Off-resonance pulsed magnetization transfer in clinical MR imaging: optimization by an analysis of transients.

Significant degrees of magnetization transfer (MT) have been observed in the magnetic resonance imaging of biological materials by conventional clinical imaging sequences, as well as by sequences specifically designed to enhance MT image contrast. Two aspects of these procedures distinguish them from the classic spectroscopic MT experiments using either so-called "hard" radiofrequency (RF) pulses of short duration and high power, or continuous wave RF irradiation of low power. First, clinical sequences must make exclusive use of "soft" pulses of intermediate length and power. Second, biological materials are modeled by a two-spin system involving magnetization transfer between a narrow and a broad homogeneous spectral component. Such materials are a relatively restricted group within the larger family of materials studied with MT by spectroscopy. The current paper addresses these two issues with a theory that gives a new transient analysis of the off-resonance pulsed MT problem for biological materials. It leads to predictions for optimal magnetization transfer in the context of medical imaging that have been verified by computer modeling.