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Evaluation of Cardiac Function and Valves by Multidetector Row Computed Tomography
Evaluation of Cardiac Function and Valves by Multidetector Row Computed Tomography Suhny Abbara, MD, Anand V. Soni, MD, and Ricardo C. Cury, MD
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dvances in multidetector computed tomography (MDCT) technology, with submillimeter isovolumetric voxel size and fast gantry rotation speeds of 330 to 350 milliseconds, enable simultaneous noninvasive evaluation of detailed coronary anatomy, global and regional left ventricular (LV), and valve function as well as myocardial perfusion. Published evidence regarding the accuracy of coronary artery evaluation with MDCT is rapidly evolving. A large number of reports have now demonstrated high diagnostic accuracy for detection of coronary artery stenosis by 64-slice MDCT as compared with invasive coronary angiography.1-7 Preliminary data regarding the global right ventricular (RV) and LV function and regional wall motion assessment,8-13 as well as myocardial perfusion assessment, have recently become available.14-21 Every retrospectively gated coronary MDCT raw dataset contains information about ventricular and valvular function and myocardial perfusion. This information is within the raw data that the scanner acquires but is lost unless dedicated functional datasets are reconstructed in addition to the standard computed tomographic angiography (CTA) image datasets. Assessments of global and regional LV function as well as aortic and mitral valve function can add significant value to the interpretation of a coronary CTA. Because this additional information can be acquired without additional radiation exposure or intravenous contrast administration, it is advisable to perform functional reconstructions in every patient that undergoes cardiac gated CTA. Computed tomography (CT) allows for true volumetric functional analysis and is less affected by patient size or acoustic window restrictions, which can be a substantial advantage over echocardiography in some patients. Compared with magnetic resonance imaging (MRI) and echocardiography, CT uses nephrotoxic contrast material and results in radiation exposure to the patient.22 Therefore cardiac CT is not routinely performed for functional assessment alone. Ventricular function assessment is an additional
Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts. Address reprint requests to Suhny Abbara, MD, Department of Radiology, Massachusetts General Hospital, 165 Cambridge Street, Suite 400, Boston, MA 02114. E-mail: [email protected]
0037-198X/08/$-see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1053/j.ro.2008.01.009
evaluation that is added to an otherwise indicated coronary CTA. Another major disadvantage of CT is the relatively poor temporal resolution compared with MRI and echocardiography, However, the temporal resolution of CT has substantially improved with the advent of the latest generation scanner, the dual-source CT scanners.23-25
CTA Acquisition A standard CT angiography protocol for the coronary arteries produces raw datasets that also may be used for reconstruction of functional LV and valve datasets. The scan protocol uses either a bolus tracking technique or a test bolus acquisition, where the scan delay is calculated depending on contrast enhancement of the aortic root. Typically, a test bolus of 20 mL of iodinated contrast followed by 40 mL of saline is injected and sequential images are acquired at the same slice location until maximal contrast enhancement of the aortic root is noted. The delay time is equal to the time to peak attenuation in the ascending aorta. For the CTA acquisition, 80 to 100 mL of contrast is injected at flow rates of 4 to 7 mL/s followed by 40 mL of saline at the same injection rate. Single-phase high-resolution CTA datasets are usually reconstructed in mid to late diastole. Some authors have proposed triphasic injections (contrast at high rate followed by contrast at lower rate followed by saline flush) or replacing the saline flush with a mixture of saline and contrast to better delineate the right ventricular endocardial border.22
Data Reconstruction Functional analysis requires additional reconstruction of multiphasic datasets. A multiphasic dataset consists of an arbitrary number of single phase datasets. Each individual phase consists of a set of axial source images (⬃100 to 500 images depending on slice thickness and overlap) throughout the heart that are reconstructed from projections that are acquired within a specific time period of the cardiac cycle. Typically the cardiac cycle is divided into 10 or 20 individual phases. A phase is generally defined by a percent number that indicates the beginning or the center of a reconstruction window (the box on the electrocardiogram) with respect to its 145
146 relative position between two adjacent R-peaks. For example, a phase of 70% refers to images that were reconstructed utilizing projections that were collected at 70% of an R-R interval. At a heart rate of 60, the R-R interval is 1000 milliseconds long and 70% means that the projections were collected at t ⫽ 700 milliseconds after the R-peak or 300 milliseconds before the subsequent R-peak. This relationship changes with heart rates and at a heart rate of 100 the R-R interval is only 600 milliseconds and 70% refers to 420 milliseconds after an R-peak or 180 milliseconds before the subsequent R-peak. Of note, it is important to consider any specific vendor’s definition of a phase when reading the literature. Some vendors (GE, Toshiba, and Phillips) refer to the center of the box (or acquisition window) with any given percent location in the R-R interval, whereas other vendors (Siemens) refer to the start of the acquisition window (Fig. 1). The length of the reconstruction window, or temporal resolution, is determined by the scanner gantry rotation speed and the reconstruction method (half scan versus multisegment reconstruction). In half-scan reconstruction the reconstruction window width or length is exactly one-half of the gantry rotation speed. For example, if the gantry completes one rotation every 350 milliseconds, then the reconstruction window (or temporal resolution) is 175 milliseconds. Multisegment reconstruction will reduce the reconstruction window width compared with half-scan reconstruction; however, this reconstruction method is only an option at higher heart rates. Some vendors offer multisegment reconstruction algorithms that may use up to four consecutive heart beats to reconstruct any given image, thereby potentially reducing the temporal resolution by one-fourth at high heart rates; others only use two consecutive beats. Because the reconstruction window width is generally wider than one-10th and one-20th of a cardiac cycle length,
S. Abbara, A.V. Soni, and R.C. Cury
Figure 2 Overlap of multiphasic reconstruction windows. The drawing illustrates a 10-phasic reconstruction from 0 to 90% of the R-R interval. Note that adjacent reconstruction windows demonstrate quite some overlap (sharing of projections), which results in temporal interpolation. Reconstruction at larger numbers of phases will create further temporal interpolation and is not likely to add substantial additional information. (Color version of figure is available online.)
there will be overlap in the projections used for each of the phases in any multiphasic dataset (Fig. 2).
Analysis and Interpretation Multiplanar datasets may be reconstructed in any cardiac plane and viewed in cine mode. Multiplanar reformations in LV short-axis, two-, three-, and four-chamber views are used for evaluation of regional wall motion abnormalities. A threshold-based real-time visualization of the lumen of the left ventricle (4D CT-Ventriculogram) is the equivalent of an invasive ventriculogram obtained during cardiac catheterization. CT ventriculography allows for visual assessment of global function and for assessment of wall motion abnormalities and aneurysms. However, it does not allow simultaneous assessment of the myocardial wall for thickness, systolic thickening, calcification, fatty metaplasia, or perfusion defects. Therefore, 2D cine reformations in standard views are an essential component that may be complemented by the ventriculographic review (Fig. 3).
Volumetric Ejection Fraction Calculation Figure 1 Definition of cardiac phase. The left reconstruction window is centered around the defining 80% mark. In this case the acquisition window starts one-half of the temporal window (box width in milliseconds) before 80% and ends at one-half of the temporal window after the 80% mark. Contrast that to the right reconstruction, where the reconstruction window starts exactly at the defining 80% mark and ends exactly one temporal window width after 80%. (Color version of figure is available online.)
