Gordon Mallinson

A virtual environment and model of the eye for surgical simulation

An anatomically detailed 3-D computer graphic model of the eye and surrounding face within a virtual environment has been implemented for use in a surgical simulator. The simulator forms part of a teleoperated micro-surgical robotic system being developed for eye surgery. The model has been designed to both visually and mechanically simulate features of the human eye by coupling computer graphic realism with finite element analysis. The paper gives an overview of the system with emphasis on the graphical modelling techniques and a computationally efficient framework for representing anatomical details of the eye and for finite element analysis of the mechanical properties. Examples of realistic images coupled to large deformation finite element model of the cornea are presented. These images can be rendered sufficiently fast for the virtual reality application.

A teleoperated microsurgical robot and associated virtual environment for eye surgery

We have developed a prototype teleoperated microsurgical robot (MSR-1) and associated virtual environment for eye surgery. Bidirectional pathways relay visual, auditory, and mechanical information between the MSR-1 master and slave. The surgeon wears a helmet (visual master) that is used to control the orientation of a stereo camera system (visual slave) observing the surgery. Images from the stereo camera system are relayed back to the helmet (or adjacent screen) where they are viewed by the surgeon. In each hand the surgeon holds a pseudotool (a shaft shaped like a microsurgical scalpel) that projects from the left and right limbs of a force reflecting interface (mechanical master). Movements of the left and right pseudotools cause corresponding movements (scaled down by 1 to 100 times) in the microsurgical tools held by the left and right limbs of the micromotion robot (mechanical slave) that performs the surgery. Forces exerted on the left and right limbs of the slave microsurgical robot via the microtools are reflected back (after being scaled up by 1 to 100 times) to the pseudotools and hence surgeon via actuators in the left and right limbs of the mechanical master. This system enables tissue cutting forces to be felt including those that would normally be imperceptible if they were transmitted directly to the surgeons hands. The master and slave subsystems (visual, auditory, and mechanical) communicate through a computer system which serves to enhance and augment images, filter hand tremor, perform coordinate transformations, and perform safety checks. The computer system consists of master and slave computers that communicate via an optical fiber connection. As a result, the MSR-1 master and slave may be located at different sites, which permits remote robotic microsurgery to become a reality. MSR-1 is being used as an experimental testbed for studying the effects of feedforward and feedback delays on remote surgery and is used in research on enhancing the accuracy and dexterity of microsurgeons by creating mechanical and visual telepresence.

A 3-D streamline tracking algorithm using dual stream functions

A methodology has been developed for constructing streamlines and particle paths in numerically generated fluid velocity fields. A graphical technique is used to convert the discretely defined flow within a cell into one represented by two three-dimensional stream functions. Streamlines are calculated by tracking constant values of each stream function, a process which corresponds to finding the intersection of two stream surfaces. The tracking process is mass conservative and does not use a time stepping method for integration, thus eliminating a computationally intensive part of traditional tracking algorithms. The method can be applied generally to any three-dimensional compressible or incompressible steady flow. Results presented compare the performance of the new method to the most commonly used scheme and show that calculation times can be reduced by an order of magnitude.<<ETX>>

The flow fields involved in hydrodynamic imaging by blind Mexican cave fish (Astyanax fasciatus). Part I: open water and heading towards a wall.

SUMMARY Blind Mexican cave fish (Astyanax fasciatus) sense the presence of nearby objects by sensing changes in the water flow around their body. The information available to the fish using this hydrodynamic imaging ability depends on the properties of the flow field it generates while gliding and how this flow field is altered by the presence of objects. Here, we used particle image velocimetry to measure the flow fields around gliding blind cave fish as they moved through open water and when heading towards a wall. These measurements, combined with computational fluid dynamics models, were used to estimate the stimulus to the lateral line system of the fish. Our results showed that there was a high-pressure region around the nose of the fish, low-pressure regions corresponding to accelerated flow around the widest part of the body and a thick laminar boundary layer down the body. When approaching a wall head-on, the changes in the stimulus to the lateral line were confined to approximately the first 20% of the body. Assuming that the fish are sensitive to a certain relative change in lateral line stimuli, it was found that swimming at higher Reynolds numbers slightly decreased the distance at which the fish could detect a wall when approaching head-on, which is the opposite to what has previously been expected. However, when the effects of environmental noise are considered, swimming at higher speed may improve the signal to noise ratio of the stimulus to the lateral line.

