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Errors in MR stereotaxy due to undetected extraneous metal objects.

MR stereotaxic procedures are being increasingly used, particularly in functional neurosurgery where very high levels of localization accuracy are required. Whilst many studies have investigated intrinsic causes of non-linearity, potential errors due to an extrinsic cause are not generally appreciated. It is not uncommon to find objects such as hair clips, paper clips and pins inside high-field magnets. They can remain undetected for long periods because they can reach positions not open to visual inspection and because they often do not produce observable deterioration in routine image quality. In this study we measured the maximum absolute positional shifts caused by such objects and found that these can be significant (> 1 mm, even up to 200 mm from one such object). Additional measurements were performed using an MR compatible Leksell stereotaxic frame to calculate actual stereotaxic coordinate errors. The encompassing nature of the frame is such that some degree of compensation for such nonlinearities is inherent, and so errors for areas of the brain more proximal to the object are found to be reduced but not eliminated. Stereotaxic coordinate errors will not be reduced in nonencompassing designs and in frameless stereotaxy. The prevalence of such objects in clinical systems and the measures required to detect their presence are discussed. The need for quality control testing before each stereotaxic procedure is highlighted.