Geometric magnification is achieved by moving the breast farther from the image receptor (closer to the x-ray tube) and switching to a small focal spot (Fig. 1-2). Placing the breast halfway between the focal spot and the image receptor (as in Fig. 1-2B) would magnify the breast by a factor of 2.0 from its actual size to the image size because of the divergence of the x-ray beam. The MQSA requires that mammography units with magnification capabilities must provide at least one fixed magnification factor of between 1.4 and 2.0 (Table 1-1). Geometric magnification makes small, high-contrast structures such as microcalcifications more visible by making them larger relative to the noise pattern in the image (increasing their signal-to-noise ratio [SNR]). Optically or electronically magnifying a contact image, as would be done with a magnifier on SFM or using a zoom factor greater than 1 on a digital mammogram, does not increase the SNR of the object relative to the background, because both are increased in size equally. To avoid excess blurring of the image with geometric magnification, it is important to use a sufficiently small focal spot (usually 0.1 mm nominal size) and not too large a magnification factor (2.0 or less). When the small focal spot is selected for geometric magnification, the x-ray tube output is decreased by a factor of 3 to 4 (to 25–40 mA) compared to that from a large focal spot (80–150 mA). This can extend imaging times for magnification mammography, even though the grid is removed in magnification mammography. The air gap between the breast and image receptor provides adequate scatter rejection in magnification mammography without the use of an antiscatter grid.

Figure 1-2 Magnification mammography improves resolution. Nonmagnified, or contact, mammography (A) and geometrically magnified mammography (B). Using a small or microfocal (0.1 mm) focal spot with the configuration shown in part B, higher spatial resolution can be obtained in the breast compared to part A, where a larger (0.3 mm) focal spot is used. C, Craniocaudal mammogram shows a possible benign mass in the inner breast. D, Microfocal magnification shows irregular borders not seen on the standard view.

Table 1-1 Mammography Focal Spot Sizes and Source-to-Image Distances

Mammography Type	Nominal Focal Spot Size (mm)	Source-to-Image Distance (cm)
Contact film-screen	0.3	≥55
Magnification	0.1	≥55

The Mammography Quality Standards Act requires magnification factors between 1.4 and 2.0 for systems designed to perform magnification mammography.

Collimators control the size and shape of the x-ray beam to decrease patient exposure to tissues beyond the compressed breast and image receptor. In mammography, the x-ray beam is collimated to a rectangular field to match the image receptor rather than the breast contour, because x-rays striking the image receptor outside the breast do not contribute to breast dose. By federal regulation, the x-ray field cannot extend beyond the chest wall of the image receptor by more than 2% of the SID. So, for a 60-cm SID unit, the x-ray beam can extend beyond the chest wall edge of the image receptor by no more than 1.2 cm.

The compression plate and image receptor assembly hold the breast motionless during the exposure, decreasing the breast thickness and providing tight compression, better separating fibroglandular elements in the breast (Fig. 1-3). The compression plate has a posterior lip that is more than 3 cm high and usually is oriented at 90 degrees to the plane of the compression plate at the chest wall. This lip keeps chest wall structures from superimposing and obscuring posterior breast tissue in the image. The compression plate must be able to compress the breast for up to 1 minute with a compression force of 25 to 45 pounds. The compression plate can be advanced by a foot-controlled motorized device and adjusted more finely with hand controls.

Figure 1-3 Schematic of a compression paddle and image receptor showing the components of the cassette holder, the compression plate, and the breast. The film emulsion faces the screen. AEC, automatic exposure control.

(Adapted from Farria DM, Kimme-Smith C, Bassett LW: Equipment, processing, and image receptor. In Bassett LW, editor: Diagnosis of diseases of the breast, Philadelphia, WB Saunders, 1997, pp 32 and 34.)

