Coned-down magnified views of borderline abnormalities, perceived either on screening mammography or on tangential views of palpable densities, add much-needed information. If a lesion is benign, it will usually look more innocuous on magnification (Figure 2.35). If it is a cancer, however, magnification should make it appear more obvious (Figure 2.36). The area in question should not be magnified any more than the size of the small focal spot will permit without excessive blur (Section 3.1.10).
Reliability of the automatic exposure control (AEC) is essential for large-volume screening, particularly for those radiologists who depend on delayed batch-processing. AEC should possess a minimum of three different sensor positions, as well as tissue-averaging
After the technologist reviews the patient's old studies to see where the densest tissue lies, the AEC should be placed under this densest tissue. If the technologist cannot do this, for instance, if the densest tissue lies in the upper outer quadrant or just under the areola in a large breast, then the density setting should be raised.
Fig. 2.36. Slight architectural distortion is visible on this screening right CC-view mammogram (left, arrow). When the C-arm is angled 10 degrees for a coned-down magnified-view mammogram (right), the border of this tissue is clearly irregular. Biopsy proved that this was a carcinoma.
Some newer AECs can average the density of the tissue over multiple locations in the breast, which makes positioning the sensor less critical.
On many units, the AEC-determined exposure time is derived from both the thickness and the density of the compressed breast. For large-volume screening to succeed, the phototimer must be exceedingly reliable. At least once a year, a physicist should check the reliability of the AEC with a phantom simulating the density of the breast across a range of breast thicknesses (2, 4, 6 and 8 cm) (ACR, 1999; MQSA, 1992).
When images are underexposed, the technologist should increase the density setting for repeat views. If, however, either the exposure for the underexposed film has reached the maximum exposure time or patient motion has occurred, the technologist should increase the operating potential but not the density setting to decrease exposure time.
Even though the grid is used for virtually every contact mam-mogram, firm compression is still necessary. The key to enlisting the patient's cooperation during compression is the technologist's ability to allay the patient's fears and explain why compression is so essential. The technologist should tell the patient that some discomfort may be experienced, but that the patient controls the degree of compression. The technologist should compress no more than the patient will permit. If the technologist takes the time to explain carefully and compassionately, she usually can win the patient's confidence. When a patient understands the reason for firm compression and realizes that the control is theirs, not the technologist's, she will almost always be willing to cooperate. But she needs to know why compression is essential. The patient needs to know that many cancers have been missed only because the breast was inadequately compressed.
The technologist must negotiate an agreement with every patient about how much compression the patient thinks she can tolerate. Some patients have exceedingly tender tissue or low pain thresholds. It is better to obtain yearly mammograms on a patient who tolerates only minimal compression than procure one mammo-gram with firm compression on a patient who objects strenuously and never returns for another mammogram.
The technologist should begin compression with a foot-controlled motorized device because this frees both hands to rotate the torso and position the patient's breast. For final compression, a hand-wheel control for example can be used by the technologist to gauge the breast's resistance, to judge the degree of the patient's discomfort, and to slow down the speed with which the paddle descends so that the patient is not frightened. The control should be sufficiently sensitive for the technologist to "feel" the degree of resistance to compression. Without such a hand-wheel control, the technologist might have difficulty in accurately determining how much compression the patient can tolerate. If a patient sees that it is the technologist and not the machine that regulates final compression, she will be less uneasy about the procedure.
Automatic decompression after exposure or the technologist's ability to press a button on the control panel and release compression immediately after exposure, or in an emergency, are also vital to the patient's comfort and safety. A release switch should also be included on the C-arm.
Because the grid improves contrast so much, some people believe that firm compression is unnecessary. This belief is incorrect. Even with the grid, firm compression offers three additional advantages:
Spot compression (Figure 2.34) spreads out the glandular tissue better for assessing questionable areas. The thinner the compressed breast and the more coned-down the area, the better the contrast. Many manufacturers supply a round, spot-compression paddle, 8 cm in diameter. A 9 cm wide, rectangular compression device (Figure 2.34) 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.
Before the technologist reviews the patient's prior images, she should keep all the foregoing factors in mind. When checking the prior images, the technologist should observe the density of the glandular tissue: the denser the tissue, the higher the operating potential should be for an additional mammogram. She also needs to determine the location of the densest tissue, so that the correct position for the AEC detector is selected. The old images should also be searched for technical imperfections. If, for example, there is motion on the images, the technologist will need more time for encouraging the patient not to move. If the patient cannot refrain from moving, the technologist needs to use a higher operating potential to reduce the exposure time. If the glandular tissue is exceedingly posterior, maximum cooperation from the patient will be needed for optimal positioning.
The final decision about the correct operating potential depends on the technologist's final evaluation of the patient just before the mammogram is initiated. This evaluation includes:
Even if a patient is cooperative and the technologist observes no motion artifacts on the radiograph, the film may still be underexposed because the tube's limitations automatically terminated the exposure. In such an instance, the technologist cannot increase the density setting because the tube's limitations will prevent a longer exposure. The only recourse, then, is to increase the operating potential.
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