Musculoskeletal ultrasonography basics
ABSTRACT
Utrasonography is emerging as a core method to evaluate musculoskeletal problems. It is best used for imaging superficial structures limited to 1 quadrant of a joint. It has several advantages over other imaging methods: lower cost, ability to perform dynamic examinations, higher spatial resolution of superficial structures, better patient comfort, and essentially no contraindications.
KEY POINTS
- Ultrasonography can be used to evaluate small fluid collections in soft tissue; joint effusions and synovitis; soft tissue masses (≤ 5 cm in diameter); tendon, ligament and muscle injuries; and peripheral nerve entrapment and lesions.
- Ultrasonography is not appropriate for survey examinations of vague or diffuse symptoms or for evaluating soft-tissue areas more than a few centimeters in diameter or more than a few centimeters deep.
- Musculoskeletal ultrasonography requires specially trained sonographers and interpreting physicians.
STRENGTHS OF MUSCULOSKELETAL ULTRASONOGRAPHY
Ultrasonography has multiple advantages:
No ionizing radiation exposure.
Portability. Unlike CT or MRI, ultrasonography equipment is portable.
Increased patient comfort. Patient positioning for an ultrasonography examination is more flexible than for MRI or CT,14 and the examination does not induce claustrophobia.8
High-resolution imaging. Ultrasonography provides very-high-resolution imaging of superficial soft tissues—in some cases, higher than MRI or CT.
Real-time dynamic examinations are possible with ultrasonography, unlike with CT or MRI, and may increase test sensitivity.4,15–18
Implanted hardware is less of a problem. Although ultrasonography cannot image beyond implanted orthopedic metallic hardware, the hardware does not obscure surrounding soft tissues as it does on CT and MRI.6,19,20 Also, ultrasonography is safe for patients with a pacemaker.8
WEAKNESSES
The main disadvantages of musculoskeletal ultrasonography are inherent to its limited field of view, making it inappropriate for a survey examination (eg, for ankle pain, knee pain, hip pain).4 Unlike CT and MRI, ultrasonography does not provide a “bird’s-eye view,” and important abnormalities can be missed during evaluation of large areas (Figure 4).
Ultrasonography also cannot evaluate bone or intra-articular structures such as cartilage, bone marrow, labrum, and intra-articular ligaments; MRI is the standard for evaluating these structures.21
Ultrasonography is time-consuming. To perform a detailed examination of the anterior, posterior, medial, and lateral aspects of the hip, knee, or ankle would require 1.5 to 2 hours of scanning time and an additional 10 to 25 minutes of image checking and interpretation.
CURRENT CLINICAL INDICATIONS
Musculoskeletal ultrasonography is best used for clinical questions regarding limited, superficial musculoskeletal problems.
Fluid collections
Ultrasonography can help evaluate small fluid collections in soft tissue. As is true for a lung opacity on chest radiography, soft-tissue fluid detected on ultrasonography is nonspecific, and results must be correlated with the clinical picture to narrow the differential diagnosis.
Fluid collections can be classified as loculated or nonloculated.
Nonloculated fluid involves more fluid than is simply interposed between tissue planes and has no wall or defined margins. It can be simple or complex in appearance: simple fluid is anechoic, and complex fluid appears more heterogeneous and may contain septations or debris.
Subcutaneous edema, which may occur postoperatively or from trauma, venous insufficiency, or inflammatory or infectious processes, appears on ultrasonography as nonloculated fluid interspersed between subcutaneous fat lobules.
Loculated fluid collections have well-defined margins or a discrete wall that does not follow normal tissue planes. They can also be simple or complex and can be caused by hematoma, abscess, or ganglion. Less commonly, neoplasms can mimic a loculated fluid collection (Figure 4).
A ganglion is a specific type of loculated fluid collection containing synovial fluid arising from a joint or tendon sheath. It tends to occur in specific locations, most commonly around the wrist, most often arising from the dorsal scapholunate ligament and volar wrist between the radial artery and flexor carpi radialis.22 On MRI, it can be difficult to distinguish between small vascular structures and a small ganglion, especially in the hands and feet.23
Ultrasonography can also help identify a Baker cyst, a specific fluid collection arising from the semimembranosus bursa between the medial head of the gastrocnemius tendon and the semimembranosus tendon. Ultrasonography can also detect inflammation, rupture, or leaking associated with a Baker cyst.24
Power Doppler is an ultrasonographic examination that can detect increased blood flow surrounding a fluid collection and determine the likelihood of an acute inflammatory or infectious cause.25
Joint effusion and synovitis
Musculoskeletal ultrasonography can help evaluate joints for effusion and synovitis. It is highly sensitive (94%) and specific (95%) for synovitis, making it superior to contrast-enhanced MRI.26,27 The area of concern should be limited to 1 quadrant of a joint (anterior, posterior, medial, or lateral); for problems beyond that, MRI should be considered.
A joint effusion appears as a distended joint capsule containing hypoechoic (complex) or anechoic (simple) joint fluid.
Complex joint fluid may contain debris and occurs with hemarthrosis, infection, and inflammation.23 Hypertrophied synovium is hypoechoic and can mimic complex joint fluid.
Power Doppler evaluation can help distinguish synovitis from joint fluid by demonstrating blood flow, a feature of synovitis but not of simple joint fluid. Power Doppler is the most sensitive means of detecting blood flow, although it does not show direction of flow.28
Using ultrasonography can help to improve disease control and minimize disabling changes by monitoring synovitis therapy. In addition, subclinical synovitis and enthesitis (inflammation of insertion sites of tendons or ligaments into bone) detected by ultrasonography may predict future disease and disease flares.29–31
Ultrasonographic guidance for a wide range of procedures is increasing rapidly.32–36 Multiple studies have shown the advantage of ultrasonography-guided aspiration and injection compared with techniques without imaging guidance.37,38
Soft-tissue masses
Accurately diagnosing soft-tissue masses can be difficult. A mass may remain indeterminate even after multiple imaging studies, requiring biopsy or surgical referral. However, for a few specific masses, ultrasonography is highly accurate and can eliminate the need for further imaging.
Ultrasonography can help evaluate soft- tissue masses no larger than 5 cm in diameter and no deeper than superficial muscular fascia. If the mass is larger or deeper than that, ultrasonography is less reliable for showing the margins of the mass and its relationship to adjacent structures (Figure 5). Further imaging by MRI may be recommended in such cases.
Fortunately, many of the most common soft-tissue masses can be accurately diagnosed with ultrasonography, including lipomas, ganglion cysts, foreign bodies, and simple fluid collections.4,39 Nerve-sheath tumors can also be diagnosed with ultrasonography if the lesion clearly arises from a nerve. Other soft-tissue masses are likely to be indeterminate with ultrasonography, requiring follow-up with MRI with contrast.
