quantitative strain analysis requires access to advanced image analysis software. Research into MR spectroscopy has delivered useful insights into cardiac energetics73 and the measurement of intramyocardial lipid.74 However, these techniques require considerable expertise and physics support and are not currently relevant to clinical practice (Fig. 7).
Post-processing A workstation to review the images offline is an essential requirement. Dedicated cardiac analysis
Figure 7 A three-dimensional b-SSFP sequence of the right coronary artery using navigator respiratory gating.
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software is required for quantitative analysis. The time taken to analyse and report a study will vary considerably according to the complexity of a case, but even a routine CMR protocol may require 30 min to complete. Cine images may be replayed to assess cardiac morphology and function. The most accurate technique for determining left ventricular volumes and mass is to trace the endo- and epicardial borders on the short axis images. A quicker method, which uses geometrical assumptions, involves tracing the endocardial border on the systolic and diastolic frames of just the two chamber view. Semi-automated techniques are available with edge-detection tools,75 but these are prone to inaccuracy. Papillary muscle should be separately demarcated and included in the mass measurement (Fig. 8). Care needs to be taken in identifying the basal section as this is a potential source of error.76 Normal ranges for ventricular function, mass and chamber size will depend on gender, age and constitution as well as the imaging planes chosen and the type of imaging sequence performed.77 However, there are limited data on physiological ranges for CMR using standardized data acquisition and analysis. 39 Perfusion sequences and wall motion abnormalities are generally appreciated qualitatively, but radial maps, analogous to those produced by radionuclide techniques, may also be generated. Valvular disease may be assessed qualitatively on the cine sequences that are sensitive to turbulent flow. Planimetry of the aortic valve area is also readily achieved and offers an alternative to transvalvular pressure gradient measurements.78
Figure 8 Edge-detection tools are employed on a workstation to identify endocardial and epicardial borders. The papillary muscle is separately defined.
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Regions of interest are drawn around the vessel wall on velocity-encoded images to derive cardiac output, peak systolic velocity and regurgitant fraction. The wealth of numerical and image data produced by a CMR study creates a potential difficulty in storing the information and imparting the salient information to clinicians. Creating reports on text-based radiology information systems may be time consuming and inefficient. Some manufacturer’s software allows the creation of structured report cards that include selected images and radial maps. Certainly an effective method of exporting images, graphs and maps to the picture archiving and communication system (PACS), as well as the written report, is a key advantage.
Running an efficient service A practised CMR team will aim to perform a myocardial viability protocol in less than an hour. However, during the initial phases two or three patients should be scheduled for a session to allow for training. A set of illustrated protocols to assist with planning image planes and administering contrast media is helpful, as is a well-organized set of sequences on the MR console. There is a wide range of indications for CMR,8 but the patients referred will depend on local service provision and clinicians’ preferences. Initially viability imaging for patients with ischaemic heart disease will probably form the majority of the referrals in a general adult centre. CMR may also be used in cases where other investigations have been equivocal or to evaluate a specific aspect of cardiac pathology. More complex cases will occasionally be referred, which may require the advice of an experienced CMR centre for interpretation. A core protocol of cine b-SSFP sequences is performed for every patient and these may be supplemented by pre-defined sets of additional sequences depending on the indication. Once the service is established a protocol for imaging under pharmacological stress may be considered. Experience from stress testing in the nuclear medicine or echocardiography departments will be invaluable, and careful consideration needs to be given to patient safety. There are a limited number of formal CMR training opportunities in the UK. For instance, a 1-week residential course is offered at Leeds General Infirmary and a 3-month attachment at The Royal Brompton Hospital in London. Specialized courses and fellowships are more widely available
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in continental Europe and North America, and CMR conferences offer opportunities for continuing medical education. The American College of Radiologists has made recommendations on the minimum training and experience required for radiologists and non-radiologists who perform CMR.79 They also recommend advanced life support accreditation for those supervising stress testing. A UK equivalent of these guidelines for sub-specialty training has not yet been made. CMR is a developing specialty and should be considered as part of a multi-technique approach to cardiac imaging. Clinico-radiological meetings are a useful way of discussing cases amongst the referring clinicians. Collaboration with cardiologists and cardiothoracic surgeons is essential in building a consistent and reliable service. Comparing CMR findings with clinical outcomes and other imaging techniques is also an important audit exercise. There are many research opportunities in this expanding field, and CMR is also likely to take a key role in measuring outcome variables of interventional studies.
Conclusion CMR has evolved from a research tool into a sophisticated and accurate method for imaging and quantifying cardiovascular disease. In particular, it is likely to play an increasingly important role in the assessment of myocardial ischaemia and viability. However, it has a wide role in both adult and paediatric cardiac diagnostics and offers a number of specific advantages over other imaging methods. Increasing demand for CMR will lead to its expansion into greater numbers of centres that provide medical or surgical cardiac services. This will have significant implications for resources and time management in imaging departments. However, with appropriate planning and training an efficient CMR service can be readily established and make an important contribution to patient care.
Acknowledgements The authors thank the patients and staff of the Imaging Department, Hammersmith Hospitals NHS Trust, where the clinical service operates. The authors also thank Julie Fitzpatrick for commenting on a draft of the manuscript. DPO’R is funded by a grant from Schering Healthcare Ltd.
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