Unilateral distribution of contrast agent injected into the main pulmonary trunk in the first cardiac cycle after injection.Even if contrast agent is injected into the main pulmonary trunk, it is unilaterally distributed mainly to the left pulmonary artery during the first cardiac cycle after injection. Therefore, accurate measurement of the right-to-left pulmonary blood flow ratio with our model should be performed over 50–90% of the R-R interval during the retention period.