SONAR and by an American Neurologist, Karl Dussik, who used ultrasound to detect brain tumors in patients [5]. Open Access *Correspondence: suephy.chen@duke.edu 5 Department of Dermatology, Duke University School of Medicine, Duke Clinic, 40 Duke Medicine Cir, Clinic 3K, Durham, NC 27710-4000, USA Full list of author information is available at the end of the article Levy et al. Ultrasound J (2021) 13:24 Page 2 of 12 Ultrasound emerged in the practice of dermatology around 1980 when two independent groups used the technique to measure skin thickness in normal and diseased skin [6]. Since then, the practice has gained popularity in several European and South American countries that incorporate its use in resident training and where its application is reimbursable [7, 8]. As the technology has improved, so has its utility. In an early study of over 4300 skin lesions, Wortsman et al. demonstrated the ability of higher frequency ultrasound (7–15 MHz) to signifcantly improve diagnostic accuracy. Compared to clinical examination alone, ultrasound corrected the referring diagnosis in 17% of cases and detected 3 missed malignancies [9]. Te application of ultrasound in dermatology is continuously evolving and increasingly studied [10, 11]. Ultrasound technology and how it works Ultrasound allows for high-resolution imaging of the skin from the stratum corneum down to the deep fascia [12]. Newer ultrasound technologies, including high-frequency ultrasound (HFUS, between 20 and 30 MHz) and ultra high-frequency ultrasound (UHFUS,>30 MHz), have further enhanced imaging clarity and expanded upon the myriad applications for use of ultrasound in dermatology [11, 13, 14]. With fast acquisition times, ultrasound images are produced in real-time, allowing for physical adjustments and optimal imaging. Many devices include color Doppler analysis for characterization of blood fow and vessel morphology. It is readily available and can be easily incorporated into clinical practice [10, 11, 15]. Te main component of the ultrasound device is the transducer—typically a handheld device containing thousands of piezoelectric crystals. Tese crystals, when stimulated by an electric voltage, generate millions of acoustic waves that pass through tissue and are refected back to the transducer. As the waves return to the transducer, they are converted back into electrical energy interpreted by a computer. Te computer then generates a onedimensional line graph in amplitude mode (A-mode) that can be used to interpret echogenicity at various distances from the probe. In brightness mode (B-mode) an image is generated with diferent intensities of brightness. Tese varying intensities are dependent upon the ultrasound waves’ ability to pass through the tissue. Bright or hyperechoic regions represent an abrupt increase in tissue density while dark or hypoechoic areas represent a decrease or no change in tissue density. A black anechoic region represents an area of tissue from which virtually no waves are refected [5]. Although dense structures refect a greater number of waves, resulting in a hyperechoic ultrasonographic structures, they may also generate an artifactual acoustic shadow due to attenuation of the ultrasound waves at the interface between two tissues of varying densities [13]. Another common artifact, acoustic enhancement, can appear as a result of relatively unimpeded passage of ultrasound waves through a preceding area of low acoustic impedance. Tis results in the appearance of a brighter structure deep to the area of low impedance [16, 17]. Te resolution and penetration of an ultrasound image depends on the frequency of the acoustic waves produced. Traditional lower-frequency ultrasound devices are ideal for evaluation of internal organs like the liver and lungs due to their penetration ability. HFUS and UHFUS waves, on the other hand, are unable to travel easily through tissue, but provide higher-resolution, superfcial detail. Tis makes HFUS and UHFUS ideal for visualizing a superfcial organ like the skin [5, 10, 11, 15, 18, 19]. Given the diverse range of probe frequencies and the variation of dermal thickness in diferent regions of the body, specifc frequencies should be considered