Modeling and Control of an MR-Safe Pneumatic Radial Inflow Motor and Encoder (PRIME).

Magnetic resonance (MR) conditional actuators and encoders are the key components for MR-guided robotic systems. In this article, we present the modeling and control of our MR-safe pneumatic radial inflow motor and encoder. A comprehensive model is developed that considers the primary dynamic elements of the system, including: 1) motor dynamics, 2) pneumatic transmission line dynamics, and 3) valve dynamics. After model validation, we present a simplified third order model that facilitates design of a first order sliding mode controller (TO-SMC). Finally, the motor hardware is tested in a 7T MRI. No image distortion or artifacts were observed. We posit the MR-safe motor and dynamic model will lower the entry barriers for researchers interested in MR-guided robots and promote wider adoption of MR-guided robotic systems.

Towards MR-Guided Robotic Intracerebral Hemorrhage Evacuation: Aiming Device Design and ex vivo Ovine Head Trial.

Stereotactic neurosurgery is a well-established surgical technique for navigation and guidance during treatment of intracranial pathologies. Intracerebral hemorrhage (ICH) is an example of various neurosurgical conditions that can benefit from stereotactic neurosurgery. As a part of our ongoing work toward real-time MR-guided ICH evacuation, we aim to address an unmet clinical need for a skull-mounted frameless stereotactic aiming device that can be used with minimally invasive robotic systems for MR-guided interventions. In this paper, we present NICE-Aiming, a Neurosurgical, Interventional, Configurable device for Effective-Aiming in MR-guided robotic neurosurgical interventions. A kinematic model was developed and the system was used with a concentric tube robot (CTR) for ICH evacuation in (i) a skull phantom and (ii) in the first ever reported ex vivo CTR ICH evacuation using an ex vivo ovine head. The NICE-Aiming prototype provided a tip accuracy of 1.41±0.35 mm in free-space. In the MR-guided gel phantom experiment, the targeting accuracy was 2.07±0.42 mm and the residual hematoma volume was 12.87 mL (24.32% of the original volume). In the MR-guided ex vivo ovine head experiment, the targeting accuracy was 2.48±0.48 mm and the residual hematoma volume was 1.42 mL (25.08% of the original volume).

Body-Mounted MR-Conditional Robot for Minimally Invasive Liver Intervention.

MR-guided microwave ablation (MWA) has proven effective in treating hepatocellular carcinoma (HCC) with small-sized tumors, but the state-of-the-art technique suffers from sub-optimal workflow due to the limited accuracy provided by the manual needle insertions. This paper presents a compact body-mounted MR-conditional robot that can operate in closed-bore MR scanners for accurate needle guidance. The robotic platform consists of two stacked Cartesian XY stages, each with two degrees of freedom, that facilitate needle insertion pose control. The robot is actuated using 3D-printed pneumatic turbines with MR-conditional bevel gear transmission systems. Pneumatic valves and control mechatronics are located inside the MRI control room and are connected to the robot with pneumatic transmission lines and optical fibers. Free-space experiments indicated robot-assisted needle insertion error of 2.6 ± 1.3 mm at an insertion depth of 80 mm. The MR-guided phantom studies were conducted to verify the MR-conditionality and targeting performance of the robot. Future work will focus on the system optimization and validations in animal trials.

Optimal Concentric Tube Robot Design for Safe Intracerebral Hemorrhage Removal.

Purpose: The purpose of this paper is to investigate the geometrical design and path planning of Concentric tube robots (CTR) for intracerebral hemorrhage (ICH) evacuation, with a focus on minimizing the risk of damaging white matter tracts and cerebral arteries. Methods: To achieve our objective, we propose a parametrization method describing a general class of CTR geometric designs. We present mathematical models that describe the CTR design constraints and provide the calculation of a path risk value. We then use a genetic algorithm to determine the optimal tube geometry for targeting within the brain. Results: Our results show that a multi-tube CTR design can significantly reduce the risk of damaging critical brain structures compared to the conventional straight tube design. However, there is no significant relationship between the path risk value and the number and shape of the additional inner curved tubes. Conclusion: Considering the challenges of CTR hardware design, fabrication, and control, we conclude that the most practical geometry for a CTR path in ICH treatment is a straight outer tube followed by a planar curved inner tube. These findings have important implications for the development of safe and effective CTRs for ICH evacuation by enabling dexterous manipulation to minimize damage to critical brain structures.

MR-Safe Cartesian Platform for Active Cardiac Shimming: Preliminary Validation.

