Advanced Cardiovascular CT Imaging

Technical Prerequisites

A priori, a high temporal resolution of the CT system is necessary to reduce motion artifacts during the cardiac cycle and ensure adequate partial volume correction. With the advent of 64-slice CT systems, temporal resolution (165 milliseconds) improved enough to allow for CTMPI in patients with low heart rates. Only with further technical advances, such as the introduction of dual-source CT (temporal resolution: first generation, 83 ms; second generation, 75 ms; third generation, 66 ms), has the scanning of individuals with higher heart rates become feasible. Development of 320-slice CT systems also allowed for cardiac image acquisition within a single heartbeat, but the moderate temporal resolution of 175 milliseconds impeded scanning at higher heart rates. Furthermore, high spatial resolution is essential to accurately assess and differentiate myocardial perfusion of the subepicardial and subendocardial layers. In addition, a motion correction algorithm is necessary to compensate for artifacts due to respiratory movements, since the required breath-hold periods are generally not realizable in clinical routine.

Measurement Principles

Testing for myocardial ischemia by means of CTMPI benefits from the principle that contrast material distribution serves as an indicator of myocardial blood flow. As opposed to normal myocardium, areas of ischemia show delayed distribution and decreased peak contrast material enhancement, while the washout is normal. In the presence of infarcted myocardium, delayed contrast material distribution, decreased peak attenuation during first pass, and slow washout can be observed. Areas of myocardium with reduced contrast material attenuation represent a perfusion defect.

Similar to cardiac stress testing in nuclear medicine, CTMPI protocols typically include acquisitions at rest and under stress conditions pharmacologically induced by vasodilative substances such as adenosine, regadenoson, dobutamine, dipyridamole, and adenosine triphosphate. The underlying reason is that reversible myocardial perfusion defects occur prior to fixed ones, and the addition of stress testing is expected to increase the sensitivity of CTMPI. Healthy coronary arteries respond to pharmacologic stress with increased perfusion and induction of hyperemic myocardium. Coronary arteries with obstructive disease have limited perfusion capacity because they are already operating within dilatory reserve to compensate for reduced perfusion.

The chronological order of the rest and stress acquisitions is variable. Residual contrast material from the rest acquisition may limit the diagnostic conclusiveness of subsequent stress imaging for detection of ischemic myocardium. In theory, when the stress acquisition is performed first, the myocardium is not contaminated with previously administered contrast material. However, continuous elevation of the heart rate may impair the quality of a subsequent rest acquisition. Furthermore, if the rest acquisition is conducted first, a cCTA study with high negative predictive value (NPV) is effectively supplied. We advocate the strategy of an initial rest acquisition because the rest acquisition allows ruling out obstructive coronary artery disease. Therefore exposure of patients without obstructive coronary artery disease to a second dose of radiation can be avoided because these do not require an additional stress test. Meinel et al. recently investigated the contributions of rest, stress, and delayed enhancement at CTMPI and concluded that rest-stress acquisition should be the protocol of choice for an assessment of myocardial blood supply.