On- and off-resonance T1ρ MRI in acute cerebral ischemia of the rat

Abstract

The ability of on-resonance T1ρ (T1ρ) and off-resonance T1ρ (T1ρoff) measurements to indicate acute cerebral ischemia in a rat model of transient middle cerebral artery (MCA) occlusion was investigated at 4.7 T. T1ρ was determined with B1 fields of 0.4, 0.8, and 1.6 G, and T1ρoff with five offset frequencies (Δω) ranging from 0-7.5 kHz at B1 of 0.4 G, yielding effective B1 (Beff) from 0.4 to 1.8 G. Diffusion, T1, and T2 were also quantified. Both T1ρ and T1ρoff acquired with Δo < 2.5 kHz showed positive contrast during the first hours of MCA occlusion in the ischemic tissue delineated by low diffusion. Interestingly, T1ρoff contrast acquired with Δω > 2.5 kHz was clearly less sensitive to ischemic alterations, and developed with a delayed time course. This discrepancy is thought to be a consequence of the frequency dependency of cross-relaxation during irradiation with spin-lock pulses. © 2003 Wiley-Liss, Inc.