Experimental study on optimization of mAs in digital mammography / 中华放射医学与防护杂志

ABSTRACT

Objective:To evaluate the effect of reducing mAs on image quality when different target/filter combinations are used in digital mammography.Methods:In different target/filter combinations, based on the mAs of automatic exposure control(AEC), the reduction of mAs by 10%, 20%, 30%, 40% and 50% respectively were used to expose the phantom with 4.4 cm thickness. The contrast to noise ratio(CNR), signal to noise ratio(SNR), figure of merit(FOM)and average glandular dose(AGD) of the resulting image were calculated. While the image quality was ensured, the optimal mAs and the corresponding AGD under the two target/filter combination were conducted by calculating the FOM. The image features of three tissues of phantom were subjectively evaluated, and the relationship between the calculated AGD and displayed AGD was compared. The mAs that meets the image quality requirements is corresponded with the density exposure gear, and the average values of the mAs for two method were calculated and compared.Results:In two targets/filters combinations, the displayed AGD was less than the calculated AGD, and Mo/Mo was underestimated by 22.5% to 23.7%. The calculated and displayed AGD values were statistically different ( F=4 982.86, 5 555.48, P<0.05). W/Rh was underestimated by 13.1% to 14.2%. The calculated and displayed AGD values were statistically different ( F=18 859.09, 15 973.55, P<0.05). In the Mo/Mo combination, when the mAs was reduced by 20%, the FOM could be increased by 9.6% for the maximum value, and the calculated AGD was decreased by 18.8%. In the W/Rh combination, when the mAs was reduced by 10%, the FOM was increased by 5.1% for the maximum value, and the calculated AGD was decreased by 11.9%. While the image quality was ensured, the mAs was reduced by 30% for evaluating simulated fibers, and by 20% for evaluating simulated specks in the two targets/filter combinations. For evaluating simulated masses, Mo/Mo combination reduced the mAs by 40%, and the W/Rh combination reduced the mAs by 30%. And the image quality scores in above were not significantly different from those in AEC method ( P>0.05). The density exposure gear(-1 to -3) could correspond with the reduced mAs(10%-30%). Conclusions:Under different target/filter combination, the tube mAs could be reduced when the image quality was ensured.