Ejection fraction can be determined by a number of methods, including variations of the area length method, Simpson’s method, and volumetric methods. Because area length methods do use geometric assumptions, they do represent only an estimate of LV volumes and left ventricular ejection fraction (LVEF) and therefore should be avoided. The main reason for that recommendation is the fact that CT datasets do contain spatially registered threedimensional information about the entire LV cavity and
Evaluation of cardiac function and valves by MDCT
Figure 3 Four-dimensional CT ventriculogram in a patient with left anterior descending (LAD) coronary artery territory acute myocardial infarction. Systolic frame demonstrates lack of contraction in the anterior-apical wall (akinesis). (Color version of figure is available online.)
therefore, unlike echocardiography, do not have to depend on any geometric assumptions to calculate ventricular volumes. Simpson’s method is based on creation of usually short-axis images through the left ventricle and determination of ventricular lumen areas on each slice. The areas are multiplied by the total slice thickness and summed, resulting in the LV volume at that particular cardiac phase. This method has been developed for cross-sectional imaging modalities that produce a number of relatively thick individual slices throughout the anatomy of interest (such as MRI). Some vendors had initially adopted cardiac MRI software for CT dataset analysis and have offered Simpson’s method based software for volume and LVEF calculation. However with MDCT datasets it is currently no longer necessary to add the labor-intensive additional steps of creating short-axis slices and to individually adjust the LV border detection. The true volumetric nature of gated MDCT allows for volumetric LVEF calculations, which utilize a threshold-based algorithm coupled with manual or automated delineation of the mitral valve plane. These algorithms require advanced software, which usually is part of cardiac packages of the major vendor’s 3D workstations (Fig. 4). The improved spatial and temporal resolution of 64-slice MDCT has resulted in more accurate measurements of LVEF and LV volumes compared with earlier generations of mechanical CT scanners. Four-slice MDCT scanners have been reported to underestimate LVEF by approximately 12%.8 This effect is due to poor temporal resolution from slower gantry rotation speeds, which results in overestimation of end-systolic volumes and underestimation of end-diastolic volumes, which leads to falsely low ejection fraction values.
147 Salm and others found excellent agreement between 16slice MDCT and echocardiography without substantial underestimation of EF.26 Abbara and coworkers demonstrated that the correlation of the 64-slice MDCT to echocardiography and single-photon emission CT appears to be superior to that of 16-slice MDCT.27 This relative improvement in correlation is likely related the improved temporal resolution of the 64-slice MDCT with faster gantry rotation times (420 milliseconds in the 16-slice MDCT versus 330 milliseconds in the utilized 64-slice MDCT scanner). Multi-segment reconstruction did not result in a significant benefit for determination of LV volumes and LVEF in patients that received beta-blocker.28 The reason for the lack of improvement is likely at least in part due to the fact that there is only little or no reduction of temporal resolution with multi-segment reconstruction at lower heart rates. RV function assessment is also possible with CTA and good correlation of right ventricular ejection fraction with radionuclide studies has been demonstrated.29 If RV functional assessment is desired, one should modify the injection protocol to a triphasic protocol. For example, assuming a scan duration of 12 seconds, one would use contrast for 12 seconds at 5 mL/s followed by contrast (or mixed saline/ contrast) for 12 seconds at 2 mL/s, followed by 20 mL of saline at 2 mL/s.
Regional Wall Motion Assessment Regional wall motion assessment is typically performed using short-axis, four-chamber, and other standard views either in thin sections or as 5-mm maximum intensity projections (MIPs). Reporting is performed utilizing the standard 17segment standardized myocardial segmentation and nomenclature for tomographic imaging of the heart.30 Each segment has to be characterized as either normal, hypokinetic (decreased systolic thickening with centripetal motion), akinetic (no systolic thickening), or dyskinetic (centrifugal systolic motion).31 Focal wall motion abnormalities are frequently seen with rest ischemia or myocardial infarction. These typically adhere to coronary territories. Wall motion abnormalities that are diffusely present are common in global disorders such as dilated cardiomyopathies. In addition to regional wall motion abnormalities, the myocardial morphology is assessed for each segment. This includes, among others, enddiastolic wall thickness, hyper-trabeculation, decreased subendocardial attenuation (perfusion defect), calcification (chronic MI), and fatty metaplasia (negative HU ¡ chronic MI).
Perfusion Analysis Perfusion analysis based on arterial phase coronary CTA is feasible.14-21 Perfusion analysis requires merely postprocessing of the acquired coronary CTA dataset without additional imaging. To evaluate images for perfusion defects, it is best to create multiplanar reformations in standard views in 5-mm sections
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Figure 4 Four-chamber, two-chamber, and short-axis views of the left ventricle with threshold-based volumetric measurement of LV lumen in systole. The highlighted voxels within the LV cavity represent the end-systolic LV volume. Note the straight line across the mitral valve is due to manual definition of this plane. If the endocardial and epicardial borders are detected, LV mass and wall thickening maps (bottom right image) can be calculated. Note inferior wall perfusion defect. (Color version of figure is available online.)
using the average weighted rendering mode and selecting a very narrow window that is leveled around normally enhanced myocardium. An alternative to average weighting is minimum intensity projection. It is important to avoid MIP because this modus has the potential to minimize the difference between the hypoenhanced perfusion defects and the normally enhanced myocardium, and therefore, it may mask perfusion defects. Delayed enhancement imaging with CT is possible; however, it requires an additional acquisition 5 to 10 minutes after the injection, and therefore, increases the radiation dose.16,32 Therefore the only role of delayed enhanced imaging currently is if there is an otherwise valid indication, but a contraindication to MRI. A potential scenario is pre-ablation mapping of LV fibrosis in a patient with intractable arrhythmias and an internal cardioverter defibrillator in place.
Valvular Function Acquired valvular heart disease remains a significant problem in the United States. While valvular heart disease can be
diagnosed by the patient’s history and cardiac auscultation, diagnostic noninvasive imaging remains vital to treatment planning and serial monitoring.33 Echocardiography is currently the most widely used noninvasive imaging modality to assess cardiac valve morphology and function. Echocardiography is widely available, cost-effective, and generally provides all the information needed to plan medical or surgical treatment. The limitation in echocardiography is its inability to obtain high-quality images in all patients due to poor acoustic windows. Cardiac MRI is an alternative imaging modality for assessment of valve disease as it too provides adequate spatial and temporal resolution and allows for quantification of flow hemodynamics.34-36 The data acquisition time and cost make cardiac MRI a suboptimal screening tool. Technical advances in ECG-gated MDCT now allow for adequate visualization and assessment of the cardiac valves.35,37 Cardiac MDCT is a comprehensive examination that can examine valvular and ventricular function through all phases of the cardiac cycle. CT provides better spatial resolution than both echocardiography and MRI at 0.4 to 0.6
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mm with near isotropic voxels.34,38,39 Hence, anatomic details of the valve leaflets, chordae tendinae, and papillary muscles are optimally visualized. Furthermore, the data acquisition time for MDCT is only 5 to 20 seconds, which is much shorter than both echo and MRI. An imaging modality with a good temporal resolution is vital for the assessment of valve motion, and with modern 64 MDCT temporal resolution is approximately 165 to 175 milliseconds at lower heart rates, which can potentially be reduced at higher heart rates if multi-segment reconstruction algorithms are used. Echocardiography and MRI provide temporal resolutions on the order of 50 milliseconds. However, newer generation scanners that use dual X-ray sources will further reduce temporal resolution of MDCT to 82 milliseconds in single segment reconstruction mode and 41 milliseconds in two-sector mode, making it more comparable to MRI and echo.23,24 Valve motion and function can be qualitatively assessed by MDCT.40 In many instances planimetry may be performed of both the opening valve area in cases of stenosis and the regurgitant orifice in cases of insufficiency. Furthermore, MDCT is the best modality to assess valve calcification, which is the major culprit of acquired valve disease in the US.38,41-45
Valve Morphology and Function All four cardiac valves function to ensure unidirectional flow from one chamber to the next. Normal valvular function allows for no impedance of forward blood flow, while the valve is open and minimal to no retrograde blood flow when the valve is closed. Valve stenosis results for obstructive narrowing of the orifice. Valve regurgitation results for malcoaptation of the leaflets with retrograde blood flow. If chronic and severe, these problems alter cardiac loading condition and result in pathologic remodeling of the ventricle. A good noninvasive imaging modality for the valves should provide information regarding valve morphology, valve function, and cardiac function.32 MDCT provides excellent visualization of the mitral and aortic valves, particularly the number of valve leaflets, leaflet thickening, opening and closing of the leaflets, and the presence of valve calcification (Fig. 5). Due to the arterial phase injection protocol and saline chaser, the right-sided pulmonic and tricuspid valves are not frequently visualized. When the indication of the MDCT is to assess the RV or pulmonary arteries, the scan protocol may be modified to provide contrast enhancement of the right-sided chambers.
