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The practice of medicine and major segments of the biological sciences have always relied on visualizations of the relationship of anatomic structure to biological function. Traditionally, these visualizations have either been direct, via vivisection and post-mortem examination, or have required extensive mental reconstruction, as in examination of serial sections. The revolutionary capabilities of new 3D and 4D imaging modalities, as well as new 3D scanning microscope technologies, underscore the vital importance of spatial visualization to these sciences. Critically important capabilities required to fully exploit the usefulness of these multi-modality, multi-dimensional image data sets are rapid, accurate 3D segmentation, robust 3D registration and interactive 3D visualization. We have been using the high resolution, multi-modality Visible Human Data Sets, both male and female, to develop, test and validate algorithms and procedures to provide such capabilities. With such capabilities in hand, we have extended them to actual patient data in a variety of clinical applications.

Computer reconstruction and rendering of multidimensional medical and histological image data obviate the taxing need for mental reconstruction and provide a powerful new visualization tool for biologists and physicians. The use of virtual reality technology opens new realms in the teaching and practice of medicine and biology by allowing the visualizations to be manipulated with intuitive immediacy similar to that of real objects, by allowing the viewer to "enter" the visualizations to take up any viewpoint, by allowing the objects to be dynamic - either in response to viewer actions or to illustrate normal or abnormal motion and by engaging other senses such as touch and hearing (or even smell) to enrich the visualization. Biomedical applications extend across a range of scale from investigating the structure of individual cells through the organization of cells in a tissue to the representation of organs and organ systems, including functional attributes, such as electrophysiological signal distribution on the surface of an organ, and are of use as instructional aids as well as basic science research tools. Clinical applications include anatomy understanding, enhanced diagnosis and surgical planning and rehearsal.

Although the greatest potential for revolutionary innovation in the teaching and practice of medicine and biology lies in dynamic, fully immersive, multi-sensory fusion of real and virtual information data streams, such technology and capabilities are still under development, and not yet generally available to the medical practitioner. There are, however, several practical applications requiring varying levels of interactivity and immersion that can be delivered now, and that will have an immediate impact on medicine and biology.

Crucial to all these applications is the facile transformation between image space organized as a rectilinear N-dimensional grid of multi-valued voxels and model space organized as surfaces approximated by multiple planar tiles. Individualized clinical procedure planning requires routine rapid conversion of patient image data into possibly dynamic models. The most complex and challenging applications, those which show the greatest promise of significantly changing the practice of medical research or treatment, require an intimate and immediate union of image and model with real-world, real-time data. It may well be that the ultimate value of VR in medicine will derive more from the sensory enhancement of real experience than from the simulation of normally sensed reality. In this paper, we describe our approach to creating useful, interactive segmentations, registrations, and visualizations from volumetric image data, using first the Visible Human Male and Visible Human Female data sets, and then extending these to several examples of both normal and pathological anatomy from 3D CT and MRI images of patients. We outline criteria and protocols for validation of these procedures. We illustrate and summarize our experience with virtual endoscopy of the colon, airway and esophagus, with prostate and neurological surgery planning and rehearsal, with anesthesia delivery, and with 3D histological analysis of tissues in the eye, in the prostate and in studies of single cells. These examples demonstrate the broad scope and variety of clinical applications of volume visualization and virtual reality technology in medicine and in biology which have been made possible through the use of the Visible Human Data Sets.

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