Diffusion Tensor Imaging (DTI) is a well-established magnetic resonance techniquethat can non-invasively interpret tissue geometry and track neural pathways bysampling the diffusion of water molecules in the brain tissue. However, it is currentlylimited to tracking large nerve fiber bundles and fails to faithfully resolve thinnerfibers. Conventional DTI studies use a diffusion time, t[subscript diff] of 30 ms - 55 ms fordiffusion measurements. This work proposes the use of DTI at long t[subscript diff] to enhancethe sensitivity of the method towards regions of low diffusion anisotropy and improvetracking of smaller fibers. The Stimulated Echo Acquisition Mode (STEAM) sequencewas modified to allow DTI measurements at long t[subscript diff] (approximately 200 ms), whileavoiding T2 signal loss. For comparison, DTI data was acquired using STEAM at theshorter value of t[subscript diff] and with the standard Double Spin Echo sequence with matchedsignal-to-noise ratio. This approach was tested on phantoms and fixed monkey brainsand then translated to in vivo studies in rhesus macaques. Qualitative and quantitativecomparison of the techniques was based on fractional anisotropy, diffusivity,three-phase plots and directional entropy. Tensor-field maps and probabilistic connectivityfronts were evaluated for all three acquisitions. Comparison of the trackednerve pathways showed that fibers obtained at long t[subscript diff] were much longer. Further,the optic tract was tracked in ex vivo fixed rhesus brains for cross validation. Theoptic tract, traced at long t[subscript diff], conformed to the well documented anatomical description,thus confirming the accuracy of tract tracing at long t[subscript diff]. The benefits ofDTI at long t[subscript diff] indeed help to realize the potential of tensor based tractographytowards studying neural development and diagnosing neuro-pathologies, albeit theimprovement is more significant ex vivo than in vivo.
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