While a variety of x-ray units have been used in mammography since its inception (Bassett et al., 1992; Gold, 1992; Vyborny and Schmidt, 1989), it is now widely recognized that quality mammog-raphy requires a dedicated mammographic x-ray unit (ACR, 1993; DHHS, 1987; Haus, 1990; Yaffe, 1991). In order to meet the stringent imaging needs of mammography such a unit must be equipped with a variety of essential features discussed in this Section. These include a small focal spot coupled with a relatively long source-to-image-receptor distance (SID) to minimize blur; a low energy x-ray beam and a specialized mammographic grid to provide high subject contrast; and specialized equipment for firm, uniform compression. Without these features, it is almost impossible to visualize small nonpalpable masses and very small microcalcifications, often the only indications of early carcinoma. Use of non-dedicated radiographic equipment can result in missing many cancers and can lead to unwarranted biopsies, and is prohibited under MQSA (1992).
A number of authors have described the need for and features of dedicated, specially designed, mammographic equipment (AAPM, 1990; NCRP, 1986). A review of these descriptions indicates that there are a number of features that should be incorporated into a dedicated mammographic unit. Probably, the most comprehensive description of the features of a dedicated mammographic unit is that prepared by ACR (1993) and is frequently cited below. Another summary of these issues appears in Seminars in Breast Disease (Haus, 1999a).
The minimum set of features for an acceptable dedicated unit has been set by MQSA (1992) regulations. These regulatory requirements are outlined in Table 3.1.
In establishing these requirements FDA drew heavily on the ACR document mentioned above (ACR, 1993). The requirements of the final regulations apply to all mammography units under the purview of MQSA (1992), whether they are used for screening or diagnostic ("problem-solving") mammography.
TABLE 3.1—MQSA equipment requirements.
Adapted from MQSA (1992) Section 900.12(b) Equipment [also published in CFR Title 21, Chapter 1, Part 900-Mammography, Subpart B-Section 900.12 (b) Equipment]
aIn this Report, the name used for this quantity is "operating potential," expressed as "kilovolt peak (kVp)" (see Glossary).
There are numerous features recommended by experts that go beyond the minimum set of features required by MQSA (1992). All of the features discussed below are highly desirable for dedicated mammographic units, particularly if the unit is used for both screening and diagnostic or problem-solving mammography. This is especially true if the unit is the only dedicated mammography unit available in a facility. All of these features are summarized in the tables at the end of each subsection.
Before purchasing a dedicated mammographic unit, there are several steps that should be undertaken. The unit's specifications should be reviewed in comparison with the critical features and specifications described below. Current owners of the unit(s) (make and model) under consideration should also be questioned with respect to the adequacy of its performance in each of these critical areas. The radiologist should also review both grid and magnification images of dense or difficult to compress breasts that have been imaged on the unit(s) under consideration. For this purpose, images should be obtained from competent radiologic colleagues rather than through the unit's manufacturer.
3.1.2 Mechanical Assembly and General Considerations
The mammographic unit should rigidly support the x-ray tube housing and image-receptor support device at opposite ends of a C-arm or similar assembly. The C-arm should be designed to allow continuous rotation to permit views to be obtained in various projections with the patient either erect or seated. The system should allow the technologist to rotate the C-arm 180 degrees relative to the vertical axis in one direction and at least 120 degrees and preferably 50 degrees in the other direction (ACR, 1993). This range of angulation allows for both routine and specialized projections, including the reverse CC view in which the breast must be compressed from below. It also insures that the technologist will always be able to compress the breast perpendicular to the long axis of the pectoralis major muscle in the MLO view and will therefore be able to include the posterior portion of the breast on the image despite differences in patient body build.
While it should be possible to position the C-arm of the mammo-graphic unit to achieve any degree of obliquity (continuously variable angulation), detents at the common positioning angles, such as 0, 30, 45, 60 and 90 degrees, on either side of vertical should also be provided to help the technologist achieve reproducible positioning. The degree of angulation of the C-arm should be indicated on the unit and should be easy to read from any position on either side of the image-receptor support.
The C-arm should be designed so that the technologist can move it high enough to accommodate a tall patient and low enough for a patient in a wheelchair. There should be room enough under the image-receptor support and counterweight for a patient's legs if they are in a wheelchair or need to be seated for the examination. In general, this will require a range of vertical motion such that the center of the image-receptor support can be positioned from 66 to 140 cm above the floor for both CC and lateral views (ACR, 1993). If only standing patients need to be accommodated, the range of vertical motion can be from 97 to 140 cm. In addition, if a patient is in a wheelchair, if they must remain seated, or if they can stand but cannot move their feet easily for different views, it is more convenient for the technologist to move the C-arm side-to-side and in-and-out in a longitudinal or transverse motion from the main body of the unit. The unit should allow this flexibility. The unit should permit the technologist to perform more than one function at a time. For example, the technologist should be able to raise the C-arm vertically at the same time that they are lifting the compression paddle.
Controls for adjusting the position and height of the C-arm and for rotating it should be readily accessible to the technologist who must use these frequently throughout the mammographic examination. The unit should be equipped with mechanical, motorized or electromagnetic locks to fix the C-arm in any required position or orientation (ACR, 1993). These locks should be strong enough to prevent motion of the C-arm when the patient leans on the unit. The locks should be released by hand or foot controls and should not release in the event of a power failure.
