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Published online before print July 13, 2007, 10.1148/radiol.2443061127
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(Radiology 2007;244:745-754.)
© RSNA, 2007

Cardiac Imaging

Aortic Valve Area Assessment: Multidetector CT Compared with Cine MR Imaging and Transthoracic and Transesophageal Echocardiography1

Anne-Catherine Pouleur, MD, Jean-Benoît le Polain de Waroux, MD, Agnès Pasquet, MD, Jean-Louis J. Vanoverschelde, MD, FESC, and Bernhard L. Gerber, MD, FESC

1 From the Division of Cardiology, Department of Cardiovascular Diseases, Cliniques Universitaires St Luc, Université Catholique de Louvain, Av Hippocrate 10/2806, B-1200 Woluwe St Lambert, Belgium. Received June 29, 2006; revision requested August 31; revision received September 19; accepted October 26; final version accepted December 18. Supported by a grant from the Fondation Nationale de la Recherche Scientifique of the Belgian government (FRSM 3.4557.02). A.C.P. supported by a personal grant from the Fondation Nationale de la Recherche Scientifique of the Belgian government. Address correspondence to B.L.G. (e-mail: Bernhard.gerber{at}clin.ucl.ac.be).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Purpose: To prospectively compare the accuracy of multidetector computed tomographic (CT) measurements of the aortic valve area (AVA) with transesophageal echocardiography (TEE) and cine magnetic resonance (MR) measurements of this area for preoperative examination of patients undergoing cardiac surgery, with transthoracic echocardiography (TTE) as the reference standard.

Materials and Methods: After giving informed consent for the institutional review board–approved study protocol, 48 patients (33 men, 15 women; mean age, 62 years ± 13 [standard deviation]) with (n = 27) or without (n = 21) aortic stenosis underwent multidetector CT, cine MR, TTE, and TEE before undergoing cardiac surgery. AVAs derived with manual planimetry by using cine short-axis multidetector CT, MR, and TEE images obtained through the aortic valve were compared among each other and with AVAs measured by using continuity equation TTE at regression and Bland-Altman analyses. The diagnostic accuracy of multidetector CT for detection of aortic stenosis was compared with that of TTE by using {kappa} statistics and receiver operating characteristic curves.

Results: Multidetector CT–derived AVA correlated highly with MR-derived (r = 0.98, P < .001), TEE-derived (r = 0.98, P < .001), and TTE-derived (r = 0.96, P < .001) AVA. Multidetector CT planimetry AVAs (mean AVA ± standard deviation, 2.5 cm2 ± 1.7) were not significantly different from MR planimetry (2.4 cm2 ± 1.8, P > .99) or TEE planimetery (2.5 cm2 ± 1.7, P = .21) AVAs, but they were significantly larger than TTE-derived AVAs (2.0 cm2 ± 1.5, P < .001). With TTE as the reference standard, multidetector CT correctly ({kappa} = 0.88, P < .001) depicted all 21 normal, six of eight mildly stenotic (AVA ≥ 1.2 cm2 and < 2.0 cm2), seven of eight moderately stenotic (AVA ≥ 0.8 cm2 and < 1.2 cm2), and 10 of 11 severely stenotic (AVA < 0.8 cm2) valves. It also correctly depicted all 14 bicuspid valves identified with TEE, eight of which were missed with TTE.

Conclusion: Multidetector CT enables accurate noninvasive assessment of the AVA.

