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Der Roboter als Hilfsmittel in der Chirurgie // International Mechatronik Forum, Linz, October 2006
(2006)
Mechanical properties of biological tissue represent important diagnostic information and are of histological relevance (hard lesions, "nodes" in organs: tumors; calcifications in vessels: arteriosclerosis). The problem is, that such information is usually obtained by digital palpation only, which is limited with respect to sensitivity. It requires intuitive assessment and does not allow quantitative documentation. A suitable sensor is required for quantitative detection of mechanical tissue properties. On the other hand, there is also some need for a realistic mechanical display of those tissue properties. Suitable actuator arrays with high spatial resolution and real-time capabilities are required operating in a haptic sensor actuator system with different applications. The sensor system uses real time ultrasonic elastography whereas the tactile actuator is based on electrorheological fluids. Due to their small size the actuator array elements have to be manufactured by micro-mechanical production methods. In order to supply the actuator elements with individual high voltages a sophisticated switching and control concept have been designed. This haptic system has the potential of inducing real time substantial forces, using a compact lightweight mechanism which can be applied to numerous areas including intraoperative navigation, telemedicine, teaching, space and telecommunication.
Mechanical properties of biological tissue represent important diagnostic information and are of histological and pathological relevance. Malignant tumors are significantly stiffer and more immobile than surrounding healthy tissue. Hard calcifications in vessels occur due to arteriosclerosis. The problem is, that such information is usually not available or can only be obtained by manual palpation, which is subjective and limited in sensitivity. It requires intuitive assessment and does not allow quantitative documentation. Unfortunately, none of the established medical imaging equipment such as magnetic resonance imaging (MRI) or X-ray computed tomography (CT) can provide direct measure of tissue elasticity. On the one hand a suitable sensor is required for quantitative measurement of mechanical tissue properties. On the other hand there is also some need for a realistic haptic display of such tissue properties. Suitable actuator arrays with high spatial resolution acting in real time are required. A haptic sensor actuator system is presented in this paper including a sensitive sensor part and an actuator array for different applications. The mechanical consistency of an object is to be locally specified using a sensor system and represented perceptibly in a remote position on an actuator system for the user. The sensor system uses ultrasound (US) elastography, whereas the actuator array is based on electrorheological (ER) fluids.
This paper reports the first step in the development of a tactile array suitable for the presentation of haptic information in virtual reality. The system is based on the electric field dependence of the viscosity of electrorheological fluids. The simulation, as well as the experimental realization of single tactels is described. The mathematical approach is based on the Eckart model (Eckart W 2000 Continuum Mech. Thermodyn. 12 341–62) and its validity is demonstrated by comparing the resulting yield stress with the experimental results from Wunderlich (2000 Dissertation Universität Erlangen-Nürnberg). Two different tactel designs are realized and the experimental results are compared with numerical simulation. The design of modification B is shown to be applicable for the realization of an actuator array with high spatial resolution.
The concept and some steps in the development of a new actuator system which enables the haptic perception of mechanically inhomogeneous virtual objects are introduced. The system consists of a two-dimensional planar array of actuator elements containing an electrorheological (ER) fluid. When a user presses his fingers onto the surface of the actuator array, he perceives locally variable resistance forces generated by vertical pistons which slide in the ER fluid through the gaps between electrode pairs. The voltage in each actuator element can be individually controlled by a novel sophisticated switching technology based on optoelectric gallium arsenide elements. The haptic information which is represented at the actuator array can be transferred from a corresponding sensor system based on ultrasonic elastography. The combined sensor-actuator system may serve as a technology platform for various applications in virtual reality, like telemedicine where the information on the consistency of tissue of a real patient is detected by the sensor part and recorded by the actuator part at a remote location.
The use of ultrasound systems for medical examination and diagnosis is nothing new. The extension of ultrasound techniques for real time elastographic analysis purposes represents a more recent development. Nevertheless, as they stand all such techniques rely on the interpretation of two-dimensional visual data displayed on a video screen. In reality however, a medical doctor will make as much use of exploratory touch as he or she does vision, making the simultaneous portrayal of both video and tactile information most desirable [Böse H, Monkman GJ, Freimuth H, Ermert H. Haptisches Sensor-Aktor-System uaf der grundlage der Echtzeitelastographie sowie von elktro- und magnetorheologischen Materialien “HASASEM” [3]. BMBF Antrag 01 IR A14D, Oktober 2000].
This paper discusses the preliminary tests and basic design parameters for single tactels using electrorheological fluids. The final aim is to produce a prototype three-dimensional tactile display comprising electrically switchable micro-machined cells whose mechanical moduli are governed by phase changes experienced by electrorheological fluids. This will be integrated with the latest elastographic ultrasonic sensor systems in order to present the human fingers with controllable surfaces capable of emulating biological tissue, muscle and bone.
The ability to image the elastic properties of tissue is potentially useful in a variety of applications. The field of elastic imaging has grown in response to the Potential use of such information in medical diagnosis. Real time ultrasound elastography represents a recent development in determining strain and elasticity distributions. Nevertheless, commonly used imaging techniques rely on the interpretation of two dimensional visual data displayed on a video screen. In reality however, physicians often prefer tactile exploration making the simultaneous portrayal of both video and haptic information most desirable.
Since the 1970's many alphanumeric to tactile data conversion methods have been investigated, mainly with the ultimate aim of assisting the blind. More recently, interest has been directed toward the display of pictures on haptically explorable surfaces - Tactile imaging. Such a system would allow surgeons to examine hard sectors contained within soft tissue, and thereby assist in operations held remotely. The expansion of ultrasound elastography to 3D formats would mean the ability to haptically explore regions of the body normally inaccessible to human hands. For three-dimensional imaging the acquisition of sequential tomographic slices using Elastography, combined with image segmentation, enables the reconstruction, quantification and visualisation of tumour volumes.
In a collaborative project between four research Institutes, the aim is to produce a prototype three dimensional tactile displays comprising electrically switchable micromachined cells, whose mechanical moduli are governed by phase changes experienced by electrorheological and/or magnetorheological fluids. This will be integrated with a sensory ultrasonic elastography in order to present the human fingers with controllable surfaces capable of emulating biological tissue, muscle and bone.
Since the 1960s many alphanumeric to tactile data conversion methods have been investigated, mainly with the ultimate aim of assisting the blind. More recently, interest has been directed toward the display of pictures on haptically explorable surfaces – tactile imaging – for a range of medical, remote sensing and entertainment purposes. This paper examines the technologies which have been utilised for haptically explorable tactile displays over the past three decades, focussing on those which appear commercially viable in the immediate future.