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DOI:10.2214/AJR.07.2069
AJR 2007; 189:197-203
© American Roentgen Ray Society

Original Research

Sixty-Four Slice CT Evaluation of Aortic Stenosis Using Planimetry of the Aortic Valve Area

Gudrun M. Feuchtner1, Silvana Müller2, Johannes Bonatti3, Thomas Schachner3, Corinna Velik-Salchner4, Otmar Pachinger2 and Wolfgang Dichtl2

1 Clinical Department of Radiology II, Innsbruck Medical University, Anichstrasse 35, A-6020 Innsbruck, Austria.
2 Clinical Department of Cardiology, Innsbruck Medical University, Innsbruck, Austria.
3 Clinical Department of Cardiac Surgery, Innsbruck Medical University, Innsbruck, Austria.
4 Clinical Department of Anesthesia and Intensive Care Medicine, Innsbruck Medical University, Innsbruck, Austria.

Received October 24, 2006; accepted after revision March 26, 2007.

 
Address correspondence to G. M. Feuchtner (gudrun.feuchtner{at}i-med.ac.at).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to evaluate planimetry of the aortic valve area with 64-slice CT in comparison with transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) in patients with aortic stenosis.

MATERIALS AND METHODS. Thirty-six patients with aortic valve disease referred for coronary 64-slice CT angiography were examined. Planimetry of the aortic valve area with 64-slice CT was compared with TTE using the Doppler continuity equation for calculation of the aortic valve area and with planimetric measurement of the aortic valve area using TEE.

RESULTS. Planimetry of the aortic valve area with CT (1.11 ± 0.42 cm2) showed a good correlation with TTE (1.05 ± 0.42 cm2) (r = 0.88, p < 0.001) in 32 patients and a good correlation with TEE (1.41 ± 1.61 cm2) (r = 0.99, p < 0.0001) in 10 patients. The mean and maximum transvalvular pressure gradients were correlated with the aortic valve area as measured with CT (r = -0.68, p = 0.0001; and r = -0.67, p = 0.0001, respectively). Beta-blockers were not given (mean heart rate, 62.5 ± 10.7 beats per minute).

CONCLUSION. MDCT allows accurate planimetry of the aortic valve area in patients with aortic stenosis. In patients referred for 64-slice CT coronary angiography, concomitant aortic stenosis can be identified and evaluated.

Keywords: aortic stenosis • echocardiography • 64-slice CT • planimetry


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Coronary angiography with 64-slice CT [1, 2] is increasingly used in clinical practice for the exclusion of coronary artery disease. Degenerative aortic stenosis is frequently present in patients with coronary artery disease because the pathologic mechanisms are similar [3]; therefore, the simultaneous evaluation of aortic valve disease would be desirable in patients referred for CT coronary angiography (Figs. 1A and 1B).


Figure 1
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  		Fig. 1A —64-slice CT of coronary arteries in 75-year-old man. MDCT allows simultaneous evaluation of coronary arteries (A) and aortic valve (B), shown in 3D by applying volume-rendering technique. Note that severe calcification (white spots) of both aortic valve and coronary arteries is frequently seen. LCA = left coronary artery, RCA = right coronary artery.

 

Figure 2
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  		Fig. 1B —64-slice CT of coronary arteries in 75-year-old man. MDCT allows simultaneous evaluation of coronary arteries (A) and aortic valve (B), shown in 3D by applying volume-rendering technique. Note that severe calcification (white spots) of both aortic valve and coronary arteries is frequently seen. LCA = left coronary artery, RCA = right coronary artery.

 
Degenerative aortic stenosis is routinely assessed in clinical practice with transthoracic echocardiography (TTE) on the basis of measurement of the aortic valve orifice area (aortic valve area) in addition to the transvalvular pressure gradients [4]. 16-MDCT has already shown promising results in the planimetric measurement of the aortic valve area in patients with aortic stenosis [5, 6]. However, technical limitations of 16-MDCT have been observed, such as time-consuming postprocessing [5] and motion artifacts in patients with high heart rates (> 65 beats per minute [bpm]) caused by limited temporal resolution ({approx} 105-250 milliseconds) [7]. Therefore, premedication with ß-blockers has been required to achieve images without motion artifacts. However, the use of ß-blockers is of concern in patients with aortic stenosis. The relatively new 64-slice CT technology provides higher temporal resolution (83-166 milliseconds) [8], which might improve image quality at higher heart rates.

