On‐ and off‐resonance T 1ρ MRI in acute cerebral ischemia of the rat

The ability of on‐resonance T 1ρ ( T 1ρ ) and off‐resonance T 1ρ ( T   1ρ off ) measurements to indicate acute cerebral ischemia in a rat model of transient middle cerebral artery (MCA) occlusion was investigated at 4.7 T. T 1ρ was determined with B 1 fields of 0.4, 0.8, and 1.6 G, and T   1ρ offwith five offset frequencies (Δω) ranging from 0–7.5 kHz at B 1 of 0.4 G, yielding effective B 1 ( B eff ) from 0.4 to 1.8 G. Diffusion, T 1 , and T 2 were also quantified. Both T 1ρ and T   1ρ offacquired with Δo < 2.5 kHz showed positive contrast during the first hours of MCA occlusion in the ischemic tissue delineated by low diffusion. Interestingly, T   1ρ offcontrast 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. Magn Reson Med 49:172–176, 2003. © 2003 Wiley‐Liss, Inc.