FIG. 6.

Pixel intensity deviation (PID) images for the dynamic field of Fig. 1(a) shot at 6 MV. (a) The (PID) image derived from a 6 MV patient field image in which intensity has been artificially increased by 2% in a 20 × 20 pixel region (the yellow rectangle). (b) The corresponding gradient scaled PID (GSPID) image, with the anomaly showing as a red rectangle. In the case of the PID image, identification of the anomaly is made more difficult by the presence of outlier PID values extending to −3 and +6. In the GSPID image the anomaly is more easily detectable because it lies outside the range of GSPIDs in the remainder of the image.