Radiology of the Breast: Technique and Normal Anatomy

TECHNIQUE AND NORMAL ANATOMY

Film-screen and Digital Radiography (Radiomammography)

The film-screen mammogram is created with x-rays, radi-ographic film, and intensifying screens adjacent to the film within the cassette; hence the term film-screen mammogra-phy. The digital mammogram is created using a similar sys-tem, but replacing the film and screen with a digital detector.

The routine examination consists of two views of each breast, the craniocaudal (C-C) view and the mediolateral oblique (MLO) view, with a total of four films. The C-C view can be considered the “top-down” view, and the MLO an an-gled view from the side (Figures 5-1, 5-2). The patient un-dresses from the waist up and stands for the examination, leaning slightly against the mammography unit. The technol-ogist must mobilize, elevate, and pull the breast to place as much breast tissue as possible on the surface of the film cas-sette holder. A flat, plastic compression paddle is then gently but firmly lowered onto the breast surface to compress the breast into as thin a layer as possible. This compression achieves both immobilization during exposure and disper-sion of breast tissue shadows over a larger area, thereby permitting better visual separation of imaged structures. Compression may be uncomfortable, and may even be painful in a small proportion of patients. However, most pa-tients accept this level of discomfort for the few seconds re-quired for each exposure, particularly if they understand the need for compression and know what to expect during the examination. Mammography has proved to be more cost-effective, while maintaining resolution high enough to demonstrate early malignant lesions, than any other breast im-aging technique. In its present state of evolution, however, the sensitivity of radiomammography ranges from 85% to 95%.

Limitations

Sensitivity is limited by three factors: (1) the nature of breast parenchyma, (2) the difficulty in positioning the organ for imaging, and (3) the nature of breast carcinoma.

The Nature of Breast Parenchyma

Very dense breast tissue may obscure masses lying within ad-jacent tissue. Masses are more easily detected in a fatty breast.

Positioning

A technologist performing mammography must include as much breast tissue as possible in the field of view for each image. The x-ray beam must pass through the breast tangen-tially to the thorax, and no other part of the body should in-trude into the field of view, so as to not obscure any part of the breast. This requires both a cooperative patient and a skilled technologist. If a breast mass is located in a portion of the breast that is difficult to include in the image, mammog-raphy may fail to demonstrate the lesion. Also, because of these practical considerations, routine mammography is not performed in markedly debilitated patients.

The Nature of Breast Carcinoma

Some breast carcinomas are seen as well-defined rounded masses or as tiny, but bright, calcifications, and are easily de-tected. Others, however, may be poorly defined and irregular, mimicking normal breast tissue. Rarely, still others may have no radiographic signs at all.

For these reasons, it must be remembered that mammog-raphy has significant limitations in detection of carcinoma. It cannot be overemphasized that any suspicious finding on breast physical examination should be evaluated further, even if the mammogram shows no abnormality. Occasionally, ad-ditional imaging may reveal an abnormality, but if not, short-term close clinical follow-up or biopsy is warranted.

Normal Structures

Normal breast is composed mainly of parenchyma (lobules and ducts), connective tissue, and fat. Lobules are drained by ducts, which arborize within lobes. There are about 15 to 20 lobes in the breast. The lobar ducts converge upon the nipple.

Parenchyma

The lobules are glandular units and are seen as ill-defined, splotchy opacities of medium density. Their size varies from 1 to several millimeters, and larger opacities result from con-glomerates of lobules with little interspersed fat. The breast lobes are intertwined and are therefore not discretely identi-fiable. This parenchymal tissue is contained between the pre-mammary and retromammary fascia.

The amount and distribution of glandular tissue are highly variable. Younger women tend to have more glandular tissue than do older women. Glandular atrophy begins infer-omedially, and residual glandular density persists longer in the upper outer breast quadrants. However, any pattern can be seen at any adult age (Figure 5-3).

Along with glandular elements, the parenchyma con-sists of ductal tissue. Only major ducts are visualized mammographically, and these are seen in the subareolar re-gion as thickened linear structures of medium density con-verging on the nipple.

Connective Tissue

Trabecular structures, which are condensations of connec-tive tissue, appear as thin ( 1 mm) linear opacities of medium to high density. Cooper’s ligaments are the sup-porting trabeculae over the breast that give the organ its characteristic shape, and are thus seen as curved lines around fat lobules along the skin-parenchyma interface within any one breast (Figure 5-4).

Fat

The breast is composed of a large amount of fat, which is lu-cent, or almost black, on mammograms. Fat is distributed in the subcutaneous layer, in among the parenchymal elements centrally, and in the retromammary layer anterior to the pec-toral muscle (Figure 5-4).

Lymph Nodes

Lymph nodes are seen in the axillae and occasionally in the breast itself (Figure 5-4).

Veins

Veins are seen traversing the breast as uniform, linear opaci-ties, about 1 to 5 mm in diameter (Figure 5-4).

