No abstract

Real-time elastography is a method for visualization of the elastic properties of soft tissue and may potentially enable differentiation between malignant and benign pathologic lesions. Our aim was to validate the method on a tissue-mimicking (TM) phantom and to evaluate the influence of different scanning parameters and investigator variability. A TM-phantom containing eight spherical inclusions with known storage modulus was examined using two different transducers on an ultrasound (US) scanner equipped with software for real-time elasticity imaging. The ultrasound transducers were moved vertically in a repetitive manner to induce strain. Two investigators performed series of standardized elastography scans applying a 0-4 categorical quality scale to evaluate the influence of seven parameters: dynamic range of elasticity, region-of-interest, frequency of transducer movement, rejection of elastogram noise, frame rate, persistence and smoothing. Subsequently, repeated examinations of four selected inclusions were performed using a visual analog scale (VAS) where investigators marked a 100 mm horizontal line representing the span in image quality based on experience from the first examination. The hardest and softest inclusions were imaged more clearly than the inclusions with elasticity more similar to the background material. Intraobserver agreement on elastogram quality was good (kappa: 0.67 - 0.75) and interobserver agreement average (kappa: 0.55 - 0.56) when using the categorical scale. The subsequent VAS evaluation gave intraclass-correlation coefficients for the two observers of 0.98 and 0.93, respectively, and an interclass-correlation coefficient of 0.93. Real-time elastography adequately visualized isoechoic inclusions with different elastic properties in a TM-phantom with acceptable intra- and interobserver agreement. Dynamic range of elasticity was the parameter with most impact on the elastographic visualization of inclusions.

PURPOSE: 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). MATERIALS AND METHODS: 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 E-dyn were recorded for each setup and the strain ratio was measured in 3 different representative elastograms for each recording situation. RESULTS: 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 intra- and interobserver reliability was high when measuring the strain ratio with a free-hand technique. CONCLUSION: 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.