Since the emergence of CR and DR, it’s become much easier for a technologist to produce diagnostically useful images with consistent optical density. At the same time, however, the development of new algorithms as well as the ability to post-process images has made it difficult for one to visually assess the quality of a digital radiograph, as most of the image quality problems encountered during the examination can now be eliminated with the new technology. The exposure index (EI) is a method provided by Kodak manufacturers to estimate the radiation exposure to the detector and thereby to offer feedback to the operator regarding the quality of the image in addition to patient dose. An improperly calibrated CR processor may result in constant under/overexposure to the patient, leading to poor overall radiographic quality or increased dose to the patient. Therefore, it is important to have the EI calibrated in order to reduce patient dose while maintaining the optimum quality of x-ray images.
The EI calibration lab was carried out in Room 6. A dosimeter was placed on the floor with its display facing the window, and the tube was moved directly above the dosimeter sensor at a distance of 172cm (68”) SID. The light was centered and collimated so that it would cover the entire sensor. A 1mm Al sheet was taped under the collimator; the added filtration was adjusted to 0.3mm Cu and kVp was set at 80 on the control panel. The dosimeter was exposed to determine an mAs value that corresponds to 10mR (0.010R), which was found to be 25 after a few trials and errors. The Al sheet was briefly removed while replacing the dosimeter with a 10"x12" cassette before it was taped under the collimator again. With the technical factors kept the same, a total of 3 cassettes were exposed, which in turn were processed using a “Pattern” setting on the Kodak CR processor. The EI values read 1840, 1860, and 1880, respectively. The numbers were then used to normalize each exposure using the following equation: EI1mr = EIExperimental - [1000 x log(ExposureExperimental)] (Samei et al., 2001) An example of the calculation would be EI1mr = 1840 - (1000 x log(10mR)) = 1840 – (1000 x 1) = 840. Therefore, the corresponding normalized values are 840, 860, and 880. The quantitative criteria for acceptance for the Kodak CR system is EI1mR-2000<+-45 for a single screen, and EI1mr-2000<+-23 for all screen averaged. There are no qualitative criteria for acceptance (Samei et al., 2001). As the quantitative data collected from the lab clearly do not meet the criteria, it is concluded that the Kodak CR processor isn't properly calibrated.
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