The invention provides for a medical instrument (100, 300, 400, 500, 600) comprising a camera system (102, 102′, 102″) for imaging a portion (418) of a subject (108) reposing on a subject support (106). The medical instrument further comprises a display system (104) for rendering a position feedback indicator (130, 900). The display system is configured such that the position feedback indicator is visible to the subject when the subject is reposing on the subject support. The execution of the machine executable instructions (120) causes a processor (114) controlling the medical instrument to: acquire (200) a base position image (122) using the camera system; repeatedly (202) acquire a subsequent image (124) using the camera system; repeatedly (204) calculate an image transformation (126) from voxels of at least a portion of the base position image to voxels of the subsequent image by inputting the base position image and the subsequent image into an image transformation algorithm (128); and repeatedly (206) render a position feedback indicator (130, 900) on the display, wherein the position feedback indicator is controlled by the image transformation.