Motorized controls for compression should be accessible on both sides of the C-arm, as well as being foot controlled, to allow for easy positioning on standard and specialized views. The control for releasing compression should also be on the C-arm, to permit quick release if the patient is feeling faint or suddenly feels that she can no longer tolerate the compression. In the event of a power failure, the compression should be released automatically. The switch for the light field should also be readily accessible, located on both sides of the C-arm or else positioned centrally where it is easy to reach.
A bar support should be available on each side of the C-arm for the patient to grip. Such a support is especially useful after the technologist has raised the patient's arm for the oblique views. The bar should extend to the height of the tube head and below the image-receptor support, so that the patient can reach it easily during positioning for any view. This bar is a necessity for assisting the patient in supporting themselves, thereby, minimizing motion. When the bar is used for CC positioning, the patient should be able to reach it without stretching. The control buttons should not be on the bar support where the patient might accidentally grasp them.
The smaller the tube housing the better. If the tube housing projects toward the patient's head, the technologist will have difficulty including the patient's chest-wall tissue on the image in the CC view. A large tube head will also interfere with positioning for magnification, especially on the CC view. Moreover, on a superola-terial-to-inferomedical oblique view, the patient's shoulder will bump into the tube head. In addition, the tube housing should be equipped with a plastic face shield. This device should be designed to prevent the patient's face or hair from projecting between the x-ray tube and the breast and should not overlap the imaging field.
The image-receptor support device should be designed to hold the cassette firmly in place with the front edge of the cassette at the chest wall and a minimum of space between the cassette edge and the chest wall (ACR, 1993). There should be <2 mm movement side-to-side and the device should be tight enough to prevent movement of the cassette during an exposure. In addition, the image-receptor support device must be designed to limit the x-ray transmission through the support to no >0.876 pGy air kerma (0.1 mR) for any exposure (CDRH, 2002b).
A radiation shield should be provided to minimize operator exposure. The operator exposure should not exceed 5 mSv y-1 (ACR, 1993). Given reasonable assumptions concerning workload and technique [6.58 mGy air kerma (750 mR)] per exposure, four exposures per patient, 40 patients per day, 5 d week1, scattered radiation at the entrance of the shield equal to 0.001 times the exposure at the breast entrance surface, a shield with an attenuation equivalent to 0.08 mm of lead at 35 kVp is appropriate to meet this standard (ACR, 1993). The shield should extend from <15 cm above the floor to a height of 1.85 m. The width of the shield should be sufficient (at least 0.6 m) to provide reasonable assurance that the technologist will not be exposed during the conduct of an examination. If the shield is movable, there should be interlocks to prevent exposure when the shield is not in place. The exposure controls should be designed so that the operator cannot make an exposure when outside the shielded area.
The unit should also provide a means for recording on the patient's images, identifying information concerning the patient, the facility where the film was taken, and the technologist who took the film. Information concerning patient position (view, angulation, etc.) and the appropriate technique variables (target-filter combination, operating potential, milliampere seconds, compressed breast thickness, compression force, etc.) should also be included. See Table 3.2 for a summary of desirable characteristics of the mechanical assembly.
TABLE 3.2—Summary of the desirable characteristics of the mechanical assembly (ACR, 1993).
• means of recording patient and technique information directly on film
A dedicated screen-film mammographic unit should have an x-ray tube with a molybdenum target and a thin beryllium window (1.5 mm thickness or less) together with an added molybdenum filter [sufficiently thick to meet the minimum half-value layer (HVL) requirements of CDRH (2002b) (ACR, 1993; AHCPR, 1994)]. This combination of target, window and filter materials has been shown to provide excellent contrast for the detection tasks present in mammography when the appropriate operating potential (<28 kVp) is employed (Beaman and Lillicrap, 1982; Feig, 1987; Jennings et al., 1981). The x-ray beam from such a system has the low-energy characteristics required to achieve high subject contrast for breasts of average density and thickness. This is due to the 17.5 and 19.7 keV characteristic x rays from the molybdenum target and the strong suppression of the spectrum at energies >20 keV because of the k-shell absorption edge of the molybdenum filter (Figure 3.1a). Inordinate amounts of filtration in the x-ray beam from a glass window or excess filtration or otherwise inappropriate added filtration would have significant negative consequences (AAPM, 1990; Yaffe, 1991). Not only would beam quality be increased resulting in a loss of subject contrast, but also tube output would be reduced, resulting in increased exposure time. Longer exposure times could lead to problems with patient motion and higher patient doses due to film reciprocity law failure. Adjustments could be made to reduce exposure time (e.g., increasing operating potential, using a higher milliampere and consequently, a larger focal spot, using a faster image receptor, etc.) but, each would have its own negative consequences for image quality (reduced contrast, increased blur, increased noise, respectively).
Alternative target and filter combinations may be employed, if they provide equivalent contrast-detail perceptibility at equal or reduced patient dose. For example, tubes with molybdenum targets and rhodium filters (Figure 3.1b), as well as those with rhodium targets and rhodium filters (Figure 3.1c), and tubes with tungsten targets and rhodium added filration (Figure 3.1d) have been used successfully in imaging patients (Beaman and Lillicrap, 1982). Such combinations are most effective in patients with larger or denser breasts. In such patients, these units can produce both better image quality and lower patient dose. In systems where the filter can be varied (for example, a molybdenum target with both molybdenum and rhodium filters), the type of filter in use should be displayed on the unit.
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