Automatic analysis of collimator structure for quality assurance

A non-destructive, automatic and quantitative method was developed in order to characterize the structure of the gamma camera collimators because they remain the weakest part of the system. As the quality of a collimator depends on the regularity of the hole shape, the authors developed software implemented on a microcomputer using the macro programming language provided by the image processing program NIH Image 1.52. A radiograph is first obtained by means of a home made rectilinear scanner and digitized with a film scanner. These data are then analyzed. The hole dimension and septal thickness are determined by applying a matched mask on the digitized image made binary. The hole inclination is estimated by the shift of the center of gravity determined from the gray level image. For foil collimators the results show a big spread; e.g. S.D. are equal to 0.98 and 0.24 mm for 2.3 mm diameter and 0.15 mm septal thickness (manufacturers data) respectively. A cast collimator with the same characteristics has S.D. equal to 0.16 and 0.08 mm. In terms of hole inclination, a value of up to 1.3 degrees was found with foil technology. This method allows to detect automatically the manufacturing defaults of a collimator. Any geometry (parallel, fan beam) may be evaluated with minor changes in the technique.