Arteries

Arteries appear as slightly thinner, uniform, linear densities and are best seen when calcified, as in patients with athero-sclerosis, diabetes, or renal disease.

Skin

Skin lines are normally thin and are not easily seen without the aid of a bright light for film-screen mammograms. Vari-ous processing algorithms with digital mammography allow better visualization of the skin.

Screening Mammography

The standard mammogram (along with appropriate history-taking) makes up the entire screening mammogram. The indi-cation for this examination is the search for occult carcinoma in an asymptomatic patient. Physical examination by the patient’s physician, known as the clinical breast examination (CBE), is an indispensable element in complete breast screening. Although the American Cancer Society no longer recommends routine breast self-examination (BSE), particular attention should be paid to lumps identified by the patient as new or enlarging. Such patients should be referred for diagnostic mammography. Table 5-1 includes guidelines for frequency.

Diagnostic Mammography

The diagnostic mammogram begins with the two-view stan-dard mammogram. Additional maneuvers are then used asappropriate in each case, dictated by history, physical exami-nation, and findings on initial mammography. Indications for diagnostic mammography are (1) a palpable mass or other symptom or sign (eg, skin dimpling, nipple retraction, or nipple discharge that is clear or bloody), and (2) a radi-ographic abnormality on a screening mammogram. Addi-tionally, patients with a personal history of breast cancer may be considered in the diagnostic category.

Other projections, magnification, and spot compression may be used to further evaluate abnormalities. These tech-niques provide better detail and disperse overlapping breast tissue so that lesions are less obscured.

Implant Views

Patients with breast implants require specialized views to best image residual breast tissue because the implants obscure large areas of the breast tissue with routine mammography. These specialized views (Eklund, “push-back,” or implant displacement view) displace the implants posteriorly while the breast tissue is pulled anteriorly as much as possible.

Computer-Aided Detection

Growing availability and affordability of computing power has led to the development of computer-aided detection (CAD). CAD utilizes complex algorithms to analyze the data from a mammogram for suspicious calcifications, masses, and architecture distortion. It then flags these areas so that the interpreting radiologist can give these areas special atten-tion. Several studies show increased cancer detection when CAD is applied, and sensitivity and specificity continue to improve as these algorithms are refined.

Ultrasonography

The indications for ultrasonography are (1) a mammographi-cally detected mass, the nature of which is indeterminate, (2) apalpable mass that is not seen on mammography, (3) a palpa-ble mass in a patient below the age recommended for routine mammography, and (4) guidance for intervention. Ultra-sonography is a highly reliable technique for differentiating cystic from solid masses. If criteria for a simple cyst are met, the diagnosis is over 99% accurate. Although certain features have been described as indicative of benign or malignant solid masses, this determination is more difficult to make and less accurate than the determination of the cystic nature of a mass.

A limitation of ultrasonography is that it is very operator-dependent. Also, it images only a small part of the breast at any one moment. Therefore, an overall inclusive survey is not possible in one image, and lesions may easily be missed.

Normal Structures

The skin, premammary and retromammary fasciae, trabecu-lae, walls of ducts and vessels, and pectoral fasciae are well seen as linear structures. The glandular and fat lobules are oval, of varying sizes, and hypoechoic relative to the sur-rounding connective tissue (Figure 5-5).

Simple cysts are anechoic (echo-free) and have thin, smooth walls. Increased echogenicity is seen deep to cysts (enhanced through-transmission). Most solid masses are hy-poechoic relative to surrounding breast tissue.

Magnetic Resonance Imaging

The role of MRI in mammography continues to expand, with common applications including (1) staging of and surgical planning for breast tumors, (2) searching for a primary tumor in patients who present with cancerous axillary lymph nodes,evaluating tumor response to neoadjuvant chemotherapy,differentiating tumor recurrence from posttreatment changes in patients with previous breast-conserving surgery and radiation, (5) screening of high-risk patients, (6) evaluating implants, and (7) evaluating difficult (dense or fibrous) breasts. In addition, the technology for MR-guided breast biopsies is in-creasingly available.

The patient lies prone on the scanner table, and a special-ized coil surrounds the breasts. Depending on the clinical question, a varying number of pulse sequences are performed to evaluate the breasts or the composition of a suspicious le-sion. Scan times can range from 30 minutes to over an hour.

MRI can show whether a lesion is solid or contains fat or fluid. Dynamic scanning after administration of intravenous contrast shows whether structures enhance and at what rate. Cancers classically enhance rapidly with subsequent “wash-out.” For instance, a lesion that enhances relatively rapidly on dynamic exam (think neovascularity) is more concerning for malignancy. If more than one suspicious lesion is identi-fied, the relative proximity of these lesions can determine whether a patient would be a good candidate for lumpec-tomy rather than mastectomy. The wide field of view allows staging by evaluating the axillary and internal mammary nodes. Figure 5-6 shows an enhancing cancerous tumor.

