MRI Compatible Manipulator

Early detection and treatment of prostate cancer is crucial in patient survival rates. Both processes require precise needle placement. MRI compatible robotic systems will play an important role in future needle manipulation.

The goal of this research is to design a manipulator that is able to position a biopsy or treatment needle in a prostate tumor under the real-time control of a surgeon while the patient is in the bore of a closed MRI.

An antagonistic position control manipulator will be developed based on bistable Dielectric Elastomer (DE) actuators. These actuators have been shown to be MRI compatible. DE actuators operate normally within an MR scanner and do not affect the image produced. They are lightweight, inexpensive and simple. Their bistable structure, using build in compliant members, allows for a simple solution.

Current methods for imaged guided placement include the ultrasound-guided needles and open MRI needle placement. Closed-bore MRI is more effective than either of these systems as it yields an image with substantially greater detail. However, the high MRI magnetic fields limit the types of tools, particularly the actuators, which can be used with the real-time imaging. Also, current open MRI methods use a guidance template. BWH uses a template that has evenly spaced holes, 5mm apart. This limits the needle accuracy in the of X-Y directions. Further, the real-time adjustment of the angle of penetration is not possible.

The proposed manipulator will meet the following specifications:

(*) The needle workspace will cover a 2cm diameter circular plate in the x’y’ plane at the perineum extending 0.5cm in the z’ direction.
(*) The manipulator’s horizontal and vertical ranges of motion will produce an effective 10cm diameter workspace in the x”y” plane, which is approximately a circular plate of 2cm thickness in the z” direction, assuming the prostate is 10 to 12cm from the perineum. This workspace will be comparable to the workspace provided by the template at BWH.

Theory of Operation :

(*) There will be two end effectors: the needle tip and the needle guide tip.
(*) The needle guide will be positioned approximately 1cm in the z-direction from the perineum before the patient is moved into the magnet, as shown above.
(*) The manipulator will be mounted to the table of the MRI between the patient’s legs on an adjustable mount.
(*) The patient and manipulator will be slid into the magnet together.
(*) The manipulator will orient the needle guide and press it against the perineum using a specified target location and needle orientation based on real-time MR images with a resolution of ±5mm in the x’y’ plane.
* The angle of the needle guide will be adjusted if necessary based on a real-time MR image.
* Once needle penetration begins, the manipulator will only move the needle in the axial direction of the needle guide a distance of 10 to 12cm with respect to the z-axis with a resolution of less than 5mm.