Real-time Video Projection in an MRI for Characterization of Neural Correlates Associated with Mirror Therapy for Phantom Limb Pain

Abstract

Mirror therapy (MT) has been proposed as an effective rehabilitative strategy to alleviate pain symptoms in amputees with phantom limb pain (PLP). However, establishing the neural correlates associated with MT therapy have been challenging given that it is difficult to administer the therapy effectively within a magnetic resonance imaging (MRI) scanner environment. To characterize the functional organization of cortical regions associated with this rehabilitative strategy, we have developed a combined behavioral and functional neuroimaging protocol that can be applied in participants with a leg amputation. This novel approach allows participants to undergo MT within the MRI scanner environment by viewing real-time video images captured by a camera. The images are viewed by the participant through a system of mirrors and a monitor that the participant views while lying on the scanner bed. In this manner, functional changes in cortical areas of interest (e.g., sensorimotor cortex) can be characterized in response to the direct application of MT.

Figures

Figure 1:. Video camera and mirror set up.

The mirror is positioned between the legs at an angle of about 45°, depending on the participant’s height and amputation level. The goal is to cover the residual limb and make it invisible to the video systems. Sandbags are used to keep the mirror in the correct position. The camera positioning is also adaptable and can be easily changed using the tripod or the adaptable hock (changes the angle of the camera).

Figure 2:. Schematic of video camera and image projection in the scanner environment.

The real-time video projection of the mirror therapy system consists of three subsystems. 1) Camera and monitor subsystem. The video is transmitted to the monitor, so the subject can watch the leg and mirror leg movements in real-time. 2) The head coil with the mirror attached. The mirror in the head coil allows the participant to watch the monitor without moving their head. The mirror is at a 45° angle at eye level. 3) The mirror and sandbags. The MRI-compatible mirror is carefully placed between the legs and the residual limb in a way that it covers the residual limb and allows for the best image to be shown.

Figure 3:. Task design.

The task design consists of three steps. During the first “leg” step, the subject is instructed to move the leg (flexing the foot) at a pace of about one movement every 2 s (10 movements in 20 s), with their eyes closed. For the second “mirror” step, the participant has to keep moving the leg (10 movements in 20 s) while looking at the video monitor displaying the online real-time mirror image of the legs. The last step instructs the subject to rest.

Figure 4:. Representative example of the cortical activations in response to mirror therapy in the MRI scanner.

(A) Comparing leg movement versus the rest condition resulted in a robust activation within the sensorimotor representation of the leg of the contralateral (i.e., left) and ipsilateral cortex. (B) The mirror condition versus rest condition also confirmed a robust contralateral and ipsilateral activation of the cortical leg sensorimotor representation, as well as occipital (i.e., visual) cortical activation associated with viewing the projected image of the moving leg.