Measurement of Magnetically Induced Torque on Lightweight Medical Devices in the Magnetic Resonance Environment for ASTM F2213

Use of medical devices in the magnetic resonance environment is regulated by international standards that include ASTM F2213 for the measurement of magnetically induced torque on the device. This standard prescribes five test methods. However, none can be directly applied to measure very low torques of slender lightweight devices such as needles.

This is a variant of the ASTM torsional spring method in which the spring is made by 2 strings that suspend the needle from its ends, the 2-string method. The magnetically induced torque on the needle causes it to rotate. The strings tilt and lift the needle. At equilibrium, the magnetically induced potential energy is balanced by the gravitational potential energy of the lift. Static equilibrium allows calculating the torque from the needle rotation angle, which is easily measured.

The same apparatus can also be configured for force measurements in the MRI according to ASTM F2052.

The device is simple and inexpensive. It may be 3D printed and the design files are shared below. The approximate price is 58 USD.

The method was tested in 1.5T and 3T MRI scanners to test bone biopsy needles. Results show that torques as low as 0.1Nmm were measured and the maximum error was 7.7%.