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hyperechoic Musculoskeletal ultrasonography is having a resurgence 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). 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. Downloaded from www.ccjm.org on August 19, 2022. For personal use only. All other uses require permission. CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 85 • NUMBER 4 APRIL 2018 285 FORNEY AND DELZELL (brighter), whereas tissues that reflect little or no sound appear hypoechoic. High-resolution imaging of superficial structures 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 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). 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 Ultrasonography involves a trade-off between image depth and resolution 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. Downloaded from www.ccjm.org on August 19, 2022. For personal use only. All other uses require permission. 286 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 85 • NUMBER 4 APRIL 2018 MUSCULOSKELETAL ULTRASOUND 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. ■ 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
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