Correlation of Pre-Operative MRI and Intra-Operative 3D Ultrasound to Measure Brain Tissue Shift David G. Gobbi * Belinda K.H. Lee, Terry M. Peters The John P. Robarts Research Institute, London ON N6A 4K8, Canada Dept. of Medical Biophysics, University of Western Ontario, London ON N6A 5K8, Canada ABSTRACT B-Mode ultrasound is often used during neurosurgery to provide intra-operative images of the brain through a craniotomy, but the use of 3D ultrasound during surgery is still in its infancy. We have developed a system that provides real-time freehand 3D ultrasound reconstruction at a reduced resolution. The reconstruction proceeds incrementally and the 3D image is overlayed, via a computer, on a pre-operative 3D MRI scan. This provides the operator with the necessary feedback to maintain a constant freehand sweep-rate, and also ensures that the sweep covers the desired anatomical volume. All of the ultrasound video frames are buffered, and a full-resolution, compounded reconstruction proceeds once the manual sweep is complete. We have also developed tools for manual taggingofhomologouslandmarksinthe3DMRIand3Dultrasoundvolumesthatuseapiecewisecubicapproximation of thin-plate spline interpolation to achieve interactive nonlinear registration and warping of the MRI volume to the ultrasound volume: Each time a homologous point-pair is identified by the user, the image of the warped MRI is updated on the computer screen after less than 0.5 s. Keywords: 3d ultrasound, brain shift, intraoperative ultrasound, image guided surgery, neurosurgery 1. INTRODUCTION 1.1. Intra-operatative Imaging Followingacraniotomy(theremovalofaskullflap,oneofthefirststepsinmanyneurosurgicalprocedures)thebrain willshiftby10mmonaverageandbyasmuchas25mm. 1,2 Forsurgeriesthatrelyonstereotacticguidanceafterthe craniotomy has been performed (e.g. removal of a tumor where the tumor boundary is not clearly visible to the eye), the presence of brain shift can result in the removal of more or less brain tissue than was necessary. In cases where the probable brain shift is greater than the precision required for the surgery, the surgeon must abandon stereotactic guidance and rely primarily on eyesight, leading longer and/or more invasive operations. The best way to perform image-guided surgery in cases where significant brain shift is probable is to perform intra-operative imaging, which provides accurate and current images of the brain. B-mode ultrasound is a commonly used imaging modality for intra-operative imaging during neurosurgical pro- cedures. It has several very favorable attributes: it is real-time (generally 30 frames/s after resampling to video), geometric distortion of the images is minimal, and its use does not require any special equipment apart the ultra- sound machine itself. These are benefits not shared by interventional MRI, which is an increasingly popular (but comparatively expensive) 3D intra-operative imaging modality. The drawbacks of B-mode ultrasound compared to MR are that it produces 2D images (reconstruction of 2D ultrasound images into a 3D ultrasound volume can be done via computer, and some ultrasound machines can perform this function themselves) and the image quality can be significantly poorer than MRI. Several groups have investigated the use of 2D intra-operative ultrasound to measure brain shift. 3–6 A summary of our own results with 2D intra-operative ultrasound are provided in previous papers. 8,9 A solution to the image quality problem is to extract the necessary geometrical information from an ultrasound image(whichisgenerallyofhigherresolutionthananMRI)andtousethisinformationtowarpapre-operativeMRI volumeintothecurrentintra-operativeshapeofthebrain. 3 Hence, though the intra-operative imaging is performed using ultrasound, surgical guidance is carried out using the (corrected) pre-operative MRI. This approach has been adopted by two companies which sell surgical guidance systems, Sofamor Danek (http://www.sofamordanek.com) Correspondence: E-mail: dgobbi@irus.rri.on.ca, tpeters@irus.rri.on.ca; Telephone: 519 663 5777x34213; Fax: 519 663 3403 Visualization, Display, and Image-Guided Procedures, Seong Ki Mun, Editor, Proceedings of SPIE Vol. 4319 (2001) © 2001 SPIE · 1605-7422/01/$15.00 264