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to the CC view for the routine, two-view mammographic study. In the MLO view, the technologist rotates the C-arm so that the image receptor is parallel to the fibers of the pectoral muscle. She places the imaging system midway between the inferior and the lateral aspect of the breast (the lower outer quadrant), and directs the radiographic beam through the breast from superomedial to inferolateral.

Screen-film mammography has a narrow recording latitude. Moreover, screen-film mammography requires a soft, low-energy x-ray beam to maximize contrast. For these reasons, the breast needs to be compressed to a uniform thickness; otherwise, the thicker areas will be underpenetrated and the thinner areas overpenetrated. Because uniform thickness is so essential, it is necessary to use a flat compression device parallel to the film (Figure 2.18).

Fig. 2.18. A flat compression device that is parallel to the image receptor and has an angle of 90 degrees at the chest-wall grips the posterior aspect of the breast and pulls the tissue forward onto the image receptor. The flat compression device also compresses the entire breast more evenly than a compression device with a curved chest-wall angle. In the mediolateral or MLO view, a 90 degree angled compression device is more successful than a curved device at gripping and pulling the tissue away from the convexity of the rib cage, and thereby images more of the posterior tissue.

Fig. 2.18. A flat compression device that is parallel to the image receptor and has an angle of 90 degrees at the chest-wall grips the posterior aspect of the breast and pulls the tissue forward onto the image receptor. The flat compression device also compresses the entire breast more evenly than a compression device with a curved chest-wall angle. In the mediolateral or MLO view, a 90 degree angled compression device is more successful than a curved device at gripping and pulling the tissue away from the convexity of the rib cage, and thereby images more of the posterior tissue.

More posterior breast tissue is visible on the MLO view than on the lateral view. On the MLO view, compression is applied parallel to the lung axis of the pectoral muscle, 30 to 60 degrees off the vertical axis. This allows better compression of the muscle than achieved on the lateral views. Thus, on the MLO view, the muscle is less likely to pull the breast toward the chest wall and off the image field.

Since the obliquity of the pectoral muscle differs from one person to the next, the angle of the oblique view may vary from 30 degrees for a patient with a short torso to 60 degrees for a tall patient with a long torso; the angle is usually about 45 degrees. The technologist should determine the correct angle by rotating the C-arm until the image receptor is parallel to an imaginary line extending from the xiphoid process to the thickest portion of the pectoral muscle anterior to the axilla. The technologist should always compress both breasts at the same angle, unless the patient has a significant anomaly such as scoliosis. The technologist should record the angle of obliquity on the image label.

Initially, the patient should stand with the ipsilateral side adjacent to the image receptor, which is usually rotated 45 degrees so that the image receptor is parallel to the fibers of the patient's pectoral muscle. The patient's anterior chest wall is, then, parallel to the image receptor. The technologist should raise the patient's arm no more than 90 degrees until it is just barely above the image receptor with the patient's elbow slightly bent and just posterior to the image receptor. The technologist should ask the patient to place her hand on the support bar of the C-arm. If the technologist pulls the patient's arm back or raises it too high, the breast skin will be too tight. When the skin is taut, it resists the technologist's attempts to pull the breast away from the chest and onto the image receptor, which might result in missing a lesion.

The technologist should face the patient, reach an arm around each side of the patient, place one hand behind the patient's back to grip her shoulder on the side being positioned, and the other hand posteriorly and beneath the breast tissue (Figure 2.19). The lateral breast skin is more mobile than the medial skin, which facilitates pulling the breast tissue medially. The technologist should lift the patient's shoulder and her breast and pull the mammary tissue upward, outward and towards the sternum to tighten the posterior skin as much as possible (Figure 2.20). Next, the technologist should place the posterior axillary line of the skin on the outer edge of the image receptor and instruct the patient to lean against the tray to keep this posterior tissue from sliding away. The

Fig. 2.19. The technologist lifts up the breast and pulls it tightly upward and outward, placing the posterior axillary line on the image receptor while with the other hand, she simultaneously lifts the patient's shoulder and rests the patient's arm along the superior edge of the image receptor.
Fig. 2.20. The patient stands adjacent to the image receptor with her anterior chest wall parallel to the image receptor. The technologist places one hand posterolaterally to the breast tissue. Next, the technologist places her other hand behind the patient's shoulder on the side to be positioned.

technologist should then rotate the patient until her sternum touches the compression device. The technologist should then walk behind the patient, slightly lift her shoulder, and recheck the posterior tissue to make sure that it has not slipped from visualization.

The technologist should return to face the patient and ask the patient to relax her shoulders and let them droop, so the technologist can pull the shoulders closer together. Many patients instinctively draw back their shoulders thereby tightening the breast skin (Figure 2.21). The technologist must recognize and counteract this posture, or else the medial tissue will be too tight to pull onto the image receptor. Relaxing the patient's shoulders and bringing them closer together will loosen the skin covering the chest wall and enable the technologist to pull the looser medial tissue into view.

