B 0 dependence of the on‐resonance longitudinal relaxation time in the rotating frame ( T 1ρ ) in protein phantoms and rat brain in vivo

On‐resonance longitudinal relaxation time in the rotating frame ( T 1ρ ) has been shown to provide unique information during the early minutes of acute stroke. In the present study, the contributions of the different relaxation mechanisms to on‐resonance T 1ρ relaxation were assessed by determining relaxation rates (R 1ρ ) in both protein phantoms and in rat brain at 2.35, 4.7, and 9.4 T. Similar to transverse relaxation rate (R 2 ), R 1ρ increased substantially with increasing magnetic field strength ( B 0 ). The B 0 dependence was more pronounced at weak spin‐lock fields. In contrast to R 1ρ , longitudinal relaxation rate (R 1 ) decreased as a function of increasing B 0 field. The present data argue that dipole–dipole interaction forms only one pathway for T 1ρ relaxation and the contributions from other physicochemical factors need to be considered. Magn Reson Med 51:4–8, 2004. © 2003 Wiley‐Liss, Inc.