At the University of Illinois, the team of Dr. Mary Rasmussen has designed a three-dimensional model of the inner ear from serial histologic sections. A powerful ImmersaDesk system, which is normally used in CAVE technology, displays the complicated interrelationships of the vital anatomic structures, embedded within the dense temporal bone, by means of true stereoscopic visualization. Surgeons and students can use a wand to make an extensive tour within the virtual temporal bone. This is done either in the same room or through networking with multiple interlinked ImmersaDesks between remote sites. In the latter case, the participants communicate by voice over standard telephone lines. The virtual temporal bone provides a time and cost saving learning tool for students as well as an ideal means for physicians to plan surgical approaches and procedures.
The temporal bone is one of the most complex parts of human anatomy for surgeons and medical students to explore. This portion of the skull contains the delicate organs of hearing and balance, the nerve which controls the muscles of facial expression, as well as blood vessels, nerves, and muscles which are involved in normal sensory and motor function. For surgical intervention in this region, the otologic specialist uses a specialized drill to enter the bone. Accurate knowledge of each anatomic structure's precise location must prevent the surgeon from erroneous drilling in order to safeguard the patient from permanent disability. The virtual temporal bone, developed by the VRML (Virtual Reality in Medicine Laboratory) team at the School of Biomedical and Health Information Sciences in Chicago, provides both trainees and surgeons with an interactive 3D visualization tool.
The 3D model is based on a single human temporal bone which has been sectioned into 630 separate slices of about 20 microns in thickness. Each section is stained, mounted on glass slides, scanned and saved as an image file. The alignment of the slides has to be performed by means of Adobe Photoshop software without any external reference points to aid in the reconstruction. Afterwards, the anatomic structures of special interest are traced and uniquely colour-coded. Small registration errors are smoothed through a filtering process before a marching cubes algorithm is applied to convert the data set into polygonal models.
To reconstruct the three small bones of hearing in the middle ear, referred to as ossicles, the researchers use Styrofoam and plaster to sculpt large models based on human ossicles which serve as a template. A standard computed tomography scans the models to produce a cross sectional data set. The marching cubes algorithm is used to transform the registered sections into polygonal models for integration into the final model. Once this model has been completed, it is ready for display on the ImmersaDesk, a transportable viewer with a drafting table format, offering a large angle, user-centred perspective. Special liquid crystal goggles support stereo vision and an associated receiver constantly tracks the exact head position, giving the user the impression of total immersion in the 3D scene. The special wand allows for real time control over the model with six degrees of freedom.
In the networking configuration, the virtual temporal bone is transmitted to the remote site by means of File Transfer Protocol (FTP). Whenever the state or position of the model changes, the information immediately is transmitted at approximately 10 frames per second, applying the User Datagram Protocol (UDP). The application programme verifies the data transfer in case different states at the participating sites should occur because of unreliable delivery. The advent of Internet2 will probably enable excellent voice communications over the network, as to replace the conventional telephone lines which still are being used. At present, the virtual temporal bone tele-immersive system has already been tested over both local and wide area networks for teaching purposes. The Virtual Reality in Medicine Laboratory presents a fully illustrated overview of this 3D educational environment on the Web.