BACKGROUND AND OBJECTIVE: Digital systems have been replacing with screen-film analogue systems in diagnostic radiology departments, rapidly. Despite the differences in the properties of new x-ray imaging detectors, the same radiographic protocols that had been used for radiographic film-screen are used for digital imaging systems, without any review yet. In this study, the image quality and the patient dose in digital imaging of the chest are evaluated and optimized.
METHODS: Two digital radiography machines from two separated hospitals (Imam Khomeini and Bu Ali Hospitals-Sari) have been used in this experimental research. Imaging and dose measurement are carried out at different source to phantom distances and kilo-voltages. For measurement of the image quality, a contrast-detail radiography (CDRAD) phantom is used. For evaluation of optimization, the Inverse Image Quality Figure per patient dose squared (IQF_inv/E²) is used.
FINDINGS: Evaluation of measured data for optimization shows that for both of these two digital radiography machines, despite of increasing in patent dose, with reducing of kilo-voltage, the IQF_inv/E² is increased. The maximum values of this parameter for Imam Khomeini and Bu Ali Hospitals are measured 0.0180 and 0.0083, respectively. 
CONCLUSION: The results of this study indicate that despite the traditional notion of using higher kilo-voltages for chest radiography, with increasing kilo-voltage, the ratio of image quality per patient dose is reduced. So, for optimization of chest radiography, as much as possible the kilo-voltage should be reduced based on the size of patient and clinical purpose.