Magnetic resonance imaging

Résumé fourni par la source

This chapter describes the physics of magnetic resonance imaging (MRI), a key diagnostic imaging modality. Initial sections cover the fundamental concepts which underpin the nuclear magnetic resonance phenomenon. Although quantum mechanical concepts are introduced, it is shown that NMR signals can be thoroughly understood using a classical approach. The origin of the T 1 and T 2 relaxation mechanisms are described, and how they relate to the local environment of hydrogen nuclei. It is shown how the application of magnetic field gradients enables images to be acquired, and the basics of an MRI pulse sequence are presented, including slice selection, phase encoding, and frequency encoding. The sequences known as gradient echo and spin echo are introduced, and the factors which contribute contrast in MRI imaging are reviewed. This is followed by an exploration of specific pulse sequences and data acquisition techniques designed to reduce imaging times. After a review of the MRI instrumentation, a section is devoted to describing the origin and appearance of the various types of artefact which can contaminate MRI images. The final sections of this chapter are devoted to a review of the many contrast mechanisms which may be exploited to visualise diagnostically valuable parameters (such as blood flow, diffusion, perfusion, and tissue susceptibility) and to the potential hazards associated with MRI.

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