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Original Research |
1 Division of Cardiology, Department of Internal Medicine, Cardiovascular
Center, University of Michigan, Ann Arbor, MI.
2 Present address: Department of Internal Medicine, Weill Cornell Medical
College, 520 E 70th St., Starr Pavilion, 4th Fl., New York, NY 10021.
3 Division of Cardiothoracic Radiology, Department of Radiology, Cardiovascular
Center, University of Michigan, Ann Arbor, MI.
Received January 21, 2008;
accepted after revision June 16, 2008.
Presented in part as a poster at the 2007 scientific sessions of the
American Heart Association, Orlando, FL.
Abstract
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MATERIALS AND METHODS. We performed retrospective evaluation of 80 consecutive patients with aortic stenosis (AS) who underwent ECG-gated 64-MDCT and transesophageal echocardiography (TEE). Valve planimetry was feasible in 80 patients with CT and in 63 patients with TEE; valve area by transthoracic echocardiography was available in 46 patients. Valve calcification grade on CT was compared with TEE. One cardiologist (echocardiography) and two radiologists (CT) independently and blindly reviewed the studies. Pearson's correlations, Spearman's rank correlations, paired Student's t tests, and weighted kappa tests were used.
RESULTS. The median valve area on TEE was 0.7 ± 0.9
cm2. There was excellent correlation (n = 80; r =
0.91, p < 0.001) and no difference (0.06 ± 0.26
cm2, p = 0.06) between CT readers. There was strong
correlation (n = 63; r = 0.84, p < 0.001) and no
difference (—0.06 ± 0.48 cm2, p = 0.33) in
valve area between CT and TEE, with a strong correlation (n = 46;
r = 0.83, p < 0.001) and small overestimation (0.17
± 0.33 cm2, p < 0.001) in valve area with CT
versus transthoracic echocardiography. The sensitivity and specificity of CT
to detect severe aortic stenosis compared with TEE were 92.1% (35/38) and
89.5% (17/19), respectively. Calcification grade had fair agreement between CT
readers and TEE (
= 0.34 and 0.37, respectively).
CONCLUSION. Aortic valve area on CT strongly correlates with echocardiography and has excellent sensitivity and specificity to detect severe stenosis. Valve calcification has fair agreement between studies. Valve area and calcification should be reported on CT angiography in patients with AS.
Keywords: aortic valve • aortic valve stenosis • CT • transesophageal echocardiography
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TEE is a semiinvasive study routinely performed in patients who are under consideration for valve surgery. TEE is useful to confirm AS severity, define valve anatomy, and evaluate the aorta and root. Invasive hemodynamics with cardiac catheterization can also be performed, but this is not recommended before valve surgery unless the noninvasive tests are inadequate or discordant [1].
MRI [2–6] is another technique that can be used to evaluate the aortic valve, with the advantage of no radiation exposure or use of iodinated contrast material. When used with ECG gating [7], CT can also evaluate the aortic valve and is less expensive. CT is also becoming more widely available, is better tolerated, and may be performed to evaluate the aorta before valve surgery.
Previous studies with earlier-generation CT scanners reported strong correlations between CT and TTE and TEE in small cohorts [8–11]. A study with 64-MDCT compared with TTE (n = 32) or TEE (n = 10) also reported strong correlation [12], whereas another found moderate correlation and an overestimation with 64-MDCT compared with TTE [13].
Data are limited comparing the AVA between 64-MDCT and TEE in AS. Furthermore, although significant echocardiographic aortic valve calcification may identify AS patients with a poor prognosis [14], there are limited data comparing this finding on CT and echocardiography. Thus, the aim of this study was to compare AVA between CT and TEE, AVA between CT and TTE, and aortic valve calcification between CT and TEE.
