Medical Device Daily Washington Editor
SAN FRANCISCO — Clinicians are keenly aware of the cost of acute coronary events triggered by the rupture of vulnerable plaques, but imaging these lesions by methods such as intravascular ultrasound are a bit more invasive than some would like, and the more expensive imaging modalities, such as SPECT, have their own obvious drawbacks. Hence, CT imaging of the coronary arteries is still of great interest in detecting these plaques.
A debate yesterday at TCT 2009 took on that very issue, and the discussion made clear that not everyone is convinced that axially projected X-rays are the answer to the need for a cost-effective and readily available imaging source to distinguish between plaques that are prone to rupture and their more stable brethren.
Taking the position of skeptic, Stephen Achenbach, MD, of the University of Erlangen (Erlangen, Germany) said "we live in an era of pretty pictures" in reference to the variety of imaging technologies, most of which offer sufficient resolution that even small plaques can be detected.
"CT can to some extent" determine the degree of calcification, Achenbach stated, and can often map the dimensions of a plaque. Some imaging studies of CT versus intravascular ultrasound (IVUS) show a strong similarity in the degree of accuracy, but that degree of similarity does not hold up across all the studies in the published literature.
"Tissue characterization is a theoretical possibility" with multi-slice CT, Achenbach observed, and several studies indicate that the latest CT machines can distinguishing between the vulnerable, lipid-rich plaques and their more stable fibrous kin, but "there are a tremendous number of influences" on CT's ability to detect plaque, Achenbach noted. Among these is the selection of a contrast agent, but a patient's body mass is also a confounder inasmuch as X-rays tend to be attenuated by an abundance of tissue between the skin and the heart. He said that the literature also suggests a lot of variance in CT's depiction of plaque density.
Achenbach took on a topic that is often tackled by the Centers for Medicare & Medicaid Services in its coverage decisions, which is basically whether a diagnostic procedure will affect a physician's treatment recommendations. "There are six or seven studies that have looked at lesions post hoc," he said, and the uncertainty around whether the detected plaques called for treatment diluted whatever value the CT images might have offered. He said that among the several recent studies that attempted to determine the prognostic power of CT detection of vulnerable plaques is one by Motoyama, et al that appeared in the Journal of the American College of Cardiology in 2007.
This study enrolled 38 patients with acute coronary syndrome and another 33 with stable angina to undergo 16-slice and/or 64-slice CT to determine whether differences in the lesion sets suggested anything about a correlation between ACS and vulnerable plaques. The study detected no difference in non-calcified plaques with densities between 30 and 150 Hounsfield units, although large calcifications were substantially more common in the angina patients. Of this study, Achenbach said that CT can detect more non-calcified components than stable lesions, but the data don't seem to offer predictive power.
Achenbach also made the case that prospective trials to determine the value of CT angiography tend to be costly considering the benefit, citing the article appearing June 30 in JACC by Motoyama, et al. He stated that this study disclosed that 35 vulnerable plaques caused no events.
Hence, "lesion-specific predictions of cardiovascular events" are still difficult to obtain partly because of lack of data regarding the reliability of non-invasive imaging equipment to render such predictions, Achenbach stated. "I think we need much more information before we can have" any confidence in CT imaging, he concluded.
Arguing for a more positive view of CT's usefulness in CT angiography was Norman Lepor, MD, of the Geffen-UCLA School of Medicine (Los Angeles), who acknowledged at the outset of his talk that he and Achenbach would "probably ... end up at the same point" of view.
Lepor noted that multi-slice CT has been around for only about five years, but remarked that the technology has generated a raft of data, thanks in part to spatial resolutions of as little as 0.4mm.
As for plaque analysis, Lepor said, "we can use CT to identify the architecture" and volume of the plaque, and stress CT with adenosine will describe the plaque's effect on coronary artery flow. "We may also get some information on pathobiology" via CT, he said.
CT imaging of non-calcified plaques can detect lesions that might be missed by other modalities, Lepor asserted, adding that the latest CT technology enhances a physician's "ability to image patients with chronic total occlusion." He insisted further that "there are certainly reasonable data" backing the idea that CT can prognosticate plaque rupture and consequent events.
"It's clear that CT has the ability to define a high negative predictive value," Lepor stated, tacitly acknowledging that the positive predictive value is lacking. He also asserted that CT could help direct therapy as well as the physiology of disruptive disease. "We need prospective follow-up" to establish the value of CT, Lepor concluded.
Mark McCarty, 703-966-3694;