Sensors for Applications in Magnetic Resonance Environments

This paper analyzes sensing methods compatible with magnetic resonance imaging (MRI) and functional MRI (fMRI) reported in the literature, and presents the three generations of MR-compatible force/torque sensors we have developed for robotic systems to interact with human motion. Conventional sensors such as camera-based measurement systems, strain gauges or commercial force/torque sensors, and optical encoders may be used, if placed sufficiently away from the imaging region and equipped with adequate shielding and filtering in order to minimize electromagnetic interference caused by electric cables, the transducer, and electronics of surrounding equipment. However, electromagnetic interference can be avoided by using light transmission over optical fibers, in which case sensitive and noisy electronic components can be placed outside the MR room, and the MR compatibility issue is restricted to the used materials. Good performance can be obtained with sensing elements made from materials adapted to the location of use, combined with reflected or differential light intensity measurement over optical fibers. We have developed various force and position sensors based on this principle, ranging from MR Safe (for a definition and discussion of the terms MR Safe and MR Conditional, see Gassert , IEEE Eng. Med. Biol. Mag., pp. 12--14, May/Jun. 2008) milled polymer probes to MR Conditional assemblies combining beryllium copper blades with a polymer body, as well as smaller aluminum probes realized through a combination of milling and electric discharge machining. It appears that, in contrast to actuators, good performance is not in tradeoff with MR compatibility.