The aim of our study was to evaluate planimetry of the aortic valve area with 64-slice CT in patients with aortic stenosis without using ß-blockers in comparison with measurement of the aortic valve area using the Doppler continuity equation with TTE and planimetric calculation of the aortic valve area with transesophageal echocardiography (TEE).


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Study Population
Thirty-six patients (Table 1) with known aortic valve disease were examined between November 2005 and September 2006. Seventeen patients before aortic valve surgery (AVS) and 19 patients from the Austrian "TASS Study" population (a prospective, randomized, double-blind study assessing the effect of statins on the progression of aortic stenosis) underwent 64-slice CT coronary angiography for the evaluation of coronary artery disease. Our institutional review board approved both studies. Written informed consent was obtained from all patients. Patient exclusion criteria were renal dysfunction (creatinine level > 1.2 mg/dL), known allergy to iodine contrast material, hyperthyroidism, left ventricular dysfunction (left ventricular ejection fraction < 35% severe aortic regurgitation (> grade 2+), previous myocardial infarction, cardiomyopathy, pregnancy, plasmocytoma, and multiple myeloma.


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  		TABLE 1: Characteristics of Patients Evaluated for Aortic Stenosis

 

CT Technique
CT data were acquired using an MDCT scanner (Sensation 64, Siemens Medical Solutions) with a 32-row detector collimation acquiring 64 x 0.6 mm slices by applying the z-axis flying-focus technique, a table translation speed of 3.8 mm per rotation, and a gantry rotation time of 0.33 second. Tube current of 120 kV and 800-900 mAs resulted in an estimated radiation exposure range of 9.4-14.8 mSv (mean, 11 mSv) [9]. The scanning direction was craniocaudal during a single inspiratory breath-hold, and the ECG signal was recorded simultaneously. A bolus of 90-120 mL of iodixanol, a nonionic iodine contrast agent, at a concentration of 320 mg I/dL (Visipaque, Amersham) was injected IV into an antecubital vein using a 20-gauge cannula at a flow rate of 5 mL/s using an automated injector. Scanning was started automatically by applying a bolus-tracking technique (ascending aorta; threshold, 100 H) as previously described for coronary CT angiography [10].

CT Image Reconstruction and Analysis
Using retrospective ECG gating, a data set containing axial slices (effective slice width, 1.0 mm; 70% overlap) was reconstructed at 5% steps during the cardiac cycle. This data set was reviewed in 4D using dedicated software (4D Inspace, Heart View, Siemens) on an external workstation (Leonardo, Siemens), and the time point of maximal aortic valve opening was identified. A series of axial slices was generated at the time point of maximal aortic valve opening with an effective slice width of 0.75 mm (60% overlap), a medium-smooth convolution kernel (B 25 f ++), and an image matrix of 512 x 512 pixels.

This image series was reviewed in 3D by applying multiplanar reformations (MPRs). The cross-sectional planimetric images of the aortic valve were reconstructed ranging from the top of the leaflets to the infundibulum. The smallest measurable aortic valve area was regarded as the effective aortic valve area. Two independent reviewers circled the effective aortic valve area with a digital caliper. Image quality was graded on a 3-point scale as 1, good (no artifacts); 2, acceptable (mild artifacts but acceptable delineation of the aortic valve area); or 3, inadequate (artifacts without continuous delineation of the aortic valve area).

Transthoracic Echocardiography
All measurements were performed using a standard sonographic system (Sequoia 256, Acuson-Siemens Medical Solutions) equipped with a 3.5-1.75-MHz transducer by an experienced observer. Doppler flow data were acquired from the left ventricular outflow tract (LVOT) and included LVOT velocity measurement using pulsed wave Doppler sonography and LVOT diameter. The peak transvalvular velocity was measured in all patients. In addition, the aortic valve area was calculated using the continuity equation approach with a Doppler velocity-time integral as in the study of Dumesnil et al. [11]. The mean and the peak transvalvular pressure gradients were calculated. Left ventricular end-diastolic volume, left ventricular end-systolic volume, and left ventricular ejection fraction were determined using Simpson's method [12].


