Graded reduction of cerebral blood flow in rat as detected by the nuclear magnetic resonance relaxation time T-2: A theoretical and experimental approach

The ability of transverse nuclear magnetic resonance relaxation time. T-2, to reveal acutely reduced CBF was assessed using magnetic resonance imaging (MRI). Graded reduction of CBF was produced in rats using a modification o f Pulsinelli's four-vessel occlusion model. The CBF in cerebral cortex was quantified using the hydrogen clearance method, and both T-2 and the trace of the diffusion tensor (D-av = 1/3 TraceD) in the adjacent cortical tissue were determined as a function of reduced CBF at 4.7 T. A previously publis hed theory, interrelating cerebral hemodynamic parameters, hemoglobin, and oxygen metabolism with T,, was used to estimate the effects of reduced CBF on cerebral T,. The MRI data show that T, reduces in a U-shape manner as a function of CBF, reaching a level that is 2.5 to 2.8 milliseconds (5% to 6% ) below the control value at CBF, between 15% and 60% of normal. This reduc tion could be estimated by the theory using the literature values of cerebr al blood volume, oxygen extraction ratio, and precapillary oxygen extractio n during compromised CBF. D-av dropped with two apparent flow thresholds, s o that a small 11% to 17% reduction occurred between CBF values of 16% to 4 5% of normal, followed by a precipitous collapse by more than 20% at CBF be low 15% of normal. The current data show that T-2 can be used as an indicat or of acute hypoperfusion because of its ability to indicate blood oxygenat ion level-dependent phenomena. on reduced CBF.