Determination of projections for subtraction radiography based on image similarity measurements

The reliability of subtraction radiography strongly depends on the ability to obtain two identical projections. Generally this is done by connecting mechanically the X-ray source, the patient and the film. However, this method has certain disadvantages, which restrict its use to small groups of patients in other than routine clinical situations. A computer-aided radiographic imaging technique (tomosynthesis) brings the reconstruction of arbitrary projections within reach. This technique can be used to reconstruct the projection required for a proper subtraction. However, in order to do so, the coordinates of the source position of one projection, relative to the source positions of the set of projections used for fomosynthesis, has to be determined. A method is described based on similarity measurements, in pairs of images (expressed in the form of standard deviations) to achieve this. The coordinates of the unknown source position could be determined with an average accuracy of 0.513 degrees (range 0.000-1.289 degrees), which is well within the range of deviations tolerable for the clinical application of subtraction radiography.