Anisotropy of the Renal Cortex and SWE Distortion: Impact of Probe Orientation on Stiffness Values
The Nature of Anisotropy and Its Manifestation in the Kidney
Anisotropy is an artifact that occurs when examining structures with an ordered fibrous architecture (tendons, ligaments, nerves, muscle bundles). When the beam does not fall perpendicularly to the fibers, the reflected signal is deflected away rather than returning to the probe — the structure appears hypoechoic or 'disappears,' mimicking pathology.
According to the Textbook of Diagnostic Sonography (Hagen-Ansert, 2023), anisotropy appears as an anechoic area or a zone of reduced echogenicity due to a decrease in the amplitude of the received signal; the degree of loss is greater the more the beam angle deviates from 90°. Although anisotropy is less common outside musculoskeletal imaging, in renal ultrasound, it manifests as a loss of clarity in the curvature of the upper pole of the right kidney.
How Probe Orientation Affects the Image
The key mechanism is the angle of incidence of the ultrasound beam. When deviating from perpendicular, part of the reflected signal is lost, distorting the image. Since shear wave elastography (SWE) relies on signal registration in the area of interest, loss of amplitude and distortion of beam geometry affect measurement quality [specific displacement values in kPa for the renal cortex are not provided in fragments — clarification needed].
Probe Tilt Technique (Heel-Toe / Angulation)
The goal is to return the beam to perpendicular incidence on the structure being examined. A slight tilt (5–15°) restores mirror reflection back to the probe, and the structure appears normally echogenic again. Diagnostic criterion: if the 'finding' disappears with tilt, it is an artifact, not pathology.
Why Other Techniques Do Not Eliminate the Artifact
| Technique | Result |
|---|---|
| Reducing Acoustic Power | Does not affect the angle of reflection, may degrade the signal |
| Adding Gel | Eliminates air gaps between skin and probe but does not change beam geometry on deep structures |
| Increasing Gain | Enhances the entire signal uniformly, including noise; the artifact is masked, not eliminated — diagnostically unreliable |
| Probe Tilt (Angulation) | Restores perpendicular incidence — eliminates the artifact |
Practical Significance for SWE Data Conversion
When comparing SWE data, it is important to remember (EFSUMB, Dietrich et al., 2017): due to the quadratic relationship between the elasticity modulus and velocity, means and standard deviations calculated for data in m/s and then converted to kPa are not equal to values calculated after primary conversion to kPa. This is critical when attempting to aggregate data and for heterogeneous tissues within the ROI.
Frequently asked questions
How to distinguish anisotropy from true pathology in renal ultrasound?
Tilt the probe (heel-toe / angulation) by 5–15°. If the hypoechoic area disappears and the structure becomes normally echogenic, it is an anisotropy artifact, not pathology.
Why does increasing gain not solve the problem of anisotropy?
Gain increases the entire signal uniformly, including noise. The anisotropic area may brighten slightly, but the artifact is masked, not eliminated — this is diagnostically unreliable.
How does anisotropy specifically manifest in the kidney?
According to Hagen-Ansert (2023) — by a loss of clarity in the outline of the curved upper pole of the right kidney; overall, it appears as an anechoic zone or an area of reduced echogenicity.
Does the choice of units (kPa vs. m/s) affect the comparability of SWE data?
Yes. Due to the quadratic relationship between modulus and velocity (EFSUMB, Dietrich et al., 2017), means and SD converted from m/s to kPa are not equal to those calculated directly in kPa — this is important when aggregating data.