OpenForce MR: A low-cost open-source MR-compatible force sensor

Measuring the force exerted by muscles during dynamic MR acquisition (either imaging or spectroscopy) provides important information for the standardization of the exercise performed in the scanner and is therefore important for reproducible results in musculoskeletal imaging. However, existing commercial solutions for such measurements are often very expensive and impractical. In this work, a novel, open-source, versatile force sensor made of non-magnetic, off-the-shelf components is presented. The sensor is based on four aluminum Wheatstone bridge load cells enclosed in a custom-built aluminum frame. These cells are connected to an Arduino microcontroller for data acquisition and serial communication with a host computer, on which a dedicated program visualizes and logs the recorded force in real time. All components were chosen to be compatible with the MR environment, commercially available, and low cost. The sensor was calibrated with a commercial dynamometer and subsequently tested in multiple MR acquisition scenarios (static morphological imaging, cine imaging during contraction, velocity-encoded imaging). The sensor correctly recorded data during all tested sequences, without cross-interference between the MR and the force acquisitions. Minor susceptibility artifacts are visible in the immediate vicinity of the sensor, but they did not impair the evaluation of the muscle of interest. In conclusion, the development of a low-cost, MR-compatible force sensor is feasible, and its usage does not interfere with MR acquisition. The full specifications of the sensor, including hardware design, firmware and host software are publicly released as open-source for the potential benefit of the whole community.