Original Research
Cardiopulmonary Imaging
October 2010

Diagnostic Value of Cardiac CT in the Evaluation of Bicuspid Aortic Stenosis: Comparison With Echocardiography and Operative Findings

Abstract

OBJECTIVE. This study was conducted to assess the diagnostic value of cardiac CT for the evaluation of patients with bicuspid aortic valve disease.
MATERIALS AND METHODS. Fifty consecutive patients with aortic stenosis who underwent surgical valve repair between September 2005 and November 2006 were examined by ECG-gated CT and echocardiography. A 64-MDCT scanner was used. The image findings regarding the number of leaflets (bicuspid or tricuspid) were compared against the intraoperative findings and were statistically analyzed by one-way univariate analysis of variance. The aortic valve area (AVA) was also measured by CT and echocardiography, and the measured values were statistically compared by use of the paired Student's t test.
RESULTS. Seventeen patients had a bicuspid aortic valve, and 33 had a tricuspid aortic valve. In 10 of the 50 patients, echocardiography was unable to depict the type of aortic valve because of extensive calcification. The sensitivity, specificity, positive predictive value, and negative predictive value for the detection of a bicuspid aortic valve were 76.5%, 60.6%, 68.4%, and 95.2%, respectively, for echocardiography and 94.1%, 100%, 100%, and 97.1%, respectively, for CT. The CT findings were not significantly different from the intraoperative findings (p = 0.99), but the echocardiographic findings were (p < 0.05). The AVA measurements obtained by CT and echocardiography were 0.940 ± 0.44 cm2 and 0.659 ± 0.234 cm2, respectively, showing a significant difference (p < 0.05).
CONCLUSION. ECG-gated cardiac CT is useful for the accurate morphologic assessment of bicuspid aortic stenosis, especially in patients with severe valve calcification.

Introduction

Bicuspid aortic valve is the most common congenital cardiac malformation. It is a major cause of aortic valve disease in young adults [13] and is typically associated with symptoms in middle age [4]. Aortic stenosis is the most frequent complication of a bicuspid aortic valve, requiring aortic valve replacement in many patients [3]. Not only surgical valve replacement but also valve repair and ascending aortic replacement are required, because aortic valve replacement alone fails to prevent progressive dilatation of the ascending aorta in patients with a bicuspid aortic valve [57]. To determine the appropriate strategy for surgical repair, accurate preoperative identification of the type of valve is necessary. The current study was conducted to assess the diagnostic value of cardiac CT for the evaluation of patients with bicuspid aortic valve disease.

Materials and Methods

Study Group

Fifty consecutive patients with aortic stenosis who underwent surgical valve repair between September 2005 and November 2006 were examined by CT and echocardiography. The study group included 23 men and 27 women with a mean age of 70 years (range, 43–83 years). Our institutional board on medical ethics granted approval to waive patient informed consent because of the retrospective nature of the study.

Standard of Reference

The intraoperative findings were considered to be the standard of reference for the presence of a bicuspid or tricuspid valve.