Aortic Valve Native Aortic Valve Abbara and coworkers showed that aortic orifice area measurements are best performed on midsystolic datasets with
Figure 5 MPR short-axis view at the aortic valve level. (A) The valve in diastole. Note minimal calcification of the leaflets. (B) The valve in systole. In this image it becomes clear that the left coronary cusp and right coronary cusp are partially fused and this valve is a functional bicuspid.
phase starts of 50 to 150 milliseconds after R-wave peak (independent of gender or age).46 The best image quality for aortic valve planimetry is if the reconstruction window starts immediately after isovolumetric contraction of the left ventricle. In ⬎90% of cases best image quality for planimetry is at midsystolic frame with phase (reconstruction window) starts of 50 milliseconds to 150 milliseconds. Typical artifacts causing image degradation are motion-related blurring and the presence of double leaflet artifacts (one leaflet shows two contours) and incomplete visualization of a cusp. The aortic valve area measured by CT in normal individuals averages 2.9 ⫾ 0.2 cm2 (1.8-5.6 cm2) and with good correlation to echocardiography.47
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with poststenotic dilation of the aortic root that requires surgical replacement. In addition, MDCT is superior to all other imaging modalities at the detection of calcium and is useful to the surgeon when planning aortic cannulation. The limitation of MDCT is its inability to assess aortic valve gradients and potentially underestimate aortic valve area in those cases of low output or low gradient aortic stenosis.
Aortic Regurgitation
Figure 6 Aortic stenosis. MPR short-axis view of the aortic valve in peak systole. Maximal excursion of all three cusps is restricted with reduction in aortic valve area. Planimetery of the valve area can be performed and revealed mild aortic stenosis in this case.
Aortic Stenosis Severe aortic stenosis (AS) is the most common valve lesion to be treated with surgical replacement.48 Aortic stenosis can be congenital (ie, bicuspid valve) or acquired (senile calcified degeneration). In addition to aortic stenosis being at the valvular level, it may also be subvalvular or supravalvular. Regardless of the etiology, the effect of the obstruction is an increase in LV afterload. In the attempt to maintain wall stress, the left ventricle remodels with concentric hypertrophy.49 The timing of surgical intervention depends on the patient’s New York Heart Association functional class and the severity of stenosis based on valve area.33 Transvalvular pressure gradients are helpful but depend on many other confounding factors such as stroke volume, ejection time, and aortic pressure. MDCT has proven an excellent imaging modality to determine aortic valve area. Direct planimetry can be obtained by determining the phase during systole where the valve is maximally open (Fig. 6). Feuchtner and coworkers were among the first group to assess the accuracy of MDCT at quantifying aortic valve area. Forty-six patients underwent MDCT with direct valve planimetry from cross-sectional images at the level of the valve from multiplanar reformats with the reconstruction window positioned mid to late systole. Results were compared with the aortic valve area derived by Trans Thoracic Echocardiography (TTE) using Doppler continuity equation. The results show MDCT to have a sensitivity of 100%, specificity of 94%, positive-predictive value of 97%, and negative-predictive value of 100%. The Bland–Altman analysis shows only slight overestimation (0.40 cm) of MDCT compared with Trans Esophageal Echocardiography (TEE).47 Similar results were obtained in two other prospectively designed trials that compared MDCT aortic valve area to both Doppler echo by TTE and direct planimetry by TEE.50,51 MDCT also gives a comprehensive examination for aortic root pathology. Severe aortic stenosis is often accompanied
Aortic regurgitation results from valve leaflet or aortic root etiologies. Valve abnormalities include congenital bicuspid valves, rheumatic deformity, bacterial endocarditis, and myxomatous degeneration. Aortic root pathology includes aortic dissection, cystic medial necrosis, Marfan’s syndrome, syphilitic aortitis, Ehlers–Danlos syndrome, and a variety of collagen vascular diseases. The hemodynamic impact of aortic regurgitation is LV volume overload with eccentric remodeling.49 In the acute setting, the left ventricle does not have time to adapt to the sudden increase in filling pressures and the result is acute pulmonary edema and shock. Chronic aortic regurgitation causes LV dilation and ultimately LV failure. Regular close follow-up is required with noninvasive imaging to correctly determine the timing of surgical intervention.33 MDCT can detect the presence of aortic regurgitation by visualizing the central valvular regurgitant orifice. Feuchtner and coworkers have shown that 16 MDCT can detect moderate and severe regurgitation in a prospective trial of 71 patients. The diagnostic criteria was a visible central valvular leakage area and the results were compared with Doppler TTE semiquantitative regurgitant jet analysis. The overall sensitivity for the identification of patients with aortic regurgitation was 81% and the specificity was 91%. For moderate to severe echo Doppler grades of aortic regurgitation, the
Figure 7 Aortic regurgitation. MPR short-axis view of the aortic valve in diastole. Note malcoaptation of the cusps causing a central regurgitant orifice. This area can be planimetered and preliminary data suggest good correlation with echo Doppler grading of aortic regurgitation.
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sensitivity and specificity of MDCT increased 95 and 96%, respectively52 (Fig. 7).
Mitral Valve Mitral Valve Stenosis The most common cause of mitral stenosis is rheumatic heart disease. More rare causes include congenital mitral stenosis with parachute deformity or valve obstruction from thrombus or tumor. A mitral valve area less than 2.5 cm2 results in abnormal diastolic mitral inflow. The result is elevation in left atrial pressure with resultant pulmonary congestion. Over time the left atrium remodels with dilation and becomes prone to thrombus formation and atrial fibrillation. Transmitral flow velocity and gradients increase with mitral stenosis; this is easily detected with Doppler echocardiography and MRI. From this hemodynamic profile a mitral valve area can be mathematically derived. Direct planimetry of the valve area can also be obtained but the precise crosssectional imaging plane at the leaflet tips is difficult to obtain with both echo and MRI. As well, the heavy degree of calcification that generally accompanies mitral stenosis causes imaging artifacts with echo and MRI. Preliminary data suggest that MDCT will prove to be an accurate and useful tool to assess mitral valve morphology and calcification. Messika-Zeitoun and coworkers examined the feasibility, accuracy, and reproducibility of mitral valve area measurements by MDCT using echocardiography as a reference standard in 29 patients. MDCT measurements were performed in early diastole (75% R-R interval). Using the four- and two-chamber views, planimetry of the true crosssectional area of the mitral valve at the leaflet tips is obtained. Results showed that Mitral Valve Area (MVA) by MDCT did not differ and correlated well with MVA by echo, with no trend for underestimation or overestimation.53
Mitral Valve Regurgitation Mitral regurgitation is the most frequently encountered valve lesion and can be acute or chronic.54 Causes of mitral regurgitation may be due to a primary abnormality in the valve apparatus (ie, leaflets, chords, papillary muscles, or mitral annulus) from such insults as rheumatic heart disease, myxomatous degeneration, infective endocarditis, or myocardial infarction. Secondary causes of mitral regurgitation result from alterations in LV geometry due to cavity dilation. Conventional qualitative techniques to grade mitral regurgitation severity include sizing of the regurgitant jet by echo color Doppler or gradient echo cine MRI. Quantitative measures to assess mitral regurgitation include calculation of the regurgitant volume or effective regurgitant orifice. MDCT is an emerging imaging modality to assess mitral regurgitation.55-57 MDCT can accurately depict most morphological causes of mitral regurgitation. Ruptured chordae tendinae is one of the rare etiologies missed by MDCT, but as spatial and temporal resolution of new generation scanners improve, these too will be diagnosed. Alkadhi and coworkers have shown that MDCT can accurately identify the cause of
Figure 8 Mitral regurgitation. MPR short-axis view of the mitral valve in systole. Note the area of leaflet malcoaptation, which results in mitral regurgitation. The regurgitant orifice area can be planimetered to grade the severity of the lesion.
mitral regurgitation and quantitatively grade mitral regurgitation severity by planimetry of the anatomic regurgitant orifice area. In a prospective study of 44 patients MDCTderived regurgitant orifice area compared very well to transesophageal echo Doppler and ventriculography grades of mitral regurgitation58 (Fig. 8).
