- Email: [email protected]
MR and CT Imaging of the Pediatric Patient With Structural Heart Disease
MR and CT Imaging of the Pediatric Patient With Structural Heart Disease Frandics P. Chan Cardiac magnetic resonance imaging (MRI) and computed tomography (CT) are imaging modalities increasingly used in the diagnosis and management of structural heart disease. They are powerful imaging tools that have individual strengths and weaknesses. Rational choice between MRI and CT should be based on a sound understanding of these issues. Management guidelines that incorporate the use of MRI and CT are currently being developed, and their utilizations are expected to grow rapidly in the future. Semin Thorac Cardiovasc Surg Pediatr Card Surg Ann 12:99-105 © 2009 Elsevier Inc. All rights reserved. KEYWORDS Congenital heart disease, Magnetic resonance imaging, Multi-detector rows, Computed tomography
Introduction
I
n the past decade, we witnessed significant advances in noninvasive cardiac imaging with magnetic resonance imaging (MRI) and computed tomography (CT). These imaging modalities now supplement information from echocardiography (ECHO) and replace certain invasive angiography. Echocardiography, having the advantages of low cost, accessibility, safety, high temporal resolution, and Doppler imaging, will remain the most effective screening tool for structural or congenital heart disease (CHD). The roles of MRI and CT are three-fold: (1) to improve diagnosis by providing 3-dimensional (3D) representation of the cardiovascular anatomy; (2) to improve accuracy of hemodynamic assessment with reproducible, quantitative measurements; and (3) to improve safety by replacing invasive catheterization whenever feasible.1 Our goals in this chapter are to explore the imaging equipment necessary for a successful imaging center for children with CHD, to present clinically available MRI and CT techniques useful for CHD, and to discuss some of their clinical applications.
Imaging Environment Ideally, a state-of-the-art cardiac imaging center should have an MRI scanner and a CT scanner because each modality has dif-
Assistant Professor, Department of Radiology, Stanford University Medical Center, Stanford, CA, USA. Address correspondence to Frandics P. Chan, MD, PhD, Stanford University Medical Center, 300 Pasteur Drive, S-066, Stanford, CA 94305-5105; e-mail: [email protected]
1092-9126/09/$-see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1053/j.pcsu.2009.01.009
ferent indications. MRI for CHD calls for a 1.5- to 3-Tesla MRI scanner equipped with a high-speed gradient system of at least 40 mT/m gradient strength and 150 mT/m/ms slew rate, a full complement of phase-array receiver coils suitable for children of varying sizes, and cardiac-specific pulse sequences. Higher field strength at 3-Tesla produces better signal-to-noise ratio that benefits low-signal applications, such as high-resolution contrast-enhanced magnetic resonance angiography (MRA), but it also worsens chemical-shift artifact in gradient-recalled echo type sequences. The overall advantage of a 3-Tesla scanner over a 1.5-Tesla scanner has not been proven for cardiac applications. Ideally, CT for CHD requires a 64-slice multi-detector row computed tomography (MDCT) scanner with a fast gantry rotation of less than 0.4 second, automatic adjustment of radiation dose or x-ray tube current to body size, and a flexible multiplanarmultiphasic reconstruction algorithm. Programmable contrast media dual injector capable of sequential or simultaneous injection of contrast agent and saline are necessary to insure consistent, flexible, and efficient delivery of intravenous contrast bolus. The high volume of data acquired from each study, often greater than a thousand images, should be stored and retrieved using a capable picture archiving and communication system (PACS). Dedicated computer workstations are needed for 3D image post-processing and quantitative measurements of ventricular volume and blood flow. To use these tools effectively requires specially trained physicians and imaging technologists.
Anesthesia Pediatric patients with CHD are often too young or too ill to cooperate for the imaging studies. Light and moderate seda99
100 tions are generally inappropriate for cardiac CT and MRI studies because of the hemodynamic instability inherent in many of these patients. Anesthesiologists specialized in cardiac care are called on to perform general anesthesia.2,3 Their roles are to monitor patient safety, to help establish venous access, to immobilize patient during the study, to suppress stimulations from MRI scanner noise and contrast administration, and to effect breath-holding when necessary for imaging. Support from knowledgeable nursing staff is crucial. All anesthesia equipments must be appropriately designed for the imaging environment. This is especially important for MRI, where the strong magnetic field can cause electromechanical equipments to malfunction, or worse, to become projectiles. These include infusion pumps, ventilators, and monitoring equipments. Should a code become necessary during an MRI study, the patient must be quickly disengaged from the MRI machine and transported to a designated magnetic field-free environment before resuscitation. The practice of anesthesia is quite different in a CT study and in an MRI study. A CT study for CHD generally utilizes angiographic technique that calls for a high rate of iodinated contrast administration during a brief 5- to 30-second scan under breath-hold. To accomplish this, an anesthesiologist may choose to use a short acting anesthetic such as propofol to suppress respiration transiently. Airway can be maintained by face mask ventilation since the open environment around the CT scanner permit continuous access to the face of the patient. In contrast, an MRI study usually consists of multiple scans over possibly an hour, and the patient is not accessible during this period. Depending on the age of the patient, the imaging protocol, and the respiratory pattern, breathholding may or may not be required. In this situation, the airway is usually secured with endotracheal intubation or laryngeal mask airway. Anesthetics by gas or by propofol continuous infusion must be maintained throughout the MRI study. Because visual contact is not possible, monitoring relies on hemodynamic measurements such as heart rate, blood pressure, ECG tracing, and arterial oxygen saturation. Anesthesia complications in either modality are uncommon, but understandably, an MRI study provokes greater anxiety than a CT study from the perspective of the anesthesiologists.4
Cardiac MRI Techniques Despite the large number of MRI pulse sequences and techniques that have been developed for cardiac imaging, four types are used routinely for CHD.5,6 They can be categorized as (1) black-blood single-phase sequence, (2) bright-blood cine sequence, (3) MR angiography, and (4) phase-contrast sequence. Nearly all cardiac MRI sequences are cardiac-gated and they assume regular, periodic motion of the heart to produce a coherent image. Most commercially available cardiac sequences also assume breath-holding to remove respiratory motion. This assumption was adopted for adults who can breath-hold voluntarily, and it can be problematic in children. Black-blood single-phase sequence produces images at a single cardiac phase that has bright soft-tissue structures,
F.P. Chan
Figure 1 Three-chamber view of a heart demonstrating an MRI black-blood image acquired with a double-inversion recovery sequence.
