Quantitative assessment of water pools by T_1ρ and T _2ρ MRI in acute cerebral ischemia of the rat

The rotating frame longitudinal relaxation magnetic resonance imaging (MRI) contrast, T_1ρ, obtained with on-resonance continuous wave (CW) spin-lock field is a sensitive indicator of tissue changes associated with hyperacute stroke. Here, the rotating frame relaxation concept was extended by acquiring both T_1ρ and transverse rotating frame (T _2ρ) MRI data using both CW and adiabatic hyperbolic secant (HSn; n=1, 4, or 8) pulses in a rat stroke model of middle cerebral artery occlusion. The results show differences in the sensitivity of spin-lock T_1ρ and T_2ρ MRI to detect hyperacute ischemia. The most sensitive techniques were CW-T_1ρ and T_1ρ using HS4 or HS8 pulses. Fitting a two-pool exchange model to the T_1ρ and T _2ρ MRI data acquired from the infarcting brain indicated time-dependent increase in free water fraction, decrease in the correlation time of water fraction associated with macromolecules, and increase in the exchange correlation time. These findings are consistent with known pathology in acute stroke, including vasogenic edema, destructive processes, and tissue acidification. Our results show that the sensitivity of the spin-lock MRI contrast in vivo can be modified using different spin-lock preparation blocks, and that physicochemical models of the rotating frame relaxation may provide insight into progression of ischemia in vivo.