© RSNA, 2007


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Assessment of the severity of aortic stenosis traditionally has been based on calculation of the aortic valve area (AVA). Aortic valve replacement is recommended when patients present with symptoms of chest pain, heart failure, or syncope and have severe aortic stenosis, which is typically defined as AVA reduced to less than 0.8 cm2 (1). In daily clinical practice, the AVA is most often calculated at Doppler transthoracic echocardiography (TTE) by using the continuity equation (2,3). The underlying principle of the continuity equation is that the flow of blood in one area must equal the flow in a second area. In practical terms, the product of the AVA and the velocity time integral (VTI) through the aortic valve equals the product of the left ventricular outflow tract (LVOT) VTI and the area. Thus, the AVA can be computed as the LVOT area multiplied by the LVOT-to–aortic VTI ratio. The continuity equation TTE is currently considered the reference standard for evaluation of the AVA. In patients with poor acoustic windows, the AVA can also be directly measured with planimetry by using either transesophageal echocardiography (TEE) (4–7) or cine magnetic resonance (MR) imaging (8–13). Although the hemodynamic significance of aortic stenosis can also be determined invasively with cardiac catheterization, retrograde catheterization of the aortic valve is associated with a substantial risk of clinically important and silent emboli and is therefore no longer recommended for systematic evaluation of aortic stenosis severity (14).

Retrospectively gated multidetector computed tomography (CT) has recently emerged as a promising method for noninvasive coronary imaging (15–17). Multidetector CT has also facilitated a widened spectrum of indications and applications other than coronary imaging. Reconstruction of cardiac images at different time points during the cardiac cycle enables visualization of the left ventricle in motion and measurement of left ventricle volume, left ventricle ejection fraction, and regional wall thickness (18–22), and in theory, it should also permit imaging of valve motion and measurement of the valve area at the time of coronary imaging.

Thus, the aim of the present study was to prospectively compare the accuracy of AVA measurements obtained with 40-section multidetector CT, TEE, and cine MR imaging for the preoperative examination of patients undergoing cardiac surgery, with TTE as the reference standard.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Patients
The study protocol was approved by the institutional review board of the Université Catholique de Louvain. Before being included in the study, patients gave consent after being informed about the study protocol, including the radiation exposure. Patients with aortic valve disease who were scheduled to undergo cardiac surgery at our institution were eligible for inclusion in the study. Exclusion criteria were as follows: hemodynamic instability, constant arrhythmia (atrial fibrillation or more than five premature heart beats per minute), New York Heart Association class IV heart disease, renal insufficiency (serum creatinine level > 1.4 mg/dL), known allergy to iodinated contrast agents, and/or any contraindication to MR imaging (ie, ferrometallic cerebral aneurysm clips, pacemaker or implantable defibrillator, or severe claustrophobia).

Accordingly, we prospectively considered 68 consecutive patients with aortic valve disease who were scheduled to undergo cardiac surgery at our institution between February and November 2005 for inclusion in this study (Fig 1). Sixteen patients were excluded because of atrial fibrillation (n = 6), a serum creatinine level higher than 1.4 mg/dL (n = 4), pacemaker implantation (n = 3), or claustrophobia (n = 3). Four patients refused to participate in the study. Therefore, the final study group consisted of 48 patients (33 men, 15 women; mean age, 62 years ± 13 [standard deviation]; age range, 35–83 years) (Table 1). Twenty-seven patients had aortic stenosis (AVA < 2 cm2). The remaining 21 patients with a normal aortic valve opening (having aortic or mitral regurgitation) served as control subjects.


Figure 1
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  		Figure 1: Outline of study protocol.

 

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  		Table 1. Clinical and Echocardiographic Characteristics of the Study Population

 
Study Protocol
Patients underwent multidetector CT, MR imaging, and TTE in random order on a single day. TEE was performed on the day of surgery intraoperatively. Because we did not want to exclude patients who had aortic regurgitation with contraindications to ß-blocker treatment, no systematic premedications were given before multidetector CT was performed. In addition, to avoid changes in hemodynamics between tests, medications were not changed between TTE, TEE, multidetector CT, and MR imaging, and patients were included regardless of their heart rate. All patients successfully completed multidetector CT, MR imaging, and TTE, all of which were performed on the same day. The mean delay between these three examinations and intraoperative TEE was 15 days ± 21 (standard deviation).

