Different magnetization transfer effects exhibited by the short and long T-2 components in human brain

Magnetization transfer ratios (MTRs) were measured separately for the two T -2 components in white matter. For both binomial and off-resonance sine MT pulses, the MTR was larger for the short T-2 component than for the long T- 2,component. This differential Mi effect disappeared for delays between the MT purse and the multi-echo pulse sequence longer than 200 msec, indicatin g exchange between the two components. When using the sine MT pulse, the ML R for the short T-2 component was similar for different white matter struct ures, whereas it varied for different white matter structures when using th e binomial pulse-a phenomenon attributed to direct saturation. When the sin e pulse frequency was brought closer to resonance, MTRs in white matter and doped water phantoms increased for both components but more so for the sho rter T-2, component. This behavior was consistent with a Bloch equation mod el of direct saturation, (C) 2000 Wiley-Liss, Inc.