The flow fields involved in hydrodynamic imaging by blind Mexican cave fish (Astyanax fasciatus). Part II: gliding parallel to a wall.

SUMMARY Blind Mexican cave fish (Astyanax fasciatus) are able to sense detailed information about objects by gliding alongside them and sensing changes in the flow field around their body using their lateral line sensory system. Hence the fish are able to build hydrodynamic images of their surroundings. This study measured the flow fields around blind cave fish using particle image velocimetry (PIV) as they swam parallel to a wall. Computational fluid dynamics models were also used to calculate the flow fields and the stimuli to the lateral line sensory system. Our results showed that characteristic changes in the form of the flow field occurred when the fish were within approximately 0.20 body lengths (BL) of a wall. The magnitude of these changes increased steadily as the distance between the fish and the wall was reduced. When the fish were within 0.02 BL of the wall there was a change in the form of the flow field owing to the merging of the boundary layers on the body of the fish and the wall. The stimuli to the lateral line appears to be sufficient for fish to detect walls when they are 0.10 BL away (the mean distance at which they normally swim from a wall), but insufficient for the fish to detect a wall when 0.25 BL away. This suggests that the nature of the flow fields surrounding the fish are such that hydrodynamic imaging can only be used by fish to detect surfaces at short range.

Wall Shear Stress is the Primary Mechanism of Energy Loss in the Fontan Connection

Long-term outcome following the Fontan operation may be affected by the amount of energy lost as blood flows through the anastomosis geometry. A method for detailed quantification of energy loss is applied to computational simulations of the flow in an atriopulmonary and a total cavopulmonary model. Five types of flow (near wall, slow recirculation, medium speed vortices, collision, and streamlined flow) are identified and their energy losses quantified. The presence of recirculation regions decreases the efficiency of the atriopulmonary model, and a region of increased energy loss is seen in the collision region in the total cavopulmonary model. However, the most significant energy loss is through wall shear stress, which is maximal in areas where there is rapid, near wall flow.

An EMG-driven neuromuscular interface for human elbow joint

This paper presents the development of a neuromuscular interface for an exoskeleton to assist the elbow joint. The interface uses electromyographic (EMG) signals obtained from the biceps and triceps to predict elbow flexion and extension movements. These movements occur in the sagittal plane and the effects of forearm weight have been incorporated. The interface uses a physiological-model-based approach to convert the EMG signals to a joint displacement and this is based on Hill-type muscle models that have traditionally been used in clinical applications or for diagnosing and managing neurological and orthopaedic conditions. Simulation results have been obtained for the performance of the interface on pre-recorded data. While the general trend of movement was correctly identified by the interface, further experiments are required to quantify the accuracy and determine real-time performance capabilities.

A Hybrid 1D and 3D Approach to Hemodynamics Modelling for a Patient-Specific Cerebral Vasculature and Aneurysm

In this paper we present a hybrid 1D/3D approach to haemodynamics modelling in a patient-specific cerebral vasculature and aneurysm. The geometric model is constructed from a 3D CTA image. A reduced form of the governing equations for blood flow is coupled with an empirical wall equation and applied to the arterial tree. The equation system is solved using a MacCormack finite difference scheme and the results are used as the boundary conditions for a 3D flow solver. The computed wall shear stress (WSS) agrees with published data.

CFD visualisation: challenges of complex 3D and 4D data fields

A review of CFD visualisation methods together with policy statements by visualisation experts conclude that the visualisation of complex 3D and 4D CFD data fields is still a major research challenge. Potential methodologies are discussed and it is suggested that a suitable approach and target for research is to understand how derived scalar and vector fields, such as dual stream functions and the Lamb vector, relate to the structure and physical characteristics of a flow. A particular focus is to use these fields to generate ‘structural’ stream functions with isosurfaces that divide the flow into self contained regions.

Mass conservative fluid flow visualization for CFD velocity fields

Mass conservation is a key issue for accurate streamline and stream surface visualization of flow fields. This paper complements an existing method (Feng et al., 1997) for CFD velocity fields defined at discrete locations in space that uses dual stream functions to generate streamlines and stream surfaces. Conditions for using the method have been examined and its limitations defined. A complete set of dual stream functions for all possible cases of the linear fields on which the method relies are presented. The results in this paper are important for developing new methods for mass conservative streamline visualization from CFD data and using the existing method.