A topologically faithful, tissue-guided, spatially varying meshing strategy for computing patient-specific head models for endoscopic pituitary surgery simulation

This paper presents a method for tessellating tissue boundaries and their interiors, given as input a map consisting of relevant tissue classes of the head, in order to produce anatomical models for finite-element-based simulation of endoscopic pituitary surgery. Our surface meshing method is based on the simplex model, which is initialized by duality from the topologically accurate results of the Marching Cubes algorithm, and which affords explicit control over mesh scale, while using tissue information to adhere to relevant boundaries. Our mesh scale strategy is spatially varying, based on the distance to a central point or linearized surgical path. The tetrahedralization stage also features a spatially varying mesh scale, consistent with that of the surface mesh.

Subject-specific non-linear biomechanical model of needle insertion into brain

The previous models for predicting the forces acting on a needle during insertion into very soft organs (such as, e.g. brain) relied on oversimplifying assumptions of linear elasticity and specific experimentally derived functions for determining needle-tissue interactions. In this contribution, we propose a more general approach in which the needle forces are determined directly from the equations of continuum mechanics using fully non-linear finite element procedures that account for large deformations (geometric non-linearity) and non-linear stress-strain relationship (material non-linearity) of soft tissues. We applied these procedures to model needle insertion into a swine brain using the constitutive properties determined from the experiments on tissue samples obtained from the same brain (i.e. the subject-specific constitutive properties were used). We focused on the insertion phase preceding puncture of the brain meninges and obtained a very accurate prediction of the needle force. This demonstrates the utility of non-linear finite element procedures in patient-specific modelling of needle insertion into soft organs such as, e.g. brain.

Robot-assisted needle placement in open MRI: system architecture, integration and validation

In prostate cancer treatment, there is a move toward targeted interventions for biopsy and therapy, which has precipitated the need for precise image-guided methods for needle placement. This paper describes an integrated system for planning and performing percutaneous procedures with robotic assistance under MRI guidance. A graphical planning interface allows the physician to specify the set of desired needle trajectories, based on anatomical structures and lesions observed in the patient's registered pre-operative and pre-procedural MR images, immediately prior to the intervention in an open-bore MRI scanner. All image-space coordinates are automatically computed, and are used to position a needle guide by means of an MRI-compatible robotic manipulator, thus avoiding the limitations of the traditional fixed needle template. Automatic alignment of real-time intra-operative images aids visualization of the needle as it is manually inserted through the guide. Results from in-scanner phantom experiments are provided.

Robot-assisted needle placement in open-MRI: system architecture, integration and validation

This work describes an integrated system for planning and performing percutaneous procedures-such as prostate biopsy-with robotic assistance under MRI-guidance. The physician interacts with a planning interface in order to specify the set of desired needle trajectories, based on anatomical structures and lesions observed in the patient's MR images. All image-space coordinates are automatically computed, and used to position a needle guide by means of an MRI-compatible robotic manipulator, thus avoiding the limitations of the traditional fixed needle template. Direct control of real-time imaging aids visualization of the needle as it is manually inserted through the guide. Results from in-scanner phantom experiments are provided.

A topologically faithful, tissue-guided, spatially varying meshing strategy for the computation of patient-specific head models for endoscopic pituitary surgery simulation

This paper presents a method for tessellating tissue boundaries and their interiors, given as input a tissue map consisting of relevant classes of the head, in order to produce anatomical models for finite element-based simulation of endoscopic pituitary surgery. Our surface meshing method is based on the simplex model, which is initialized by duality from the topologically accurate results of the Marching Cubes algorithm, and which features explicit control over mesh scale, while using tissue information to adhere to relevant boundaries. Our mesh scale strategy is spatially varying, based on the distance to a central point or linearized surgical path. The tetrahedralization stage also features a spatially varying mesh scale, consistent with that of the surface mesh.

Towards MRI guided surgical manipulator

BACKGROUND

The advantages of surgical robots and manipulators are well recognized in the clinical and technical community. Precision, accuracy and the potential for telesurgery are the prime motivations in applying advanced robot technology in surgery. In this paper critical interactions between Magnetic Resonance Imaging equipment and mechatronic devices are discussed and a novel Magnetic Resonance compatible surgical robot is described.

MATERIAL AND METHODS

Experimental results of the effects from several passive (metallic materials) and active (ultrasound motors) mechanical elements are demonstrated. The design principles for Magnetic Resonance compatible robots are established and the compatibility of the proposed robot is assessed by comparing images taken with and without the robot's presence within Signa SP/I GE Medical Systems scanner.

RESULTS

The results showed that, in principle, it is possible to construct precision mechatronic devices intended to operate inside MR scanner. Use of such a device will not cause image shift or significant degradation of signal-to-noise-ratio. An MR compatible surgical assist robot was designed and constructed. The robot is not affected by the presence of strong magnetic fields and is able to manoeuvre during imaging without compromising the quality of images. A novel image-guided robot control scheme was proposed. As a part of the control scheme, biomechanics-based organ deformation model was constructed and validated by in-vivo experiment. It has been recognised that for robust control of an image guided surgical robot the precise knowledge of the mechanical properties of soft organs operated on must be known. As an illustration, results in mathematical modelling and computer simulation of brain deformation are given.

CONCLUSION

The novel MR compatible robot was designed to position and direct an axisymmetric tool, such as a laser pointer or a biopsy catheter. New Robot control system based on the prediction of soft organ deformation was proposed.

