Estimation of intraoperative brain surface movement

It is becoming increasingly common for surgical navigation systems to be used intraoperatively to enable the surgeon to ascertain his or her position within the patient with respect to features in registered preoperative images. These systems assume that the skull and its contents behave as a rigid body between imaging and surgery, and during surgery. We have used the ACUSTAR I surgical navigation system to measure shifts in the brain surface relative to the skull between imaging and surgery on five patients. The preoperative images are registered to the coordinates of the surgical localizer using four fiducial markers screwed into the outer table of the skull. The brain surface is delineated from preoperative MR images, and the distance between this surface and brain surface points recorded intraoperatively is calculated. The median shift of points on the brain surface ranged from 0.3 mm to 7.4 mm. In all cases, the direction of this shift corresponds to a “sinking” of the brain intraoperatively compared to its preoperative position. We consider possible changes in CNS volume that might account for these shifts.