Meshless Mechanics and Point-Based Visualization Methods for Surgical Simulations

Computer-based modeling and simulation practices have become an integral part of the medical education field. For surgical simulation applications, realistic constitutive modeling of soft tissue is considered to be one of the most challenging aspects of the problem, because biomechanical soft-tissue models need to reflect the correct elastic response, have to be efficient in order to run at interactive simulation rates, and be able to support operations such as cuts and sutures. Mesh-based solutions, where the connections between the individual degrees of freedom (DoF) are defined explicitly, have been the traditional choice to approach these problems. However, when the problem under investigation contains a discontinuity that disrupts the connectivity between the DoFs, the underlying mesh structure has to be reconfigured in order to handle the newly introduced discontinuity correctly. This reconfiguration for mesh-based techniques is typically called dynamic remeshing, and most of the time it causes the performance bottleneck in the simulation. In this dissertation, the efficiency of point-based meshless methods is investigated for both constitutive modeling of elastic soft tissues and visualization of simulation objects, where arbitrary discontinuities/cuts are applied to the objects in the context of surgical simulation. The point-based deformable object modeling problem is examined in three functional aspects: modeling continuous elastic deformations with, handling discontinuities in, and visualizing a point-based object. Algorithmic and implementation details of the presented techniques are discussed in the dissertation. The presented point-based techniques are implemented as separate components and integrated into the open-source software framework SOFA. The presented meshless continuum mechanics model of elastic tissue were verified by comparing it to the Hertzian non-adhesive frictionless contact theory. Virtual experiments were setup with a point-based deformable block and a rigid indenter, and force-displacement curves obtained from the virtual experiments were compared to the theoretical solutions. The meshless mechanics model of soft tissue and the integrated novel discontinuity treatment technique discussed in this dissertation allows handling cuts of arbitrary shape. The implemented enrichment technique not only modifies the internal mechanics of the soft tissue model, but also updates the point-based visual representation in an efficient way preventing the use of costly dynamic remeshing operations

Research on real-time physics-based deformation for haptic-enabled medical simulation

This study developed a multiple effective visuo-haptic surgical engine to handle a variety of surgical manipulations in real-time. Soft tissue models are based on biomechanical experiment and continuum mechanics for greater accuracy. Such models will increase the realism of future training systems and the VR/AR/MR implementations for the operating room

Investigation of the use of meshfree methods for haptic thermal management of design and simulation of MEMS

This thesis presents a novel approach of using haptic sensing technology combined with virtual environment (VE) for the thermal management of Micro-Electro-Mechanical-Systems (MEMS) design. The goal is to reduce the development cycle by avoiding the costly iterative prototyping procedure. In this regard, we use haptic feedback with virtua lprototyping along with an immersing environment. We also aim to improve the productivity and capability of the designer to better grasp the phenomena operating at the micro-scale level, as well as to augment computational steering through haptic channels. To validate the concept of haptic thermal management, we have implemented a demonstrator with a user friendly interface which allows to intuitively "feel" the temperature field through our concept of haptic texturing. The temperature field in a simple MEMS component is modeled using finite element methods (FEM) or finite difference method (FDM) and the user is able to feel thermal expansion using a combination of different haptic feedback. In haptic application, the force rendering loop needs to be updated at a frequency of 1Khz in order to maintain continuity in the user perception. When using FEM or FDM for our three-dimensional model, the computational cost increases rapidly as the mesh size is reduced to ensure accuracy. Hence, it constrains the complexity of the physical model to approximate temperature or stress field solution. It would also be difficult to generate or refine the mesh in real time for CAD process. In order to circumvent the limitations due to the use of conventional mesh-based techniques and to avoid the bothersome task of generating and refining the mesh, we investigate the potential of meshfree methods in the context of our haptic application. We review and compare the different meshfree formulations against FEM mesh based technique. We have implemented the different methods for benchmarking thermal conduction and elastic problems. The main work of this thesis is to determine the relevance of the meshfree option in terms of flexibility of design and computational charge for haptic physical model

