How It Works
CardinalSim allows users to feel the virtual structures during simulated dissections through the use of haptic interfaces and a method for 6 degree-of-freedom haptic rendering.
The integration of a 6-degree-of-freedom (DOF) haptic rendering algorithm allows for more realistic interaction of instruments with structures, replicating constraints that a surgeon is likely to face during actual dissections. Using this approach, contact is not only detected between the tip of a virtual instrument and structures (as is the case using conventional 3 DOF algorithms), but the entirety of the instrument is constrained by its contact with anatomy. This produces a considerably more stable and realistic experience during surgical rehearsal.
Example of 6 DOF interaction of a virtual drill with a temporal bone model. Contact is detected between the tip of the drill and the surface of the bone, allowing the user vary pressure to control the removal of removal of bone volume. In addition, collisions are also detected between the entirety of the drill and the specimen, eliminating interpenetration between the objects.
Using this algorithm, the experience of 6 DOF haptic interaction can be replicated with even less expensive 3 DOF haptic interfaces.
Our studies have indicated that when navigating incomplex environments, such as those encountered during temporal bone surgery, the addition of 6 DOF haptic rendering significantly improves completion of complex tasks.
Forsslund J, Chan S, Selesnick J, Salisbury K, Silva R, Blevins NH. The Effect of Haptic Rendering Degrees of Freedom on Task Performance in Virtual Environments, Stud Health Technol Inform 2013; 184:129-35.
A method for six degree-of-freedom haptic rendering of isosurface geometry embedded within sampled volume data is presented. The algorithm uses a quasi-static formulation of motion constrained by multiple contacts to simulate rigid-body interaction between a haptically controlled virtual tool, represented as a point-sampled surface, and volumetric isosurfaces. Unmodified volume data, such as CT or MR images, can be rendered directly with this approach, making it particularly suitable for applications in medical or surgical simulation.
The distinguishing characteristics of the presented method are:
- The algorithm executes at haptic update rates of 1000 Hz.
- A constraint-based approach allows for distributed contact using a massless proxy, enabling the rendering of very stiff contacts.
- Isosurfaces within volumetric data of any type (eg. CT scans) can be rendered directly at sub-voxel resolution without any preprocessing.
Key functional components are depicted in the diagram below.
The geometry of the virtual environment exists as an isosurface within a sampled volume. A central differencing scheme is used to estimate the normals on the surface for computing contact constraints and for shading in the visual rendering. The virtual tool is represented as a point shell derived from its polygonal model.
Every vertex of the point shell surface is queries against the volume intensity field during collision detection. A point’s path is subdivided into feature-sized segments to detect and enforce non-penetration. Interval bisection is used to refine the contact position.
Each contact imposes a constraint on the proxy’s motion:
Minimization of the “acceleration energy” (Gauss’ Principle) subject to the contact constraints yields the correct motion path for the proxy.
The method was tested using a variety of haptic devices including the Force Dimension sigma.7 and our custom-built 6-DOF µHaptic device depicted below. Performance characteristics of the algorithm on four different data sets were collected.
[image: Force Dimension Sigma 7 and 6-DOF µHaptic device]
The simulation remained stable throughout interactions that included hooking, wedging, and prying, even with a coupling stiffness set as high as 5000 N/m. The tool could be moved quickly in free space or in contact without feeling effects of artificial mass, inertia, or viscosity.
Physical interaction with the world using a rigid tool is inherently a six degree-of-freedom task. Our haptic rendering algorithm provides a means for exploring isosurfaces embedded within volumetric data using an arbitrarily shaped virtual instrument. The algorithm prevents object interpenetration and allows quick movements of the tool without conveying artificial mass or inertia, thus enhancing the perceived realism of the virtual object interaction.
Chan, S., Conti, F., Blevins, N. H., & Salisbury, K. Constraint-based six degree-of-freedom haptic rendering of volume-embedded isosurfaces. Proc. IEEE World Haptics Conference (2011).