Multidetector CT
Retrospective electrocardiographically gated cardiac multidetector CT was performed by using a 40-section system (Brilliance; Philips Medical Systems, Cleveland, Ohio) after the intravenous bolus injection of 120 mL of iomeprol (Iomeron 400; Bracco Diagnostics, Milan, Italy) at a rate of 4 mL/sec followed by a 60-mL saline bolus chaser. Imaging was initiated according to the automatic detection of a greater than 100-HU increase in attenuation due to the arrival of the contrast material bolus in the descending aorta. The tube rotation speed was 420 msec; the detector collimation, 40 x 0.625 mm; the table pitch, 0.20–0.24, depending on the heart rate; the tube voltage, 120 kV; and the effective tube current, 600 mAs. The mean radiation exposure was 11.8 mSv ± 2.2 (range, 7.4–15.8 mSv), as derived from volumetric CT dose index (40 mGy) and mean dose-length product (692 mGy·cm ± 127) estimates, which were validated by means of comparison with phantom measurements (23). Electrocardiographic dose modulation was not used.

For every patient, 10 transverse data sets were reconstructed every 10% of the cardiac cycle by using a retrospective multisegment electrocardiographic gating algorithm involving one to four cardiac cycles for reconstruction, with subsegment reconstruction angles varying between 45° and 180° and temporal resolution varying between 52 and 210 msec, depending on the patient's heart rate (24,25). Reconstruction parameters were as follows: an image matrix of 512 x 512 pixels, a field of view of 280 mm, and a section thickness of 1 mm in 0.5-mm increments. With use of a three-chamber view and an oblique coronal view of the LVOT for reorientation, the cine CT data sets were resectioned into six consecutive cross sections of 5 mm thickness, with no intersection gap, through the aortic valve, starting at the level of the LVOT and ending at the level above the tip of the valve (Fig 2).


Figure 2
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  		Figure 2: Image reconstruction to obtain cross-sectional multidetector CT images of the aortic valve at the level of the valve tips during early systole. Three-chamber long-axis view (left image), short-axis view at the level of the LVOT (middle image), and serial short-axis planes through the aortic valve (right image). Two reviewers measured the AVA by precisely delineating the edges of the maximal opening of the aortic valve during systole on each image plane. Only the smallest of these measurements were retained as AVA values. Ao = aorta, LA = left atrium, LV = left ventricle, RV = right ventricle.

 
MR Imaging
MR imaging was performed by using a 1.5-T magnet (Intera CV; Philips Medical Systems, Best, the Netherlands) equipped with a five-element cardiac synergy coil. With use of a three-cavity view and an oblique coronal view of the LVOT as localizer images, four contiguous cross-sectional cine MR images of the aortic valve—starting at the outflow tract and ending at the valve tips—were acquired. Images were acquired by using a multisection cine steady-state free precession echo sequence with retrospective vectorcardiographic gating during multiple breath holds. Twenty-five cine phase images were acquired with a temporal resolution varying between 25 and 50 msec. The section thickness was 5 mm with no intersection spacing. Imaging parameters were as follows: 3.4/1.7 (repetition time msec/echo time msec), 60° flip angle, 32-cm field of view, image matrix of 160 x 256 pixels, sensitivity encoding acceleration factor of 1.6, and 10 lines acquired per cardiac cycle.

TTE Reference Standard
Two-dimensional Doppler TTE was performed by using a Sonos 7500 system (Philips Medical Systems, Andover, Mass) equipped with a broadband transducer. All images were acquired together by two physicians (A.C.P., A.P.) with 3 and 15 years of experience in echocardiography, respectively. The LVOT diameter was measured on the parasternal long-axis view in midsystole, parallel to the valve plane and immediately adjacent to the aortic leaflet insertion into the annulus. The LVOT velocity was recorded from the apical window by positioning the pulsed wave Doppler sample volume in the outflow tract, proximal to the aortic valve. Proper positioning of the sample volume was ensured by verifying the presence of smooth spectral velocity curves, which were associated with an aortic valve closing click. Care was taken to optimize the alignment of the ultrasound beam with the blood flow and to avoid sampling in the transvalvular jet or the proximal flow convergence region by excluding velocity curves with spectral broadening at peak ejection. The maximal velocity across the aortic valve was measured with continuous wave Doppler ultrasonography (US) from the apical window.

