Mark A. Ganter

Real-time finite element modeling for surgery simulation: an application to virtual suturing

Real-time finite element (FE) analysis can be used to represent complex deformable geometries in virtual environments. The need for accurate surgical simulation has spurred the development of many of the new real-time FE methodologies that enable haptic support and real-time deformation. These techniques are computationally intensive and it has proved to be a challenge to achieve the high modeling resolutions required to accurately represent complex anatomies. We present a new real-time methodology based on linear FE analysis that is appropriate for a wide range of surgical simulation applications. A methodology is proposed that is characterized by high model resolution, low preprocessing time, unrestricted multipoint surface contact, and adjustable boundary conditions. These features make the method ideal for modeling suturing, which is an element common to almost every surgical procedure. We describe constraints in the context of a Suturing Simulator currently being developed.

3D-printed mechanochromic materials.

We describe the preparation and characterization of photo- and mechanochromic 3D-printed structures using a commercial fused filament fabrication printer. Three spiropyran-containing poly(ε-caprolactone) (PCL) polymers were each filamentized and used to print single- and multicomponent tensile testing specimens that would be difficult, if not impossible, to prepare using traditional manufacturing techniques. It was determined that the filament production and printing process did not degrade the spiropyran units or polymer chains and that the mechanical properties of the specimens prepared with the custom filament were in good agreement with those from commercial PCL filament. In addition to printing photochromic and dual photo- and mechanochromic PCL materials, we also prepare PCL containing a spiropyran unit that is selectively activated by mechanical impetus. Multicomponent specimens containing two different responsive spiropyrans enabled selective activation of different regions within the specimen depending on the stimulus applied to the material. By taking advantage of the unique capabilities of 3D printing, we also demonstrate rapid modification of a prototype force sensor that enables the assessment of peak load by simple visual assessment of mechanochromism.

Skeleton-based modeling operations on solids

The skeleton is a lower-dimensional geometric abstraction that is useful for performing a number of important geometric operations on solid models. In this paper we develop skeleton-based algorithms that demonstrat,e the utility of the skeleton in addressing: (1) 1 evel-of-detail control, the generation of hierarchical representations that preserve overall shape but blur local boundary features; (2) hexahedral mesh generation, the decomposit,ion of a 3D shape into a collection of block elements suitable for finite element analysis; (3) shape interpolation and morphing, the generation of an “int>ermediate” shape from two given 3D shapes and the generation of a sequence of shapes that smoothly transform one shape into another; and (4) shape synthesis, the generation of an optimal shape from specifications of functional performance requirements and constraints. Besides our goal of providing novel solutions to these problems of significant, practical importance, we seek to illustrate the general usefulness of the skeleton as an intermediate geometric descrig tion that should be more widely implemented in commercial CAD systems.

An accelerated triangulation method for computing the skeletons of free-form solid models

Abstract Shape skeletons are powerful geometric abstractions that provide useful intermediate representations for a number of geometric operations on solid models including feature recognition, shape decomposition, finite element mesh generation, and shape design. As a result there has been significant interest in the development of effective methods for skeleton generation of general free-form solids. In this paper we describe a method that combines Delaunay triangulation with local numerical optimization schemes for the generation of accurate skeletons of 3D implicit solid models. The proposed method accelerates the slow convergence of Voronoi diagrams to the skeleton, which, without optimization, would require impraelically large sample point sets and resulting meshes to attain acceptable accuracy. The Delaunay triangulation forms the basis for generating the topological structure of the skeleton. The optimization step of the process generates the geometry of the skeleton patches by moving the vertices of Delaunay tetrahedra and relocating their centres to form maximally inscribed spheres. The computational advantage of the optimization scheme is that it involves the solution of one small optimization problem per tetrahedron and its complexity is therefore only linear (O(n)) in the number of points used for the skeleton approximation. We demonstrate the effectiveness of the method on a number of representative solid models.

Implicit reconstruction of solids from cloud point sets

Chek T. Lirnl George M. Turkiyyah2 hark A. G’ante# Duane W. storti~ University of Washington Seattle, WA 98195 {ctlim@u, george@ce,ganter@u, storti@u}.Washington.edu This paper describes a new technique that combines numerical optimization methods with triangulation methods for generating mathematical representations of solids from 3D point data. The solid representation obtained takes the form of an algebraic function whose level surface closely approximates the surface described by the data, The algebraic function is obtained via Implicit Solid Modeling, a constructive scheme for approximating Boolean volume set operations on implicitly defined primitive volumes, and is comprised of a blended union of spherical primitives. The parameters of the algebraic function are the spatial locations and radii of the spheres as well as the parameters that describe the blending of these primitives, Fitting an implicit solid model to a data set is formulated as a sequence of non-linear optimization problems of an increasing number of variables. The cost function we employ in these optimizations is a weighted combination of discrepancies in location (distance from points to boundary of reconstructed object), discrepancies in surface normals, and desired curvature characteristics of the reconstructed solid. Since a set of trivariate data points without any connectivity information is ambiguous, an infinite number of solids, in principle, can be constructed to fit them. Different characteristics of the solid can be specified through the cost function to create the most desirable interpretation of the data. The starting point of the optimization—corresponding to the starting configuration of the primitives—is determined by performing a 3D Delaunay triangulation on the data set, and is based on the locations and sizes of the resulting tetrahedral. The effectiveness of the algorithm is demonstrated through the reconstruction of several sample data sets, including a molar and a femur. Tradeoffs between accuracy and compactness of the representations are also examined. 1Department of Mechanical Engineering, FU-10 ‘Department of Civil Engineering, FX-10. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association of Computing Machinery.To copy otherwise, or to republish, requires a fee andlor specific permission. Solid Modeling ’95, Salt Lake City, Utah USA

