Since the main diagnostic reason for performing coronary angiography has always been the identification of significant stenoses, it should not be surprising that the bulk of early clinical research efforts with CT coronary angiography has been to assess its ability to identify significant stenoses. Sometimes the clinical correlation between CT angiography and the catheterization are excellent as in the patient illustrated below:
Most clinical images are less striking and less clearcut. The technical limitations of the current generation of CT scanners, however, should make it clear that CT angiography is not a replacement for invasive angiography. As is discussed elsewhere on this site, the main diagnostic strengths of CT imaging are the identification and characterization of plaque and the ability to produce unlimited cross sectional arterial images.
Below are cross-sectional images of the ostium and mid and distal segments of a left main coronary artery with extensive plaque and significant stenosis. Comparable images from catheterization (cath) and intravascular ultrasound (IVUS) are included:
Let us consider the optimal spatial resolution of CT angiography (0.4 mm) as a "unit" and assume that we are making measurements in the range of the coronary arterial tree:
A large proximal artery with a diameter of 4 mm would be 10 units across. If our accuracy is +/- 1 "unit," the best we can do is +/- 10% or a range of 20%. For a vessel with a diameter of 2 mm, our accuracy would be proportionately worse.
For this reason, CT angiograms of the coronary arteries should never report stenoses as specific percentages. CT stenoses are usually reported as some variation of mild (0-40%), moderate or borderline (40-70%), severe or significant (>70%), and occluded.
Although more studies have appeared since the following table was prepared, the basic conclusion has not changed significantly: CT coronary angiography is the best non-invasive test for accurately detecting significant stenoses, but it is not a replacement for traditional angiography in the catheterization laboratory.
The course of anomalous coronary arteries is sometimes difficult to demonstrate in the catherization laboratory because neighboring structures may not be well visualized. The volumetric imaging formats of CT angiography are unusually well suited to demonstrate such arteries.
The VRT image below shows the course of an anomalous circumflex artery which arises from the right coronary artery and runs posterior to the aorta before entering the left A-V groove.
The following VRT image from a CT angiogram demonstrates bilateral coronary artery aneurysms from Kawaski disease:
The following VRT image from a CT angiogram of a middle-aged woman with chest pain demonstrates a very marked myocardial bridge of the LAD artery:
The MPR image from CT and the image from catheterization are below: