1. A position measurement system comprising: an array of photosensors configured to be disposed on a medically operative member configured to interact with or acquire data from a subject disposed in an examination region, the array of photosensors arranged to view the examination region; and a position-determining member configured to determine a position of an optically detectable marker disposed with the subject in the examination region based on light from the optically detectable marker sensed by the array of photosensors.