DOI: 10.1148/rg.266065035
RadioGraphics 2006;26:1769-1784
© RSNA, 2006
Cardiac Valve Assessment with MR Imaging and 64-Section Multi–Detector Row CT1
Jens Vogel-Claussen, MD,
Harpreet Pannu, MD,
Philip J. Spevak, MD,
Elliot K. Fishman, MD and
David A. Bluemke, MD, PhD
1 From the Russell H. Morgan Department of Radiology and Radiological Science (J.V.C., H.P., E.K.F., D.A.B.) and the Departments of Pediatrics and Medicine (P.J.S.), The Johns Hopkins Hospital, MRI, Room 143 (Nelson Basement), 600 N Wolfe St, Baltimore, MD 21287. Recipient of a Cum Laude award for an education exhibit at the 2005 RSNA Annual Meeting. Received March 21, 2006; revision requested April 17 and received August 4; accepted August 11. J.V.C. was supported by the Radiological Society of North America Research and Education Foundation; P.J.S. is a stockholder of General Electric; E.K.F. was supported by Siemens AG and General Electric, is a member of the advisory boards for Siemens AG and General Electric, and is a cofounder of Hip Graphics; D.A.B. was supported by EPIX Pharmaceuticals and is a consultant for the Bracco Group and Schering AG (Berlex). H.P. has no financial relationships to disclose.
Address correspondence to D.A.B. (e-mail: dbluemke{at}jhmi.edu).
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Abstract
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A variety of noninvasive techniques are available to assess cardiac valve morphologic features and function, with echocardiography currently being the most widely used modality for this purpose. Technical advances in electrocardiographically gated multi–detector row computed tomography (CT) and magnetic resonance (MR) imaging allow the noninvasive visualization of the cardiac valves. At present, 64-section multi–detector row CT and MR imaging are commonly being used for comprehensive examination of the heart. Information about the cardiac valves is routinely provided by MR imaging of cardiac function or coronary CT angiography. Thus, the interpreting physician may have additional information available that can aid in making the diagnosis.
Supplemental movie clips are available at http://radiographics.rsnajnls.org/cgi/content/full/26/6/1769/DC1.
© RSNA, 2006
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Introduction
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Various noninvasive technologies are available to assess cardiac valve morphologic features and function. Echocardiography is most widely used for this purpose because it is readily available, is cost effective, and, in most circumstances, provides all the information required for planning therapeutic options (1,2). However, limitations of this modality may include a restricted field of view in patients with emphysema, interobserver variability, and low reliability in the measurement of pulmonary valve regurgitant volume. Sixty-four–section multi–detector row computed tomography (CT) and magnetic resonance (MR) imaging have been less frequently used to evaluate the cardiac valves. However, information about cardiac valves is routinely provided by MR imaging of cardiac function or coronary CT angiography (3). Thus, the interpreting physician may have additional information available regarding cardiac valve morphologic features and function that can contribute to the overall patient diagnosis. Tailored velocity-encoded MR imaging sequences are available and allow absolute quantification of velocity profiles.
In this article, we compare 64-section multi–detector row CT and MR imaging in terms of spatial and temporal resolution, use of contrast agents, assessment of valve anatomy and function, work flow, and radiation and safety. We emphasize assessment of the cardiac valves with state-of-the-art CT and MR imaging techniques, including contrast material–enhanced electrocardiographically (ECG) gated cardiac multi–detector row CT, black blood MR imaging, steady state free precession (SSFP) cine MR imaging, and phase-contrast MR imaging, along with various postprocessing techniques. We also discuss the benefits, limitations, and potential pitfalls of the aforementioned CT and MR imaging techniques.
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Cardiac MR Imaging versus 64-Section Multi–Detector Row CT
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The fundamental differences between cardiac MR imaging and 64-section multi–detector row CT are summarized in Table 1 and discussed in detail in the following sections.
Spatial and Temporal Resolution
Multi–detector row CT has a higher spatial resolution than does MR imaging. With 64-section multi–detector row CT, near-isotropic 4D data sets with 0.4–0.6 mm spatial resolution can be acquired in a few seconds. With the current SSFP MR imaging technique, 2D cine images with an in-plane spatial resolution of 1–2 mm are routinely obtained. 
Depending on the number of views per segment, a temporal resolution of 20–50 msec can be achieved with SSFP MR imaging (Fig 1; Movies 1, 2 at radiographics.rsnajnls.org/cgi/content/full/26/6/1769/DC1). The temporal resolution of 64-section multi–detector row CT is limited by the gantry rotation time (approximately 0.33–0.5 seconds); with nonsegmented reconstruction, the temporal resolution of multi–detector row CT is one-half the gantry rotation time (165–250 msec) (Fig 1; Movies 1, 2) (4). Recently, one manufacturer introduced a 64-section multi–detector row CT scanner with two x-ray tubes and two detector arrays at right angles; this scanner may achieve a temporal resolution of up to 85 msec without cardiac segmentation.
