Roch M. Comeau

Transcranial Magnetic Stimulation during Positron Emission Tomography: A New Method for Studying Connectivity of the Human Cerebral Cortex

We describe a new technique permitting the mapping of neural connections in the living human brain. The method combines two well established tools of brain research: transcranial magnetic stimulation (TMS) and positron emission tomography (PET). We use TMS to stimulate directly a selected cortical area while simultaneously measuring changes in brain activity, indexed by cerebral blood flow (CBF), with PET. The exact location of the stimulation site is achieved by means of frameless stereotaxy. In the first study using this technique, we found significant positive correlations between CBF and the number of TMS pulse trains at the stimulation site, namely the left frontal eye field (FEF) and, most importantly, in the visual cortex of the superior parietal and medial parieto-occipital regions. The pattern of these distal effects was consistent with the known anatomic connectivity of the monkey FEF. We suggest that the combined TMS/PET technique offers an objective tool for assessing the state of functional connectivity without requiring the subject to engage in any specific behavior.

Intraoperative ultrasound for guidance and tissue shift correction in image-guided neurosurgery

We present a surgical guidance system that incorporates pre-operative image information (e.g., MRI) with intraoperative ultrasound (US) imaging to detect and correct for brain tissue deformation during image-guided neurosurgery (IGNS). Many interactive IGNS implementations employ pre-operative images as a guide to the surgeons throughout the procedure. However, when a craniotomy is involved, tissue movement during a procedure can be a significant source of error in these systems. By incorporating intraoperative US imaging, the target volume can be scanned at any time, and two-dimensional US images may be compared directly to the corresponding slice from the pre-operative image. Homologous points may be mapped from the intraoperative to the pre-operative image space with an accuracy of better than 2 mm, enabling the surgeon to use this information to assess the accuracy of the guidance system along with the progress of the procedure (e.g., extent of lesion removal) at any time during the operation. Anatomical features may be identified on both the pre-operative and intraoperative images and used to generate a deformation map, which can be used to warp the pre-operative image to match the intraoperative US image. System validation is achieved using a deformable multi-modality imaging phantom, and preliminary clinical results are presented.

Diagnosis of subtle focal dysplastic lesions: curvilinear reformatting from three-dimensional magnetic resonance imaging

Focal cortical dysplasia is a frequent cause of medically intractable partial epilepsy. These lesions are being increasingly identified by high quality images provided by magnetic resonance imaging (MRI), resulting in improved seizure control of surgically treated patients. Small dysplastic lesions are often missed by conventional MRI methods. The identification of subtle structural abnormalities by rectilinear slices is often limited by the complex convolutional pattern of the brain. We developed a method of curvilinear reformatting of three‐dimensional MRI data that improves the anatomical display of the gyral structure of the hemispheric convexities. It also reduces the asymmetric sampling of gray–white matter that may lead to false‐positive results. We present 5 patients in whom conventional two‐dimensional and three‐dimensional MRI with multiplanar reformatting was initially considered normal. Subsequent studies using curvilinear reformatting identified lesions in all. Four patients underwent surgery with histological diagnosis of focal cortical dysplasia. Three patients are seizure‐free and 1 had significant improvement in seizure control. These results indicate that an increase in the detection of subtle focal dysplastic lesions may be accomplished when one improves the anatomical display of the brain gyral structure by performing curvilinear reformatting. Ann Neurol 1999;46:88–94

Ultrasound/MRI Overlay with Image Warping for Neurosurgery

Performing a craniotomy will cause brain tissue to shift. As a result of the craniotomy, the accuracy of stereotactic localization techniques is reduced unless the brain shift can be accurately measured. If an ultrasound probe is tracked by a 3D optical tracking system, intra-operative ultrasound images acquired through the craniotomy can be compared to pre-operative MRI images to quantify the shift. We have developed 2D and 3D image overlay tools which allow interactive, real-time visualization of the shift as well as software that uses homologous landmarks between the ultrasound and MRI image volumes to create a thin-plate-spline warp transformation that provides a mapping between pre-operative imaging coordinates and the shifted intra-operative coordinages. Our techniques have been demonstrated on poly vinyl alcohol cryogel phantoms which exhibit mechanical and imaging properties similar to those of the human brain.

