Capabilities and technical limitations
Ultrasound diagnostics is ideally suited for examining superficial soft tissues and bone contours. Although ultrasound is heavily attenuated at depth and penetrates poorly into bone, the method excellently visualizes the cortical layer (cortex) and periosteum (periosteum). This enables the physician to quickly identify fractures and superficial aggressive lesions.
The primary limitation remains the loss of signal quality when attempting to "look" deeper. If it is necessary to examine deeply located structures or perform a biopsy, specialists use a low-frequency curvilinear probe (curvilinear probe).
Features of working with a low-frequency probe:
- Advantages: significantly expanded field of view, improved visualization at depth, and the ability to safely guide a needle during biopsy.
- Disadvantages: due to the lower wave frequency, spatial resolution inevitably decreases, which can complicate the detailed assessment of pathological lesion characteristics.
Diagnostic pitfall: anisotropy artifact
When examining structures with a distinct parallel linear fibrillar organization (such as ligaments, tendons, and muscles), a specific optical effect frequently occurs — anisotropy (anisotropy).
The essence of the phenomenon lies in the dependence of the reflected signal quality on the angle of incidence of the wave. If the ultrasound beam hits the fibers non-perpendicularly, part of the signal is scattered. On the monitor screen, such an area appears unnaturally dark (hypoechoic).
In clinical practice, this is a classic trap (pitfall). An inexperienced diagnostician may mistakenly interpret a hypoechoic focus as inflammatory tissue degeneration or even a rupture. The main prevention measure for such errors is strict technique control: the physician must constantly adjust the probe angle to achieve ideal perpendicularity of the beam to the anatomical bundles.
Ultrasonographic presentation in tumors and granulomatous processes
Ultrasound provides valuable information regarding the response of bone structures to aggressive pathologies. Let us examine this through classic clinical examples:
- Sarcoidosis of a finger phalanx. Doppler mode reveals a hyperemic, dumbbell-shaped hypoechoic mass that aggressively invades through a defect in the bone cortex. Simultaneously, radiographs will show intraosseous granulomas (well-defined radiolucent areas) and associated surrounding soft tissue edema.
- Osteosarcoma of the distal femur. The method is extremely sensitive to periostitis reactions. On axial views, periosteal elevation with active new bone formation in the form of spicules ("sun-ray" sign or sun-ray spiculation) can be seen. On longitudinal views at the proximal edge of the malignant lesion, a periosteal cuff — the Codman triangle (Codman triangle) — is clearly visualized.
Compression elastography
Ultrasound elastography (Elastography) is a specialized mode that generates images based on the elastic (mechanical) properties of soft tissues. In musculoskeletal imaging, compression elastography (strain elastography) is the most widely used.
Mechanics and physical basis of the method: The algorithm is based on manual tissue compression by the ultrasound probe itself.
- Pressing the probe causes physical displacement or deformation of the tissues (strain).
- The device software measures the magnitude of this displacement by comparing the image at rest (uncompressed) with the moment of maximum pressure (compressed).
- Next, the elastic modulus ($E$) is calculated using the basic physical formula: $E$ is equal to the ratio of the applied mechanical stress (stress) to the resulting strain.
Important assumption: the method works correctly only if the mechanical stress from the physician's hand is distributed uniformly throughout the entire block of tissue being examined. As a result, the software calculates the elastic modulus for each pixel, color-coding the region of interest based on tissue density.
Briefly: answers to topical questions
What artifacts, other than anisotropy, are encountered in musculoskeletal ultrasound?
In addition to anisotropy, acoustic shadowing is encountered during musculoskeletal ultrasound examinations.
- Acoustic shadowing (acoustic shadowing) is an effect that can be produced by echogenic calcium deposits in calcific tendinopathy.
It is important to distinguish ultrasound artifacts from magnetic resonance artifacts so as not to mistake them for pathology. For example, MRI of tendons features the magic angle artifact (magic angle artefact), which, like anisotropy in ultrasound, should not be mistaken for signs of tendinopathy.
What types of high-frequency probes are used to examine superficial musculoskeletal structures?
High-frequency linear probes with a frequency of 10 MHz and higher are used to examine superficial musculoskeletal structures.
They provide high spatial resolution of soft tissues. The choice of probe depends on the depth of the structures being examined:
| Probe type | Administration | Features |
|---|---|---|
| High-frequency linear probes | Superficial structures | Frequency 10 MHz and higher, high spatial resolution |
| Low-frequency curvilinear probe (curvilinear probe) | Deep structures | Larger field of view, reduced spatial resolution |
To understand the essence of anisotropy, imagine a pocket mirror: if you direct a beam of light at it at an angle, it reflects to the side, and the mirror appears dark. Similarly, ultrasound "slides off" tendon fibers at an incorrect angle, making them dark (hypoechoic) on the screen.
Class and exam questions
Can ultrasound be used for a comprehensive evaluation of the internal bone structure?
No, the method has strict limitations in assessing the bone tissue itself due to high ultrasound absorption. However, it is exceptionally well-suited for examining its surface—the periosteum and cortical layer.
Why do orthopedists switch from a high-frequency probe to a low-frequency curvilinear probe?
This is necessary for visualizing deeply located structures. A low-frequency probe increases the field of view and penetration depth, although this comes at the expense of reduced spatial resolution.
How to distinguish a real tendon rupture from an anisotropy artifact?
It is necessary to smoothly adjust the probe tilt. If the dark (hypoechoic) area disappears once strict perpendicularity of the beam to the fibers is achieved, it was anisotropy rather than a rupture.
What is the calculation of the elastic modulus in elastography based on?
It is calculated as the ratio of mechanical stress (generated by probe compression) to the degree of physical tissue deformation, assuming a uniform load distribution.
What else to study on this topic
- Ultrasound targeting techniques for diagnostic and therapeutic injections
- Radiographic and ultrasonographic correlations in bone sarcoidosis
- Types of periosteal reactions: from Codman's triangle to sun-ray spiculation
- Physical principles and limitations of compression elastography (strain elastography)
- Image optimization using low-frequency probes in traumatology
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