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Figure 1a. Mitral valve anatomy at 64-section multi–detector row CT versus 1.5-T MR imaging. Four-dimensional 64-section multi–detector row CT scan (a) shows better image resolution than a 1.5-T SSFP MR image (b), but the MR image shows better temporal resolution. Arrow indicates the posterior leaflet of the mitral valve, which is more clearly defined on the CT scan. The two images were obtained in two different patients.
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Figure 1b. Mitral valve anatomy at 64-section multi–detector row CT versus 1.5-T MR imaging. Four-dimensional 64-section multi–detector row CT scan (a) shows better image resolution than a 1.5-T SSFP MR image (b), but the MR image shows better temporal resolution. Arrow indicates the posterior leaflet of the mitral valve, which is more clearly defined on the CT scan. The two images were obtained in two different patients.
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Contrast Agents
For coronary CT angiography, intravenous contrast material is needed to visualize valve motion. With SSFP MR imaging, the intrinsic T2 weighting of blood provides excellent contrast between the low-signal-intensity myocardium and the valves, so that intravenous contrast material is not required.
Valve Anatomy and Function
Valve motion and function can be assessed semi-quantitatively with both multi–detector row CT and MR imaging. In addition, phase-contrast MR imaging allows accurate quantification of flow across a cardiac valve. Because of its better spatial resolution, 4D multi–detector row CT is superior in the detailed depiction of leaflet anatomy at times during the cardiac cycle when the valve is quiescent. The higher temporal resolution of cardiac SSFP cine MR imaging allows visualization of rapid valve movements, such as abnormal valve motion or ventricular dyskinesis. Calcium is much better visualized with multi–detector row CT than with MR imaging; therefore, valve calcifications are evaluated and quantified with the former modality.
Work Flow
Coronary CT angiography allows acquisition of a 4D data set of the heart in less than 15 seconds, whereas cardiac MR imaging examinations last for approximately 30–40 minutes depending on the sequences and protocol used. The 4D multi–detector row CT data set can be manipulated and displayed in any desired plane at postprocessing with use of dedicated maximum-intensity-projection (MIP) or volume-rendering (VR) software. However, considerable time (usually 10–20 minutes) is spent transferring, reconstructing, manipulating, and analyzing the 4D multi–detector row CT data set. At present, MR imaging is routinely performed with SSFP cine and black blood sequences. The 2D planes are prescribed individually during the cardiac MR imaging examination and are not postprocessed to achieve different views that would otherwise be available from a three-dimensional (3D) data set. Newer cardiac 3D SSFP MR imaging techniques are now available but have lower temporal and spatial resolution than do 2D methods. Complex studies such as congenital cardiac MR imaging often require close cooperation between the radiologist and the technologist to identify complex anatomy, prescribe imaging planes, and plan phase-contrast MR imaging.
Radiation and Safety
Multi–detector row CT makes use of ionizing radiation, whereas there is no radiation involved in MR imaging. With the current 64-section multi–detector row CT technology, the radiation dose for a cardiac CT angiographic examination is quite high (13–15 mSv for males and 18–21 mSv for females [5–7]) and significantly higher than for conventional catheter-directed coronary angiography (6). The high radiation dose presently limits the number of serial examinations that can be performed (8), but x-ray current modulation reduces the radiation dose for coronary CT angiography by approximately 40% (9); the impact on valve evaluation has not yet been determined. Patients with claustrophobia or other contraindications for MR imaging such as pacemakers, internal cardiac defibrillators, insulin pumps, or spinal cord stimulators usually cannot undergo MR imaging and are better suited for a cardiac multi–detector row CT examination.
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Cardiac Valve Assessment with MR Imaging
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For cardiac valve assessment with MR imaging, there are three principal techniques: black blood imaging, SSFP cine imaging, and phase-contrast imaging (10).
Black Blood MR Imaging
Black blood MR imaging remains the first step in assessing cardiac chamber and valve morphologic features, such as thickening of the valve leaflets (11). A fast spin-echo double inversion recovery MR imaging sequence is used to acquire black blood images. With the fast spin-echo double inversion recovery technique, a nonselective 180° inversion is first applied, followed by a section-selective reinversion pulse (Fig 2) (12,13). An inversion time is allowed to elapse prior to image acquisition. During this time interval, in-plane blood leaves and is replaced by blood from outside the imaging section. ECG-gated fast spin-echo imaging is then performed when the longitudinal magnetization of the inflowing blood is nearly zero, so that the vascular signal is nulled (7). The inversion time needed to null blood is 400–600 msec and depends on the heart rate (13). If intravenous gadolinium-based contrast material has been administered, the null point for blood is typically reduced to approximately 200 msec. Both proton-density–weighted and T2-weighted black blood images of the heart can be obtained with this method (Fig 3).
