Asian Cardiovasc Thorac Ann 2000;8:46-49
© 2000 Asia Publishing EXchange Pte Ltd
Electron-Beam Computed Tomography for Symptomatic Coronary Disease
Bernard Kwok Wing Kuin, MRCP,
Lim Yean Teng, FRCP,
Quek Swee Tian, FRCR,1,
Lenny Tan Kheng Ann, FRACR,1
Cardiac Department
1 Department of Diagnostic Imaging
National University Hospital
Singapore, Republic of Singapore
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For reprint information contact: Bernard Kwok Wing Kuin, MRCP Tel: 65 436 7546 Fax: 65 227 3562 email: bernard_kwok{at}nhc.com.sg National Heart Centre, 17 Third Hospital Avenue, Mistri Wing, Singapore 168752, Republic of Singapore.
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Abstract
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Forty-two symptomatic patients underwent both electron-beam computed tomo-graphic calcium scoring and coronary angiography. Correlation between coronary artery calcium score and angiographic coronary disease showed a high specificity (90%) but low sensitivity (50%). The low negative predictive value of 36% suggests that electron-beam computed tomography is not useful in symptomatic patients.
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Introduction
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Electron-beam computed tomography (EBCT) derives from a new generation computed tomography (CT) scanner. Its enhanced capabilities make it ideally suited for ultrafast scanning. Published studies have shown that it is an excellent tool for detecting and quantifying coronary calcium deposits.1 There is also an intimate and direct relation between coronary atherosclerotic plaque area and EBCT coronary calcium.2,3 This study sought to determine if a relationship exists between coronary calcium deposits and angiographically significant atherosclerosis in symptomatic patients.
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Patients and Methods
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Forty-two symptomatic patients were included in this study. Their clinical characteristics are listed in Table 1
. Following informed consent, the patients underwent diagnostic coronary angiography and EBCT scanning on the same day. Patients with previous cardiac interventions (coronary angioplasty, coronary bypass surgery) or renal failure were excluded. Coronary angiography was performed for the usual indications: recent myocardial infarction; unstable angina pectoris; or a positive stress test. All angiograms were obtained via the right femoral artery approach with selective engagement of the right and left coronary arteries using different catheters. Image acquisition was accomplished with a Siemens Bicor image intensifier (Siemens, Munich, Germany). Significant coronary artery disease was defined as luminal diameter loss of 50% or more in any of the 3 major coronary arteries or their respective large branches. The angio-graphic results were interpreted by experienced consultant cardiologists.
EBCT was performed according to an established protocol.1 Noncontrast axial images of 3-mm thickness were acquired using the Imatron C-150 ultrafast CT scanner (Imatron, San Francisco, CA, USA). On average, 20 to 50 contiguous slices were required for a complete cardiac scan. The images were electrocardiogram-gated to the late diastolic phase (80% of R-R interval). Deposits of area greater than 1 mm2 and a CT Hounsfield Unit (a measure of CT density) of 130 or more were chosen as indicative of the presence of calcium and identified as such on the images. The calcium score was then derived from the product of the area and a factor (ranging from 1 to 4) depending on the maximum calcium density within the area. The coronary artery calcium score was examined, which is the total amount of calcium expressed as the total calcium score of the entire heart. This method of calcium quantification was described by Agatston and colleagues1 as: calcium score = area x factor (1 to 4); coronary artery calcium score = sum of calcium scores of all image slices. Both the cardiologists and radiologists were blinded to each others' interpretation of angiograms and EBCT scan images, respectively. Following coronary angiography, patients were classified as having no or only mild coronary artery disease (CAD) or significant CAD (single, double, or triple-vessel disease). As sug-gested by Arad and colleagues,4 a coronary artery calcium score threshold of 160 was used to optimize the sensitivity and specificity of the test.
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Results
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The mean (± standard deviation) age of the 42 patients was 55 ± 10 years. There were 33 males. Nine patients who did not have significant coronary calcium showed no or only mild CAD on coronary angiography. Sixteen patients with significant calcification had significant CAD. There were 16 false-negative results and 1 false-positive. Table 2 shows the results in a 2 by 2 format. From these data, the sensitivity was calculated as 50% and the specificity as 90%.
