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The effect of cross relaxation on the NMR behaviour of bovine white matter Bjarnason, Thorarin Albert


In vitro experiments on 15 white matter samples from 5 bovine brains were performed on a ¹HNMR spectrometer at 24 and 37°C. A four pool model including intra/extracellular water, myelin water, non-myelin tissue, and myelin tissue was proposed to represent the NMR behaviour of bovine white matter. A cross relaxation correction was introduced to compensate for shifting of the measured data points and T₂ times over the duration of the CPMG measurement due to cross relaxation. This correction was found to be slight, providing evidence that myelin water fractions measured using a multi-echo technique are near physical values. At 24°C the cross relaxation times between myelin tissue and myelin water, myelin water and intra/extracellular water, and intra/extracellular water and non-myelin tissue were found to be approximately 227, 2064, and 402ms, respectively. At 37°C these same cross relaxation times were 158, 1021, and 170ms, respectively. The cross relaxation time between the two mobile pools at 24°C was found to be roughly two times bigger than the larger T₁ value of the measured bi-exponential T₁ times for white matter. However, at 37°C this cross relaxation time was of the same order as the measured mono-exponential T₁ value. Thus, one can conclude that cross relaxation is responsible for merging the two T₁ peaks at the higher temperature. The cross relaxation time was over order of magnitude larger than the measured T₂ times and hence unable to merge the T₂ peaks together. The exchange rate'between myelin water and myelin was found to be 11.8s⁻¹ at 37°C while the exchange rate between intra/extracellular water and non-myelin tissue was found to be 6.8s⁻¹. These exchange rates are of similar magnitude; indicating that the interaction between intra/extracellular water and non-myelin tissue cannot be ignored.

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