| A bone biopsy is a procedure in which a small sample of bone is taken from the body and looked at under a microscope for cancer, infection, or other bone disorders to care of oncology patients as a minimally invasive surgery. In traditional bone biopsy surgeries, it depends on the surgeon’s experience to determine the depth and invasion site, which may increase the pain and cost for patients due to re-operation, even the risk or medical malpractice. Image-guided robot is the integrated technology of robotics, imaging technology, electronics and automation which offers distinct advantages for both patients and physicians. MRI (Magnetic resonance imaging)-guided robot was proposed to improve the accuracy and success rate of bone biopsy in this paper, and the goal of this paper is to provide some key technologies for developing MRI-guided bone biopsy robot.The compatibility of MRI-guided robot consists of MR-compatibility and structural compatibility. The MRI theories of magnetic resonance (MR) principle, and MRI principle and the effects of paramagnetic and diamagnetic substances on MR images were introduced at the beginning. Also, the methods to evaluate the quality of MR images were discussed from the point of view of signal-noise ratio, spatial resolution and contrast. An optimized bone biopsy procedure was proposed to achieve the structural compatibility by optimizing the operating requirements of bone biopsy procedure in closed-MR scanner. The equivalent model of MR scanner were built to analyze the magnetic flux density and magnetic induction line of static magnetic field and eddy current field, which provides solution to control the movement of surgical robot and path planning of surgical tool in MR scanner.In order to accurately model the force of needle insertion into soft tissue, it is necessary to separate force data into components from different sources. The Neo-Hookean hyperelastic material model was chosen for finite element analysis as it is known to closely imitate the mechanical behaviour of biological soft tissues. A series of experiments were employed to prove the feasibility of simulation by discussing the effects of tool diameter and the geometric shape on insertion force and the effects of insertion speed on deformation of soft tissue. In bone drilling experiments, the comparison of manual and automatic operation indicated that automatic drilling process is more stable, which is helpful to reduce oscillation in bone biopsy. The drilling experiments also concluded that the maximum rotational speed500r/min was limited for bone biopsy with robotic operation, and the maximum drilling force93N,102N and116N should be controlled under diameter of2.5mm,4mm and5mm respectively. To reduce pain, deformation of soft tissue and the risk of breaking bone during robotic operation, the feed rate of insertion into soft tissue and bone should be set in1~2mm/s.The real3D model of porcine femur bone was completed by the following operation: scanning of fresh porcine femur bone with GE LightSpeed64slice CT scanner, saving images in DICOM standard file, modeling real3D bone with Mimics, optimizing meshes, smoothing3D model and assigning material properties based on threshold values. Then, a simplified3D model of bone biopsy tool was utilized to analyze the insertion force during the process before removing stylet. The simulation results demonstrate the feasibility of the proposed method system of3D modeling, mesh optimization and finite element analysis.Due to the limitation of MR-compatible commercial force sensor, a Flexiforce sensor-based force feedback system was designed to provide solution for the safety of bone biopsy robot. By designing a universal mounting device of Flexiforce sensor, the calculation and MR-compatibility test were performed in regular situation and MR room separately. The results show the presented mounting device of Flexiforce sensor is MR-compatible, and the mechanical vibration has influence on force acquisition which was improved by the proposed measures. Meanwhile, the achievement demonstrates the significance of eliminating vibration in developing bone biopsy robot to record stable force data.A Flexiforce sensor-based force feedback testing device driven by US motor was manufactured with non-ferromagnetic materials. A comparative force feedback system was constructed to test dynamic force during soft tissue insertion. It could be concluded that the Flexiforce sensor-based feedback systems could be used in dynamic test. The MR-compatibility tests were performed with single US motor, double US motors and multiple US motors under different mounting direction of US motor and different configurations. The normalized SNR was used to estimate the effect of mounting distance of US motor on MR-compatibility. It was found that the mounting distance should be up to20cm or more to obtain MR-compatible images, and using more US motors may reduce SNR.The tests of the presented device show that the main part did not create significant artifacts or distortion, and MR-compatibility could be improved by reducing the usage of metallic materials. Finally, the purpose that using MR images to guide bone biopsy and implementing the operation by robot was proven as the assigned site and depth were successfully reached in the guide test. |
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