such as myocardium and vessel wall, but dark blood signal inside ventricles and vessel lumen. Traditionally, blackblood imaging was implemented with T1-weighted spinecho sequence, and it was the first cardiac sequence available for clinical use. Today, it is implemented with breath-held, double-inversion recovery sequence that yields better contrast and fewer artifacts (Fig. 1). Black blood images are useful for evaluating morphology and connections of the cardiovascular structures. The disadvantage is that it provides no temporal information about cardiac motion. Bright-blood cine sequence produces a series of images that can be played as a movie loop showing cardiac motion. As its name implies, it produces bright signal inside ventricles and vessel lumen. Today, bright-blood cine imaging is implemented by a steady-state free precession (SSFP) sequence (Fig. 2). Images produced by this sequence are T2-weighted, and unlike older gradient-recall echo cine sequence, the SSFP sequence is free of signal fluctuation from inflow effect. In a typical examination, cine images are acquired in the principal cardiac planes. Both structural anatomy and motion of the cardiovascular system can be evaluated at once in a manner similar to gray-scale ECHO. Finally, ventricles from these bright-blood images can be segmented for volume and ejection fraction measurements. MR angiography can be divided into contrast-enhanced technique and non-contrast-enhanced technique. Contrastenhanced MRA produces high-resolution, 3D images where the contrast filled blood vessel is bright (Fig. 3). In typical applications, this technique is not cardiac gated and it is used for evaluating non-cardiac structures, such as the aorta and the pulmonary arteries. This technique requires the administration of gadolinium-based contrast agent. Although gadolinium contrast agent is safe in most situations, it is contra-
MR and CT Imaging
Figure 2 Four-chamber view of a heart demonstrating an MRI bright-blood image acquired with a steady-state free-precession cine sequence. The right ventricle is enlarged from free pulmonary regurgitation after tetralogy of Fallot repair.
indicated in patients with renal failure as it can worsen renal function, especially in high dose. In adults, a rare condition known as nephrogenic systemic fibrosis has been associated with gadolinium contrast use in patients with severe renal failure (creatinine clearance ⬍ 30 cc/min).7,8 This is a debilitating disease with no known treatment. Reported cases of nephrogenic systemic fibrosis are rare in children, and their
101
Figure 4 A maximal intensity projection image of a non-contrastenhanced MR angiography showing the coronary arteries.
association with gadolinium contrast has not yet been established. However, prudence dictates that gadolinium contrast should not be used in children with renal failure.9 MR angiography can be obtained without gadolinium contrast agent if it can distinguish blood and soft tissue based on their intrinsic differences. The most important sequence in this category is a cardiac-gated, 3D, navigated-echo, SSFP sequence. Like the SSFP cine sequence, it is T2-weighted and blood inside a vessel is bright by the virtue of its long T2 value. This non-contrast-enhanced MRA technique was developed originally for coronary artery imaging (Fig. 4) but it has been extended to image other vessels. Compared with contrast-enhanced MRA, it has a long acquisition time and is more prone to produce artifacts. The non-contrast technique is useful when contrast-enhanced MRA is contraindicated. Phase-contrast sequence generates quantitative velocity information that is functionally similar to Doppler ECHO imaging. But unlike ECHO, velocity information can be acquired in arbitrary direction relative to the imaging plane. Phase-contrast sequence for CHD is used primary to quantify flow and pressure gradient estimated from velocity by the Bernoulli equation. To measure flow, an imaging plane is placed perpendicular to the vessel lumen such that both the cross-section of the vessel and the through-plane velocity of blood are imaged. Total flow is calculated by summing velocity across the luminal cross-section. In this manner, cardiac output, shunt ratio, and valvular regurgitation (Figs. 5 and 6) can be quantified.10
Cardiac CT Techniques Figure 3 A maximal intensity projection image of a contrast-enhanced MR angiography showing a tight juxtaductal coarctation with numerous collateral arteries.
