Magnetization transfer effects in multislice MR imaging.

A theoretical model is presented which describes the effects of magnetization transfer in multislice MR imaging of a tissue-mimicking phantom composed of cross-linked agar gel. The model is successful in explaining differences between single and multislice image signal intensities observed for the agar gel but not seen in a simple aqueous solution. Magnetization transfer leads to a reduction in the image signal intensity of a slice of interest due to off-resonance RF irradiation arising from 90 degrees and 180 degrees pulses intended for neighboring slices. The contribution of magnetization transfer to multislice MR imaging depends on the amount of off-resonance RF irradiation during the imaging sequence repetition interval. For the tissue-mimicking agar gel, conventional spin-echo multislice imaging gave rise to a negligible image signal intensity reduction (< or = 2%); however, fast spin-echo (FSE) imaging, which employs up to 16 times as many RF pulses per slice, exhibited as much as a 13% reduction in image signal intensity (13 slices). The reduction in multislice image signal intensity due to magnetization transfer is sample specific and is shown to be more dramatic for in vivo human leg muscle (10% for conventional spin echo, 40% for FSE) where magnetization transfer rates are greater than in the cross-linked agar gel.