MR Safe actuator design

MRI is one of the imaging techniques to diagnosis prostate cancer (PCa) and guide an intervention. Prostate biopsies performed with the MRI are time-consuming and difficult to perform accurately because of the space constraint caused by the closed bore of the MR scanner. MR guided manipulators address this problem by providing accessibility and position accuracy. However, the design of manipulators are limited in material and actuation type due to the magnetic field of the MRI. State-of-the-art MR safe pneumatic actuators show high potential results, but lack of bandwidth (i.e. low output velocity and/or accuracy), because they are connected to 6 meter long hoses between the control room and the MR scanner. A new design is of the actuators is required: one that can increase the velocity of the actuators, while maintaining the required biopsy accuracy.

What did I do?

For my thesis I designed and constructed an MR safe stepper motor that has a higher output velocity then state-of-the-art MR safe stepper motors, while it is still able to achieve a high position accuracy. The stepper motor is completely MR safe, which means no magnetic and conductive materials are used. The actuation is done by pneumatics, because the actuator needs to be free of electronics to not interfere with the imaging of the MR scanner.

Software and hardware used

• Solidworks
  The designs are created with Solidworks combined with Git for source control. The solidworks parts and assemblies are equation-driven, which means that a change in one module, updates all the other modules. This design method reduces the magic numbers in a part or assembly to the bare minimum, such that interface errors between the modules and there parts are avoided.
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• 3D printers and lasercutter
  All the designed parts are 3D printed in Vero transparant material with the Objet260 Connex3 or in PLA with the Ultimaker 2 or Leapfrog Creatr HS. The plexiglass used in the actuators are lasercutted. The bearings and screws are bought (the nuts are custom-made in PLA).
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• Arduino Uno
  To control the actuator, a pneumatic setup is created with a manifold with solenoid switching valves. These valves are connected to an Arduino Uno. The Arduino is connected to the laptop to create a serial connection.
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• Python
  A connection to the serial bus is established with a  python script. It is used to send commands and receive data to/from the Arduino. The python script pre-processed and records the data on the laptops hard-disk.
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• Matlab
  Several scripts are written in Matlab to further process and analyse the data.
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• Matlab 2 Tikz
  My thesis is written in LaTeX, therefore Matlab-2-tikz is used to create vector images of the data in the report.

Organisations

I had the opportunity to do my graduation at DEMCON, a high-tech mechatronic company, together with the research group RAM at University of Twente. In an earlier project, they developed together the needle position system MIRIAM. I used this system as a starting point for my requirements, where the ultimate goal would be interchanging the current actuators with my pneumatic actuator. However, due to the complexity of creating medical approved actuators, this is more likely a few years project, then a graduation project of less then a year.