Electron-beam CT was the first CT scanner dedicated to cardiac imaging. However, cardiac CT applications did not become widespread until after the development of MDCT scan-
F.P. Chan
102
Figure 5 An MRI phase-contrast velocity image showing the pulmonary systolic forward flow in black through the pulmonary valve.
ner equipped with cardiac imaging capability. This type of scanner was originally developed for imaging coronary artery disease in adults, but it was found useful in imaging CHD. All cardiac CT studies use angiographic techniques that call for a high rate of iodinated contrast injection into a peripheral venous access. Thin-section axial images are acquired when the contrast bolus opacifies the cardiovascular structure of interest. Unlike conventional CT angiography, image acquisition in cardiac CT study is synchronized with the cardiac motion. Two types of cardiac synchronization techniques are available: prospective ECG-gating and retrospective ECGgating. Detailed explanations of these techniques can be found elsewhere.11 In prospective ECG-gating, the scanner turns on the x-ray tube and acquires image data only at a predetermined phase in the cardiac cycle, represented as a moment within an R-wave to R-wave interval in an ECG tracing. The reconstructed images are a 3D representation of the heart at a single cardiac phase. In retrospective ECG-gating, the scanner scans the heart continuously while the ECG tracing is recorded independently. After the scanning has completed, a reconstruction algorithm extracts a subset of the acquired data relevant to a cardiac phase and reconstruct the images. Because the acquired data contain information throughout the cardiac cycle, it is possible to reconstruct images at multiple cardiac phases, producing a movie of the beating heart. Retrospective ECG gating can be combined with a multi-sector reconstruction technique to improve temporal resolution, thereby producing clearer images than prospective ECG-gating, especially in children with high heart rates. However, these advantages of retrospective ECG-gating come at a price. Because the x-ray tube is turned on throughout the scan, retrospective ECG-gating incurs much greater radiation dose than prospective-ECG gating.12
Radiation exposure from CT studies is an important concern in children because of its oncogenic effect.13 Compared with other x-ray based imaging procedures, CT incurs some of the highest radiation dose per study. Among different CT protocols, cardiac-gated CT angiography produces the highest radiation dose, on the order of 10 to 30 mSv14,15 compared with annual background radiation of 3 mSv. There is increasing evidence that at this level of radiation, a small but increased lifetime attributable risk of cancer exists. The decision to perform a pediatric CT, like any medical procedures, must be based on a reasonable risk-benefit analysis. There must be a welldefined diagnostic question that can be better answered by CT at a lower risk than other alternatives. For many types of CHD that carry significant modality and morbidity, the benefit from diagnostic information gained from CT usually outweighs its risk. Still, meticulous care must be taken to lower the radiation dose As Low As Reasonably Achievable, or the ALARA principle.16 In pediatric CT, these measures include the use of low tube-voltage technique, the adjustment of tube-current to body size, the use of geometric dose modulation,17 the choice of ungated study over gated study, and the choice of prospective ECG-gating over retrospective ECG-gating whenever feasible.
MRI Versus CT Cardiac MRI and CT each have unique strengths and weaknesses. The choice of one over the other depends on the diagnostic question to be answered and the general condition of the patient (Fig. 7). Both MRI and CT are capable of evaluating the anomalous anatomy found in CHD. CT has supe-
Figure 6 An MRI phase-contrast velocity image showing the pulmonary diastolic regurgitant flow in white.
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Figure 7 A diagram showing shared and unique indications for cardiac CT and MRI.
rior spatial resolution and it is better at imaging small structures, such as the coronary arteries. CT is the modality of choice for imaging the pulmonary parenchyma and the peripheral pulmonary vessels, such as the detection of pulmonary sequestration, pulmonary vascular stenosis, and pulmonary embolism. Patient accessibility at the CT scanner, together with its short study time, makes CT a safer test for critically ill patients. Patients with implanted pacemakers or defibrillators cannot undergo MRI, making CT an alternative choice. In contrast, MRI has superior temporal resolution and it is better at analyzing moving structures, such as the cardiac valves and the ventricular walls. It is the modality of choice for quantifying ventricular volumes and ejection fractions. Using the phase contrast technique, MRI is unique in its ability to measure blood flow. Special MRI techniques are available to determine myocardial perfusion and viability. The greatest advantage of MRI over CT is that it is radiationfree. For studies that must be repeated, cumulative radiation
Figure 8 A 3D volume-rendered CT image of a double aortic arch constricting a segment of the trachea (arrows).
Figure 9 A 3D volume-rendered CT image of a patent ductal arteriosus.
dose from CT can be substantial and efforts should be made to utilize MRI if possible.
