It is a kind of infection that diffuses on the skin and in subcutaneous tissues. There might be injury on the skin. It may be characterized with local sensitivity, pain, edema, and erythema. The most common pathogens are gram (+) aerobic cocci (S. aureus, S. pneumoniae, S. pyogenes) and aerobic and anaerobic gram (−) bacteria. US guides in visualizing the progression of local infection and identifying the necessary interventional procedures [13]. In the US, edematous area (echogenicity increase in heterogeneity) and fibrous connective tissue (increase in hyperechogenicity) become more evident. Increased vascularization pattern helps the diagnosis in CDUS [14].

Cellulitis is treated with antibiotics. In the event that abscess develops, surgical intervention is required. In some cases, an erythematous line, in other words lymphangitis, might extend from cellulitis plaque to the proximal.

It is the collection of pus in the dermis or in deeper layers. Majority of the skin abscesses are polymicrobial, and S. aureus is the only factor in 25 % of the cases. Surgical drainage is the most effective treatment in the abscess. Systemic antibiotic treatment is rarely required. US appearance is quite changing (Fig. 8.2). It changes from anechoic fluid collection to irregular hypoechogenicity, to internal echoes, to hyperechoic sediments, to septa, and even to gas formation. Occasionally, only posterior acoustic enhancement is detected. In CDUS, increased vascularization in the abscess wall and adjacent tissues supports the diagnosis. In a PD study, peripheral vascularity increase was observed in 90 % of the cases [15].

It is a rarely seen progressive soft tissue infection that emerges following minor injuries [16]. Most of the infections are polymicrobial [17]. S. pyogenes is the most common pathogen. Chronic diseases (diabetes and immunosuppressive), IV medication administration, and malignity are among the various factors that increase the predisposition to this disease. The diagnostic value of the US is low at the early period, and fascia gets thicker and fluid collects in deep fascia. The fact fluid collected is measured 4 mm at the thickest part is significant in necrotizing fasciitis [18]. In the advanced cases, the possible abscess and gas formation can be indicated [19].

Early diagnosis, wide-spectrum antibiotic treatment, debridement which cleans all necrotic tissues, regulating liquid-electrolyte balance, regulating the oxygenation of the infected area, nutritional support, and analgesia promotion constitute the basis of the treatment [20].

The subcutaneous soft tissue changes that emerge following the trauma might range from minor hemorrhage to fat necrosis, hematoma, and abscess. While the hemorrhage is visualized as a hyperechogenic structure at the early period, fat necrosis is visualized as the hypoechoic areas developed secondary to the necrosis within hyperechoic focus. However, in hematoma, US findings change in time [21]. While it appears to be pseudo-solid at the beginning, it becomes anechoic in time. Within almost a month, hematoma is absorbed. If open wound and abscess formation are observed in hematoma emerged following a trauma, the presence of foreign objects should be investigated. Morel-Lavallee syndrome is the intense seroma that collects between deep subcutaneous soft tissue and fascia in traumas that occur in the lateral side of the upper calf. US illustrates a plenty amount of liquid collection on the echogenic fascia [22]. It is possible to detect the fat necrosis that occurs in this disease and the other affected deep-located structures.

Foreign bodies might be seen in subcutaneous soft tissue generally following open wounds or perforating traumas. The decision of removing the foreign body is made in line with the size, location, structure, accessibility, and mechanic and inflammatory effects of the object. In case that the pieces that cannot be removed keep staying within the wound and the body cannot figure this foreign object out, a fibrous capsule surrounds it. Inflammation, infection (wound infection, cellulitis, toxicity, tenosynovitis, bursitis, osteomyelitis), and mechanical damage might occur. Foreign object lesions do not have a real neoplasm but are traumatic reactive masses [23]. The sensitivity of the US in foreign bodies is 43–98 % and specificity ranges between 59 and 98 % [24]. It can show wood, bone, sea urchin, and other plantlike objects. A hypoechoic halo like a band occurs around the foreign object in US. It helps it to be differentiated from the neighboring soft tissues. This hypoechoic halo shows increased vascularization pattern in CDUS [25]. Calcification deteriorates the appearance with sesamoid bone and tendon acoustic shadowing. It might be difficult to place probe in places such as interdigital sites, where foreign bodies are found very commonly.

The muscles, which are located around the skeleton, which enable movement, and that we move voluntarily, are skeletal muscles. They make up averagely 40 % of body weight. Skeletal muscle is made up of the combination of hundreds of muscle cells called skeletal muscle fibers with the height ranging between 1 mm and 30 cm. Skeletal muscles have two components, which are muscle fibrils and stromal connective tissue. Fibrils make up fascicles, and fascicles make up muscles. Since the skeletal muscles are structures that can extend from one joint to another, long linear probes are used when they are examined. Two adjacent muscle tissues appear as hyperechoic band formation (Fig. 8.3).

While muscles are visualized as homogenous, multiple, thin parallel echoes on the longitudinal plane, they are more irregular, dispersed thin echoes on the transverse plane. Bright echogenic connective tissue fascia is present around the muscle.

The pathologies of muscles can be assessed with US. These are strain/tear, hematoma, myositis ossificans, myositis, compartment syndrome, rhabdomyolysis, hernia, and tumors [26].

US can detect the presence, the location, and the severity of the anatomic damage. US demonstrates the volume increase, edema, or fluid-pus collection in a detailed manner [27] (Fig. 8.4).

Tendons connect the muscles to the bones. They help the bones move by transferring the load of the muscle to the bone. Water constitutes 70 % of the wet weight of the tendon. The most visible cells in the tendon are fibroblasts. Tendon is made up of Type I collagen fibrils. Fibrils combine and make up fibers and fibers combine and make up fascicles, which contain blood, vessels, and nerves. This structure helps the tendon to stress and stretch [34].

US is the primary option in tendon pathologies because it allows for dynamic examination. It should be performed along with both longitudinal and axial planes. In longitudinal examination, tendons appear as hyperechoic bands similar to stria, with uniform thickness, and as thin parallel fibril echoes. In transverse examination, round or oval echoic spots with homogenous distribution are visible.

Synovium is visualized as thin hypoechoic frame at the anterior and posterior sides of the tendon. Paratenon (fibrous connective tissue) is visualized as ambiguous hyperechoic tissue that continues with subcutaneous fatty tissue.

Tendon pathologies include tendinosis and partial tear, tendon dislocation, full-thickness tears (Figs. 8.8 and 8.9), inflammatory processes (tenosynovitis and peritendinitis), tendon tumors, and postoperative findings.