The correction of stereotactic inaccuracy caused by brain shift using an intraoperative ultrasound device

Abstract

Cranial stereotactic systems which utilize preoperative computed tomography (CT) or magnetic resonance imaging (MRI) data sets to guide surgery are subject to inaccuracy introduced by the intraoperative movement of the brain (brain shift). Although these systems allow precise navigation initially during a procedure, brain shift resulting from surgical intervention can lead to progressive degradation in accuracy, with the greatest inaccuracy occurring when deep structures are manipulated. One method of addressing this issue is with the use of an intraoperative scanning device such as CT or MRI; however, such scanners are costly and restrict surgical access. We have developed an alternative intraoperative imaging device consisting of an ultrasound unit coupled to a stereotactic system to quantify the degree of brain shift. This system determines the orientation of ultrasound images produced by the device and reformats the pre-operative CT or MRI images to match the ultrasound image. By comparing the position of specific structures on the two images, the amount of shift can be determined. Furthermore, this system is being expanded to include the aquisition of three-dimensional ultrasonic volumes.

We have used this device on a series of patients (n=23) to determine the position of specific intracranial structures (e.g. vessels, sulci, gyri) prior to and after surgery. Cases in which hematoma or tumors were removed had the highest average shift (9.5mm and 7.9 mm, respectively); whereas, implantation of electrodes for the recording of seizures had the least amount of shift (2.9 mm). As the degree of shift of specific intracranial structures can vary greatly, we have classified the structures into three levels-low (0–2.9 mm), moderate (3.0–6.9 mm), and high (>7.0 mm). In addition, increased age and the use of diuretics during surgery lend to an increase in shift. Using these results, a model of response of the brain to surgery is being developed to correct for brain shift in a automated fashion. We conclude that brain shift can be corrected using an inexpensive intraoperative ultrasound device to improve the accuracy and reliability of stereotactic intracranial surgery.