Accuracy of thalamotomy target determination using axial images only

The target point for thalamotomy is generally determined in relationship to the intercommissural line. Standard axial computed tomography (CT) images can show both the anterior and posterior commissures. However, these commissures may not always be visible on the same axial image, as the plane in which the two commissures are located may not be the same as the plane of scanning. Mathematical calculations were performed to determine the degree of error that would occur if the target point determination was performed using axial images alone, even when the two commissures were not in the same plane as the CT images. The degree of error was found to be directly proportional to the distance between the plane of scanning for the two commissures. For each millimeter separation of these planes an error of approximately 0.11 mm in the z-coordinate (caudo-cephaled distance) and 0.04 mm in the y-coordinate (antero-posterior distance) was observed. These errors were felt to be negligible, especially since the final target point determination is based on physiological responses.