1. Apparatus for acquiring an elongated radiographic image comprising: a flat panel electronic detector of radiographic images, the detector having a known length; a transport mechanism for mounting the detector for movement in a direction parallel to the known length so that the detector can be positioned in sequential contiguous partially-overlapping positions to acquire a radiation image greater in length than the detector length; a frame supporting the transport mechanism, the frame having an edge substantially parallel to the direction of movement of the detector; and at least one marker disposed on the frame proximate the edge in a region corresponding with each partially-overlapping position of the detector.