Right-Sided Valves The tricuspid and pulmonic valves are generally not well visualized due to the arterial phase injection protocol and saline bolus. There are, however, certain right-sided pathologies that can be detected by MDCT. Tricuspid regurgitation is seen by MDCT with incomplete leaflet coaptation and the presence of refluxing contrast into the hepatic veins during the first pass of contrast. Secondary clues to the presence of tricuspid regurgitation include a dilated IVC as well as right atrial and ventricular dilation. Furthermore, the presence of pulmonary arterial enlargement may represent underlying pulmonary hypertension as the cause. Likewise, the presence of LV dilation represents underlying left heart failure as the cause, which is the most common etiology of tricuspid regurgitation. Congenital abnormalities of the tricuspid valve such as Ebstein’s anomaly are easily detected by MDCT. Imaging features noted are apical displacement of the septal leaflet and “sail-like” anterior leaflet deformity with atrialization of the right ventricle. Regarding the pulmonic valve, clues on MDCT to pulmonic stenosis are poststenotic dilation in cases of classic pulmonic stenosis. Pulmonic valve dysplasia is generally associated with a hypoplastic pulmonic trunk.
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Valve Prosthesis Mechanical valve prostheses are well visualized with MDCT and their function can easily be assessed on the 4D multiphase datasets. Complications of valve prostheses include “frozen” leaflet from thrombus or pannus, valve dehiscence, infective endocarditis, and paravalvular abscess. Traditionally, fluoroscopy is used to assess prosthesis function. Mechanical leaflet motion and opening angles can be easily visualized with fluoroscopy. Gated MDCT is proving to be an excellent modality to assess valve prosthesis. Using the 4D multiphase dataset in systole and diastole, the mechanical prosthesis can be easily visualized with the right window and level settings. Similar to fluoroscopy, leaflet motion and opening angles can be derived. Regarding paravalvular abscess, contrast collections and tissue stranding are clues on MDCT.
Conclusion The role of MDCT in cardiac disease is increasing and the technique now has an established role in the assessment of coronary artery disease. MDCT is an excellent tool to evaluate ventricular function and has great potential in evaluation of aortic and mitral valves. Because information on cardiac function is present in every retrospectively gated CTA dataset and reconstruction of functional images comes without the need of additional radiation or contrast, it is advisable to perform functional assessment in every coronary CTA. MDCT scanners with improvement on temporal resolution, such as dual-source CT, have the greatest promise to greatly enhance the value of CT for evaluation of these cardiac parameters and structures; however, further studies are needed to clarify the accuracy of the technique and its role in the clinical decision making process.
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S. Abbara, A.V. Soni, and R.C. Cury 8. Juergens KU, Grude M, Fallenberg EM, et al: Using ECG-gated multidetector CT to evaluate global left ventricular myocardial function in patients with coronary artery disease. AJR Am J Roentgenol 179:15451550, 2002 9. Lessick J, Mutlak D, Rispler S, et al: Comparison of multidetector computed tomography versus echocardiography for assessing regional left ventricular function. Am J Cardiol 96:1011-1015, 2005 10. Ferencik M, Gregory SA, Butler J, et al: Analysis of cardiac dimensions, mass and function in heart transplant recipients using 64-slice multidetector computed tomography. J Heart Lung Transplant 26:478-484, 2007 11. Raman SV, Shah M, McCarthy B, et al: Multi-detector row cardiac computed tomography accurately quantifies right and left ventricular size and function compared with cardiac magnetic resonance. Am Heart J 151:736-744, 2006 12. Schlosser T, Pagonidis K, Herborn CU, et al: Assessment of left ventricular parameters using 16-MDCT and new software for endocardial and epicardial border delineation. AJR Am J Roentgenol 184:765-773, 2005 13. Heuschmid M, Rothfuss JK, Schroeder S, et al: Assessment of left ventricular myocardial function using 16-slice multidetector-row computed tomography: comparison with magnetic resonance imaging and echocardiography. Eur Radiol 16:551-559, 2006 14. Hoffmann U, Millea R, Enzweiler C, et al: Acute myocardial infarction: contrast-enhanced multi-detector row CT in a porcine model. Radiology 231:697-701, 2004 15. Nieman K, Cury RC, Ferencik M, et al: Differentiation of recent and chronic myocardial infarction by cardiac computed tomography. Am J Cardiol 98:303-308, 2006 16. Mahnken AH, Bruners P, Katoh M, et al: Dynamic multi-section CT imaging in acute myocardial infarction: preliminary animal experience. Eur Radiol 16:746-752, 2006 17. Mahnken AH, Koos R, Katoh M, et al: Assessment of myocardial viability in reperfused acute myocardial infarction using 16-slice computed tomography in comparison to magnetic resonance imaging. J Am Coll Cardiol 45:2042-2047, 2005 18. Lardo AC, Cordeiro MA, Silva C, et al: Contrast-enhanced multidetector computed tomography viability imaging after myocardial infarction: characterization of myocyte death, microvascular obstruction, and chronic scar. Circulation 113:394-404, 2006 19. Paul JF, Wartski M, Caussin C, et al: Late defect on delayed contrastenhanced multi-detector row CT scans in the prediction of SPECT infarct size after reperfused acute myocardial infarction: initial experience. Radiology 236:485-489, 2005 20. Gerber BL, Belge B, Legros GJ, et al: Characterization of acute and chronic myocardial infarcts by multidetector computed tomography: comparison with contrast-enhanced magnetic resonance. Circulation 113:823-833, 2006 21. Buecker A, Katoh M, Krombach GA, et al: A feasibility study of contrast enhancement of acute myocardial infarction in multislice computed tomography: comparison with magnetic resonance imaging and gross morphology in pigs. Invest Radiol 40:700-704, 2005 22. Juergens KU, Fischbach R: Left ventricular function studied with MDCT. Eur Radiol 16:342-357, 2006 23. Reimann AJ, Rinck D, Birinci-Aydogan A, et al: Dual-source computed tomography: advances of improved temporal resolution in coronary plaque imaging. Invest Radiol 42:196-203, 2007 24. Johnson TR, Nikolaou K, Wintersperger BJ, et al: Dual-source CT cardiac imaging: initial experience. Eur Radiol 16:1409-1415, 2006 25. Flohr TG, McCollough CH, Bruder H, et al: First performance evaluation of a dual-source CT (DSCT) system. Eur Radiol 16:256-268, 2006 26. Salm LP, Schuijf JD, de Roos A, et al: Global and regional left ventricular function assessment with 16-detector row CT: comparison with echocardiography and cardiovascular magnetic resonance. Eur J Echocardiogr 7:308-314, 2006 27. Abbara S, Chow BJ, Pena AJ, et al: Assessment of left ventricular function with 16- and 64-slice multi-detector computed tomography. Eur J Radiol 2007 Sep 7:[Epub ahead of print]
Evaluation of cardiac function and valves by MDCT 28. Juergens KU, Maintz D, Grude M, et al: Multi-detector row computed tomography of the heart: does a multi-segment reconstruction algorithm improve left ventricular volume measurements? Eur Radiol 15: 111-117, 2005 29. Delhaye D, Remy-Jardin M, Teisseire A, et al: MDCT of right ventricular function: comparison of right ventricular ejection fraction estimation and equilibrium radionuclide ventriculography, part 1. AJR Am J Roentgenol 187:1597-1604, 2006 30. Cerqueira MD, Weissman NJ, Dilsizian V, et al: Standardized myocardial segmentation and nomenclature for tomographic imaging of the heart: a statement for healthcare professionals from the Cardiac Imaging Committee of the Council on Clinical Cardiology of the American Heart Association. Circulation 105:539-542, 2002 31. Savino G, Zwerner P, Herzog C, et al: CT of cardiac function. J Thorac Imaging 22:86-100, 2007 32. Mahnken AH, Muhlenbruch G, Gunther RW, et al: Cardiac CT: coronary arteries and beyond. Eur Radiol 17:994-1008, 2007 33. Bonow RO, Carabello BA, Kanu C, et al: ACC/AHA 2006 guidelines for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (writing committee to revise the 1998 Guidelines for the Management of Patients With Valvular Heart Disease): developed in collaboration with the Society of Cardiovascular