Design and Characterization of MRI-compatible Plastic Ultrasonic Motor

TL;DR

This work tackles the challenge of MRI-compatible actuation for MR-guided procedures by introducing a fully plastic ultrasonic motor (USM) that minimizes eddy-current–related artifacts and SNR loss. It combines an Ultem stator with a PZT-5H ring and a four-phase high-voltage drive, followed by comprehensive MRI and mechanical testing to map how stator geometry affects speed and torque. Key findings include a maximum speed of 436.67 rpm and a torque of 0.0348 Nm for an optimized 48-teeth, beveled-stator design, with spinning-condition SNR reductions around 13–16%—an improvement over prior plastic motors. The work outlines concrete design guidelines (teeth count, notch size, edge bevel, and surface finish) and points to material and geometry optimizations to advance toward a semi-commercial MRI-compatible rotary motor for MR-guided robotics.

Abstract

Precise surgical procedures may benefit from intra-operative image guidance using magnetic resonance imaging (MRI). However, the MRI's strong magnetic fields, fast switching gradients, and constrained space pose the need for an MR-guided robotic system to assist the surgeon. Piezoelectric actuators can be used in an MRI environment by utilizing the inverse piezoelectric effect for different application purposes. Piezoelectric ultrasonic motor (USM) is one type of MRI-compatible actuator that can actuate these robots with fast response times, compactness, and simple configuration. Although the piezoelectric motors are mostly made of nonferromagnetic material, the generation of eddy currents due to the MRI's gradient fields can lead to magnetic field distortions causing image artifacts. Motor vibrations due to interactions between the MRI's magnetic fields and those generated by the eddy currents can further degrade image quality by causing image artifacts. In this work, a plastic piezoelectric ultrasonic (USM) motor with more degree of MRI compatibility was developed and induced with preliminary optimization. Multiple parameters, namely teeth number, notch size, edge bevel or straight, and surface finish level parameters were used versus the prepressure for the experiment, and the results suggested that using 48 teeth, thin teeth notch with 0.39mm, beveled edge and a surface finish using grit number of approximate 1000 sandpaper performed a better output both in rotary speed and torque. Under this combination, the highest speed reached up to 436.6665rpm when the prepressure was low, and the highest torque reached up to 0.0348Nm when the prepressure was approximately 500g.

Figures (30)

• Figure 1: The CAD drawing of the MRI-compatible USM.
• Figure 2: Components of MRI-compatible USM.
• Figure 3: Enclosed and finished status of the motor. A hole at the side was for rotary motion observation.
• Figure 4: Detailed design of plastic stator assembly.
• Figure 5: A remake high voltage USM driving system was configured. This system consists of an MRI-compatible HV motor driver, a 24V power supply, and a function generator. An oscilloscope is connected to the output testing pin for monitoring the signals.