Figure 3
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  		Fig. 2A —Planimetry of aortic valve area (AVA) performed after identifying maximal aortic valve opening using 4D dynamic imaging (see Fig. S1, cine CT, at www.ajronline.org). MDCT images show tricuspid valve in 79-year-old woman with moderate aortic stenosis (aortic valve area, 1.1 cm2) (A and C) and bicuspid valve in 53-year-old-man with severe aortic stenosis (aortic valve area, 0.98 cm2) (B and D). A and B were reconstructed with multiplanar reformations, C and D with volume rendering using 1-mm slab.

 


Figure 4
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  		Fig. 2B —Planimetry of aortic valve area (AVA) performed after identifying maximal aortic valve opening using 4D dynamic imaging (see Fig. S1, cine CT, at www.ajronline.org). MDCT images show tricuspid valve in 79-year-old woman with moderate aortic stenosis (aortic valve area, 1.1 cm2) (A and C) and bicuspid valve in 53-year-old-man with severe aortic stenosis (aortic valve area, 0.98 cm2) (B and D). A and B were reconstructed with multiplanar reformations, C and D with volume rendering using 1-mm slab.

 


Figure 5
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  		Fig. 2C —Planimetry of aortic valve area (AVA) performed after identifying maximal aortic valve opening using 4D dynamic imaging (see Fig. S1, cine CT, at www.ajronline.org). MDCT images show tricuspid valve in 79-year-old woman with moderate aortic stenosis (aortic valve area, 1.1 cm2) (A and C) and bicuspid valve in 53-year-old-man with severe aortic stenosis (aortic valve area, 0.98 cm2) (B and D). A and B were reconstructed with multiplanar reformations, C and D with volume rendering using 1-mm slab.

 


Figure 6
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  		Fig. 2D —Planimetry of aortic valve area (AVA) performed after identifying maximal aortic valve opening using 4D dynamic imaging (see Fig. S1, cine CT, at www.ajronline.org). MDCT images show tricuspid valve in 79-year-old woman with moderate aortic stenosis (aortic valve area, 1.1 cm2) (A and C) and bicuspid valve in 53-year-old-man with severe aortic stenosis (aortic valve area, 0.98 cm2) (B and D). A and B were reconstructed with multiplanar reformations, C and D with volume rendering using 1-mm slab.

 


Figure 7
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  		Fig. 3A —Planimetry of aortic valve area with 64-slice CT versus transthoracic echocardiography (TTE) using continuity equation for calculation of aortic valve area (in cm2) with Doppler velocity-time integral in 32 patients. Linear regression analysis illustrates good correlation between both imaging techniques.

 


Figure 8
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  		Fig. 3B —Planimetry of aortic valve area with 64-slice CT versus transthoracic echocardiography (TTE) using continuity equation for calculation of aortic valve area (in cm2) with Doppler velocity-time integral in 32 patients. Bland-Altman plot implies good intertechnique agreement.

 


Figure 9
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  		Fig. 3C —Planimetry of aortic valve area with 64-slice CT versus transthoracic echocardiography (TTE) using continuity equation for calculation of aortic valve area (in cm2) with Doppler velocity-time integral in 32 patients. Planimetry of aortic valve area with CT versus TEE (n = 10) shows high concordance on Bland-Altman plot.

 
Transesophageal Echocardiography
Planimetry of the aortic valve area was performed by an experienced reviewer with a multiplane phased-array 2-7-MHz TEE transducer on a sonography unit (iE33 echocardiography system, Philips Medical Systems) before aortic valve surgery.

Statistical Analysis
Statistical analysis was performed using SSPS software, version 8.0. The correlations between the aortic valve area measured by CT, TTE, and TEE and the transvalvular pressure gradient were determined using linear regression analysis and the Pearson's correlation coefficient. A two-tailed probability value of less than 0.05 was considered statistically significant. Bland-Altman analysis [13] was performed to evaluate intertechnique agreement by plotting the difference in aortic valve area between CT and TTE against aortic valve area averages. The mean of the difference with a bias of ± 1.96 SD denotes the limits of agreement. Interobserver variability of aortic valve area and left ventricular functional measurements was computed as a percentage of the mean differences between the corresponding observations divided by the average of all observations.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Results for planimetry of the aortic valve area by 64-slice CT (Figs. 2A, 2B, 2C, and 2D) versus TTE and TEE are shown in Figures 3A, 3B, and 3C.

