Michael E. Leventon

Statistical shape influence in geodesic active contours

A novel method of incorporating shape information into the image segmentation process is presented. We introduce a representation for deformable shapes and define a probability distribution over the variances of a set of training shapes. The segmentation process embeds an initial curve as the zero level set of a higher dimensional surface, and evolves the surface such that the zero level set converges on the boundary of the object to be segmented. At each step of the surface evolution, we estimate the maximum a posteriori (MAP) position and shape of the object in the image, based on the prior shape information and the image information. We then evolve the surface globally, towards the MAP estimate, and locally, based on image gradients and curvature. Results are demonstrated on synthetic data and medical imagery, in 2D and 3D.

Level set based segmentation with intensity and curvature priors

A method is presented for segmentation of anatomical structures that incorporates prior information about the intensity and curvature profile of the structure from a training set of images and boundaries. Specifically the authors model the intensity distribution as a function of signed distance from the object boundary, instead of modeling only the intensity of the object as a whole. A curvature profile acts as a boundary regularization term specific to the shape being extracted, as opposed to simply penalizing high curvature. Using the prior model, the segmentation process estimates a maximum a posteriori higher dimensional surface whose zero level set converges on the boundary of the object to be segmented. Segmentation results are demonstrated on synthetic data and magnetic resonance imagery.

Multi-modal Volume Registration Using Joint Intensity Distributions

The registration of multimodal medical images is an important tool in surgical applications, since different scan modalities highlight complementary anatomical structures. We present a method of computing the best rigid registration of pairs of medical images of the same patient. The method uses prior information on the expected joint intensity distribution of the images when correctly aligned, given a priori registered training images. We discuss two methods of modeling the joint intensity distribution of the training data, mixture of Gaussians and Parzen windowing. The fitted Gaussians roughly correspond to various anatomical structures apparent in the images and provide a coarse anatomical segmentation of a registered image pair. Given a novel set of unregistered images, the algorithm computes the best registration by maximizing the log likelihood of the two images, given the transformation and the prior joint intensity model. Results aligning SPGR and dual-echo MR scans demonstrate that this algorithm is a fast registration method with a large region of convergence and sub-voxel registration accuracy.

Utilizing segmented MRI data in image-guided surgery

While the role and utility of Magnetic Resonance Images as a diagnostic tool are well established in current clinical practice, there are a number of emerging medical arenas in which MRI can play an equally important role. In this article, we consider the problem of image-guided surgery, and provide an overview of a series of techniques that we have recently developed in order to automatically utilize MRI-based anatomical reconstructions for surgical guidance and navigation.

Clinical Experience with a Hich Precision Image-Guided Neurosurgery System

We describe an image-guided neurosurgery system which we have successfully used on 70 cases in the operating room. The system is designed to achieve high positional accuracy with a simple and efficient interface that interferes little with the operating room’s usual procedures, but is general enough to use on a wide range of cases. It uses data from a laser scanner or a trackable probe to register segmented MR imagery to the patient’s position in the operating room, and an optical tracking system to track head motion and localize medical instruments. Output visualizations for the surgeon consist of an “enhanced reality display,” showing location of hidden internal structures, and an instrument tracking display, showing the location of instruments in the context of the MR imagery. Initial assessment of the system in the operating room indicates a high degree of robustness and accuracy.

Experimentation with a transcranial magnetic stimulation system for functional brain mapping

We describe functional brain mapping experiments using a transcranial magnetic stimulation (TMS) device. This device, when placed on a subjects scalp, stimulates the underlying neurons by generating focused magnetic field pulses. A brain mapping is then generated by measuring responses of different motor and sensory functions to this stimulation. The key process in generating this mapping is the association of the 3-D positions and orientations of the TMS probe on the scalp to a 3-D brain reconstruction such as is feasible with a magnetic resonance image (MRI). We have developed a registration system which not only generates functional brain maps using such a device, but also provides real-time feedback to guide the technician in placing the probe at appropriate points on the head to achieve the desired map resolution. Functional areas we have mapped are the motor and visual cortex. Validation experiments focus on repeatability tests for mapping the same subjects several times. Applications of the technique include neuroanatomy research, surgical planning and guidance, treatment and disease monitoring, and therapeutic procedures.

