1. A method for producing a magnetic resonance angiogram of a patient using an MRI system, the steps comprising: a) injecting the patient with a contrast agent which flows into a region of interest in the patient; b) acquiring NMR image data for a series of NMR time course data sets during a time resolved phase of an examination, during which the contrast agent makes a first passage through the patient's arteries in the region of interest; c) reconstructing a corresponding series of image frames depicting NMR signal magnitude in an array of voxels during the first passage of contrast agent; d) calculating a plurality of arterial contrast enhancement reference curves by: i) dividing the array of voxels into a plurality of regions; ii) identifying a set of arterial voxels in each region; and iii) computing an arterial contrast enhancement reference curve for each region by averaging the signals from identified arterial voxels therein; and e) producing the magnetic resonance angiogram from the acquired NMR image data by segmenting arteries in each region of the angiogram using information contained in the series of NMR time course images and the calculated arterial contrast enhancement curve for the corresponding region.