Applications The 3D imaging capability of MRI and CT helps answer anatomic questions not resolved by ECG or projectional catheter angiography. Greater accuracy and reproducibility of hemodynamic measurement with MRI help physicians follow disease progression and determine timing of intervention. Indications are numerous and growing but, for illustration, examples will be presented in the following categories: anomalies of the great vessels, anomalies of the coronary arteries, abnormal ventricular and valvular functions, and complex structural anomalies. Evaluation of the great vessels is a common indication for MRI and CT because these extracardiac vessels are not well seen by ECHO, and imaging with catheter angiography is invasive. Aortic coarctation is historically imaged with contrast-enhanced MRA (Fig. 3). Vascular rings, double aortic arch, and pulmonary sling can be diagnosed equally well with CT or MRI, although CT visualizes the constricted trachea better, which can help the surgeon determine the length of the trachea that may need repair (Fig. 8). Accurate size measurement of a patent ductal arteriosus is helpful for planning interventional device closure (Fig. 9). In neonates with pulmonary atresia with ventricular septal defect, both CT and MRI can determine ductal-dependent pulmonary flow but CT can better identify tiny native pulmonary arteries, major aortopulmonary collateral arteries, and their pulmonary distributions. Both CT and MRI can help diagnose total anomalous pulmonary venous return (TAPVR) and determine the pulmonary vein to systemic vein connection. A minority of patients develop pulmonary vein stenoses after TAPVR repair. These patients can be quite ill
104
F.P. Chan
Figure 10 A multiplanar reformatted CT image showing multiple tight pulmonary vein stenoses (arrows) after repair for total anomalous pulmonary venous return.
Figure 12 A multiplanar reformation CT image showing an atretic right coronary artery (arrow) after an arterial-switch procedure for transposition of the great arteries.
with poor oxygen saturation. The stenoses are readily visualized on CT (Fig. 10). Until the recent past, anomalous coronary arteries can be imaged only with catheter coronary angiography. They are now better evaluated with MR or CT coronary angiography because their anatomic relationships with other cardiac structures are better visualized. Numerous variations of anomalous coronary arteries exist, but the lethal ones are usually those that have an inter-arterial course (Fig. 11).
With currently available scanners, CT usually images the coronary arteries more reliably than MRI, but CT incurs substantial radiation dose. In older children and adolescents, their coronary arteries are large enough to be visualized with MRI. In small children with tiny coronary arteries, CT remains the better choice. In addition to structural anomalies, CT or MR coronary angiography is useful in evaluating coronary aneurysm caused by Kawasaki disease and coronary stenosis after arterial-switch repair for transposition of the great arteries (Fig. 12). MRI is the method of choice for the evaluation of cardiac function and blood flow. A frequent indication is the assessment of pulmonary regurgitation, right ventricular dilatation (Fig. 2), and right ventricular ejection fraction after tetralogy of Fallot repair. The information will help determine the optimal timing for surgical placement of the prosthetic pulmonary valve. For patients with abnormal ventricular development, such as atrioventricular canal defect, accurate measurements of the left and right ventricular volumes is helpful in deciding between one- or two-ventricle repair. After pulmonary banding for ventricular training, the growth in ventricular mass can be measured with CT or MRI. Finally, longitudinal follow-up in ventricular function is helpful in detecting early ventricular failure after a single ventricle repair. Many complex structural anomalies or surgical repair cannot be adequately represented by 2-dimensional images obtained with ECHO and projectional catheter angiography. They benefit from the 3D imaging capability of CT and MRI. Examples include conjoined twins sharing cardiac structures, heterotopic heart transplant (Fig. 13), and complex malalignment of cardiac valves and ventricular chambers. In these problem-solving cases, the choice
Figure 11 A multiplanar reformatted CT image showing an aberrant right coronary artery with an inter-arterial course (arrow).
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References
Figure 13 A multiplanar reformation CT image showing heterotopic heart transplant in a patient with pulmonary hypertension. The donor’s heart is the smaller one (arrow). The atria of the hearts are surgically connected. This CT study maps out the path for a transjugular biopsy of the transplanted heart.
of imaging modality and protocol must be individualized by experienced imager to best suite the clinical problems at hand.
Conclusion Cardiac MRI and CT are established imaging modalities used in the management of CHD. Indications for cardiac MRI and CT are rapidly expanding. In the meanwhile, the risks associated with imaging tests, such as radiation dose from CT, are being closely scrutinized. Advancements in our knowledge in these areas will refine the use of these important diagnostic tools.
