7.11
Imaging and Diagnosis of Cardiac Valves J. Willmann
tures and no additional multi-slice CT scanning is required for evaluation of the cardiac valves.
In this section, the capability of multi-slice CTA in imaging and diagnosis of the aortic and mitral valves is described.
7.11.2
Technical Considerations
To assess the anatomical details of the cardiac valves, the use of contrast medium is suggested. However, in the quantification of aortic or mitral valve cal- cification by multi-slice CT scanning, intravenous contrast medium is not required. The technical imaging protocols for the different generations of multi-slice CT scanners are summarized in Chap- ter 6 of this book.
For contrast-enhanced multi-slice CT scanning, optimal contrast can be obtained either by the test bolus or bolus-tracking method. In the test bolus method, the optimal delay time of a multi-slice CT scan is determined by visually evaluating the contrast material at the level of the aortic valve by obtaining ten consecutive transverse images after intravenous administration of 20 ml of contrast medium. The time of optimal contrast at the level of the aortic valve is chosen as the delay time for the subsequent multi-slice CT scan of the heart. In the bolus-tracking technique, repetitive measurements of contrast enhancement at the level of the aortic valve are obtained after intravenous administration of the total amount of contrast material. When a preset contrast enhancement level (between 100 and 150 HU) is reached, the multi-slice CT scan is initi- ated automatically.
Multi-slice CT scanning is done with a total of 80–120 ml non-ionic iodinated contrast material administered intravenously using a power injector at a flow rate of 3–5 ml/s and followed by a saline chaser of 30–50 ml at the same flow rate.
The digital ECG file of the patient obtained during multi-slice CT scanning is used to retrospectively reconstruct the multi-slice CT data set at different reconstruction intervals within the cardiac cycle.
The aortic and mitral valves are best evaluated by MPRs, which are obtained parallel and perpendicu- lar to the aortic and mitral valve annulus.
C o n t e n t s
7.11.1 Introduction 281
7.11.2 Technical Considerations 281 7.11.3 Valvular Morphology 282 7.11.3.1 Aortic Valve 282 7.11.3.2 Mitral Valve 283 7.11.4 Valvular Disease 283 7.11.4.1 Aortic Valve 283 7.11.4.2 Mitral Valve 285 7.11.5 Limitations 285 7.11.6 Conclusion 286 References 286
7.11.1 Introduction
With the introduction of multi-slice CT technol- ogy in 1998 and the subsequent continuous devel- opment of the technique, an increasing number of patients with cardiac disease are being examined using multi-slice CT. The technology combines high temporal and spatial resolution, which allows a non- invasive assessment of the moving heart with high morphological details. Combined with retrospective ECG gating, the clinical indications of multi-slice CT are considered to be the detection and quantifica- tion of coronary artery stenosis and calcification, evaluation of CABG patency and stenosis, as well as the diagnosis of pericardial abscess formation, constrictive pericarditis, and cardiac tumors and thrombus (Hahn 2004, Willmann 2004). A com- prehensive work-up of patients with cardiac symp- toms also includes assessment of the cardiac valves.
Important information about the cardiac valves can be obtained from the same single multi-slice CT data set acquired for imaging other cardiac struc-
7.11.3
Valvular Morphology
7.11.3.1 Aortic Valve
The aortic valve consists of the aortic valve annu- lus and the three aortic valve cusps (right and left cusps as well as non-coronary, posterior cusp) with their free edges (Fig. 7.80). Bicuspid aortic valve (two cusps) is the most common congenital anomaly, with an estimated incidence between 0.9 and 2%
in the general population. In bicuspid aortic valve, there is usually one larger (conjoined) cusp that con- tains a shallow ridge (raphe) representing the line of congenital fusion of the two cusps. Usually there are two complete commissures. The presence of a partially fused commissure (also called high raphe) predisposes towards eventual stenosis.
Optimal image quality of the anatomical details of the aortic valve can be obtained during diastole (closed aortic valve). This corresponds to a recon- struction interval between 50 and 70% of the cardiac cycle. To assess the aortic valve orifice, the multi- slice CT data set is reconstructed during systole
(opened aortic valve) at an optimal reconstruction interval between 0 and 20% of the cardiac cycle.
