Primary Authors: Johnna Torres, Katienne Yau, Vi Dinh
Many patients present to the emergency department, urgent care, and clinic with complaints of the skin and soft tissue. It is the goal of the provider to diagnose these patients and appropriately treat them. Though the gold standard imaging for diagnosing soft tissue pathology remains computed tomography (CT), beside point-of-care ultrasound provides a quick, affordable, and non-invasive option for initial work up. It also easily allows for manual compression, muscle contraction, and extremity movement with real-time imaging.
In this article, we will systematically perform an ultrasound examination of soft tissue. We will cover:
- Soft tissue indications and preparation
- Anatomy overview
- Common pathologies seen
Table of Contents
Soft Tissue Ultrasound Indications
Ultrasound can help distinguish between numerous pathologies, including infection, benign or neoplastic masses, pseudomasses, and lymphadenopathy. It can also be utilized during procedures such as incision and drainage of an abscess or removal of a foreign body. It is an excellent next step when there is uncertainty about a subcutaneous finding or the diagnosis is unclear.
Soft Tissue Ultrasound Preparation:
Patient Prep
Patient positioning is dependent on the area of interest (erect, supine, prone, decubitus). It is important to consider patient comfortability and clinician accessibility when scanning. Place the patient in a position that allows maximum exposure to the area of interest.
Machine Prep
If possible, place the machine to the right of the patient. As most subcutaneous structures are relatively superficial, the higher-frequency linear probe will provide appropriate balance between depth and resolution while allowing visualization of the entire lesion and surrounding structures. A lower-frequency curvilinear probe may be used if scanning deeper structures, large lesions, or patients with higher body mass index. Use the preset MSK exam setting if available.
Scanning Technique:
When performing soft tissue ultrasound, grip and stabilization are especially important as the potentially small surface areas require fine, controlled movements. Place the probe directly on the point of interest. Images should be obtained in both longitudinal and transverse views while sweeping through the entire lesion (or at least two planes 90° apart). Only light pressure should be applied to avoid compressing small vessels. Oftentimes, it is helpful to also scan a short distance from the area of interest to appreciate the appearance of the uninvolved anatomy. If needed, the contralateral part of the patient’s body can also be scanned for comparison.
Alternative Techniques:
Some soft tissues, particularly the hands and feet, can be difficult to ultrasound due to their uneven contour. Irregular body surfaces such as fingers, toes, or web spaces may require excessive pressure or manipulation to obtain optimal views. While application of additional ultrasound gel can help, it is still difficult to scan without causing discomfort.
Two alternative options to improve visualization of the soft tissue structures are using a water bath or using a store bought or homemade stand-off pad. When using the water bath, the patient’s hand or foot is immersed in a basin of lukewarm water. The extremity should be mostly, but not completely, submerged.
A stand-off pad can be purchased or made using a bag of saline or a glove filled with water or ultrasound gel. The fluid acts as an excellent medium to propagate transmission of the ultrasound. These techniques replace the need for direct contact between the transducer and patient.
Brief Anatomy Overview and Normal Ultrasound Findings:
Familiarity with the appearance of normal soft tissue is essential before diagnosing abnormal findings on ultrasound. The most superficial layer of the soft tissue is the skin, composed of the epidermal and dermal layers. Skin appears as a thin, hyperechoic line immediately under the probe. Deep to the skin is the subcutaneous tissue which is primarily composed of connective and adipose tissue with interspersed blood vessels and nerves. Subcutaneous tissue appears as hypoechoic fat lobules interspersed with hyperechoic connective tissue septa. The blood vessels and nerves in this layer can be recognized as anechoic circular or tubular structures, depending on the orientation of the probe compared to the structure. On either side of the subcutaneous tissue are fascial layers, intuitively called the superficial and deep fascia. These layers appear as thin, hyperechoic lines within and below the subcutaneous tissue. Deep to these structures are muscle and/or bone, depending on the location being examined. Muscles are hypoechoic with a striated pattern surrounded by hyperechoic fascia. Bones are hyperechoic structures with shadowing artifacts.