1. Lee T, Tsai IC, Fu YC, et al: Using multidetector-row CT in neonates with complex congenital heart disease to replace diagnostic cardiac catheterization for anatomical investigation: Initial experiences in technical and clinical feasibility. Pediatr Radiol 36:1273-1282, 2006 2. Aronson LA, Spaeth JP: Frontiers in pediatric anesthesia: cardiac anesthesia. Int Anesthesiol Clin 44:33-49, 2006 3. Serafini G, Zadra N: Anaesthesia for MRI in the paediatric patient. Curr Opin Anaesthesiol 21:499-503, 2008 4. Odegard KC, DiNardo JA, Tsai-Goodman B, et al: Anaesthesia considerations for cardiac MRI in infants and small children. Paediatr Anaesth 14:471-476, 2004 5. Valente AM, Powell AJ: Clinical applications of cardiovascular magnetic resonance in congenital heart disease. Cardiol Clin 25:97-110, 2007 6. Gutierrez FR, Ho ML, Siegel MJ: Practical applications of magnetic resonance in congenital heart disease. Magn Reson Imaging Clin N Am 16:403-35, 2008 7. Thomsen HS, Marckmann P, Logager VB: Update on nephrogenic systemic fibrosis. Magn Reson Imaging Clin N Am 16:551-560, 2008 8. Bhave G, Lewis JB, Chang SS: Association of gadolinium based magnetic resonance imaging contrast agents and nephrogenic systemic fibrosis. J Urol 180:830-835, 2008 9. Mendichovszky IA, Marks SD, Simcock CM, et al: Gadolinium and nephrogenic systemic fibrosis: time to tighten practice. Pediatr Radiol 38:489-496, 2008 10. Gatehouse PD, Keegan J, Crowe LA, et al: Applications of phase-contrast flow and velocity imaging in cardiovascular MRI. Eur Radiol 15: 2172-2184, 2005 11. Chan FP: Cardiac MDCT, in Fishman EK, Jeffrey RB (eds): Multidetector CT. Philadelphia, PA; Lippincott Williams and Wilkins; 2004, pp 129-158 12. Tsai IC, Chen MC, Jan SL, et al: Neonatal cardiac multidetector row CT: Why and how we do it. Pediatr Radiol 38:438-451, 2008 13. Brenner D, Elliston C, Hall E, et al: Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 176:289-296, 2001 14. Hollingsworth CL, Yoshizumi TT, Frush DP, et al: Pediatric cardiacgated CT angiography: Assessment of radiation dose. AJR Am J Roentgenol 189:12-18, 2007 15. Hurwitz LM, Reiman RE, Yoshizumi TT, et al: Radiation dose from contemporary cardiothoracic multidetector CT protocols with an anthropomorphic female phantom: Implications for cancer induction. Radiology 245:742-750, 2007 16. Shah NB, Platt SL: ALARA: Is there a cause for alarm? Reducing radiation risks from computed tomography scanning in children. Curr Opin Pediatr 20:243-247, 2008 17. Herzog C, Mulvihill DM, Nguyen SA, et al: Pediatric cardiovascular CT angiography: Radiation dose reduction using automatic anatomic tube current modulation. AJR Am J Roentgenol 190:1232-1240, 2008
Introduction
I
n the past decade, we witnessed significant advances in noninvasive cardiac imaging with magnetic resonance imaging (MRI) and computed tomography (CT). These imaging modalities now supplement information from echocardiography (ECHO) and replace certain invasive angiography. Echocardiography, having the advantages of low cost, accessibility, safety, high temporal resolution, and Doppler imaging, will remain the most effective screening tool for structural or congenital heart disease (CHD). The roles of MRI and CT are three-fold: (1) to improve diagnosis by providing 3-dimensional (3D) representation of the cardiovascular anatomy; (2) to improve accuracy of hemodynamic assessment with reproducible, quantitative measurements; and (3) to improve safety by replacing invasive catheterization whenever feasible.1 Our goals in this chapter are to explore the imaging equipment necessary for a successful imaging center for children with CHD, to present clinically available MRI and CT techniques useful for CHD, and to discuss some of their clinical applications.
Imaging Environment Ideally, a state-of-the-art cardiac imaging center should have an MRI scanner and a CT scanner because each modality has dif-
Assistant Professor, Department of Radiology, Stanford University Medical Center, Stanford, CA, USA. Address correspondence to Frandics P. Chan, MD, PhD, Stanford University Medical Center, 300 Pasteur Drive, S-066, Stanford, CA 94305-5105; e-mail: [email protected]
1092-9126/09/$-see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1053/j.pcsu.2009.01.009
ferent indications. MRI for CHD calls for a 1.5- to 3-Tesla MRI scanner equipped with a high-speed gradient system of at least 40 mT/m gradient strength and 150 mT/m/ms slew rate, a full complement of phase-array receiver coils suitable for children of varying sizes, and cardiac-specific pulse sequences. Higher field strength at 3-Tesla produces better signal-to-noise ratio that benefits low-signal applications, such as high-resolution contrast-enhanced magnetic resonance angiography (MRA), but it also worsens chemical-shift artifact in gradient-recalled echo type sequences. The overall advantage of a 3-Tesla scanner over a 1.5-Tesla scanner has not been proven for cardiac applications. Ideally, CT for CHD requires a 64-slice multi-detector row computed tomography (MDCT) scanner with a fast gantry rotation of less than 0.4 second, automatic adjustment of radiation dose or x-ray tube current to body size, and a flexible multiplanarmultiphasic reconstruction algorithm. Programmable contrast media dual injector capable of sequential or simultaneous injection of contrast agent and saline are necessary to insure consistent, flexible, and efficient delivery of intravenous contrast bolus. The high volume of data acquired from each study, often greater than a thousand images, should be stored and retrieved using a capable picture archiving and communication system (PACS). Dedicated computer workstations are needed for 3D image post-processing and quantitative measurements of ventricular volume and blood flow. To use these tools effectively requires specially trained physicians and imaging technologists.
