Toshikatsu Washio

Measurement of the Tip and Friction Force Acting on a Needle during Penetration

We present the tip and friction forces acting on a needle during penetration into a canine prostate, independently measured by a 7-axis load cell newly developed for this purpose. This experimental apparatus clarifies the mechanics of needle penetration, potentially improving the development of surgical simulations. The behavior of both tip and friction forces can be used to determine the mechanical characteristics of the prostate tissue upon penetration, and the detection of the surface puncture, which appears in the friction force, makes it possible to estimate the true insertion depth of the needle in the tissue. The friction model caused by the clamping force on the needle can also be determined from the measured friction forces.

Robotic needle insertion: effects of friction and needle geometry

Two experiments were performed to determine the effects of friction and needle geometry during robotic needle insertion into soft tissues. In Experiment I, friction forces along the instrument axis were characterized during needle insertion into bovine liver under CT fluoroscopic imaging. Because the relative velocity of the tissue and needle affect viscous and Coulomb friction, the needle insertion process was segmented into several phases of relative motion: none, partial and complete. During the complete relative motion phase, it was found that Coulomb friction accounts for the majority of needle force. In Experiment II, insertion forces along and orthogonal to the needle axis were measured during insertion into a silicone rubber phantom with a consistency similar to liver. The effects of needle diameter and tip type (bevel, cone, and triangle) on insertion force were characterized. A bevel tip causes more needle bending and is more easily affected by tissue density variations. Forces for larger diameter needles are higher due to increased cutting and friction forces. These results may be used in the control of needle insertion for robot-assisted percutaneous therapies.

A Model for Relations between Needle Deflection, Force, and Thickness on Needle Penetration

A force-deflection model of needle penetration is proposed and evaluated experimentally. The force at the fixed end of the needle and the needle deflection were measured using a force sensor and a bi-plane X-ray imaging system, and the model was evaluated with the data. We define a physical quantity ?, which we call infinitesimal force per length, analogous to traction (force per surface area). The model predicts ? to be constant over the length of the inserted portion of the needle. However the results indicate that this assumption does not fully account for the real deflection. It is strongly suggested that there is an additional degree of freedom: a moment or a rotational force acting on the needle.

Endoscope Manipulator for Trans-nasal Neurosurgery, Optimized for and Compatible to Vertical Field Open MRI

This paper preliminarily reports the robotic system working inside the gantry of vertical field Open MRI. This manipulator is new in terms of the application to vertical field Open MRI, cost effectiveness, accuracy and stiffness sufficient for endoscope manipulation. The endoscope manipulation for trans-nasal neurosurgery under MR-guidance was selected as the sample task. The endoscope operation in MR-gantry might provide the surgeon(s) with real-time feedback of MR image to endoscopic image and the reverse. This facilitates the comprehensive understanding, because MRI compensates the vision lost through narrow opening of keyhole surgery with global view. So this surgery is a good motivation for combination of MRI and robotic systems. In this paper, the design and implementation are presented and preliminary test shows good MR-compatibility, accuracy and stiffness.

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.

Modeling shear modulus distribution in magnetic resonance elastography with piecewise constant level sets

Magnetic resonance elastography (MRE) is designed for imaging the mechanical properties of soft tissues. However, the interpretation of shear modulus distribution is often confusing and cumbersome. For reliable evaluation, a common practice is to specify the regions of interest and consider regional elasticity. Such an experience-dependent protocol is susceptible to intrapersonal and interpersonal variability. In this study we propose to remodel shear modulus distribution with piecewise constant level sets by referring to the corresponding magnitude image. Optimal segmentation and registration are achieved by a new hybrid level set model comprised of alternating global and local region competitions. Experimental results on the simulated MRE data sets show that the mean error of elasticity reconstruction is 11.33% for local frequency estimation and 18.87% for algebraic inversion of differential equation. Piecewise constant level set modeling is effective to improve the quality of shear modulus distribution, and facilitates MRE analysis and interpretation.

Numerical simulations and lab tests for design of MR-compatible robots

A numerical simulation of the magnetic field in the imaging volume of a magnetic resonance imaging (MRI) scanner and a method for quick searches for electromagnetic noise source are proposed as part of the design and development process for surgical robots that are compatible with MRI. Although MR compatibility is a major challenge for development of such robots, no simulation method was known that could predict it. This paper demonstrates the potential of two basic techniques that are useful in predicting the magnetic and electromagnetic compatibility. First, the distortion of the magnetic field caused by the robot can be computed by a finite element method. Simulation of the resonance signal distribution showed good agreement with experimentally obtained signal distributions. Second, a noise source and its strength can be visualized by a spectrum analyzer

Evaluation of accuracy of non-linear finite element computations for surgical simulation: study using brain phantom

In this paper, the accuracy of non-linear finite element computations in application to surgical simulation was evaluated by comparing the experiment and modelling of indentation of the human brain phantom. The evaluation was realised by comparing forces acting on the indenter and the deformation of the brain phantom. The deformation of the brain phantom was measured by tracking 3D motions of X-ray opaque markers, placed within the brain phantom using a custom-built bi-plane X-ray image intensifier system. The model was implemented using the ABAQUSTM finite element solver. Realistic geometry obtained from magnetic resonance images and specific constitutive properties determined through compression tests were used in the model. The model accurately predicted the indentation force–displacement relations and marker displacements. Good agreement between modelling and experimental results verifies the reliability of the finite element modelling techniques used in this study and confirms the predictive power of these techniques in surgical simulation.

A simple method for MR elastography: a gradient-echo type multi-echo sequence

To demonstrate the feasibility of a novel MR elastography (MRE) technique based on a conventional gradient-echo type multi-echo MR sequence which does not need additional bipolar magnetic field gradients (motion encoding gradient: MEG), yet is sensitive to vibration. In a gradient-echo type multi-echo MR sequence, several images are produced from each echo of the train with different echo times (TEs). If these echoes are synchronized with the vibration, each readouts gradient lobes achieve a MEG-like effect, and the later generated echo causes a greater MEG-like effect. The sequence was tested for the tissue-mimicking agarose gel phantoms and the psoas major muscles of healthy volunteers. It was confirmed that the readout gradient lobes caused an MEG-like effect and the later TE images had higher sensitivity to vibrations. The magnitude image of later generated echo suffered the T2 decay and the susceptibility artifacts, but the wave image and elastogram of later generated echo were unaffected by these effects. In in vivo experiments, this method was able to measure the mean shear modulus of the psoas major muscle. From the results of phantom experiments and volunteer studies, it was shown that this method has clinical application potential.

Accuracy of Non-linear FE Modelling for Surgical Simulation: Study Using Soft Tissue Phantom

In this chapter, we evaluated the accuracy of non-linear FE modelling in application to surgical simulation. We compared experiment and FE modelling of indentation of the soft tissue phantom. The evaluation was done in terms of soft tissue phantom deformation and the forces acting on the indenter. The soft tissue phantom deformation was measured by tracking 3D motions of X-ray opaque markers placed in the direct neighbourhood under the indenter with a custom-built bi-plane X-ray image intensifiers (XRII) system. The modelling of soft tissue phantom indentation was conducted using the ABAQUS/standard finite element solver. Specific constitutive properties of the soft tissue phantom determined through semi-confined uniaxial compression tests were used in the model. The model accurately predicted the indentation force–displacement relations and marker displacements. The agreement between modelling and experimental results verifies the reliability of our FE modelling techniques and confirms the predictive power of these techniques in surgical simulation.