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Musculoskeletal ultrasonography basics

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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.


 

References

Ultrasonography has been used to evaluate musculoskeletal problems for decades but has only recently become more widely available in the United States. Advances in technology and physician familiarity are increasing its role in orthopedic imaging.

See related editorial

No single imaging method can yield all musculoskeletal diagnoses. Like any imaging technique, ultrasonography has strengths and weaknesses specific to orthopedics. Radiography, computed tomography (CT), and magnetic resonance imaging (MRI) play important roles for investigating musculoskeletal problems and are complementary to each other and to ultrasonography.

To help clinicians make informed decisions about ordering musculoskeletal ultrasonography, this article reviews the basic physics underlying ultrasonography, its advantages and disadvantages compared with other imaging methods, and common clinical applications.

CLASSIC TECHNOLOGY MAKING A RESURGENCE

The first reports of the use of musculoskeletal ultrasonography appeared in the 1970s for investigating the rotator cuff,1–3 actually preceding reports of its use in obstetrics and gynecology.4 In the 1980s, reports emerged for evaluating the Achilles tendon.5,6 After that, its popularity in the United States plateaued, likely because of the advent of MRI, lower reimbursement and greater variability in interpretation compared with MRI, as well as a lack of physicians and sonographers trained in its use.7,8

Musculoskeletal ultrasonography is currently experiencing a resurgence. Although it remains a specialized service more commonly available in large hospitals, its use is increasing rapidly, and it will likely become more widely available.

SPECIAL TRAINING REQUIRED

Musculoskeletal ultrasonography is simply an ultrasonographic examination of part of the musculoskeletal system. But because not all ultrasonographic transducers offer sufficient resolution for musculoskeletal evaluation and not all sonographers and imaging physicians are familiar with the specialized techniques, musculoskeletal ultrasonography often has a separate designation (eg, “MSKUS,” “MSUS”). At Cleveland Clinic, it is offered through the department of musculoskeletal imaging by subspecialty-trained musculoskeletal radiologists and specially trained musculoskeletal ultrasonographers with 4 to 5 years of training in the technique.

Musculoskeletal ultrasonography is also performed by physician groups with specialized training, including sports medicine physicians, rheumatologists, physiatrists, neurologists, and orthopedic surgeons. The American Institute of Ultrasound in Medicine offers voluntary accreditation for practice groups using musculoskeletal ultrasonography. Certification in musculoskeletal radiology is offered to sonographers through the American Registry for Diagnostic Medical Sonography.

SONOGRAPHY HAS UNIQUE QUALITIES

Ultrasonography uses high-frequency sound waves to generate images. The transducer (or probe) emits sound from the many piezoelectric elements at its surface, and the sound waves travel through and react with tissues. Sound reflected by tissues is detected by the transducer and converted to an image. Objects that reflect sound appear hyperechoic (brighter), whereas tissues that reflect little or no sound appear hypoechoic.

High-resolution imaging of superficial structures

In ultrasonography, a trade-off exists between image resolution and penetration depth

Figure 1. In ultrasonography, a trade-off exists between image resolution and penetration depth. The superficial patellar tendon (A, arrow) can be seen with high resolution, demonstrating its fine internal structure. The much deeper iliopsoas tendon cannot be seen with the same high resolution because of its deep location
(B, arrow).

Ultrasonography involves a fundamental trade-off between image resolution and imaging depth. Higher-frequency sound waves do not penetrate far into tissues but generate a higher-resolution image; lower-frequency sound waves can penetrate much further but yield a lower-resolution image. Although high-resolution imaging of deep structures with ultrasonography is not possible (Figure 1), many musculoskeletal structures are located superficially and are amenable to ultrasonographic evaluation.

Be aware of artifacts

Ultrasonography of the posterior thigh in a patient with obesity

Figure 2. Ultrasonography of the posterior thigh in a patient with obesity. Because subcutaneous fat attenuates sound waves, examination of soft tissues greater than a few centimeters in thickness is nondiagnostic.

Some materials attenuate sound very little, such as simple fluid. Low attenuation results in artifacts on ultrasonography, making tissues behind the simple fluid appear brighter than neighboring tissues. These artifacts may be reported as “increased through transmission” or “posterior acoustic enhancement.” Conversely, metal and bone reflect all sound waves that reach them, rendering any structures beyond them invisible. This “shadowing” creates a problem for imaging of structures in or near bone. Subcutaneous fat also attenuates sound waves, limiting the use of ultrasonography for patients with obesity (Figure 2).

On ultrasonography, anisotropy causes a hypoechoic defect of the articular supraspinatus tendon fibers

Figure 3. On ultrasonography, anisotropy causes a hypoechoic defect of the articular supraspinatus tendon fibers (A, arrow). With improved transducer angle, anisotropy is decreased and intact fibers can be seen (B, arrow). Sonographers and interpreting physicians must be careful not to mistake aberrations due to anisotropy for tissue disease.

Ultrasonography is also subject to artifacts depending on the direction of the transducer, a phenomenon known as anisotropy. Aniso­tropy causes highly ordered tissues such as tendons and ligaments to sometimes appear hypoechoic,9,10 which is also the appearance of diseased or disrupted tendons and ligaments (Figure 3).11 Anisotropy is minimized when the transducer is held perpendicularly to the structure of interest.11

High-frequency linear transducer sharpens images

High-frequency linear transducers reduce anisotropy because their flat surface keeps sound waves more uniformly perpendicular to the structure of interest.4,7 Their development has allowed imaging of superficial structures that is superior to that of MRI. A high-frequency linear transducer offers more than twice the spatial resolution of a typical 1.5T MRI examination of superficial tissue.12,13

Operator experience is critical

Ultrasonography examinations, more than other imaging tests, are dependent on operator experience. A solid understanding of musculoskeletal anatomy is imperative. Because the probe images only a thin section of tissue (about the thickness of a credit card), referencing adjacent structures for orientation is more difficult with ultrasonography than with CT or MRI.

The accuracy of ultrasonography is highly dependent on acquiring and interpreting images, whereas the accuracy of MRI is dependent primarily on image interpretation.7 Interpreting physicians must check that sonographers capture relevant targets.

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Musculoskeletal ultrasonography has arrived

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