MR compatible force sensing system for real-time monitoring of wrist moments during fMRI testing

Abstract

Functional magnetic resonance imaging (fMRI) of brain function is used in neurorehabilitation to gain insight into the mechanisms of neural recovery following neurological injuries such as stroke. The behavioral paradigms involving the use of force motor tasks utilized in the scanner often lack the ability to control details of motor performance. They are often limited by subjectiveness, lack of repeatability, and complexity that may exclude evaluation of patients with poor function. In this paper we describe a novel MR compatible wrist device that is capable of measuring isometric forces generated at the hand and joint moments along wrist flexion-extension and wrist ulnar-radial deviation axes. Joint moments measured by the system can be visually displayed to the individual and used during target matching block or event related paradigms. Through a small set of pilot testing both inside and outside the MR environment, we have found that the force tracking tasks and performance in the scanner are reproducible, and that high quality force and moment recordings can be made during fMRI studies without compromising the fMRI images. Furthermore, the device recordings are extremely sensitive making it possible for individuals with poor hand and wrist function to be tested.

Figures

Fig. 1

Wrist module developed for use during fMRI testing. Subjects lightly grasp the plastic handle extending from the load cell and exert wrist flexion–extension or ulnar–radial deviation forces against it. The arrows denote the direction of the applied forces for wrist flexion or extension. The forearm is supported by four padded bumpers which helps minimize forces from propagating up the arm necessitating contraction of proximal arm muscles. The plastic wedge is mounted to a plastic base which the subject lays onto anchor the device.

Fig. 2

Graphical user interface. (a) The operator screen shows the subject target display as well as the joint moments exerted by the subject along wrist flexion–extension and ulnar–radial deviation axes, as well as forearm pronation–suppination. (b) The subject target display consists of a target (rectangle shown in white) and a cursor (bar shown in yellow). The subject is instructed to exert a joint moment along a particular joint axis (e.g. wrist flexion) and move the cursor into the target for a specified amount of time. In between trials, the cursor remains locked in the center of the screen for the subject to fixate on.

Fig. 3

Mean maximum isometric wrist flexion torques for subjects S1–S4 for sessions 1 and 2. There were no statistical differences in maximum joint torque between sessions. Note the error bars represent 95% confidence intervals.

Fig. 4

Group mean muscle activation patterns for target locations ranging from 10 to 50% maximum wrist flexion torque. It can be seen that while the flexor carpi radialis scales with increasing exertion level, the proximal arm muscles remain quiet due to the forearm bumpers preventing the forces generated at the wrist from propagating up the arm (see text for details).

Fig. 5

Illustration of the wrist device being used inside the scanner during fMRI testing. The wrist module is normally located just inside the scanner bore, dependent on the size of the test subject.

Fig. 6

Section of wrist flexion moments generated during an fMRI scan. The shaded rectangles represent the target, where the vertical component represents the target level (e.g. 20 ±4% maximum) while the width of the shaded region represents the timeout period (e.g. the maximum time the target will be displayed). The vertical dashed lines represent when the targets appear.

Fig. 7

A representative scan of the motor cortex during a wrist flexion behavioral task located at 20% maximum exertion level. See text for interpretation of results.