TEE Examination
Multiplanar TEE was performed in the operating room by using a 7.5-MHz multiplanar probe connected to the Sonos 7500 system. All images were acquired together by the same two physicians (A.C.P., A.P.), who had 3 and 15 years of experience in TEE, respectively. To define the optimal level for placement of the transducer for subsequent planimetry of the aortic valve on the short-axis view, the leaflet tips were first positioned in the center of the two-dimensional sector on the long-axis view of the ascending aorta and the aortic valve (usually between 110° and 160°). Once the proper level was determined, the transducer was held stable and the US array was steered to obtain a short-axis view of the valve. This was usually possible at 30°–60°. Minimal probe manipulation was then applied to ensure visualization of the smallest aortic valve orifice.

Image Interpretation
Images were downloaded to dedicated workstations (Brilliance Extended Workspace, version 1.05; View Forum R4.1; and XCelera 1.2 [Philips Medical Systems for all] for analysis of multidetector CT, MR, and echocardiographic images, respectively) for subsequent analysis. All measurements were independently performed in duplicate by two observers (A.C.P., B.G.) who were blinded to patient-identifying information. These readers had 3 and 5 years of experience, respectively, with all four noninvasive imaging methods. To avoid recalling the patient's images, the two blinded readers analyzed the multidetector CT, MR, TTE, and TEE images on separate days. To assess intraobserver variability, one of the readers repeated the measurements obtained with all four techniques 1 month after the first readings.

The quality of the multidetector CT, MR, and TEE images was graded independently by the two readers (A.C.P., B.G.) by using a four-point ordinal Likert scale (26). Grade 1 meant poor (nondiagnostic, delineation of anatomic details of the valve not possible); grade 2, moderate (diagnostic, poor visibility of the anatomic details of the aortic valve cusps and edges); grade 3, good (good visibility of the anatomic details of the aortic valve); and grade 4, excellent (excellent visibility and differentiation of the anatomic details of the aortic valve) quality. The average grade for both readers was reported. Severity of aortic valve calcification at multidetector CT was computed by using a volumetric score, which represented a measure of the volume of all aortic valve pixels with an absorption rate higher than 450 HU.

The duration of the aortic valve opening during systole was computed by the same two readers on the basis of the duration of the aortic VTI at TTE. With use of multidetector CT, MR imaging, and TEE, the two reviewers measured the AVA by means of planimetry. This was done by precisely delineating the edges of the maximal systolic opening of the aortic valve on each imaging plane. Only the smallest of these measurements were retained. With TTE, the AVA was calculated by using the continuity equation (2,3). This was done by measuring the VTI of the continuous Doppler signal through the aortic valve (VTIAV), the subvalvular VTI of the pulsed wave Doppler signal in the LVOT (VTILVOT), and the diameter of the LVOT (DLVOT) on parasternal two-dimensional images. The continuity equation–derived AVA (AVACE) was computed by using the following formula:

Formula

For each modality, the valve opening was semiquantitatively judged (27) to be normal (AVA ≥ 2.0 cm2), mildly stenotic (AVA ≥ 1.2 cm2 and < 2.0 cm2), moderately stenotic (AVA ≥ 0.8 cm2 and < 1.2 cm2), or severely stenotic (AVA < 0.8 cm2). In addition, for each modality, the valve morphology was recorded by both reviewers in consensus as bicuspid or tricuspid.

