CT (computed tomography) has been used for over 15 years to determine heart attack risk by measuring the amount of calcium in the coronary arteries, an indicator of the presence of atherosclerotic plaques.

Coronary calcium is used only as a noninvasive measure of risk, kind of like blood pressure and cholesterol. It gives no information on narrowing of coronary arteries like conventional angiography or CT angiography.

Now new, super-fast CT machines provide three-dimensional images that are almost as clear as those provided by conventional coronary angiography to detect the presence of plaques and narrowings in the coronary arteries.

More centers around the country are purchasing the new $1.5 million, 64-slice, CT scanners and promoting CT angiograms. But are the benefits from CT angiography worth the costs to patients?

Compared with CT angiography, conventional angiography is more expensive and more invasive — it involves injecting a contrast material into a catheter that is inserted into an artery in the groin and threaded into the coronary arteries.

At a cost of about $1,000 per procedure, CT angiography is cheaper than conventional angiography but it has several serious drawbacks. One is that the patient is exposed to about three to four times more radiation than with a conventional coronary angiogram.

Because it is noninvasive, a CT angiogram may look tempting to people who want to learn whether they have dangerous heart disease. And a completely normal CT angiogram will certainly let the person know that he or she is highly unlikely to have significant narrowing of the coronary arteries.

Unfortunately, a second drawback is that the CT may also detect small plaques or other unimportant arterial abnormalities, which may in turn lead to further tests that are unnecessary, expensive, and possibly invasive. With further technical improvements, however, CT angiography may one day replace the invasive and more expensive conventional coronary angiography.

For these reasons, CT angiograms are not recommended for worried people who have no symptoms of heart disease but wonder about possible dangers lurking in their arteries.

But if someone has chest pains or other heart disease symptoms, or an inconclusive exercise stress test, then he or she may be a suitable candidate for CT angiography. A normal CT scan would eliminate the need to carry out further tests.