The anatomical details of the aortic valve are best assessed using contrast medium. In a prospective study, 25 patients with aortic valve stenosis were examined using 4-slice CT. The annulus and cusps of the aortic valve, including the free edges of the cusps, were better visualized on contrast-enhanced than on non-enhanced multi-slice CT (Willmann 2002a). Multi-slice CT also allows reliable differen- tiation between the bicuspid and tricuspid aortic valves. There was 100% agreement between multi- slice CT and echocardiography with respect to aortic valve morphology using contrast-enhanced multi-slice CT, whereas there was a mismatch between multi-slice CT and echocardiography in two patients examined by non-enhanced multi-slice CT (Willmann 2002a). Further studies using the newer 16- and 64-slice CT technologies are needed to assess whether multi-slice CT reconstructed during diastole also differentiates between a tricus- pid and a bicuspid aortic valve with a high raphe.
Multi-slice CT also allows accurate measurement of the diameter of the aortic valve annulus (Fig. 7.81a).
With this approach, the diameter correlates highly
Fig. 7.80. Multi-planar reconstruction of a tricuspid aortic valve reconstructed parallel to the aortic valve annulus. Aor- tic valve annulus (white arrowhead), aortic valve cusp (arrow) with free edge of the aortic valve cusp (black arrowhead). Ret- rospective ECG-gated reconstruction of multi-slice CT data set were obtained at 60% of the cardiac cycle
Fig. 7.81. Visualization of the aortic valve based on a 16-slice CT examination. MPR perpendicu- lar to the aortic valve annulus allows accurate preoperative assessment of aortic valve annulus diameter
32.4mm (2D)
with intraoperative measurement, despite a mean overestimation of 0.7 mm on multi-slice CT data sets (Willmann 2002a). However, also in this appli- cation, improved accuracy can be expected with the sub-millimeter resolution of 16- and 64-slice CT scanners (Fig. 7.81b).
7.11.3.2 Mitral Valve
The anatomical details of the mitral valve and its apparatus include the anterior (aortic) and poste- rior (mural) mitral valve leaflets, the anterolateral and posteromedial commissures, the mitral valve annulus, tendinous cords, and papillary muscles (Fig. 7.82). The anterior leaflet is in fibrous conti- nuity with the aortic valve and meets the posterior leaflet to form an arc-shaped closure line called the zone of apposition. Due to the fast and complex movement of the mitral valve within the cardiac cycle, imaging of the valve by multi-slice CT is chal- lenging. The best image quality of the anatomical details of the mitral valve can be obtained during time intervals within the cardiac cycle, during which
there is the least movement of the valve. This cor- responds to either systole (closed valve; best image quality between 0 and 20% of the cardiac cycle) or diastole (open valve; best image quality between 50 and 70% of the cardiac cycle), whereas image quality is degraded during the transitional time.
The image quality of the mitral valve and its appa- ratus was assessed on multi-slice CT scans in a pro- spective study of 20 patients with mitral valve disease who underwent 4-slice CT and echocardiography preoperatively (Willmann 2002b). Good to excellent image quality of the mitral valve annulus and its leaf- lets were obtained in 15 of 20 patients (75%). In 19 of 20 patients (95%), visibility of the papillary muscles was good or excellent. The visibility of the tendinous cords was inferior. In 14 of 20 patients (70%), they were not or only moderately visible (Willmann 2002b).
The small size of the tendinous cords, between 0.5 and 1 mm, and their high anatomical variability may have contributed to the non-diagnostic image quality in a large percentage of patients in this study. It can be expected that the improved spatial and temporal resolution of 16- and 64-slice CT scanners will allow reliable delineation of the entire mitral valve mor- phology in a higher proportion of patients (Fig. 7.83).