Statistical Analyses
Statistical analyses were performed by using SPSS, version 11.5, software (SPSS, Chicago, Ill). Values are reported as means ± standard deviations. Interobserver reliability for measurement of the AVA with each technique was assessed by using two-way random single-measure intraclass correlation coefficient analysis. Intraobserver and intrasubject reliability was assessed by using one-way random two-measures intraclass correlation coefficient analysis. The average of the measurements obtained by both readers was used for further analysis. Linear regression analysis and Bland-Altman plots (28), with assessment of systematic bias and 95% confidence intervals, were used to compare the AVAs measured by using multidetector CT, MR imaging, TEE, and TTE. Systematic differences in measurements between the four modalities were assessed by using one-way repeated-measures analysis of variance. Individual differences between the examinations were compared post hoc by using the Bonferroni test. Agreement on semiquantitative grades of aortic stenosis severity among the four methods was expressed by using {kappa} statistics. The sensitivity and specificity of different imaging modalities for the identification of moderate or severe (AVA < 1.2 cm2) and severe (AVA < 0.8 cm2) aortic stenoses were computed by using TTE as the reference standard. The relative diagnostic accuracy of the different modalities was evaluated by comparing the areas under the receiver operating characteristic curves. All tests were two sided, and P < .05 was considered to indicate statistical significance.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Patient Characteristics
Intraoperative TEE could not be performed in two patients, in whom the probe could not be introduced into the esophagus. Another patient died during a traffic accident before undergoing surgery and TEE. Twenty-four (50%) patients received chronic ß-blocker treatment. Ten patients had contraindications to heart rate–lowering medications because of the presence of severe aortic regurgitation. Since there were no changes in medication between examinations, the mean heart rate was not significantly different (P = .95, analysis of variance) during TTE (69 beats per minute ± 14), TEE (69 beats per minute ± 14), multidetector CT (69 beats per minute ± 13), and MR imaging (69 beats per minute ± 14). The mean duration of AVA opening computed by using the continuity equation TTE VTI was 297 msec ± 40.

Feasibility of Aortic Valve Imaging and Planimetry with Multidetector CT
Multidetector CT enabled visualization and planimetry of the aortic valve in all 48 patients (Fig 3). The mean image quality grade was 3.7 ± 0.6 (P = .72 at comparison with TEE and MR imaging, analysis of variance), and no patient had either an image quality grade lower than 2 or noninterpretable images. Calcifications were detected in 26 valves with both CT and TEE. The mean calcification mass size was 599 mg ± 929. Calcification mass correlated moderately in an inverse exponential relationship with AVA (r = 0.72, P < .001, curve not shown). The presence of calcification resulted in minor artifacts but never hindered multidetector CT planimetry of AVA. Therefore, planimetry of AVA could be performed in all 48 multidetector CT examinations.


Figure 3
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  		Figure 3: Short-axis multidetector CT (left), MR (middle), and TEE (right) images of the aortic valve at maximal opening during systole in four patients.

 
Aortic Valve Morphology at TTE, TEE, and MR Imaging
TTE-derived AVAs ranged from 0.5 to 6.1 cm2 (mean, 2.0 cm2 ± 1.5) (Table 2). Twenty-one patients had normal aortic valves, eight had mild aortic stenosis, eight had moderate aortic stenosis, and 11 had severe aortic stenosis (Table 3). At TEE, 31 tricuspid valves and 14 bicuspid valves were identified. The presence of all bicuspid valves was confirmed by means of direct inspection during surgery. Multidetector CT and MR imaging correctly depicted the valve morphology in all cases. In contrast, eight bicuspid valves were missed with TTE. The mean image quality grades with TEE and MR imaging were 3.6 ± 0.6 and 3.7 ± 0.6, respectively. Therefore, planimetry of the AVA was successfully performed in all 45 patients who underwent TEE and in all 48 patients who underwent MR imaging.


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  		Table 2. Comparisons of AVA Values Measured with Multidetector CT, MR Imaging, TEE, and TTE

 

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  		Table 3. Comparison of Semiquantitative Multidetector CT and TTE Assessments of Aortic Stenosis Severity

 
Comparison of AVAs Derived with Multidetector CT, MR Imaging, TTE, and TEE
Multidetector CT–derived AVA correlated highly with MR-derived (r = 0.98), TEE-derived (r = 0.98), and TTE-derived (r = 0.96) AVA values (P < .001 for all comparisons) (Table 2, Fig 4). Planimetric measurements of AVA with multidetector CT, MR imaging, and TEE did not significantly differ from each other (Table 2). Yet all three planimetric methods yielded significantly higher AVA values than did continuity equation TTE (P < .001, analysis of variance and post hoc Bonferroni test).