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Discussion
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EBCT employs a stationary source and detector com-bination along with a rotating electron beam. Giving superior spatial and temporal resolution, it is ideal for detecting calcification. Although fluoroscopy as well as conventional and spiral CT scanning can identify calcium deposits, only EBCT can quantify the amount or volume of calcium. Further advantages are that EBCT allows electrocardiogram-gating similar to that employed in nuclear cardiac scans and a short image-acquisition time. A single image can be acquired in 100 msec, compared to 1 to 2 seconds with conventional CT scanners. Therefore, scanning of the entire heart can be completed in 1 or 2 breath-holds. Studies have shown that there is little interstudy variation in individual patients and the inter- and intra-observer variability is excellent.5 These make EBCT the ideal tool for detecting and quantifying coronary calcium. Calcium deposition is a surrogate of atherosclerosis. There is a high correlation between plaque volume and calcium content.2,3 However, it is unclear if plaque burden can be equated with coronary luminal stenosis.6
Of the 42 patients in this study, 8 presented after a recent myocardial infarction (Q or non-Q), 17 presented with unstable angina, and 17 had coronary angiography for chest pain with positive stress tests. As the study consisted of only symptomatic individuals, it is not surprising that 76% (32/42) were proven to have angiographically significant CAD. Of these, only 50% (16/32) had signi-ficant coronary artery calcium scores. The 16 cases of false-negative results and 1 false-positive case showed that in symptomatic subjects, the positive predictive value of EBCT was 94%, while the negative predictive value was only 36%. This is in contrast to most published studies.7–13 The sensitivity of EBCT has been quoted as between 85% and 100% and the specificity has ranged from 41% to 76%. This study showed a lower sensitivity (50%) but very high specificity (90%). This could be related to the unique make-up of our study subjects that included symptomatic patients with angina pectoris and even postinfarction patients.
The recommendation of Arad and colleagues4 of a coronary artery calcium score threshold of 160 was based on a study of asymptomatic patients. The choice of a coronary artery calcium score threshold of 100 as the level of significance in this study would have resulted in a higher sensitivity of 63% with no difference in the specificity of 90% (Table 2
). The negative predictive value, although better, would be still low at 43%. This low negative predictive value suggests that EBCT is likely to be of limited use in symptomatic patients.
On the contrary, the use of EBCT to detect asymptomatic coronary artery disease has shown more promise. A long-term study showed that coronary calcification on EBCT was highly predictive of cardiovascular events.4,14 The primary limitation of EBCT is that although it is able to detect and quantify calcium in atheromatous plaque, the plaque may not be luminally obstructed (i.e., angio-graphically stenotic). Furthermore, a negative study does not rule out the absence of atherosclerotic plaque. Perhaps Rumberger15 put this best: "[Nearly] all calcium is plaque, but not all plaque is calcified".
Recent studies using EBCT with intravenous contrast material have shown greater promise.16,17 While promising the ability to visualize coronary artery anatomy non-invasively, only 79% to 85% of the CT studies were technically interpretable. Furthermore, only high-grade lesions in the proximal coronary tree could be accurately assessed. Hence, the technique is not yet robust enough to replace conventional coronary angiography in the delineation of coronary anatomy. A cautious attitude should be adopted towards the use of EBCT. In its statement for health professionals, the American Heart Association Writing Group advised that the "role of EBCT as a screening tool in asymptomatic patients....is not yet clearly defined".18
The current use of EBCT in cardiology is primarily in detecting and quantifying the extent of coronary calcification. From this study, it appears that EBCT is not useful in symptomatic patients. Further refinement in calcium scoring may improve this. Contrast-enhanced EBCT providing noninvasive visualization of coronary anatomy may prove to be an exciting and promising new imaging modality.
Presented in part at the 12th ASEAN Congress of Cardiology, Manila, Philippines, December 6–10, 1998.
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References
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Abstract
Introduction
