The major challenge for virtual reality simulation of surgical procedures lies in the realistic rendering of organ deformation when touched by medical instruments, and the resulting degree of haptic feedback, that is experienced by the surgeon. This is especially true in the delicate case of abdominal trauma, where open surgery is required. At HT Medical, based in Maryland, Dr. Morten Bro-Nielsen and his team have developed a testbed for simulation of open surgery from the front to remove a shattered kidney. The experimental approach involved the most innovative technologies to generate deformable organ models and a lot of high performance computational power to simulate complex actions, like cutting, bleeding, and force feedback. Fully realistic virtual scenes are impossible to create at present, so some of the surgical steps have to be substituted or suggested with a set of multimedia interfaces.
Basically, there are two methods to simulate organ deformation. The older one, applied in the KISMET simulator in Karlsruhe for instance, uses simple surface-based mass-spring models. These are provided with a volumetric behaviour through the introduction of parent nodes, connecting nodes on different sides of the object. A more sophisticated and complex system implements finite element modelling (FEM) techniques for a faster and more accurate result. In the HT Abdominal Trauma Simulator (HATS) testbed however, the researchers preferred to introduce standard mass-spring systems because these mathematically explicit models are better suited to cut the surface to accommodate incisions.
The algorithm has to take into account a dozen different cutting operations on individual triangles. For tubular models, such as the arteries, a linear mass-spring model is used as the spine of the tubular structure, and based on connected contours. Maybe, the team will still try to deform, by means of the Fast Finite Elements method, the organ model of the shattered kidney after its removal. To simulate the blood flow on the surface of the polygonal models, the scientists have developed a diffusion-style algorithm, which can even modify the polygonal surface, thus creating the illusion that the blood lies on top of it. In order to detect organ collision and to generate force feedback, the researchers selected the GHOST software package, designed by SensAble Technologies. The Phantom device offers 6 degrees of freedom for motion, with 3 degrees of freedom for haptic feedback.
The HATS testbed configuration consists of a patient mannequin, lying on the surgery table. A computer monitor is horizontally installed at the place where the patient's abdomen is located, to serve as the exposed operative field. The rest of the virtual body is covered by blue surgical draping. The software architecture has been divided over two platforms. The virtual reality simulation software runs on an SGI Onyx 2 with shared memory parallel processing, and includes four processors. The collision detection and force feedback functions have been put on an SGI Impact. Both systems are being interconnected through the Parallel Virtual Machine (PVM) communication protocol to perform the real time distributed simulation processing. In addition, a Graphical User Interface (GUI) provides patient records as well as training instructions and guidance.
The virtual scene offered by the Abdominal Trauma Simulator project, sponsored by the Defense Advanced Research Projects Agency (DARPA), has been conceived to look as real as possible, using all the current most advanced technologies. Nevertheless, perfect life-like simulator building for open surgery still remains a dream but slowly, the walls of technical limitation are crumbling down. Both the KISMET Endoscopic Surgery Trainer and the HT Abdominal Trauma Simulator, which was presented to the audience of this year's "Medicine Meets Virtual Reality" Conference in San Diego, figure as milestones on the way towards authentic virtual surgery simulation.