Application of computed radiography for portal imaging in radiation oncology

In radiation therapy the treatment portal fields are conventionally imaged with a silver-halide film sandwiched in a cassette between lead and/or copper screens. The tissue contrast in these images is low due to the high energy of the photons used for treatment. Digital processing could be used to enhance the display contrast in the portal image. The authors investigated the use of a computed radiography (CR) system to obtain the digital portal image. CR is extensively employed in diagnostic radiology to acquire digital radiographic images that are archived in a picture archival communication system (PACS). In CR, the film is replaced with a photostimulable phosphor plate (PSP). The energy deposited by the X-ray beam in the PSP is read out using a scanning laser beam. The PSP is scanned in approximately 30 seconds. The PSP is prepared for reuse by exposing it to light for approximately 50 seconds, which erases any residual image on the plate. The display contrast of the digital portal images obtained with the CR system was enhanced using regional histogram-equalization techniques. These images were compared to images on radiographic film, exposed together with the PSP in the same cassette. The contrast-enhanced CR images allowed the observer to more readily identify anatomical structures in the image. Acquiring both digital treatment portal and radiographic simulation images is important for establishing a PACS for radiation oncology. Existing software could be used to register these digital portal and simulation images which would facilitate verification of treatment setup