Evaluation of analog contrast enhancement and digital unsharp masking in low-contrast portal images.

An experimental high-contrast sensitivity storage phosphor imaging system was used to produce double-exposure localization portal images of abdominal and pelvic treatment fields. The images were contrast enhanced by using an analog windowing technique and edge enhanced with a digital unsharp masking routine. A laser printer was used to print the storage phosphor images onto film. Conventional images were obtained by placing film in the cassette with the storage phosphor plates prior to exposure. Four radiation oncologists rated the storage phosphor and conventional films for perceptibility of anatomical detail needed to verify the placement of the treatment field. Contrast enhancement alone did not result in a significant improvement in perceptibility over unprocessed conventional film (p greater than 0.20). However, the combination of contrast and edge enhancement did result in a significant improvement over conventional film (p less than 0.05).