Banded matrix approach to Finite Element modelling for soft tissue simulation

Realistic deformation of computer-simulated anatomica structures is computationally intensive. As a result, simple methodologies not based in continuum mechanics have been employed for achieving real-time deformation of virtual anatomy. Since the graphical interpolations and simple spring models commonly used in these simulations are not based on the biomechanical properties of tissue structures, these ‘quick and dirty” methods typically do not represent accurately the complex deformations and force-feedback interactions that can take place during surgery. Finite Element (FE) analysis is widely regarded as the most appropriate alternative to these methods. Extensive research has been directed toward applying this method to modelling a wide range of biological structures, and a few simple FE models have been incorporated into surgical simulations. However, because of the highly computational nature of the FE method, its direct application to real-time force-feedback and visualisation of tissue deformation has not been practical for most simulations. This limitation is due primarily to the overabundance of information provided by the standard FE approaches. If the mathematics are optimised through preprocessing to yield only the information essential to the simulation task, run-time computation requirements can be reduced drastically. We are currently developing such methodologies, and have created computer demonstrations that support real-time interaction with soft tissue. To illustrate the efficacy and utility of these fast “banded matrix” FE methods, we present results from a skin suturing simulator which we are developing on a PC-based platform.

The guide to glass 3D printing: developments, methods, diagnostics and results

Purpose – This purpose of this paper is to provide an overview of the steps and processes behind successfully adapting novel materials, namely virgin glass and recycled glass, to three‐dimensional printing (3DP).Design/methodology/approach – The transition from 3DP ceramic systems to glass systems will be examined in detail, including the necessary modifications to binder systems and printing parameters. The authors present preliminary engineering data on shrinkage, porosity, and density as functions of peak firing temperature, and provide a brief introduction to the complexities faced in realizing an adequate and repeatable firing method for 3D printed glass.Findings – Shrinkage behavior for the 3D printed recycled glass showed significant anisotropy, especially beyond peak firing temperatures of 730°C. The average shrinkage ratios for the slow‐ and fast‐axes to the Z‐axis were 1:1.37 and 1:2.74, respectively. These extreme differences can be attributed to the layer‐by‐layer production method and binder ...

Virtual reality for dermatologic surgery: virtually a reality in the 21st century.

In the 20th century, virtual reality has predominantly played a role in training pilots and in the entertainment industry. Despite much publicity, virtual reality did not live up to its perceived potential. During the past decade, it has also been applied for medical uses, particularly as training simulators, for minimally invasive surgery. Because of advances in computer technology, virtual reality is on the cusp of becoming an effective medical educational tool. At the University of Washington, we are developing a virtual reality soft tissue surgery simulator. Based on fast finite element modeling and using a personal computer, this device can simulate three-dimensional human skin deformations with real-time tactile feedback. Although there are many cutaneous biomechanical challenges to solve, it will eventually provide more realistic dermatologic surgery training for medical students and residents than the currently used models.

Creating fast finite element models from medical images.

The procedure for creating a patient-specific virtual tissue model with finite element (FE) based haptic (force) feedback varies substantially from that which is required for generating a typical volumetric model. In addition to extracting geometrical and texture map data to provide visual realism, it is necessary to obtain information for supporting a FE model. Among many differences, FE-based VR environments require a FE model with appropriate material properties assigned. The FE equation must also be processed in a manner specific to the surgical task in order to maximize deformation and haptic computation speed. We are currently developing methodologies and support software for creating patient-specific models from medical images. The steps for creating such a model are as follows: 1) obtain medical images and texture maps of tissue structures; 2) extract tissue structure contours; 3) generate a 3D mesh from the tissue structure contours; 4) alter mesh based on simulation objectives; 5) assign material properties, boundary nodes and texture maps; 6) generate a fast (or real-time) FE model; and 7) support the tissue models with task-specific tools and training aids. This paper will elaborate on the above steps with particular reference to the creation of suturing simulation software, which will also be described.

Skeleton-based three-dimensional geometric morphing

Abstract In this paper, we describe a method for generating geometric morphs between general 3D solid models. The method is based on the Euclidean skeleton and is capable of generating morphs between shapes that possess different feature sets and different topology. The essential concept that enables the morphing method is utilization of the trimmed skeleton of the symmetric difference as an intermediate shape. The intermediate shape is a valid solid model whose boundary does not self-intersect and is everywhere equidistant from the boundaries of the source shapes. We apply the skeleton-based intermediate shape generation procedure recursively to produce a sequence of shapes, referred to as a morph history, that gradually transform between the initial and target shapes. The method is sufficiently robust to handle significant changes in geometry and topology, such as the creation and annihilation of protrusions, indentations, internal holes and handles, and produces intuitive morph histories. The skeleton also establishes a correspondence between points on the boundaries of the source and target objects. Interpolation between corresponding points is performed to enable fast generation of a morph history consisting of a sequence of valid solid models. For source and target models that are sufficiently close, this interpolative morphing scheme generates results comparable to those obtained by the recursive skeletonization procedure, but with improved computational efficiency. The boundary point correspondence generated by the skeleton enables morphing with surface attributes (e.g., color, texture, surface roughness, and transparency). The skeleton-based procedure also allows for morphing between open curves or surfaces. A modification of the basic procedure allows the user to control the morph by specifying corresponding feature sets on the initial and final objects. Examples are presented to demonstrate the capabilities of the methods described.