Anatomy. The NAC is a specialized structure of the breast, richly vascularized and innervated, composed of terminal duct lobular units that are the functional unit of the breast lobule. The lactiferous ducts drain milk of 15–20 lobes through 9–20 orifices in the nipple. The surrounding areola contains sebaceous glands (Montgomery’s glands) that provide lubrication.  [5] (Fig.1) Microscopically, the nipple's epidermis is composed of keratinized stratified squamous epithelium, densely populated with melanocytes. Below this layer, the lactiferous ducts are lined by a bi-layered epithelium comprising luminal and myoepithelial cells, supported by dense connective stroma.  [6]

Normal nipple morphology and enhancement on breast MRI. First, as with any MRI procedure, the use of a high-field-strength magnet (1.5 or 3 T), a dedicated breast coil, proper timing for dynamic sequences, and correct nipple positioning are essential to achieve high-quality MRI images and avoid artifacts. [7] In a retrospective observational study, Gao et al. found that the majority of normal of nipples are symmetric (96%) and everted (75%), less commonly flat (23%), and inverted (2%) (Fig.2). First postcontrast T1-weighted subtraction MR images were used to assess enhancement. Gao et al. demonstrate that physiologic nipple enhancement (Fig.3) most commonly exhibit superficial linear enhancement (SLE, 96%), corresponding to the thin linear layer of the epidermidis highly vascularized. Below that, a non-enhancing zone (NEZ), made up of the connective tissue of the dermis, is present in 93% of cases. Deeper to that, there’s an internal nipple enhancement (INE), which can be linear or patchy and represents the lactiferous duct bundles and smooth muscle. Typically, the enhancement pattern of the NACs is symmetrical and persistent. However, in some cases it may appear asymmetric (Fig.4) especially during the early phase and become more symmetric in the delayed phase [8].

Benign Pathologies.

• Duct ectasia

Ductal ectasia is a dilatation of the mammary ductal system greater than 3 mm that typically involves the retroareolar ducts bilaterally and symmetrically. [5] On MRI, ductal ectasia appears as tubular structures with a segmental distribution, showing high signal intensity on T2-weighted imagces, and high signal intensity also on T1-weighted images in case of proteinaceous or hematic content. (Fig. 5) [3,5]

• Periductal mastitis and abscess

In periductal mastitis, the duct walls typically demonstrate enhancement following contrast administration. MRI may also reveal either enhancing or non-enhancing masses, depending on the extent of the inflammatory process in the surrounding tissues. Additional findings, such as skin thickening and prominent blood vessels, are often observed.  [2,3] A common complication of inflammatory processes is abscess formation. On MRI, breast abscesses generally appear as fluid collections with peripheral rim enhancement on contrast-enhanced sequences and exhibit hyperintensity on T2-weighted imaging. Surrounding edema and/or fistula formation may also be present. (Fig.6) [5]

• Papillary lesions

Papillary lesions can be classified as central, typically presenting as solitary papillomas, or peripheral, often manifesting as multiple lesions (papillomatosis) (Fig.7) associated with a higher malignancy risk. On T2-weighted images, they usually appear as oval, well-circumscribed intraductal masses within dilated ducts, though less frequently, they may present as irregular masses. [9] In dynamic sequences, these lesions commonly exhibit rapid contrast uptake followed by washout, though some papillomas demonstrate a plateau-type enhancement curve. [10]

• Nipple adenoma

Nipple adenoma is a rare benign lesion originating from the lactiferous ducts within the nipple. It often presents as a palpable mass or nipple discharge and can mimic malignancy clinically and radiologically. On MRI, nipple adenomas typically appear as well-circumscribed, hyperintense masses on T2-weighted images, with homogeneous or a peripheral rim enhancement pattern secondary enhancement following contrast administration. However, their imaging features can overlap with those of malignant lesions, necessitating histopathological confirmation for a definitive diagnosis. [11,12]

Malignant Pathologies.

• Breast cancer

Invasive ductal carcinoma (IDC) and ductal carcinoma in situ (DCIS) are the most common breast cancer types and frequently involve the nipple-areolar complex (NAC). Clinically the NAC involvement is often associated with nipple retraction, discharge, and palpable mass. However clinical sign, mammography either ultrasound are inadequate for detecting NAC invasion, whereas MRI, with its high negative predictive value (94.8%) and ability to identify suspicious enhancement patterns, plays a crucial role in preoperative planning and optimizing surgical outcomes.  Breast MRI is particularly effective for evaluating NAC involvement and measuring the lesion-to-nipple distance. (Fig.8) A lesion-to-nipple distance greater than 1 cm typically qualifies patients for nipple-sparing mastectomy (NSM), a procedure that offers superior cosmetic results but carries risks of residual tumor cells and potential recurrence. (Fig.9) [2, 5,8]

• Paget's disease of the breast (PD)

PD is a malignant involvement of the nipple epidermis, often associated with underlying DCIS or IDC. Clinical features consist in erythema, scaling, and ulceration. On MRI, PD typically presents as asymmetric thickening and enhancement NAC with associated dermal thickening. Additional findings suggestive of underlying malignancy, such as masses or non-mass enhancement, may also be observed. (Fig.10)  MRI’s high sensitivity and ability to assess both the NAC and underlying breast tissue make it indispensable for detecting associated multifocal or multicentric disease and guiding clinical management. [5,8]