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Assessment of environmental disturbances to the static magnetic field in magnetic resonance installations

Department of Medical Physics, St George's Hospital, Blackshaw Road, London SW17 0QT, UK

The static magnetic field of MRI scanners can be affected by environmental factors. Magnetic resonance spectroscopy and functional imaging with single-shot echo-planar imaging (EPI) are particularly vulnerable to the movement of lifts, vehicles, trains and other large metallic masses in the vicinity. This work investigates the sensitivity of two different imaging techniques to assess disturbances of the static magnetic field: (i) phase changes in gradient-echo images of a uniform test object; and (ii) image displacement along the phase encoding direction in single-shot EPI images. For the latter a hexane sample was used, and the separation between CH₂ and CH₃ signals was taken as a reference. Both techniques were evaluated in a site known to be free of any significant environmental disturbances and validated by inducing a magnetic field disturbance. Both techniques provide valuable information in acceptance tests, allowing MRI users to evaluate and manage the environmental conditions surrounding a scanner. The single-shot EPI technique was found to be highly sensitive, being expected to detect magnetic field fluctuations down to 0.005 parts per million (ppm). The phase images method was found to be less sensitive (0.02 ppm) but is more easily available. The single-shot EPI technique was used in acceptance tests and environmental disturbances to the magnetic field of the order of 0.04 ppm were measured at the isocentre on two separate occasions.