ABSTRACT
This final chapter of the text covers an unusual and exceptionally powerful technique – magnetic resonance imaging (MRI). The chapter begins by introducing MRI as an anatomical and functional imaging approach that monitors radiofrequency (RF) signals associated with the magnetism of protons in soft tissues. This introduction is followed by a review of essential basics from quantum and classical mechanics, including magnetic moments, angular momentum, spin, quantum energy levels in an external magnetic field, and torque on a magnetic moment. After the review, the chapter addresses the complex nature of MRI imaging using a mix of ideas from classical mechanics and quantum mechanics. Important topics include (1) the preferential alignment of the magnetic moments of protons in a patient upon exposure to an external magnetic field, (2) the resulting creation of a net equilibrium magnetization, which is perturbed by the input of on-resonance RF radiation, and (3) the subsequent evolution of the non-equilibrium magnetization, which creates the MRI signal. The most important topic is “frequency encoding,” which is an unusual method that MRI uses to map proton distribution in the body. Later sections of the chapter discuss the three steps in three-dimensional MRI imaging (slice selection, phase encoding, and frequency encoding), Fourier-based image reconstruction, resolution, pulse sequences, and contrast. This last topic reveals that the primary strength of MRI, its exquisite soft-tissue contrast, has its origin in relaxation-time-weighted encoding of tissue brightness. The chapter closes with a discussion of prominent applications of MRI, including detection of brain abnormalities, angiography, diagnosis of soft tissue injuries, functional MRI, and planning surgical procedures on the brain.
