Modelling Conception of the Simulation Environment

The basic new development is the modelling concept of the simulation environment. Goal is to achieve a highly realistic three-dimensional simulation of human soft tissue behaviour under effect of external stimulations. This leads to a system of 'deformable objects' with specified geometrical shape and natural physical/mechanical behaviour. Another critical subject is the realistic simulation of the interaction between deformable objects and instruments and the manipulation of the virtual tissues in realtime.

These demands require a new, homogeneous modelling concept to get an efficient simulation. For the core of the simulation environment, following three topics must be integrated:

Physical Modelling

Imitation of the living soft tissue

For simulation of the physical properties, the construction of an equivalent mathematical model is necessary which is based mainly on principles of mechanics. Goal is a simulation model with high-realistic mechanical and physical behaviour, resulting in 'deformable objects'. We use a physically-based method: the model mass is discretized to zero-dimensional mass-knots. These mass knots are connected by 'binding elements'. Examples of such force-transferring binding elements are: This method leads to a simple finite-element-system, a so called 'nodal net model'

Based on nodal nets, the MESD-procedure (methods of modelling for realtime capable simulation and manipulation of deformable objects using physically-based nodal systems) was developed. This models are characterized by the application of basic physical equations on discrete object components (mass knots and binding elements). The MESD-procedure is optimized for realtime capable simulation of body movements and deformation behaviour.
A net with n mass knots results in a system of n coupled non-linear differential equations 2. order (mathematically corresponding to a damped spring mass pendulum). This differential equation system must be solved numerically in realtime.

'Nodal net'-system (JPEG 79 kB)
(red: simple mass knots, blue: central knots, brown: couple knots, yellow: binding elements)

Object Primitives (JPEG 75 kB)

Geometrical Modelling

Description of the 3D geometric shapes of the organs, tissue and vessels and graphical representation

The graphical representation consists of the visualization of the simulation results on a display using geometrical models of the deformable objects. Goal is the realistic imitation of the endoscopic view. At the present stage we use 'SOFTIMAGE' for tissue modelling (B-Spline surfaces). A KISMET data-converter was developed to process the data into the KISMET- specific format. After conversion, each part is further interactively processed inside KISMET to add realistic color and lighting parameters as well as surface texture. Because of efficiency, we internally use pure surface representation of the geometries. Two methods were developed, an approach based on free-form surfaces (NURBS, Non Uniform Rational B-Spline Surface) as well as the direct output of polygonal nets (extended polyhedrons). The aim is a fast graphical data processing to obtain a frame rate which makes interactive manipulation possible.

To connect the elastodynamical model with the geometrical model, the positions of the mass knots are projected onto the corresponding controlpoints (NURBS) or vertices of the polygonal net (extended polyhedron). Additionally, the resulting net can be refined by generating additional vertices (interpolation) to enhance the visual quality.

Graphical and Geometrical Models (JPEG 91 kB)
(upper row: left: nodal net, middle: NURBS, right: textured surface / lower row: left: simple polyhedron, middle: refined polyhedron, left: Gouraud-shaded polyhedron)

Model Interaction

Interactive manipulation of the deformable objects by the operator using the simulated instruments

An important demand for interactive systems is the possibility of direct influencing the simulation scenario including the immediate reaction of the concerned objects. Therefore the developed methods necessarily have to be realtime capable. The stimulation is done by the user by means of the simulated MIS-instruments.

. The interaction can be divided in three steps:

Especially the reactions of the deformable objects dependent on the manipulation have to be specified and simulated. Refering to object modifications, it is necessary to change and adapt the structure of the models. In addition to the physical properties, rule-based behaviour models are used which define the object's reaction on different manipulations. The greatest difficulties are the realtime demand, the temporal discretization (because of computer simulation) and the spatial discretization (because of nodal net model).

We have implemented so far several typical surgical tasks like grasping, cutting, coagulating and applicatio of clips.

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This page is maintained by Christian Kuhn, FZK/IAI-SK
Last modification: January 24, 1997.