Application of digital image change detection to diagnosis and follow-up of cancer involving the lungs

The difference image produced by digitally subtracting one member of a pair of sequential PA chest radiographs from the other has been investigated as a possible tool for detecting primary lung cancer and for measuring response to treatment of cancer that is metastatic to the lungs. The resultant images show a variation from a uniform neutral gray level wherever significant changes exist between conjugate regions on the two x-rays; such changes occur, for example, when a lesion has grown during the interval between the two radiographic examinations. Such difference images might improve the ability of the radiologist to locate clinically significant features in the original radiographs, which might otherwise be overlooked because of the faintness of the detail. Certain demands of the process place restrictions on the repeatability of patient positioning during X-ray exposure. A method is suggested for facilitating more exact repositioning and also for producing usable difference images directly.