Figure 4
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  		Figure 4: Bland-Altman plots of linear regression and limits of agreement between AVA estimates derived with multidetector CT (MDCT) and those derived with MR imaging, TEE, and TTE. AVA derived with multidetector CT planimetry correlated well with AVA derived with MR planimetry, TEE planimetry, and continuity equation TTE. Plots show no significant difference (bias) in AVA derived by using multidetector CT planimetry versus that derived by using MR planimetry and TEE planimetry but significantly overestimated planimetric values compared with continuity equation TTE values. Diff = difference.

 
Diagnostic Accuracy of Multidetector CT in Detection of Moderate and Severe Aortic Stenoses
Agreement between multidetector CT and TTE in the detection of aortic stenosis was excellent ({kappa} = 0.88, P < .001) (Table 3) and similar to the agreement between TTE and MR imaging ({kappa} = 0.79, P < .001) and between TTE and TEE ({kappa} = 0.80, P < .001). The diagnostic accuracy of multidetector CT in the identification of moderate or severe (AVA < 1.2 cm2) and severe (AVA < 0.8 cm2) aortic stenoses detected at TTE was compared with the diagnostic accuracies of MR imaging and TEE by using receiver operating characteristic analysis. Areas under the receiver operating characteristic curves were similarly high for the multidetector CT, MR, and TEE identification of moderate (areas under curve: 1.00, 0.99, and 1.00, respectively; P = .89) and severe (areas under curve: 0.98, 0.99, and 0.99, respectively; P = .68) aortic stenoses. Accordingly, the sensitivity, specificity, and overall diagnostic accuracy for the detection of moderate and severe aortic stenoses were similarly high among the three techniques (Fig 5). As the data in Table 4 show, multidetector CT measurement of AVA—similar to MR, TEE, and TTE measurements—had high reproducibility.


Figure 5A
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  		Figure 5a: Sensitivity, specificity, and diagnostic accuracy of multidetector CT (white bars), MR imaging (hatched bars), and TEE (black bars)—as compared with continuity equation TTE—in the detection of (a) moderate or severe (AVA < 1.2 cm2) and (b) severe (AVA < 0.8 cm2) aortic stenoses. Multidetector CT, MR imaging, and TEE had equally high sensitivity for the detection of moderate and severe aortic stenoses. N.S. = nonsignificant difference.

 

Figure 5B
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  		Figure 5b: Sensitivity, specificity, and diagnostic accuracy of multidetector CT (white bars), MR imaging (hatched bars), and TEE (black bars)—as compared with continuity equation TTE—in the detection of (a) moderate or severe (AVA < 1.2 cm2) and (b) severe (AVA < 0.8 cm2) aortic stenoses. Multidetector CT, MR imaging, and TEE had equally high sensitivity for the detection of moderate and severe aortic stenoses. N.S. = nonsignificant difference.

 

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  		Table 4. Inter- and Intraobserver Reproducibility of AVA Measurements Obtained by Using Different Techniques

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Several study investigators have reported on the identification of calcifications of the aortic valve at nonenhanced electron beam CT or multidetector CT performed for coronary calcium scoring (29–33). A nonlinear relationship between severity of aortic valve calcification and AVA has been described (31,32) and was confirmed in our present work. The presence of severe aortic valve calcification at nonenhanced electron beam CT or multidetector CT may facilitate the identification of patients who have aortic stenosis versus healthy patients with moderately high diagnostic accuracy (29,32). In addition, it has been suggested that the presence of more severe valvular calcification indicates faster progression of aortic stenosis (34).

