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 accura cy are required. Whilst many studies have investigated intrinsic cause s of non-linearity, potential errors due to an extrinsic cause are not generally appreciated. It is not uncommon to find objects such as hai r clips, paper clips and pins inside high-held magnets. They can remai n undetected for long periods because they can reach positions not ope n to Visual inspection and because they often do not produce observabl e deterioration in routine image quality. In this study we measured th e 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 reduce d but not eliminated. Stereotaxic coordinate errors will not be reduce d in non-encompassing designs and in frameless stereotaxy. The prevale nce of such objects in clinical systems and the measures required to d etect their presence are discussed. The need for quality control testi ng before each stereotaxic procedure is highlighted.