MAGNETIZATION-TRANSFER OR SPIN-LOCK - AN INVESTIGATION OF OFF-RESONANCE SATURATION PULSE IMAGING WITH VARYING FREQUENCY OFFSETS

PURPOSE: To characterize near-resonance saturation pulse MR imaging on a 1.5-T scanner in order to gain insight into underlying mechanisms t hat alter tissue contrast and to optimize the technique for neuroimagi ng. METHODS: Off-resonance saturation pulses were applied to T1-weight ed, spin-density-weighted, and T2-weighted sequences at frequency offs ets ranging from 50 Hz to 20 000 Hz down field from water resonance. S uppression ratios were determined at each offset for phantom materials (MnCl2 solution, gadopentetate dimeglumine, corn oil, water, and agar ), normal brain structures, and a variety of brain lesions. RESULTS: S ignal suppression of MnCl2 on T1-weighted images occurred at offsets o f less than 2000 Hz even though no macromolecules were present in the solution. Only those phantom materials and tissues with short or inter mediate T1 relaxation times and relatively large T1/T2 ratios were sen sitive to changing frequency offsets. Suppression of brain increased f rom approximately 20% at 2000 Hz offset to approximately 45% when the offset was reduced to 300 Hz. In human subjects, the net effect of red ucing the frequency offset was to increase T2 contrast on T1-weighted, spin-density-weighted, and T2-weighted images. Distilled water and co ntrast material did not suppress except at very low offsets (<300 Hz). A frequency offset of 300 Hz was optimal for maximizing conspicuity b etween most contrast-enhancing lesions and adjacent brain while preser ving anatomic detail. CONCLUSION: Suppression of MnCl2 indicates that magnetization transfer is not the sole mechanism of contrast in near-r esonance saturation MR imaging. Spin-lock excitation can reasonably ex plain the behavior of the phantom solutions and the increase in T2 con trast of tissues achieved as the frequency offset is decreased from 20 00 Hz to 300 Hz. Below 300 Hz, saturation is presumably caused by spin -tip effects. With our pulse design, an offset of 300 Hz is optimal fo r many routine clinical imaging examinations.