Three-axis MR-conditional robot for high-intensity focused ultrasound for treating prostate diseases transrectally

Abstract

Background: A prototype magnetic resonance image (MRI)-conditional robot was developed for navigating a high-intensity focused ultrasound (HIFU) system in order to treat prostate cancer transrectally.

Materials and methods: The developed robotic device utilizes three PC-controlled axes: a linear axis for motion along the rectum, an angular axis for rotation in the rectum, and a linear axis to lift the robot up and down. Experiments with the system were performed in a 1.5-T MRI system using gel phantoms.

Result: The robot was successfully operated in a 1.5-T clinical MRI system. The effect of piezoelectric motors and optical encoders was quantified based on the reduction of signal to noise ratio. Discrete and overlapping lesions were created accurately by moving the HIFU transducer with the robotic device.

Conclusion: An MRI-conditional HIFU robot was developed which can create controlled thermal lesions under MRI guidance. The intention is to use this robot transrectally in the future for the treatment of prostate cancer.

Figure 1

Complete CAD drawing and photo of the positioning device. (A) Complete CAD drawing of the positioning device, and (B) photo of the positioning device with the three PC-controlled stages.

Figure 2

Main window of the software and electronic system. (A) Software used to control the positioning device and the therapeutic system which is guided by MRI, and (B) electronic system.

Figure 3

Photo of the 3D positioning device as placed on the table of the GE MRI scanner.

Figure 4

Graphs of different accuracy tests. (A) Measured distance vs. the number of cycles of the encoder for the forward linear motion for the X-axis, (B) measured distance vs. the number of cycles for the forward linear motion for the Z-axis, and (C) measured angle vs. the number of cycles of the encoder for the angular axis.

Figure 5

T1-W FSPGR imaging of the gel phantom with and without activation of motor. (A) T1-W FSPGR imaging of the gel phantom without activation of motor, encoder, and transducer and (B) T1-W FSPGR imaging of the gel phantom with activation of motor, encoder, and transducer.

Figure 6

Signal to noise ratio (SNR) measured for different conditions using T1-W FSPGR using the liquid phantom.

Figure 7

Discrete thermal lesions created in the gel phantom. Using the linear stage of the positioning device imaged using T2-W FSE in a 1.5-T GE scanner. The intensity used was 1,500 W/cm² (spatial average in situ) for 10 s. The spacing between the lesions was 10 mm. The bar corresponds to 5 mm. (A) Perpendicular to the transducer beam and (B) parallel to the transducer beam. The bar corresponds to 10 mm.

Figure 8

Strategy of creating overlapping lesions and thermal lesions and the grid ablation from a plane. (A) Strategy of creating overlapping lesions by moving the linear axis X and the angular axis θ (example of five linear steps and three angular steps). (B) Overlapping thermal lesions created in gel phantom using both the linear and angular stages of the positioning device. The intensity used was 1,200 W/cm² (spatial average in situ) for 10 s. The ablation grid was 10 × 5 with spacing between lesions of 2 mm for the linear stage and 3° for the angular stage. The plane is perpendicular to the ultrasonic beam. (C) The plane is parallel to the ultrasonic beam.