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Figure 2. Diagram illustrates the double inversion recovery black blood MR imaging technique. Blood flowing into the image plane appears black; however, in-plane–flowing blood or slow-flowing blood may not be entirely suppressed with this technique. FSE = fast spin echo, TI = inversion time.
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Figure 3. Tetralogy of Fallot in a 6-month-old patient with cyanosis. Black blood MR image of the heart demonstrates the key features of tetralogy of Fallot, a congenital cardiac malformation: a large, malalignment-type ventricular septal defect (VSD); an overriding aorta (AO); and right ventricular hypertrophy (RVH) due to pulmonic stenosis.
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Cine MR Imaging
Cardiac chamber function and valve motion are assessed with SSFP cine MR imaging. Cardiac motion is displayed in a cine loop of 20–30 frames covering the entire R-R interval (Figs 1b, 4; Movies 3–5). With SSFP MR imaging, cardiac chamber volumes at end diastole and end systole are readily measured. In addition, qualitative evaluation of valve regurgitation or stenosis can be made. Typical imaging parameters at 1.5 T include a repetition time of 3–4 msec, an echo time of 1–2 msec, a flip angle of 45°–90°, and segmented k-space acquisition. The temporal resolution depends on the number of phase-encoded lines (views per segment) obtained during each R-R interval. Increasing the number of views per segment reduces the acquisition time of the cine MR imaging sequence and, thus, the duration of the breath hold. However, temporal resolution decreases as the number of views per segment increases (Fig 5).
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Figure 4a. Cine loops of normal valves at SSFP MR imaging. SSFP MR images show the tricuspid valve (arrow in a) with the anterior papillary muscle originating from the moderator band (arrowhead in a), a normal mitral valve (arrow in b), and a normal aortic valve (arrowhead in c). The image in b is a vertical long-axis two-chamber view.
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Figure 4b. Cine loops of normal valves at SSFP MR imaging. SSFP MR images show the tricuspid valve (arrow in a) with the anterior papillary muscle originating from the moderator band (arrowhead in a), a normal mitral valve (arrow in b), and a normal aortic valve (arrowhead in c). The image in b is a vertical long-axis two-chamber view.
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Figure 4c. Cine loops of normal valves at SSFP MR imaging. SSFP MR images show the tricuspid valve (arrow in a) with the anterior papillary muscle originating from the moderator band (arrowhead in a), a normal mitral valve (arrow in b), and a normal aortic valve (arrowhead in c). The image in b is a vertical long-axis two-chamber view.
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Figure 5. Segmented k-space acquisition technique in SSFP cine MR imaging. Images A–C demonstrate segmented k-space acquisition in systolic and diastolic segments with an echo train length of 10. D shows Fourier transform SSFP cine images in systole and diastole. The shorter the echo train length within a segment, the longer the image acquisition time and the higher the temporal resolution of the SSFP cine MR images.
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For example, a repetition time of 3 msec multiplied by 16 lines per segment yields a temporal resolution of 48 msec. If 128 lines of k space are acquired per image, the cine image is acquired in eight heartbeats but temporal resolution decreases markedly. Parallel imaging can routinely be used in conjunction with SSFP cine MR imaging, so that breath-hold times are further reduced by a factor of two or more.
At SSFP MR imaging, the blood pool has high signal intensity due to the favorable T2/T1 ratio of the blood pool compared with stationary tissue in sequences with a large (>20°) flip angle. With smaller flip angles, the bright blood is predominantly due to the higher spin density of the blood pool compared with stationary tissue (14). Abnormal flow patterns, such as turbulence and acceleration of flow within the normally high-signal-intensity blood pool, cause dephasing of spins, resulting in a decreased MR imaging signal. With SSFP MR imaging sequences, the signal arises from the next-to-last = radiofrequency pulse. Thus, only the spins that have not moved over the interval of two radiofrequency pulses are refocused to generate high signal. As a result, areas of turbulence or valvular stenosis with rapidly flowing blood demonstrate low signal intensity at SSFP MR imaging (Figs 6, 7; Movies 6, 7) (14).
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Figure 6. Aortic regurgitation in a 58-year-old woman. Cine MR image shows a signal void (jet) in the left ventricular outflow tract due to turbulent flow. Arrow indicates aortic regurgitation. There was also anterior motion (flutter) of the anterior mitral valve leaflet during diastole due to the turbulent regurgitant flow in the left ventricular outflow tract.
