Cardiovascular CT Poised to Take Imaging Center Stage
CT Fractional Flow Reserve
Fractional flow reserve (FFR) is defined as the maximal blood flow through a diseased artery to the blood flow in the hypothetical case that this artery is normal. CT-based FFR relies on the use of some complicated math and modeling to calculate FFR from CT data and actually traces its roots back to the air flow modeling that goes into designing airplane wings.
All of the data needed can be gleaned from a single-phase static CT image. Geometry can be extracted from CCTA anatomic data. Boundary conditions can also be determined. Resting coronary blood flow can be calculated from myocardial mass. Mean blood pressure can be estimated from brachial artery pressure. Coronary microcirculatory resistance can be derived from morphometry data. Lastly, fluid properties include the viscosity and density of blood. "Putting it all together, we can come out with a calculated assessment of things like coronary flow and FFR ... now, we can do anatomy and function," said Dr. Matthew J. Budoff, director of cardiology at the University of California at Los Angeles School of Medicine in Torrance.
Until now, FFR was determined through invasive coronary angiography, which was the only method for specific determination of coronary artery lesions (lesion-specific ischemia). Values of 0.80 and lower or 0.75 and lower are considered diagnostic of lesion-specific ischemia. "It tells us whether there is a physiologic significance of the lesion," Dr. Budoff said.
At the same time, stenosis seen on CCTA has been an unreliable predictor of lesion-specific ischemia in trials. "Just seeing that anatomical stenosis doesn’t mean it’s functionally significant," said Dr. Budoff. CCTA results in a lot of false positives when it comes to physiologic significance.
FFR estimates throughout the entire coronary tree. "Because it relies on the entire coronary anatomy, it’s actually less sensitive to artifacts and less sensitive to things like calcification than individual segment assessment."
The first trial of FFR-CT was in the Diagnosis of Ischemia-Causing Stenoses Obtained via Noninvasive Fractional Flow Reserve (DISCOVER-FLOW) trial. In the study, the researchers compared the accuracy of CCTA with FFR-CT (invasive FFR served as the reference).
The results showed that the coronary stenoses that cause ischemia can be identified noninvasively with computer analysis of CCTA to construct FFR for specific lesions (Eur. Heart J. Cardiovasc. Imaging 2012 Jul 15 [doi: 10.1093/ehjci/jes130]).
The upshot of this study is that "when we think it’s a high-grade stenosis [with FFR-CT], it is a physiologically significant stenosis," Dr. Budoff said. "So we’ve dramatically improved the ability of CT to correlate with the most invasive and probably the most definitive way, currently, of assessing physiologic significance, and that’s FFR."
CT-based fractional flow reserve (FFR) is now a step closer to clinical use with the results of the Determination of FFR by Anatomic CT Angiography (DeFACTO) trial presented at the ESC meeting and simultaneously published (JAMA 2012 Aug. 26 [doi:10.1001/2012.jama.11274]). The addition of CT-based FFR information to CT alone improved diagnostic accuracy of stenoses, compared with invasive angiography and FFR.
"We’ve dramatically improved the ability of CT to correlate with ... FFR," said Dr. Matthew J. Budoff.
Although FFR-CT plus CT narrowly failed to meet the trial’s primary end point – diagnostic accuracy greater than 70% for the lower bound of the 95% confidence interval, the per-patient performance diagnostic accuracy of FFR-CT plus CT was 73% with a 95% CI of 67%-78%.
Nevertheless, the addition of FFR-CT "demonstrated superior diagnostic performance characteristics, as compared with CT stenosis alone in all patients, in all vessels and also in vessels of intermediate stenosis severity," Dr. Min said during a press conference.