Screen-Film Mammography Image Acquisition

In SFM, the image receptor assembly holds a screen-film cassette in a carbon-fiber support with a moving antiscatter grid in front of the cassette and an AEC detector behind it. Screen-film image receptors are required to be 18 × 24 cm and 24 × 30 cm in size to accommodate various sized breasts (Box 1-5). Each size image receptor must have a moving antiscatter grid composed of lead strips with a grid ratio (defined as the ratio of the lead strip height to the distance between strips) between 3.5 : 1 and 5 : 1. The reciprocating grid moves back and forth in the direction perpendicular to the grid lines during the radiographic exposure to eliminate grid lines in the image by blurring them out. One manufacturer uses a hexagonal-shaped grid pattern to improve scatter rejection; this grid is also blurred by reciprocation during exposure. Use of a grid improves image contrast by decreasing the fraction of scattered radiation reaching the image receptor. Grids increase the required exposure to the breast by approximately a factor of 2 (the Bucky factor), due to attenuation of primary as well as scattered radiation. Grids are not used with magnification mammography. Instead, in magnification mammography, scatter is reduced by collimation and by rejection of scattered x-rays due to a significant air gap between the breast and the image receptor.

Box 1-5 Compression Plate and Imaging Receptor

Both 18 × 24-cm and 24 × 30-cm sizes are required

A moving grid is required for each image receptor size

The compression plate has a posterior lip >3 cm and is oriented 90 degrees to the plane of the plate

Compression force of 25–45 pounds

Paddle advanced by a foot motor with hand compression adjustments

Collimation to the image receptor, not the breast contour

The AEC system, also known as the phototimer, is calibrated to produce a consistent film optical density (OD) by sampling the x-ray beam after it has passed through the breast support, grid, and cassette. The AEC detector is usually a D-shaped sensor that lies along the midline of the breast support and can be positioned by the technologist closer to or farther from the chest wall. If the breast is extremely thick or inappropriate technique factors are selected, the AEC will terminate exposure at a specific backup time (usually 4–6 seconds) or mAs (300–750 mAs) to prevent tube overload or melting of the x-ray track on the anode.

Screen-film cassettes used in mammography have an inherent spatial resolution of 18 to 21 lp/mm. Such resolution is achieved typically by using a single-emulsion film placed emulsion side down against a single intensifying screen that faces upward toward the breast in the film cassette. The single-emulsion film with a single intensifying screen is used to prevent the parallax unsharpness and cross-over exposure that occur with double-emulsion films and double-screen systems. One manufacturer has introduced a double-emulsion film with double-sided screens (EV System, Carestream Health, Toronto; formerly Eastman Kodak Healthgroup) with a thinner film emulsion and screen on top to minimize parallax unsharpness. Most screen-film processing combinations have relative speeds of 150 to 200, with speed defined as the reciprocal of the x-ray exposure required to produce an OD of 1.0 above base plus fog (which is 1.15–1.2, because base plus fog OD is 0.15–0.2).

Film processing involves development of the latent image on the exposed film emulsion. The film is placed in an automatic processor that takes the exposed film and rolls it through liquid developer to amplify the latent image on the film, reducing the silver ions in the x-ray film emulsion to metallic silver, thereby resulting in film darkening in exposed areas. The developer temperature ranges from 92°F to 96°F. The film is then run through a fixer solution containing thiosulfate (or hypo) to remove any unused silver and preserve the film. The film is then washed with water to remove residual fixer, which if not removed can cause the film to turn brown over time. The film is then dried with heated air.

Film processing is affected by many variables, the most important of which are developer chemistry (weak or oxidized chemistry makes films lighter and lower contrast), developer temperature (too hot may make films darker, too cool lighter), developer replenishment (too little results in lighter, lower-contrast films), inadequate agitation of developer, and uneven application of developer to films (causes mottling) (Table 1-2).

Table 1-2 Variables Affecting Image Quality of Screen-Film Mammograms

Film too dark	Developer temperature too high Wrong mammographic technique (excessive kVp or mA) Excessive plus-density control
Film too light	Inadequate chemistry or replenishment Developer temperature too low Wrong mammographic technique
Lost contrast	Inadequate chemistry or replenishment Water to processor turned off Changed film
Film turns brown	Inadequate rinsing of fixer
Motion artifact	Movement by patient Inadequate compression applied Inappropriate mammographic technique (long exposures)

For positioning, the technologist tailors the mammogram to the individual woman’s body habitus to get the best image. The breast is relatively fixed in its medial borders near the sternum and the upper breast, whereas the lower and outer portions of the breast are more mobile. The technologist takes advantage of the mobile lower outer breast to obtain as much breast tissue on the mammogram as possible. The two views obtained for screening mammography are the craniocaudal (CC) and mediolateral oblique (MLO) projections. The names for the mammographic views and abbreviations are based on the ACR Breast Imaging Reporting and Data System (ACR BI-RADS®), a lexicon system developed by experts for standard mammographic terminology. The first word in the mammographic view indicates the location of the x-ray tube, and the second word indicates the location of the image receptor. Thus, a CC view would be taken with the x-ray tube pointing at the breast from the head (cranial) down through the breast to the image receptor in a more caudal position.

