Magnetic resonance imaging (MRI) has emerged as one of the most powerful diagnostic tools in the radiology clinic. The chief strengths of MRI are its ability to provide cross-sectional images of anatomical regions in any arbitrary plane and its excellent soft-tissue contrast. MRI has the ability to provide functional as well as anatomical information. The nuclear energy states of certain atoms interact with incident radio frequency photons in the presence of a static magnetic field. The radio frequency emission by tissue that follows the absorption of photons can be exploited to generate images. The phenomenon responsible for this, called the nuclear magnetic resonance (NMR) effect, was discovered almost simultaneously by Purcell and Bloch in 1946. It was recognized from the start that the nuclear magnetic resonance effect could be used to probe the electronic structure of molecules. NMR has since emerged as an invaluable analytical tool for molecular structure determination. It was only in the early 1980s that the nuclear magnetic resonance effect was exploited for imaging. In this chapter we will lay a foundation for an understanding of MRI by examining the basis of the resonance effect.
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