Magnetization transfer imaging with pulsed off-resonance saturation: variation in contrast with saturation duty cycle

Magnetization transfer contrast (MTC) can be generated in magnetic resonance images of water-containing macromolecular structures. Results obtained with the pulsed off-resonance saturation technique for achieving MTC are presented, allowing comparison of this technique with others. The magnetization transfer effect is demonstrated in newborn piglet brain and bovine muscle tissue, as well as in human subjects. MTC increases nonlinearly with the duty cycle of the off-resonance irradiation, approaching a maximum value corresponding to continuous irradiation. However, approximately half of the available contrast can be obtained at a given saturation offset frequency, with 16% irradiation duty cycle. Examples of MTC in various human tissues demonstrate the specificity of this technique at power levels within U.S. Food and Drug Administration guidelines. Plots of MTC versus the irradiation offset frequency demonstrate the effects of magnetization transfer and are compared with the curve generated by a control phantom that does not show magnetization transfer. Magnetization transfer imaging with pulsed off-resonance saturation is effective and easily implemented and may be useful in the investigation of saturation transfer phenomena in tissue.