Anesthesia Pediatric patients with CHD are often too young or too ill to cooperate for the imaging studies. Light and moderate seda99
100 tions are generally inappropriate for cardiac CT and MRI studies because of the hemodynamic instability inherent in many of these patients. Anesthesiologists specialized in cardiac care are called on to perform general anesthesia.2,3 Their roles are to monitor patient safety, to help establish venous access, to immobilize patient during the study, to suppress stimulations from MRI scanner noise and contrast administration, and to effect breath-holding when necessary for imaging. Support from knowledgeable nursing staff is crucial. All anesthesia equipments must be appropriately designed for the imaging environment. This is especially important for MRI, where the strong magnetic field can cause electromechanical equipments to malfunction, or worse, to become projectiles. These include infusion pumps, ventilators, and monitoring equipments. Should a code become necessary during an MRI study, the patient must be quickly disengaged from the MRI machine and transported to a designated magnetic field-free environment before resuscitation. The practice of anesthesia is quite different in a CT study and in an MRI study. A CT study for CHD generally utilizes angiographic technique that calls for a high rate of iodinated contrast administration during a brief 5- to 30-second scan under breath-hold. To accomplish this, an anesthesiologist may choose to use a short acting anesthetic such as propofol to suppress respiration transiently. Airway can be maintained by face mask ventilation since the open environment around the CT scanner permit continuous access to the face of the patient. In contrast, an MRI study usually consists of multiple scans over possibly an hour, and the patient is not accessible during this period. Depending on the age of the patient, the imaging protocol, and the respiratory pattern, breathholding may or may not be required. In this situation, the airway is usually secured with endotracheal intubation or laryngeal mask airway. Anesthetics by gas or by propofol continuous infusion must be maintained throughout the MRI study. Because visual contact is not possible, monitoring relies on hemodynamic measurements such as heart rate, blood pressure, ECG tracing, and arterial oxygen saturation. Anesthesia complications in either modality are uncommon, but understandably, an MRI study provokes greater anxiety than a CT study from the perspective of the anesthesiologists.4
Cardiac MRI Techniques Despite the large number of MRI pulse sequences and techniques that have been developed for cardiac imaging, four types are used routinely for CHD.5,6 They can be categorized as (1) black-blood single-phase sequence, (2) bright-blood cine sequence, (3) MR angiography, and (4) phase-contrast sequence. Nearly all cardiac MRI sequences are cardiac-gated and they assume regular, periodic motion of the heart to produce a coherent image. Most commercially available cardiac sequences also assume breath-holding to remove respiratory motion. This assumption was adopted for adults who can breath-hold voluntarily, and it can be problematic in children. Black-blood single-phase sequence produces images at a single cardiac phase that has bright soft-tissue structures,
F.P. Chan
Figure 1 Three-chamber view of a heart demonstrating an MRI black-blood image acquired with a double-inversion recovery sequence.
such as myocardium and vessel wall, but dark blood signal inside ventricles and vessel lumen. Traditionally, blackblood imaging was implemented with T1-weighted spinecho sequence, and it was the first cardiac sequence available for clinical use. Today, it is implemented with breath-held, double-inversion recovery sequence that yields better contrast and fewer artifacts (Fig. 1). Black blood images are useful for evaluating morphology and connections of the cardiovascular structures. The disadvantage is that it provides no temporal information about cardiac motion. Bright-blood cine sequence produces a series of images that can be played as a movie loop showing cardiac motion. As its name implies, it produces bright signal inside ventricles and vessel lumen. Today, bright-blood cine imaging is implemented by a steady-state free precession (SSFP) sequence (Fig. 2). Images produced by this sequence are T2-weighted, and unlike older gradient-recall echo cine sequence, the SSFP sequence is free of signal fluctuation from inflow effect. In a typical examination, cine images are acquired in the principal cardiac planes. Both structural anatomy and motion of the cardiovascular system can be evaluated at once in a manner similar to gray-scale ECHO. Finally, ventricles from these bright-blood images can be segmented for volume and ejection fraction measurements. MR angiography can be divided into contrast-enhanced technique and non-contrast-enhanced technique. Contrastenhanced MRA produces high-resolution, 3D images where the contrast filled blood vessel is bright (Fig. 3). In typical applications, this technique is not cardiac gated and it is used for evaluating non-cardiac structures, such as the aorta and the pulmonary arteries. This technique requires the administration of gadolinium-based contrast agent. Although gadolinium contrast agent is safe in most situations, it is contra-
MR and CT Imaging
Figure 2 Four-chamber view of a heart demonstrating an MRI bright-blood image acquired with a steady-state free-precession cine sequence. The right ventricle is enlarged from free pulmonary regurgitation after tetralogy of Fallot repair.
indicated in patients with renal failure as it can worsen renal function, especially in high dose. In adults, a rare condition known as nephrogenic systemic fibrosis has been associated with gadolinium contrast use in patients with severe renal failure (creatinine clearance ⬍ 30 cc/min).7,8 This is a debilitating disease with no known treatment. Reported cases of nephrogenic systemic fibrosis are rare in children, and their
101
Figure 4 A maximal intensity projection image of a non-contrastenhanced MR angiography showing the coronary arteries.
