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Fig. 4 —Mean attenuation measurements on MDCT angiography (MDCTA) at various points of pulmonary artery, heart, and aorta. From top to bottom, protocol groups are chest pain, dissection, gated, and pulmonary embolism (PE). Although locations are physiologically aligned on x-axis, scanning could not follow temporal sequence of blood circulation. Thus, distal thoracic aorta, which is physiologically last point in temporal sequence of contrast passage, was scanned first in caudal–cranial scanning mode. Considering this mode of scanning, mean values of distal thoracic aorta show that patients were scanned earliest in PE group (allowing for scanning early in the pulmonary circulation) and latest in chest pain group, as indicated by higher (later occurrence) attenuation value chosen to trigger scan. Dissection group, being in-between, allowed for a more physiologic following of IV contrast circulation at in-between time interval to produce satisfactory attenuation in both pulmonary arteries and aorta. Intravascular contrast level was sustained in gated protocol by lower injection rate. RV = right ventricle, PA = main pulmonary artery, PA/L = distal pulmonary artery to right lower lobe, PA/U = distal pulmonary artery to right upper lobe, LV = left ventricle, Ao/As = ascending aorta, Ao/Ar = aortic arch, Ao/De = mid descending thoracic aorta, Ao/Di = distal thoracic aorta.