Integration of intra-operative 3D ultrasound with pre-operative MRI for neurosurgical guidance

When image-guided neurosurgery is used in procedures that require a craniotomy, targeting accuracy can often be compromised because of the brain shift that occurs due to pressure, gravitational and resection effects. Errors between the positions of homologous structures in the pre-operative images and within the brain itself of up to 25 mm have been reported. We have recently completed a study of the use of tracked 2D intra-operative ultrasound, integrated with 3D MRI as a means of visualizing and measuring the shift of the brain tissue during neurosurgical procedures, as well as correcting the pre-operative MR images on a slice-by-slice basis to conform with the intra-operative ultrasound images. More than 15 surgical cases have been performed thus far with the 2D system. We are extending this study to incorporate tracked 3D ultrasound. To date we have developed new tools for real-time overlay of the 3D ultrasound volumes and with the pre-operative MRI volumes. These facilities include a stereoscopic virtual-reality view of the ultrasound probe with live ultrasound video superimposed over a 3D-rendered MRI of the brain, as well as 3D ultrasound/MRI transparency overlay views. In addition, algorithms to automatically extract homologous landmarks from MRI and 3D ultrasound images are under development. These landmarks will be used to automatically generate nonlinear warp transformations to correct the pre-operative MRI as well as surgical target coordinates for brain shift.