A Fully Actuated Robotic Assistant for MRI-Guided Precision Conformal Ablation of Brain Tumors

Abstract

This paper reports the development of a fully actuated robotic assistant for magnetic resonance imaging (MRI)-guided precision conformal ablation of brain tumors using an interstitial high intensity needle-based therapeutic ultrasound (NBTU) ablator probe. The robot is designed with an eight degree-of-freedom (DOF) remote center of motion (RCM) manipulator driven by piezoelectric actuators, five for aligning the ultrasound thermal ablator to the target lesions and three for inserting and orienting the ablator and its cannula to generate a desired ablation profile. The 8-DOF fully actuated robot can be operated in the scanner bore during imaging; thus, alleviating the need of moving the patient in or out of the scanner during the procedure, and therefore potentially reducing the procedure time and streamlining the workflow. The free space positioning accuracy of the system is evaluated with the OptiTrack motion capture system, demonstrating the root mean square (RMS) error of the tip position to be 1.11±0.43mm. The system targeting accuracy in MRI is assessed with phantom studies, indicating the RMS errors of the tip position to be 1.45±0.66mm and orientation to be 1.53±0.69°. The feasibility of the system to perform thermal ablation is validated through a preliminary ex-vivo tissue study with position error less than 4.3mm and orientation error less than 4.3°.

Keywords: MRI-guided robot; image-guided therapy; robot-assisted neurosurgery; ultrasound thermal ablation.

Fig. 1:

(Top) ACOUSTx needle-based therapeutic ultrasound (NBTU) ablator in a configuration with two separate transducer sections along the axis of the instrument. The ablator is designed to be inserted within a plastic implant catheter to prevent from bending, as well as return the water flow. (Bottom) Schematic diagram of the US ablator in a configuration with two 90° directional acoustic sectors, labeled as the US transducers, and two solenoid coils on both ends.

Fig. 2:

Image processing steps for tracking coils localization (a) original image, (b) mask for first tracking coil, (c) mask containing both the tracking coils and (d) calculated coil locations overlaid with red cross marks. Unit: pixel (pixel spacing: 0.39mm x 0.39mm)

Fig. 3:

3D CAD model of the ultrasonic ablator manipulator, showing the kinematic configuration of degrees of freedom modeled after those of a traditional stereotactic frame and its dimensions at home position.

Fig. 4:

3-DOF ablator driver module. (a) exploded view showing: 1) driver base 2) ablator insertion stage 3) gears 4) ablator 5) ablator clamp 6) cannula 7) cannula guide 8) cannula insertion stage 9) thumb screws, (b) components in the semitransparent block are covered with a sterile drape and the remaining parts are made of bio-compatible and sterilizable materials, and (c) assembly of the ablator driver covered with a sterile plastic drape.

Fig. 5:

CAD model of the head frame adjustment module with patient placed in supine position and the fiducial frame attached to the platform. The robot sits beside the patient’s head attached to the platform. Note that the fiducial frame is designed to be attached proximal to the head during the registration phase of the procedure; in this figure it is intentionally located at the end of the platform to clearly demonstrate the head frame.

Fig. 6:

D-H frame assignment of the 8-DOF ablator manipulator. The origin of robot frame F_Rob is defined at the robot platform with x-y-z axes aligned with scanner’s RAS frame. The base frame F_Base is defined as the RCM point at home position. The tip frame F_Tip is defined at the tip of the ablator, with z-axis pointing along the ablator’s longitudinal axis.

Fig. 7:

Reachable workspace of the robot overlaid on a representative human brain.

Fig. 8:

Clinical workflow of MRI-guided robot-assisted thermal ablation of brain tumors.

Fig. 9:

Experimental setup of free space accuracy evaluation with OptiTrack motion capture system. Note that, six identical cameras are used in this study to capture the 6-D needle pose, and only two of them are visible in this figure.

Fig. 10:

A representative plot of five insertion pathways intersecting at a given target location based on the segmentation of MR image data. For the total 15 trials, the RMS error of the tip position is 1.45mm, orientation error is 1.53°, and RCM intersection error is 0.75mm.

Fig. 11:

Experiment setup of active tracking coils localization showing 3D printed template, ablator inserted through one of the guiding holes, and the tracking coil tuning box.

Fig. 12:

Ablator localization results showing position errors in R, A and S axes and orientation errors around R-axis and S-axis. The RMS errors of tip position less than 1.1mm and orientation less than 2.3°.

Fig. 13:

Ex-vivo chicken breast tissue with two thermal ablation foci about 30mm apart. The chicken breast tissue is modeled into a gelatin phantom to reduce movement during the insertions. The tissue is cut open right after the experiment to inspect the ablation treated foci, which has visible changes from the normal tissue, i.e changing to solid white color in the direction of the active ultrasound sector.

Fig. 14:

Experimental setup for ultrasound-based thermal ablation on an ex-vivo lamb brain. The lamb head is fixed at the head frame adjustment module via screws to prevent head movement during the procedures. The NBTU ablator is placed by the robot manipulator locked in place on the side.

Fig. 15:

3D MR image volume showing the US ablator inserted inside an ex-vivo lamb brain. The red line represents the actual US ablator segmented from MR volume images.