Computed tomography (CT) is a technique that can fully evaluate both cardiac structure and function. Recent advances in imaging allow for evaluation of not only relatively stationary anatomy, such as the thoracic aorta, but also rapidly moving structures, such as the myocardium and coronary arteries. This imparts the ability to noninvasively evaluate for significant coronary artery disease (CAD), myocardial and pericardial abnormalities, and aortic pathology. When combined with electrocardiographic (ECG) gating, freeze-frame images of the heart can be obtained, obviating most of the motion artifact. This is particularly important in contrast-enhanced CT angiography of the coronary arteries and quantifying coronary artery calcium. The advances in spatial and temporal resolution and image reconstruction software have also helped in the evaluation of cardiac structures including coronary veins, saphenous vein grafts, atria, ventricles, and pulmonary arteries and veins with precise definition of their spatial relationships for the comprehensive assessment of a variety of cardiovascular disease processes. This chapter details the current and future role of cardiac CT for the assessment of cardiovascular physiology and pathology.
Multidetector Computed Tomography
Advancements in CT technology have made it possible to noninvasively image the beating heart. Multidetector CT (MDCT) scanners produce images by rotating an x-ray tube around a circular gantry through which the patient advances on a moving table. Improvements in gantry rotation speeds and the development of partial reconstruction algorithms have reduced effective single-image acquisition time to <200 ms. The coronary arteries move independently throughout the cardiac cycle and even at slow heart rates exhibit significant translational motion of up to 60 mm/s for the right coronary artery (RCA) and 20 to 40 mm/s for the left anterior descending (LAD) and circumflex coronary arteries (Fig. 22–1).1,2 Image acquisition <50 ms is truly required to completely avoid cardiac motion artifacts.1,3
Coronary artery velocity varies substantially throughout the cardiac cycle, depending on whether the heart rate is relatively slow (72 beats/min) (A) or fast (89 beats/min) (B). The greatest motion occurs in the right coronary artery (RCA), followed by the left circumflex (LCX) and left anterior descending (LAD) coronary arteries. A. A biphasic pattern of rest is found during end-systole (at 40%-50% of the R-R interval) and mid-diastole (at 70%-80% of the R-R interval). B. A monophasic rest period pattern was found near end-systole (at 40%-60% of the R-R interval). Reproduced with permission from Lu et al.1
With MDCT systems, temporal resolution may be further improved by selecting specific partial image sector data from different heartbeats and detector rings to reconstruct a complete 240-degree image data set. With retrospective gating, several thousand images can be acquired during a single cardiac study, allowing one to pick and choose images with the least amount of motion-related distortion prior to final image reconstruction.
The typical radiation exposure from an electron-beam CT (EBCT) study is <1.0 rad,4 whereas MDCT scanners using retrospective gating can increase exposure approximately 13-fold.5 Prospective gating during either spiral or nonspiral acquisitions uses ECG-gated image triggering only at specific temporal locations in the cardiac cycle, thereby significantly reducing radiation exposure to <3 mSv, with reports as low as <1 mSv. Gating works relatively well at slow heart rates (ie, <60 beats/min), where the R-R interval is >1000 ms and the fastest imaging protocols are used. However, ...