1. A method, comprising: displaying selected views of a 3D image of a portion of a patient's vasculature derived from 3D MRI volume data; selecting a particular image plane of the 3D MRI volume data from a set of available image planes, wherein the selected image plane is the one most nearly perpendicular to the direction of blood flow within the selected blood vessel; calculating a 2D image corresponding to the selected image plane from the 3D MRI volume data; defining a region of interest around a selected blood vessel in the 2D image, refining the region of interest to obtain the border of the selected blood vessel, and calculating a center point of the blood vessel; deriving a set of center points of the selected blood vessel from the 3D MRI volume data as the selected image plane is moved generally along the length of the selected blood vessel; employing a line-fitting algorithm to fit the derived center points of the blood vessel to one or more lines; selecting the line of the one or more lines with the maximum length; defining a location and slice orientation for phase contrast magnetic resonance angiography corresponding to a center and an endpoint of the selected line; and, performing phase contrast magnetic resonance angiography using the selected location and slice orientation.