Soft Tissue Ultrasound Pathology
Edema
On ultrasound, edema appears as increased echogenicity with loss of distinction between the layers of soft tissue. Though ultrasound can reliably identify the presence of edema, it is important to note that edema from infectious causes appears similar to edema from non-infectious causes. Clinical correlation is key to interpreting the results of the scan. For example, edema from infectious causes is often unilateral with localized pain, erythema, and warmth. On the other hand, edema from non-infectious causes is often bilateral and affects multiple areas of the body. If the latter is suspected, it is helpful to obtain cardiac, pulmonary, and abdominal ultrasounds to narrow the differential diagnoses.
Cellulitis
Cellulitis is an infection of the deep dermis and subcutaneous tissue, and its appearance on ultrasound depends on the stage and severity. Initially, cellulitis may present with generalized edema and loss of the distinction between the layers of soft tissue. As the infection progresses, the superficial hyperechoic fat lobules become surrounded and separated by increasing amounts of anechoic or hypoechoic fluid. This is often described as “cobblestoning”. However, this is a non-specific finding that indicates inflamed tissue; therefore, clinical correlation is again necessary. Color doppler may be useful in this setting because inflamed tissue will often have increased vascularity and blood flow. With time, cobblestoning may lead to development of fluid tracts that appear as anechoic or hypoechoic spaces within the subcutaneous tissue. These tracts have the potential to form abscesses with a background of cellulitis.
Cysts/Abscesses
Cysts are sac-like structures that contain liquid, gaseous, or semisolid material while abscesses are pus-filled infections. Both can take on a variety of sizes and shapes and can look similar on ultrasound. However, compared to a cyst that will appear uniformly anechoic, an abscess will appear anechoic or hypoechoic with hyperechoic debris within the sac-like structure. Due to the presence of fluid, posterior acoustic enhancement may be present with both cysts and abscesses. In addition, the walls of an abscess may be distinct and hyperechoic or irregular and indistinguishable from adjacent tissue. When evaluating for an abscess, dynamic scanning with gentle compression of the probe may swirl the contents, which can be a diagnostic “squish sign.” If considering performing incision and drainage on an abscess, color or power Doppler may be helpful in identifying nearby blood vessels.
Necrotizing Fasciitis
Necrotizing fasciitis is a rapidly progressive, life-threatening infection with necrosis of the subcutaneous tissue, fascial planes, and muscle. Timely recognition and diagnosis is key to preserving life and limb. For this reason, using ultrasound to quickly diagnose this condition can be pivotal in providing the highest level of patient care. Ultrasound findings that are consistent with necrotizing fasciitis include thickening of the fascia and subcutaneous tissue, abnormal fluid accumulation in the deep fascia layer, and in extreme cases, subcutaneous air. A fluid layer measuring greater than 4 mm deep to the fascia greatly increases the likelihood of this pathology. There may also be hyperechoic pockets of gas with dirty posterior shadowing. This air can form an image of A-lines, similar to findings expected in the lungs. We have proposed a mnemonic for these criteria of “STAFF”, standing for Subcutaneous Thickening, Air, and Fascial Fluid.
Lipoma
Lipomas are a type of benign tumor composed of adipocytes. Fat is anechoic on ultrasound, so echogenicity of a lipoma depends on the presence of connective tissue or other reflective surfaces within the tumor. As a result, lipomas may appear hyper-, iso-, or hypoechoic when compared with adjacent soft tissues. A characteristic feature of lipomas is gently curved echogenic lines parallel to the skin. If color doppler is applied to these structures, there is typically no detectable blood flow. In addition, lipomas are often compressible.