OBJECTIVE: Implanted Cardioverter Defibrillators (ICDs) induce a large (100 parts per million) inhomogeneous magnetic field in the magnetic resonance imaging (MRI) scanner which cannot be corrected by the scanner's built-in shim coils, leading to significant image artifacts that can make portions of the heart unreadable. To compensate for the field inhomogeneity, an active shim coil capable of countering the field deviation in user-defined regions was designed that must be optimally placed at patient-specific locations. We aim to develop and evaluate an MR-safe robotic solution for automated shim coil positioning. METHODS: We designed and fabricated an MR-safe Cartesian platform that holds the shim coil inside the scanner. The platform consists of three lead screw stages actuated by pneumatic motors, achieving decoupled translations of 140 mm in each direction. The platform is made of plastics and fiberglass with the control electronics placed outside the scanner room, ensuring MR safety. Mechanical modeling was derived to provide design specifications. RESULTS: Experiments show that the platform achieves less than 2 mm average motion error and 0.5 mm repeatability in all directions, and reduces the adjustment time from 5 min to a few seconds. Phantom and animal trials were conducted, showing that the proposed system is able to position a heavy shim coil ( kg) for improved ICD artifact suppression. CONCLUSION: This robotic platform provides an effective method for reliable shim coil positioning inside the scanner. SIGNIFICANCE: This work contributes to improving cardiac MRI quality that could facilitate accurate diagnosis and treatment planning for patients with implanted ICDs.

Balanced-force shim system for correcting magnetic-field inhomogeneities in the heart due to implanted cardioverter defibrillators.

Background: In the US, 1.4 million people have implanted ICDs for reducing the risk of sudden death due to ventricular arrhythmias. Cardiac MRI (cMR) is of particular interest in the ICD patient population as cMR is the optimal imaging modality for distinguishing cardiac conditions that predispose to sudden death, and it is the best method to plan and guide therapy. However, all ICDs contain a ferromagnetic transformer which imposes a large inhomogeneous magnetic field in sections of the heart, creating large image voids that can mask important pathology. A shim system was devised to resolve these ICD issues. A shim coil system (CSS) that corrects ICD artifacts over a user-selected Region-of-Interest (ROI), was constructed and validated. Methods: A shim coil was constructed that can project a large magnetic field for distances of ~15 cm. The shim-coil can be positioned safely anywhere within the scanner bore. The CSS includes a cantilevered beam to hold the shim coil. Remotely controlled MR-conditional motors allow 2 mm-accuracy three-dimensional shim-coil position. The shim coil is located above the subjects and the imaging surface-coils. Interaction of the shim coil with the scanner's gradients was eliminated with an amplifier that is in a constant current mode. Coupling with the scanners' radio-frequency (rf) coils, was reduced with shielding, low-pass filters, and cable shield traps. Software, which utilizes magnetic field (B0) mapping of the ICD inhomogeneity, computes the optimal location for the shim coil and its corrective current. ECG gated single- and multiple-cardiac-phase 2D GRE and SSFP sequences, as well as 3D ECG-gated respiratory-navigated IR-GRE (LGE) sequences were tested in phantoms and N = 3 swine with overlaid ICDs. Results: With all cMR sequences, the system reduced artifacts from >100 ppm to <25 ppm inhomogeneity, which permitted imaging of the entire left ventricle in swine with ICD-related voids. Continuously acquired Gradient recalled echo or Steady State Free Precession images were used to interactively adjust the shim current and coil location. Conclusion: The shim system reduced large field inhomogeneities due to implanted ICDs and corrected most ICD-related image distortions. Externally-controlled motorized translation of the shim coil simplified its utilization, supporting an efficient cardiac MRI workflow.

Open Source MR-Safe Pneumatic Radial Inflow Motor and Encoder (PRIME): Design and Manufacturing Guidelines.

Actuators and encoders used in MR-guided robotic interventions are subject to strict requirements to ensure patient safety and MR imaging quality. In this paper, we present an open source computer aided design (CAD) of our MR-safe Pneumatic Radial Inflow Motor and Encoder (PRIME). PRIME is a parametrically designed motor that enables scalability based on torque and speed requirements for a wide range of MR-guided robotic procedures. The design consists of five primary modifiable parameters that define the entire motor geometry. All components of the motor are either 3D printed or available off-the-shelf. Quadrature encoding is achieved using a 3D printed housing and four fiber optic cables. Benchtop experiments were performed to validate the performance of the proposed design. To the best of our knowledge, this is the first open source MR-safe pneumatic motor and encoder in the field. We aim to share the design and manufacturing guidelines to lower the entry barriers for researchers interested in MR-guided robotics.

Review of Robot-Assisted HIFU Therapy.

