Chapter 3 - Gaussian Modeling of the Diffusion Signal

Abstract

This chapter provides a very basic introduction to the concept of diffusion MRI as a probe of tissue microstructure. Diffusion is an essential physical process for the normal functioning of living systems. Diffusion-weighted MR sequences are made sensitive to diffusion by the addition of magnetic field gradients, i.e. the magnetic field is made to vary in a linear manner over the volume of interest. The concept of Gaussian diffusion is explored and the effect of diffusion on the MR signal is explained. Microstructure fundamentally affects the apparent diffusion properties of water and so non-invasive quantification of diffusion acts as a sensitive probe to any changes in cellular structures that alter the displacement per unit time. Microstructure fundamentally affects the apparent diffusion properties of water and so non-invasive quantification of diffusion acts as a sensitive probe to any changes in cellular structures that alter the displacement per unit time. The most useful clinical application to date is the use of the diffusion-weighted scan in acute ischemia in which there is a reduction in the voxel-averaged displacement of water molecules per unit time. It also explores how tissue microstructure affects the random walk of water molecules undergoing diffusion and why, in highly ordered tissue, the single ADC model is inadequate for characterizing the diffusion process. This motivates the introduction of the diffusion tensor model.