Peripheral Nerve Sheath Tumors
Peripheral nerve sheath tumors are neoplasms that include schwannomas, neurofibromas, and malignant peripheral nerve sheath tumors. In most cases, a mass in direct continuity with a peripheral nerve is a pathognomonic finding for this pathology. On ultrasound, these appear as well-defined, round or oval, hypoechoic structures with increased posterior through-transmission. They can be confirmed by the presence of flow on color or power Doppler. Transducer pressure usually elicits symptoms. Although it can be difficult to differentiate schwannomas from neurofibromas, the former tends to be round, smooth, and vascularwhile the latter is fusiform, lobulated, and hypovascular. In addition, the involved nerve is characteristically delineated and eccentric with schwannomas compared to infiltrative and central with neurofibromas.
Foreign Bodies
Ultrasound is useful in the evaluation of potential foreign bodies within soft tissue, particularly radiolucent objects that are undetectable by plain radiography or CT imaging. Foreign bodies initially appear hyperechoic to the surrounding soft tissue. Over time, soft tissue reactions may produce a hypoechoic halo surrounding the foreign body. This is referred to as a “halo sign.” Artifacts are helpful in both identifying and locating the object and determining the type of material. It is important to note that the surface attributes of the foreign body contribute to the appearance of the object on ultrasound rather than its internal composition. For example, a foreign body with a smooth and flat surface produces posterior reverberation while a foreign body with an irregular and curved surface produces posterior shadowing. For optimal visualization, the probe must be parallel to the foreign body. Therefore, attempts should be made to scan the area of interest from various angles in order to eliminate anisotropy. Once the foreign body is located, the size of the object and depth from the skin may be measured. It is also important to locate any blood vessels or nerves near the foreign body that may be at risk if the object is removed.
Lymph Nodes
Lymph nodes are small, oval-shaped organs found along lymphatic vessels that filter particles from lymph. Though not often visualized, ultrasound findings of lymph nodes can help to provide information about potential underlying pathology. They are also often mistaken for another structure on ultrasound, appearing similar to a vessel on DVT ultrasound. For these reasons, being able to recognize the presence of lymph nodes on ultrasound is a key skill for a proficient ultrasound user.
Normal lymph nodes appear on ultrasound as oval, hypoechoic structures with an average transverse diameter of 8 mm or less. To determine if a lymph node has a normal ultrasonographic appearance, four aspects are appreciated. These include normal size, normal S/L ratio, an echogenic hilus, and hilar vascularity.
- As already noted, appropriate lymph node size is 8 mm or less measured transversely. An experienced sonographer can typically identify lymph nodes as small as 2 mm. A lymph node greater than 8 mm is more likely to be reactive, although there is some amount of normal variation. Likewise, a node that is 8 mm or less could also be reactive yet within normal range for size. It is also typical for nodes in the submandibular and upper cervical areas to be somewhat larger due to their proximity to the oral cavity. Because of these variations, it is important to monitor trends rather than make a diagnosis from a single exam.
- Short/long (S/L) ratio refers to the short axis to long axis ratio. This can be calculated by measuring the lymph node transversely and longitudinally. S/L ratio of less than 0.5 (twice as long as it is wide) is considered normal, with the exception of the submandibular and parotid nodes, which are often more round than oval and therefore have a higher S/L ratio.
- An echogenic hilus refers to a hyperechoic structure located at the center of the node and is continuous with surrounding soft tissue structures. This represents the nodal center which is composed of fatty tissue and vasculature.
- On color doppler or power doppler, a normal node will display vascularity emerging from the hilum, or apparent avascularity. This normal appearing hilar vascularity is in contrast with peripheral vascularity seen in many malignant lymph nodes.
Various lymph node pathologies can be noted on ultrasound including reactive lymph nodes, metastatic lymph nodes, and lymphoma. When compared to normal lymph nodes, reactive lymph nodes are generally enlarged with a preserved S/L axis, less distinct echogenic hilum, and increased hypoechoicity. These benign reactive nodes have preserved or reduced hilar vascularity, without peripheral vascularity. Metastatic lymph nodes and lymphoma generally have a round and hypoechoic appearance. They lack an echogenic hilum and demonstrate peripheral or mixed vascularity rather than the classic hilar blood flow. It is not uncommon to see intranodal calcifications or cystic necrosis in these metastatic lymph nodes.
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