7.11.4
Valvular Disease
7.11.4.1 Aortic Valve
Multi-slice CT is highly accurate method for the detection and quantification of calcification. From the point of view of cardiology and cardiac surgery, accurate assessment of aortic valve calcification by multi-slice CT is of particular interest. The amount of aortic valve calcification has been identified as a strong predictor for both the progression and the outcome of patients with aortic valve stenosis (Otto 1999, Rosenhek 2000). The significant association between aortic valve calcification and hypercholes- terolemia suggest an important role of lipids in the etiology of aortic valve calcification. Lipid-lowering pharmacological therapy with HMG CoA reductase inhibitors was shown to have a positive therapeutic effect on the natural history of calcific aortic val-
Fig. 7.82. Multi-planar reconstruction of a mitral valve ob- tained perpendicular to the mitral valve annulus demonstrates the mitral valve leafl et (arrow), mitral valve annulus (small arrow), and tendinous cord (arrowhead). P Papillary muscle
P LV
LA
vular disease (Shavelle 2002). Therefore, an imag- ing modality for accurate quantification of aortic valve calcium in patients with aortic valve stenosis is desirable.
From a surgical point of view, preoperative knowledge of the extent and exact localization of the aortic valve and annulus calcification is of para- mount interest, since extensive calcification may lead to technical difficulties during surgery. Rein- sertion of the coronary arteries after aortic root replacement, as well as placement and fixation of the aortic valve prosthesis into the aortic valve annulus are complicated by calcification of the aortic valve root and aortic valve annulus (Mullany 2000). In addition, preoperative awareness of the presence of aortic valve calcification extending to the interven- tricular septum may be useful, as this finding pre- dicts the need for permanent pacing postoperatively (Boughaleb 1994). Finally, the long-term perfor- mance of a biological aortic valve prosthesis has been demonstrated to be related to calcification of the aortic valve cusps, and accurate quantification of aortic valve calcification may predict the long-term performance of the biological aortic valve prosthesis (Melina 2001).
Only a few studies have as yet addressed the value of multi-slice CT for quantification of aortic valve calcification (Willmann 2002a, Morgan-Hughes 2003, Cowell 2003). Based on a semi-quantitative grading scale for aortic valve calcification, an agree- ment of 84% between multi-slice CT findings and visual evaluation of the calcification status of the aortic valves as assessed during surgery was demon- strated (Fig. 7.83) (Willmann 2002a). The volumet- ric quantification technique for calcium assessment described by Callister et al. (Callister 1998), was shown to be highly reliable in the quantification of aortic valve calcium. Mean inter-scan variability was only 7.9% for quantification of aortic valve cal- cification in 50 patients who underwent multi-slice CT (Morgan-Hughes 2003); however, in that study, the inter-scan variability was higher (10–20%) in patients with low to moderate aortic valve calcium (Morgan-Hughes 2003).
A moderate but significant correlation between semi-quantitative assessment of aortic valve calci- fication by multi-slice CT and the mean pressure gradient of the aortic valve, as obtained using trans- thoracic Doppler echocardiography, could be dem- onstrated in a study of 25 patients who underwent both multi-slice CT and surgery (Willmann 2002a).
In another study with 50 patients, a highly signifi- cant relation between aortic valve calcification, as determined by multi-slice CT, and the instanta- neous peak velocity gradient across the aortic valve as well as the aortic valve area was demonstrated (Willmann 2002a). Cowell et al. (Cowell 2003), in another study, concluded that heavy aortic valve calcification, as seen on multi-slice CT, suggests the presence of sever aortic valve stenosis requir- ing urgent cardiologic assessment, whereas patients with a lesser degree of aortic valve calcification may be screened for aortic stenosis and monitored for disease progression.
The accuracy of aortic valve calcification mea- surement was validated in a histomorphometric study of EBCT (Pohle 2004). The mean difference between the amount of calcium of the explanted aortic valves, as determined by EBCT and histo- morphometric analysis, was as low as –0.5% ± 5.9%
(Pohle 2004). While similar results can be assumed for multi-slice CT, this needs to be verified by pro- spective studies.
Fig. 7.83. Multi-planar reconstruction of a 16-slice cardiac CT examination along the long axis of the heart through the mi- tral valve plane. The increased spatial resolution of 16-slice CT enables improved visualization of the mitral valve fl aps (arrows) compared to 4-slice CT
7.11.4.2 Mitral Valve
Multi-slice CT allows assessment of the morphologi- cal pathologies typically seen in patients with mitral valve disease, including thickening of the mitral valve leaflets, mitral valve annulus calcification (MAC), and calcifications of the leaflets. Accord- ing to echocardiographic criteria (Davidoff 2001) mitral valve leaflets are considered abnormally thickened if their free edges are > 2 mm in diameter;
thickening of the leaflets is > 5 mm is considered to be extensive thickening. The presence of MAC and calcifications of the mitral valve leaflets are diag- nosed when isolated or confluent areas of calcifica- tions are identified on the mitral valve annulus or leaflets. If the thickness of MAC is > 5 mm, this is referred to as heavy calcification (Adler 1998).
