Hsu K, et al. - This research investigates the hypothesis that the empiric protocol more consistently produces diagnostic quality images of both the pulmonary arteries and the aorta with lower radiation exposure. Compared to a timing bolus protocol, a simple empiric timing protocol for CT pulmonary angiography has robust performance. It was demonstrated that empiric timing preserves the required high diagnostic quality for assessment of the pulmonary arteries with the added benefits of aortic enhancement and lower radiation exposure.