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Original Research |
1 Department of Diagnostic Imaging, Sheba Medical Center, Tel Hashomer, Ramat
Gan 52621, Israel.
2 Heart Institute, Sheba Medical Center, Ramat Gan, Israel.
3 Department of Cardiac Surgery, Sheba Medical Center, Ramat Gan, Israel.
Received July 28, 2007;
accepted after revision January 15, 2008.
Address correspondence to E. Konen
(eli.konen{at}sheba.health.gov.il).
Abstract
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MATERIALS AND METHODS. Twenty sequential patients with 23 mechanical prosthetic valves were evaluated with an ECG-gated 40- or 64-MDCT scanner. Multiplanar reformation, maximal-intensity-projection, volume-rendering, and volume-averaging techniques were used for visualization of valve leaflets in systole and diastole. The visibility of each mechanical valve was evaluated by consensus of a radiologist and a cardiologist using a subjective 5-point scale (0–4). MDCT findings were correlated with fluoroscopic opening and closing angle measurements and echocardiographic pressure gradient measurements in 11 and 19 valves, respectively.
RESULTS. The series included 18 bileaflet and five single-leaflet mechanical valves. The visibility score for the bileaflet mechanical valves was excellent (score of 4) in all 18 cases, but it was lower for single-leaflet valves (mean score, 2.8; range, 1–4) (p = 0.04). Bland-Altman plots showed high agreement between MDCT and fluoroscopy for measurements of opening and closing angles of bileaflet mechanical valves. In four patients, a stuck valve was seen on MDCT and was confirmed by fluoroscopy. Doppler echocardiography showed increased transvalvular pressure in two of the four patients with a stuck mitral valve and increased transaortic pressure in four patients with normal prosthetic aortic valve motion.
CONCLUSION. Our preliminary results suggest that MDCT is a promising technique for functional evaluation of bileaflet mechanical valves, allowing reliable measurements of opening and closing leaflet angles. However, the role of MDCT in the evaluation of single-leaflet valves might be limited.
Keywords: aortic valve • cardiac imaging • MDCT • mitral valve • prosthetic valves
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ECG-gated cardiac CT angiography is a novel and promising noninvasive technique for coronary artery disease evaluation. We hypothesized that the same raw data could be further processed, as presented here, for the evaluation of mechanical prosthetic valves without a need for additional contrast medium or ionizing radiation. Moreover, prosthetic valves can be evaluated using the same CT data processing scanning only at valve level and without contrast injection when coronary visualization is not requested. Because CT is a 3D technique, it should circumvent the occasional limitations of the techniques mentioned earlier to allow optimal visualization of prosthetic valves. The aim of this retrospective study was to report our initial experience using MDCT for the evaluation of single-leaflet and bileaflet mechanical prosthetic valves.
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Complete body data were available for 15 of the 20 patients: Average height was 169 cm (range, 153–185 cm); average weight, 78.8 kg (range, 55–112 kg); average body surface area, 1.9 m2 (range, 1.6–2.2 m2); and average body mass index, 27.5 kg/m2 (range, 21.8–37.2 kg/m2).
All patients underwent retrospectively ECG-gated CT using either a 40-MDCT (seven patients with eight prosthetic valves) or a 64-MDCT (13 patients with 15 prosthetic valves) scanner (Brilliance, Philips Medical Systems). Scanning was performed with both scanner types at 120 kV using 800–1,000 mAs with a detector collimation of 0.625 mm and gantry rotation speed of 0.42 second. The minimal slice thickness was 0.67 mm, and the reconstruction interval was 0.4 mm. Using retrospective ECG gating, reconstructions were obtained in 10 cardiac phases of the R-R interval period. In the 14 patients who were referred for coronary artery evaluation, 80–100 mL of contrast medium (iomeprol [Iomeron 400, Bracco Imaging]) was injected IV at a rate of 4–5 mL/s using an automatic injector. The remaining six patients, two of whom had two mechanical valves each, were referred for a questionable stuck valve be cause of suboptimal visualization of leaflets during fluoroscopy. In these cases, the scanning area was limited merely to the valve level and the exami nation was performed without contrast material. Retrospective review and analysis of the patients' imaging data were approved by the local ethics committee.
