A mobile radiography apparatus includes radiopaque markers disposed in a radiation path that extends from an x-ray source to a digital radiographic detector, which detector is mechanically uncoupled from the x-ray source or x-ray sources. A processing system calculates a position of the detector relative to the x-ray source or x-ray sources according to identified marker positions in acquired x-ray projection images, and reconstructs a volume image according to the acquired x-ray projection images.