Impact of exposure parameters on chest image quality of mobile digital radiography

Objective To assess the impact of different exposure tube voltage and mAs on pixel mean value,noise and contrast to noise ratio(CNR) of image in regions of interest,in order to recommend the best exposure parameters to reduce radiation dose on the condition of good image quality.Methods One group kept the mAs constantly(the mAs was used in conventional bedside X-ray photography) and raised the tube voltage from 55 kVp to 120 kVp by step of 5 kVp.Four suitable kVp values were selected out from first experiment and the mAs was raised from 1.00 to 5 by 7.00 steps as the second experiment.The pixel mean value and noise of the image in region of interest were measured and recorded in 2 density areas,and the CNR was calculated.Results The pixel mean value of the image in region of interest was not influenced by the exposure tube voltage or mAs.The exposure condition affected the noise and CNR: When tube voltage was 80 kVp,the noise decreased and CNR remarkably as tube voltage raised.When tube voltage was ≥80 kVp,increase of tube voltage affected the noise and CNR indistinctly.Keeping the tube valtage in 4 different values,the mAs influenced the noise and CNR: When mAs was 2.00,the noise decreased remarkably as the mAs raised.When mAs was ≥2.00,raising mAs affected less and less on the noise and CNR.Conclusion The suggested tube voltage of the chest radiography by DR is from 75 kVp to 85 kVp,depending on patients' form,and the suggested mAs is from 2.00 to 3.0 0 for obtaining better image quality and less patients' X-ray dose.