Author + information
- Frank J. Rybicki, MD, PhD⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Frank J. Rybicki, Cardiac CT, Applied Imaging Science Laboratory, Brigham and Women's Hospital, 75 Francis Street, Boston, Massachusetts 02115
Multidetector computed tomography (MDCT) technology has rapidly evolved, and coronary imaging is largely responsible for current computed tomography (CT) hardware releases that have extended beyond “64-generation” scanners (1) to offer superior temporal resolution, volume coverage, and lower patient radiation exposure. Because of limitations largely related to spatial resolution, the aforementioned technical parameters, and reimbursement, clinical use of CT has been conservative. This contributes to the challenge in accumulating and acting on “current” clinical evidence; “post-64” technology hardware releases can outpace our ability to study large patient populations with a single imaging platform.
Many cardiac CT studies to date have focused on coronary morphology, specifically the morphology of the coronary lumen determined by CT with catheterization data as reference. Although technology advances have delivered incremental improvement in test characteristics for CT, many patients with a 50% stenosis in a coronary artery require additional information, such as from single-photon emission CT, to extend beyond the evaluation of the lumen alone.
The CT research has already begun to extend beyond the morphology of the lumen. One step from the lumen is the coronary artery wall, where plaque components can be classified by Hounsfield unit (HU) groups. The next step studies the myocardium itself, including the relatively recent introduction of pharmacological stress CT that can be used to test the hypothesis that CT perfusion data correlate with more-established methods to determine the hemodynamic significance of coronary lumen narrowing.
As new CT technology enables studies to gravitate from the lumen to the myocardium, it remains important to recognize that the “A” in CTA is for “angiography” that, in turn, is determined by the kinetics of the iodinated contrast media imaged within the coronary arteries. Although less comprehensively studied than lumen stenoses, as pointed out by Chow et al. (2) in this issue of the Journal, properties of coronary lumen enhancement likely hold far more information than currently reported clinically. Differences in proximal versus distal coronary artery opacification have been recognized for wide-area detector CT, where the entire coronary tree is evaluated at one point in time within a single heartbeat (3). The theoretical advantage of this 320-detector row approach with respect to enhancement studies over multiple heartbeats is that the latter has more complex kinetics as contrast enters and leaves the coronary arteries during the multi-heartbeat acquisition.
Coronary contrast opacification gradients (4) are defined as coronary HU differences divided by a property of the coronary artery such as distance from the ostium or the coronary short-axis diameter. Gradients are inherently more complex than opacification differences, because they are normalized—for example, the distance gradient (in units of HU/cm) across a single lesion in the left anterior descending artery is calculated by dividing the HU difference across the lesion by the length of the lesion in question. Preliminary work suggests that gradients across lesions are larger than across normal coronary arteries (4), in theory providing additional contrast-based information beyond the morphology of lumen stenoses.
The work by Chow et al. (2) adds 2 new important components to advanced studies of coronary enhancement. By focusing on coronary HU differences, the group provides a new normalization, with the enhancement of the aorta. This logical approach is important for normalizing scans over multiple heartbeats. Moreover, for future applications where differences and/or gradients are studied in stress CT with high heart rates (e.g., >65 to 70 beats/min), this normalization step might be critical. The second novel, significant aspect of this study is the correlation between coronary contrast opacification differences and abnormal (Thrombolysis In Myocardial Infarction flow grade <3) resting coronary flow. As noted by the authors, coronary CTA estimates of coronary blood flow and new algorithms to assess for functional coronary stenoses are highly desirable, and this work strongly supports future studies to maximize information within the lumen from CT angiograms.
Coronary imaging will define the next generation of “state-of-the-art” in CT, and future generations of hardware technology are likely to change the way we think about image acquisition. While we collectively study these gains and new applications, it is essential to recognize, optimize, and study the properties of coronary artery enhancement to maximize our understanding of CT angiography and optimally use it for patient care.
Dr. Rybicki has research grants with Toshiba Medical Systems Corporation, Bracco Diagnostics, and Vital Images.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
- American College of Cardiology Foundation
- Chow B.J.W.,
- Kass M.,
- Gagné O.,
- et al.
- Steigner M.L.,
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- et al.