Simultaneously Measuring Axonal Diameter Distribution and Direction of Rat Brain Using Q-Space Diffusion Tensor Magnetic Resonance Imaging

Jun-Cheng Weng¹

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. The study proposed a novel technique, q-space diffusion tensor imaging (qDTI), combined with two image reconstruction methods based on the assumption to simultaneously map axonal diameter distribution and direction of rat brain. One was tensor-based method. The 3D Gaussian displacement distribution could be obtained directly by the displacement tensor. The other was displacement projection method. The effective axonal diameter was defined as the average of several displacements projected to the direction of the fiber section. They provided MR images in which physical parameters of water diffusion such as the mean displacement and maximum diffusivity of water molecules were used as image contrast. Our results demonstrated that two qDTI methods both produced reasonable distribution of effective axonal diameters and directions in rat brain.