Flow, radiofrequency pulse sequences, and gradient magnetic fields: basic interactions and adaptations to angiographic imaging.

ABSTRACT

The basic process of MRI consists of two essential, relatively independent components (1) excitation in the form of a radiofrequency pulse sequence, and (2) signal sampling and localization, that is, forming the MR image through the use of field gradients. The presence of motion (blood flow) during either excitation or sampling results in two types of corresponding effects (1) time-of-flight effects, and (2) spin phase phenomena. These effects can be manipulated through the use of special coils, pulse sequences, gradients, and postprocessing techniques to provide angiographic images in which simple motion provides the basis for contrast.