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Two fellowship-trained cardiothoracic radiologists independently evaluated the CT studies and an experienced independent level 3 certified cardiologist reviewed the TEE and TTE studies. The readers were blinded to all clinical data and information regarding other imaging technique results, including observations from the other readers. AVA was measured using planimetry with CT and TEE and by the continuity equation with TTE. Aortic valve calcification was graded on a standardized scale (Table 1) and findings on CT compared with TEE.
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Patients with left ventricular (LV) dysfunction (ejection fraction < 50% on TEE) were excluded from the sensitivity and specificity analyses for the detection of moderate and severe AS with TEE and CT because the assessment of the severity of AS on echocardiography may not be accurate in this cohort. These patients were included in all other analyses.
CT Technique and Evaluation
Image acquisition—No pharmacologic agents were used for
heart rate control on aorta-specific examinations (n = 79). For the
one coronary CT angiography examination, the heart rate was 
65 beats per
minute (bpm) at baseline, so no pharmacologic agent was used for heart rate
control. A timing bolus of 15 mL was used in all patients to determine the
optimal scanning start time. Iodinated IV nonionic contrast material (either
135 mL of iopromide 370 mg I/mL [Ultravist, Bayer HealthCare] or 95 mL of
iodixanol 320 mg I/mL [Visipaque, GE Healthcare]) was used for aortic or
coronary CT angiography, with a flow rate of 4 mL/s.
CT examinations were performed on a 64-MDCT scanner (VCT, GE Healthcare) using retrospective ECG gating. The CT examinations were performed in the craniocaudal direction and ECG gating was turned off at the upper abdominal aorta. CT scanner detector collimation width was 0.625 mm, detector coverage was 40 mm, reconstructed slice thickness was 1.25 mm, and the slice interval was 1.25 mm. Gantry rotation time was 0.35 second and the scan pitch ranged between 0.16 and 0.20 (adjusted per heart rate). Depending on the patient size, the maximum tube current ranged between 450 and 700 mA with a fixed tube voltage of 120 kVp. In patients with no tachyarrhythmia, dose modulation was applied in CT examinations using maximum tube current during LV diastole (between 60 and 80% of the R-R interval) and 20% of the maximum tube current during the remainder of the cardiac cycle. The radiation dose was available only in 32 study patients and included the cumulative dose for the thorax, abdomen, and pelvis in 28 of these 32 study patients. The effective dose was calculated by multiplying the dose–length product by the tissue weighting factor of 0.015. In our current practice, the typical effective radiation dose for an aorta-specific examination for thorax portions alone is approximately 25 mSv.
Image reconstruction—CT images were retrieved from the radiology electronic database and were reviewed on a PACS workstation (Advantage Windows Workstation, version 4.3_05 with CardIQ software, GE Healthcare). The retrieved images were obtained throughout the entire cardiac cycle at 5–10% increments of the R-R cycle. As originally archived in the radiology CT electronic database, 79 of these examinations had 1.25-mm thick images and one examination had 2.5-mm thick images. Aortic valve evaluation was performed using multiplanar reformatted images of the aortic root; neither maximum nor minimum intensity projection images were used.
Image analysis—The aortic valve was centered on a sagittal plane, with the reformatting plane used to generate a paracoronal oblique long-axis view of the aortic valve, aortic root, and left ventricular outflow tract (LVOT). On this view, another reformatting plane was used to generate a double-oblique short-axis view of the aortic valve (Figs. 1A, 1B, 1C, and 1D). These short-axis plane images were evaluated in all phases of the cardiac cycle by panning through the aortic valve. For the measurement of AVA, the images that best represented the maximal valve opening in mid LV systole were chosen. Planimetry was then carefully performed by measuring the entire area of contrast material present between the aortic leaflets excluding valve calcifications (Figs. 2A, 2B, 3A, 3B, and 3C). Aortic valve calcification was graded using both short- and long-axis views of the aortic valve.
