Computed radiography X-ray exposure trends

Computed radiography provides correct optical density on film, independent of the incident radiation exposure, but it can result in under- or overexposure of the imaging plate. In the current study, we evaluated the radiation exposure trends of computed radiography over a 2-year period for portable chest examinations to determine and compare the radiographic techniques of the computed radiography system relative to conventional screen-film detectors.

A Fuji computed radiography system was interfaced to a digital workstation to track system usage and examination demographics, including examination type and sensitivity number. Hard-copy films were used for diagnosis. The sensitivity number, a value inversely related to incident exposure on the imaging plate, was used to determine whether the proper techniques were used by the technologists.

The initial use of the computed radiography system revealed a broad distribution of exposures being used; complaints regarding noisy films (e.g., underexposure) resulted in subsequent overexposure for a significant number of films. A quality-control audit indicating excessive exposure resulted in educational feedback and a tighter distribution of exposures within the optimal range as determined by our radiologists. The average technique was approximately equivalent to a 200-speed system.

Computed radiography provides excellent dynamic range and rescaling capabilities for proper film optical density, and thus fewer repeat examinations. However, underexposure results in suboptimal image quality that is related to excessive quantum mottle. Overexposure requires film audits to limit unnecessary radiation exposure. In general, the optimal exposures are achieved with approximately 1.5-2 times the incident detector exposure of a 400-speed rare-earth system. The ability of computed radiography to reduce radiation exposure is unlikely when compared with a typical rare-earth screen-film combination (400 speed) in terms of adequate image quality for the diagnosis of subtle, low-contrast findings. For certain diagnostic procedures (e.g., nasogastric tube placement verification), lower exposures can be tolerated.