Standard state-of-the-art CT imaging systems, although much faster than their earlier counterparts, still have limited temporal resolution for the beating heart. Therefore, investigations of intracardiac structures‚ except within the atria, pericardial space, or aorta‚ are limited. CT has been useful for evaluating suspected constrictive pericarditis or other pericardial diseases (determining pericardial thickness) and intracardiac thrombi (usually in the pulmonary arteries or atria, where there are less cardiac motion artifacts). In many centers, CT has become the first-line assessment for abnormalities of the ascending and descending aorta, especially in suspected aortic dissection.
What are the limitations of standard CT imaging?
Cardiac evaluation by CT is primarily limited by insufficient temporal resolution to allow stop-frame assessment of cardiac structures, therefore limiting evaluations of LV chamber thickness, contraction, and intracardiac structures.
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Additionally, this technique requires administration of an iodinated contrast agent to opacify the intravascular regions and to identify abnormalities within these structures. These contrast agents can provoke adverse reactions. Recently, the introduction of Fastrak Rapid CT (Picker International) provides assessment of multiple cardiac slices within 10-20 cardiac beats at a temporal resolution of approximately 50 ms. Unlike standard CT, this modality has high resolution for intracardiac structures and masses. It also allows bolus contrast injections to study intracardiac transit times and coronary artery bypass graft flow. Unfortunately, rapid CT imaging is not widely available and is of limited general use except for thoracic pathology.
How does CT compare to other imaging modalities for the diagnosis of aortic dissection?
A recent comparison of aortic dissection detected by multiple imaging modalities found that, overall, CT had a 93% sensitivity and 87% specificity for detecting dissecting aortic aneurysm. The difficulty in defining a second lumen and the potential false-positive of artifacts resulting from the streaming of contrast and other issues reduced the sensitivity of standard CT. Overall, the sensitivity and specificity are acceptable for a screening test for aortic dissection. Additionally, CT allows assessment of the ascending aorta as well as aortic arch and descending aorta; however, this evaluation requires administration of intravenous contrast. This potentially renal toxic contrast, when combined with either total ischemic time or cardiopulmonary bypass pump use at surgical dissection repair, can induce acute and prolonged renal failure postoperatively.