ABSTRACT
Conventional magnetic resonance imaging (MRI) as applied in clinical settings can provide a number of different types of image, including high spatial resolution structural images and images of brain perfusion, based on the nuclear magnetic resonance (NMR) properties of water protons and other nuclides found in brain tissue (Young, 1988). The MRI technique relies on the fact that protons behave differently in the presence of a strong magnetic field. In the presence of an ambient magnetic field, the constantly spinning and moving protons tend to become aligned along the main axis of the field. Images are created by applying a series of brief energy pulses, radio frequency (RF) pulses, through the tissue. Each RF pulse contains enough energy to momentarily disrupt or tilt the protons in a particular plane. Measurements of the resultant change from one energy state to another are used to create images of the tissue. High resolution structural images are constructed based on measures of the energy released as pulsed protons relax back to their aligned state (called T1 relaxation). Different body tissue types demonstrate different T1 relaxation times, with lipids showing longer relaxation times than water (Bloch, 1946; Hahn, 1950). These relaxation differences are then mapped to produce T1-weighted structural images of the brain with high contrast between grey matter, white matter, and cerebrospinal fluid (Fig. 2.4). The RF field must be pulsed in such a way as to sequentially perturb multiple planes, or slices throughout the tissue, resulting in a series of in-plane images that can be stacked to recreate a 3D volume. Parameters such as slice thickness and in-plane resolution affect the length of the scanning procedure and the quality of the acquired images.