Anesthesiologists: endorsed by the Society for Cardiovascular Angiography and Interventions and the Society of Thoracic Surgeons. Circulation 114:e84-e231, 2006 34. Vogel-Claussen J, Pannu H, Spevak PJ, et al: Cardiac valve assessment with MR imaging and 64-section multi-detector row CT. Radiographics 26:1769-1784, 2006 35. Rozenshtein A, Boxt LM: Computed tomography and magnetic resonance imaging of patients with valvular heart disease. J Thorac Imaging 15:252-264, 2000 36. Gilkeson RC, Markowitz AH, Balgude A, et al: MDCT evaluation of aortic valvular disease. AJR Am J Roentgenol 186:350-360, 2006 37. Boxt LM, Lipton MJ, Kwong RY, et al: Computed tomography for assessment of cardiac chambers, valves, myocardium and pericardium. Cardiol Clin 21:561-585, 2003 38. Koos R, Kuhl HP, Muhlenbruch G, et al: Prevalence and clinical importance of aortic valve calcification detected incidentally on CT scans: comparison with echocardiography. Radiology 241:76-82, 2006 39. Mahnken AH, Koos R, Wildberger JE, et al: [Value of cardiac multislice spiral CT for the assessment of degenerative aortic stenosis: comparison with echocardiography]. Rofo 176:1582-1588, 2004 40. Willmann JK, Weishaupt D, Lachat M, et al: Electrocardiographically gated multi-detector row CT for assessment of valvular morphology and calcification in aortic stenosis. Radiology 225:120-128, 2002 41. Koos R, Mahnken AH, Sinha AM, et al: Aortic valve calcification as a marker for aortic stenosis severity: assessment on 16-MDCT. AJR Am J Roentgenol 183:1813-1818, 2004
153 42. Morgan-Hughes GJ, Owens PE, Roobottom CA, et al: Three dimensional volume quantification of aortic valve calcification using multislice computed tomography. Heart 89:1191-1194, 2003 43. Cowell SJ, Newby DE, Burton J, et al: Aortic valve calcification on computed tomography predicts the severity of aortic stenosis. Clin Radiol 58:712-716, 2003 44. Wagner S, Selzer A: Patterns of progression of aortic stenosis: a longitudinal hemodynamic study. Circulation 65:709-712, 1982 45. Stewart BF, Siscovick D, Lind BK, et al: Clinical factors associated with calcific aortic valve disease. Cardiovascular Health Study. J Am Coll Cardiol 29:630-634, 1997 46. Abbara S, Pena AJ, Maurovich-Horvat P, et al: Feasibility and optimization of aortic valve planimetry with MDCT. AJR Am J Roentgenol 188:356-360, 2007 47. Feuchtner GM, Dichtl W, Friedrich GJ, et al: Multislice computed tomography for detection of patients with aortic valve stenosis and quantification of severity. J Am Coll Cardiol 47:1410-1417, 2006 48. Carabello BA: Clinical practice. Aortic stenosis. N Engl J Med 346:677682, 2002 49. Grossman W, Jones D, McLaurin LP: Wall stress and patterns of hypertrophy in the human left ventricle. J Clin Invest 56:56-64, 1975 50. Bouvier E, Logeart D, Sablayrolles JL, et al: Diagnosis of aortic valvular stenosis by multislice cardiac computed tomography. Eur Heart J 27: 3033-3038, 2006 51. Alkadhi H, Wildermuth S, Plass A, et al: Aortic stenosis: comparative evaluation of 16-detector row CT and echocardiography. Radiology 240:47-55, 2006 52. Feuchtner GM, Dichtl W, Schachner T, et al: Diagnostic performance of MDCT for detecting aortic valve regurgitation. AJR Am J Roentgenol 186:1676-1681, 2006 53. Messika-Zeitoun D, Serfaty JM, Laissy JP, et al: Assessment of the mitral valve area in patients with mitral stenosis by multislice computed tomography. J Am Coll Cardiol 48:411-413, 2006 54. Carabello BA: Mitral valve regurgitation. Curr Probl Cardiol 23:202241, 1998 55. Alkadhi H, Bettex D, Wildermuth S, et al: Dynamic cine imaging of the mitral valve with 16-MDCT: a feasibility study. AJR Am J Roentgenol 185:636-646, 2005 56. Lembcke A, Wiese TH, Enzweiler CN, et al: Quantification of mitral valve regurgitation by left ventricular volume and flow measurements using electron beam computed tomography: comparison with magnetic resonance imaging. J Comput Assist Tomogr 27:385-391, 2003 57. Lembcke A, Borges AC, Dohmen PM, et al: Quantification of functional mitral valve regurgitation in patients with congestive heart failure: comparison of electron-beam computed tomography with cardiac catheterization. Invest Radiol 39:728-739, 2004 58. Alkadhi H, Wildermuth S, Bettex DA, et al: Mitral regurgitation: quantification with 16-detector row CT—initial experience. Radiology 238: 454-463, 2006
A
dvances in multidetector computed tomography (MDCT) technology, with submillimeter isovolumetric voxel size and fast gantry rotation speeds of 330 to 350 milliseconds, enable simultaneous noninvasive evaluation of detailed coronary anatomy, global and regional left ventricular (LV), and valve function as well as myocardial perfusion. Published evidence regarding the accuracy of coronary artery evaluation with MDCT is rapidly evolving. A large number of reports have now demonstrated high diagnostic accuracy for detection of coronary artery stenosis by 64-slice MDCT as compared with invasive coronary angiography.1-7 Preliminary data regarding the global right ventricular (RV) and LV function and regional wall motion assessment,8-13 as well as myocardial perfusion assessment, have recently become available.14-21 Every retrospectively gated coronary MDCT raw dataset contains information about ventricular and valvular function and myocardial perfusion. This information is within the raw data that the scanner acquires but is lost unless dedicated functional datasets are reconstructed in addition to the standard computed tomographic angiography (CTA) image datasets. Assessments of global and regional LV function as well as aortic and mitral valve function can add significant value to the interpretation of a coronary CTA. Because this additional information can be acquired without additional radiation exposure or intravenous contrast administration, it is advisable to perform functional reconstructions in every patient that undergoes cardiac gated CTA. Computed tomography (CT) allows for true volumetric functional analysis and is less affected by patient size or acoustic window restrictions, which can be a substantial advantage over echocardiography in some patients. Compared with magnetic resonance imaging (MRI) and echocardiography, CT uses nephrotoxic contrast material and results in radiation exposure to the patient.22 Therefore cardiac CT is not routinely performed for functional assessment alone. Ventricular function assessment is an additional
Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts. Address reprint requests to Suhny Abbara, MD, Department of Radiology, Massachusetts General Hospital, 165 Cambridge Street, Suite 400, Boston, MA 02114. E-mail: [email protected]
0037-198X/08/$-see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1053/j.ro.2008.01.009
evaluation that is added to an otherwise indicated coronary CTA. Another major disadvantage of CT is the relatively poor temporal resolution compared with MRI and echocardiography, However, the temporal resolution of CT has substantially improved with the advent of the latest generation scanner, the dual-source CT scanners.23-25
CTA Acquisition A standard CT angiography protocol for the coronary arteries produces raw datasets that also may be used for reconstruction of functional LV and valve datasets. The scan protocol uses either a bolus tracking technique or a test bolus acquisition, where the scan delay is calculated depending on contrast enhancement of the aortic root. Typically, a test bolus of 20 mL of iodinated contrast followed by 40 mL of saline is injected and sequential images are acquired at the same slice location until maximal contrast enhancement of the aortic root is noted. The delay time is equal to the time to peak attenuation in the ascending aorta. For the CTA acquisition, 80 to 100 mL of contrast is injected at flow rates of 4 to 7 mL/s followed by 40 mL of saline at the same injection rate. Single-phase high-resolution CTA datasets are usually reconstructed in mid to late diastole. Some authors have proposed triphasic injections (contrast at high rate followed by contrast at lower rate followed by saline flush) or replacing the saline flush with a mixture of saline and contrast to better delineate the right ventricular endocardial border.22
