Figure D shows image acquired using the same x-ray tube voltage (32 kV), but different filters. The image on the left in Figure D was obtained using the Rh filter, and required an exposure of 40 mAs (EI = 460). For this image obtained using Rh/32 kV, the incident air kerma was determined to be 6.6 mGy, and the average glandular dose was 0.88 mGy. When a Mo filter is selected (Figure D – right) and the x-ray tube voltage is kept at the same 32 kV value, the mAs increased to 42 mAs (vs 40 mAs for the Rh filter), the entrance air kerma increased to 15 mGy (vs 6.6 mGy for the Rh filter), and the average glandular dose increased to 1.1 mGy (vs 0.88 mGy for the Rh filter). For both images shown in Figure D, the exposure index value was the same at a value of 460. This example quantifies the increase in dose when using x-ray beams that have lower average photon energy, as is achieved by switching from an Rh filter to a Mo filter.
|Figure D. Images of the anthropomorphic phantom acquired in manual mode using the same 32 kV, and a constant detector radiation intensity (EI = 460): Left Rhodium filter (click here for the full size image); Right Molybdenum filter (click here for the full size image).|
The manual mode of operating the digital mammography system can also be used to illustrate the effect of x-ray tube voltage on image quality and patient dose for a fixed x-ray beam filter material. Figure E shows the anthropomorphic phantom imaged in manual mode using the same x-ray beam filter material (Molybdenum). The image on the left was acquired using 28 kV, which required a total exposure of 74 mAs, and resulted in an exposure index of 480. The entrance air kerma at 28 kV was 21 mGy, and the resultant average glandular dose was 1.4 mGy. When the x-ray tube voltage increased from 28 to 32 kV (Figure E, right) the exposure was terminated at a shorter time (42 mAs), but with no marked change in the detector exposure (EI = 460). With the increased penetration achieved by using a higher 32 kV, the image on the right only required an incident air kerma of 15 mGy (vs 21 mGy at 28 kV), and the average glandular dose at this higher voltage was also reduced at 1.1 mGy (vs 1.4 mGy at 28 kV).
|Figure E. Images acquired of the anthropomorphic breast phantom using a constant x-ray beam filter (Mo), but the x-ray tube voltage selected manually by the operator: Left 28 kV (click here for the full size image); Right 32 kV (click here for the full size image).|