Marieke van Rosmalen

Chapter 1 14 realignment appear dark as it does not retain signal ( Figure 1.2A ). T2-weighted images require a long TE and TR and highlight differences in the T2 relaxation times of tissues. Tissues with a longer T2 relaxation time will retain signal and appear bright ( Figure 1.2B ). MRI of the peripheral nervous system, e.g. the brachial or lumbosacral plexus, is often based on T2-weighted images. T2-weighted imaging with fat suppression (e.g. spectral presaturation with inversion recovery (or SPIR)) is an excellent technique to visualize pathology of peripheral nerves, and the brachial or lumbosacral plexus ( Figure 1.2C ). 8 Figure 1.2 Basic pulse sequences of MRI Examples of the healthy brachial plexus visualized in a T1 weighted image (A), a T2 weighted image (B) and T2 weighted imaging with fat suppression (C). Quantitative MRI techniques MRI is a versatile technique that can provide qualitative as well as quantitative information on (nervous) tissues. T1- and T2-weighted imaging, as described in the previous paragraph, provides qualitative information on anatomical tissues and generates an image. Advanced quantitative MRI techniques do not only produce an image, but also generate a quantitative parameter. One of these quantitative techniques is diffusion tensor imaging (DTI). DTI gives quantitative information on microstructural integrity that correlates with histological findings. 9–11 DTI measures diffusion of water in tissue in a number of different directions. Diffusion rates of biological tissues are not the same in every direction, which means the tissue is not isotropic but rather anisotropic . The direction and magnitude of the diffusion can be expressed by the diffusion tensor. From this diffusion tensor eigenvalues and eigenvectors can be derived. Eigenvectors express the direction of the diffusion, and eigenvalues express the magnitude of the diffusion. In this way, the degree of diffusion of water can be calculated along the main axis (axonal diffusivity, AD) or perpendicular to the nervous tissue (radial diffusivity, RD). AD is determined by the eigenvalue λ 1 and RD is determined by the mean of the eigenvalues λ 2 and λ 3 ( Figure 1.3 ). The mean diffusivity (MD) is calculated by the mean of