Magnetic resonance imaging of stereotactic radiosurgical lesions in the internal capsule

Confirmation of radiosurgical lesions in white matter has earlier been difficult to obtain, which has hampered the evaluation of clinical outcome in relation to the site and the size of the lesions. In this investigation 7 consecutive patients subjected to bilateral stereotactic gamma capsulotomy for intractable anxiety disorders were re-examined several years after treatment. The protocol included both magnetic resonance imaging (MRI) and computed tomography (CT) interpreted under blind conditions and prospective psychiatric evaluations performed by two independent evaluators who had not been involved in the selection and the treatment of the patients. The lesions were clearly visible with MRI in all patients who improved after treatment. MRI proved to be more accurate than CT both in detecting and in defining the size and the configuration of the lesions. The extent of the tissue reaction following the irradiation seemed to be best defined in T2 weighted images. A high correlation (p less than 0.01) was obtained between ratings of clinical outcome and radiosurgical precision as reflected by MRI. It is concluded that MRI may provide the clinician with more information than CT does. In the future MRI may also facilitate the determination of a radiation threshold value for white and grey matter lesions and provide a more detailed knowledge of the time course of the development of such lesions.