Medicine is a science of uncertainty and an art of probability -Osler

CT detection
Atherosclerotic heart disease is the number one cause of death. Methods of detecting coronary artery disease prior to fatal events are needed so that appropriate measures can be taken to reduce risk. Anatomic studies have established that coronary calcification is invariably located near areas of advanced atherosclerotic disease. A direct relation between the extent of coronary calcification and the severity of stenotic lesions or frequency of myocardial infarction is consistently observed in autopsy series. The more extensive the calcification, the more frequent and more severe the degree of stenosis. This relationship is recognized in all age groups and both sexes, but is more marked in younger patients.

CT and in particular, electron-beam CT (EBCT) is the most sensitive radiographic method to detect coronary artery calcification. The value of EBCT can be summarized as follows:

 
Absence of Detectable Coronary Artery Calcification EBCT

• Does not absolutely rule out the presence of atherosclerotic plaque, including unstable plaque
• Highly unlikely in the presence of significant luminal obstructive disease
• Observation made in the majority of patients who have had both angiographically normal coronary arteries and EBCT scanning
• Testing is gender independent
• May be consistent with a low risk of a cardiovascular event in the next 2-5 years 

Presence of Detectable Coronary Artery Calcification EBCT

• Confirms the presence of coronary atherosclerotic plaque
• The greater the amount of calcification (i.e., calcium area or calcium score), the greater the likelihood of obstructive disease, but there is no one-to-one relation, and findings may not be site specific
• Total amount of calcification correlates best with total amount of atherosclerotic plaque, although the true "plaque burden" is underestimated
• A high calcium score may be consistent with moderate to high risk of a cardiovascular event within the next 2-5 year 

Electron Beam CT (EBCT) Protocol

• Imatron Ultrafast CT
• No contrast
• 3 mm slice thickness
• High resolution volume mode
• 100 ms scan time
• EKG gating, triggered at 80% of the RR interval
• Breath hold
• Supine position
• Radiation exposure < 500 mrem
• Total procedure time 10 min

Coronary calcification score (Agatston)

• Threshold CT density > 130 HU for pixel areas > 1mm2
• Lesion Score 1 = 130 - 199, 2 = 200 - 299, 3 = 300 - 399, 4 > 400
• Score each region of interest by multiplying the density score and the area
• Total coronary calcium score determined by adding up each lesion score for all sequential slices

Coronary Artery Anatomy
Quicktime Movies
 Normal Cardiac Cine
 Coronary Artery Calcification

Calculate the Probability of Coronary Artery Disease

Select age, gender, patient presentation, coronary artery calcification at CT

Prediction of Coronary Heart Disease
For the prior probability of coronary artery disease, the above model uses age, gender and clinical presentation. Other predictive models could be used for prior probability. Based on data from the Framingham Study, predictive models have been derived from the blood pressure, total cholesterol, LDL cholesterol, and HDL cholesterol, diabetes, smoking, gender and age. To calculate risk based on these models choose one of the following.

• Male, Total cholesterol category
• Female, Total cholesterol category
• Male, LDL-C category
• Female, LDL-C category

Likelihood ratios 

From 6 participating centers, EBCT used in 709 patients (427 with angiographic significant disease). Likelihood ratios (95% confidence interval) calculated from data provided in Table 2.

Note that in older subjects, the presence of calcium does not raise the likelihood of significant coronary artery disease as much as it does in younger patients. Conversely note that the absence of calcium markedly decreases the likelihood of coronary artery disease in the older patient population.