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Visibility of the valves during the heart cycle was subjectively estimated by consensus of an experienced radiologist (5 years of experience with cardiac CT) and an experienced cardiologist (30 years of experience in cardiac catheterization) using the following 5-point scale: score of 0, mechanical leaflet was not visible; 1, leaflet was hardly seen; 2, leaflet was identified in some of the reconstructed phases but opening and closing angle measurements could not be obtained reliably; 3, images allowed opening and closing angle measurements but images were partially blurred; and 4, visualization of valvular angles was excellent in both end-diastole and end-systole, allowing reliable measurements. Because of the clinical indications for the MDCT examinations, the readers were not blinded to fluoroscopy results.
Correlation of MDCT findings with cardiac fluoroscopy was available for 11 valves. Cinefluoroscopy was performed using single-plane cardiac catheterization equipment with a C-arm unit. The C-arm was rotated to obtain a tangential view of the prosthetic valve. The examination was considered successful when a projection was obtained with the x-ray beam parallel to both the valve ring plane and the tilting axis of the disk or disks, enabling full visualization of the valve leaflet or disk in profile and allowing calculation of the valve opening angle.
For both CT and cineradiography, opening and closing valve angles were defined by frame-by-frame analysis of a single cardiac cycle. For bileaflet valves, opening and closing angles were measured between the two leaflets in the fully open and closed positions [10] (Fig. 1A, 1B). For single-leaflet valves, measurements of the lesser angle between the disk and the valve strut at full opening and full closure were recorded (Fig. 2A, 2B). A valve was defined as stuck when motion of one valve leaflet was absent. Prosthetic valve obstruction was diagnosed when motion of a leaflet or leaflets was persistently restricted, with a calculated opening angle of more (for bileaflet valve) or less (for disk valves) than the values for a normal valve, as specified by the manufacturer.
Transthoracic Doppler echocardiography was available in 19 patients. Doppler echocardiography measurements included transvalvular velocity in meters per second and calculation of the peak and mean pressure gradients using the modified Bernoulli equation (pressure gradient [mm Hg] = velocity2 x 4). Valve leaflet motion was qual itatively assessed.
Leaflet angle measurements obtained with MDCT and those obtained with fluoroscopy were compared using a two-tailed paired Student's t test and Bland-Altman analysis. Data were expressed as means and SDs. Visibility scores for single-leaflet versus bileaflet mechanical valves were compared using the two-sided Fisher's exact test.
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The mean opening and closing angles in the remaining patients with normal-functioning valves were in accordance with the manufacturer's in vitro data and the clinical literature [10, 11] (Table 1).
Echocardiography showed a stuck leaflet in three of the four patients with a stuck valve with no evidence of a thrombus or pannus in any of the cases. In the remaining cases with available correlation, valvular morphology and function were unremarkable. An increased transmitral pressure gradient (mean > 10 mm Hg) was found on Doppler echocardiography in two of the four patients with a stuck mitral valve (Table 1). In addition, an increased transaortic pressure gradient (mean > 30 mm Hg) was noted on Doppler echocardiography in four patients with normal prosthetic aortic valve motion. In two of these patients, significant regurgitant flow was identified on Doppler echocardiography: One of those patients had a single leaflet valve that could hardly be visualized (visibility score = 1), whereas in the other patient, perivalvular dehiscence was clearly identified on MDCT and was subsequently confirmed at surgery (Fig. 6A, 6B, 6C, 6D).
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ECG-gated MDCT is a novel technique increasingly used for coronary artery
disease evaluation. The same raw data can be used for functional prosthetic
valve assessment. Teshima et al.
