Measurement of intraoperative brain surface deformation under a craniotomy

Abstract

We measured the deformation of the dura and brain surfaces between the time of imaging and the start of surgical resection for 21 patients. All patients underwent intraoperative functional mapping, allowing us to measure brain surface motion at two times that were separated by nearly an hour after opening the dura but before resection. The positions of the dura and brain surfaces were recorded and transformed to the coordinate space of a preoperative MR image using the Acustar neurosurgical navigation system. The mean displacements of the dura and the first and second brain surfaces were 1.2, 4.4, and 5.6 mm, respectively, with corresponding mean volume reductions under the craniotomy of 6, 22, and 29 ml. The maximum displacement was greater than 10 mm in approximately one-third of the patients for the first brain surface measurement and one-half of the patients for the second. In all cases the direction of brain shift corresponds to a “sinking” of the brain intraoperatively, compared with its preoperative position. We observed two patterns of the brain surface deformation field depending on the inclination of the craniotomy with respect to gravity. Separate measurements of brain deformation within the closed cranium caused by changes in patient head orientation with respect to gravity suggested that less than 1 mm of the brain shift recorded intraoperatively could have resulted from the change in patient orientation between the time of imaging and the time of surgery. These results suggest that intraoperative brain deformation is an important source of error that needs to be considered when using neurosurgical navigation systems.