This paper provides an overview of current robot-assisted high-intensity focused ultrasound (HIFU) systems for image-guided therapies. HIFU is a minimally invasive technique that relies on the thermo-mechanical effects of focused ultrasound waves to perform clinical treatments, such as tumor ablation, mild hyperthermia adjuvant to radiation or chemotherapy, vein occlusion, and many others. HIFU is typically performed under ultrasound (USgHIFU) or magnetic resonance imaging guidance (MRgHIFU), which provide intra-operative monitoring of treatment outcomes. Robot-assisted HIFU probe manipulation provides precise HIFU focal control to avoid damage to surrounding sensitive anatomy, such as blood vessels, nerve bundles, or adjacent organs. These clinical and technical benefits have promoted the rapid adoption of robot-assisted HIFU in the past several decades. This paper aims to present the recent developments of robot-assisted HIFU by summarizing the key features and clinical applications of each system. The paper concludes with a comparison and discussion of future perspectives on robot-assisted HIFU.

Non-Metallic MR-Guided Concentric Tube Robot for Intracerebral Hemorrhage Evacuation.

OBJECTIVE: We aim to develop and evaluate an MR-conditional concentric tube robot for intracerebral hemorrhage (ICH) evacuation. METHODS: We fabricated the concentric tube robot hardware with plastic tubes and customized pneumatic motors. The robot kinematic model was developed using a discretized piece-wise constant curvature (D-PCC) approach to account for variable curvature along the tube shape, and tube mechanics model was used to compensate torsional deflection of the inner tube. The MR-safe pneumatic motors were controlled using a variable gain PID algorithm. The robot hardware was validated in a series of bench-top and MRI experiments, and the robot's evacuation efficacy was tested in MR-guided phantom trials. RESULTS: The pneumatic motor was able to achieve a rotational accuracy of 0.32°±0.30° with the proposed variable gain PID control algorithm. The kinematic model provided a positional accuracy of the tube tip of 1.39 ± 0.54 mm. The robot was able to evacuate an initial 38.36 mL clot, leaving a residual hematoma of 8.14 mL after 5 minutes, well below the 15 mL guideline suggesting good post-ICH evacuation clinical outcomes. CONCLUSION: This robotic platform provides an effective method for MR-guided ICH evacuation. SIGNIFICANCE: ICH evacuation is feasible under MRI guidance using a plastic concentric tube, indicating potential feasibility in future live animal studies.

MR-Tracked Deflectable Stylet for Gynecologic Brachytherapy.

Brachytherapy is a radiation based treatment that is implemented by precisely placing focused radiation sources into tumors. In advanced interstitial cervical cancer bracytherapy treatment, this is performed by placing a metallic rod ("stylet") inside a hollow cylindrical tube ("catheter") and advancing the pair to the desired target. The stylet is removed once the target is reached, followed by the insertion of radiation sources into the catheter. However, manually advancing an initially straight stylet into the tumor with millimeter spatial accuracy has been a long-standing challenge, which requires multiple insertions and retractions, due to the unforeseen stylet deflection caused by the stiff muscle tissue that is traversed. In this paper, we develop a novel tendon-actuated deflectable stylet equipped with MR active-tracking coils that may enhance brachytherapy treatment outcomes by allowing accurate stylet trajectory control. Herein we present the design concept and fabrication method, followed by the kinematic and mechanics models of the deflectable stylet. The hardware and theoretical models are extensively validated via benchtop and MRI-guided characterization. At insertion depths of 60 mm, benchtop phantom targeting tests provided a targeting error of 1. 23 ± 0. 47 mm, and porcine tissue targeting tests provided a targeting error of 1. 65 ± 0. 64 mm, after only a single insertion. MR-guided experiments indicate that the stylet can be safely and accurately located within the MRI environment.

Minimal artifact actively shimmed metallic needles in MRI.

PURPOSE: Signal voids caused by metallic needles pose visualization and monitoring challenges in many MRI applications. In this work, we explore a solution to this problem in the form of an active shim insert that fits inside a needle and corrects the field disturbance (ΔB0 ) caused by the needle outside of it. METHODS: The ΔB0 induced by a 4 mm outside-diameter titanium needle at 3T is modeled and a two-coil orthogonal shim set is designed and fabricated to shim the ΔB0 . Signal recovery around the needle is assessed in multiple orientations in a water phantom with four different pulse sequences. Phase stability around the needle is assessed in an ex-vivo porcine tissue dynamic gradient echo experiment with and without shimming. Additionally, heating of the shim insert is assessed under 8 min of continuous operation with 1A current and concurrent imaging. RESULTS: An average recovery of ~63% of lost signal around the needle across orientations is shown with active shimming with a maximum current of 1.172 A. Signal recovery and correction of the underlying ΔB0 is shown to be independent of imaging sequence. Needle-induced phase gradients outside the perceptible signal void are also minimized with active shimming. Temperature rise of up to 0.9° Celsius is noted over 8 min of continuous 1A active shimming operation. CONCLUSION: A sequence independent method for minimization of metallic needle induced signal loss using an active shim insert is presented. The method has potential benefits in a range of qualitative and quantitative interventional MRI applications.