Fig. 4.2. Typical x-ray emission spectra used in screen-film mammography. For the molybdenum (Mo) target, a 0.03 mm molybdenum filter is used and a 26 kVp setting is shown (solid line). For greater penetration, a rhodium (Rh) filter can be used as shown here (dotted line). For very dense breasts, a Rh target and Rh filter x-ray source operated at 30 to 32 kVp may be useful. The dominant characteristic x-ray peaks (Ka) occur at 17.4 keV for a molybdenum target and at 20.2 keV for a rhodium target (Haus, 1999b).
characteristic emissions at 20.2 and 23.2 keV. The Rh/Rh combination is most effective with very dense, difficult-to-penetrate breasts, providing some dose reduction, while preserving as much subject contrast as possible in these difficult to image breasts. It is also possible to provide a suitable spectrum for imaging the dense breast with a tungsten target tube and various metallic filters, such as aluminum or rhodium. While this does not provide the quasi-monoenergetic x rays available with molybdenum and rhodium targets, careful choice of operating potential and filter material and thickness can yield an excellent result in terms of contrast and dose.
For screen-film mammography, operating potentials between 22 and 32 kVp are used depending on breast tissue thickness and composition, the target and filter materials, and exposure time constraints. Hendrick and Berns (1999) have shown that optimum technique factors for screen-film mammography in terms of contrast-detail perceptibility are Mo/Mo with low operating potentials (22 to 25 kVp) for thin breasts (<5 cm), Mo/Rh with intermediate operating potentials (26 to 30 kVp) for thicker breasts (5 to 7 cm) and Rh/Rh or some equivalent harder x-ray beam at 28 to 32 kVp for very thick breasts (>7 cm). At each breast thickness, a sufficiently hard beam should be used to obtain adequate film optical density with exposure times <2 s (Figure 4.3).
For digital systems, it has been found that because of the ability to adjust display contrast, it may be advantageous to employ slightly higher energy x-ray beams than are used with screen film. Except for small breasts, tungsten or rhodium targets with various filters, selected according to breast composition and thickness, appear to provide a better compromise between SNR and dose than obtained using molybdenum target tubes (Fahrig and Yaffe, 1994; Venkatakrishnan et al., 1999).
220.127.116.11.1 Scattered radiation. In soft tissues, even at the low energies used in mammography, scattered radiation is an important mechanism that depletes the primary beam due to Compton interactions of x rays with breast tissue. Scattered x rays that escape the breast and are recorded by the image receptor reduce image contrast. The amount of scattered radiation recorded compared to the useful, directly-transmitted x-ray intensity is characterized by the S/P. It is not unusual for S/P to be greater than one (Barnes and Brezovich, 1978).
For screen-film systems, scattered x rays recorded by the image receptor have the following effects: (1) to reduce image contrast, (2) to "use up" some of the available recording range or latitude of the film, and (3) to add noise to the image, thereby reducing its SNR, a measure of the information content of the mammogram.
In digital mammography the same factors affecting subject contrast apply. The effect of scattered radiation on the final radiographic contrast, however, is somewhat different. Because of the fact that x rays may scatter multiple times within the breast, their spatial distribution is diffuse [i.e., mainly affecting the low spatial frequency part of the modulation transfer function (MTF)]. For this reason, in digital systems, much of the contrast can be recovered by viewer adjustment of the computer image display. Similarly, the i*
Fig. 4.3. Images of an anthropomorphic breast phantom acquired at varying operating potentials and approximately the ^ same optical density, illustrating a slight dependence of contrast on operating potential (Haus, 1999b). 4
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