An interactive meshless cutting model for nonlinear viscoelastic soft tissue in surgical simulators

In this paper, we present a novel interactive cutting simulation model for soft tissue based on the meshless framework. Unlike most existing methods that consider the cutting process of soft tissue in an over simplified manner, the presented model is able to simulate the complete cutting process that includes three stages: deformation before cutting open (DBCO), cutting open (CO), and deformation after cutting open (DACO). To characterize the complicated physical and mechanical properties of soft tissue, both nonlinearity and viscoelasticity were incorporated into the equations governing the motion of soft tissue. A line contact model was used for simulating the cutting process after analyzing the two major types of surgical instruments, i.e., scalpel and electrostick. The cutting speed and angle were taken into account in order to improve haptic rendering. Biomechanical tests and simulation experiments verified the validity of the introduced model. Specifically, the displacement vs. cutting force curves can be divided into three segments corresponding to the three stages of the cutting process. The results were also applied in a liver cutting simulating system and satisfactory visual effect and haptic feedback were achieved

Haptic Interaction with 3D oriented point clouds on the GPU

Real-time point-based rendering and interaction with virtual objects is gaining popularity and importance as di�erent haptic devices and technologies increasingly provide the basis for realistic interaction. Haptic Interaction is being used for a wide range of applications such as medical training, remote robot operators, tactile displays and video games. Virtual object visualization and interaction using haptic devices is the main focus; this process involves several steps such as: Data Acquisition, Graphic Rendering, Haptic Interaction and Data Modi�cation. This work presents a framework for Haptic Interaction using the GPU as a hardware accelerator, and includes an approach for enabling the modi�cation of data during interaction. The results demonstrate the limits and capabilities of these techniques in the context of volume rendering for haptic applications. Also, the use of dynamic parallelism as a technique to scale the number of threads needed from the accelerator according to the interaction requirements is studied allowing the editing of data sets of up to one million points at interactive haptic frame rates

Real-time hybrid cutting with dynamic fluid visualization for virtual surgery

It is widely accepted that a reform in medical teaching must be made to meet today's high volume training requirements. Virtual simulation offers a potential method of providing such trainings and some current medical training simulations integrate haptic and visual feedback to enhance procedure learning. The purpose of this project is to explore the capability of Virtual Reality (VR) technology to develop a training simulator for surgical cutting and bleeding in a general surgery

Trajectory planning with task constraints in densely filled environments

In this paper the problem of computing a rigid object trajectory in an environment populated with deformable objects is addressed. The problem arises in Minimally Invasive Robotic Surgery (MIRS) from the needs of reaching a point of interest inside the anatomy with rigid laparoscopic instruments. We address the case of abdominal surgery. The abdomen is a densely populated soft environment and it is not possible to apply classical techniques for obstacle avoidance because a collision free solution is, most of the time, not feasible. In order to have a convergent algorithm with, at least, one possible solution we have to relax the constraints and allow collision under a specific contact threshold to avoid tissue damaging. In this work a new approach for trajectory planning under these peculiar conditions is implemented. The method computes offline the path which is then tested in a surgical simulator as part of a pre-operative surgical plan

