Inferring Microstructural Properties Using Angular Double Pulsed Gradient Spin Echo NMR in Orientationally Unknown Tissue

We present an analytical water diffusion model for inferring important microstructural properties such as axon radius, orientation, and packing density using low-q angular Double Pulsed Gradient Spin Echo (double-PGSE) NMR, taking into account finite gradient pulses. The MR signal attenuation obtained from Single Pulsed Gradient Spin Echo (single-PGSE) NMR reflects the underlying microstructural properties that restrict the molecular diffusion within. Estimating these properties using single-PGSE, however, requires prior knowledge of axon orientation and high q-values, inhibiting clinical application of these methods. Feasibility of estimating orientationally invariant fibers are emerging, although the acquisition protocol needs to be optimized for the specific axon radius to be estimated. Our simulation results are the first to use low-q angular double-PGSE experiments without prior knowledge of the axon orientation to demonstrate the feasibility of estimating axon radii of the typical human brain tissue range (1 to 5μm). Along with axon radius, we were also able to infer other important microstructural properties such as axon orientation, axon packing density and water diffusivity.