CT Coronary Calcium Assessment
Abstract & Commentary
Synopsis: Electrocardiographically triggered single-slice conventional CT yields measurements of coronary artery calcium that are equivalent to scores derived from electron beam CT.
Source: Becker CR, et al. Helical and single-slice conventional CT versus electron beam CT for the quantification of coronary artery calcification. AJR Am J Roentgenol 2000;174: 543-547.
Becker and associates examined 210 patients referred for coronary artery calcification. All patients underwent two CT scans. The first scan was electron beam CT with electrocardiographic (ECG) triggering, using a slice thickness of 3 mm. For the second scan, patients were randomly assigned to one of five CT protocols: helical CT with a pitch of 1, helical CT with a pitch of 2, single-slice CT, single-slice CT with ECG triggering, or repeat electron-beam CT with ECG triggering. Patients in groups one through four were studied using a single-detector CT (Somatom Plus 4A; Siemens, Forchheim, Germany). Slice thickness was 3 mm for all groups. Effective scan time was 1000 ms for groups 1 and 2, 500 ms for groups 3 and 4, and 100 ms for group 5.
In groups 1 and 2, the mean calcium scores evaluated by electron beam CT and helical CT (pitch of 1 or 2) were significantly different. In groups 3 and 4, no significant difference was present between the mean scores obtained with electron beam CT and single-slice CT, with or without ECG triggering. All groups had a high linear correlation between the two scan protocols, with regression coefficients ranging from 0.934 to 0.993. Mean variability was lowest for patients who underwent repeat electron beam CT (22.1%) or ECG-triggered single-slice conventional CT (25.4%). Variability ranged from 42.2% to 61.4% in the other groups. Becker et al conclude that the variability among calcium scoring measurements using electron beam CT and ECG-triggered single-slice conventional CT is within the range reported in this and other studies for repeat measurements using electron beam CT.
Comment by Gautham P. Reddy, MD
Some investigators have advocated the use of coronary artery calcium assessment by electron beam CT to screen for coronary artery disease (CAD) or to identify CAD in patients with atypical chest pain. Calcium identified by electron beam CT corresponds closely to coronary artery calcification detected on histopathologic examination.1 The "calcium score" is derived from the area and density of coronary calcification. Electron beam CT can acquire ECG-triggered images in 100 ms, which allows imaging of the heart and coronary arteries with relatively little cardiac motion artifact. Recently, ECG triggering has become available on conventional CT scanners, which have an effective scan time of as low as 500 ms. The study by Becker et al shows that a conventional CT can produce scores that are substantially equivalent to the results of electron beam CT for coronary artery calcium assessment. In the near future, ECG-triggered helical CT and multidetector-row helical CT, which permits volumetric imaging, may allow coronary calcium assessment that is even more reproducible than single-slice conventional CT or electron beam CT.
Becker et al do not address the clinical use of CT calcium assessment, which remains controversial. Several large studies have shown that electron beam CT has a high negative predictive value, ranging from 84% to 100%, for angiographically detected coronary artery stenoses.2-4 Studies have also been performed to correlate coronary calcium scores with clinical outcome. Detrano et al5 followed symptomatic patients for a mean duration of 31 months and found that patients with a calcium score of 100 or more were at greater risk for coronary events, such as myocardial infarction (MI) or sudden death. Arad et al6 followed self-referred asymptomatic individuals for a mean period of 19 months and showed that the CT calcium score was a predictor of coronary events.
No clear link has been established between calcification and plaque rupture, an important cause of cardiac events. Furthermore, studies of CT calcium assessment have had a relatively limited follow-up period, and they have relied primarily on self-selection of patients (which can introduce selection bias). Therefore, some of the issues regarding CT calcium assessment will remain unresolved until a large clinical outcome trial is performed that is free of selection bias and allows for a follow-up period of several years.
It has been proposed that calcium assessment may be most useful in two groups: asymptomatic individuals at high risk for CAD and patients who present with atypical chest pain.7 If the CT scan showed minimal or no coronary calcification, significant atherosclerosis would be unlikely. If the scan demonstrated moderate or severe calcification, however, the patient could be advised to reduce cardiac risk if asymptomatic or be referred for further testing if symptomatic.
References
1. Rumberger JA, et al. Coronary artery calcium area by electron-beam computed tomography and coronary atherosclerotic plaque area. A histopathologic correlative study. Circulation 1995;92:2157-2162.
2. Agatston AS, et al. Quantification of coronary artery calcium using ultrafast computed tomography. J Am Coll Cardiol 1990;15:827-832.
3. Breen JF, et al. Coronary artery calcification detected with ultrafast CT as an indication of coronary artery disease. Radiology 1992;185:435-439.
4. Budoff MJ, et al. Ultrafast computed tomography as a diagnostic modality in the detection of coronary artery disease: A multicenter study. Circulation 1996;93:898-904.
5. Detrano R, et al. Prognostic value of coronary calcification and angiographic stenoses in patients undergoing coronary angiography. J Am Coll Cardiol 1996;27:285-290.
6. Arad Y, et al. Predictive value of electron beam computed tomography of the coronary arteries. 19-month follow-up of 1173 asymptomatic subjects. Circulation 1996;93:1951-1953.
7. Rumberger JA, et al. Electron beam computed tomography and coronary artery disease: Scanning for coronary artery calcification. Mayo Clin Proc 1996;71:369-377.
Coronary calcium assessment by electron beam CT:
a. has a high positive predictive value for angiographically detected coronary stenoses.
b. corresponds closely to calcification on histopathologic exam.
c. can predict plaque rupture.
d. is substantially equivalent to helical CT with ECG triggering.
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