拟人化的幻影

One important characteristic of any digital imaging system is to vary the amount of radiation used to generate any image. The gray scale appearance of digital images, unlike those of analog screen-film systems, is not affected by how much radiation was used. 相应的, unlike screen-film systems, digital images can never be "under-exposed" (i.e.(如全白)或“过度曝光”(如.e.，全黑). When radiation intensity increases, quantum mottle will decrease and 反之亦然.

The two images depicted in 图F were obtained using a Molybdenum x-ray beam filter and an x-ray tube voltage of 28 kV. 图F中的左图使用了40ma. 入口空气质量为6.5 mGy, and the resultant average glandular dose was 0.69年mGy. The right image in 图G shows the effect of further reducing of the radiation intensity, 使用的辐射量只有4.7 ma，入口风量为0.80 mGy, and an average glandular dose of 0.077年mGy. 正如预期的, when the filter and x-ray tube voltage are kept constant, there is a direct and simple proportionality between the mAs, 入口风量, 平均腺体剂量. 图G shows region of interest that illustrate the marked increase in quantum mottle as the amount of radiation that is used to generate these images is progressively reduced.

图F. Images acquired of the anthropomorphic phantom in manual mode using identical exposure conditions (i.e., Mo/28 kV) except the total amount of radiation used to generate the image: Left 40 mAs ( 查看完整尺寸的图像 ); Right 4.7 ma /EI 90 ( 查看完整尺寸的图像 ).

 

图G. Regions of interest of the anthropomorphic phantom images acquired in manual mode using identical exposure conditions (i.e., Mo/28 kV) except the total amount of radiation used to generate the image: Left 40 mAs; Right 4.7马斯.

 

ACR认证幻影

Images depicted in 图H were obtained in the manual mode using a Molybdenum x-ray beam filter and an x-ray tube voltage of 25 kV. 图H中的左图使用了110ma. The 入口风量 was 13 mGy, and the resultant average glandular dose was 1.7 mGy. The right image in 图H shows the effect of reducing of the radiation intensity, 使用的辐射量只有26毫安, or a less than a quarter of the value used for the image on the left. The image shown on the right in 图H had an 入口风量 of 2.7 mGy, and an average glandular dose of 0.34 mGy. 图我 shows region of interest that illustrate the marked increase in quantum mottle as the amount of radiation that is used to generate these images is progressively reduced, and illustrate how the visibility of lesions is reduced when less radiation is used to generate any digital image because of the increase in quantum mottle.

图H. Images acquired of the ACR accreditation image quality phantom in manual mode using identical conditions (i.e., Mo/28 kV) except the total amount of radiation used to generate the image: Left 110 mAs ( 查看完整尺寸的图像 ); Right 26 mAs ( 查看完整尺寸的图像 ).

 

图我. Region of interest of the images shown in 图H, and acquired in manual mode using identical conditions (i.e., Mo/28 kV) except the total amount of radiation used to generate the image: Left 110 mAs; Right 26 mAs.

数字电影与银幕电影

图J shows an image of the ACR认证幻影 acquired on a digital imaging system using the techniques normally used in screen-film mammography (i.e., Mo滤波器+ 25kv), but using the auto timed mode of operation to ensure that the detector dose is the one normally used for this system (i.e., EI = 450). The total exposure used was 110 mAs; the 入口风量 is 13 mGy, 腺体的平均剂量是1.7 mGy. This image can be directly compared with Figure B, where the auto filter mode also selected the Molybdenum x-ray beam filtration, but a higher x-ray tube voltage (29 kV vs 25 kV). In the example shown in 图J, use of a lower x-ray tube voltage (i.e., lower photon energy) will result in a higher average glandular dose (at 25 kV the dose is 1.而在29千伏时，剂量是1.5 mGy).

Digital systems generally use higher photon energies (kV) than analog screen-film. Digital data can compensate loss of subject contrast at higher kVs by increasing the image display contrast. 相应的, digital images use higher x-ray beam qualities because these reduce patient doses, and can compensate for the corresponding reduction in subject contrast by increasing the image display contrast. National surveys show that the introduction of digital mammography has resulted in a corresponding downward trend in radiation doses. At the same time, the ability to digitally improve (i.e., enhance) mammograms has also appeared to improve diagnostic imaging performance of this imaging modality.

图J. Image acquired of the ACR认证幻影 using manually selected values of filter (i.e. Mo)和x射线管电压(i.e., 25 kV) that are normally used in screen-film mammography. 生成的图像使用了110毫安, 产生的暴露指数为450, and resulted in an average glandular dose of 1.7毫格( 查看完整尺寸的图像 ).