Accuracy of MRI-guided stereotactic thalamic functional neurosurgery

Our goal was to evaluate the accuracy of stereotactic technique using MRI i n thalamic functional neurosurgery. A phantom study was designed to estimat e errors due to MRI distortion. Stereotactic mechanical accuracy was assess ed with the Suetens-GybelsVandermeulen (SGV) angiographic localiser. Three- dimensional MRI reconstructions of 86 therapeutic lesions were performed. T heir co-ordinates were corrected from adjustments based on peroperative ele ctrophysiological data and compared to those planned. MR image distortion ( maximum: 1 mm) and chemical shift of petroleum oil-filled localiser rods (2 .2 mm) induced an anterior target displacement of 2.6 mm (at a field streng th of 1.5 T: frequency encoding bandwidth of 187.7 kHz, an T1-weighted imag es). The average absolute error of the stereotactic material was 0.7 mm for anteroposterior (AP), 0.5 mm for mediolateral (ML) and 0.8 mm for dorsoven tral (DV) co-ordinates (maximal absolute errors: 1.6 mm, 2.2 mm and 1.7 mm, respectively; mean euclidean error: 1 mm). Three-dimensional MRI reconstru ctions showed an average absolute error of 0.8 mm, 0.9 mm and 1.9 mm in AP, hit and DV coordinates, respectively (maximal absolute errors: 2.4 mm, 2.7 mm and 5.7 mm, respectively; mean euclidean error: 2.3 mm). MRI distortion and chemical-shift errors must be determined by a phantom study and then c ompensated for The most likely explanation for an average absolute error of 1.9 mm in the DV plane is displacement of the brain under the pressure of the penetrating electrode. When this displacement is corrected for by micro electrode recordings and stimulation data, MRI offers a high degree of accu racy and reliability for thalamic stereotaxy.