Data Reconstruction Functional analysis requires additional reconstruction of multiphasic datasets. A multiphasic dataset consists of an arbitrary number of single phase datasets. Each individual phase consists of a set of axial source images (⬃100 to 500 images depending on slice thickness and overlap) throughout the heart that are reconstructed from projections that are acquired within a specific time period of the cardiac cycle. Typically the cardiac cycle is divided into 10 or 20 individual phases. A phase is generally defined by a percent number that indicates the beginning or the center of a reconstruction window (the box on the electrocardiogram) with respect to its 145
146 relative position between two adjacent R-peaks. For example, a phase of 70% refers to images that were reconstructed utilizing projections that were collected at 70% of an R-R interval. At a heart rate of 60, the R-R interval is 1000 milliseconds long and 70% means that the projections were collected at t ⫽ 700 milliseconds after the R-peak or 300 milliseconds before the subsequent R-peak. This relationship changes with heart rates and at a heart rate of 100 the R-R interval is only 600 milliseconds and 70% refers to 420 milliseconds after an R-peak or 180 milliseconds before the subsequent R-peak. Of note, it is important to consider any specific vendor’s definition of a phase when reading the literature. Some vendors (GE, Toshiba, and Phillips) refer to the center of the box (or acquisition window) with any given percent location in the R-R interval, whereas other vendors (Siemens) refer to the start of the acquisition window (Fig. 1). The length of the reconstruction window, or temporal resolution, is determined by the scanner gantry rotation speed and the reconstruction method (half scan versus multisegment reconstruction). In half-scan reconstruction the reconstruction window width or length is exactly one-half of the gantry rotation speed. For example, if the gantry completes one rotation every 350 milliseconds, then the reconstruction window (or temporal resolution) is 175 milliseconds. Multisegment reconstruction will reduce the reconstruction window width compared with half-scan reconstruction; however, this reconstruction method is only an option at higher heart rates. Some vendors offer multisegment reconstruction algorithms that may use up to four consecutive heart beats to reconstruct any given image, thereby potentially reducing the temporal resolution by one-fourth at high heart rates; others only use two consecutive beats. Because the reconstruction window width is generally wider than one-10th and one-20th of a cardiac cycle length,
S. Abbara, A.V. Soni, and R.C. Cury
Figure 2 Overlap of multiphasic reconstruction windows. The drawing illustrates a 10-phasic reconstruction from 0 to 90% of the R-R interval. Note that adjacent reconstruction windows demonstrate quite some overlap (sharing of projections), which results in temporal interpolation. Reconstruction at larger numbers of phases will create further temporal interpolation and is not likely to add substantial additional information. (Color version of figure is available online.)
there will be overlap in the projections used for each of the phases in any multiphasic dataset (Fig. 2).
Analysis and Interpretation Multiplanar datasets may be reconstructed in any cardiac plane and viewed in cine mode. Multiplanar reformations in LV short-axis, two-, three-, and four-chamber views are used for evaluation of regional wall motion abnormalities. A threshold-based real-time visualization of the lumen of the left ventricle (4D CT-Ventriculogram) is the equivalent of an invasive ventriculogram obtained during cardiac catheterization. CT ventriculography allows for visual assessment of global function and for assessment of wall motion abnormalities and aneurysms. However, it does not allow simultaneous assessment of the myocardial wall for thickness, systolic thickening, calcification, fatty metaplasia, or perfusion defects. Therefore, 2D cine reformations in standard views are an essential component that may be complemented by the ventriculographic review (Fig. 3).
Volumetric Ejection Fraction Calculation Figure 1 Definition of cardiac phase. The left reconstruction window is centered around the defining 80% mark. In this case the acquisition window starts one-half of the temporal window (box width in milliseconds) before 80% and ends at one-half of the temporal window after the 80% mark. Contrast that to the right reconstruction, where the reconstruction window starts exactly at the defining 80% mark and ends exactly one temporal window width after 80%. (Color version of figure is available online.)
Ejection fraction can be determined by a number of methods, including variations of the area length method, Simpson’s method, and volumetric methods. Because area length methods do use geometric assumptions, they do represent only an estimate of LV volumes and left ventricular ejection fraction (LVEF) and therefore should be avoided. The main reason for that recommendation is the fact that CT datasets do contain spatially registered threedimensional information about the entire LV cavity and
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Figure 3 Four-dimensional CT ventriculogram in a patient with left anterior descending (LAD) coronary artery territory acute myocardial infarction. Systolic frame demonstrates lack of contraction in the anterior-apical wall (akinesis). (Color version of figure is available online.)
therefore, unlike echocardiography, do not have to depend on any geometric assumptions to calculate ventricular volumes. Simpson’s method is based on creation of usually short-axis images through the left ventricle and determination of ventricular lumen areas on each slice. The areas are multiplied by the total slice thickness and summed, resulting in the LV volume at that particular cardiac phase. This method has been developed for cross-sectional imaging modalities that produce a number of relatively thick individual slices throughout the anatomy of interest (such as MRI). Some vendors had initially adopted cardiac MRI software for CT dataset analysis and have offered Simpson’s method based software for volume and LVEF calculation. However with MDCT datasets it is currently no longer necessary to add the labor-intensive additional steps of creating short-axis slices and to individually adjust the LV border detection. The true volumetric nature of gated MDCT allows for volumetric LVEF calculations, which utilize a threshold-based algorithm coupled with manual or automated delineation of the mitral valve plane. These algorithms require advanced software, which usually is part of cardiac packages of the major vendor’s 3D workstations (Fig. 4). The improved spatial and temporal resolution of 64-slice MDCT has resulted in more accurate measurements of LVEF and LV volumes compared with earlier generations of mechanical CT scanners. Four-slice MDCT scanners have been reported to underestimate LVEF by approximately 12%.8 This effect is due to poor temporal resolution from slower gantry rotation speeds, which results in overestimation of end-systolic volumes and underestimation of end-diastolic volumes, which leads to falsely low ejection fraction values.
147 Salm and others found excellent agreement between 16slice MDCT and echocardiography without substantial underestimation of EF.26 Abbara and coworkers demonstrated that the correlation of the 64-slice MDCT to echocardiography and single-photon emission CT appears to be superior to that of 16-slice MDCT.27 This relative improvement in correlation is likely related the improved temporal resolution of the 64-slice MDCT with faster gantry rotation times (420 milliseconds in the 16-slice MDCT versus 330 milliseconds in the utilized 64-slice MDCT scanner). Multi-segment reconstruction did not result in a significant benefit for determination of LV volumes and LVEF in patients that received beta-blocker.28 The reason for the lack of improvement is likely at least in part due to the fact that there is only little or no reduction of temporal resolution with multi-segment reconstruction at lower heart rates. RV function assessment is also possible with CTA and good correlation of right ventricular ejection fraction with radionuclide studies has been demonstrated.29 If RV functional assessment is desired, one should modify the injection protocol to a triphasic protocol. For example, assuming a scan duration of 12 seconds, one would use contrast for 12 seconds at 5 mL/s followed by contrast (or mixed saline/ contrast) for 12 seconds at 2 mL/s, followed by 20 mL of saline at 2 mL/s.