CT

After oral informed consent was obtained, CT was performed to assess the severity of atherosclerotic changes by detecting plaque and calcification of the ascending aortic wall, to measure the diameter of the ascending aorta, or to evaluate the degree of coronary artery disease to minimize the risk of embolic or ischemic events during surgical repair [811]. The retrospective ECG-gating technique was used.
Contrast-enhanced CT with semiautomatic selection of the helical pitch and rotation time (heart navigation technology) was performed during breath-holding at end-inspiration using a 64-MDCT scanner (Aquilion, Toshiba). The imaging parameters were a slice thickness of 0.5 mm, a gantry rotation speed of 0.35–0.4 s/rotation, a tube voltage of 135 kV, a tube current of 300–350 mA (with the tube current modulation method), and a reconstruction field of view of 200 mm. IV contrast material (iohexol [Omnipaque 350, Daiichi-Sankyo] or iopamidol [Iopamiron 370, Nihon Schering]) was administered at a dose of 0.525 g of iodine/kg of body weight at an injection rate of 1.575 g of iodine/s via a 22-gauge peripheral IV catheter placed in a right antecubital vein. In all patients, the injection of contrast material was followed by a 30-mL saline flush. The scan start time was controlled by the manual bolus-tracking method with a threshold of 200 HU in the left ventricle. The scan delay after the trigger was 5 seconds because of patient table movement and the mechanical delay. ECG-triggered dose modulation was not used because the software version of the CT scanner used at the time of this study did not include this function.
The optimal cardiac phase for evaluating aortic valve morphology was selected using images from 15% to 35% at 5% intervals for the systolic phase and from 65% to 75% at 5% intervals for the diastolic phase. All reconstructed images were reviewed independently by two radiologists with more than 10 years of experience who were blinded to the echocardiographic and intraoperative findings, with any discrepancies resolved by consensus.
The aortic valve area (AVA) near the end-systolic phase when the aortic valve was almost fully open was also manually traced by a radiologist with more than 10 years of experience. The AVA measurements obtained by CT were compared with the estimated AVA values calculated by Doppler echocardiography.
The type of valve was identified on the basis of the following diagnostic features: the number of leaflets and their balance, the presence of a raphe (which was evaluated in terms of its extension to either the edge of the leaflet or the aortic annulus), and the shape of the opening (triangular or oval) (Appendix 1). The morphologic findings obtained by CT were compared with the intraoperative findings.

Echocardiography

In all patients, comprehensive echocardiographic studies, including 2D M-mode scanning and Doppler measurements, were performed by experienced sonographers using various diagnostic ultrasound systems (SSD 6500, ALOKA; Sequoia c512, Siemens Healthcare; or Sonos 550, Philips Healthcare). The images were reviewed by cardiologists with more than 10 years of experience who were blinded to the CT findings.

Statistical Analysis

The AVA measurements obtained by CT and echocardiography were compared with each other and analyzed by use of the paired Student's t test. Findings were considered statistically significant if the resulting p values were less than 0.05 by the two-tailed test. The nonweighted Cohen's kappa coefficient was used to assess interobserver agreement between the two radiologists in the interpretation of the CT scans.
To assess diagnostic accuracy, the numbers of leaflets determined by CT and by echocardiography were compared with the intraoperative findings and were analyzed by one-way univariate analysis of variance. The differences in the findings were considered statistically significant if the resulting p values were less than 0.05. All statistical tests were performed using PASW statistics 18 (SPSS, Inc.) for Microsoft Windows.

Results

According to echocardiographic assessment, all of the patients included in this study had moderate to severe aortic stenosis with calcification. The transvalvar maximum pressure gradient and mean (± SD) pressure gradient measured by echocardiography were 102 ± 29 and 59 ± 18 mm Hg, respectively. The traced AVA in CT was 0.940 ± 0.44 cm2, and the estimated AVA by Doppler echocardiography was 0.659 ± 0.234 cm2 (Fig. 1). A fair correlation (r = 0.44) and a significant difference (p < 0.05) were observed between CT and Doppler echocardiography. According to the intraoperative findings, 17 patients had a bicuspid aortic valve and 33 patients had a tricuspid aortic valve.
The kappa score for interobserver agreement between the two radiologists in the interpretation of the CT scans was 0.953. The sensitivity, specificity, positive predictive value, and negative predictive value of cardiac CT were 94.1%, 100%, 100%, and 97.1%, respectively (Table 1). The diagnostic accuracy of CT was 98% (49/50).
TABLE 1: Diagnostic Accuracy of CT in Identifying Bicuspid Aortic Valve
Operation
CTBicuspidTricuspidTotal
Bicuspid16016
Tricuspid
1
33
34
Total
17
33
50
Fig. 1 Differences in aortic valve area measurements (in cm2) obtained by CT and Doppler echocardiography.
In 10 (20%) of the 50 patients (seven with a tricuspid valve and three with a bicuspid valve), the type of valve could not be clearly identified by echocardiography because of severe acoustic shadows caused by extensive calcification (Fig. 2). The sensitivity, specificity, positive predictive value, and negative predictive value of echocardiography were 76.5%, 60.6%, 68.4%, and 95.2%, respectively (Table 2). The diagnostic accuracy of echocardiography was 66% (33/50). The CT findings were not significantly different from the intraoperative findings (p = 0.99), but the echocardiographic findings were (p < 0.05).
TABLE 2: Diagnostic Accuracy of Echocardiography in Identifying Bicuspid Aortic Valve
Transthoracic EchocardiographyOperation
BicuspidTricuspidTotal
Bicuspid13619
Tricuspid12021
Nonassessable
3
7
10
Total
17
33
50
In 49 (98%) of the 50 patients, cardiac CT correctly identified the type of aortic valve (Fig. 2). In one patient, both cardiac CT and echocardiography were unable to distinguish between a raphe and an adhesive commissure, because the raphe extended the entire distance from the annulus to the edge of the leaflet (Fig. 3). At surgery, the valve was found to be bicuspid (Fig. 3C).

