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Locally, patients undergoing prostate radiotherapy are monitored for prostate displacement using an autoscan 4D transperineal ultrasound (TPUS) probe during treatment. However, the existing TPUS probe and autoscan probe kit (ASPK) are not MRI-compatible. Hence, uncertainties due to variations in patient position and delineation of the prostate gland based on CT/MRI fusion may be introduced during treatment planning. This project aims to develop a MRI-compatible pseudo TPUS probe and ASPK to reproduce patient position during MRI to facilitate image fusion and prostate gland contouring during treatment planning.

The fabricated MRI-safe TPUS probe and ASPK were designed and produced in components with certain parts required to provide mechanical functions for angulation and height adjustments with similar accuracy of the existing probe used during treatment. Major components were modelled separately for producing parts using MRI-safe materials and assembly. The first prototype was printed as a singular part in both FDM and PolyJet. Selected materials must not introduce any magnetic field interference or contra-indicate MRI procedures (i.e. must not get heated up by magnetic waves). Selected manufacturing process had to consider production of parts that can ensure the rigidity and accuracy of the structure to withstand pressure on the probe at the transperineal region (i.e. Computerized Numerical Control (CNC) machined vs 3D printing). CNC process was eventually selected as 3D-printed parts tend to create material warpage that reduces the accuracy of the design and the material strength required.

Materials that passed both image test and metal detector test: PVC, PP, ABS, Wood Meranti, Polycarbonate, with Aluminium Composite failing both image and metal detector test. Acrylonitrile Butadiene Styrene (ABS) was selected as the MRI-safe material for development of the prototype due to its durability, rigidity, strength, corrosion and impact-resistant. A test scan was performed on the MRI scanner and found to be safe and artefact-free for target used cases. (Figure 1). Two rounds of prototyping were conducted to arrive at the final product (Figure 2). 

Figure 1: Test scans images from the MRI Scanner on the final design probe and ASPK setup
(Material passed the image test and dimension accuracy was achieved).

Figure 2: Final CNC machined prototype of the MRI-safe probe and ASPK (with commercial knee rest).

Final CNC-machined MRI-safe probe and ASPK was developed for subsequent TPUS prostate patients requiring CT/MRI fusion during radiotherapy treatment planning.  Moving forward, the effectiveness of this gadget on the reproducibility of patient position during MRI to facilitate image fusion for contouring of the prostate gland will be assessed in this novel clinical workflow for prostate cancer patients. Additionally, the gadget can also be used for patient education to improve compliance during setup.