Interaction model between elastic objects for haptic feedback considering collisions of soft tissue

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Summary

The simulation of organ–organ interaction is indispensable for practical and advanced medical VR simulator such as open surgery and indirect palpation. This paper describes a method to represent real-time interaction between elastic objects for accurate force feedback in medical VR simulation. The proposed model defines boundary deformation of colliding elements based on temporary surface forces calculated by temporary deformation. The model produces accurate deformation and force feedback considering collisions of objects as well as prevents unrealistic overlap of objects. A prototype simulator of rectal palpation is constructed on general desktop PC with a haptic device, PHANToM. The system allows users to feel different stiffness of a rear elastic object located behind another elastic object. The results of experiments confirmed the method expresses organ–organ interaction in real-time and produces realistic and perceivable force feedback.

Introduction

Virtual reality (VR) technologies enable physicians to interact with flexibly customized simulated environments based on visual, auditory and haptic feedback without potentially harmful contact with real patients. For this reason, VR-based simulation has attracted considerable attention as a key technology for the advancement of medical treatments and improvement of quality of human life. In the field of medicine, VR simulators are applied for uses such as education, therapy and rehabilitation, procedural training, surgical planning, rehearsal, and interoperative support [1], [2]. Though many simulators have been developed, and a few even commercialized [3], [4], [5], [6], most have dealt with single organ objects without handling collisions between multiple organ objects [4], [5], [6], [7], [8], [9]. This makes them unsuitable for VR simulation of the human body, a system with many organs which often collide. Haptic feedback is especially important in delicate surgical pressures requiring fine sensations, especially when slightly excessive pressure can injure a patient. Haptic sensation is also significant in palpation: as the physician examines the characteristics of an organ beneath the body surface with the tips of his or her fingers, collisions of the soft tissues are inevitable. This paper proposes a model of interaction between soft tissues, in order to provide a virtual environment simulating haptic feedback from the collisions of soft tissues.

Palpation and surgery simulations require the use of physics-based deformable models to accurately calculate the deformation and force caused by physical action on soft tissue. This adds considerably to the challenge of simulation, however, as physics-based deformation models generally require more computations than the geometry-based deformation models used in computer graphics. A comprehensive simulation of multiple characteristics of soft tissue all at once is tremendously difficult. Elasticity, a property related to force and displacement, is one of the most important characteristics of soft tissue. Accordingly, this paper treats soft tissues as elastic objects and seeks to model the interactions between them. The interaction model presented here must perform three important functions.

  • 1.

    To allow interactive manipulation in real-time.

  • 2.

    To take into account the physical properties of colliding objects.

  • 3.

    To produce an adequate visual reality.

Interactive manipulation, an operation performed in both palpation and surgery, requires real-time computation of soft tissue deformation and reaction forces. When a soft elastic object and a hard object collide, the former deforms more, as shown in Fig. 1. In addition, the reaction forces during the collision depend on the extent of the deformations in the collision area. Thus, the interaction model must represent the deformation on the basis of the physical properties of both elastic objects. Visual reality is one of the most important functions for effective simulation. Excessive invasion of colliding objects must be avoided.

In this paper, we propose an interaction model between elastic objects that performs the above functions. After describing the proposed model, we evaluate its performance and validity by applying it to the development of a rectal palpation simulator.

Section snippets

Background

The simulation of physical phenomena has been a key technology to enhance visual and haptic reality in medical simulations. Studies in biomechanics and computer graphics in the field have devoted particularly close attention to soft tissue modelling [10], [11], [12], [13]. Many kinds of physics-based deformable models have been proposed for deformation and force feedback [13], [14], [15], [16], [17], [18], [19]. One such model, the mass-spring model, represents an object as points of mass

Design considerations

There are two ways to represent multiple organs: with a single elastic object representing multiple organs or with multiple elastic objects representing multiple organs.

Several simulators [26], [27] use the former method, treating multiple organs as a single elastic object. Methods which rely on the filling of finite elements into gaps between elastic objects are unsuitable in situations where contact regions often change. When the contact regions of organs are changed, the topology of each

Interaction model between elastic objects

Our interaction model between elastic objects focuses on physics-based force feedback taking into account the physical properties of colliding objects. We define “interaction” as the process which determines the deformation in the collision area based on the physical properties of the colliding objects. Though the extent of deformation by collisions is theoretically unsolved, as mentioned earlier, it presumably depends on the physical properties of the colliding objects. Thus, the deformation

Structure of the system

As previously mentioned, collisions between multiple organs are especially important to consider in the haptic displays of palpation simulations. Our group addresses this challenge by developing a rectal palpation simulator and using it to evaluate the proposed model. Rectal palpation is a very common and important manipulation in urology. A physician inserts his or her index finger through the anus of the patient and palpates the prostate gland indirectly through the rectal wall to assess the

Calculation time

We examined the calculation time when a sphere-shaped object A and cubic object B collide. Both objects are in contact and a moving point pushes a point of object A from the opposite side of the contact region. The calculation times for deformation, interaction, and total computation of one cycle are shown in Fig. 8.

The horizontal axis indicates the number of component nodes of both objects A and B. The vertical axis indicates the calculation time (ms). Only about 10% of the nodes of both

Lessons learned and future plans

Nakao et al. [31] have reported that skilled cardiovascular surgeons can accurately recall and identify the relative levels of stiffness of a normal aorta and hardened aorta through the sense of touch. In the present experiments using a simulator of rectal palpation, we had the tests performed by skilled urologists on the assumption that they could accurately recall the tactile sensations of the fingertip when pushing the prostate through the rectal wall. Their answers indicated that the

Acknowledgements

This research was supported by Grant-in-Aid for Scientific Research (S) (16100001) and Young Scientists (A) (16680024) from The Ministry of Education, Culture, Sports, Science and Technology, Japan.

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