Arterial input functions (AIFs) measured from dynamic contrast enhanced (DCE) MRI following low dose (0.015 mmol/kg) contrast media were compared with AIFs from DCE CT as ‘gold standard’. Twenty prostate cancer patients received CT and MRI scans on the same day. To correct for different temporal resolution and sampling periods, an empirical mathematical model was used to fit the AIFs and calculate numerical AIFs. Convolution was performed to correct for differences in CT and MRI injection times (~1.5s vs. 30s). MRI and CT AIFs were very similar. Therefore, AIFs can be accurately measured by MRI following low dose contrast agent injection.