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Soft Tissue Ultrasound

Quick Image Reference

Figure 1: To image deep structures such as the thigh muscles as shown here, a low frequency curvilinear transducer may be used.

Figure 2: Normal skin, subcutaneous tissue, and fascia.

Figure 2A: Normal skin, subcutaneous tissue, and fascia. (Slider )

Figure 3: Longitudinal image of muscle.

Figure 4: Transverse image of muscle.

Figure 5: Early cellulitis.

Figure 6: Cobblestone appearance of advanced cellulitis.

Figure 7: Small ganglion cyst.

Brian Euerle, MD, RDMS

I.  Introduction and Indications

Many patients come to the emergency department with complaints related to the soft tissues, e.g., infection, injury, and abnormal masses. Traditionally, computed tomography and magnetic resonance have been used when imaging studies are needed in these patients; however, ultrasound is generally more readily available and in some instances is the preferred imaging study.(1)

The most common use of bedside ultrasound in patients with soft-tissue abnormalities is in the evaluation of infections, including cellulitis, abscess, and necrotizing fasciitis. Other soft-tissue indications include the evaluation of cysts and lymph nodes. Ultrasound is also used to locate foreign bodies.

Although the majority of abscesses are treated with incision and drainage, in certain cases, usually because of cosmesis, treatment with needle aspiration and antibiotics may be an option.(3,7) Ozseker and colleagues(7) found that ultrasound-guided aspiration and irrigation of breast abscesses was preferred to surgical drainage for abscesses with a diameter less than 3 cm.(7) Ultrasound provides dynamic real-time guidance for needle aspiration, resulting in increased success.

II.  Anatomy

The structures that are imaged in soft-tissue bedside ultrasound are primarily the skin, subcutaneous tissue, fascia, and muscle.

The skin consists of two layers: the superficial epidermis and the deeper, thicker dermis. The subcutaneous tissue, located beneath the dermis, consists of connective tissue septa and fat lobules.(2) Fascia, a deeper structure, is a dense, fibrous membrane.(3) Muscle consists of long muscle fibers grouped together into fascicles. Various layers of connective tissue surround the individual muscle fibers, fascicles, and entire muscles.

III.  Scanning Technique, Normal Findings and Common Variants


Equipment

Selection of the appropriate transducer is an important aspect of soft-tissue bedside ultrasonography. Because most structures to be imaged are relatively superficial, a high-frequency (7–12 MHz) linear transducer is most useful and provides a good balance between imaging depth and resolution. On occasion, deeper structures need to be visualized; in these cases, a lower-frequency transducer is required. If a lower-frequency linear transducer is not available, then a curvilinear transducer may be used (Figure 1).

Figure 1: To image deep structures such as the thigh muscles as shown here,
a low frequency curvilinear transducer may be used.



Scanning Technique
The sonographer’s grip on the transducer is especially important in soft-tissue ultrasound because fine, controlled movements of the transducer are often required. The transducer should be held such that part of the hand, such as the small finger or ulnar aspect, rests against the patient’s body to stabilize the transducer.(4)

In many cases, it is helpful to begin scanning a short distance away from the area of interest to gain an appreciation of the appearance of the normal, uninvolved anatomy. Then the transducer can be slid toward the area of interest. It can be helpful for the sonographer to place a finger of the hand not holding the transducer on the area of interest and then slide the transducer until it contacts the finger.

Images should be obtained in both longitudinal and transverse planes, which will provide the most information and allow accurate localization.(5) It may be helpful to view the contralateral side of the patient’s body to obtain information about the normal appearance of structures.(5)

Normal Findings
With the equipment that is typically used for bedside ultrasonography, the epidermis and dermis cannot be differentiated. They appear together as a thin, hyperechoic layer.(2) The subcutaneous layer appears hypoechoic on ultrasound, with two components: hypoechoic fat interspersed with hyperechoic linear echoes running mostly parallel to the skin, which represent connective tissue septa (Figure 2).(2) Veins and nerves may be visualized within the subcutaneous layer.

 

Figure 2: Normal skin, subcutaneous tissue, and fascia.

Figure 2A: Normal skin, subcutaneous tissue, and fascia. ( Slider )

Fascia appears as a linear hyperechoic layer. Its thickness may vary depending on the location.(3)

Muscle fascicles can be visualized as hypoechoic cylindrical structures, with hyperechoic connective tissue (perimysium) surrounding them (Figures 3 and 4).(3) A characteristic feature of muscle is that its appearance on ultrasound can change with contraction; it appears thicker and more echoic on short-axis planes when contracted.(6)

 

Figure 3: Longitudinal image of muscle.

Figure 4: Transverse image of muscle.

Lymph nodes are hypoechoic and generally oval.(7)

IV. Pathology

Cellulitis
Cellulitis is the pathologic condition most frequently encountered during soft-tissue bedside ultrasonography. The ultrasound appearance of cellulitis varies depending on the stage and severity. The initial appearance may be generalized swelling and increased echogenicity of the skin and subcutaneous tissues (Figure 5).(8,9) As cellulitis progresses and the amount of subcutaneous fluid increases, hyperechoic fat lobules become separated by hypechoic fluid-filled areas. This later stage of cellulitis is most typical and has been described as having a cobblestone appearance (Figure 6).(8) The appearance of fluid-filled interlobar septae is not specific for cellulitis but rather of generalized subcutaneous edema, which may result from other conditions such as congestive heart failure.(8)


Figure 5: Early cellulitis.

Figure 6: Cobblestone appearance of advanced cellulitis.