After the technologist has rotated the patient's upper torso 90 degrees (Figure 2.22), she needs to ask the patient to move her feet (Figure 2.23) to match the rotation of her upper body; this will stabilize the patient and help prevent breast motion during the exposure. With both hands, the technologist should pull and lift the breast up and away from the chest wall while applying motorized compression with the foot pedal (Figure 2.24). Not until the compression device has firmly gripped the breast tissue should the technologist remove her hand from under the compression paddle (Figure 2.25). This maneuver is necessary to prevent gravity from pulling the breast down and making the ducts droop instead of

Fig. 2.21. This patient is instinctively drawing back her shoulders thereby tightening the skin of the breast (left). Hunching the shoulders (right) loosens the skin in the anterior chest wall, so more medial tissue can be positioned under the compression device.
Fig. 2.22. The technologist rotates the patient until the compression plate touches the sternum while continuing, at the same time, to pull the breast upward and away from the chest.
  1. 2.23. For the MLO view, the patient's feet initially are parallel to the image receptor. After the sternum is rotated towards the film tray, it is often difficult for the patient to keep her balance if her feet are at right angles to her torso (A). If her feet are rotated 90 degrees to match the torso position (B), it will be easier for the patient to tolerate compression without motion.
  2. 2.23. For the MLO view, the patient's feet initially are parallel to the image receptor. After the sternum is rotated towards the film tray, it is often difficult for the patient to keep her balance if her feet are at right angles to her torso (A). If her feet are rotated 90 degrees to match the torso position (B), it will be easier for the patient to tolerate compression without motion.
Fig. 2.24. Beginning compression with the motorized foot control frees both the technologist's hands for positioning the breast.

radiating straight out from the nipple like the spokes of a wheel. Its importance must be stressed because a cancer is identifiable when its spicules radiate at right angles to the ducts. If the ducts droop, discerning these spicules can be difficult (Figure 2.26). The technologist should then apply final compression with a hand-wheel control or with a similarly swiftly responsive control, which enables her to reduce compression quickly if the patient becomes uncomfortable. The technologist may need a rubber spatula or wooden ruler to hold very small breasts in position while bringing the compression paddle into place. Finally, to open up the inframam-mary crease, the technologist should pull the tissue beneath the inframammary crease down and away from the chest wall (Figure 2.27). The posterior axillary line, the sternum, the clavicle, and the humeral head should mark the boundaries of the imaged tissue (Figure 2.28).

Fig. 2.25. The technologist pulls the breast away from the chest wall, removing her hand only after the compression device has gripped the breast.
  1. 2.26. When the ducts overlap in a poorly compressed breast (A), spicules radiating from a small cancer are more difficult to see. Lifting the breast up and away from the chest wall for the MLO view allows the ducts to radiate in straight lines from the nipple, permitting the spicules radiating from a small cancer (B-arrow) to be more readily perceptible.
  2. 2.26. When the ducts overlap in a poorly compressed breast (A), spicules radiating from a small cancer are more difficult to see. Lifting the breast up and away from the chest wall for the MLO view allows the ducts to radiate in straight lines from the nipple, permitting the spicules radiating from a small cancer (B-arrow) to be more readily perceptible.
Fig. 2.27. To help visualize the inframammary crease, the technologist pulls the tissue beneath the inframammary fold after the compression plate is brought down.
Fig. 2.28. The completed MLO position.

When the MLO projection is done correctly, Bassett et al. (1993) state that the pectoral muscle is usually visible (Figure 2.29) and should be identifiable within 1 cm of the nipple line or below it in 81 percent of patients. But in patients whose positioning is limited by a physical impairment such as arthritis, a stroke, an injury to the shoulder, kyphoscoliosis, or atrophy of the shoulder muscles, the pectoral muscle may not be identifiable. Ideally, the inframam-mary crease should also be discernible. But thin patients do not have enough subcutaneous adipose tissue to pull onto the image receptor, so that in actual practice, the inframammary crease is visible in only approximately 50 percent of patients.

Fig. 2.29. This well-positioned MLO mammogram demonstrates the pectoral muscle (black arrows) and the inframammary crease (white arrows). The breasts have been pulled upward and outward to prevent them from drooping.

If the patient's abdomen protrudes and hinders the technologist from compressing the breast, the technologist should ask the patient to move her abdomen away from the image receptor by stepping back from the tray. The patient should then bend the upper part of her body forward and upward from her waist toward the image receptor to prevent her abdomen from interfering with compression.

The importance of the MLO view is to image as much posterior tissue as possible. It is important for the technologist to concentrate on pulling as much of the flexible posterolateral glandular tissue as possible onto the film tray, rather than positioning the image-receptor tray in the midaxilla behind the pectoral muscle. On the MLO view, the latissimus dorsi muscle will be imaged in approximately 10 percent of patients (Figure 2.30).

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