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The extent and degree of decreased MR imaging signal in areas of turbulence can be measured and correlated with the degree of regurgitation (15–17). Unfortunately, the extent and degree of the signal void vary considerably with different repetition times, echo times, and flip angles. For example, the extent of the signal void resulting from valvular turbulence increases with increasing echo time, the latter making additional time available for protons to acquire different phase shifts (18). In addition, pixel size, section thickness, and window settings affect the extent of the signal void (18,19). Moreover, the shape of the valvular orifice and the size of the receiving chamber affect the size of the jet. Consequently, cine MR imaging is mainly used for qualitative assessment for the presence of valvular dysfunction.
Because of the excellent temporal resolution of SSFP cine MR imaging, abnormal valve motion is easily evaluated. For example, systolic anterior motion of the ventral leaflet of the mitral valve in idiopathic hypertrophic obstructive cardiomyopathy is readily depicted. During late systole, the left ventricular outflow tract is narrowed, causing blood to accelerate. The acceleration of blood in the outflow tract is associated with reduced pressure, causing the anterior mitral valve leaflet to move toward the hypertrophic septum (Fig 8, Movie 8) (20). In aortic regurgitation, anterior motion (flutter) of the anterior mitral valve leaflet can be observed during diastole due to the turbulent regurgitant flow in the left ventricular outflow tract (Fig 6, Movie 6) (21).
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Figure 8. Idiopathic hypertrophic obstructive cardiomyopathy in a 25-year-old woman with moderate subaortic stenosis. SSFP cine MR image shows a signal void (jet) in the left ventricular outflow tract due to turbulent flow (arrow). Systolic anterior motion of the anterior leaflet of the mitral valve caused by negative pressure in the left ventricular outflow tract was also present. The systolic anterior motion of the mitral valve led to mitral regurgitation, which caused moderate to severe dilatation of the left atrium (arrowhead).
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Phase-Contrast MR Imaging
Phase-contrast MR imaging is a well-known but often underused method for obtaining quantitative information on blood flow (22,23). Magnetic moments (spins) moving along a magnetic field gradient acquire a shift in their phase of rotation relative to stationary spins. For linear field gradients, the amount of this phase shift (
) is proportional to the velocity of the moving spin as expressed in the following equation (24): 
= 
x m x v, where = phase difference of two interleaved measurements, = gyromagnetic ratio, m = product of gradient area and time between bipolar gradients, and v = velocity.
Phase shifts of stationary tissue are zero due to the use of a bipolar gradient. By repeating the measurement of the MR imaging signal with an inverted bipolar gradient, phase shifts induced by other sequence parameters are eliminated (25). The phase difference between these two data sets is used for a voxel-by-voxel calculation of tissue velocities.
The information from the phase-contrast MR imaging yields magnitude and phase images. The magnitude image is used for anatomic correlation and is a bright blood gradient-echo image. On the phase image, the gray-scale value of each voxel is the velocity of that voxel. The thoracic wall is generally not moving during acquisition and can be used as a reference tissue for zero velocity (Fig 9; Movies 9, 10). Phase-contrast MR imaging can accurately help assess both flow volume and direction of flow across valve planes or any other vascular channel.
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Figure 9a. (a) Magnitude phase-contrast MR image used for anatomic correlation. (b) On a velocity MR image, the gray-scale value of each voxel represents the velocity value of that voxel. Black areas represent caudal blood flow in the descending aorta (arrowhead), white areas represent cephalad flow in the ascending aorta (arrow). The thoracic wall demonstrates an intermediate signal intensity that corresponds to no flow.
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Figure 9b. (a) Magnitude phase-contrast MR image used for anatomic correlation. (b) On a velocity MR image, the gray-scale value of each voxel represents the velocity value of that voxel. Black areas represent caudal blood flow in the descending aorta (arrowhead), white areas represent cephalad flow in the ascending aorta (arrow). The thoracic wall demonstrates an intermediate signal intensity that corresponds to no flow.
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The regurgitant fraction in a malfunctioning valve can also be calculated and plotted on flow volume graphs (Figs 10, 11).
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Figure 10a. Normal aortic valve. (a) On a velocity phase-contrast MR image, orthogonal measurements of the ascending aorta at the level of the pulmonary bifurcation (outlined in red) have been performed. (b) Flow volume graph of one cardiac cycle illustrates a normal flow curve.
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Figure 10b. Normal aortic valve. (a) On a velocity phase-contrast MR image, orthogonal measurements of the ascending aorta at the level of the pulmonary bifurcation (outlined in red) have been performed. (b) Flow volume graph of one cardiac cycle illustrates a normal flow curve.
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Figure 11a. (a, b) Velocity (a) and magnitude (b) phase-contrast MR images obtained in a patient with tetralogy of Fallot repair, pulmonary artery outflow tract reconstruction, and absence of the pulmonary valve show the main pulmonary artery (outlined in red). (c) Flow volume graph of one cardiac cycle illustrates a 26% regurgitant fraction of the pulmonary artery outflow tract.
