Kazuya Sase

Design and evaluation of an encountered-type haptic interface using MR fluid for surgical simulators

A novel encountered-type haptic interface for surgical simulators is proposed. This interface has a container of MR (Magneto–Rheological) fluid, and an operator puts a surgical instrument into the fluid and can feel resistance force. The advantage of this interface is that an operator can move an instrument freely when it does not contact with MR fluid and change instruments easily. If an instrument is mounted mechanically on a haptic interface driven by servomotors, it is difficult to change surgical tools. On the other hand, the developed device does not require a procedure for changing tools and can increase a sense of reality. However, MR fluid cannot display large deformation of a tissue since its elastic region is small. Therefore, a container of the fluid is moved by servomotors. In this paper, concept and design of the interface and performance evaluations are described. In order to specify required display force, cutting force of a liver is analysed, and the maximum force is about 1.6 [N]. The device is designed based on this required force. Relationship between coil current and display force is measured, and the interface can exert 2.7 [N] when the current is 1 [A]. In addition, the validness of the proposed scheme using servomotors is evaluated.

Development of a haptic interface using MR fluid for displaying cutting forces of soft tissues

In open abdominal surgical procedures, many surgical instruments, e.g., knives, cutting shears and clamps, are generally used. Therefore, a haptic interface should display reaction force of a soft biological tissue through such a surgical instrument. Simplest solution for this difficulty is that an actual instrument is mechanically mounted on the traditional haptic interface driven by servomotors. However, operators lose a sense of reality when they change the instrument since they must perform a procedure which is not required in actual surgery for attaching/detaching the instrument to/from the haptic interface. Therefore, a novel haptic interface using MR (Magneto-Rheological) fluid is developed in this research. Rheological property of MR fluid can be changed in a short time by applied magnetic flux density. By cutting the fluid using a surgical instrument, operators can feel resistance force as if they cut tissue. However, MR fluid cannot display large deformation of soft tissues since elastic region of MR fluid is small. Therefore, a container of the fluid is moved by a motion table driven by servomotors. In this paper, concept and design of the haptic interface and performance evaluations are described.

Identification of mechanical properties of brain parenchyma for brain surgery haptic simulation

Dissection and removal of lesion area are fundamental operations in brain surgery. In order to correctly reproduce dissection and removal in brain surgery haptic simulation, mechanical properties of brain tissue must be investigated. In this work, porcine brains are used as specimens. Mechanical properties such as density, Poissons ratio, Youngs modulus, damping coefficient, and fracture stress of porcine brain parenchyma are measured and identified. Since haptic simulations require real-time computation of deformation and fracture of brain tissue, a linear finite element model and Rayleigh damping model are used. The Rayleigh damping coefficients are identified by solving optimization problems, so that the error between experimental results and simulation results is minimized.

Optimization of retraction in neurosurgery to avoid damage caused by deformation of brain tissues

In neurosurgery, effects of deformation should be considered to avoid damaging brain tissues. The goal of this study is to develop an automatic path planner considering the deformation of brain tissues. This paper shows a scheme which combines FEM (Finite Element Method) and an optimization method for optimization of retraction in order to approach a deep part of a brain. Also, evaluations of two optimization results are discussed. One optimization is for retraction of a simple shape model for comparing two solvers, Pattern Search and Genetic Algorithm. Pattern Search Algorithm obtained maximum view size for the simple model when the principal stress of the tissue is not more than the threshold 500 (Pa). The other optimization is for retraction of a brain fissure model. Based on the result of the simple shape model, Pattern Search Algorithm is used for this optimization. It successfully generated optimal position and posture of a spatula for opening the fissure model which has same mechanical property with the human brain. These results show the effectiveness of the proposed scheme.

Embedding Segmented Volume in Finite Element Mesh with Topology Preservation

The generation of a patient-specific finite element (FE) model of organs is important for preoperative surgical simulations. Although methods for generating a mesh from a 3D geometric model of organs are well established, the reproduction of complex structures, such as holes, branches, and jaggy boundaries, remains difficult. To approximate the deformation of complex structures, an approach for embedding a fine geometry in a coarse volumetric mesh can be used. In this paper, we introduce a volume embedding method that preserves the topology of a complicated structure on the basis of segmented medical images. Our evaluation shows that the generated FE model precisely reproduces the topology of a human brain according to a segmented medical image.

Haptic rendering of contact between rigid and deformable objects based on penalty method with implicit time integration

The handling of contacts between virtual objects is a fundamental problem of 6 DOF haptic rendering in virtual environments. Especially, the haptic rendering of the contact between rigid and deformable objects is challenging because the physics simulation with high computational burden must be calculated in real time with good stability. In this paper, we introduce a fast and stable contact handling method that can be used for 6 DOF haptic rendering. The two-way contact response is formulated based on a penalty method. To enhance the numerical stability in large time step, the penalty-based contact force is integrated using implicit time integration. The method was evaluated by experiments using a haptic device combined with a physics simulator based on finite element method.

Experimental and numerical analysis of damage fracture mechanics of brain parenchyma

In this paper, a damage and fracture model of brain parenchyma is proposed for a haptic brain surgery simulation. It is assumed that microscopic damage begins by von Mises yield criterion, and the microscopic damage grows rate in proportion to volume strain. Tensile tests with two different strain rate were conducted using porcine brain parenchyma (tensile velocities: 0.1 (mm/s) and 1.0 (mm/s), mean length of specimens: 15 (mm)). Mechanical properties and proposed damage model parameters were identified by solving optimization problem with fitted curves of experimental data at 0.1 (mm/s). The proposed model and identified parameters were verified by comparing the simulation result and experimental data with tensile velocity of 1.0 (mm/s). A conventional damage model, simplified Lemaitre model, was implemented for comparison. Tensile simulations were performed with two models, proposed model and simplified Lemaitre model, with tensile velocity of 0.1 (mm/s). The simulation results were compared with the experimental result. It is confirmed that the proposed model well reproduce damage and fracture mechanics of brain parenchyma, while the simplified Lemaitre model could not well reproduce the ductility.

A viscoelastic model of brain parenchyma for haptic brain surgery simulations

This paper presents a viscoelastic model of brain parenchyma which is based on the generalized Maxwell model but considers also inertial force. The proposed model is implemented in finite element method (FEM). The viscoelastic behavior of brain parenchyma was confirmed by stress relaxation tests using porcine brain parenchyma. The stress relaxation characteristics were measured at strain 0.1, 0.2 and 0.3. The strains were applied by strain rates of 0.1 (s−1) and 1.0 (s−1). Viscoelastic parameters were identified by solving an optimization problem using experimental data and dynamic simulations. In order to verify the consistency and quality of the experimental data and the dynamic simulation, stress relaxation simulations were performed using the identified parameters.

Securing an optimum operating field without undesired tissue damage in neurosurgery

Graphical Abstract In neurosurgery, surgeons sometimes retract brain tissue to prepare an operating field around a lesion. In addition, they are required to plan a safe surgical pathway for deep-brain regions while considering tissue damage caused by excessive stress. The goal of this study is to develop a technique for automatically generating a surgery pathway for lesions in the deep-brain region, focusing on securing an operating field around the lesion as a first step and also considering brain tissue deformation. In previous studies, securing the operating field has been treated as a single-objective optimization problem in order to maximize the viewable area of the lesion. However, in this study, the task of securing the operating field is formulated as a multi-objective optimization problem. Using a technique that combines finite element analysis and an optimization method, the principal stress on the brain is constrained to less than a certain threshold value, and the position and orientation of the surgical instrument are optimized for safe retraction of the brain according to various weighting factors.