Frictional Contact in Interactive Deformable Environments

L\u2019uso di simulazioni garantisce notevoli vantaggi in termini di economia, realismo e di flessibilit\ue0 in molte aree di ricerca e in ambito dello sviluppo tecnologico. Per questo motivo le simulazioni vengono usate spesso in ambiti quali la prototipazione di parti meccaniche, nella pianificazione e nell\u2019addestramento di procedure di assemblaggio e disassemblaggio inoltre, di recente, le simulazioni si sono dimostrate validi strumenti anche nell\u2019assistenza e nell\u2019addestramento ai chirurghi, in particolare nel caso della chirurgia laparoscopica. La chirurgia laparoscopica, infatti, \ue8 considerata lo standard per molte procedure chirurgiche. La principale differenza rispetto alla chirurgia tradizionale risiede nella notevole limitazione che ha il chirurgo nell\u2019interagire e nel percepire l\u2019ambiente in lavora, sia nella vista che nel tatto. Questo rappresenta una forte limitazione per il chirurgo a cui \ue8 richiesta una lunga fase di addestramento prima di poter ottenere la necessaria destrezza per intervenire in laparoscopia con profitto. Queste limitazioni, d\u2019altra parte, rendono la laparoscopia il candidato ideale per l\u2019introduzione della simulazione nell\u2019addestramento. Attualmente sono disponibili in commercio dei software per l\u2019addestramento alla laparoscopia, tuttavia essi sono in genere basati su modelli rigidi, o modelli che comunque mancano del necessario realismo fisico. L\u2019introduzione di modelli deformabili migliorerebbe notevolmente l\u2019accuratezza e il realismo delle simulazioni. Nel caso dell\u2019addestramento il maggior realismo permetterebbe all\u2019utente di acquisire non solo le conoscenze motorie basilari ma anche capacit\ue0 e conoscenze di pi\uf9 alto livello. I corpi rigidi, infatti, rappresentano una buona approssimazione della realt\ue0 solo in situazioni particolari ed entro intervalli di sollecitazioni molto ristretti. Quando si considerano materiali non ingegneristici, come accade nelle simulazioni chirurgiche, le deformazioni non possono essere trascurate senza compromettere irrimediabilmente il realismo dei risultati. L\u2019uso di modelli deformabili tuttavia introduce notevole complessit\ue0 computazionale per il calcolo della fisica che regola le deformazioni e limita fortemente l\u2019uso di dati precalcolati, spesso utilizzati per velocizzare la fase di identificazione delle collisioni tra i corpi. I ritardi dovuti all\u2019uso di modelli deformabili rappresentano un grosso limite soprattutto nelle applicazioni interattive che, per consentire all\u2019utente di interagire con l\u2019ambiente, richiedono il calcolo della simulazione entro intervalli di tempo molto ridotti. In questa tesi viene affrontato il tema della simulazione di ambienti interattivi composti da corpi deformabili che interagiscono con attrito. Vengono analizzati e sviluppati differenti tecniche e metodi per le diverse componenti della simulazione: dalla simulazione di modelli deformabili, agli algoritmi di identificazione e soluzione delle collisioni e alla modellazione e integrazione dell\u2019attrito nella simulazione. In particolare vengono valutati i principali metodi che rappresentano lo stato dell\u2019arte nella modellazione di materiali deformabili. L\u2019analisi considera i fondamenti fisici su cui i modelli si basano e quindi sul grado di realismo che possono garantire in termini di deformazioni modellabili e la semplicit\ue0 d\u2019uso degli stessi (ovvero la facilit\ue0 di comprensione del metodo, la calibrazione del modello e la possibilit\ue0 di adattare il modello a situazioni differenti) ma viene considerata anche la complessit\ue0 computazionale di ciascun metodo in quanto essa rappresenta un fattore estremamente importante nella scelta e nell\u2019uso dei modelli deformabili nelle simulazioni. Il confronto dei differenti modelli e le caratteristiche identificate hanno motivato lo sviluppo di un metodo innovativo per fornire un\u2019interfaccia comune ai vari metodi di simulazione dei tessuti deformabili. Tale interfaccia ha il vantaggio di fornire dei metodi omogenei per la manipolazione dei diversi modelli deformabili. Ci\uf2 garantisce la possibilit\ue0 di scambiare il modello usato per la simulazione delle deformazioni mantenendo inalterati le altre strutture dati e i metodi della simulazione. L\u2019introduzione di tale interfaccia unificata si dimostra particolarmente vantaggiosa in quanto permette l\u2019uso di un solo metodo per l\u2019identificazione delle collisioni per tutti i differenti modelli deformabili. Ci\uf2 semplifica molto l\u2019analisi e la definizione dei requisiti di tale modulo software. L\u2019identificazione delle collisioni tra modelli rigidi generalmente precalcola delle partizioni dello spazio in cui i corpi sono definiti oppure sfrutta la suddivisione del corpo analizzato in parti convesse per velocizzare la simulazione. Nel caso di modelli deformabili non \ue8 possibile applicare tali tecniche a causa dei continui cambiamenti nella configurazione dei corpi. Dopo che le