iaea human health series publications - SEDIM

(4) There should be no unexpected variation in apparent texture or magnitude of the noise across the uniformimage 21 . If the place to place variation changes over time, areas where the noise appears to be less mayindicate degradation in the detector causing a loss of sharpness. Increases in noise may indicate other detectorproblems. Document such problems by measuring the local SNR ((pixel value – offset)/standard deviation) inROIs of about 100 mm 2 or less located in an area of ‘normal’ noise level and compare these with the area(s)where noise levels are observed to have changed.(5) For CR, bear in mind that the heel effect is not compensated for by the CR reader’s processing algorithms, sosome non-uniformity due to the X ray beam is to be expected.8.5.5.5. Recommendations and corrective actions(1) If dead pixels or other unacceptable artefacts are noted, or if significant non-uniformity is present, the serviceperson should be contacted to investigate and correct the problem.(2) CR systems have no flat fielding and will require the service engineer to investigate unacceptable artefacts ina manner similar to that for screen film imaging.8.5.5.6. Time frame for corrective actionFor severe artefacts, corrective action should be taken immediately, before any further patients are imaged.For minor artefacts, corrective action should be taken at the next routine servicing of equipment.21Note that variations in signal level (especially in CR systems) and noise due to the heel effect are to be expected.104