and perpendicular scan in the center of each lesion or anatomical area with and without pressure. Ten, 3D-views were simulated using two sweeps in both planes over 3–8 s for a total of three to eight 2D-slices in each plane. Normal skin of the non-sun exposed medial upper arm of a middle-aged man demonstrated a hyperechoic epidermal entry echo and hyperechoic dermis with horizontally arrayed collagen bundles of the reticular dermis. Tese superfcial structures are followed by the hypoechoic subcutis transected by a hyperechoic band of superfcial fascia and the hypoechoic biceps muscle invested by the hyperechoic deep fascia (Fig. 1). Te thickened stratum corneum over the thenar eminence demonstrated a hyperechoic bilaminar entry echo (Fig. 2). Ultrasound of a frst digit nail also demonstrated a bilaminar appearance with hyperechoic dorsal and ventral plates separated by a hypoechoic band (Fig. 3). Finally, a basal cell carcinoma on the medial mandibular cheek of an elderly man demonstrated a hypoechoic lesion in the superfcial dermis with subtle posterior acoustic enhancement (Fig. 4). Infammatory skin diseases Sclerosing disorders One of the earliest dermatologic applications of ultrasound was in scleroderma [61]. Due to the pathological increase in collagen deposition, ultrasound can help differentiate between the infammatory and sclerotic phases of the disease [62]. In the infammatory phase there is a reduction in echogenicity of the skin and dermal thickening, whereas in the sclerotic phase there is increased echogenicity and thickening [63]. Finally, ultrasonographic features of sclerodermatous skin in the atrophic phase demonstrates thinning of the dermis with an echogenicity similar to unafected surrounding skin [64]. Te modifed Rodnan score is used to measure skin hardening. Skin lesions are scored on a 4-point scale from 0 to 3. Tis score is applied to 17 discrete areas of the body after evaluation by a clinician. A study found that 18-MHz ultrasound is more sensitive than clinical assessment in measuring skin hardening, with results of ultrasound providing the additional benefts of being quantitative, valid, and reproducible [65]. In a study of 16 patients with morphea and four with extragenital lichen sclerosis, the results of HFUS demonstrated unique features of extragenital lichen sclerosis characterized by widened, polycyclic, hyperechoic and disorganized epidermal entry echo relative to that seen in morphea [62]. Another study using UHFUS to measure response to treatment demonstrated a statistically signifcant decrease in dermal thickness and density after treatment with ultraviolet A1 phototherapy [66]. Psoriasis Currently, evaluation of psoriasis in a clinical setting includes the use of the Psoriasis Epidemiology Screening Tool, a validated screening tool designed to identify patients who should be referred to a rheumatologist for possible co-existing psoriatic arthritis. Rheumatologists, on the other hand, use the Classifcation Criteria for Psoriatic Arthritis (CASPAR) to characterize psoriatic arthritis disease burden. A 2015 study used ultrasound (6–18 MHz) to determine the prevalence of enthesitis, a feature in the pathogenesis of psoriatic arthritis, among psoriasis patients with and without psoriatic arthritis as defned by CASPAR criteria [67]. Te study found at least one ultrasonographic sign of enthesitis in approximately half the patients with psoriatic arthritis, but also in Levy et al. Ultrasound J (2021) 13:24 Page 7 of 12 almost half of the psoriasis patients without a diagnosis of psoriatic arthritis [67]. Furthermore, after these patients were treated for 6 months with systemic therapy, ultrasonographic features of enthesitis in both of these groups improved, suggesting subclinical joint involvement and underdiagnosis of psoriatic arthritis among psoriasis patients [67]. In a 2013 study, researchers found that 78% of patients with psoriasis had at least one nail abnormality identifed by ultrasound: loss of ultrasonographic architecture, pitting, or both [68]. Tis study suggested that OCT was better than ultrasound at 9–14 MHz for identifying features of psoriatic nails, but both were more sensitive than clinical examination alone [68]. In a later study