The quality of portal images can be improved by the use of computed radiography(CR)with storage phosphor technology instead of the conventional screen-film system. The frequency processing of portal images, however, sometimes produces artifacts in the field margin, which may decrease the precision of verification of the irradiation field. The object of our study was to clarify the optimum parameters of frequency processing for CR portal imaging. The FCR-7000 system with 4MV X-ray was employed. Our results showed that the usual parameters of frequency processing and irradiation field size, were useful in verifying the irradiation field. However, artifacts are likely to have an effect, especially extreme parameters of frequency processing and small fields. In addition, the following wre important in selecting the parameters of frequency processing:1)the band of frequency should be selected according to field width and frequency(RN3:0.25cycle/mm), 2)the type of frequency should be used the graininess emphasis, 3)the emphasis should be minimized(RE3.0).