Discussion

Echocardiography is the standard diagnostic procedure for the evaluation of patients with valvular disease. The presence of severe calcification (which is frequently associated with aortic valve disease) may lead to misdiagnosis because of the resulting acoustic shadows. Although transesophageal echocardiography is a useful diagnostic alternative, such studies are contraindicated in patients with severe aortic stenosis, because the stress of transesophageal echocardiography often exacerbates hemodynamic instability. Intraoperative transesophageal echocardiography is also useful, regardless of the severity of aortic stenosis. By obtaining Doppler measurements, it is possible to diagnose valvular disease and to evaluate its severity even in patients with extensive calcification [12]. However, in patients with aortic stenosis, dilatation and dissection of the ascending aorta are often observed, and it is therefore also important to determine the presence and severity of an aortic pathologic abnormality. In addition, there are significant differences in the complications, such as progressive aortic dilation, aneurysm formation, and dissection in the distal ascending aorta, between patients with a bicuspid aortic valve and those with a tricuspid aortic valve [3]. Also, even in patients with a bicuspid aortic valve without dilatation of the ascending aorta, valve replacement is not able to prevent long-term dilatation of the ascending aorta [5]. At our institution, we therefore perform replacement of the dilated ascending aorta at the same time as replacement of the bicuspid valve in such patients. The preoperative morphologic evaluation of the aortic valve, together with assessment of the ascending aorta, has become even more important because tricuspidalization of a bicuspid aortic valve and annuloplasty have become common procedures.
Fig. 2 73-year-old man with aortic stenosis due to bicuspid aortic valve. A, Multiplanar reformatted image obtained by MDCT clearly depicts calcified bicuspid aortic valve with severe stenosis. B, Virtual endoscopic image (cranial view) obtained by MDCT shows bicuspid aortic valve. Leaflets are almost same size and show dense calcifications. C, Transthoracic echocardiography is not able to depict type of valve because of presence of acoustic shadow from densely calcified leaflet. D, Densely calcified bicuspid aortic valve was confirmed at time of surgery.
MDCT with ECG-gated reconstruction has made it possible to evaluate heart disease, and CT is also applied to the diagnosis of aortic valve disease [1322]. A number of studies have focused on the presence of calcification of the valve or annulus in patients with valvular disease, because this is an important predictor of severe valvular disease or ischemic heart disease. However, some of these reports were based on evaluation by conventional CT without ECG-gated reconstruction, and there have been relatively few reports concerning the morphologic assessment of the aortic valve using CT with ECG gating. Several studies on the morphologic assessment of the normal aortic valve have reported good reproducibility in the measurement of valve areas by MDCT with ECG-gated reconstruction [15, 22]. In the current study, we assessed the usefulness of MDCT with ECG-gated reconstruction in aortic stenosis and observed a very good correlation with intraoperative findings. In particular, the diagnostic capabilities of CT were found to be superior to those of transthoracic echocardiography in patients with extensively calcified aortic valves.
The AVA measurements obtained by CT were significantly different from those obtained by Doppler echocardiography. The severity of aortic stenosis was assessed by transthoracic echocardiography with Doppler measurements, because a number of studies have reported that AVA measurements are not useful for evaluating the degree of severity [23]. As in other reports [2427], the AVA measurements obtained by CT were larger than those obtained by Doppler echocardiography. In our study, a good correlation was not observed, with significantly different AVA measurements obtained by CT and Doppler echocardiography. Because of limitations in the selection of the cardiac phase, we used 5% of the R-R interval for assessment, and insufficient selection of the cardiac phase might have been one factor responsible for the difference in AVA measurements. On the other hand, aortic valve motion was markedly limited in our study group, and selection of the cardiac phase may therefore not have been a major factor for the difference.
Fig. 3 74-year-old man with bicuspid aortic valve in whom valve type was incorrectly identified. A, Multiplanar reformatted image obtained by MDCT depicts severely calcified aortic valve. B, Virtual endoscopic image obtained by MDCT shows structure of leaflets in perspective view. Valve shows severe stenosis and calcification, but three leaflets are seen to be of same size. C, Intraoperative findings showed that valve was bicuspid. Raphe with severe calcifications mimicked commissure.
The estimated AVA measurements obtained by Doppler echocardiography in other studies were larger than those in the current study. This finding suggests that there were differences in patient selection between the current study and other studies. We included patients with aortic stenosis requiring surgical repair, and the number of patients with severe aortic stenosis was therefore greater in the current study than in other studies. In patients with aortic stenosis requiring surgical repair, the AVA measurements obtained by CT are thought to be less reliable than those obtained by Doppler echocardiography.
There is also the issue of radiation exposure in the evaluation of aortic valve disease, because the best cardiac phase for the evaluation of valve disease is different from that for the evaluation of the coronary arteries. The lack of dose modulation control results in greater radiation exposure compared with well-controlled coronary artery CT angiography. Despite the disadvantages with regard to radiation exposure and AVA measurement in patients with severe aortic stenosis, assessment of the aortic valve by CT still has advantages in the evaluation of aortic valve morphology with simultaneous assessment of diseases of the aorta and coronary arteries.
In conclusion, we found that aortic valve assessment by cardiac CT with ECG gating is useful for accurately identifying bicuspid aortic stenosis in patients with severe aortic valve stenosis requiring surgical repair, especially those with extensively calcified aortic valves.
APPENDIX 1: CT Features Used to Distinguish Between the Two Types of Valves