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The AVA can be calculated using the continuity equation [AVA =
(VTILVOT / VTIaortic valve) x
r2], where VTI = the velocity–time
integral and r = the radius of the LVOT. Because the stroke volume
must be identical at both the LVOT and the aortic valve, by measuring the
velocity–time integral at the LVOT and at the aortic valve and by
estimating the LVOT area, the AVA can be determined. The LVOT diameter was
measured 1 cm caudal to the aortic valve annulus, using the parasternal
long-axis view on TTE.
Statistical Analysis
Weighted kappa and Bland-Altman tests were performed with MedCalc, version
9.3.0.0, statistical software because these tests are not available with SPSS.
All other statistical analysis was performed using SPSS, version 15.0 for
Windows. The Pearson correlation coefficient was used to analyze the
interobserver correlation between the CT readers and to compare AVA obtained
by CT and echocardiography. Bland-Altman plots were used to compare results
between tests. Paired Student's t tests were used to compare
differences between the groups. The sensitivity and specificity of CT to
detect moderate or severe AS as defined by TEE were also determined. For
comparisons between AVA on TEE and the calcium grade on CT, corrected
Spearman's rank correlation was used. The agreement in calcification grades
between groups was calculated using a weighted kappa test. The Wilcoxon's
signed rank test was used to compare the difference in valve calcification
grade between CT and TEE. A p value of < 0.05 was considered
significant.
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After excluding one patient with severe motion artifact on CT, there were 80 cases available for comparison. The AVA was measurable by planimetry with TEE in 63 of 80 patients and by planimetry with CT in all 80 patients. A TTE study was available in 53 patients, with measurement of AVA by continuity equation possible in 46 patients. Determination of AVA by TEE or TTE was possible in 79 of 80 patients. In 17 patients, planimetry of the aortic valve by TEE could not be accurately obtained secondary to poor visualization of the stenotic orifice area because of intense leaflet calcification or an inability to obtain a true short-axis view of the valve. In seven patients, determination of AVA using TTE was not possible because of poor visualization of the LVOT. Calcification did not prevent determination of the AVA with CT in any patient.
There was excellent interobserver correlation (Fig. 4A) of AVA with CT as measured by the two readers (n = 80, r = 0.91, p < 0.001), with no significant difference observed (mean difference, 0.06 cm2; SD, 0.26 cm2; p = 0.06) and Bland-Altman plot concordance in 75 of 80 cases (Fig. 4B).
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Aortic valve calcification could be determined on all 80 CT examinations
and on 77 TEE examinations (Tables
2,
3,
4). There was good
interobserver agreement between CT readers ( = 0.72), with fair
agreement between each CT reader and TEE ( = 0.34 and 0.37). Agreement
within one grade was noted in 78 of 80 patients (97.5%) between CT readers and
in 59 and 57 of 77 patients (76.6% and 74.0%) between individual CT readers
and TEE. Calcification grade was higher with each CT reader compared with TEE
(p < 0.001), with the mean grade 0.64 and 0.79 points higher for
each reader, respectively. There was a relationship between the valvular
calcification grade and the severity of AS. A smaller AVA on TEE was
associated with a higher calcification grade with each CT reader (r =
—0.38, p = 0.002; r = —0.50, p <
0.001). Smaller AVA on CT was also associated with a higher CT calcification
grade with each reader (r = —0.37, p = 0.001;
r = —0.51, p < 0.001).
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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When TTE is not sufficient to determine the severity of AS, often due to poor visualization of the aortic valve, additional testing may be necessary. TEE is commonly performed in this situation because it allows improved imaging of the aortic valve and an assessment of AVA by planimetry, as validated by several studies [15–17].
This present study shows that the AVA obtained by planimetry on ECG-gated 64-MDCT has a strong correlation to the AVA obtained by TEE, with no significant difference between the two techniques. Furthermore, in patients with normal LV function and a high prevalence of significant AS, CT can detect moderate or severe AS with high sensitivity and specificity in comparison with TEE.