collisioni tra i corpi sono state riconosciute e che i punti di contatto sono stati identificati e necessario risolvere le collisioni tenendo conto della fisica sottostante i contatti. Per garantire il realismo \ue8 necessario assicurare che i corpi non si compenetrino mai e che nella simulazione delle collisioni tutti i fenomeni fisici di interesse coinvolti nel contatto tra i corpi vengano considerati: questi includono le forze elastiche che si esercitano tra i corpi e le forze di attrito che si generano lungo le superfici di contatto. L\u2019innovativo metodo proposto per la soluzione delle collisioni garantisce il realismo della simulazione e l\u2019integrazione con l\u2019interfaccia proposta per la gestione unificata dei modelli. Una caratteristica importante dei tessuti biologici \ue8 il comportamento anisotropico, dovuto, in genere, alla loro struttura fibrosa. In questa tesi viene proposto un nuovo metodo per aggiungere l\u2019anisotropia al comportamento dei modelli massa molla. Il metodo ha il vantaggio di mantenere la velocit\ue0 computazionale e la semplicit\ue0 di implementazione dei modelli massa molla classici e riesce a differenziare efficacemente la risposta del modello alle sollecitazioni lungo le differenti direzioni. Le tecniche descritte sono state integrate in due applicazioni che forniscono la simulazione della fisica di ambienti con corpi deformabili. La prima delle due implementa tutti i metodi descritti per la simulazione dei modelli deformabili, identifica le collisioni con precisione e le risolve fornendo la possibilit\ue0 di scegliere il modello di attrito pi\uf9 adatto, dimostrando cos\uec la fattibilit\ue0 dell\u2019approccio proposto. La limitazione principale di tale simulatore risiede nell\u2019alto tempo di calcolo richiesto per la simulazione dei singoli passi di simulazione. Tale limitazione \ue8 stata superata in una seconda implementazione che sfrutta il parallelismo intrinseco delle simulazioni fisiche per ottimizzare gli algoritmi e che, quindi, riesce a sfruttare al meglio la potenza computazionale delle architetture hardware parallele. Al fine di ottenere le prestazioni richieste per la simulazione di ambienti interattivi con ritorno di forza, la simulazione \ue8 basata su un algoritmo di identificazione delle collisioni semplificato, ma implementa gli altri metodi descritti in questa tesi. L\u2019implementazione parallela sfrutta le capacit\ue0 di calcolo delle moderne schede video munite di processori altamente paralleli e ci\uf2 permette di aggiornare la scena ogni millisecondo. Questo elimina ogni discontinuit\ue0 nel ritorno di forza reso all\u2019utente e nell\u2019aggiornamento della grafica della scena, inoltre garantisce il realismo necessario alla simulazione fisica sottostante. Le applicazioni implementate provano la fattibilit\ue0 della simulazione della fisica di interazioni complesse tra corpi deformabili. Inoltre, l\u2019implementazione parallela della simulazione rappresenta un promettente punto di partenza per la realizzazione di simulazioni interattive che potr\ue0 essere utilizzato in ambiti di ricerca differenti, quali l\u2019addestramento di chirurghi o la prototipazione rapida.The use of simulations provides great advantages in term of economy, realism, and adaptability to user requirements in many research and technological fields. For this reason simulations are currently exploited, for example, in prototyping of machinery parts, in assembly-disassembly test or training and, recently, simulations have also allowed the development of many useful and promising tools for the assistance and learning of surgical procedures. This is particularly true for laparoscopic intervention. Laparoscopy, in fact, represents the gold standard for many surgical procedures. The principal difference from standard surgery is the reduction of the surgeon ability to perceive the surgical scenario, both from visual and tactile point of view. This represents a great limitation for surgeons who undergo long training before being able to perform laparoscopic intervention with proficiency. This, on the other hand, makes laparoscopy an excellent candidate for the use of simulations for training. Some commercial training softwares are already available on the market, but they are usually based on rigid body models that completely lack the physical realism. The introduction of deformable models may leads to a great increment in terms of realism and accuracy. And, in the case of laparoscopy trainer it may allow the user to learn not only basic motor skills, but also higher level capabilities and knowledge. Rigid bodies, in fact, represents a good approximation of reality only in some situations and in very restricted ranges of solicitations. In particular, when non engineering materials are involved, as happens in surgical simulations, deformations cannot be neglected without completely loosing the realism of the environment. The use of deformable models, however, is limited for the high computational costs involved in the computation of the physics undergoing the deformations and because of the