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Figure 11b. (a, b) Velocity (a) and magnitude (b) phase-contrast MR images obtained in a patient with tetralogy of Fallot repair, pulmonary artery outflow tract reconstruction, and absence of the pulmonary valve show the main pulmonary artery (outlined in red). (c) Flow volume graph of one cardiac cycle illustrates a 26% regurgitant fraction of the pulmonary artery outflow tract.
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Figure 11c. (a, b) Velocity (a) and magnitude (b) phase-contrast MR images obtained in a patient with tetralogy of Fallot repair, pulmonary artery outflow tract reconstruction, and absence of the pulmonary valve show the main pulmonary artery (outlined in red). (c) Flow volume graph of one cardiac cycle illustrates a 26% regurgitant fraction of the pulmonary artery outflow tract.
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Phase-contrast cine MR imaging will typically generate 20–30 cine frames covering the entire cardiac cycle. For phase-contrast MR imaging, the technologist must set the flow-sensitizing gradients at a level greater than or equal to the expected peak velocity (threshold value, encoding velocity). If the blood velocities exceed the prescribed encoding velocity, aliasing artifact occurs, substantially complicating the analysis of the phase-contrast data set (23).
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Cardiac Valve Assessment with 64-Section Multi–Detector Row CT
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Because of the excellent spatial resolution of multi–detector row CT angiography, anatomic details of the valve leaflets, chordae tendinae, and papillary muscles can be visualized (Fig 1a, Movie 1). These structures may also be depicted at MR imaging; however, the inferior spatial resolution of this modality sometimes makes it difficult to visualize the fine detail of the chordae tendinae or papillary muscle trabeculations.
For multi–detector row CT of the heart, a scan delay is determined by (a) injecting a test bolus of 20 mL of contrast material followed by 40 mL of saline solution, (b) measuring the time to peak attenuation in the ascending aorta, and (c) adding 6 seconds to this delay. Eighty milliliters of nonionic contrast material is injected at a rate of 4 mL/sec, followed by a 40-mL saline flush. Ten sets of images are reconstructed through the cardiac cycle at 10% intervals from 0%–90% of the R-R interval. In our experience, it has been most feasible for cardiac valve evaluation to upload the entire 4D data set (0%–100% reconstruction at 10% intervals) and use thin-slab MIP or VR to create reformatted images in any plane desired (26). With the reverse ramp technique, the contrast-enhanced blood appears black and the soft-tissue-density valve leaflets are highlighted (Figs 12, 13; Movies 11–20) (27). Some physicians use endoluminal 3D tools to display the anatomy of the cardiac valves (3). Table 2 outlines the protocol we use for coronary CT angiography at our institution.
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Figure 12a. MIP reformatted images from 64-section multi–detector row CT data show a normal aortic valve (arrow) in closed (a–c, e) and open (d, f) position. Inverted gray-scale (reverse ramp) 4D multi–detector row CT images (b, e, f) are often useful for depicting valve motion more clearly.
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Figure 12b. MIP reformatted images from 64-section multi–detector row CT data show a normal aortic valve (arrow) in closed (a–c, e) and open (d, f) position. Inverted gray-scale (reverse ramp) 4D multi–detector row CT images (b, e, f) are often useful for depicting valve motion more clearly.
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Figure 12c. MIP reformatted images from 64-section multi–detector row CT data show a normal aortic valve (arrow) in closed (a–c, e) and open (d, f) position. Inverted gray-scale (reverse ramp) 4D multi–detector row CT images (b, e, f) are often useful for depicting valve motion more clearly.
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Figure 12d. MIP reformatted images from 64-section multi–detector row CT data show a normal aortic valve (arrow) in closed (a–c, e) and open (d, f) position. Inverted gray-scale (reverse ramp) 4D multi–detector row CT images (b, e, f) are often useful for depicting valve motion more clearly.
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Figure 12e. MIP reformatted images from 64-section multi–detector row CT data show a normal aortic valve (arrow) in closed (a–c, e) and open (d, f) position. Inverted gray-scale (reverse ramp) 4D multi–detector row CT images (b, e, f) are often useful for depicting valve motion more clearly.
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Figure 12f. MIP reformatted images from 64-section multi–detector row CT data show a normal aortic valve (arrow) in closed (a–c, e) and open (d, f) position. Inverted gray-scale (reverse ramp) 4D multi–detector row CT images (b, e, f) are often useful for depicting valve motion more clearly.
