Modeling and modification of medical 3D objects. The benefit of using a haptic modeling tool

The Computer Laboratory of the medical faculty in Goteborg (Mednet) has since the end of 1998 been one of a limited numbers of participants in the development of a new modeling tool together with SensAble Technologies Inc [http:¿www.sensable.com/]. The software called SensAble FreeForm was officially released at Siggraph September 1999. Briefly, the software mimics the modeling techniques traditionally used by clay artists. An imported model or a user defined block of "clay" can be modified using different tools such as a ball, square block, scrape etc via the use of a SensAble Technologies PHANToM haptic arm. The model will deform in 3D as a result of touching the "clay" with any selected tool and the amount of deformation is linear to the force applied. By getting instantaneous haptic as well as visual feedback, precise and intuitive changes are easily made. While SensAble FreeForm lacks several of the features normally associated with a 3D modeling program (such as text handling, application of surface and bumpmaps, high-end rendering engines, etc) it's strength lies in the ability to rapidly create non-geometric 3D models. For medical use, very few anatomically correct models are created from scratch. However, FreeForm features tools enable advanced modification of reconstructed or 3D scanned models. One of the main problems with 3D laserscanning of medical specimens is that the technique usually leaves holes or gaps in the dataset corresponding to areas in shadows such as orifices, deep grooves etc. By using FreeForms different tools, these defects are easily corrected and gaps are filled out. Similarly, traditional 3D reconstruction (based on serial sections etc) often shows artifacts as a result of the triangulation and/or tessellation processes. These artifacts usually manifest as unnatural ridges or uneven areas ("the accordion effect"). FreeForm contains a smoothing algorithm that enables the user to select an area to be modified and subsequently apply any given amount of smoothing to the object. While the final objects need to be exported for further 3D graphic manipulation, FreeForm addresses one of the most time consuming problems of 3D modeling: modification and creation of non-geometric 3D objects.