These findings are not unexpected because both TEE and CT directly measure the valve area by planimetry and would be expected to provide similar results. Interestingly, after excluding one patient with severe motion artifact on CT, planimetry of the AVA was possible in all 80 patients with CT but in only 63 patients with TEE, suggesting a potential advantage of CT. Accurate assessment of AVA by planimetry with TEE depends on obtaining a true short-axis view of the valve, which may be difficult in some cases. CT allows more flexibility in finding the optimal plane to obtain an accurate short-axis view of the valve.
A subgroup of patients had TTE studies available, and comparison provided a similarly strong correlation between the AVA by TTE and by CT, with a small overestimation with CT compared with TTE. The small difference between TTE and CT is not unexpected given the differences between methods. This small overestimation of the AVA with CT is consistent with previous studies [6, 13].
In addition to AVA, aortic valve calcification is an important prognostic finding. Progressive valve calcification is the most common cause of AS and represents an active, progressive disease process that may result in decreased leaflet mobility and AVA [1]. Among patients with asymptomatic AS, significant valve calcification on echocardiography was associated with an increased risk for death or valve replacement [14]. Furthermore, a small study reported that aortic valve calcification on CT was associated with adverse clinical events [18]. Significant correlations between valvular calcification with CT and AS severity on cardiac catheterization [19] and TTE [20] have been previously reported, and nonlinear relationships between aortic valve calcification and AVA as well as valve hemodynamics have been described [21].
Furthermore, quantitative assessment of aortic valve calcification with CT [21, 22] and electron beam CT [23] have been found to be highly reproducible. This study shows that grading of aortic valve calcification with CT is reproducible and has good interobserver agreement, particularly within one grade. There is only fair agreement between CT and TEE, with a higher valve calcification grade using CT. This may be related to the differences in imaging calcium between these two techniques. On echocardiography, calcification can only be inferred by the presence of high echodensity and acoustic shadowing. In contrast, CT can distinguish calcification from fibrosis and other tissue. However, as has been shown with coronary CT angiography, calcifications may appear larger on CT due to blooming artifact. This study compares CT to TEE, which generally provides a higher-resolution image of the aortic valve than is available with TTE. Although the clinical significance of aortic valve calcification detected by CT is not well established, CT is the reference standard for evaluating calcium and would be expected to be superior to echocardiography.
The relationship of higher aortic valve calcification grade to a smaller AVA is expected given the role of calcification in AS. Previously published findings have reported stronger relationships [19–21], although the majority of these determined quantified aortic valve calcification. This study graded aortic valve calcification and used contrast-enhanced studies only because unenhanced studies were not routinely performed for aortic-protocol CT studies and hence not available. This prevented the use of quantified calcium scoring and may explain differences from previously published studies.
The majority of the patients in this study had normal LV systolic function. Although LV function does not affect measurement of AVA by planimetry with TEE or CT, the area obtained by this method may not reflect the severity of AS. In addition, valvular gradients may be underestimated in low stroke volume status. In patients with LV dysfunction, the aortic valve may not open fully as it would in normal stroke volume status, and therefore AVA by planimetry may overestimate the severity of AS. Further testing, such as low-dose dobutamine stress echocardiography, may be useful to assess the severity of AS in patients with LV dysfunction, low gradients, and AVA by planimetry suggesting severe AS.
Limitations of CT include the need for IV contrast material and ionizing radiation, although assessment of the aortic valve did not require additional contrast material or radiation exposure beyond that required for the aortic or cardiac CT. The retrospective design is a limitation of this study, including the retrospective evaluation of CT and echocardiography studies. Additional limitations include the use of a single academic center, and the use of highly experienced readers may not reflect conditions available at other sites.
These results suggest that CT may play a useful role in the evaluation of patients with AS. Furthermore, aortic valve calcification can be assessed with high reproducibility using CT, although the prognostic implications of aortic valve calcification on CT are not yet established. In patients with known or suspected AS, the aortic valve should be evaluated on ECG-gated 64-MDCT of the heart and thoracic aorta. The AVA and aortic valve calcification may provide important and incremental value in the clinical evaluation and management of these patients.
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