The agreement and operating characteristics of corrected coronary opacification (CCO) differences in identifying abnormal resting coronary flow seem to be very good and indicate its potential role in the assessment of functional coronary stenosis and measurement of stress coronary flow, according to a proof-of-concept study published in the March 15 issue of the Journal of the American College of Cardiology (JACC).
The utility of CT angiography (CTA) is somewhat compromised by its limited ability to predict the hemodynamic significance of coronary stenosis, and cardiac imaging stakeholders are interested in the development of “a single imaging modality with the ability to evaluate for both anatomical and functional stenosis,” according to Benjamin J.W. Chow, MD, of the division of cardiology at the University of Ottawa Heart Institute in Ontario, and colleagues.
Chow and colleagues hypothesized that partial correction of the lack of temporal uniformity in 64-slice CTA images may provide a pathway to measure CCO to indirectly estimate coronary blood flow.
The proof-of-concept study entailed a retrospective review of obstructive and nonobstructive vessels of 52 consecutive patients who had presented for routine CTA and subsequent invasive coronary angiography.
An observer measured coronary luminal attenuation values in Hounsfield units (HU) in normal arteries and those with obstructive CAD ( >50 percent diameter stenosis), explained Chow and colleagues. The researchers then corrected for known normal variation in opacification of the coronary arteries by normalizing each coronary luminal attenuation measurement to the descending aorta on the same slice prior to calculating CCO variability. Invasive coronary angiography images provided data about the percentage diameter stenosis of each artery and Thrombolysis In Myocardial Infarction (TIMI) flow grade and frame count for each artery of interest.
Chow and colleagues reported mean CCO of 0.979 in normal arteries and determined that a pre- and post-stenosis CCO difference of more than 0.184 was abnormal, testing the threshold in arteries with varying degrees of stenosis, TIMI flow grade and TIMI frame count.
When they assessed arteries with obstructive CAD, they found the CCO difference was significantly greater (p = 0.004). They noted that “the proportion of abnormal CCO differences increased with worsening diameter stenosis.” The researchers additionally found that CCO differences in arteries with TIMI flow grade <3 were significantly higher than those with normal TIMI flow grade 3 and normal arteries.
Consequently, Chow and colleagues indicated that CCO differences may be employed to identify abnormal (TIMI flow grade <3) resting coronary flow. The researchers conjectured that the method might be applied to detect abnormal flow in patients with unevaluable segments, or it could be employed in conjunction with pharmacologic stress to help measure functional stenosis.
The researchers emphasized the need for additional study and validation of the findings in larger cohorts. Furthermore, they explained that research to understand the incremental value and potential utility of CCO difference in conjunction with CT stress imaging is needed.
In an accompanying JACC commentary, Frank J. Rybicki, MD, PhD, of the noninvasive cardiovascular imaging program at Brigham and Women’s Hospital and Harvard Medical School in Boston, reiterated the value of the study.
Rybicki also highlighted two important components related to the Canadian study. Chow and colleagues provided a new normalization that can be used in future applications by focusing on coronary HU differences. In addition, “this work strongly supports future studies to maximize information within the lumen from CT angiograms” by offering CCO differences as a potential new algorithm to assess for functional coronary stenosis.