Although MRI is quite sensitive (good for detecting dis-ease), it is relatively nonspecific. This is due to the overlap-ping imaging characteristics of both benign and malignant processes. Like cancer, some benign breast structures show enhancement, although usually with a slower rate.

Because of the relatively low specificity, screening with MRI is best used in patients with a higher probability of disease. The 2007 American Cancer Society recommenda-tions include annual MRI breast screening of patients with a lifetime risk of 20% or greater.

Normal Structures

Tissues are differentiated by their pattern of change on dif-ferent pulse sequences. The skin, nipple and areola, mam-mary fat, breast parenchyma, and connective tissue are normally seen, in addition to the anterior chest wall, in-cluding musculature, ribs and their cartilaginous portions, and portions of internal organs. Small calcifications are not visible, and small solid nodules may not be detected. Cystic structures are well seen. Normal implants appear as cystic structures with well-defined walls. Their location is deep to the breast parenchyma or subpectoral, depending on the surgical technique that was used to place the implants. In-ternal signal varies and depends on implant contents, either silicone or saline.

Ductography

Ductography, or galactography, uses mammographic imag-ing with contrast injection into the breast ducts. The indica-tion for use is a profuse, spontaneous, nonmilky nipple discharge from a single duct orifice. If these conditions are not present, the ductogram is likely to be of little help. The purpose is to reveal the location of the ductal system in-volved. The cause of the discharge is frequently not identi-fied. Occasionally, an intraluminal abnormality is seen, but findings have low specificity.

The patient lies in supine position while the discharging duct is cannulated with a blunt-tipped needle or catheter under visual inspection and with the aid of a magnifying glass. A small amount of contrast material (usually not more than 1 mL) is injected gently by hand into the duct. Several mammographic images are then made. The procedure re-quires about 30 minutes and is not normally painful.

Normal Structures

Just deep to the opening of the duct on the nipple, the duct ex-pands into the lactiferous sinus. After a few millimeters, the duct narrows again and then branches as it enters the lobe con-taining the glands drained by this ductal system. The normal caliber of the duct and its branches is highly variable, but nor-mal duct walls should be smooth, without truncation or abrupt narrowing. With high-pressure injection, the lobules, as well as cystically dilated portions of ducts and lobules, may opacify.

Image-Guided Needle Aspiration and Biopsy

The indications for needle aspiration and biopsy of breast le-sions are varied and are variably interpreted by radiologists and referring physicians. Two categories are discussed here.

The first indication is aspiration of cystic lesions to con-firm diagnosis, to relieve pain, or both. Nonpalpable cysts re-quire either ultrasound or mammography to be seen. A fine needle (20- to 25-gauge) usually suffices to extract the fluid. The cystic fluid is not routinely sent for cytology unless it is bloody.

The second indication concerns solid lesions. Needle biopsy is used in this case (1) to confirm benignity of a lesion carrying a low suspicion of malignancy mammographically,to confirm malignancy in a highly suspicious lesion prior to initiating further surgical planning and treatment, and to evaluate any other relevant mammographic lesion for which either follow-up imaging or surgical excision is a less desirable option for further evaluation.

Guidance for needle biopsy can be accomplished with stereotactic mammography, ultrasound, and MR. Imaging modality for needle guidance is selected on the basis of lesion characteristics, availability of technology, and personal pref-erence of the radiologist. Ultrasound and mammography are the most commonly used techniques.

Large core biopsy (typically 14-, 11-, or 8-gauge) has been shown to be more accurate for nonpalpable lesions than fine needle aspiration (20-gauge or smaller) and is often com-bined with vacuum assistance to further increase tissue yield.

Mammographic guidance is most easily and accurately performed with a stereotactic table unit. Lesions of only a few millimeters can be successfully biopsied. With stereotac-tic tables, the patient lies prone with the breast protruding through an opening in the table surface. A needle is mechan-ically guided to the proper location in the breast with com-puter assistance. The entire procedure requires 30 minutes to 1 hour.

Image-Guided Needle Localization

When a nonpalpable breast lesion must be excised, imaging is used to guide placement of a needle into the breast, with the needle tip traversing or flanking the lesion. Either ultrasono-graphic or mammographic guidance can be used, and the choice again depends on lesion characteristics and personal preference. Once the needle is in the appropriate position, a hook wire is inserted through the needle to anchor the device in place. This prevents migration during patient transport and surgery. After needle placement, the patient is taken to the operating theater for excision of the lesion by the surgeon.

Biopsy Specimen Radiography

When a lesion is excised from the breast, a surgical specimen can be radiographed to document that the mammographic abnormality was removed. This practice is routinely followed with needle-localized lesions, but palpable lesions excised may also be radiographed to confirm that the specimen con-tains an abnormality that may have been present on the mammogram.