Compartmental study of diffusion and relaxation measured in vivo in normal and ischemic rat brain and trigeminal nerve.

The correlation between the apparent diffusion coefficient (ADC) and T(2) of water in rat brain and trigeminal nerve was investigated using a hybrid diffusion-weighted-CPMG imaging sequence. Little dependence of ADC on T(2) was found in brain regions of interest, which is postulated to be due to rapid exchange between intra- and extracellular water. Conversely, the ADC of water in trigeminal nerve was found to change significantly with echo time (TE). Parallel to the nerve and with a constant diffusion time (t(diff) = 10.8 ms), the ADC increased by approximately 30% between TEs of 25 ms and 185 ms; perpendicular to the nerve, the ADC decreased by a similar amount over the same range of TE. Measurements made following the onset of global ischemia yielded lower ADCs, with similar dependence on TE. Observations that transverse relaxation of water in nerves is multiexponential have previously been interpreted in terms of microanatomical compartments in slow exchange. In the context of this interpretation, our data suggest that diffusional anisotropy is greater outside than within the myelinated axons. Further, data following the onset of global ischemia suggest that the mechanism(s) by which ADC is reduced affect most or all microanatomical environments of nerve, at least insofar as they are represented over the TE domain investigated. Magn Reson Med 43:837-844, 2000.