To image flowing materials, magnetic resonance preconditioning pulses are applied in an upstream region (28). For scanning a subject, an RF pulse calibration sequence is performed by generating a corresponding magnetic resonance data line (36.sub.1, . . . , 36.sub.n) in each of a plurality of slices (40.sub.1, . . . , 40.sub.n) along a vessel. A processor (54) determines a signal intensity for each slice (56), fits the intensities for the family of slices to a curve (58), and adjusts an RF pulse profile with spatial position in accordance with the curve which is dynamically dependent on the scanned subject. In a subsequent imaging sequence with the adjusted tip angles, data lines from each of the slices are received (52) and reconstructed (62) into an image representation stored in the memory (64).