for optimal visualization of lesions in various anatomic locations [15]. Normal skin One of the frst dermatologic applications of ultrasound measured the thickness of healthy skin [20]. While this original study used a 15-MHz transducer (and not meeting current standards for HFUS), the study was among the frst to highlight the beneft of higher frequencies. Subsequent research has demonstrated that the three layers of the skin have distinct echogenicity, or corresponding levels of brightness (Fig. 1) [21]. Te epidermis is mainly composed of keratin, a dense, fbrous structural protein that strongly refects ultrasound waves; therefore, the epidermis is hyperechoic. Te dermis is also hyperechoic, though to a lesser extent than the epidermis, due to its high content of collagen. Finally, the subcutis is hypoechoic due to the presence of fat lobules that allow for the unimpeded passage of ultrasound waves [2, 21]. Te dermis can be further distinguished on ultrasound based on the arrangement of collagen and elastin bundles; the upper 20% of the hyperechoic dermis corresponds to the papillary dermis and contains irregularly arranged, thinner collagen and elastin bundles. Te lower 80% of the dermis is made up of more regularly arranged collagen and elastin. Correspondingly, ultrasound at frequencies>10 MHz demonstrates a reticular dermis with hyperechoic strands of thicker collagen bundles interposed with hypoechoic extracellular matrix [2, 5, 22]. An important consideration when using ultrasound to study healthy skin is persistence of a well-defned highamplitude echo between the ultrasound gel and the corneal layer of the epidermis. Tis has been attributed to the impedance jump of the ultrasound waves from the Levy et al. Ultrasound J (2021) 13:24 Page 3 of 12 ultrasound gel to the callused epidermal cells [2]. Tis refection is thus referred to as the epidermal entry echo. Tus, the epidermal entry echo is not a direct visualization of the epidermis per se, but an artifact representing the epidermis [23]. Ultrasound studies examining healthy skin have demonstrated that while skin thickness is inversely proportional to age, echogenicity tends to increase with age corresponding to increase collagen production [24–28]. One notable exception is sun-damaged skin, as frst described in 1988 by Querleux et al., where there is presence of a subepidermal low-echogenic band (SLEB). Te SLEB was described as a dark band subjacent to the epidermal entry echo [29]. Tough not specifc to sun damage, its presence in more than 50% of adults over the age of 40 and its prominence in highly sun-exposed areas like the face and dorsal forearms have been correlated to solar elastosis, a histologic marker of cutaneous sun damage [25]. Similar fndings have been recently demonstrated in patients treated with radiotherapy for head and neck cancers [30]. Glabrous skin, hair follicles, and the scalp Compared to non-glabrous skin, glabrous epidermis appears as a bright bilaminar structure when visualized by ultrasound (Fig. 2). Te bilaminar structure is thought to be caused by sound waves frst entering the thick, compact stratum corneum before entering the normally hyperechoic epidermis itself [9]. Tis bilaminar appearance can be difcult to visualize at lower frequencies in which the stratum corneum is poorly resolved from the entry echo, but is discernable in UHFUS ranges [31]. Understanding the structure and characteristics of normal glabrous skin has allowed for diferentiation between abnormalities in glabrous skin in conditions like Pachyonychia Congenita and other palmoplantar keratodermas [32–34]. At other anatomic sites, hair follicles appear as oblique hypoechoic bands on ultrasound imaging. Multiple studies have demonstrated ultrasound can be used to estimate the phase of hair follicle growth, identify infammation of hair follicles, and assess hair density [18, 35, 36]. Hair follicle growth phase is estimated based on band termination relative to the subcutis with anagen phase hairs terminating in the deeper dermis and telogen phase hairs terminating more superfcially. An infamed hair follicle will appear thicker and darker relative to a normal hair follicle. A number of studies have suggested that the ultrasonographic fndings