## 4 Discussion
### Dose Optimization  
As suggested in the literature, optimization is needed when typical patient doses exceed the corresponding established DRLs or doses associated with significant variations that cannot be explained among hospitals or individual patients. In X-ray mammography, AGD depends on several factors, such as the following: (1) parameters that affect the incident air kerma (e.g., exposure factors, beam quality, and focus-to-skin distance) and (2) mammography-related parameters (e.g., breast thickness and breast gradually). The  
Figure 4: Mean glandular dose as a function of compressed breast thickness (CBT): (**a**) M4, CC View; (**b**) M4, MLO view.  
correlation between MGD and CBT demonstrates the importance of using AEC in all mammography procedures. Dose variations provide clear evidence that radiation dose optimization is possible without increasing the quality of diagnostic information.  
As shown in Tables 2 and 3, a high MGD was associated with high exposure and average breast thickness. Almost seven of the eight mammography devices used 28 kV or higher for their examination. The use of high kV increases the beam penetrability and spatial resolution required for mammography. Hospital M6 used the lowest tube voltage (25 kV), which corresponded to the lowest average breast thickness (29 mm), in contrast to the dose used at hospital M2, which corresponded to 52 mm. High MGD per woman was associated with high mAs values (M1 and M8). Implementing a technique chart for M1 and M7 is essential for the dose optimization of these manual devices using fixed exposure factors. In contrast, the lowest doses are shown for devices M5, M6, and M7, which correspond to the lowest deployed mAs. A high MGD is generally associated with high mAs values, irrespective of whether the device detector is an SF, CR, or DR device. The median dose for the craniocandal view images was lower than that for the mediolateral oblique view because the thickness of the CC images was lower than that of the MLO images. Generally, MGD is higher in the MLO view because of the greater inclusion of the pectoral muscle in the MLO view, which is denser and has higher attenuation and hence higher radiation dose.  
As presented in Table 2, only clinics M2 and M8 are using AEC where exposure factors (kV and mAs) are selected based on the CBT. Hospitals M1, M3, M6, and M7 are utilizing fixed exposure factors irrespective of the CBT. The devices are based on CR or SF mammography and are associated with higher doses than DR systems. As a result, the transition from SF to DR is essential for reducing the radiation doses (Table 3).  
An analysis of variance (ANOVA), which involves two hypotheses, was performed: (1) null hypothesis--the means of all levels are equal; (2) alternative hypothesis--the means of one or more levels are different. A one way ANOVA test was performed on the AGD values determined at eight clinics for the CC and MLO views. The population means were deemed significantly different at a significance level of 0.05. The results could be explained by the different type of mammography units involved in the study sample, as well as the technology involved.