Major strides are being made in achieving the Holy Grail of cardiology - namely, a non-invasive means (i.e., without having to do a heart catheterization) of visualizing the coronary arteries.
Various types of CT scanning have proven useful for detecting the amount of calcium that has been deposited in the coronary arteries, which gives a general indication of the amount of atherosclerosis that is present. (Read about calcium scanning here.) But calcium scanning is not useful in revealing whether actual, significant blockages are present in the coronary arteries.
Over the past few years, however, CT technology has advanced significantly. Today, CT imaging machines have been updated to allow much better visual resolution of the coronary arteries. These modern CT scans - called multislice CT scans, or MSCT scans - can not only quantify calcium scores, but also can allow remarkably accurate imaging of the coronary arteries themselves, and often, blockages in the coronary arteries can be seen quite clearly. The quality of the image with MSCT scans is partially related to the number of "slices" taken by the camera. Currently, 16-slice and 64-slice CT imagers are commercially available.
How good are multislice CT scans?
It depends on what you're looking for. If you want to know whether coronary artery disease is present or not, the test is very good. If the test says it cannot find coronary artery disease, then there is a 98 - 99% chance that, in fact, no coronary artery disease is present. (That is, the MSCT scan has a very high negative predictive value, similar to the calcium scans.) If a person has significant blockages in the coronary arteries, there is over a 90% chance the MSCT scan will detect one or more.
However, the MSCT scan cannot visualize the entire coronary artery tree. A recent multi-center study showed that only 71% of the important segments of the coronary arteries could be evaluated by MSCT scan. Other studies have claimed that up to 88% of segments can be evaluated. In any case, if a blockage exists in a coronary artery there is a 12 - 29% chance that the MSCT will miss it.
Limitations of multislice CT scans
To have a MSCT scan, a patient needs to have a resting heart rate that is regular (so, among other things, no atrial fibrillation allowed) and no faster than 60 - 70 beats per minute. The patient must be able to hold his/her breath for at least 15 seconds, and cannot be allergic to contrast dye. Furthermore, the visual resolution of the coronary arteries with MSCT scans will be relatively poor in patients with significant calcium deposits.
With today's technology, MSCT scans expose the patient to quite a bit of radiation. Radiation doses with MSCT scans are 30 - 50 times higher than with a chest x-ray, 5 - 10 times higher than with simple calcium scans, and approximately the same as with a cardiac catheterization.
The bottom line
While the images obtained with MSCT scans have greatly improved the ability of CT scanning to visualize the coronary arteries, they are still not sufficient to serve as a replacement for a catheterization procedure, and thus are not suitable for making a definitive anatomic diagnosis of obstructive coronary artery disease. Should MSCT scans ought to be used as a screening tool for coronary artery disease? Click here for an analysis of the controversy surrounding noninvasive testing for coronary artery disease.
Sources:
Gerber TC, Manning WJ. Noninvasive coronary arteriography with cardiac computed tomography and cardiovascular magnetic resonance. www.uptodate.com, April, 2006 (subscription required)
Garcia MJ, Lessick J, Hoffmann MHK et al. Accuracy of 16-row multidetector computed tomography for the assessment of coronary artery stenosis. JAMA 2006; 296:403-411.
