Jeffrey HsinImage Overlay: Surgical Navigation System of TomorrowJeffrey Hsin3/8/01CIS 2Image Overlay: Surgical Navigation System of TomorrowThe demand for improved surgical precision and accuracy has brought forth the implementation of computer integrated surgical systems. A great deal of research has been invested in the possibility of using 3-D images of a particular surgical area in hopes to provide the surgeon with more surgical guidance. The focus of this discussion is on the 3-D image-overlay navigation system and its potential to be applied for clinical use inthe near future. Although surgical navigation systems have been widely used in many surgical fields including: neurosurgery, orthopedics, microsurgery, obstetrics, plastic surgery and other specialties, there still are many aspects about the system in need of refinement. Thepresent day technique, when using 3-D information, remains inconvenient for surgeons. The process requires that the surgeon view pre-operational 3-D images in the form of two-dimensional sectional images displayed away from the surgical area. This entails a great deal of effort for the surgeon to perceive 3-D information. The surgeon must (i) view the computer display showing the image (ii) reconstruct and formulate the 3-D image in his head (Results vary based on this step since the quality of reconstruction of the image in the surgeon’s mind is dependent upon his surgical experience as well as his anatomical knowledge). (iii) The surgeon must also be able to relate the 3-D image, now stored in his head, with the actual patient’s body when conducting the operation. Such procedures are disruptive in the flow of surgery and leave room for considerable margins of error. What a surgeon needs is a surgical navigation system that can eliminate such imprecise and arduous steps.3-D image overlay navigation is a display technique that will superimpose computer-generated images over the surgeon’s direct view of the patient’s surgical area. In other words with respect to the surgeon’s point of view, images will appear to be insideof the patient. The image overlay system is composed of four parts: computer graphic workstation, semi-transparent display, position tracking system, and software to calculate object positions and to transform images. The computer graphics station is responsible for the transformation as well as there-displaying of images in real-time, a minimum of thirty updates per second. The current workstation that is used in image overlay proto-types is the Indiglo-II R10K, made by Silicon Graphics, Inc. Thus far, this workstation has been able to re-display images with great efficiency. The semi-transparent display is either a half-silvered mirror or beam splitter glass.The display enables the surgeon to look through the glass while reflecting an image produced from a computer screen across from the display. The setup enables the surgeon to see the integration of a virtual image with the actual patient. Something worth noting is that, in pre-clinical trials with the image-overlay, the semi-transparent display was so effective that surgeons forgot that it was even there. Consequently, there was a great dealof accidental contact of surgical tools with the display, causing the glass display to shatter. Cotton pads have been placed on surgical tools to avoid damage to the displays in the case of contact. In terms of a tracking system, the OptoTrak 3020 has been widely used in image-overlay proto-types. The OptoTrak 3020 has the capability to make accurate spatial calculations and to keep up with the graphic workstation’s update speed. The trackerutilizes three cameras to triangulate the positions of LEDs (Light emitting diodes), which are attached to the objects getting tracked. When used as a part of the image overlay system, Optotrak keeps track of: the surgeon’s eye position, the display, the patient, as well as any surgical instruments involved in the operation. This is a crucial component asit provides real-time information on the whereabouts of each tracked object and its spatialrelationship with respect to the other objects being tracked.Software is needed to properly register image data to the external environment and to calculate transformations of the images so that they appear to be a part of the patient. Three-dimensional shapes of objects (usually obtained from multiple camera views) are matched to geometric features in image data such as a CT scan. This producesa registration transformation where all objects from these different sources are transformed into the same coordinate system. The software is also responsible for display of 3-D rendered images placed on the 2-D display given its coordinates with respect to the position of the surgeon’s eyepiece. At its current stage of development, image-overlay technology is focused on two specific areas of medical application: intra-operative guidance and surgical education. The system can assist the surgeon in the placement of the acetabular and femoral prosthetic implant components during total hip replacement surgery. In this particular surgical application, the image overlay would work in conjunction with the Hipnav system, a pre-clinical trial with simple feedback. Based on CT data, as well as bio-mechanic and kinematics simulation data, the image overlay will be able to display the proper implant positions as well as graphical indications such as arrows to guide the surgeon on which way he should move his instrument. Thus image-overlay has thepotential for greater accuracy in less time for the total hip replacement. Another application under investigation is the proper placement of screws or intramedullary rods in bone fragments. Image-overlay would allow the surgeon to visualize the positions and orientations of bone-fragments, generated by CT scan data. Bones are rigid bodies; hencemodels of the bone structure, built from pre-operative CT scan data, will remain consistent with the actual bone structure of the patient during surgery. Researchers have also been experimenting with ways to incorporate image-overlay with neurosurgical procedures. Based on MRI data, the image overlay system should be able to display the three-dimensional locations of tumors, vessels, as well as other critical areas in the brain. These models would allow the surgeon to pre-operatively plan out his approach, so as to minimize surgical exposure in