Fig. 3.10. (Top) Scatter-to-primary ratio (S/P) as a function of Lucite® (duPont, Wilmington, Delaware) phantom thickness for a 14 cm diameter radiation field at 32 kVp. (Bottom) S/P as a function of field size for 3 and 6 cm thick Lucite® phantoms at 32 kVp (Barnes, 1994).
The compressed breast thickness should be displayed on the unit and the display should be accurate to within 0.5 cm (ACR, 1993). This degree of accuracy will be impossible to achieve if the compression device does not remain rigid and flat when compression is applied. The display or scale should also be usable for both grid and nongrid work and must therefore correct for any differences in image-receptor support thickness in these instances. The accurate display of compressed breast thickness will allow documentation that will help insure consistency from exam-to-exam on the same patient and will be helpful in estimating patient dose.
The compression device must be rigid enough not to deform excessively (>1 cm), when maximum compression is applied and thick enough not to crack under firm compression (ACR, 1993). At the same time, it must not be so thick as to attenuate and harden the x-ray beam excessively and it must be transparent to light to facilitate proper positioning. The material used for the compression device should be such that, if the device fails (e.g., cracks) minimal injury is caused to the patient. In addition, the edges of the compression device should be smooth for patient comfort. The corners of the posterior edge of the compression device should be slightly rounded to prevent a sharp edge that might be uncomfortable for the patient. But, only the most posterior 2 mm can be rounded. More rounding will result in underpenetration of the posterior aspect of the breast. The support at the sides of the compression device should be slender and strong, occupying as little space as possible, to make it easier for the technologist to pull the glandular tissue onto the film. The support should neither obscure imaged glandular tissue nor push the patient's arm away. Compression devices should be available in various sizes so that the overall size of the compression device will always correspond to the size of the breast and that appropriate compression can be applied in special circumstances (e.g., spot compression).
The posterior (chest-wall) edge of the compression device should be bent upward at a sharp 85 degree angle along the posterior border (AHCPR, 1994; Logan and Norlund, 1979; NCRP, 1986) and should be at least 3 to 4 cm high (Figure 2.7) in order to push back the axillary fat fold, which overlies the posterior aspect of the breast in the CC view, and to prevent excess tissue high on the chest wall from overlapping the film (ACR, 1993; Feig, 1987; NCRP, 1986). This added height also helps prevent the plastic from fracturing during firm compression. The sharp posterior angle allows the compression device to grip the posterior aspect of the breast tissue during compression rather than allowing it to slide out from underneath, as will occur with a compression device that has a more gently angled curvature. Most importantly, the edge of the compression device that is adjacent to the chest wall should be straight (Feig, 1987; Yaffe, 1991). A contoured chest-wall edge on the compression device will interfere with proper positioning and compression, particularly on the MLO view.
This compression device design enables improved visualization of the posterior aspect of the breast provided the device is properly aligned. When properly positioned, the vertical (chest-wall) edge of the compression device will lie in the plane defined by the chest-wall edge of the image receptor and the ray from the focal spot perpendicular to that edge. The chest-wall edge of the compression device should remain in this plane as the compression device is moved vertically through its full range of motion with respect to the image-receptor support device.
The compression device should be aligned with the posterior edge of the image receptor within one percent of the SID (ACR, 1999). If the compression device does not project far enough to be properly aligned with the chest-wall edge of the film, the edge of the compression device will project onto the film image and the thicker posterior breast tissue beyond the compression device will be undercompressed. On the other hand, if the compression device projects beyond the edge of the image receptor, it will push breast tissue away, failing to properly image the whole breast. Many mammographic units arrive at the mammography site with a misaligned compression device. For this reason, the compression device should provide a means of adjustment so that any misalignment can be readily corrected. During mammographic examinations, pressure from the patient's ribs will be exerted on the compression device and may result in misalignment.
Dedicated mammography units should be equipped with powered compression systems (electric, pneumatic or hydraulic) controlled by foot pedals to allow the technologist to use both hands to position the breast while applying compression (ACR, 1993). The foot pedals should be conveniently accessible from either side of the patient and should allow both application and release of compression force. Such a motorized compression system should be immediately responsive, should not delay or reverse, and should not slip after final compression is applied. The power drive on the compression system should not be excessively noisy. There is considerable debate over the degree of compression that such systems should provide (AAPM, 1990; Sullivan et al., 1991), but the current consensus is that the maximum force should be 200 N (newtons) (45 pounds) (ACR, 1993; DHHS, 1987). In a study of 560 patients who determined their own compression force (Sullivan et al., 1991), the compression force applied during mammography ranged from 49 to 186 N (mean = 127 N, mode = 108 N) which suggests that the 200 N maximum is quite adequate.
There should be a readout of the applied compression force visible to the technologist during positioning, although such readouts may be prone to error (Clark et al., 1990). It should also be remembered that the force applied by the compression device is not a good predictor of the adequacy of compression nor of the patient's level of discomfort (Eklund, 1991).
As noted above, the technologist should begin compression with the foot controlled motorized device keeping her hands free to rotate the patient's torso and position the breast. For final compression, however, fine control is essential (Feig, 1987). This is typically provided by a hand control that will allow the technologist to gauge the breast's resistance and judge the degree of patient discomfort so the compression will not be too firm. This approach has significant advantages in terms of patient acceptance. It will also allow the technologist to slow down the speed with which the compression device descends so that the patient is not frightened. The hand control should be sufficiently sensitive for the technologist to "feel" the degree of resistance to compression. Without a hand control, the technologist may have difficulty in accurately determining how much compression the patient can tolerate.
As noted above, the technologist's ability to release the compression device instantly after exposure or in an emergency is vital to the patient's comfort and safety. The release switch should be on the C-arm. If the patient sees that it is the technologist and not the machine that regulates compression, they will be less uneasy about the procedure. Alternatively, the release switch can be on the control console which has the advantage of allowing somewhat quicker release of compression. Some systems have an automatic compression release feature. This may be useful under some conditions to minimize the time during which the breast is under compression. If such an automatic release feature is provided, there should be a means of overriding it when appropriate, such as in localization procedures. The compression device should be designed to release the compression automatically in the event of a power interruption.
A small compression device is also necessary to spot-compress questionable areas and spread out the glandular tissue so it can be better visualized. Significantly better compression can be applied locally to a restricted area than can be applied to the breast as a whole (Figure 2.35b) (Barnes, 1994). The thinner the compressed breast and the more coned-down the area imaged, the better the contrast. For this reason, a dual-focus compression device has been developed. This compression device incorporates a raised section on the patient support that compresses the breast from below, in concert, with an identically sized spot-compression paddle positioned conventionally above the breast. A 9 cm wide rectangular compression device is useful in spot compression of slightly larger, nonspecific problematic areas. It is also helpful in compressing areas of the breast and axilla that are difficult to position. Table 3.12 presents desirable characteristics of compression devices.
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