of an infamed hair follicle may improve diagnostics by identifying early signs of adnexal pathologies like hidradenitis suppurativa [35, 37, 38]. In addition to identifying changes in hair follicle appearance, ultrasound has also been used to characterize scalp cysts; cysts typically appear as round and dark Fig. 1 Normal skin from the anterior arm using a 50-MHz HFUS probe (Longport, Inc. EPISCAN). Note the fne hyperechoic epidermal entry echo. Deep to the entry echo is the hyperechoic dermis measuring about 1 millimeter (mm) with thicker horizontal bands in the deep dermis corresponding to the organized, horizontally arrayed collagen bundles in the reticular dermis. Deep to the dermis is the subcutis composed primarily of hypoechoic fat globules. Within this layer is the hyperechoic superfcial fascia. Deep to the subcutis is the hyperechoic deep fascia overlying the biceps muscle Levy et al. Ultrasound J (2021) 13:24 Page 4 of 12 structures. Trichilemmal cysts characteristically contain hyperechoic areas within their round, dermal anechoic structures, corresponding to the dense keratinous and oily inner-cystic debris [35]. Pilomatricomas appear targetoid with a solid hyperechoic nodule thought to correspond to cystic calcifcations or giant cells surrounded by a hypoechoic rim [35]. The nail Te dorsal and ventral plates of the nail are visible as separate entities on ultrasound, generating the second instance of a hyperechoic bilaminar linear structure (Fig. 3). Te matrix of the nail is hypoechoic and ventral to the hyperechoic proximal nail fold. Te nail bed is normally hypoechoic and just deep to the hyperechoic ventral nail plate. Finally, the surface of the distal phalanx appears as a hyperechoic linear structure [5, 22, 39]. Ultrasonography in neoplastic disease Nonmelanoma skin cancers Te main dermatologic use of ultrasound is in the pre-operative assessment of non-melanoma and melanoma skin cancers [40, 41] and detection of early neoplasms [42]. In general, cutaneous neoplasms appear sonographically as hypoechoic lesions in a relatively hyperechoic epidermis and/or dermis [43]. Te most prevalent human malignancy, BCC, has several histologic subtypes including superfcial, nodular, Fig. 2 Glabrous skin from the palm using a 50-MHz HFUS probe (Longport, Inc. EPISCAN). Note the bilaminar hyperechoic entry echo due to the thickened stratum corneum in this anatomic location Fig. 3 Longitudinal view of a normal nail using a 50-MHz HFUS probe (Longport, Inc. EPISCAN). Note the bilaminar appearance of the nail plate consisting of the hyperechoic dorsal and ventral nail plates separated by a hypoechoic band. The top-most hyperechoic horizontal line represents the probe membrane Levy et al. Ultrasound J (2021) 13:24 Page 5 of 12 micronodular and morpheaform [44]. Te most common BCC histologic subtype—nodular—sonographically appears as a hypoechoic lesion with well-defned borders and is usually found in the dermis, though occasionally can extend deeper into the subcutis (Fig. 4) [42, 45]. Some histologic subtypes of BCC that have demonstrated more aggressive biologic behavior have been shown to feature small hyperechoic dots seen within the hypoechoic tumor. Such spots are referred to as “fower cotton” spots [13, 46]. Tough the cause of this artifactual fnding is unknown, fower cotton spots are thought to correlate with clusters of apoptotic neoplastic cells within the tumor [13, 46]. Risk of aggressive BCC phenotype, such as micronodular or morpheaform, and risk of recurrence have been correlated to increasing numbers of fower cotton spots [47]. Furthermore, the presence of fower cotton spots is important in diferentiating a BCC from melanoma as melanoma lacks these hyperechoic inclusions [48, 49]. Te clinical relevance of ultrasound in the examination of suspected SCC is less well defned. Ultrasound has been used to identify SCCs, which also appear as hypoechoic lesions [42, 50]. However, in 2010 Wortsman et al. analyzed 75 non-melanoma skin cancers of which 18 were cutaneous SCCs. She demonstrated a modest but signifcant increase in diagnostic accuracy with the addition of higher frequencies of ultrasound [9]. An additional