reduction in pre computable data in particular for collision detection between bodies. This represents a very limiting factor in interactive environments where, to allow the user to interactively control the virtual bodies, the simulation should be performed in real time. In this thesis we address the simulation of interactive environment populated with deformable models that interact with frictional contacts. This includes the analysis and the development of different techniques which implement the various parts of the simulation: mainly the methods for the simulation of deformable models, the collision detection and collision solution techniques but also the modelling and the integration of suitable friction models in the simulation. In particular we evaluated the principal methods that represent the state of the art in soft tissue modeling. Our analysis is based on the physical background of each method and thus on its realism in terms of deformations that the method can mimic and on the ease of use (i.e. method understanding, calibration and ability to adapt to different scenarios) but we also compared the computational complexity of different models, as it represents an extremely important factor in the choice and in the use of models in simulations. The comparison of different features in analyzed methods motivated us to the development of an innovative method to wrap in a common representation framework different methodologies of soft tissue simulation. This framework has the advantage of providing a unified interface for all the deformable models and thus it provides the ability to switch between deformable model keeping unchanged all other data structures and methods of the simulation. The use of this unique interface allows us to use one single method to perform the collision detection phase for all the analyzed deformable models, this greatly helped during the identification of requirements and features of such software module. Collision detection phase, when applied to rigid bodies, usually takes advantage of pre computation to subdivide body shapes in convex elements or to construct partitions of the space in which the body is defined to speed up the computation. When handling deformable models this is not possible because of the continuous changes in bodies shape. The collision detection method used in this work takes into account this problem and regularly adapt the data structures to the body configuration. After collisions have been detected and contact points have been identified on colliding bodies, it is necessary to solve the collision in a physics based way. To this extent we have to ensure that objects never compenetrate during the simulation and that, when solving collisions, all the physical phenomena involved in the contact of real bodies are taken into account: this include the elastic response of bodies during the contact and the frictional force exerted between each pair of colliding bodies. The innovative method for solving collision that we describe in this thesis ensures the realism of the simulation and the seamless interaction with the common framework used to integrate deformable models. One important feature of biologic tissues is their anisotropic behavior that usually comes from the fibrous structure of these tissues. In this thesis we propose a new method to introduce anisotropy in mass spring model. The method has the advantages of preserving the speed and ease of implementation of the model and it effectively introduces differentiation of the model behavior along the chosen directions. The described techniques have been integrated in two applications that allows the physical simulation of environments populated with deformable models. The first application implements all the described methods to simulate deformable models, it performs precise collision detection and solution with the possibility to chose the most suitable friction model for the simulation. It demonstrates the effectiveness of the proposed framework. The main limitation of this simulator, i.e. its high computation time, is tackled and solved in a second application that exploits the intrinsic parallelism of physical simulations to optimize the implementation and to exploit parallel architecture computational power. To obtain the performances required for an interactive environment the simulation is based on a simplified collision detection algorithm, but it features all the other techniques described in this thesis. The parallel implementation exploits graphic cards processor, a highly parallel architecture that update the scene every milliseconds. This allows the rendering of smooth haptic feedback to the user and ensures the realism of the physics simulation. The implemented applications prove the feasibility of the simulation of complex interactions between deformable models with physics realism. In addition, the parallel implementation of the simulator represents a promising starting point for the development of interactive simulations that can be used in different fields of research, such as surgeon training or fast prototyping