Baumert et al (35) in the examination of subjects with normal aortic valves and Feuchtner et al (36) and Alkhadi et al (26) in the examination of patients with aortic stenosis reported the feasibility of performing direct planimetry of AVA with contrast material–enhanced retrospectively gated 16-section multidetector CT at the time of coronary imaging. Our current study results obtained by using 40-section multidetector CT confirm their results. Our results demonstrate that multidetector CT planimetric measurements of AVA are highly reproducible and correlate strongly with MR and TEE planimetric measurements of AVA and with TTE measurements of AVA obtained by using the continuity equation. Accordingly, multidetector CT, as compared with TTE, facilitated the accurate identification of patients who had moderate or severe aortic stenosis with sensitivity, specificity, and diagnostic accuracy values approaching 100%. The excellent performance and reproducibility of multidetector CT are most likely related to the even higher spatial resolution and tissue characterization capabilities of 40-section multidetector CT, compared with those of 16-section multidetector CT, which allow precise delineation of the free edges of the valve. It is interesting that the limited temporal resolution of multidetector CT was not found to be of clinical importance—probably because the time during which the aortic valve remains fully opened (mean duration, 297 msec ± 40) is much longer than the temporal resolution (52–210 msec) of the scanner.

We compared three planimetric AVA measurement approaches with the continuity equation TTE AVA measurement method. Although we observed excellent correlations between the planimetric and continuity equation AVA measurements, confirming earlier findings (7,11–13,26,35,36), all three planimetric measurements were found to be overestimations of the continuity equation–derived AVA. A potential explanation for this overestimation could be that planimetric techniques and the continuity equation are used to measure different aortic valve orifices. Planimetric techniques are used to measure the true dimensions of the anatomic orifice, whereas the continuity equation is used to measure the "effective" orifice area—that is, the orifice of the vena contracta (37). This orifice is always smaller than the actual anatomic orifice, because blood flow tends to stream centrally to the anatomic orifice. Additional explanations for the observed differences between the two techniques are related to the timing of the measurements. Planimetric measurements are usually obtained at the time of maximal valve opening, whereas continuity equation measurements represent an integration (or average) of measurements obtained through the duration of aortic valve opening during systole.

Because multidetector CT is limited owing to radiation exposure and the injection of potentially nephrotoxic contrast material, it probably will not become a first-line method of assessing aortic stenosis severity. However, our study results demonstrate that because of the high diagnostic accuracy of multidetector CT, it could be an alternative to TTE in patients with poor acoustic windows or whenever echocardiographic estimates of the AVA leave the clinician in doubt about the severity of aortic stenosis.

Our study had limitations. It was performed in patients with well-preserved systolic function and sinus rhythm. The performance of the various imaging techniques was not investigated in patients with severely altered systolic function and low-grade aortic stenosis or in patients with arrhythmias such as atrial fibrillation, and it could be worse in these groups. Images were acquired by using a 40-section system with a 420-msec rotation time. Newer systems with larger numbers of detector rows and faster rotation times, which result in higher spatial resolution and higher temporal resolution, might generate even better results.

In summary, our study results show that multidetector CT can be used to measure the AVA and detect aortic stenosis at the time of noninvasive coronary imaging with accuracy similar to that of MR imaging and TEE. Multidetector CT has become far more than a simple anatomic technique dedicated to coronary imaging. Results of relatively recent studies have demonstrated its capability for assessment of global and regional contractile function (18–21) and for characterization of myocardial infarction (38–40). Our study results confirm the usefulness of multidetector CT in assessing aortic valve morphology and stenosis severity and thus its further expanding role in cardiac imaging.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE
 References
 


   IMPLICATION FOR PATIENT CARE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
References
 


   FOOTNOTES
 

Abbreviations: AVA = aortic valve area • LVOT = left ventricular outflow tract • TEE = transesophageal echocardiography • TTE = transthoracic echocardiography • VTI = velocity time integral

Author contributions:Guarantors of integrity of entire study, A.C.P., B.L.G.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, A.C.P., J.L.J.V., B.L.G.; clinical studies, A.C.P., J.B.l.P.d.W., A.P., B.L.G.; statistical analysis, A.C.P., B.L.G.; and manuscript editing, A.C.P., J.L.J.V., B.L.G.

Authors stated no financial relationship to disclose.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 

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