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Fig. 2.31. This 37 y old woman's calcifications are not as identifiable on her left CC nongrid mammographic view (A) as they are on the same projection with the grid (B).

Fig. 2.32. With firm compression, the small cancer (arrow) is obvious on this CC-view mammogram (A), because compression displaces the islands of glandular tissue so that the border of the carcinoma stands out. With less firm compression, the mammogram (B) on the same patient produces less displacement of the glandular tissue, which makes the lesion more difficult to perceive.

Fig. 2.32. With firm compression, the small cancer (arrow) is obvious on this CC-view mammogram (A), because compression displaces the islands of glandular tissue so that the border of the carcinoma stands out. With less firm compression, the mammogram (B) on the same patient produces less displacement of the glandular tissue, which makes the lesion more difficult to perceive.

Fig. 2.33. If a posterior cancer were present in this patient's small breast, it might not be imaged because her arm must be raised above the edge of the large compression device. This stretches the skin and makes it difficult to pull the posterior aspect of the breast into view or onto the image receptor.

Fig. 2.33. If a posterior cancer were present in this patient's small breast, it might not be imaged because her arm must be raised above the edge of the large compression device. This stretches the skin and makes it difficult to pull the posterior aspect of the breast into view or onto the image receptor.

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).

2.5.3 Reliability of the Automatic Exposure Control

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

Fig. 2.34. (A) If the questionable area is small and if its location can be ascertained precisely, the round 5 cm spot-compression device can be used to separate the glandular tissue. It provides the best compression and with coning to this small area, the most contrast with magnification. (B) The rectangular, 9 cm wide spot-compression device permits imaging of a larger area of breast tissue, but the price is slightly poorer compression and contrast.

Fig. 2.34. (A) If the questionable area is small and if its location can be ascertained precisely, the round 5 cm spot-compression device can be used to separate the glandular tissue. It provides the best compression and with coning to this small area, the most contrast with magnification. (B) The rectangular, 9 cm wide spot-compression device permits imaging of a larger area of breast tissue, but the price is slightly poorer compression and contrast.

capability. Most units have as many as 11 separate density control settings. The unit should offer the technologist the option of either setting the operating potential, target material and filter manually before positioning, or permitting the x-ray machine to choose the tube potential, target material and filter based on breast thickness or measured attenuation. If the technologist does preset the operating potential, target material and filter, the unit should not be able to override the selection and change the technologist's choice. The design of the AEC must be such that the technologist can choose the best position for placing the sensor. The sensor should not be larger than a small breast. If the sensor extends beyond the breast, the image may be underexposed.

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. 2.35. Four tiny calcifications are visible on one contact grid study (A). A 2x magnified coned-down radiograph (B) shows benign rim calcium indicating that biopsy is unnecessary. One year later, the presence of further calcifications verified that these were benign (C).

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.

2.5.4 Compression

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.

Firm compression is obligatory (Logan and Norlund, 1979) for the following reasons:

• It minimizes geometric blurring. The larger the focal spot, the more essential is straight, firm breast compression that reduces object-film distance.

• It reduces scatter by making the tissue thinner, and thus it enhances subject contrast (Barnes and Brezovich, 1977). Increased subject contrast aids the discovery of calcifications; it makes the outlines of mass densities more identifiable.

• It diminishes motion blurring. If compression is inadequate, significant motion blurring may obscure the image. Most patients are willing to tolerate 2 or 3 s of firm compression.

• It reduces x-ray exposure. When breast compression is minimal, a proportionately higher breast entrance exposure is necessary. Firm compression thus decreases both the breast entrance exposure and the mean glandular dose.

• It reduces the dynamic range requirement of the image receptor (e.g., film), which means that more information can be recorded on the image with greater contrast.

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:

• It provides more uniform film optical density. Nonuniform thickness of glandular tissue produces a wide range of film optical density. Firm compression flattens the tissue, reducing the variations in the density of the image. The grid does not flatten tissue. Greater compression allows the mammog-rapher to use a lower operating potential, which enhances contrast even further and facilitates the discovery of subtle calcifications, low-density masses, asymmetries, and architectural distortion.

• It accentuates the difference in optical density between normal and malignant tissue. Cancers are usually denser than normal glandular tissue. Compression accentuates this difference in density because it flattens the more-elastic glandular tissue, but the radius of a cancer, which ordinarily is less distensible, usually remains unchanged.

• It separates tissue elements. By pushing the islands of overlapping glandular tissue apart, firm compression permits better imaging of the margins of suspicious lesions (Figure 2.32).

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.

2.5.5 Technical Decisions

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:

• How much the breast can be compressed (the operating potential can be lower if the breast is compressed more thinly).

• How dense is the glandular tissue on previous mam-mograms. Dense tissue necessitates a higher operating potential.

• How long is the exposure that the patient can tolerate before motion becomes a problem. A higher operating potential may be necessary to shorten the exposure time.

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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