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Surgical Simulators

In a strange twist of logic, endoscopic surgeons are starting to benefit from the technologies used in flight simulators. Researchers at Rensselaer Polytechnic Institute, at Harvard Medical School, Albany Medical Centre, and the Massachusetts Institute of Technology have combined to work on surgical simulators.

The task at hand is the development of a surgical simulator for minimally invasive surgery, that uses the standard endoscopic tools the surgeons would use in an actual surgery, and will use VR technology to add in full sensation. In other words, surgeons will feel the correct levels of resistance on the tools as they slide past different types of tissue. Resting a tool on the virtual liver say, they will actually feel the liver below the tool with full haptic feedback - size, shape, deformity, the pressure will work as though it was physical. The spleen, the kidneys, a rib... Everything feels real.

Likewise, the tools can manipulate the virtual organs, opening them up, lifting and moving to the side where length of tendon and blood supply allows, all the time feeling like they are moving physical objects.

The point of the task is to create a virtual surgery that is indistinguishable from an actual surgery, facilitating surgeon training in a way never before possible - practising an operation 'for real' over and over again, until they are confident the patient will survive.

"The most important single factor that determines the success of a surgical procedure is the skill of the surgeon," said Suvranu De, assistant professor of mechanical, aerospace, and nuclear engineering and director of the Advanced Computational Research Lab at Rensselaer. It is therefore not surprising, he notes, that more people die each year from medical errors in hospitals than from motor vehicle accidents, breast cancer, or AIDS, according to a 2000 report by the Institute of Medicine.

"The sense of touch plays a fundamental role in the performance of a surgeon," De said. "This is not a video game. People's lives are at stake, so when training surgeons, you better be doing it well."

One of the key challenges this goal has to overcome, is the sheer, raw computation required. To program the realism of touch feedback from a surgical probe navigating through soft tissue, the researchers must develop efficient computer models that perform 30 times faster than real-time graphics, solving complex sets of partial differential equations about a thousand times a second.

The major challenge to current technologies is the simulation of soft biological tissues, according to De. Such tissues are heterogeneous and viscoelastic, meaning they exhibit characteristics of both solids and liquids - similar to chewing gum or silly putty. And surgical procedures such as cutting and cauterising are almost impossible to simulate with traditional techniques.

To overcome these barriers, De's group has developed a new computational tool called the Point-Associated Finite Field (PAFF) approach, which models human tissue as a collection of particles with distinct, overlapping zones of influence that produce co-ordinated, elastic movements.

A single point in space models each spot, while its relationship to nearby points is determined by the equations of physics. The localised points migrate along with the tip of the virtual instrument, much like a roving swarm of bees.

This method enables the program to rapidly perform hundreds of thousands of calculations for real-time touch feedback, making it superior to other approaches, according to the researchers. "Our approach is physics-based," De said.
"The technologies that are currently available for surgical simulation are mostly graphical renderings of organs, and surgeons are not very happy with them."

The same physics-based technology can be used to model blood flow and the generation of smoke during cauterisation, which is often used to burn tissue and stop haemorrhaging.

Currently, the team uses video image feedback from past, actual endoscopic surgical procedures to enhance the visual realism of the VR. However, this of course has the distinct disadvantage that the actions of the surgeon on the body are not replicated precisely on the screen. Currently, the 'swarms of bees' particle engine does not visualise well in real-time.

Additionally, the researchers are currently conducting experiments on human cadavers - yes, dead bodies - to evaluate the mechanical properties of human organs, something that has not been done before. This data, together with the particle swarming technology, is being developed to be combined in a giant database of human anatomy for virtual replication.

After developing a successful prototype, De hopes to apply the model to a much wider class of medical procedures. "The grand vision," he said, "is to develop a palpable human - a giant database of human anatomy that provides real-time interactivity for a variety of uses, from teaching anatomy to evaluating injuries in a variety of scenarios. In the long run, a better simulator could even help in the design of new surgical tools and techniques."

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