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Original Research |
1 All authors: Department of Radiology, Massachusetts General Hospital, 165 Cambridge St., Ste. 400, Boston, MA 02114.
Received February 12, 2006;
accepted after revision May 30, 2006.
Address correspondence to S. Abbara
(sabbara{at}partners.org).
Abstract
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MATERIALS AND METHODS. Retrospectively gated MDCT was performed in 57 patients with the following parameters: gantry rotation time, 420 milliseconds; tube voltage, 120 kV; tube current, 550 mAs with tube current modulation; and slice collimation, 16 x 0.75 mm. From 72 to 100 mL of contrast agent (320 g/mL3) was injected IV at 4-5 mL/s. High-resolution data sets were obtained for planimetry at phase starts of 0, 50, 100, 150, and 200 milliseconds after the R wave peak and were assessed for aortic opening area and the presence of artifacts.
RESULTS. In 41% of patients, the cardiac phase with the largest aortic opening area was at 50 milliseconds after the R wave peak. The area of the aortic opening measured at 0 milliseconds after the R peak was 2.7 ± 0.8 cm2 (mean ± SD); at 50 milliseconds, 2.9 ± 0.2 cm2; at 100 milliseconds, 2.9 ± 0.7 cm2; at 150 milliseconds, 2.8 ± 0.7 cm2; and at 200 milliseconds, 2.4 ± 0.8 cm2. The image quality was best at 50 milliseconds after the R peak in 42% of patients, 100 milliseconds in 29%, 150 milliseconds in 20%, 0 milliseconds in 7%, and 200 milliseconds in 2%. The aortic valve appeared closed in three patients at 0 milliseconds and in four patients at 200 milliseconds. Fewer artifacts were present in the midsystolic phases (i.e., 50-150 milliseconds) ("double-leaflet" artifact, 5-13%; "incomplete contour" artifact, 20-26%) than in the early (0 milliseconds) and late (200 milliseconds) systolic phases (double-leaflet artifact, 38% and 43% of patients; incomplete contour artifact, 76% and 73%, respectively).
CONCLUSION. Aortic valve planimetry is best performed at phase starts of 50-100 milliseconds after the R peak because the area of the aortic opening is widest and image quality is best at that phase.
Keywords: aorta • aortic valve • cardiac imaging • coronary artery disease • heart disease • MDCT • planimetry
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Unlike aortic valve assessment on echocardiography, MRI, and cardiac catheterization, which derive an index of functional aortic valve area by pressure or velocity measurements, the assessment of aortic stenosis on MDCT is purely anatomic and is performed through direct anatomic planimetry of the valve in midsystole, when the valve cusps are open and relatively quiescent [4-9]. Although the questions of clinical utility and role of aortic valve planimetry using MDCT require further studies, several technical questions related to the technique require clarification. The aim of this study was thus to investigate the technical feasibility of aortic valve planimetry using MDCT and to determine whether any technical factors in the acquisition or reconstruction of images impact image quality significantly. In particular, because the area of the aortic valve must be measured in midsystole, we sought to determine the temporal characteristics of the ideal acquisition window in which to perform planimetry.
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The contrast agent used was iodixanol (320 g/mL3) [Visipaque, GE Healthcare]), which was injected IV at a rate of 4-5 mL/s. After an initial test bolus, a study contrast bolus (range, 72-100 mL) was injected with timing tailored to yield peak arterial opacification. For the test-bolus acquisition, a region of interest was placed above the aortic valve in the ascending thoracic aorta. Aortic opacification was measured in 2-second intervals until a decline in aortic contrast concentration was visible. The exact timing of peak opacification was then assessed by reviewing the Hounsfield unit data in each image.
The coronary CT angiography acquisition was reconstructed using a multisegment reconstruction algorithm. This algorithm uses projection data from two consecutive heart beats to generate one image in order to minimize the temporal resolution for that image. The resulting temporal resolution is heart rate-dependent and ranges from 105 to 210 milliseconds (one fourth to one half of the gantry rotation speed).
Aortic Valve Orifice Area Estimation
A multiphase data set consisting of 10 phases at 10% intervals ranging from
0% to 90% of the cardiac cycle was reconstructed for visual assessment of
aortic valve motion (Fig. 1A,
1B). A slice thickness of 1.5
mm with an increment of 1.5 mm was used to keep the size of this data set
manageable. In addition, high-resolution images, each with a heart
rate-dependent reconstruction window length of 105-210 milliseconds, were
reconstructed for planimetry at five different systolic phases (Fig.
2A,
2B). In the first set of
high-resolution images, the acquisition was started with a phase start
beginning with the isovolumetric contraction immediately on the R wave. In the
second to fifth midsystolic sets, the start points for the reconstruction
windows were shifted by 50 milliseconds to cover phase starts from 0 to 200
milliseconds.
The data sets were transferred to an offline workstation (Leonardo, Siemens Medical Solutions) for planimetry and analyzed by a single experienced MDCT reviewer.
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The area of the aortic valve opening was found by scrolling through the short-axis images toward the tip of the cusps until the smallest opening was found. Using a manual region-of-interest tool at that slice location, planimetry was then performed by tracing the inside borders of the coronary cusps.
In each of these five cardiac phases (0-200 milliseconds phase start), the aortic valve orifice area was traced and reported in square centimeters (cm2) (Fig. 3A, 3B).
