1. A computer-aided bone densitometry system comprising: an x-ray source and detector opposable about a patient to produce signals indicating x-ray attenuation by bone of the patient; a computer receiving the signals and executing a stored program to: (a) collect a first and second bone density image at a first and second time; (b) determine a correspondence in bone areas in corresponding regions of the first and second bone density images; and (c) provide an indication to an operator of possible patient positioning error when a predetermined correspondence in bone areas is not found.