To pass ACR accreditation clinical image review, the MLO mammogram must show most of the breast tissue in one projection, with portions of the upper inner and lower inner quadrants partially excluded (Fig. 1-4). Clinical evaluation of the MLO view should show fat posterior to the fibroglandular tissue and a large portion of the pectoralis muscle, which should be concave and extend inferior to the posterior nipple line (PNL). The PNL describes an imaginary line drawn from the nipple to the pectoralis muscle or film edge and perpendicular to the pectoralis muscle. The PNL should intersect the pectoralis muscle in the MLO view in more than 80% of women. Although the technologist tries to avoid producing skin folds on the film when possible, they are seen occasionally but do not usually cause problems for the radiologist reading the film. The MLO view should show adequate compression, exposure, contrast, and an open inframammary fold, in which both the lower portion of the breast and a portion of the upper abdominal wall should be seen.

Figure 1-4 Positioning for a normal mediolateral oblique (MLO) mammogram. By convention, the view type and side (R, L) labels are placed near the axilla. On a properly positioned MLO view, the inferior aspect of the pectoralis muscle should extend down to the posterior nipple line, an imaginary line drawn from the nipple back to and perpendicular to the pectoral muscle (double arrow). The anterior margin of the pectoralis muscle should be convex in a properly positioned MLO view. Ideally, the image shows fat posterior to the glandular tissue (star). The open inframammary fold (arrow) and abdomen are displayed with the breast pulled up and away from the chest.

To pass ACR accreditation clinical image review, the CC view should include the medial posterior portions of the breast without sacrificing the outer portions (Figs. 1-5 and 1-6). With proper positioning technique, the technologist should be able to include the medial portion of the breast without rotating the patient medially by lifting the lower medial breast tissue onto the image receptor. The pectoralis muscle should be seen when possible on the CC view. On the CC view, the PNL extends from the nipple to the pectoralis muscle or the edge of the film, whichever comes first, perpendicular to the pectoralis muscle or film edge. For a given breast, the length of the PNL on the CC view should be within 1 cm of its length on the MLO view.

Figure 1-5 Normal craniocaudal (CC) mammogram. The posterior nipple line (PNL) on the CC view is the distance between the nipple and the posterior aspect of the image. The PNL on the CC view (double arrow) should be within 1 cm of the PNL on the mediolateral oblique (MLO) view. The goal is to include posterior medial tissue (excluded on the MLO view) (arrow) and as much retroglandular fat (star) as possible.

Figure 1-6 Improper positioning. A, Inadequate pectoralis muscle and sagging breast tissue on this full-field digital mediolateral oblique view show that the posterior nipple line (PNL) requirements are not met, and the craniocaudal (CC) view is rotated laterally (B). Note the calcifying fibroadenoma on the left. C, In a second patient with a fatty breast, the pectoralis muscle is concave but just barely meets PNL requirements. D, The CC views are adequate.

Clinical images are evaluated on positioning, compression, contrast, proper exposure, random noise (radiographic mottle or quantum mottle produced by varying numbers of x-rays contributing to the image in different locations, even with a uniform object), sharpness, and artifacts (or structured noise). Imaging on a phantom is helpful in evaluating most of these factors, except for positioning and compression (Fig. 1-7). Adequate exposure (to achieve adequate film OD) and adequate contrast (OD difference) are important to ensure detection of subtle abnormalities (Fig. 1-8). Artifacts seen on clinical images include processing artifacts (roller marks, wet pressure marks, guide shoe marks), white specklike artifacts from dust or lint between the fluorescent screen and film emulsion, grid lines from incomplete grid motion, motion artifacts from patient movement (made more likely by longer exposure times), skin folds from positioning, tree static caused by static electricity from low humidity in the dark room, or film handling artifacts (fingerprints, crimp marks, or pressure marks) (Figs. 1-9 to 1-12).