Researchers optimise presentation of virtual organ models
KARLSRUHE, Germany: During minimally invasive operations, a surgeon has to rely on the information displayed on the screen: a virtual 3-D model of the respective organ shows where a tumour is located and where sensitive vessels can be found. Soft tissue, such as the tissue of the liver, deforms during breathing or when the scalpel is applied. Endoscopic cameras record in real time how the surface deforms, but do not show the deformation of deeper structures, such as tumours. German scientists have now developed a real-time-capable computation method to adapt the virtual organ to the deformed surface profile.
The principle appears to be simple: based on computer tomography image data, scientists at the Karlsruhe Institute of Technology (KIT) construct a virtual 3-D model of the respective organ, including the tumour, prior to surgery. During the operation, cameras scan the surface of the organ and generate a rigid profile mask. To this virtual mould, the 3-D model is to fit snugly, like jelly to a given form. The Young Investigator Group of Dr Stefanie Speidel analysed this geometrical problem of shape adaptation from a physical perspective. “We modelled the surface profile as electrically negatively and the volume model of the organ as electrically positively charged,” Speidel explained. “Both thus attract each other and the elastic volume model slides into the immovable profile mask.” The adapted 3-D model then reveals to the surgeon how the tumour has moved with the deformation of the organ.
Simulations and experiments using a close-to-reality phantom liver have demonstrated that the electrostatic-elastic method even works when only parts of the deformed surface profile are available. This is the usual situation. The human liver is surrounded by other organs and, hence, only partly visible by endoscopic cameras. “Only those structures that are clearly identified as parts of the liver by our system are assigned an electric charge,” said Dr Stefan Suwelack who, as part of Speidel’s group, wrote his PhD thesis on this topic. Problems only arise if far less than half of the deformed surface is visible. In order to stabilise computation in such cases, the KIT researchers can use clear reference points, such as crossing vessels. Their method, however, in contrast to other methods, does not rely on such references from the outset.
In addition, the model of the KIT researchers is more precise than conventional methods because it also considers biomechanical factors of the liver, such as the elasticity of the tissue. So, for instance, the phantom liver used by the scientists consists of two different silicones: a harder material for the outer shell of the liver and a softer material for the inner tissue.
As a result of their approach, the young scientists also succeeded in accelerating the computation process. As they described shape adaptation by electrostatic and elastic energies, they found a single mathematical formula. Using this formula, even conventional computers equipped with a single processing unit work so quickly that the method is competitive. Unlike conventional computation methods, however, the new method is also suited for parallel computers. Using such a computer, the Young Investigator Group now plans to model organ deformations stably in real time.
The study, titled “Physics-based shape matching for intraoperative image guidance”, was published in the Medical Physics journal.