In computerized radiography (CR) imaging, collimation is frequently employed to shield the body parts from unnecessary radiation exposure and minimize radiation scattering using x-ray opaque material. The radiation field is therefore the diagnostic region of interest which has been exposed directly to x-rays. We present an image analysis system for the recognition of the collimation, or equivalently, detection of the radiation field. The purpose is to (1) facilitate optimal tone scale enhancement, which can be driven only by the diagnostically useful part of the image data, and (2) minimize the viewing flare caused by the unexposed area. This system consists of three stages of operations: (1) pixel-level detection and classification of collimation boundary transition pixels; (2) line-level delineation of candidate collimation blades; and (3) region- level determination of the collimation configuration. This system has been reduced to practice and tested over 807 images of 11 exam type and a success rate in excess of 99% has been achieved for tone scale enhancement and masking. However, in general, these false negative cases have no significant impact on either tone scale or flare minimization because of the intrinsic nature of the algorithm. Due to the novel design of the system, its computational efficiency lends itself to on-line operations.