Regional Wall Motion Assessment Regional wall motion assessment is typically performed using short-axis, four-chamber, and other standard views either in thin sections or as 5-mm maximum intensity projections (MIPs). Reporting is performed utilizing the standard 17segment standardized myocardial segmentation and nomenclature for tomographic imaging of the heart.30 Each segment has to be characterized as either normal, hypokinetic (decreased systolic thickening with centripetal motion), akinetic (no systolic thickening), or dyskinetic (centrifugal systolic motion).31 Focal wall motion abnormalities are frequently seen with rest ischemia or myocardial infarction. These typically adhere to coronary territories. Wall motion abnormalities that are diffusely present are common in global disorders such as dilated cardiomyopathies. In addition to regional wall motion abnormalities, the myocardial morphology is assessed for each segment. This includes, among others, enddiastolic wall thickness, hyper-trabeculation, decreased subendocardial attenuation (perfusion defect), calcification (chronic MI), and fatty metaplasia (negative HU ¡ chronic MI).
Perfusion Analysis Perfusion analysis based on arterial phase coronary CTA is feasible.14-21 Perfusion analysis requires merely postprocessing of the acquired coronary CTA dataset without additional imaging. To evaluate images for perfusion defects, it is best to create multiplanar reformations in standard views in 5-mm sections
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Figure 4 Four-chamber, two-chamber, and short-axis views of the left ventricle with threshold-based volumetric measurement of LV lumen in systole. The highlighted voxels within the LV cavity represent the end-systolic LV volume. Note the straight line across the mitral valve is due to manual definition of this plane. If the endocardial and epicardial borders are detected, LV mass and wall thickening maps (bottom right image) can be calculated. Note inferior wall perfusion defect. (Color version of figure is available online.)
using the average weighted rendering mode and selecting a very narrow window that is leveled around normally enhanced myocardium. An alternative to average weighting is minimum intensity projection. It is important to avoid MIP because this modus has the potential to minimize the difference between the hypoenhanced perfusion defects and the normally enhanced myocardium, and therefore, it may mask perfusion defects. Delayed enhancement imaging with CT is possible; however, it requires an additional acquisition 5 to 10 minutes after the injection, and therefore, increases the radiation dose.16,32 Therefore the only role of delayed enhanced imaging currently is if there is an otherwise valid indication, but a contraindication to MRI. A potential scenario is pre-ablation mapping of LV fibrosis in a patient with intractable arrhythmias and an internal cardioverter defibrillator in place.
Valvular Function Acquired valvular heart disease remains a significant problem in the United States. While valvular heart disease can be
diagnosed by the patient’s history and cardiac auscultation, diagnostic noninvasive imaging remains vital to treatment planning and serial monitoring.33 Echocardiography is currently the most widely used noninvasive imaging modality to assess cardiac valve morphology and function. Echocardiography is widely available, cost-effective, and generally provides all the information needed to plan medical or surgical treatment. The limitation in echocardiography is its inability to obtain high-quality images in all patients due to poor acoustic windows. Cardiac MRI is an alternative imaging modality for assessment of valve disease as it too provides adequate spatial and temporal resolution and allows for quantification of flow hemodynamics.34-36 The data acquisition time and cost make cardiac MRI a suboptimal screening tool. Technical advances in ECG-gated MDCT now allow for adequate visualization and assessment of the cardiac valves.35,37 Cardiac MDCT is a comprehensive examination that can examine valvular and ventricular function through all phases of the cardiac cycle. CT provides better spatial resolution than both echocardiography and MRI at 0.4 to 0.6
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mm with near isotropic voxels.34,38,39 Hence, anatomic details of the valve leaflets, chordae tendinae, and papillary muscles are optimally visualized. Furthermore, the data acquisition time for MDCT is only 5 to 20 seconds, which is much shorter than both echo and MRI. An imaging modality with a good temporal resolution is vital for the assessment of valve motion, and with modern 64 MDCT temporal resolution is approximately 165 to 175 milliseconds at lower heart rates, which can potentially be reduced at higher heart rates if multi-segment reconstruction algorithms are used. Echocardiography and MRI provide temporal resolutions on the order of 50 milliseconds. However, newer generation scanners that use dual X-ray sources will further reduce temporal resolution of MDCT to 82 milliseconds in single segment reconstruction mode and 41 milliseconds in two-sector mode, making it more comparable to MRI and echo.23,24 Valve motion and function can be qualitatively assessed by MDCT.40 In many instances planimetry may be performed of both the opening valve area in cases of stenosis and the regurgitant orifice in cases of insufficiency. Furthermore, MDCT is the best modality to assess valve calcification, which is the major culprit of acquired valve disease in the US.38,41-45
Valve Morphology and Function All four cardiac valves function to ensure unidirectional flow from one chamber to the next. Normal valvular function allows for no impedance of forward blood flow, while the valve is open and minimal to no retrograde blood flow when the valve is closed. Valve stenosis results for obstructive narrowing of the orifice. Valve regurgitation results for malcoaptation of the leaflets with retrograde blood flow. If chronic and severe, these problems alter cardiac loading condition and result in pathologic remodeling of the ventricle. A good noninvasive imaging modality for the valves should provide information regarding valve morphology, valve function, and cardiac function.32 MDCT provides excellent visualization of the mitral and aortic valves, particularly the number of valve leaflets, leaflet thickening, opening and closing of the leaflets, and the presence of valve calcification (Fig. 5). Due to the arterial phase injection protocol and saline chaser, the right-sided pulmonic and tricuspid valves are not frequently visualized. When the indication of the MDCT is to assess the RV or pulmonary arteries, the scan protocol may be modified to provide contrast enhancement of the right-sided chambers.
Aortic Valve Native Aortic Valve Abbara and coworkers showed that aortic orifice area measurements are best performed on midsystolic datasets with
Figure 5 MPR short-axis view at the aortic valve level. (A) The valve in diastole. Note minimal calcification of the leaflets. (B) The valve in systole. In this image it becomes clear that the left coronary cusp and right coronary cusp are partially fused and this valve is a functional bicuspid.
phase starts of 50 to 150 milliseconds after R-wave peak (independent of gender or age).46 The best image quality for aortic valve planimetry is if the reconstruction window starts immediately after isovolumetric contraction of the left ventricle. In ⬎90% of cases best image quality for planimetry is at midsystolic frame with phase (reconstruction window) starts of 50 milliseconds to 150 milliseconds. Typical artifacts causing image degradation are motion-related blurring and the presence of double leaflet artifacts (one leaflet shows two contours) and incomplete visualization of a cusp. The aortic valve area measured by CT in normal individuals averages 2.9 ⫾ 0.2 cm2 (1.8-5.6 cm2) and with good correlation to echocardiography.47
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with poststenotic dilation of the aortic root that requires surgical replacement. In addition, MDCT is superior to all other imaging modalities at the detection of calcium and is useful to the surgeon when planning aortic cannulation. The limitation of MDCT is its inability to assess aortic valve gradients and potentially underestimate aortic valve area in those cases of low output or low gradient aortic stenosis.
Aortic Regurgitation
Figure 6 Aortic stenosis. MPR short-axis view of the aortic valve in peak systole. Maximal excursion of all three cusps is restricted with reduction in aortic valve area. Planimetery of the valve area can be performed and revealed mild aortic stenosis in this case.
