Human Brain Mapping of Orientationally Invariant Axonal Diameter using Q-Space Diffusion Tensor MRI

Jun-Cheng Weng^1,2

Fundamental relationships between diffusion tensor imaging (DTI) and q-space imaging can be derived which establish conditions when these two complementary MR methods are equivalent. When the 3D displacement distribution is measured by q-space imaging with large displacement and small q vector, the result is similar to 3D Gaussian assumed in DTI. Combing displacement information from q-space imaging and fiber direction from DTI, distribution of axonal diameters and directions could be derived at the same time. Based on the assumption, the study proposed a novel technique, q-space diffusion tensor imaging (qDTI), to map orientationally invariant axonal diameter distribution of human brain. The goal could be achieved with any of two image reconstruction methods described below. One was tensor-based method. The 3D Gaussian displacement distribution could be obtained directly from the displacement tensor. The other was displacement projection method. The fiber directions were first calculated from conventional DTI, and the mean displacement as well as maximum diffusivity of water molecules along specific direction were then obtained with q-space imaging. The effective axonal diameter was defined as the average of several displacements projected to the direction of the fiber cross section. Our results demonstrated that two qDTI methods both produced reasonable distribution of orientationally invariant axonal diameters in human brain.