Determining air kerma from pixel values in digital mammography

The pixels in digital detectors used in x-ray imaging can be considered as small dose meters. In mammography, the image detector area is commonly only partly covered by tissue and the incident air kerma could be monitored by using pixel values from the detector elements in the unattenuated primary beam area. The measured air kerma could be used for patient dose estimation. The air kerma and radiation quality dependence of the response of pixel values within a large exposure range for two direct digital detector types was experimentally investigated for this purpose. Using these calibration results, air kerma was measured from clinical images and compared with tube output calculation. The differences between the two methods were typically below 2% and measurements based on pixel values can therefore be recommended.