Magnetic Resonance Imaging of the Brain

Abstract

Magnetic resonance imaging (MRI) examinations of the brain can be performed with several coil types, depending on the design of the MRI unit and the information required. Traditionally, MRI examinations of the brain are performed with quadrature (i.e., circularly polarized) head coils. These volume coils are closely shaped around the head of the patient and usually present a so-called “bird-cage” configuration. Many coils are split in half, for easier patient access and positioning. Recently, phased-array head coils have become the standard of practice for state-of-the-art high-resolution MRI of the brain. Phased-array head coils contain multiple small coil elements, which are arranged in an integrated design which surrounds the head (e.g., 8-, 12- or even 32-channel head coils). Data from the individual coils are integrated by special software to compensate for the nonuniform distribution of the signal-to-noise ratio (SNR) between the peripheral and central parts of the brain. The major advantage of a multichannel, phased-array head coil is that it allows the application of parallel acquisition techniques (PAT), which can be used to speed up MRI. The concept is to reduce the number of phase-encoding steps by switching a field gradient for each phase-encoding step. Skipping, for example, every second phase-encoding line accelerates the acquisition speed by a factor of two. This is called the acceleration or PAT factor. The trade-off for this increased imaging speed is a decrease in SNR. Image reconstruction with PAT techniques is more complicated, and several algorithms have been described, depending on whether image reconstruction takes place before (SMASH, GRAPPA (generalized autocalibrating partially parallel acquisition)) or after (SENSE) Fourier transform of the image data.