Digital subtraction is a common technique used in radiographic studies of periapical lesions and other dental disorders for which the treatment must be evaluated over time. This paper presents a fast and reliable parallel image registration method for subtracting two digitized radiographs where an unpredicted mismatch is present. An optimal affine transformation is found using an adaptive genetic algorithm (GA) as the optimization strategy and a correlation ratio as the similarity measure. The parallel GA implemented takes advantage of the CPU idle cycles of a computational grid, resulting in an application that exploits an existent infrastructure and with a computational time of about twenty seconds when processing pairs of standard intra-oral radiographs.