T₁ and magnetization transfer at 7 Tesla in acute ischemic infarct in the rat

T₁ and magnetization transfer at a field strength of 7 Tesla were used to discriminate between water accumulation and protein mobilization in tissue undergoing infarction. Twelve rats subjected to acute stroke via intraluminal suture occlusion of the middle cerebral artery, and 19 controls, were studied. In MRI studies to 6 hr post-ictus, serial data acquisition allowed the measurement of cerebral blood flow (CBF), apparent diffusion coefficient of water (ADC(w)), equilibrium magnetization (M₀) and T₁, and equilibrium magnetization and T₁ under an off-resonance partial saturation of the macromolecular pool (M(sat) and T(1sat)). Using these parameters, the apparent forward transfer rate of magnetization between the free water proton pool and the macromolecular proton pool, k(fa), was calculated. Regions of interest (ROIs) were chosen using depressed areas in maps of the ADC(w). T₁ measurements in bovine serum albumin at 7T were not affected by the mobility of the macromolecular pool (P > 0.2), but magnetization transfer between free water and protein depended strongly on the mobility of the macromolecular pool (P < 0.001). For 6 hr after ictus, k(fa) uniformly and strongly decreased in the region of the infarct (P < 0.0001). Ratios (ischemic/non- ischemic) of parameters M₀, M(sat), T₁, and T(1sat) all uniformly and strongly increased in the infarct. The ratio T₁/T(1sat) in the region of infarction showed that a progressive accumulation of free water in the region of interest was the major (>80%) contribution to the decrease in k(fa). There also existed a small contribution due to changes at the water-macromolecular interface, possibly due to proteolysis (P = 0.005).