Visual hemifield mapping using transcranial magnetic stimulation coregistered with cortical surfaces derived from magnetic resonance images

The perception of a visual stimulus can be inhibited by occipital transcranial magnetic stimulation. This visual suppression effect has been attributed to disruption in the cortical gray matter of primary visual cortex or in the fiber tracts leading to V1 from the thalamus. However, others have suggested that the visual suppression effect is caused by disruption in secondary visual cortex. Here the authors used a figure-eight coil, which produces a focal magnetic field, and a Quadropulse stimulator to produce visual suppression contralateral to the stimulated hemisphere in five normal volunteer subjects. The authors coregistered the stimulation sites with magnetic resonance images in these same subjects using optical digitization. The stimulation sites were mapped onto the surface of the occipital lobes in three-dimensional reconstructions of the cortical surface to show the distribution of the visual suppression effect. The results were consistent with disruption of secondary visual cortical areas.

Three-Dimensional Reconstruction and Surgical Navigation in Pediatric Epilepsy Surgery

We have used MRI-based three-dimensional (3D) reconstruction and a real-time, frameless, stereotactic navigation device to facilitate the removal of seizure foci in children suffering from intractable epilepsy. Using this system, the location of subdural grid and strip electrodes is recorded on the 3D model to facilitate focus localization and resection. Ten operations were performed, including 2 girls and 8 boys ranging in age from 3 to 17, during which 3D reconstruction and surgical instrument tracking navigation was used. In all the cases, the patients tolerated the procedure well and showed no postoperative neurological deficits. We believe this to be a valuable tool for a complete and safe resection of seizure foci, thereby reducing the incidence of postoperative neurological deficits and significantly improving the overall quality of life of the patients.

Design considerations for a computer-vision-enabled ophthalmic augmented reality environment

We have initiated studies towards the design and implementation of an ophthalmic augmented reality environment in order to allow for a) more precise laser treatment for ophthalmic diseases, b) teaching, c) telemedicine, and d) real-time image measurement, analysis, and comparison. The proposed system is being designed around a standard slit-lamp biomicroscope. The microscope will be interfaced to a CCD camera, and the image sent to a video capture board. A single computer workstation will coordinate image capture, registration, and display. The captured image is registered with previously stored, montaged photographic and angiographic data, with superposition facilitated by funduslandmark-based fast registration algorithms. The computer then drives a high intensity, VGA resolution video display with adjustable brightness and contrast attached to one of the oculars of the slitlamp biomicroscope. Preliminary studies with a modified binocular operating microscope interfaced to a Sun Ultral Workstation and an IBM-compatible PC demonstrates proof-of-principle. Robust, accurate fundus image montaging is accomplished with Hausdorff-distance-based methods. For photographic and angiographic data where the vessel gray levels vary from light to dark, and intensity-based correlation methods fail, image-preprocessing with smoothing, edge-detection, and thresholding facilitates registration. Non-real-time registration (∼ 0.4–4.0 CPU seconds) is achieved by non-optimized simple template matching (translation only, Matrox Inspector) or Hausdorff-distance-based (translation, rotation, and scale) algorithms performed on edge-detected fundus photographic and angiographic images, and on images of a model eye. Successful registration and image overlay of color, monochromatic, and angiographic images is demonstrated. To our knowledge, these studies represent the first investigation towards design and implementation of an ophthalmic augmented reality environment.

Non-invasive functional brain mapping using registered transcranial magnetic stimulation

The authors describe a method for mapping the functional regions of the brain using a transcranial magnetic stimulation (TMS) device. This device, when placed on a subjects scalp, stimulates the underlying neurons by generating focused magnetic field pulses. A brain mapping is then generated by measuring responses of different motor and sensory functions to this stimulation. The key process in generating this mapping is the association of the 3D positions and orientations of the TMS probe on the scalp to a 3D brain reconstruction such as is feasible with a magnetic resonance image (MRI). The authors perform this matching process by (1) registering the subjects head position to an a priori MRI scan, (2) tracking the 3D position/orientation of the TMS probe, (3) transforming the TMS probe position/orientation to the MRI coordinate frame, and (4) tracking movements in the subjects head position to factor out any head motion. The resultant process generates a high resolution, accurate brain mapping which supports surgical planning, surgical guidance, neuroanatomy research, and psychiatric therapy. When compared to other functional imaging modalities, this approach exhibits much lower cost, greater portability, and more direct active control over the functional areas being studied.