Research of using image noise characteristic to evaluation image quality in digital mammography

Objective To find the proper way of assessing the image noise characteristic through the analysis of noise characteristic in the digital mammographic images.Methods The CDMAM phantom was firstly exposed with different filter materials and tube voltages in the range of 23 to 35 kVp at a fixed tube current 90 mAs,then exposed with different filter materials and tube current in the range of 20 to 200 mAs at a fixed tube voltage 29 kVp.The parameters such as background standard deviation,joint standard deviation,subtracted standard deviation and relative standard deviation were calculated and measured from both 60 pixels of signal disc and background beside of the disk for all the images.The IQFinvs were calculated for the achieved images.Non-parametric rank sum tests were used to compare the difference of the background standard deviation,joint standard deviation and subtracted standard deviation,t test was used to compare the IQFinvs between Mo and Rh filers in each group.Results With the same exposure parameters in the test above,the maximum value was the background standard deviation,while the subtracted standard deviation was the minimum,and the value of joint standard deviation was in the middle. With other parameters unchanged,the values of three kind of standard deviation increased with the increasing of tube voltage or tube current,while the differences between them getting more significant.Under the condition of 90 mAs and Mo filter material,background standard deviation,joint standard deviation,subtracted standard deviation were 10.91,10.34 and 9.87,the results were 11.24,10.57 and 9.96 while Rh filter material was used,significant difference was found ( x2 =26.0,P ＜ 0.01 ). Under the condition of 29 kVp and Mo filter material,background standard deviation,joint standard deviation,subtracted standard deviation were 12.30,11.61 and 11.05,the results were 12.29,11.58 and 10.87 while Rh filter material was used,significant difference was found( x2 =20.0,P ＜ 0.01 ).The IQFinvs were 149.14 ± 23.87 and 139.16 ± 23.14 under the condition of 90 mAs and Mo filter material ( t =9.129,P ＜ 0.01 ).The IQFinvs were 149.75 ±34.85 and 145.2 ±34.64 while the parameters were 29 kVp and Rh filter material(t =1.304,P ＞0.05 ).The value of relative standard deviation decreased with the increasing of tube voltage or tube current,and the image quality got better with the decreasing of relative standard deviation.Conclusion Relative standard deviation is suitable not only for the expression of the digital mammographic images,but also for the evaluation of the image quality. Key words: Mammography ;  Noise;  Comparative study