There’s increasing evidence that CT is becoming better at identifying ‘high risk’ features of atherosclerotic plaque in the coronary arteries. In a recent paper by Motoyama and colleagues, over 1000 patients were imaged at baseline - generating a total of 10,000 coronary segments for analysis. These middle-aged subjects (mean age 64) all had CT imaging for known or suspected coronary artery disease. This painstaking effort yielded some worthwhile results. Subjects were followed for an average of 27 months and those with acute coronary syndromes were identified. Two plaque characteristics emerged that were associated with an adverse outcome - the presence of positive outward arterial remodelling (PR) and an area of plaque with low attenuation (LAP). Cut-off values for these two features were based on previous studies that validated cardiac CT against IVUS.
Although the absolute number of events was very small (just 15 in the entire cohort), the presence of PR and LAP increased the hazard ratio for an event to over 22. Of the 1059 subjects imaged, only 45 had both high-risk features in the same plaque; these subjects had a 22% chance of an event during the next 2 years. Compare this to the <0.5% event rate in those with plaques without these two features. Another fascinating conclusion was that those patients harboring plaques with the largest LAP or RP volumes had earlier ACS events. Reassuringly, no patient identified as having zero plaque disease by CT developed subsequent ACS during the follow-up period.
Additionally, there is evidence that ANY degree of coronary atherosclerosis on cardiac CT bestows an adverse prognosis. On the contrary, if cardiac CT reveals no atherosclerosis, the 1 and 3 year event rates are as low as 0.5% and 1.2% respectively (Matsumoto N, et al Circ J. 2007;71(12):1898-1903. Pundziute G, et al JACC 2007;49(1):62-70).
How does this translate into practical approaches to CT, and especially guidelines for the appropriate use of the technology?
It’s clear that with the rapid development of CT hardware, combined with more intuitive plaque characterization algorithms that the use of CT will likely continue to increase. Non-invasive monitoring of plaque volume over time by CT is already possible, published by Burgstahler in 2007, where statin therapy caused a reduction in plaque volume of approaching 25%. We are likely to see increased use being made of this technology by pharmaceutical companies searching for novel surrogate markers of drug efficacy. As ever, the radiation dose is the Achilles heel of the technology, although the CT vendors are tackling this with the current generation of machines.