Intramammary lymph nodes are separate from axillary lymph nodes and are completely surrounded by breast tissue, typically in the upper outer quadrant. Approximately 5 % of patients undergoing mammography and up to 28 % of breast specimens will exhibit these lymph nodes. Characteristic features of a normal intramammary node include a fatty hilum, diameter less than 1 cm, and circumscribed margins. An intramammary lymph node may become abnormal and lose its hilum or enlarge secondary to hyperplasia, metastatic disease, or inflammation. A benign intramammary lymph node demonstrates reniform morphology with a fatty hilum on MRI. The cortex will be T1 hypointense and T2 hyperintense with rapid, homogeneous enhancement; a type III washout kinetic pattern can be seen, as demonstrated in this case.

Normal intramammary lymph nodes are usually seen as oval or reniform, smoothly marginated, fat-containing masses. They are often seen adjacent to vessels within the upper outer quadrant, although 28 % of intramammary lymph nodes are found in other locations. They are usually nonpalpable and are found incidentally on breast MRI, where they appear hyperintense on T2-weighted images. They typically demonstrate homogenous enhancement with rapid initial and washout delayed kinetics. Non–fat-saturated T1-weighted images are usually helpful to confirm the presence of fat within the hilum. Biopsy can be avoided if the mass demonstrates classic characteristics of a benign lymph node (Figs. 2.3 and 2.4).

Simple cysts result from dilatation and effacement of the terminal duct lobular unit. They are commonly seen incidentally on MRI and follow fluid signal on all sequences (appearing very hyperintense on T2-weighted images and intermediate to hypointense on T1-weighted images). On post-contrast images, cysts typically do not enhance, but the periphery of the cyst may enhance if there is surrounding inflammation. Not infrequently, cysts may have intermediate or high signal on T1-weighted images owing to proteinaceous or hemorrhagic contents (Figs. 2.5, 2.6, 2.7, and 2.8).

Phase-encoded motion artifact commonly seen in breast imaging is due to physical movement of the patient and manifests as ghosting in the direction of phase encoding, which is usually in the direction of the short axis. The spacing between the artifacts is related to the TR and frequency of the motion. Unlike Gibbs or truncation artifacts, the motion artifacts usually extend across the entire field of view. Possible solutions to phase-encoded motion artifact include cardiac/respiratory gating, switching phase and frequency encoding directions, increasing the number of signal averages, and shortening the scan time when patient motion is causing the artifact.

Normal ducts usually are not well visualized on breast MRI. In the setting of a chronic obstructive process, however, they become dilated and present as high-signal-intensity, branching tubular structures on T1-weighted images and as hypointense structures on T2-weighted images (owing to proteinaceous or hemorrhagic material). Dilated ducts do not typically enhance, but enhancement of the ductal wall may be seen because of underlying inflammation. It is important to remember that assessment of enhancement should not depend solely on the subtracted images, as misregistration artifact may occur (Figs. 2.11 and 2.12).

Signal void secondary to air can be seen on MR images on T1- and T2-weighted sequences (Figs. 2.13, 2.14, and 2.15). The etiology in healthy patients is unclear, but it is commonly iatrogenic (for example, following MRI-guided biopsy or ductography).

Normal ducts usually are not seen on MR imaging, as they blend in with the background parenchyma. In cases of inflammation, infection, or obstruction, however, ducts filled with protein or blood will be visible on non-enhanced T1-weighted images as high-signal-intensity, branching tubular structures without associated enhancement. Therefore, subtraction images are very helpful when pre-contrast images demonstrate a hyperintense ductal pattern, to exclude abnormal enhancement. Otherwise, subtle areas of ductal wall enhancement may be overlooked, and a potential underlying carcinoma can be missed.

Fibroadenomas are frequently seen on breast MRI and are best identified using morphologic features, classically a smooth margin with hypointense internal septa on T2-weighted images. Although less than 40 % of fibroadenomas demonstrate nonenhancing internal septations, if present they can be predictive of benignity (>95 %). MRI enhancement kinetics alone is neither specific nor sensitive because of the overlap in enhancement characteristics between benign and malignant lesions (Figs. 2.18, 2.19, 2.20, and 2.21).

Phyllodes tumors are rare and constitute 0.3–1.0 % of all breast tumors. They have a high incidence of local recurrence after surgery and potential for hematogenous metastases. Because of their rapid growth, phyllodes tumors tend to have circumscribed margins on mammography and sonography, so it is difficult to distinguish them from fibroadenomas on the basis of imaging. Large phyllodes tumors have a characteristic morphologic appearance on breast MRI, including smooth margins, internal cysts, and septations. They typically demonstrate low T1-weighted signal intensity and appear somewhat bright on T2-weighted images. Literature has shown that contrast enhancement pattern can be variable, so differentiation of phyllodes tumors and fibroadenomas on breast MRI remains unreliable (Figs. 2.26, 2.27, 2.28, 2.29, 2.30, and 2.31).

Breast infection/abscess can result from unresolved local infection, obstruction of the duct near the nipple and acute, chronic or lactational mastitis. They are usually palpable, often associated with erythema, edema, and induration or overlying skin and pain. Depending on the water content of the collection, MR T2-weighted signal may vary. The overlying thickened skin may be intermediate or bright in signal depending on the degree of edema. Post-contrast T1-weighted images usually demonstrate a non-enhancing central, round, or irregular mass surrounded by an early, intensely enhancing rim. Abscess may be confused with malignancy due to the enhancement kinetics of the rim and overlying skin thickening, as illustrated in this case. In addition, focal skin thickening and axillary lymphadopathy can also be present. In case of persistent clinical concern with no resolution with antibiotics, surgical management may be appropriate.

Breast infection/abscess can result from unresolved local infection, iatrogenic in etiology, obstruction of the duct near the nipple, and acute, chronic or lactational mastitis. Depending on the water content of the collection, MR T2-weighted signal may vary but often is hyperintense. The overlying thickened skin may be intermediate or bright in signal depending on the degree of edema. Post-contrast T1-weighted images usually demonstrate a non-enhancing central, round, or irregular mass surrounded by an early, intensely enhancing rim. All these features are illustrated in this case.