Linear regression analysis showed a good correlation between planimetry of the aortic valve area by 64-slice CT (1.11 ± 0.42 [SD] cm2) and the aortic valve area as measured with TTE (1.05 ± 0.42 cm2; r = 0.88; p < 0.001) in 32 patients (Fig. 3A). The Bland-Altman plot implies a good intertechnique concordance between CT and TTE, placing 30 of 32 patients between the limits of agreement (0.47-0.35) (Fig. 3B). The mean pressure gradient (36.3 ± 17.1 mm Hg) and the maximal pressure gradient (56.8 ± 23.7 mm Hg) correlated significantly with the aortic valve area as quantified with 64-slice CT (r = -0.68, p = 0.0001; and r = -0.67, p = 0.0001, respectively) (Figs. 4A and 4B). Planimetry of the aortic valve area with TEE (1.41 ± 1.61 cm2) showed a high correlation with 64-slice CT (r = 0.99, p < 0.0001) in 10 patients, and the Bland-Altman plot confirmed good concordance (Fig. 3C).


Figure 10
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  		Fig. 4A —Planimetry of aortic valve area with CT versus transthoracic echocardiography (TTE). Planimetry of aortic valve area on CT versus mean (A) and maximum (B) transvalvular pressure gradients on TTE show moderate correlation.

 

Figure 11
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  		Fig. 4B —Planimetry of aortic valve area with CT versus transthoracic echocardiography (TTE). Planimetry of aortic valve area on CT versus mean (A) and maximum (B) transvalvular pressure gradients on TTE show moderate correlation.

 
The mean interobserver variability of the aortic valve area measurement on CT was 4.8% (absolute variability, 0.51 ± 0.03 cm2). The mean postprocessing time was 6.3 minutes (range, 4.1-10.8 minutes) for the physician. Images were reconstructed at 12% of the R-R interval in 34 of 36 patients and at 10% and 16% in one patient each.

The mean heart rate was 62.5 ± 10.7 bpm (range, 42-88 bpm) during the CT scanning. None of the patients received ß-blockers. Thirty-four patients were in sinus rhythm, two of whom had multiple premature beats requiring ECG editing; one additional patient had atrial fibrillation and another patient had a right ventricular pacemaker. Image quality was good in 29 patients, acceptable in seven (reasons for mild artifacts were atrial fibrillation, a pacemaker, and premature beats), and insufficient in none of the patients.


Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Diagnostic imaging of degenerative aortic stenosis based on planimetry of the aortic valve area using 16-MDCT has been previously reported [5, 6]. However, postprocessing was time-consuming and ß-blockers were required [5] in up to 60% of patients referred for coronary CT angiography [14] to achieve appropriate image quality without motion artifacts in patients with high heart rates (> 65 bpm). In general, ß-blockers are contraindicated in patients with aortic stenosis because of their negative influence on hemodynamics and cardiac output.

In contrast to a previous study using 16-MDCT [5], this study shows that 64-slice CT allows accurate planimetry of the aortic valve area (Figs. 5A, 5B, 5C, and 5D) and good correlation with both TTE and TEE without using ß-blockers. This might be explained by improved temporal resolution, which is achieved by faster gantry rotation times, thus leading to a reduction of motion artifacts at higher heart rates. In addition, our data suggest that image quality may be improved when compared with a previously published study using 16-MDCT [5]. Postprocessing is facilitated and less time-consuming when using new software that enables 4D imaging, which permits a dynamic display of valvular motion (cine CT) during the entire cardiac cycle. This dynamic display is based on stepwise image reconstruction at arbitrary percentages (e.g., every 5% of the R-R interval) (Fig. S1, cine CT, available at www.ajronline.org) that can be applied for both 16- and 64-slice CT data sets [15, 16]. When the clips are reviewed, the time point of the aortic valve opening can be identified; after the valve is opened, it stays open until the end of systole and allows imaging of the valve orifice. As the result, the aortic valve area could be accurately quantified at 12% of the R-R interval in most of our patients.


