MRI-powered Actuators for Robotic Interventions

Design of a Patient-Specific Respiratory-Motion-Simulating Platform for In Vitro 4D Flow MRI

Four-dimensional (4D) flow magnetic resonance imaging (MRI) is a leading-edge imaging technique and has numerous medicinal applications. In vitro 4D flow MRI can offer some advantages over in vivo ones, especially in accurately controlling flow rate (gold standard), removing patient and user-specific variations, and minimizing animal testing. Here, a complete testing method and a respiratory-motion-simulating platform are proposed for in vitro validation of 4D flow MRI. A silicon phantom based on the hepatic arteries of a living pig is made. Under the free-breathing, a human volunteer's liver motion (inferior-superior direction) is tracked using a pencil-beam MRI navigator and is extracted and converted into velocity-distance pairs to program the respiratory-motion-simulating platform. With the magnitude displacement of about 1.3 cm, the difference between the motions obtained from the volunteer and our platform is ≤ 1 mm which is within the positioning error of the MRI navigator. The influence of the platform on the MRI signal-to-noise ratio can be eliminated even if the actuator is placed in the MRI room. The 4D flow measurement errors are respectively 0.4% (stationary phantom), 9.4% (gating window = 3 mm), 27.3% (gating window = 4 mm) and 33.1% (gating window = 7 mm). The vessel resolutions decreased with the increase of the gating window. The low-cost simulation system, assembled from commercially available components, is easy to be duplicated.

NITINOL-based actuator for device control even in high-field MRI environment

Background

The magnetic resonance imaging (MRI) environment with its high-strength magnetic fields requires specialized and sometimes sophisticated solutions for otherwise simple problems. One of these problems is MR-compatible actuator mechanisms that transfer a signal into an action.

Purpose

Normal actuators are based on a magnetic effect (eg, relays) and will typically not work in magnetic fields exceeding 1000 G, eg, inside the bore of an MR scanner. To enable the use of clinical devices inside the MRI, eg, for interventional procedures, there is a need for fully compatible actuators.

Patients and methods

Various actuators were compared for the purpose as a simple on-off switch within an MRI. NITNOL wire as an actuator showed the highest potential because of its simplicity and reliability. We tested the possible force achieved by the NITINOL wire related to the respective energy consumption, to provide a travel range of 2 mm.

Results

Compared to other actuators, the NITNOL wire is cheaper and requires less space. In the switching process however, there is a delay due to the time required for the heating of the wire up to the transformation temperature. The NITINOL switch shows a reliable behavior with regard to the generated force and the switching path over the entire measurement. Significant artifacts, caused by the NITNOL wire could not be detected in the MRI.

Conclusion

NITINOL wires can be repeatedly used, are relatively easy to implement and could be an economic alternative to other more complicated actuator technologies.

MRI-powered biomedical devices

Magnetic resonance imaging (MRI) is beneficial for imaging-guided procedures because it provides higher resolution images and better soft tissue contrast than computed tomography (CT), ultrasound, and X-ray. MRI can be used to streamline diagnostics and treatment because it does not require patients to be repositioned between scans of different areas of the body. It is even possible to use MRI to visualize, power, and control medical devices inside the human body to access remote locations and perform minimally invasive procedures. Therefore, MR conditional medical devices have the potential to improve a wide variety of medical procedures; this potential is explored in terms of practical considerations pertaining to clinical applications and the MRI environment. Recent advancements in this field are introduced with a review of clinically relevant research in the areas of interventional tools, endovascular microbots, and closed-loop controlled MRI robots. Challenges related to technology and clinical feasibility are discussed, including MRI based propulsion and control, navigation of medical devices through the human body, clinical adoptability, and regulatory issues. The development of MRI-powered medical devices is an emerging field, but the potential clinical impact of these devices is promising.

Teleoperation System with Hybrid Pneumatic-Piezoelectric Actuation for MRI-Guided Needle Insertion with Haptic Feedback

This paper presents a surgical master-slave tele-operation system for percutaneous interventional procedures under continuous magnetic resonance imaging (MRI) guidance. This system consists of a piezoelectrically actuated slave robot for needle placement with integrated fiber optic force sensor utilizing Fabry-Perot interferometry (FPI) sensing principle. The sensor flexure is optimized and embedded to the slave robot for measuring needle insertion force. A novel, compact opto-mechanical FPI sensor interface is integrated into an MRI robot control system. By leveraging the complementary features of pneumatic and piezoelectric actuation, a pneumatically actuated haptic master robot is also developed to render force associated with needle placement interventions to the clinician. An aluminum load cell is implemented and calibrated to close the impedance control loop of the master robot. A force-position control algorithm is developed to control the hybrid actuated system. Teleoperated needle insertion is demonstrated under live MR imaging, where the slave robot resides in the scanner bore and the user manipulates the master beside the patient outside the bore. Force and position tracking results of the master-slave robot are demonstrated to validate the tracking performance of the integrated system. It has a position tracking error of 0.318mm and sine wave force tracking error of 2.227N.

An MRI-powered and controlled actuator technology for tetherless robotic interventions

This paper presents a novel actuation technology for robotically assisted MRI-guided interventional procedures. In the proposed approach, the MRI scanner is used to deliver power, estimate actuator state and perform closed-loop control. The actuators themselves are compact, inexpensive and wireless. Using needle driving as an example application, actuation principles and force production capabilities are examined. Actuator stability and performance are analyzed for the two cases of state estimation at the input versus the output of the actuator transmission. Closed-loop needle position control is achieved by interleaving imaging pulse sequences to estimate needle position (transmission output estimation) and propulsion pulse sequences to drive the actuator. A prototype needle driving robot is used to validate the proposed approach in a clinical MRI scanner.

Tracking and position control of an MRI-powered needle-insertion robot

The excellent imaging capabilities of MRI technology are standardizing this modality for a variety of interventional procedures. To assist radiologists, MRI compatible robots relying on traditional actuation technologies are being developed. Recently, a robot that is not only MRI compatible but also MRI powered was introduced. This surgical robot is imaged and actuated through interleaved MRI pulses, and can be controlled to perform automated needle insertion. Using the electromagnetic field generated by the MRI scanner, the robot can exercise adequate forces to puncture tissue. Towards the goal of automation, this paper reports results on tracking and control of an MRI-powered robot tagged with a fiducial marker. Tracking is achieved using non-selective RF pulses and balanced gradient readouts. To suppress the signal received from the tissue, spoiler gradients and background suppression are introduced. Their effects on tracking are quantified and are used to optimize the algorithm. Subsequently, a Kalman filter is employed for robustness. The developed algorithm is used to demonstrate position controlled needle insertion ex vivo.