Stereovision-based integrated system for point cloud reconstruction and simulated brain shift validation.

Image-guided surgery (IGS)1 provides a standard of care platform for guiding surgeons during brain tumor resection. Unfortunately, commercial IGS navigation systems do not have mechanisms to account for nonrigid tissue deformations, which commonly arise from cerebrospinal fluid drainage, tissue swelling due to edema, tissue contraction due to hyperosmotic drugs, or tissue retraction/resection.2 Solutions like intraoperative magnetic resonance (iMR) imaging,3 intraoperative computed tomography (iCT),4 and intraoperative ultrasound (iUS)5 have been proposed to compensate for soft-tissue changes. However, deficiencies like ionizing radiation of iCT, high cost of iMR, and poor image contrast/quality of iUS have compelled researchers to look for alternatives. Another cost effective method is to use sparse data acquired intraoperatively to drive a biomechanical model to update and register preoperative images to the intraoperative field.6 Laser range scanners (LRS) and stereoscopic microscopes are two widely used surface data acquisition techniques.7,8 Both devices can be used in an operating room (OR) and generate three-dimensional (3-D) cortical surface cloud data as well as a texture map. Typically, the microscope is continuously used during the whole surgery. Thus, it can provide high-resolution, consistent intraoperative information in near real time with very limited interruption to the surgical workflow. Performing a single LRS acquisition is more disruptive because it takes about 15 to 30 s and requires the surgical microscope to be moved away from the surgical field of view (FOV). Standard stereomicroscopes allow surgeons to appreciate the 3-D nature of their domain when performing surgery. This facilitates the physical execution of surgery. With respect to tracked microscopes, once calibrated, these will typically track the focal point of the scope in 3-D space. This allows one to probe the tissue one point at a time to determine the associated imaging data. If one wanted to measure the optical field quickly, such as in measuring the distribution of deformations on the brain surface, the process of refocusing the focal point for each measurement would be highly impractical. The proposed work herein is focused at a tracked stereo-pair reconstruction; put more simply, the system herein is focused at measuring the whole visible brain surface dynamically throughout surgery capturing multidimensional displacements. These measurements may subsequently be used within nonrigid image-to-physical registration methodologies to account for soft-tissue brain shift, which can compromise neuronavigation systems. The proposed tracked stereo-scope solution allows for those measurements potentially to be continually captured throughout surgery without disrupting workflow and with optimal visibility of the tissue of interest. In the recent work, we demonstrated using position-fixed stereo-pair cameras for surface measurements. This work used two identical Grasshopper digital cameras produced by Point Grey Research, Inc. (Richmond, British Columbia, Canada) and served as an initial testing prototype for our mock cortical surface environment.9 Here, we have extended those approaches to a surgical microscope that is used clinically at our institution. We have developed an interactive environment that permits acquisition of stereo-image capture from the microscope, calibration of the cameras, and adjustment of parameters used for 3-D point cloud reconstruction. We have also equipped the microscope with a rigid body tracking star (Microscope Tracking Array, Brainlab Inc., Westchester, Illinois) to permit tracking of the microscope position within an IGS system. We show that with this device we can register 3-D point clouds that are acquired with the microscope in different positions thus permitting an intraopertaive calculation of cortical surface displacement. While the availability of microscope focal point tracking is possible with commercial IGS systems (although accuracy on these commercial systems is not widely reported), the implementation of a fully 3-D tracked approach necessary for measuring full cortical surface field displacements is lacking. The methods we are utilizing for stereovision reconstruction are well-referenced and our goal is to explore this within the scope environment and within the context of brain shift measurement.