Ultrasound Probe Tracking for Real-Time Ultrasound/MRI Overlay and Visualization of Brain Shift

Stereotactic techniques are prevalent in neurosurgery. A fundamental assumption of stereotaxis is that the brain is a rigid body. It has been demonstrated, however, that following a craniotomy the brain tissue will shift by 10 mm on average. We are investigating intra-operative ultrasound, using an optical tracking system to record the position and orientation of the ultrasound probe, as a method of measuring and correcting for brain shift. We have determined that the accuracy to which ultrasound image coordinates can be tracked (including the errors involved in calibration) is better than 0.5 mm within the ultrasound image plane, and better than 2 mm perpendicular to the plane. We apply two visualization methods to compare the ultrasound and the pre-operative MRI: the first is real-time overlay of the ultrasound with the co-planar MR slice, and the second is the real-time texture mapping of the ultrasound video into a 3D view with the MRI. Our technique is demonstrated on a poly vinyl alcohol cryogel phantom.

Automatic non-linear MRI-ultrasound registration for the correction of intra-operative brain deformations

Objective: Movements of brain tissue during neurosurgical procedures reduce the effectiveness of using pre-operative images for intra-operative surgical guidance. In this paper, we explore the use of acquiring intra-operative ultrasound (US) images for the quantification of and correction for non-linear brain deformations. Materials and Methods: We will present a multi-modal registration strategy that automatically matches pre-operative images (e.g., MRI) to intra-operative US to correct for these deformations. The strategy involves using the predicted appearance of neuroanatomical structures in US images to build “pseudo ultrasound” images based on pre-operative segmented MRI. These images can then be non-linearly registered to intra-operative US using cross-correlation measurements within the ANIMAL package. The feasibility of the theory is demonstrated through its application to clinical patient data acquired during 12 neurosurgical procedures. Results: Results of applying the method to 12 surgical cases, including those with brain tumors and selective amygdalo-hippocampectomies, indicate that our strategy significantly recovers from non-linear brain deformations occurring during surgery. Quantitative results at tumor boundaries indicate up to 87% correction for brain shift. Conclusions: Qualitative and quantitative examination of the results indicate that the system is able to correct for non-linear brain deformations in clinical patient data.

Integrated MR and ultrasound imaging for improved image guidance in neurosurgery

We present a surgical guidance system that incorporates preoperative image information (e.g. MRI or CT) and intraoperative ultrasound imaging to detect brain tissue deformation during image guided neurosurgery. Many interactive IGNS implementations involve using pre-operative image information (e.g. MRI or CT) as a guide to the surgeons throughout the procedure. Tissue movement during a procedure can be a significant source of error in these systems. By incorporating intraoperative imaging, the target volume can be scanned at any time, and mapped into the pre- operative image space. The surgeon can use this information to assess the accuracy of the guidance system at any time during the procedure. In addition, the system can be used to provide updated information of the progress of this procedure (e.g. extent of lesion removal). Validation results using a deformable multimodality imaging phantom are presented as well as initial examples of the system used in surgery.

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.

Multimodality interactive stereoscopic image-guided neurosurgery

We demonstrate the use of integrated multi-modality data (MRI, MRA, DSA, PET and live video) and 3D stereoscopic imaging in the context of image-guided neurosurgery. We consider here the integration of anatomical data (MRI), vascular data (DSA and MRA) and functional data (PET) derived from the patient undergoing the surgical procedure. In addition live video images are merged with renderings of the data stored in the computer. The integration of multimodality data provides the surgeon with interactive and intuitive access to a comprehensive overview of the brain structures on which surgery is being performed. Ready access to this information enhances the surgeons ability to avoid critical vessels and structures of functional significance.

Interactive stereoscopic image-guided neurosurgery

We demonstrate the use of multi-modality and 3-D stereoscopic imaging in the context of image-guided neurosurgery. We consider here the integration of anatomical data (MRI), vascular data (DSA), and functional data (PET) derived from the same patient. Our workstation, which has a stereoscopic 3-D image display, is interfaced to a hand-held probe whose position coordinates in real space are constantly relayed to the computer during the procedure. This enables the probe to be visualized in images relating to individual or combined modalities during the surgical procedure. The integration of multi-modality data in this manner provides the surgeon with a comprehensive overview of brain structures on which he is performing surgery, or through which he is passing probes or cannulas, enabling critical vessels and/or structures of functional significance to be avoided.