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:
- geometrical modelling - graphical representation
- physical modelling (elastodynamics)
- model interaction - manipulation
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:
- elastic element (spring)
- viscous element (damper)
- plastic element
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:
- collision recognition:
check for space sections simultaneously used by two objects
- interaction management:
handling of different interaction types, evaluation (depending on type of effector)
- model modification:
Basic manipulation of the deformable object, change of the model structure
(i.e. cutting), important: efficient data structure
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.
Back to page 'Karlsruhe Endoscopic Surgery Trainer'
This page is maintained by
Christian Kuhn, FZK/IAI-SK
E-Mail: kuhn@iai.fzk.de
Last modification: January 24, 1997.