Examinations were performed on an Acuson S3000 system (Siemens,
Mountain View,
CA,
USA).
All examinations were done by two investigators on an elasticity phantom (Elasticity QA,
model 049A,
CIRS (CIRS,
Virginia,
USA)) (Figure 1).
Twenty targets of varying diameter (16.7,
10.4,
6.5,
4.1 and 2.5 mm) and stiffness (80,
45,
14,
and 8 kPa),
placed at a depth of 3.5 cm were scanned.
Both strain- and shear-wave-elastography was performed ten times for each of the targets scanned,
yielding 200 measurements for each method used.
For the shear-wave examinations the region of interest (ROI) was placed in the middle of the target,
using a color-depiction of the elasticity of the target and its surroundings (Figure 2).
For the strain-elastography both strain-ratios and strain-histogram analysis was performed.
Strain-ratios were calculated from grey scale strain-elastogram still-frames (Figure 3),
strain-histogram analysis was performed on color elastography videos (Figure 4 and 5) at a length of minimum five seconds.
Strain-histogram analysis was performed using the free software ImageJ (downloaded at nih.gov) [8].
The units for shear-wave measurements were m/s,
while both strain-ratios and strain-histogram analysis were unit less,
ranging from 0 to indefinite and 0 to 255 respectively.
Statistics: Statistical analysis was done using the software SAS version 9.3 (Copyright © 2012,
SAS Institute Inc.,
SAS Campus Drive,
Cary,
North Carolina 27513,
USA).
To evaluate the impact of size on elastographic assessments of target stiffness,
mixed models analysis was performed including target stiffness,
target size and the interaction thereof as model parameters.
The significance level was set at 0.05.
To evaluate the performance of each elastographic method at different levels of target stiffness,
data was divided on a binary scale three times with one data division between each of the target elasticities.
The three resulting sets of binary data were used for ROC-curve analysis,
allowing for direct comparison of the three methods at different levels of target elasticity.