Computed radiography exposure indices in mammography

Background. Studies indicate that computed radiography (CR) can lead to increased radiation dose to patients. It is therefore important to relate the exposure indicators provided by CR manufacturers to the radiation dose delivered so as to assess the radiation dose delivered to patients directly from the exposure indicators. Aim. The aim of this study was to investigate the performance of an Agfa CR system in order to characterise the dose indicators provided by the system. Method. The imaging plate response was characterised in terms of entrance exposure to the plate and the digital signal indicators generated by the system (SAL - scanning average level; and lgM - logarithmic median) for different beam qualities. Several exposures were performed on a mammography unit, and the digital signal, expressed in terms of SAL and lgM for each image, was correlated with the entrance exposure on a standard American College of Radiology (ACR) phantom. From this correlation, a relationship between the Agfa dose indices (SAL and lgM) and the average glandular dose (AGD) in mammography could be established. An equation was derived to calculate the AGD delivered to the patient as a function of the exposure indicator, lgM, and the kV. Results. The results indicated that the measured AGD at 28 kV for a standard breast thickness during routine calibration with the ACR phantom was 1.58 mGy (lgM=1.99). This dose value lay within 1.5% of the value calculated using the derived equation for a standard Perspex thickness of 4.2 cm using the automatic exposure control (AEC) (1.56 mGy). The standard error in using this equation was calculated to be 8.3%.