Figure 12
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  		Fig. 5A —Planimetry of aortic valve areas (AVA) in different shapes. Normal aortic valve (area, 3.2 cm2) in 53-year-old woman.

 

Figure 13
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  		Fig. 5B —Planimetry of aortic valve areas (AVA) in different shapes. Stenotic triangular aortic valve (area, 0.81 cm2) in 70-year-old man with severe aortic stenosis and severe calcification (white spots) of tricuspid valve.

 

Figure 14
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  		Fig. 5C —Planimetry of aortic valve areas (AVA) in different shapes. Slotlike appearance of aortic valve (area, 0.67 cm2) in 79-year-old woman with severe aortic stenosis and bicuspid valve.

 

Figure 15
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  		Fig. 5D —Planimetry of aortic valve areas (AVA) in different shapes. Spotlike aortic valve (area, 0.55 cm2) in 66-year-old woman with symptomatic, critical severe aortic stenosis of functional bicuspid valve in whom surgery is indicated.

 
The main advantage of 64-slice CT over TTE is that CT allows a measurement of the "true" anatomic aortic valve orifice area (so-called planimetry), similar to TEE and cardiac MRI. Therefore, measurement of the aortic valve area with 64-slice CT is not adversely influenced by hemodynamics such as left ventricular dysfunction and severe aortic regurgitation, which are limiting factors in the assessment of aortic stenosis on TTE based on Doppler flow parameters (such as the continuity equation and the transvalvular pressure gradients). Therefore, 64-slice CT might be considered a comprehensive imaging technique under those conditions.

Our study has some limitations. Radiation exposure of cardiac 64-slice CT scans ranges between 9.4 and 14.8 mSv (mean, 11 mSv) [8] to a maximum of 21 mSv [2], which is greater than the range of cardiac catheterization [17] but comparable to a myocardial SPECT scan ({approx} 20 mSv) [18]. ECG dose modulation, which reduces the radiation exposure by approximately 45-48% [19], is not recommended in this specific setting because the tube output is reduced during systole, thus increasing image noise (e.g., if planimetry of the aortic valve area is required in patients with limited echocardiographic windows).

In patients undergoing coronary CT angiography, unenhanced CT calcium score scanning, which has previously often been performed, can be used to determine the presence and severity of aortic valve calcification [20, 21]. In patients with aortic valve calcification, ECG dose modulation should not be applied. Aortic valve stenosis commonly develops in older patients (> 65 years) [3]; and available data suggest that a radiation dose received during CT may not significantly increase the lifetime risk of cancer in this age group [22]. However, in younger patients (< 60 years), the radiation dose from cardiac CT when ECG pulsing is not used is a matter of concern, and the benefit from cardiac CT should be carefully considered for each individual.

Iodine contrast agents should not be administered to patients with renal dysfunction, known allergies, and hyperthyroidism. The maximal heart rate in our study population was 88 bpm; therefore, the performance of 64-slice CT at heart rates greater than 88 bpm remains unknown. The low heart rate (mean, 62.5 bpm) in our study population in whom ß-blockers were not administered might be explained by the unique lifestyle of our population, who live in an alpine region (Innsbruck, Austrian Alps) characterized by hiking (the equivalent of endurance training). We acknowledge also that the small number of investigated patients may lead to a bias.

In conclusion, TEE will remain the primary diagnostic imaging technique for assessing patients with aortic stenosis in clinical practice because it is widely available, radiation-free, and reliable. However, planimetry of the aortic valve area with 64-slice CT is accurate and has been improved; pretreatment with ß-blockers can be avoided in patients with heart rates up to 88 bpm. Postprocessing is facilitated using dedicated software that enables 4D cine imaging of valvular motion. With respect to the emerging use of 64-slice CT coronary angiography in outpatients for the evaluation of coronary artery disease, planimetry of the aortic valve area is recommended in all patients who present with previously unknown aortic valve calcification to distinguish between nonstenotic aortic valve sclerosis and aortic stenosis.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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