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Magnetic resonance imaging
  1. Andrew J Farrall
  1. Consultant Neuroradiologist, Division of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Crewe Road, Edinburgh EH4 2XU, UK; Andrew.Farrall@ed.ac.uk

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    Magnetic resonance (MR) imaging is one of the few routinely tangible manifestations of quantum theory—fortunately it can be understood without understanding quantum theory. In fact, given the space confines granted this article, I will ignore quantum theory completely, provided you are willing to make some assumptions and leaps of faith that would bring physicists to their knees, weeping.

    MR imaging terminology is filled with seemingly impenetrable acronyms like FLAIR and STIR; phrases like “T1-weighted” and “T2-weighted” are bandied about; and “gradient echo” rather than “spin echo” sequences may be applied to patients. This article will provide a much simplified and limited discussion of the principles underlying these and other common MR terms.

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    Magnets

    The main MR unit magnetic field is typically 1.5 Tesla strong (about 30,000 times the Earth’s magnetic field) in clinical settings. It is usually generated by an electric current continuously circulating through a superconductor (cooled with liquid helium). The magnetic field points in a direction, effectively having a “north” and a “south” pole, just as the Earth has a north and south pole; the field strength is conventionally denoted “Bo” (pronounced “bee zero”) (fig1) (Bo is simply a physics convention and isn’t short for anything; basically at the beginning of time, and if you look in all the physics texts, the words were “and let the main magnetic field be denoted by Bo …”). Also by convention, and useful for further discussion, a three dimensional grid is superimposed on the magnet system with the z axis aligned in the same direction as Bo, and the x and y axes in planes perpendicular to the z axis (fig 1).

    Figure 1

    Magnetic resonance scanner schematic diagram to illustrate magnetic field orientation, proton alignment and magnetisation, and grid coordinates used to describe proton motion and location. …

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