TEMPORAL AND REGIONAL CHANGES DURING FOCAL ISCHEMIA IN RAT-BRAIN STUDIED BY PROTON SPECTROSCOPIC IMAGING AND QUANTITATIVE DIFFUSION NMR IMAGING

The early development of focal ischemia after permanent occlusion of t he right middle cerebral artery (MCA) was studied in six rats using in terleaved measurements by diffusion-weighted NMR imaging (DWI) of wate r and two variants of proton spectroscopic imaging (SI), multiecho SI (TE: 136, 272, 408 ms) and short TE SI (TE: 20 ms). Measurements on a 4.7-T NMR imaging system were performed between the control phase and approximately 6 h postocclusion. In the center of the ischemic lesion of all rats, the apparent diffusion coefficient (ADC) decreased rapidl y to 84.4 +/- 4.2% (mean +/- SD) of the control values approximately 2 min postocclusion. Approximately 6 h postocclusion, the ADC was reduc ed to 67.1 +/- 5.9%. In contrast, large differences between the animal s were observed for the temporal increase of lactate (Lac) in the ipsi lateral hemisphere. The maximum Lac signal was reached in four rats af ter 0.5-1.5 h, and in two rats was not reached even after 6 h postoccl usion. Six h postocclusion, SI spectra measured at a TE of 136 ms reve aled a decrease in the CH3 signal of N-acetylaspartate (NAA) to 67 +/- 13% of the control values. Differences were observed between the spat ial regions of decreased NAA and increased Lac. In the lesions, a T-2 relaxation time of Lac of 292 +/- 40 ms, considering a J-coupling cons tant of 6.9 Hz, was measured. Furthermore, a prolongation;of the T-2 o f the CH3 signal of creatine/phosphocreatine (Cr/PCr) was observed in the lesion, from 163 +/- 22 ms during control to 211 +/- 41 ms approxi mately 6 h postocclusion. The experiments proved that DWI and proton S I are valuable tools to provide complementary information on processes associated with brain infarcts.