Exact evaluation and quantification of mitral valve leaflet thickness is possible with multi-slice CT, with an agreement compared to echocar- diography and surgery of 95–100%, as shown in a study with 20 patients with mitral valve disease (Willmann 2002b).
A potential indication of multi-slice CT for func- tional assessment of the mitral valve in patients who may not undergo echocardiography or MRI is the diagnosis of residual systolic anterior movement of the mitral valve and tendinous cords. In a study of ten patients with hypertrophic obstructive cardio- myopathy who underwent percutaneous translumi- nal septal myocardial ablation, there was excellent
agreement between 4-slice CT and echocardiog- raphy with regard to the diagnosis of residual sys- tolic anterior movement of the mitral valve (de Vos 2004). Assessment of the mitral valve on video loops obtained from multi-slice CT images reconstructed at different reconstruction intervals of the cardiac cycle may be useful for this indication.
Multi-slice CT may also provide a non-invasive alternative for trans-thoracic or trans-esophageal echocardiography in the early identification of peri- valvular abscesses complicating endocarditis, since timely surgery is needed before widespread destruc- tion of the valve occurs (Willmann 2002b).
7.11.5 Limitations
A major limitation of multi-slice CT scanning of the cardiac valves is the fact that functional imaging of the valves, including calculation of a pressure gradient along the aortic and mitral valve, is not possible with multi-slice CT. A second limitation is related to the radiation exposure of the patient during scanning. Since the data are acquired with an overlapping helical pitch and continuous radia- tion exposure, there is a considerable amount of applied radiation (Hunold 2003). Improvements in the amount of exposure have been achieved with the recent generation multi-slice CT scanners. While the spatial resolution of 4- and 16-slice CT scanners are usually sufficient to visualize the valvular morphol-
Fig. 7.84a–c. Semi-quantitative assessment of aortic valve calcifi cation on MPR obtained parallel to the aortic valve annulus in three different patients. a Mild aortic valve calcifi cation, b moderate aortic valve calcifi cation, c heavy aortic valve calcifi cation
b
a c
ogy, the temporal resolution is often insufficient to allow for motion-free imaging of the systolic phase, and thus does not allow assessment of valvular func- tion. The increased spatial and temporal resolution of the latest 64-slice CT scanners holds promise that also functional defects of the valves can be visual- ized (Fig. 7.85).
7.11.6 Conclusion
Multi-slice CT is a continuously evolving technology that is increasingly used for cardiovascular appli- cations, in particular to assess the coronary arter- ies and CABGs. A comprehensive work-up of these patients, however, also includes assessment of the cardiac valves. Some of the important information about the valves can also be obtained from the same multi-slice CT scan obtained during imaging of the coronary arteries or CABGs, obviating the need of an additional scan.
Since the velocity of the blood and thus the pres- sure gradient along the aortic and mitral valve cannot be determined from multi-slice CT imaging, information provided by this approach about the cardiac valves is mostly restricted to morphological diagnosis. For example, multi-slice CT is particu- larly valuable for monitoring aortic valve calcium in patients on lipid-lowering therapy. Due to the radia- tion exposure inherent with the technique, however, the indications for repetitive multi-slice CT scans
need to be restricted to patients who cannot be assessed thoroughly with echocardiography or MRI.
Accurate quantification of aortic valve area, early detection of perivalvular abscess formation, and diagnosis of residual systolic anterior movement of the mitral valve and tendinous cords in patients with hypertrophic obstructive cardiomyopathy after per- cutaneous transluminal septal myocardial ablation may become other diagnostic niches of multi-slice CT in cardiac imaging.
Fig. 7.85a–c. Cardiac 64-slice CT examination of a patient with aortic valve disease. Images were reconstructed during the diastolic phase at 60% of the RR-interval. VRT reconstruction (a) and MPR (b) reveal extensive calcifi cation. MPR through the valve plane (c) demonstrates incomplete closing of the aortic valve (arrow)
b
a c
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