to image flowing materials , magnetic resonance preconditioning pulses are applied in an upstream region . for scanning a subject , an rf pulse calibration sequence is performed by generating a corresponding magnetic resonance data line in each of a plurality of slices along a vessel . a processor determines a signal intensity for each slice , fits the intensities for the family of slices to a curve , 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 and reconstructed into an image representation stored in the memory .