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Figure 13a. (a–e) MIP reformatted images from multi–detector row CT data demonstrate a normal mitral valve (arrow in a and b) and a pulmonary valve that is located anterior, superior, and to the left of the aortic valve (arrow in c–e). Because saline chaser is administered to our coronary CT angiography patients, there is reduced contrast in the right ventricle (arrowhead in e). Reverse ramp multi–detector row CT images are shown in b and d. (f) MIP reformatted image from multi–detector row CT data demonstrates the tricuspid valve (arrow), which is difficult to evaluate owing to the reduced contrast (cf e).
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Figure 13b. (a–e) MIP reformatted images from multi–detector row CT data demonstrate a normal mitral valve (arrow in a and b) and a pulmonary valve that is located anterior, superior, and to the left of the aortic valve (arrow in c–e). Because saline chaser is administered to our coronary CT angiography patients, there is reduced contrast in the right ventricle (arrowhead in e). Reverse ramp multi–detector row CT images are shown in b and d. (f) MIP reformatted image from multi–detector row CT data demonstrates the tricuspid valve (arrow), which is difficult to evaluate owing to the reduced contrast (cf e).
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Figure 13c. (a–e) MIP reformatted images from multi–detector row CT data demonstrate a normal mitral valve (arrow in a and b) and a pulmonary valve that is located anterior, superior, and to the left of the aortic valve (arrow in c–e). Because saline chaser is administered to our coronary CT angiography patients, there is reduced contrast in the right ventricle (arrowhead in e). Reverse ramp multi–detector row CT images are shown in b and d. (f) MIP reformatted image from multi–detector row CT data demonstrates the tricuspid valve (arrow), which is difficult to evaluate owing to the reduced contrast (cf e).
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Figure 13d. (a–e) MIP reformatted images from multi–detector row CT data demonstrate a normal mitral valve (arrow in a and b) and a pulmonary valve that is located anterior, superior, and to the left of the aortic valve (arrow in c–e). Because saline chaser is administered to our coronary CT angiography patients, there is reduced contrast in the right ventricle (arrowhead in e). Reverse ramp multi–detector row CT images are shown in b and d. (f) MIP reformatted image from multi–detector row CT data demonstrates the tricuspid valve (arrow), which is difficult to evaluate owing to the reduced contrast (cf e).
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Figure 13e. (a–e) MIP reformatted images from multi–detector row CT data demonstrate a normal mitral valve (arrow in a and b) and a pulmonary valve that is located anterior, superior, and to the left of the aortic valve (arrow in c–e). Because saline chaser is administered to our coronary CT angiography patients, there is reduced contrast in the right ventricle (arrowhead in e). Reverse ramp multi–detector row CT images are shown in b and d. (f) MIP reformatted image from multi–detector row CT data demonstrates the tricuspid valve (arrow), which is difficult to evaluate owing to the reduced contrast (cf e).
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Figure 13f. (a–e) MIP reformatted images from multi–detector row CT data demonstrate a normal mitral valve (arrow in a and b) and a pulmonary valve that is located anterior, superior, and to the left of the aortic valve (arrow in c–e). Because saline chaser is administered to our coronary CT angiography patients, there is reduced contrast in the right ventricle (arrowhead in e). Reverse ramp multi–detector row CT images are shown in b and d. (f) MIP reformatted image from multi–detector row CT data demonstrates the tricuspid valve (arrow), which is difficult to evaluate owing to the reduced contrast (cf e).
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Valve Morphologic Features
Contrast-enhanced multi–detector row CT is superior to unenhanced multi–detector row CT for the characterization of abnormal valve anatomy (28), routinely providing excellent visualization of the mitral and aortic valves (Figs 12, 13; Movies 11–20) and thereby allowing good evaluation of congenital or acquired structural anomalies of these valves (Figs 14 [Movie 21], 15). The number of valve leaflets, leaflet thickness, opening and closing of the leaflets, and presence of valve calcification can be observed. Because of the saline chaser technique, evaluation of the tricuspid valve and sometimes of the pulmonary valves may be limited because of reduced contrast between the blood pool and the valve in the right side of the heart (Fig 13, Movies 15–20). To reduce this effect, one can inject a mixture of contrast material and saline chaser or use a slow infusion of contrast material following the initial bolus. With these techniques, some contrast material will be in the right side of the heart and artifacts due to concentrated intravenous contrast material will be minimized.
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Figure 14a. Bicuspid aortic valve in a 61-year-old man. (a, b) Endoluminal VR reformatted images from 64-section multi–detector row CT data show a bicuspid aortic valve in open (a) and closed (b) position. (c, d) MIP reformatted images from 64-section multi–detector row CT data show the aortic valve (arrow) in systole (c) and diastole (d). Bicuspid valves occur in 2% of the population (29), and one-half of affected patients develop at least mild aortic stenosis by the age of 50 years. However, the patient in this case had no significant stenosis or aortic valve calcifications.