Mechanical properties of brain tissue in-vivo: experiment and computer simulation

Realistic computer simulation of neurosurgical procedures requires incorporation of the mechanical properties of brain tissue in the mathematical model. Possible applications of computer simulation of neurosurgery include non-rigid registration, virtual reality training and operation planning systems and robotic devices to perform minimally invasive brain surgery. A number of constitutive models of brain tissue, both single-phase and bi-phasic, have been proposed in recent years. The major deficiency of most of them, however, is the fact that they were identified using experimental data obtained in vitro and there is no certainty whether they can be applied in the realistic in vivo setting. In this paper we attempt to show that previously proposed by us hyper-viscoelastic constitutive model of brain tissue can be applied to simulating surgical procedures. An in vivo indentation experiment is described. The force-displacement curve for the loading speed typical for surgical procedures is concave upward containing no linear portion from which a meaningful elastic modulus might be determined. In order to properly analyse experimental data, a three-dimensional, non-linear finite element model of the brain was developed. Magnetic resonance imaging techniques were used to obtain geometric information needed for the model. The shape of the force-displacement curve obtained using the numerical solution was very similar to the experimental one. The predicted forces were about 31% lower than those recorded during the experiment. Having in mind that the coefficients in the model had been identified based on experimental data obtained in vitro, and large variability of mechanical properties of biological tissues, such agreement can be considered as very good. By appropriately increasing material parameters describing instantaneous stiffness of the tissue one is able, without changing the structure of the model, to reproduce experimental curve almost perfectly. Numerical studies showed also that the linear, viscoelastic model of brain tissue is not appropriate for the modelling brain tissue deformation even for moderate strains.

New UWA robot--possible application to robotic surgery

This research is motivated by the need to design a Nuclear Magnetic Resonance Image guided surgical robot. The achievement of this objective requires the solution of two problems: design and construction of a magnetic resonance compatible mechanical manipulator and development of the appropriate robot control system. It is beneficial to keep robot actuators outside the magnet. Therefore, the parallel architecture should be used for the mechanical manipulator. Newly developed University of Western Australia Robot satisfies this requirement. Moreover, it has substantially larger workspace and torsional stiffness when compared to existing parallel configurations such as the Delta. The plausible method of dealing with the delays in the robot control system caused by the image analysis is the prediction of the deformation based on the mathematical model of the organ mechanical and geometric properties. The hyper-viscoelastic constitutive models offer a good way of representing non-linear stress-strain and stress-strain rate relations of soft tissues such as the brain. The numerical values for material constants for brain tissue are given. Additional advantage of the proposed model is that it can be easily implemented in commercially available finite element codes and immediately applied to large-scale computer simulations.

Constitutive modelling of brain tissue: experiment and theory

Recent developments in computer-integrated and robot-aided surgery--in particular, the emergence of automatic surgical tools and robots--as well as advances in virtual reality techniques, call for closer examination of the mechanical properties of very soft tissues (such as brain, liver, kidney, etc.). The ultimate goal of our research into the biomechanics of these tissues is the development of corresponding, realistic mathematical models. This paper contains experimental results of in vitro, uniaxial, unconfined compression of swine brain tissue and discusses a single-phase, non-linear, viscoelastic tissue model. The experimental results obtained for three loading velocities, ranging over five orders of magnitude, are presented. The applied strain rates have been much lower than those applied in previous studies, focused on injury modelling. The stress-strain curves are concave upward for all compression rates containing no linear portion from which a meaningful elastic modulus might be determined. The tissue response stiffened as the loading speed increased, indicating a strong stress-strain rate dependence. The use of the single-phase model is recommended for applications in registration, surgical operation planning and training systems as well as a control system of an image-guided surgical robot. The material constants for the brain tissue are evaluated. Agreement between the proposed theoretical model and experiment is good for compression levels reaching 30% and for loading velocities varying over five orders of magnitude.

Development of a computer-aided surgery system: three-dimensional graphic reconstruction for treatment of liver cancer

Simulation of the needle puncture and volume estimation for the tumors in the liver were carried out with the three-dimensional image reconstruction system, which consists of a medical image acquisition system, a data processing system, and a graphic display. A set of sliced-image data from a computerized tomography and/or a magnetic resonance imaging was used to reconstruct the liver, the vessels, and the tumors of the patients with liver cancer. A good agreement of anatomic locations of both the intrahepatic vessels and the tumors between the reconstructed liver model and the echography done intraoperatively was observed. Surgical simulations with these graphic models clearly indicated safety areas for needle puncture in the laser coagulation therapy. In addition liver volumes were calculated within 3% of error in comparison to the measured values. These results indicate that the computer-aided surgery system is a highly promising method that avoids cumbersome stereoscopic recognition of the anatomical location of the diseased area and the vessels, before and after surgery.

Pathological features of peripheral facial paralysis caused by malignant tumour

It has been reported that tumour invasion of the facial canal does not necessarily produce facial paralysis. The purpose of this paper is to present the facial nerve pathology in cases of facial paralysis caused by malignant neoplasms. Forty-seven temporal bones acquired from patients who died of malignant neoplasms were used for this analysis. Of the 47, 18 temporal bones (38.3%) had temporal bone metastasis or invasion. In 6 cases, the tumour destroyed the bony facial canals exposing the facial nerves to the invading tumour cells. The records of the patients showed that only half of the patients had facial paralysis. Those in whom tumour cells invaded the facial nerve beyond the epineural sheath had complete facial paralysis before death.