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.
A few years ago we discussed the "ultrafast" CT scan (also called the electron beam CT scan, or EBCT scan), and considered the appropriate use and the limitations of this popular test. In summary, EBCT scans are less useful for actually creating an accurate image of the coronary arteries than they are for creating a "calcium score," which reflects the degree of coronary artery disease a person has. (When calcium deposits are present in the lining of the coronary arteries - a feature readily detected by ultrafast CT scans - that is a clear indication that plaques are present in the coronary arteries. The more calcium, the more coronary artery disease.) However, while the calcium score can be important, EBCT scans have not been particularly 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 imagers have been fitted with rotating gantries that allow much better visual resolution of the coronary arteries. These modern CT scans - called multislice CT scans, or MSCT scans - often allow remarkably accurate imaging of the coronary arteries, 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 EBCT 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 (which is required for either EBCT or MSCT scans.) 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 a fair amount of radiation. Radiation doses with MSCT scans are 30 - 50 times higher than with a chest x-ray, 5 - 10 times higher than with EBCT scans, and approximately the same as with a cardiac catheterization.
The bottom line
While the images obtained with MSCT scans are far better than were ever possible with EBCT scans, they are still not sufficient to serve as a replacement for a catheter procedure, and thus are not suitable for making a definitive anatomic diagnosis of 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.
