Nuclear magnetic resonance imaging of the orbit.

SUMMARY Nuclear magnetic resonance is a noninvasive imaging technique which does not employ ionising radiations. The authors report its application to imaging and tissue characterisation in the normal orbit. The application of x-ray computed tomography (CT) to the investigation of orbital disease has radically changed the diagnostic approach to patients thought to have retrobulbar masses. ' Previously available investigative techniques were either relatively unin-formative (plain films, phlebography) or potentially hazardous (orbitography).2 Applications of ultra-sound have also developed, but the technique has shortcomings, particularly in disease at the apex of the orbit and in its walls.3 X-ray CT also has shortcomings in the orbit: it is relatively nonspecific as regards histology, which is usually inferred from other data. ' It is expensive to install, and sophisticated studies with modem scanners involve substantial radiation to the eye, which cannot be avoided by screening. The radiation dose, while lower than that of comparable techniques, is still considerable,4 and recent studies indicate that, with repeated examinations a cataractogenic dose is easily reached.' Nuclear magnetic resonance (NMR) imaging uses no ionising radiation and is very sensitive to tissue differences. At present relatively few centres have the requisite apparatus, but it seems probable that within a few years it will become a major part of the imaging armamentarium. This paper reports preliminary experience of orbital imaging with NMR after a brief explanation of the principles ofNMR imaging. More complete reviews of NMR are available for the interested reader. 8 Principles ofNMR imaging Nuclei such as that of hydrogen which have an odd number of protons or neutrons tend when placed in a strong magnetic field to become aligned with tha