association with gadolinium contrast has not yet been established. However, prudence dictates that gadolinium contrast should not be used in children with renal failure.9 MR angiography can be obtained without gadolinium contrast agent if it can distinguish blood and soft tissue based on their intrinsic differences. The most important sequence in this category is a cardiac-gated, 3D, navigated-echo, SSFP sequence. Like the SSFP cine sequence, it is T2-weighted and blood inside a vessel is bright by the virtue of its long T2 value. This non-contrast-enhanced MRA technique was developed originally for coronary artery imaging (Fig. 4) but it has been extended to image other vessels. Compared with contrast-enhanced MRA, it has a long acquisition time and is more prone to produce artifacts. The non-contrast technique is useful when contrast-enhanced MRA is contraindicated. Phase-contrast sequence generates quantitative velocity information that is functionally similar to Doppler ECHO imaging. But unlike ECHO, velocity information can be acquired in arbitrary direction relative to the imaging plane. Phase-contrast sequence for CHD is used primary to quantify flow and pressure gradient estimated from velocity by the Bernoulli equation. To measure flow, an imaging plane is placed perpendicular to the vessel lumen such that both the cross-section of the vessel and the through-plane velocity of blood are imaged. Total flow is calculated by summing velocity across the luminal cross-section. In this manner, cardiac output, shunt ratio, and valvular regurgitation (Figs. 5 and 6) can be quantified.10
Cardiac CT Techniques Figure 3 A maximal intensity projection image of a contrast-enhanced MR angiography showing a tight juxtaductal coarctation with numerous collateral arteries.
Electron-beam CT was the first CT scanner dedicated to cardiac imaging. However, cardiac CT applications did not become widespread until after the development of MDCT scan-
F.P. Chan
102
Figure 5 An MRI phase-contrast velocity image showing the pulmonary systolic forward flow in black through the pulmonary valve.
ner equipped with cardiac imaging capability. This type of scanner was originally developed for imaging coronary artery disease in adults, but it was found useful in imaging CHD. All cardiac CT studies use angiographic techniques that call for a high rate of iodinated contrast injection into a peripheral venous access. Thin-section axial images are acquired when the contrast bolus opacifies the cardiovascular structure of interest. Unlike conventional CT angiography, image acquisition in cardiac CT study is synchronized with the cardiac motion. Two types of cardiac synchronization techniques are available: prospective ECG-gating and retrospective ECGgating. Detailed explanations of these techniques can be found elsewhere.11 In prospective ECG-gating, the scanner turns on the x-ray tube and acquires image data only at a predetermined phase in the cardiac cycle, represented as a moment within an R-wave to R-wave interval in an ECG tracing. The reconstructed images are a 3D representation of the heart at a single cardiac phase. In retrospective ECG-gating, the scanner scans the heart continuously while the ECG tracing is recorded independently. After the scanning has completed, a reconstruction algorithm extracts a subset of the acquired data relevant to a cardiac phase and reconstruct the images. Because the acquired data contain information throughout the cardiac cycle, it is possible to reconstruct images at multiple cardiac phases, producing a movie of the beating heart. Retrospective ECG gating can be combined with a multi-sector reconstruction technique to improve temporal resolution, thereby producing clearer images than prospective ECG-gating, especially in children with high heart rates. However, these advantages of retrospective ECG-gating come at a price. Because the x-ray tube is turned on throughout the scan, retrospective ECG-gating incurs much greater radiation dose than prospective-ECG gating.12
Radiation exposure from CT studies is an important concern in children because of its oncogenic effect.13 Compared with other x-ray based imaging procedures, CT incurs some of the highest radiation dose per study. Among different CT protocols, cardiac-gated CT angiography produces the highest radiation dose, on the order of 10 to 30 mSv14,15 compared with annual background radiation of 3 mSv. There is increasing evidence that at this level of radiation, a small but increased lifetime attributable risk of cancer exists. The decision to perform a pediatric CT, like any medical procedures, must be based on a reasonable risk-benefit analysis. There must be a welldefined diagnostic question that can be better answered by CT at a lower risk than other alternatives. For many types of CHD that carry significant modality and morbidity, the benefit from diagnostic information gained from CT usually outweighs its risk. Still, meticulous care must be taken to lower the radiation dose As Low As Reasonably Achievable, or the ALARA principle.16 In pediatric CT, these measures include the use of low tube-voltage technique, the adjustment of tube-current to body size, the use of geometric dose modulation,17 the choice of ungated study over gated study, and the choice of prospective ECG-gating over retrospective ECG-gating whenever feasible.
MRI Versus CT Cardiac MRI and CT each have unique strengths and weaknesses. The choice of one over the other depends on the diagnostic question to be answered and the general condition of the patient (Fig. 7). Both MRI and CT are capable of evaluating the anomalous anatomy found in CHD. CT has supe-
Figure 6 An MRI phase-contrast velocity image showing the pulmonary diastolic regurgitant flow in white.
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Figure 7 A diagram showing shared and unique indications for cardiac CT and MRI.
rior spatial resolution and it is better at imaging small structures, such as the coronary arteries. CT is the modality of choice for imaging the pulmonary parenchyma and the peripheral pulmonary vessels, such as the detection of pulmonary sequestration, pulmonary vascular stenosis, and pulmonary embolism. Patient accessibility at the CT scanner, together with its short study time, makes CT a safer test for critically ill patients. Patients with implanted pacemakers or defibrillators cannot undergo MRI, making CT an alternative choice. In contrast, MRI has superior temporal resolution and it is better at analyzing moving structures, such as the cardiac valves and the ventricular walls. It is the modality of choice for quantifying ventricular volumes and ejection fractions. Using the phase contrast technique, MRI is unique in its ability to measure blood flow. Special MRI techniques are available to determine myocardial perfusion and viability. The greatest advantage of MRI over CT is that it is radiationfree. For studies that must be repeated, cumulative radiation
Figure 8 A 3D volume-rendered CT image of a double aortic arch constricting a segment of the trachea (arrows).
