Ultrasound elastography

Working Principle and Essence of US Elastography

Ultrasound elastography (USE) is a technology that expands the capabilities of traditional ultrasound. The machine calculates the elasticity of the tissue being examined relative to neighboring areas. As a result of software data processing, an elastogram is created—a detailed map of relative elastic moduli.

The primary diagnostic value of the method lies in the fact that it provides a direct visual assessment of the elastic properties of tissues. In addition, the physician can obtain semiquantitative indicators: for this purpose, the software compares the stiffness of the pathological focus with the characteristics of a healthy (reference) zone. To date, the method is most successfully and routinely applied in two areas: for the non-invasive assessment of the stage of liver fibrosis, as well as for the differential diagnosis of breast masses. As for musculoskeletal medicine and orthopedics, clinical experience here is still in the early stages of data accumulation.

Computed Tomography (CT) in Diagnostic Imaging

Unlike elastography, computed tomography (CT) has long been the standard in orthopedics. CT serves as a crucial adjunct to standard radiography in the evaluation of fractures. In addition, the method is used to monitor orthopedic implants in situations where a complex three-dimensional structure cannot be adequately analyzed on a flat 2D image, and also serves as a navigation tool for interventional procedures.

A special place is occupied by dual-energy CT (DECT), used for the diagnosis of crystal arthropathies of the appendicular skeleton.

• Mechanism: The scanner differentiates calcium salts based on their relative attenuation of X-rays at two different peak voltages (typically 80 and 140 kVp).
• Technology: On modern single-source, single-detector scanners, this is achieved through very rapid voltage switching («kV switching») during a single scan. This guarantees perfect co-registration of the acquired datasets.
• Value in Gout: DECT is capable of detecting soft tissue urate deposits, clearly distinguishing uric acid crystals from other types of calcifications (which look identical on conventional radiographs or standard CT). The method is extremely useful in the acute phase of the disease, when serum urate levels may be normal, joint aspiration is "dry", and classical radiographic signs have not yet formed.

Metal Artifact Reduction (MAR)

When performing CT in patients with metal implants, a serious problem arises: the metal causes disproportionate attenuation of low-energy X-ray photons. Standard image reconstruction (filtered back-projection) mathematically assumes uniform attenuation, which inevitably leads to a beam-hardening artifact (beam-hardening artefact). Strong beam attenuation by metal also causes marked noise (photon starvation effect, or photon starvation).

There are various ways to solve this problem, but each has limitations:

1. Increasing kVp: Reduces the proportion of absorbed low-energy photons, but negatively affects the patient due to an increase in radiation dose.
2. DECT Monochromatic Reconstruction: Allows image generation based on a single virtual kVp. Increasing this virtual value reduces the beam-hardening artifact, but provokes an increase in noise.
3. Hardware Noise Reduction Methods: Reducing the pitch (pitch), narrowing the collimation, and increasing the tube current (mA). The drawback of all these actions is an increase in radiation dose.
4. Software Solutions: Using multiplanar reconstruction (MPR), increasing the reconstructed slice thickness, or slice averaging methods.
5. Manufacturer-Specific Algorithms: These include iterative reconstruction and segmentation methods. At the same time, it is important to note: there is no compelling clinical evidence that these complex tools work more effectively than competent baseline optimization of standard scanner parameters.

Magnetic Resonance Imaging (MRI) of the Musculoskeletal System

Routine MRI (MRI) of the musculoskeletal system is based on classical fast-spin echo (fast-spin echo) and gradient-echo (gradient-echo) sequences. Thanks to improved spatial resolution from stronger magnets and modern coils, invasive MR arthrography is nowadays increasingly and successfully replaced by standard MRI.

Current Trends in MRI:

• Gaining Popularity: Diffusion-weighted imaging (DWI), motion artifact reduction techniques, ultrashort echo time (TE) MRI, and cartilage mapping.
• Not Proven Effective: Dynamic contrast-enhanced MRI (DCE-MRI).

Principles of MRI Protocol Selection: The golden rule of diagnostics is that every joint must be scanned in all three planes using a combination of different sequences:

• PD (Proton Density) and T2-Weighted (T2W): Ideal for evaluating fibrocartilage (e.g., menisci), the ligamentous apparatus, and tendons.
• PD (Fat-Suppressed / Fat-Saturated): Are the preferred choice for studying hyaline articular cartilage.
• T1-Weighted (T1W) and T2-Weighted Fat-Suppressed: Essential for detailed evaluation of the bone marrow.

Optimization of Scanning Parameters: When using a T2W fat-suppressed sequence, excessively long echo times should be avoided. A TE value within 60–70 ms is quite sufficient for reliable visualization of bone marrow edema, while the signal-to-noise ratio is significantly better than with a long TE of 120 ms. Intermediate-weighted MR (Intermediate weighted MR) is also useful, occupying a niche between PD and T2W (TE around 35 ms). It is extremely informative for differentiating hyaline cartilage, fibrocartilage, and synovial fluid. When scanning the ankle joint, the physician must take into account that this area is anatomically prone to inhomogeneous fat signal suppression.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Principles of elastogram formation and elasticity modulus calculation
• Physics of Dual-Energy CT (DECT) and kV switching
• Nature of the beam-hardening artifact from metal
• Selection of MRI sequences (PD, T1W, T2W) for different types of cartilage

Other topics in the "Musculoskeletal System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.