TABLE 2.

Local strains predicted by the FEA and from ultrasound elastography (US) for the last three steps of applied compression on the top of the three-layered gel construct.

	Strain top 8% gel			Strain middle 4% gel			Strain bottom 8% gel
Comp. (μm)	FEA	US	%	FEA	US	%	FEA	US	%
80	0.00551	0.00686 ± 0.00118	19.6	0.01483	0.01225 ± 0.00185	–21.0	0.00596	0.00620 ± 0.00174	3.87
120	0.00815	0.00911 ± 0.00133	10.5	0.02237	0.01995 ± 0.00243	–12.1	0.00868	0.00853 ± 0.00197	–1.75
160	0.01086	0.01100 ± 0.00145	1.27	0.03003	0.02806 ± 0.00284	–7.02	0.01149	0.01063 ± 0.00203	–8.09
160	0.01086	0.0109	0.367	0.03003	0.02792	–7.56	0.01149	0.0105	–9.42

Each 40 μm compression increment translates to a global strain of 0.44%. The first step is not shown. Local strains in rows two though four were computed under the assumption the SOS was constant in the gels, which is a common assumption in elastography. In the last row, the predicted strain is corrected for the decrease in SOS with compression. For these hydrogels, the effect of compression-dependent SOS on the strains is small. Percent differences in strain between two methods, FEA and US, were, on average, lowest for the largest, 160 μm, compression step. The percent difference was calculated using $(\overline{\mathrm{US}}-\mathrm{FEA})∕\overline{\mathrm{US}}\times 100\%$, where $\overline{\mathrm{US}}$ was the mean of the strains computed from ultrasound elastography.