Figure 9 A 3D volume-rendered CT image of a patent ductal arteriosus.
dose from CT can be substantial and efforts should be made to utilize MRI if possible.
Applications The 3D imaging capability of MRI and CT helps answer anatomic questions not resolved by ECG or projectional catheter angiography. Greater accuracy and reproducibility of hemodynamic measurement with MRI help physicians follow disease progression and determine timing of intervention. Indications are numerous and growing but, for illustration, examples will be presented in the following categories: anomalies of the great vessels, anomalies of the coronary arteries, abnormal ventricular and valvular functions, and complex structural anomalies. Evaluation of the great vessels is a common indication for MRI and CT because these extracardiac vessels are not well seen by ECHO, and imaging with catheter angiography is invasive. Aortic coarctation is historically imaged with contrast-enhanced MRA (Fig. 3). Vascular rings, double aortic arch, and pulmonary sling can be diagnosed equally well with CT or MRI, although CT visualizes the constricted trachea better, which can help the surgeon determine the length of the trachea that may need repair (Fig. 8). Accurate size measurement of a patent ductal arteriosus is helpful for planning interventional device closure (Fig. 9). In neonates with pulmonary atresia with ventricular septal defect, both CT and MRI can determine ductal-dependent pulmonary flow but CT can better identify tiny native pulmonary arteries, major aortopulmonary collateral arteries, and their pulmonary distributions. Both CT and MRI can help diagnose total anomalous pulmonary venous return (TAPVR) and determine the pulmonary vein to systemic vein connection. A minority of patients develop pulmonary vein stenoses after TAPVR repair. These patients can be quite ill
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F.P. Chan
Figure 10 A multiplanar reformatted CT image showing multiple tight pulmonary vein stenoses (arrows) after repair for total anomalous pulmonary venous return.
Figure 12 A multiplanar reformation CT image showing an atretic right coronary artery (arrow) after an arterial-switch procedure for transposition of the great arteries.
with poor oxygen saturation. The stenoses are readily visualized on CT (Fig. 10). Until the recent past, anomalous coronary arteries can be imaged only with catheter coronary angiography. They are now better evaluated with MR or CT coronary angiography because their anatomic relationships with other cardiac structures are better visualized. Numerous variations of anomalous coronary arteries exist, but the lethal ones are usually those that have an inter-arterial course (Fig. 11).
With currently available scanners, CT usually images the coronary arteries more reliably than MRI, but CT incurs substantial radiation dose. In older children and adolescents, their coronary arteries are large enough to be visualized with MRI. In small children with tiny coronary arteries, CT remains the better choice. In addition to structural anomalies, CT or MR coronary angiography is useful in evaluating coronary aneurysm caused by Kawasaki disease and coronary stenosis after arterial-switch repair for transposition of the great arteries (Fig. 12). MRI is the method of choice for the evaluation of cardiac function and blood flow. A frequent indication is the assessment of pulmonary regurgitation, right ventricular dilatation (Fig. 2), and right ventricular ejection fraction after tetralogy of Fallot repair. The information will help determine the optimal timing for surgical placement of the prosthetic pulmonary valve. For patients with abnormal ventricular development, such as atrioventricular canal defect, accurate measurements of the left and right ventricular volumes is helpful in deciding between one- or two-ventricle repair. After pulmonary banding for ventricular training, the growth in ventricular mass can be measured with CT or MRI. Finally, longitudinal follow-up in ventricular function is helpful in detecting early ventricular failure after a single ventricle repair. Many complex structural anomalies or surgical repair cannot be adequately represented by 2-dimensional images obtained with ECHO and projectional catheter angiography. They benefit from the 3D imaging capability of CT and MRI. Examples include conjoined twins sharing cardiac structures, heterotopic heart transplant (Fig. 13), and complex malalignment of cardiac valves and ventricular chambers. In these problem-solving cases, the choice
Figure 11 A multiplanar reformatted CT image showing an aberrant right coronary artery with an inter-arterial course (arrow).
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References
Figure 13 A multiplanar reformation CT image showing heterotopic heart transplant in a patient with pulmonary hypertension. The donor’s heart is the smaller one (arrow). The atria of the hearts are surgically connected. This CT study maps out the path for a transjugular biopsy of the transplanted heart.
of imaging modality and protocol must be individualized by experienced imager to best suite the clinical problems at hand.
Conclusion Cardiac MRI and CT are established imaging modalities used in the management of CHD. Indications for cardiac MRI and CT are rapidly expanding. In the meanwhile, the risks associated with imaging tests, such as radiation dose from CT, are being closely scrutinized. Advancements in our knowledge in these areas will refine the use of these important diagnostic tools.
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