Where two materials with very different susceptibilities are closely adjacent, there may be severe distortion of the magnetic field, causing artefactual loss or exaggeration of the magnetic resonance signal. This is clearly seen in an image acquired with a metallic clip placed close to the scalp ( Plate.7). Ferromagnetic (highly paramagnetic) materials that may cause artefacts include metallic dental fillings and plates, hairgrips, ear or nose rings, and even some cosmetics. A less obvious form of susceptibility artefact arises if more (diamagnetic) tissue is situated at one end of the image field of view than the other. A field gradient is set up across the image resulting in signal loss. This is termed bulk susceptibility artefact and is often seen in images of both the head and neck ( Plate. . .. 7).

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