Techniques and Normal Anatomy : Computed Tomography

Computed Tomography

Cardiac computed tomographic angiography has undergone a revolution over the past decade. Owing to improved detec-tors, increased detector rows, and decreased scan times, breath-hold imaging of the heart can now be performed in many cases without pharmaceutical intervention (Figure 3-9). The major indications for cardiac CT are to evaluate the coronary arteries in subjects with indeterminate nuclear stress tests, to characterize or confirm coronary or cardiac anomalies, to assess location and patency of bypass grafts, and, in some cases, to assess for the presence of atherosclerotic dis-ease in subjects presenting to the emergency department with atypical chest pain. The last is often performed with an ex-tended coverage of the chest to concomitantly evaluate for pul-monary embolism and aortic dissection (triple rule-out). At the present time, some physicians also use the measurement of calcium in the coronary arteries detected at unenhanced ECG-gated CT to stratify the risk of future cardiovascular events (Figure 3-10). Contrast administration is mandatory when there are questions related to intrinsic cardiac anatomy or abnormalities of the thoracic aorta such as dissection or for evaluation of the pulmonary arteries for pulmonary em-bolism. For many of these applications, rapid administration of contrast is necessary (up to 4–5 mL per second), and a well-functioning large-bore (at least 18- to 20-gauge) IV catheter must be present to ensure a high-quality study.

The major drawback to CT for cardiac imaging at present is the use of ionizing radiation, which without careful man-agement can be 4 to 5 times higher than for a standard chest CT. Fortunately, many techniques have been developed that lower radiation exposure. These include, but are not limited to, limiting scan distance, pulsing the x-rays to limit exposure during systole, prospectively gating rather than retrospec-tively reconstructing oversampled data, and modulating the x-ray beam based on patient size.