mediolateral oblique and craniocaudal views, are acquired. Whereas screening mammography attempts to detect breast cancer in the asymptomatic population, diagnostic mammography procedures are performed to assess and delineate lesions identified by screening mammography. The diagnostic mammographic examination may include additional x-ray projections, magnification views, spot compression views, tomosynthesis, ultrasound, magnetic resonance imaging (MRI), mammoscintigraphy, or breast CT.
and decreases with higher energies. Low x-ray energies provide the best differential attenuation between breast tissues; however, the high absorption at these low energies results in higher radiation doses. Detection of microcalcifications in the breast is also important because they are often early markers of breast cancer. The need to visualize microcalcifications requires x-ray tubes with small focal spots and very high-resolution detectors. Enhancing contrast sensitivity, reducing dose, and providing the spatial resolution necessary to depict microcalcifications impose challenging requirements on mammographic equipment and detectors. Therefore, dedicated x-ray equipment design, specialized x-ray tubes, breast compression devices, antiscatter grids, digital x-ray detectors, and automatic exposure control (AEC) subsystems are essential for mammography (Fig. 8-3).
of the basic concepts regarding image quality, x-ray production, and radiography are presented in Chapters 4, 6, and 7, respectively. The applications of these concepts to mammography, together with many new topics specific to mammography, are presented here.
digital values on the anterior (anode) side of the field will be consistently less. For fixed, non-removable digital flat-panel detectors, the reduced fluence on the anode side of the field can be corrected with “flat-fielding” procedures to adjust the digital number uniformity across the FOV (see Section 8.5). However, the quantum noise (and thus the standard deviation of the digital numbers) will be higher in this area due to reduced x-ray fluence.
above the magnification platform. Orientation of the bar pattern parallel and perpendicular to the cathode-anode direction of the x-ray tube yields measurements of overall effective resolution including the focal spot length and width dimensions, the acquisition geometry, and the detector sampling characteristics. Orientation at a 45° angle yields a resolution measurement up to a factor of 1.4 times higher, resulting
from the increased physical distance between bars of the bar pattern relative to the detector element sampling array and when the focal spot does not limit resolution. Without correction for bar pattern spacing at an angle, the reported resolution can exceed the theoretical maximum resolution of a detector defined by the Nyquist sampling criterion (see Chapter 4). A resolution bar pattern is shown in Figure 8-6B for contact and magnification images using 0.3- and 0.1-mm focal spots, respectively. The resolving capability of the imaging system is limited by the component that
causes the most blurring. In magnification mammography, this is generally the focal spot, whereas in contact mammography, it may be the detector element size. In clinical breast imaging, patient motion can be the limiting factor.
TABLE 8-1 NOMINAL FOCAL SPOT SIZE AND MEASURED TOLERANCE LIMITS OF MAMMOGRAPHY X-RAY TUBES SPECIFIED BY IEC STANDARDS
thin, uniform sheets of pure metal comprised of elements with a K-absorption edge energy (Chapter 3) between 20 and 26 keV, including Mo (20.0 keV), Rh (23.2 keV), and Ag (25.5 keV). At the lowest x-ray energies in the spectrum, the beam attenuation by these filters is very high. The attenuation decreases as the x-ray energy increases up to the K-edge of the element constituting the filter. For x-ray energy just above the K-edge, photoelectric absorption by interaction with the K-shell electron creates a step increase of ˜6× in attenuation, and then decreases with higher x-ray energies (Fig. 8-8A). Selective transmission of x-rays in a narrow band from about 15 keV up to the K-absorption edge energy of the filter is achieved with the added filter. In Figure 8-8B, an unfiltered Mo target spectrum and a superimposed attenuation curve for a Mo filter are shown. Note: the characteristic x-ray energies produced by the Mo target occur at the lowest attenuation by the filter in this energy range.
▪ FIGURE 8-7 The x-ray spectrum of a mammography x-ray tube is composed of bremsstrahlung (with a continuous photon energy fluence) and characteristic (discrete energy) radiation (see Chapter 6 for more details). A Mo anode tube operated at 30 kV creates the continuous spectrum (upper right) as well as characteristic radiation with photon energies of 17.5 and 19.6 keV (middle right). On the lower right, the “unfiltered” composite spectrum transmitted through 1 mm of Be (tube port material) has a large fraction of low-energy x-rays that deposits high dose without contributing to the image, and a substantial fraction of high-energy x-rays that reduces subject contrast of the breast tissues. The ideal spectral energy range (shaded in green) is from ˜15 to ˜25 keV, depending on breast tissue composition and thickness.
0.05 mm, as shown in Figure 8-11B. Consequently, the output fluence rate of a W target is lower than Mo or Rh targets mainly because of the doubled filter thickness (0.05 mm compared to 0.025 mm). Higher fluence rate for a W target is achieved by using an Al filter of 0.5-0.7 mm thickness to sufficiently attenuate the L x-rays of lower energy and permit a larger fraction of higher energy x-rays in the transmitted spectrum. In breast tomosynthesis implemented by one vendor, requirements for rapid image capture are satisfied by the W/Al selection, and the loss of tissue subject contrast by the higher energy spectrum is offset by image reconstruction processing with reduced superimposition of tissues in the tomographic images, as well as digital contrast enhancement (see Section 8.5).
which the system is designed to operate. With the introduction of W targets and lower output rates, each manufacturer has set a lower acceptable output rate limit for a designated kV and filter for this test.
area is defined by the lowest signal region (highest attenuation) over the active digital detector area. The AEC sensor is wired to a charge to voltage amplifier that produces voltage in proportion to the accumulated x-ray exposure. The voltage comparator circuit has a calibrated reference voltage as one input, and the AEC sensor voltage as the other input. During an exposure, the sensor voltage increases from zero volts to a voltage equal to the reference voltage, which triggers the comparator circuit to terminate the exposure. The reference voltage produces a known incident x-ray fluence to the detector and thus determines the radiation exposure to the breast, as well as the quantum mottle in the digital image.
A fully automatic AEC mode that sets the optimal kV and filtration (and target material on some systems) from a short test exposure of approximately 100 ms to determine the penetrability of the breast.
Automatic kV selection with a short test exposure, with user-selected target and filter values.
Automatic time of exposure using manually set target, filter, and kV values.
exposure. In situations such as imaging breast implants, however, the “auto” selection should not be used, except for implant displaced views. An example of a sensor position inadvertently set in the open field (Fig. 8-16B left image) results in an unacceptably noisy image. The patient returns to a different unit for a repeat study performed under “auto” mode (Fig. 8-16B right image) with appropriate image quality.
size of the image receptor (18 × 24 cm or 24 × 30 cm) and is flat and parallel to the breast support table. The compression paddle has a right-angle edge at the chest wall to produce a flat, uniform breast thickness when an adequate force of 111 to 200 newtons (25 to 44 lb) is applied. A smaller “spot” compression paddle (˜7-cm diameter) produces more compression over a specific region where the tissue surrounding the paddle bulges out, allowing more aggressive compression over a limited FOV (Fig. 8-17C). Alternatives to the flat compression paddle include a “flex” paddle that is spring-loaded on the anterior side to tilt and accommodate variations in breast thickness from the chest wall to the nipple, and a curved paddle, providing a more comfortable yet adequate compression of the breast.
TABLE 8-2 TECHNIQUE CHART FOR A DIGITAL MAMMOGRAPHY SYSTEM WITH SELENIUM DETECTOR FOR 50% GLANDULAR 50% ADIPOSE COMPOSITION