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Figure 14b. Bicuspid aortic valve in a 61-year-old man. (a, b) Endoluminal VR reformatted images from 64-section multi–detector row CT data show a bicuspid aortic valve in open (a) and closed (b) position. (c, d) MIP reformatted images from 64-section multi–detector row CT data show the aortic valve (arrow) in systole (c) and diastole (d). Bicuspid valves occur in 2% of the population (29), and one-half of affected patients develop at least mild aortic stenosis by the age of 50 years. However, the patient in this case had no significant stenosis or aortic valve calcifications.
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Figure 14c. Bicuspid aortic valve in a 61-year-old man. (a, b) Endoluminal VR reformatted images from 64-section multi–detector row CT data show a bicuspid aortic valve in open (a) and closed (b) position. (c, d) MIP reformatted images from 64-section multi–detector row CT data show the aortic valve (arrow) in systole (c) and diastole (d). Bicuspid valves occur in 2% of the population (29), and one-half of affected patients develop at least mild aortic stenosis by the age of 50 years. However, the patient in this case had no significant stenosis or aortic valve calcifications.
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Figure 14d. Bicuspid aortic valve in a 61-year-old man. (a, b) Endoluminal VR reformatted images from 64-section multi–detector row CT data show a bicuspid aortic valve in open (a) and closed (b) position. (c, d) MIP reformatted images from 64-section multi–detector row CT data show the aortic valve (arrow) in systole (c) and diastole (d). Bicuspid valves occur in 2% of the population (29), and one-half of affected patients develop at least mild aortic stenosis by the age of 50 years. However, the patient in this case had no significant stenosis or aortic valve calcifications.
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For patients with dysfunction of a single valve, contrast-enhanced cardiac multi–detector row CT (and SSFP cine MR imaging) can be used to calculate the regurgitant volume. In normal hearts, the right ventricular stroke volume (right ventricular diastolic volume – right ventricular systolic volume) equals the left ventricular stroke volume. The difference between the stroke volume of the regurgitant ventricle and that of the normal ventricle is the regurgitant volume. If, for example, mitral and aortic regurgitation coexist, it is important to recognize that the total volume of regurgitation will be calculated. This calculation will be inaccurate if regurgitation exists on both sides of the heart (30).
Valve Calcification
A major advantage of ECG-gated multi–detector row CT over MR imaging is the accurate and reproducible assessment and quantification of valve calcification (31,32).
The latter is achieved using the same principles that apply to coronary artery calcium scoring, with either electron beam CT or multi–detector row CT. It has been shown that quantification of aortic valve calcification with contrast-enhanced images is not reliable, since contrast material may simulate calcification (33). Therefore, unenhanced ECG-gated multi–detector row CT is the preferred method. Because valve calcifications are associated with dysfunction, quantitative multi–detector row CT may be used to monitor the natural history of affected patients in conjunction with the evaluation of valve morphologic features and motion assessment.
Calcific aortic stenosis may result from gradual progressive calcification of a congenitally bicuspid valve in younger patients or may represent "degenerative" calcification of a morphologically normal valve. Degenerative valve calcification is usually seen in patients over 65 years old. The CT diagnosis of aortic stenosis is based on the demonstration of left ventricular hypertrophy, mild to moderate dilatation of the ascending aorta ("post-stenotic dilatation"), calcification of the aortic valve, and limited motion and reduced area of the aortic valve in diastole at 4D multi–detector row CT (Fig 16). Calcific deposits are commonly distributed along the commissural edges of the leaflets.
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Figure 16a. Aortic valve calcifications. Sixty-four–section multi–detector row CT scans show minimal aortic valve calcifications without significant aortic stenosis (arrow in a), moderate aortic valve calcifications (arrow in b), and severe aortic valve calcifications causing significant aortic stenosis (arrow in c and d). Note also the concentric left ventricular hypertrophy (arrowhead in d).
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Figure 16b. Aortic valve calcifications. Sixty-four–section multi–detector row CT scans show minimal aortic valve calcifications without significant aortic stenosis (arrow in a), moderate aortic valve calcifications (arrow in b), and severe aortic valve calcifications causing significant aortic stenosis (arrow in c and d). Note also the concentric left ventricular hypertrophy (arrowhead in d).
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Figure 16c. Aortic valve calcifications. Sixty-four–section multi–detector row CT scans show minimal aortic valve calcifications without significant aortic stenosis (arrow in a), moderate aortic valve calcifications (arrow in b), and severe aortic valve calcifications causing significant aortic stenosis (arrow in c and d). Note also the concentric left ventricular hypertrophy (arrowhead in d).