Color and power Doppler are advanced ultrasound modalities that can help determine whether the inflammation characteristic of cellulitis is present. However, their use in the emergency department is beyond the skills of most emergency practitioners, who will instead rely on the physical examination for clinical correlation.

Tayal and colleagues studied the use of bedside ultrasound in emergency department patients with signs of cutaneous soft-tissue infection but no signs of obvious abscess.(10) The use of ultrasound was found to change the management of a significant percentage of the study group. By routinely utilizing ultrasound in the evaluation of these patients, the physicians were able to avoid unnecessary drainage procedures as well as detect occult abscesses. This research supports the use of ultrasound in the evaluation of most patients with cellulitis.

Necrotizing Fasciitis
Necrotizing fasciitis is an uncommon, severe, life-threatening infection that involves the subcutaneous tissue, fascia, and muscle. On ultrasound it appears as thickened distorted fascia with adjacent hypoechoic fluid collection, along with swelling of the subcutaneous tissue and muscle.(8) Small foci of gas can appear as bright echogenic areas with posterior acoustic shadowing.(2,8,11)

Morrison et al(11) used ultrasound to evaluate emergency department patients with Fournier’s gangrene and found subcutaneous scrotal gas in all cases. The authors concluded that bedside ultrasound was very useful in the evaluation of these patients.

Cysts
Ultrasound can help differentiate solid from cystic masses. Ganglion cysts are a common cystic mass that arises from tendon sheaths and joints (Figure7). They are filled with a gelatinous material. On ultrasound they appear anechoic and may cause posterior acoustic enhancement.(12) It may be possible to trace the cyst to its origin from the tendon sheath or joint space.

Figure 7: Small ganglion cyst.

Lymph Nodes
Normal and reactive lymph nodes have the same ultrasound appearance: they are hypoechoic, with an echogenic hilus, and are generally oval.(7) [hyperlink to ENT-Lymphadenitis] Malignant lymph nodes are usually hypoechoic, without an echogenic hilus, and round. This distinction is beyond the skill and experience of most emergency physicians, who should not attempt to differentiate these two entities. However, they can use ultrasound to identify a mass as a probable lymph node.

Foreign Bodies
Ultrasound is useful in the evaluation of wounds and other soft tissue that may contain foreign bodies. [hyperlink to Foreign Bodies] It is especially useful in the search for radiolucent foreign bodies, which are difficult or impossible to detect by plain radiography or computed tomography.

 

 

V.  Pearls and Pitfalls

  • A high-frequency linear transducer is most useful for soft-tissue ultrasonography. In some cases, a lower-frequency curvilinear transducer may be needed to visualize deeper structures.

  • Holding the transducer in such a way that part of the examiner’s hand rests on the patient will stabilize the transducer and allow the fine, controlled movements that are necessary.

  • Start scanning on nearby, uninvolved body areas to gain an appreciation of normal anatomy. Scanning the contralateral corresponding body part can also help in this regard.

  • Cellulitis typically has a cobblestone appearance on ultrasound.
    Necrotizing fasciitis appears as thickened, distorted fascia with adjacent fluid and may have small echogenic foci of gas.

  • Cysts appear anechoic with posterior acoustic shadowing.
    Normal and reactive lymph nodes appear hypoechoic, with an echogenic hilus, and are generally oval.



 

VI.  References

  1. Nazarian LN.
    The top 10 reasons musculoskeletal sonography is an important complementary or alternative technique to MRI. Am J Roentgenol. 2008;190(6):1621-6.

  2. Valle M, Zamorini MP.
    Skin and subcutaneous tissue. In: Bianchi S, Martiloni C, eds. Ultrasound of the Musculoskeletal System. Springer: Berlin; 2007:19-43.

  3. O’Neill J.
    Introduction to musculoskeletal ultrasound. In: O’Neill J, ed. Musculoskeletal Ultrasound: Anatomy and Technique. Springer: New York; 2008:3-17.

  4. Jacobson JA.
    Fundamentals of Musculoskeletal Ultrasound. Saunders: Philadelphia; 2007.

  5. Kaplan PA, Matamoros A Jr, Anderson JC.
    Sonography of the musculoskeletal system. Am J Roentgenol. 1990;155(2):237-45.

  6. Zamorini MP, Valle M.
    Muscle and tendon. In: Bianchi S, Martinoli C, eds. Ultrasound of the Musculoskeletal System. Springer: Berlin; 2007:45-96.

  7. Ahuja AT, Ying M.
    Sonographic evaluation of cervical lymph nodes. Am J Roentgenol. 2005;184(5):1691-9.

  8. Chau CL, Griffith JF.
    Musculoskeletal infections: ultrasound appearances. Clin Radiol. 2005;60(2):149-59.

  9. Chao HC, Lin SJ, Huang YC, Lin TY.
    Sonographic evaluation of cellulitis in children. J Ultrasound Med. 2000;19(11):743-9.

  10. Tayal VS, Hasan N, Norton HJ, Tomaszewski CA.
    The effect of soft-tissue ultrasound on the management of cellulitis in the emergency department. Acad Emerg Med. 2006;13(4):384-8.

  11. Morrison D, Blaivas M, Lyon M.
    Emergency diagnosis of Fournier’s gangrene with bedside ultrasound. Am J Emerg Med. 2005;23(4):544-7.

  12. Hwang S, Adler RS.
    Sonographic evaluation of the musculoskeletal soft tissue masses. Ultrasound Q. 2005;21(4):259-70.
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