When prostate cancer is suspected, biopsies of the questionable tissue are taken. This has to be done using MRI imaging, in the limited area available between the patient’s legs. In the current, manual procedure, the doctor inserts the biopsy needle rectally; the patient is given preventive antibiotics because of the risk of infection. On average some four attempts are needed before the needle reaches exactly the right position. The alternative is a robot which can introduce the needle through the external skin. This makes the procedure faster, more accurate, more sterile and more patient-friendly.
This automation is the objective of MIRIAM (Minimally Invasive Robotics In An MR environment), a project by the University of Twente (UT) and several partners, including the Radboudumc and DEMCON. DEMCON’s task was to develop a robot system for placing the needle accurately. In consultation with the UT researchers and Radboudumc specialists, DEMCON drew up the programme of requirements in terms of functionality, available space and the required degrees of freedom.
For the system concept a so-called Stewart platform (familiar in movement simulators) was chosen, which hangs suspended from an upper plate by five activated arms. The movements of these five arms are controlled by a system of ball and socket and line joints, threaded spindles and piezoelectric motors. An extensive mechanical tolerance analysis established the foundation for a very precise construction.
The doctor indicates the point of interest, and the operating system recognises the robot and the patient’s position. The system positions the robot in the correct orientation, and the robot then introduces the needle gradually into the prostate, autonomously. Taking the biopsies occurs in accordance with a clever concept in which pneumatics builds up the mechanical tension needed to perform an effective cutting movement. The biopsy needle has an angled tip which enables active control: rotating the needle points it in a different direction.
A significant design challenge was the MRI environment. The various robot components could not be allowed to influence the quality of the MRI images, and thus the magnetic field. This required the use of high-end MRI-compatible materials. Wherever there were any doubts, DEMCON could test on a manikin in the Radboudumc. The arms of the Stewart platform are manufactured in carbon, and synthetic screws (by PEEK) are reliably holding the construction together. Ceramic was used in bearings and on the treads of the piezoelectric motors for sufficient accuracy and rigidity.
Other design requirements involved ergonomics, sterility and appearance. The construction was chosen in such a way that patients of different sizes could adopt a comfortable position, and that there would always be sufficient space to introduce the needle. The design also took account of cleaning the system and easily being able to sterilise the parts which come in contact with the patient. The industrial design produced a high-quality appearance. Thus both the doctor and the patient can be assured of MIRIAM’s quality.