1. A method for imaging a vascular system of a patient comprising the steps of: introducing a bolus of contrast agent into the patient; selecting a slab at a position of a desired three-dimensional region and using an EPI sequence to acquire a two-dimensional k-space data set; taking a two-dimensional k-space data set of the patient at a same position of the desired three-dimensional region; thresholding the two-dimensional k-space data to retain a predetermined percentage of pixels; identifying locations of interest of the patient in the two-dimensional k-space; applying a three-dimensional acquisition sequence to acquire only those points selected by the thresholding step to form a mask; forming a mask from the two-dimensional k-space image to prospectively select data samples for the acquisition of three-dimensional MRA data set of the locations of interest; acquiring a three-dimensional MRA data set of the locations of interest of the two-dimensional k-space data set of the selected data samples; and imaging the contrast agent from the three-dimensional MRA as it passes through the vascular system of the patient.