Aerodynamics in Cardiac CT.

I t seems not long ago that invasive coronary angiography was considered as the undisputed diagnostic standard and management determining test for patients with ischemic heart disease. Registry data have shown that reliance on noninvasive testing for myocardial ischemia, positioned as a gatekeeper to restrict referral to the catheterization laboratory, is far from optimal in clinical practice. 1 Coronary computed tomographic (CT) angiography (CCTA), which was developed and validated during the past decade, allows noninvasive visual confirmation of the presence or absence of coronary artery disease (CAD). Meanwhile the cardiology community has rediscovered an appreciation for functional parameters of CAD, particularly for clinical decision making. Cardiac CT is arguably the most accurate noninvasive technique to exclude CAD, with sensitivities and negative predictive values >95% as reported in meta-analyses. 2 Unfortunately, CCTA has a tendency to overestimate the severity of CAD, particularly in the presence of coronary calcifications. More than ever before, we are aware of the complex and unpredictable relation between angiographic stenosis and myocardial ischemia. Compared with invasive fractional flow reserve (FFR cath), which is currently considered as the clinical standard of lesion-specific hemodynamic significance , both invasive and noninvasive coronary angiography overestimate the functional severity of CAD. 3,4 Current clinical practice guidelines recommend consideration of both anatomic and functional parameters in the treatment of stable CAD. There seems to be no prognostic benefit from revascularizing hemodynamically nonsignificant stenosis, whereas on the other hand, mechanical treatment of myocardial ischemia is difficult in the absence of focal narrowing of the epicardial coronary branches. In that context, a diagnostic modality able to provide both angiographic and functional information during a single, noninvasive examination would be highly desirable. For decades, (computer) simulations of air flow around objects have defined the design of aircrafts and automobiles. Based on the shape of the design and the boundary conditions, air velocity, pressure, and mechanical behavior can be predicted by a computer. Based on similar principles, the developers of CT-based computation of fractional flow reserve (FFR CT) have found a way to estimate the flow and pressure within the coronary arteries and calculate pressure gradients for stenotic lesions during simulated peripheral vasodilation. 5 Although the complex methodology and inherent limitations of these algorithms are fully comprehended by few, several studies have demonstrated that FFR simulations based on CT images are feasible, and that the numbers correlate with FFR Cath. The largest of these technical validation studies …