Aortic Stenosis Severe aortic stenosis (AS) is the most common valve lesion to be treated with surgical replacement.48 Aortic stenosis can be congenital (ie, bicuspid valve) or acquired (senile calcified degeneration). In addition to aortic stenosis being at the valvular level, it may also be subvalvular or supravalvular. Regardless of the etiology, the effect of the obstruction is an increase in LV afterload. In the attempt to maintain wall stress, the left ventricle remodels with concentric hypertrophy.49 The timing of surgical intervention depends on the patient’s New York Heart Association functional class and the severity of stenosis based on valve area.33 Transvalvular pressure gradients are helpful but depend on many other confounding factors such as stroke volume, ejection time, and aortic pressure. MDCT has proven an excellent imaging modality to determine aortic valve area. Direct planimetry can be obtained by determining the phase during systole where the valve is maximally open (Fig. 6). Feuchtner and coworkers were among the first group to assess the accuracy of MDCT at quantifying aortic valve area. Forty-six patients underwent MDCT with direct valve planimetry from cross-sectional images at the level of the valve from multiplanar reformats with the reconstruction window positioned mid to late systole. Results were compared with the aortic valve area derived by Trans Thoracic Echocardiography (TTE) using Doppler continuity equation. The results show MDCT to have a sensitivity of 100%, specificity of 94%, positive-predictive value of 97%, and negative-predictive value of 100%. The Bland–Altman analysis shows only slight overestimation (0.40 cm) of MDCT compared with Trans Esophageal Echocardiography (TEE).47 Similar results were obtained in two other prospectively designed trials that compared MDCT aortic valve area to both Doppler echo by TTE and direct planimetry by TEE.50,51 MDCT also gives a comprehensive examination for aortic root pathology. Severe aortic stenosis is often accompanied
Aortic regurgitation results from valve leaflet or aortic root etiologies. Valve abnormalities include congenital bicuspid valves, rheumatic deformity, bacterial endocarditis, and myxomatous degeneration. Aortic root pathology includes aortic dissection, cystic medial necrosis, Marfan’s syndrome, syphilitic aortitis, Ehlers–Danlos syndrome, and a variety of collagen vascular diseases. The hemodynamic impact of aortic regurgitation is LV volume overload with eccentric remodeling.49 In the acute setting, the left ventricle does not have time to adapt to the sudden increase in filling pressures and the result is acute pulmonary edema and shock. Chronic aortic regurgitation causes LV dilation and ultimately LV failure. Regular close follow-up is required with noninvasive imaging to correctly determine the timing of surgical intervention.33 MDCT can detect the presence of aortic regurgitation by visualizing the central valvular regurgitant orifice. Feuchtner and coworkers have shown that 16 MDCT can detect moderate and severe regurgitation in a prospective trial of 71 patients. The diagnostic criteria was a visible central valvular leakage area and the results were compared with Doppler TTE semiquantitative regurgitant jet analysis. The overall sensitivity for the identification of patients with aortic regurgitation was 81% and the specificity was 91%. For moderate to severe echo Doppler grades of aortic regurgitation, the
Figure 7 Aortic regurgitation. MPR short-axis view of the aortic valve in diastole. Note malcoaptation of the cusps causing a central regurgitant orifice. This area can be planimetered and preliminary data suggest good correlation with echo Doppler grading of aortic regurgitation.
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sensitivity and specificity of MDCT increased 95 and 96%, respectively52 (Fig. 7).
Mitral Valve Mitral Valve Stenosis The most common cause of mitral stenosis is rheumatic heart disease. More rare causes include congenital mitral stenosis with parachute deformity or valve obstruction from thrombus or tumor. A mitral valve area less than 2.5 cm2 results in abnormal diastolic mitral inflow. The result is elevation in left atrial pressure with resultant pulmonary congestion. Over time the left atrium remodels with dilation and becomes prone to thrombus formation and atrial fibrillation. Transmitral flow velocity and gradients increase with mitral stenosis; this is easily detected with Doppler echocardiography and MRI. From this hemodynamic profile a mitral valve area can be mathematically derived. Direct planimetry of the valve area can also be obtained but the precise crosssectional imaging plane at the leaflet tips is difficult to obtain with both echo and MRI. As well, the heavy degree of calcification that generally accompanies mitral stenosis causes imaging artifacts with echo and MRI. Preliminary data suggest that MDCT will prove to be an accurate and useful tool to assess mitral valve morphology and calcification. Messika-Zeitoun and coworkers examined the feasibility, accuracy, and reproducibility of mitral valve area measurements by MDCT using echocardiography as a reference standard in 29 patients. MDCT measurements were performed in early diastole (75% R-R interval). Using the four- and two-chamber views, planimetry of the true crosssectional area of the mitral valve at the leaflet tips is obtained. Results showed that Mitral Valve Area (MVA) by MDCT did not differ and correlated well with MVA by echo, with no trend for underestimation or overestimation.53
Mitral Valve Regurgitation Mitral regurgitation is the most frequently encountered valve lesion and can be acute or chronic.54 Causes of mitral regurgitation may be due to a primary abnormality in the valve apparatus (ie, leaflets, chords, papillary muscles, or mitral annulus) from such insults as rheumatic heart disease, myxomatous degeneration, infective endocarditis, or myocardial infarction. Secondary causes of mitral regurgitation result from alterations in LV geometry due to cavity dilation. Conventional qualitative techniques to grade mitral regurgitation severity include sizing of the regurgitant jet by echo color Doppler or gradient echo cine MRI. Quantitative measures to assess mitral regurgitation include calculation of the regurgitant volume or effective regurgitant orifice. MDCT is an emerging imaging modality to assess mitral regurgitation.55-57 MDCT can accurately depict most morphological causes of mitral regurgitation. Ruptured chordae tendinae is one of the rare etiologies missed by MDCT, but as spatial and temporal resolution of new generation scanners improve, these too will be diagnosed. Alkadhi and coworkers have shown that MDCT can accurately identify the cause of
Figure 8 Mitral regurgitation. MPR short-axis view of the mitral valve in systole. Note the area of leaflet malcoaptation, which results in mitral regurgitation. The regurgitant orifice area can be planimetered to grade the severity of the lesion.
mitral regurgitation and quantitatively grade mitral regurgitation severity by planimetry of the anatomic regurgitant orifice area. In a prospective study of 44 patients MDCTderived regurgitant orifice area compared very well to transesophageal echo Doppler and ventriculography grades of mitral regurgitation58 (Fig. 8).
Right-Sided Valves The tricuspid and pulmonic valves are generally not well visualized due to the arterial phase injection protocol and saline bolus. There are, however, certain right-sided pathologies that can be detected by MDCT. Tricuspid regurgitation is seen by MDCT with incomplete leaflet coaptation and the presence of refluxing contrast into the hepatic veins during the first pass of contrast. Secondary clues to the presence of tricuspid regurgitation include a dilated IVC as well as right atrial and ventricular dilation. Furthermore, the presence of pulmonary arterial enlargement may represent underlying pulmonary hypertension as the cause. Likewise, the presence of LV dilation represents underlying left heart failure as the cause, which is the most common etiology of tricuspid regurgitation. Congenital abnormalities of the tricuspid valve such as Ebstein’s anomaly are easily detected by MDCT. Imaging features noted are apical displacement of the septal leaflet and “sail-like” anterior leaflet deformity with atrialization of the right ventricle. Regarding the pulmonic valve, clues on MDCT to pulmonic stenosis are poststenotic dilation in cases of classic pulmonic stenosis. Pulmonic valve dysplasia is generally associated with a hypoplastic pulmonic trunk.
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Valve Prosthesis Mechanical valve prostheses are well visualized with MDCT and their function can easily be assessed on the 4D multiphase datasets. Complications of valve prostheses include “frozen” leaflet from thrombus or pannus, valve dehiscence, infective endocarditis, and paravalvular abscess. Traditionally, fluoroscopy is used to assess prosthesis function. Mechanical leaflet motion and opening angles can be easily visualized with fluoroscopy. Gated MDCT is proving to be an excellent modality to assess valve prosthesis. Using the 4D multiphase dataset in systole and diastole, the mechanical prosthesis can be easily visualized with the right window and level settings. Similar to fluoroscopy, leaflet motion and opening angles can be derived. Regarding paravalvular abscess, contrast collections and tissue stranding are clues on MDCT.
Conclusion The role of MDCT in cardiac disease is increasing and the technique now has an established role in the assessment of coronary artery disease. MDCT is an excellent tool to evaluate ventricular function and has great potential in evaluation of aortic and mitral valves. Because information on cardiac function is present in every retrospectively gated CTA dataset and reconstruction of functional images comes without the need of additional radiation or contrast, it is advisable to perform functional assessment in every coronary CTA. MDCT scanners with improvement on temporal resolution, such as dual-source CT, have the greatest promise to greatly enhance the value of CT for evaluation of these cardiac parameters and structures; however, further studies are needed to clarify the accuracy of the technique and its role in the clinical decision making process.
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