A typical coronary angiogram performed as part of a catheterization exhibits high temporal and spatial resolution but can image only the lumen of the artery. The presence of plaque is inferred from stenotic filling defects which encroach upon the lumen.
CT angiography, by comparison, can image not only the contrast in the arterial lumen but also any plaque which may be present. Furthermore, the plaque can be characterized by its component parts: calcific, fibrotic, and lipid-rich.
CT can image arteries not only in long axis, as is done in invasive
angiography, but also in cross-section, similar to intracoronary
ultrasound. The difference in the two techniques is that intracoronary ultrasound has to be performed at specific sites. With CT angiography, the reading physician can re-create the cross-sectional view anywhere within the coronary tree.
This is a traditional long-axis view of an artery with typical mixed atherosclerotic plaque:
One aspect of this image which is noteworthy is that the plaque on the bottom surface of the artery actually extends out beyond the wall of the vessel, a phenomenon known as positive remodeling, yet another aspect of arterial plaque which can rarely be appreciated with traditional angiography.
This is the same plaque seen in a cross-sectional image, a view which would require intracoronary ultrasound in the catheterization laboratory:
The arterial lumen occupies the upper half of the vessel and the mixed plaque the lower half. CT allows an approximate characterization of the components of plaque based on their differential X-ray attenuation (measured in Hounsfeld units, HU). Below is the same image with samples of the attenuation in each area:
The mean value of 110 HU is typical of fibrotic plaque and the mean value of 464 HU is typical of calcific plaque, substantially greater than the contrast-enhanced lumen at 375 HU.
Plaque which is unusually susceptible to rupture is usually characterized as having a lipid-rich core, a thin fibrous cap, positive remodeling, and active inflammation. CT angiography can identify lipid-rich plaque because of its low attenuation and positive remodeling. It does not have sufficient spatial resolution to measure cap thickness nor can it assess inflammatory changes. The most that can be said at this time is that CT can identify plaque which may be vulnerable.
The following lesion has an unusually large area with low attenuation (-66 HU) and substantial positive remodeling but not hemodynamically significant stenosis:
One would expect such a patient to be on extremely aggressive lipid-lowering therapy. Until tests are available which better identify truly vulnerable plaque, there is currently no data to support interventional therapy in such patients.