Tricuspid Valve
    Three leaflets are clearly visualized in systolic phase image.
    Leaflets are balanced, about 120° for each leaflet.
    Indentation between two leaflets is seen when the leaflets are in contact with each other.
    There is continuity of the contacting edges of the leaflets, from the edge to the annulus.
    Location of calcification (calcification on the ventricular side) is suggestive of rheumatic aortic valve disease.
Bicuspid Valve
    Two leaflets without a raphe are clearly visualized.
    There are two unbalanced leaflets with an anomalously dilated sinus of Valsalva.
    No indentation is seen at the edge of the anomalous leaflet.
    Raphe is clearly identified.
    There are two balanced leaflets, about 180° for each leaflet.

Footnotes

This work is supported by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science, and Technology (grant 19591433).
Address correspondence to R. Tanaka ([email protected]).

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 895 - 899
PubMed: 20858815

History

Submitted: June 10, 2009
Accepted: January 24, 2010

Keywords

  1. aortic stenosis
  2. bicuspid aortic valve
  3. CT
  4. echocardiography

Authors

Affiliations

Ryoichi Tanaka
Department of Cardiovascular Radiology, Memorial Heart Center, Iwate Medical University, 1-2-1 Chuodori, Morioka, Iwate 020-8505, Japan.
Kunihiro Yoshioka
Department of Cardiovascular Radiology, Memorial Heart Center, Iwate Medical University, 1-2-1 Chuodori, Morioka, Iwate 020-8505, Japan.
Hiroyuki Niinuma
Department of Cardiology, Memorial Heart Center, Iwate Medical University, Iwate, Japan.
Satoshi Ohsawa
Department of Cardiovascular Surgery, Memorial Heart Center, Iwate Medical University, Iwate, Japan.
Hitoshi Okabayashi
Department of Cardiovascular Surgery, Memorial Heart Center, Iwate Medical University, Iwate, Japan.
Shigeru Ehara
Department of Radiology, Iwate Medical University, Iwate, Japan.

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