Real-Time Elastography: Strain Ratio Measurements Are Influenced by the Position of the Reference Area

Real-time elastography (RTE) is an ultrasound-based method for the visualization of relative strain distribution in soft tissues. Strain ratio is a semi-quantitative measurement of strain differences between two user-defined areas in an elastogram. The aim of this study was to evaluate the impact of the size and location of a reference area when measuring the strain ratio of focal lesions in a tissue-mimicking phantom and in normal liver tissue. We also investigated whether the strain ratio was affected by changing the scanner parameter: elasticity dynamic range (E-dyn).

Two investigators individually collected data by scanning 4 spherical inclusions with different elasticity in a phantom in which the elastic modulus was known in both the lesions and the background. Subsequently, a liver scan was performed in-vivo using the same scanning protocol. Five different setups with changes in reference area position or size were tested. All eight levels of the scanner setting Edyn were recorded for each setup and the strain ratio was measured in 3 different representative elastograms for each recording situation.

The four inclusions had significantly different mean strain ratio levels (p < 0.01) when compared to the surrounding material. Changing the position of the reference area to a deeper position influenced the strain ratio measurements significantly for all phantom lesions and in the liver. Changing the size of the reference area, while keeping the center depth unchanged, did not influence the mean strain ratio levels significantly. The strain ratio was independent of the E-dyn parameter setting. The intraand interobserver reliability was high when measuring the strain ratio with a free-hand technique.

Strain ratio provides reproducible measurements of inclusions representing different elastic contrasts using a free-hand technique in vitro. Changes in the distance of the reference areas to the ultrasound probe, representing the stress source, seem to have a significant impact on strain ratio measurements.