For each phase the image quality was rated on a subjective scale from 0 to 10, with zero being uninterpretable and 10 being excellent. The presence and type of artifacts were recorded for each phase. The recorded types of artifacts included the "double-leaflet" artifact (present if at least one valve cusp was illustrated with two contours or lines within an image), "incomplete contour" artifact (present if < 80% of the leaflet circumference was visible in the image selected for planimetry), and image blurring (present if any portion of the valve leaflet illustrated unsharpness).
Statistical Analysis
Demographic analyses were performed on the study population. Descriptive
exploratory analysis was performed on the aortic opening area and imaging
quality data in all patients. Continuous data are presented as means ±
SD, with ranges provided when appropriate, and categoric data are presented as
proportions. The data for aortic opening area and image quality were also
assessed stratified by patient age (> 65 vs < 65 years) and sex. All
analyses were performed using statistics software (version 11.0, SPSS).
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Aortic Orifice Area Assessment
In 83% of the patients, the largest average aortic valve area was seen in
one of the images from the midsystolic data sets (phase start, 50-150
milliseconds after R wave peak) (Fig.
4). The results were similar when stratified by patient sex and
age (detailed data not shown). The area of the aortic valve measured with a
phase start in isovolumetric contraction (phase start, 0 milliseconds after R
wave peak) averaged 2.7 ± 0.8 cm2 (range, 1.5-4.6
cm2), whereas in isovolumetric relaxation (phase start, 200
milliseconds after R wave), it averaged 2.4 ± 0.8 cm2
(range, 1.0-4.5 cm2). On the other hand, aortic valve area was
highly consistent in all three midsystolic frames, averaging 2.9 ± 0.2
cm2 (range, 1.8-5.6 cm2) with a 50-millisecond delay
after the R wave, 2.9 ± 0.7 cm2 (range, 1.8-4.9
cm2) with a 100-millisecond delay, and 2.8 ± 0.7
cm2 (range, 1.8-4.6 cm2) with a 150-millisecond delay.
However, whereas no differences were seen between the planimetric aortic
opening area in the various frames of the midsystolic data set, differences
were observed in image quality.
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Double-leaflet artifacts and incomplete contour artifacts were present in substantially fewer cases in the midsystolic phases (double-leaflet artifact, 5-13%; incomplete contour, 20-26% of cases) than in the early and late systolic phases (double-leaflet artifact, 38% and 43%; incomplete contour artifact, 76% and 73% of cases, respectively) (Fig. 6).
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In most patients, aortic valve disease is currently assessed using transthoracic echocardiography. With modern scanners and appropriate attention to detail, the area of the aortic valve can be measured directly in most patients. It should be noted, however, that the measurement of aortic valve area by pressure (cardiac catheterization) or velocity (Doppler imaging and phase-contrast MRI) gradients across the valve is inherently a measure of functional valve area [4-8]. This functional valve area reflects the physiologic effects of coexisting valvular heart disease—most notably, aortic regurgitation and mitral regurgitation—and other variables affecting ventricular loading. Direct planimetry of the aortic valve using MDCT, on the other hand, is a purely anatomic measurement of valve area in systole and is not influenced by the flow dynamics to which that the valve is subjected. It is thus possible that an assessment of functional valve area using echocardiography or MRI and an assessment of anatomic valve area using MDCT could be integrated into a highly complementary data set addressing different aspects of the condition. This may become particularly useful in the research setting as more therapies are developed to treat calcific aortic stenosis medically; however, this needs to be studied further.
Previously, Willmann et al. [11] reported their results on patients who were to undergo aortic valve surgery. Fifteen patients underwent unenhanced MDCT, and 25 underwent contrast-enhanced MDCT. The aortic valve was visualized almost free of motion artifacts on all of the MDCT images. The image quality and diagnostic confidence for assessing aortic valve morphology were significantly superior on contrast-enhanced images compared with unenhanced images. Both unenhanced and contrast-enhanced CT showed good agreement with the surgical findings with regard to quantification of the degree of aortic valve calcification. However, measurements of the diameter of the aortic valve annulus using the contrast-enhanced images were more reliable [11].
Several limitations in our study require discussion. First, we assessed patients who had neither significant aortic valve disease nor aortic valve calcification. Whether the accuracy of aortic valve area planimetry is compromised in calcified abnormal valves remains to be studied. Second, a large and rapid fluid bolus (100 mL at 4-5 mL/sec) is required to acquire these images. Such a fluid challenge may not be tolerated by some patients with severe aortic stenosis and highly hypertrophied noncompliant left ventricles. The inability of aortic valve area planimetry using MDCT to measure the physiologically and clinically relevant functional valve area was discussed earlier in this article. It is thus unlikely that MDCT could be used as the sole technique with which to evaluate aortic stenosis; however, its combination with either echocardiography or MRI may provide additive information. Finally, we assumed that the anatomic valve orifice is planar and lies fully in the plane of the image.
In summary, the role of MDCT in the assessment of cardiac disease is increasing and the technique now has an established role in the assessment of coronary artery disease. We show in this study that MDCT can potentially be used to evaluate stenotic aortic valves, which often coexist with coronary artery disease. Additional studies need to be performed to further clarify the accuracy of the technique and its role in the clinical decision-making process.
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