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, 44 (3), 424-9

Ultrasound Echo Is Related to Stress and Strain in Tendon

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Ultrasound Echo Is Related to Stress and Strain in Tendon

Sarah Duenwald et al. J Biomech.

Abstract

The mechanical behavior of tendons has been well studied in vitro. A noninvasive method to acquire mechanical data would be highly beneficial. Elastography has been a promising method of gathering in vivo tissue mechanical behavior, but it has inherent limitations. This study presents acoustoelasticity as an alternative ultrasound-based method of measuring tendon stress and strain by reporting a relationship between ultrasonic echo intensity (B-mode ultrasound image brightness) and mechanical behavior of tendon in vitro. Porcine digital flexor tendons were cyclically loaded in a mechanical testing system while an ultrasonic echo response was recorded. We report that echo intensity closely follows the applied cyclic strain pattern in time with higher strain protocols resulting in larger echo intensity changes. We also report that echo intensity is related nonlinearly to stress and nearly linearly to strain. This indicates that ultrasonic echo intensity is related to the mechanical behavior in a loaded tissue by an acoustoelastic response, as previously described in homogeneous, nearly incompressible materials. Acoustoelasticity is therefore able to relate strain-dependent stiffness and stress to the reflected echo, even in the processed B-mode signals reflected from viscoelastic and inhomogeneous material such as tendon, and is a promising metric to acquire in vivo mechanical data noninvasively.

Figures

Figure 1
Figure 1
Test setup. The tendon is held in place by a stationary metal block and a moveable soft tissue grip, which is connected to the load cell. The ultrasound transducer is submerged in the PBS bath and held in place to image the tendon during mechanical testing.
Figure 2
Figure 2
Strain input for mechanical testing, shown for the 4% strain test. The first three cycles give rise to history dependent transient effects. Tissue has generally reached steady-state by the final three cycles. Data from the final three cycles are used in this study.
Figure 3
Figure 3
Echo intensity change during cyclic loading to 2, 4, and 6% strain for a single specimen. Echo intensity follows a cyclic pattern for all three strain levels, and the increase in echo intensity depends on strain (greater for higher strains).
Figure 4
Figure 4
Average change in echo intensity (percent) and average load reached during cycles to 2, 4, and 6% strain for eight specimens. Bars indicate one standard deviation from the mean. Arrows indicate the corresponding axes. Both mean load reached and mean echo intensity change during cyclic loading increase with strain.
Figure 5
Figure 5
Echo intensity change and strain vs. stress during one half-cycle to 4% strain (stretch from 0 to 4%) for a. one specimen (with logarithmic curve fits) and b. all eight specimens. Open circles represent ultrasonic echo intensity data points and x's represent strain data from the MTS test frame; arrows point to the corresponding axes. The stress-echo intensity relationship follows a similar shape to the stress-strain relationship.
Figure 5
Figure 5
Echo intensity change and strain vs. stress during one half-cycle to 4% strain (stretch from 0 to 4%) for a. one specimen (with logarithmic curve fits) and b. all eight specimens. Open circles represent ultrasonic echo intensity data points and x's represent strain data from the MTS test frame; arrows point to the corresponding axes. The stress-echo intensity relationship follows a similar shape to the stress-strain relationship.
Figure 6
Figure 6
Echo intensity change versus time during one half-cycle of repeatability testing (in the same fashion as the results in Figure 2) to 4% strain. One specimen was subjected to three cyclic loading tests, and the echo intensity of each loading test was analyzed three times, for a total of nine echo intensity analyses. Trial numbers and corresponding symbols are labeled on the top of the graph. The dotted black line indicates the average of all nine analyses; black bars indicate one standard deviation.
Figure7
Figure7
Echo intensity and strain changes versus stress during one half-cycle (same data as in Figure 4b) plotted with a. logarithmic curve fit of echo intensity data [echo = 11.066 + 6.893log(stress)] and b. logarithmic curve fit of strain [strain = 3.8521 + 2.2337log(stress)] (top) and the linear echo-based approximation [strain = 3.7 + 2.29log(stress), or 1/3*(echo percent change)] (bottom). The linear approximation fits the experimental strain data nearly as well as the curve fit of the strain data, with no significant difference in RMSE. Open circles represent ultrasonic echo intensity data points and x's represent strain data from the MTS test frame; arrows point to the corresponding axes.
Figure7
Figure7
Echo intensity and strain changes versus stress during one half-cycle (same data as in Figure 4b) plotted with a. logarithmic curve fit of echo intensity data [echo = 11.066 + 6.893log(stress)] and b. logarithmic curve fit of strain [strain = 3.8521 + 2.2337log(stress)] (top) and the linear echo-based approximation [strain = 3.7 + 2.29log(stress), or 1/3*(echo percent change)] (bottom). The linear approximation fits the experimental strain data nearly as well as the curve fit of the strain data, with no significant difference in RMSE. Open circles represent ultrasonic echo intensity data points and x's represent strain data from the MTS test frame; arrows point to the corresponding axes.

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