### Ultrasound Diagnostics in Dermatology and Cosmetology  
The last decade has been characterized by the popularization and wider use of modern ultrasound equipment. The development of high-frequency and ultra-high-frequency transducers has made it possible to detect anatomical changes with high resolution, even comparable to histologic examination [16]. There has been a transition from the use of ultrasound in experimental research to its application in clinical practice. Despite the fact that publications on the use of ultrasound for measuring skin thickness appeared more than 40 years ago in 1979 [17], the development of this diagnostic method and active application only began with the advent of high-frequency and ultra-high-frequency transducers. The number of publications in this field began to grow, and guidelines for conducting surveys appeared with the accumulation of experience and the analysis of the research results [18; 19]. The research work on the evaluation of the characteristics of the devices and transducers available on the market, the technical requirements for them [20], and various methods of analyzing ultrasound images of the skin have made it possible to select transducers depending on the purpose of the study (Table 1) [21]. The EFSUMB Committee (European Federation of Societies for Ultrasound in Medicine and Biology) has developed a series of statements on a coordinated position regarding the main areas of dermatological ultrasound (technical requirements, normal skin and its appendages, inflammatory, and tumor skin diseases) and requirements for aesthetic dermatology and practical training [22; 23]; an international working group, DERMUS (Dermatological Ultrasound), has also been established, consisting of doctors who work on a regular basis and publish articles on dermatological ultrasound [24]. All of these factors influence the daily practice of dermatologists, as they provide important information for practical applications [25]. In aesthetic cosmetology, skin examination is useful at every stage of treatment, from planning and monitoring to evaluating the results of treatment and possible complications [26]. Dermal fillers are clearly visible on ultrasound images; hyaluronic acid is the most homogeneous and is easily identified as a rounded anechoic space [27]. Currently, the number of procedures associated with the injection of botulinum toxin type A [28; 29] and skin fillers is steadily increasing; unfortunately, the number of complications is also growing accordingly [30; 31]. The most difficult to diagnose are late complications, manifested by nodules or granulomas, presenting diagnostic and therapeutic problems due to the lack of uniform standards or guidelines. With the advent of high-frequency ultrasound imaging, it became possible to detect and identify fillers and granulomas [32; 33], as did introducing, under the control of ultrasound, hyaluronidase into areas of deposits and incorrect correction with hyaluronic acid [34; 35]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Frequency (MHz)** & **Depth of Penetration (mm)** & **Visualized Structures** \\ \hline
7.5 MHz & \(>\)40 & Lymph nodes, deep structures \\
10 MHz & 35 & Epidermis, dermis, subcutis \\
20 MHz (high frequency & 10 & Epidermis, dermis, part of \\ ultrasound) & & subcutaneous tissue \\
50 MHz & 3–4 & Epidermis and dermis \\
75 MHz & 3 & Epidermis and part of dermis \\
100 MHz & 1.5 & Epidermis only \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Visualized structures according to the used sonographic frequency (MHz) and penetration depth (mm). There have been publications on the use of ultrasound control in anti-cellulite therapy [36; 37]. The authors noted the smoothing of the boundaries between the skin and subcutaneous tissue and a decrease in the strands penetrating into the dermis with ultrasonic control [38], thereby assessing the effectiveness of treatment in these studies.