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Figure 16d. Aortic valve calcifications. Sixty-four–section multi–detector row CT scans show minimal aortic valve calcifications without significant aortic stenosis (arrow in a), moderate aortic valve calcifications (arrow in b), and severe aortic valve calcifications causing significant aortic stenosis (arrow in c and d). Note also the concentric left ventricular hypertrophy (arrowhead in d).
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In a review of 109 patients who underwent CT and echocardiography, Lippert et al (34) found incidental aortic valve calcification in 30% of patients. Valve calcification was significantly more prevalent in older individuals: Nearly 50% of patients over 75 years old had aortic valve calcification at CT. These authors also found aortic valve calcification in 15% of patients under 55 years of age. Higher aortic valve calcium scores were associated with higher aortic valve jet velocities. A valve score of 150 was 100% sensitive for discriminating between valve jet velocities under 2.5 m/sec and those over 2.5 m/sec.
In a recent study, Liu et al (35) examined 115 patients over the age of 60 years in whom aortic valve calcifications were seen at chest multi–detector row CT performed without intravenous contrast material or electrocardiographic gating. The Agatston and volumetric scores of the aortic valve calcification correlated well with increased mean gradients across the aortic valve at echocardiography (r = 0.76 and 0.78, respectively), particularly for calcifications in the peripheral left posterior commissure and the central right-left commissure. These areas may represent the sites of greatest mechanical stress. However, 13.3% of patients with an increased gradient across the aortic valve at echocardiography had an Agatston score of less than 50 (35).
Valve Prosthesis
Aortic valve prostheses are usually well visualized with multi–detector row CT, and function can be assessed on the 4D reconstructed images (3). Mechanical valves are durable but require anticoagulation. Alternatively, homografts from human donors or a heterograft bioprosthesis from porcine valves or bovine pericardium may be used, but they have limited durability after 10 years. Depending on the type of valve replacement, the leaflets may be difficult to assess due to streak artifacts caused by surgical clips and wires at the aortic root, especially in valve bioprostheses (Fig 17). Surgical valve procedures may also sometimes be depicted at MR imaging (Fig 18; Movies 22, 23).
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Figure 17a. Aortic valve replacement. (a–c) Sixty-four–section multi–detector row CT scans show a mechanical bileaflet valve (arrow) during diastole (a) and systole (b, c). (d, e) Sixty-four–section multi–detector row CT scans obtained in a different patient with an aortic valve bioprosthesis (arrow) reveal how metallic streak artifacts can make evaluation of the leaflet challenging.
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Figure 17b. Aortic valve replacement. (a–c) Sixty-four–section multi–detector row CT scans show a mechanical bileaflet valve (arrow) during diastole (a) and systole (b, c). (d, e) Sixty-four–section multi–detector row CT scans obtained in a different patient with an aortic valve bioprosthesis (arrow) reveal how metallic streak artifacts can make evaluation of the leaflet challenging.
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Figure 17c. Aortic valve replacement. (a–c) Sixty-four–section multi–detector row CT scans show a mechanical bileaflet valve (arrow) during diastole (a) and systole (b, c). (d, e) Sixty-four–section multi–detector row CT scans obtained in a different patient with an aortic valve bioprosthesis (arrow) reveal how metallic streak artifacts can make evaluation of the leaflet challenging.
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Figure 17d. Aortic valve replacement. (a–c) Sixty-four–section multi–detector row CT scans show a mechanical bileaflet valve (arrow) during diastole (a) and systole (b, c). (d, e) Sixty-four–section multi–detector row CT scans obtained in a different patient with an aortic valve bioprosthesis (arrow) reveal how metallic streak artifacts can make evaluation of the leaflet challenging.
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Figure 17e. Aortic valve replacement. (a–c) Sixty-four–section multi–detector row CT scans show a mechanical bileaflet valve (arrow) during diastole (a) and systole (b, c). (d, e) Sixty-four–section multi–detector row CT scans obtained in a different patient with an aortic valve bioprosthesis (arrow) reveal how metallic streak artifacts can make evaluation of the leaflet challenging.
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Figure 18a. Valve prosthesis in a 55-year-old patient with a history of transmural myocardial infarction and left ventricular reconstruction surgery (Dor procedure). Horizontal (a) and vertical (b) long-axis SSFP cine MR images demonstrate a functional mitral valve (arrow) with no evidence of a jet in the left atrium, findings that suggest mitral regurgitation. A prosthetic ring (arrowhead in b) was inserted at the mitral annulus for treatment of regurgitation.
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Figure 18b. Valve prosthesis in a 55-year-old patient with a history of transmural myocardial infarction and left ventricular reconstruction surgery (Dor procedure). Horizontal (a) and vertical (b) long-axis SSFP cine MR images demonstrate a functional mitral valve (arrow) with no evidence of a jet in the left atrium, findings that | |
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Abstract
Introduction
