Chapter 4 - Principle of diffusion tensor imaging

Abstract

Nuclear magnetic resonance (NMR)/ magnetic resonance imaging (MRI) can measure diffusion constants along an arbitrary axis. This unique feature of NMR/MRI may not be important if free water diffusion is measured inside a sample tube, because the same diffusion constant regardless of measurement orientation is expected. However, when water diffusion inside a living system is measured, it is often found that the diffusion process has directionality. Anisotropic diffusion is of great interest because it carries information about the underlying anatomical architecture of living tissues. This chapter demonstrates the results of two diffusion constant measurements using the right-left and anterior-posterior gradient orientations. The results clearly show that the diffusion process inside the brain is highly anisotropic. The six parameters needed to define an ellipsoid are rather more complicated. Three lengths for the longest, shortest, and middle axes that are perpendicular to each other are needed. A simpler way is to use three unit vectors to define the orientation of the principal axes. Diffusion tensor imaging (DTI) characterizes the diffusion ellipsoid from multiple diffusion constant measurements along different directions. Measurement of diffusion anisotropy inside the white matter is of great interest because water tends to diffuse along axonal tracts and, thus, the anisotropy carries unique anatomical information of axonal architecture.