[13] showed the usefulness of
8-MDCT in the diagnosis of a pannus in two patients with prosthetic valve
dysfunction. Using the latest 64-MDCT scanners, the total scanning time is
only approximately 10 seconds. Compared with fluoroscopy, MDCT, which
intrinsically is a 3D technique, has the potential to offer optimal
visualization of the valve profiles independent of valve position, the
patient's physical characteristics, limitation of C-arm motion, and the
operator's skills. Our preliminary results suggest that MDCT is a reliable
technique for functional evaluation of bileaflet mechanical valves and might
be helpful in some, but not all, cases with single-leaflet mechanical valves.
Furthermore, MDCT can be performed without contrast material injection and
through a limited span centered only to the level of the prosthetic valve,
thus allowing a shorter scanning time (
5 seconds) and a significant
reduction in the patient's radiation exposure.
The visibility of bileaflet valves on MDCT, presently the most commonly used mechanical valves, was excellent in all 18 cases in our series when using the MPR postprocessing technique, and MDCT measurements showed good correlation with fluoroscopic measurements of opening and closing angles. However, in two of the five single-leaflet valves, opening and closing angles could not be identified with MDCT. The difficulty in visualizing single-leaflet mechanical valves can be explained by the radiolucency of the disk, which has only a thin metallic rim on its edges. This thin metallic line can barely be depicted with postprocessing techniques even when using thick volumes that include the whole contour of the disk. In addition, earlier single-leaflet valves, such as the Björk-Shiley valve manufactured before 1975, do not contain this rim and are totally radiolucent. In comparison, in bileaflet mechanical valves, the whole leaflet is metallic to allow easy delineation of its exact position during each phase of the cardiac cycle using reformations with thin volumes.
Compared with other techniques, MDCT may have additional value because it can show perivalvular structures as well as cardiac and mediastinal abnormalities. In one patient with aortic valve replacement, suspected aortic regurgitation was suboptimally visualized on echocardiography. MDCT showed a normal-functioning bileaflet prosthetic aortic valve with an annular dehiscence causing perivalvular leak, which was confirmed at surgery. In another patient with a stuck mitral prosthetic valve, MDCT could show also the cause: A rim of soft tissue was clearly identified along the edge of the stuck leaflet suggestive of a pannus or thrombus. However, in three additional cases in our series with a stuck valve, artifacts caused by the dense metallic valvular rings did not allow adequate visualization of the soft tissues adjacent to the edges of the leaflet. We extrapolate that for similar reasons MDCT might be limited in showing questionable vegetations attached to or close to the metallic leaflets or ring.
One of the main limitations of our study is that both the radiologist and the cardiologist were not blinded to clinical history and available imaging results from previous studies when performing measurements on MDCT or fluoroscopy; this is due to the clinical setup of our study. Although the unblinded retrospective comparison might lessen the generalizability of the study, we think that the potential bias when performing measurements based on MDCT reformations and fluoroscopy is minimal. Another limitation of our study is that visualization of the leaflets during fluoroscopy was suboptimal in six patients and fluoroscopy was used as a gold standard. This problem is common in retrospective clinical research and can be resolved by a prospective study using both techniques on all patients. In addition, our series included a relatively small number of patients with mechanical valves, especially those with a single leaflet. Further larger series are required to evaluate the role of MDCT in patients with single-leaflet mechanical valves not only with regard to complications, such as a stuck valve but also with regard to other rare complications, such as broken struts, that have been encountered in some patients with the Björk-Shiley single-leaflet valves [14].
In conclusion, our preliminary experience suggests that ECG-gated MDCT is a helpful and reliable technique for functional mechanical prosthetic valve evaluation, mainly in patients with bileaflet valves. It can be performed in conjunction with preoperative coronary artery evaluation when repeated valvular replacement is planned or can be used as a stand-alone evaluation in conjunction with or as an alternative to traditional cinefluoroscopy. Further prospective studies are needed to confirm these preliminary results.
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