How to Read Quality and Reliability Maps in 2D-SWE: A Practical Guide for Physicians

The Purpose of Quality Maps in 2D-SWE

Some manufacturers provide a quality measure in 2D-SWE. This measure evaluates the collected raw data and provides information on the reliability of shear wave speed (SWS) measurement (ACR/SRU, 2024). SWS is assessed using B-mode to track micro-movements of disturbed tissues within the ROI or a larger 2D field of view.

Confidence Map

The confidence map can be adjusted to allow the B-mode image to 'shine through,' or the corresponding pixel is set to black. Typically, this occurs as the signal-to-noise ratio deteriorates with depth, limiting 2D-SWE penetration (Dietrich et al., EFSUMB, 2017). Thus, black or 'dropout' zones on the map indicate areas of unreliable data that should be excluded from measurement.

Wavefront Assessment

An additional approach to quality assessment is implemented as a display of recorded wave arrival times in a 'wavefront' style. The concept is that if consecutive lines are parallel (even if they are uneven), the quality is acceptable; if the lines are not parallel, the quality is unacceptable (Dietrich et al., EFSUMB, 2017).

Technical Conditions for Correct Measurement

The operator places the region of interest in an area free of large vessels, at a depth of more than 2 cm but not deeper than 8 cm (Sporea et al., EFSUMB Course Book, 2018). According to EFSUMB recommendations, the elastographic box is placed avoiding superficial liver sections and discrete structures; the recommended depth is from 3 to 7 cm (Cosgrove et al., 2013). The 2D-SWE signal stabilizes after 3–4 seconds, after which an image can be obtained (Cosgrove et al., 2013).

For 2D-SWE.GE, the ROI is placed at least 10 mm below the liver capsule, in an area free of vessels; at least 10 shear wave frames are acquired, and measurement ROIs are chosen to exclude obvious artifacts (Sporea et al., EFSUMB Course Book, 2018).

Number of Measurements and Result Presentation

The latest EFSUMB recommendations prescribe obtaining at least three measurements for 2D-SWE.SSI; the result is expressed as a median along with the interquartile range (IQR) (Sporea et al., EFSUMB Course Book, 2018).

Reliability Criteria and Their Impact

Initially, the manufacturer did not recommend quality criteria, but when applying the same criteria as in transient elastography (IQR<30% and SR>60%), the proportion of reliable measurements may decrease to 71.3% (Sporea et al., EFSUMB Course Book, 2018).

Reproducibility and Training

The 2D-SWE technology, like conventional ultrasound, can be operator-dependent. When comparing measurements on the same day, ICCs were 0.95 and 0.93 for expert and novice operators, and for measurements on different days — 0.84 and 0.65; interobserver agreement was 0.88 (Ferraioli et al., WFUMB, 2015). It is recommended that a novice perform at least 50 supervised studies and measurements to achieve stable results (Ferraioli et al., WFUMB, 2015).