Image Contrast Mechanisms in Diffusion-Weighted and Diffusion-Tensor Imaging

Abstract

The previous chapter discussed the fundamentals of magnetic resonance imaging as well as its applications in brain grey- and white-matter imaging. The major learning point was the use of a single solitary hydrogen proton for MR imaging. The concepts of MR active nucleus and its utilization to form an image of the human body with the help of magnets, gradient coils, radiofrequency coils and image encoding were also discussed. In the current chapter, we will study the various components of image contrast and weighting employed in magnetic resonance imaging in detail. Image weighting and contrast play a vital role in interpreting MR images, unlike other counterpart imaging modalities which possess standard image contrast. MRI varies in image weighting and contrast for individual pulse sequences which in turn depend upon key parameters like time of repetition (TR), time of echo (TE), turbo factor, time of inversion (TI), the b-value and flip angle which are also known as extrinsic contrast parameters. The intrinsic contrast parameters affecting MRI image contrast are the apparent diffusion coefficient (ADC), T1 recovery, T2 decay, proton density (PD) and flow. Hence, understanding this mechanism forms the basis for interpreting clinical images in MRI.