Assessment of the Mechanical Properties of Soft Tissue Phantoms Using Impact Analysis

Abstract

Skin physiopathological conditions have a strong influence on its biomechanical properties. However, it remains difficult to accurately assess the surface stiffness of soft tissues. The aim of this study was to evaluate the performances of an impact-based analysis method (IBAM) and to compare them with those of an existing digital palpation device, MyotonPro^®. The IBAM is based on the impact of an instrumented hammer equipped with a force sensor on a cylindrical punch in contact with agar-based phantoms mimicking soft tissues. The indicator Δt is estimated by analyzing the force signal obtained from the instrumented hammer. Various phantom geometries, stiffnesses and structures (homogeneous and bilayer) were used to estimate the performances of both methods. Measurements show that the IBAM is sensitive to a volume of interest equivalent to a sphere approximately twice the punch diameter. The sensitivity of the IBAM to changes in Young's modulus is similar to that of dynamic mechanical analysis (DMA) and significantly better compared to MyotonPro. The axial (respectively, lateral) resolution is two (respectively, five) times lower with the IBAM than with MyotonPro. The present study paves the way for the development of a simple, quantitative and non-invasive method to measure skin biomechanical properties.

Keywords: elastography; impact analysis; instrumentation; mechanical properties; soft tissues.

Figure 1

Experimental set-up of the impact-based analysis method (IBAM). During a measurement, the instrumented hammer impacts the punch, which is vertically guided. The lower part of the punch is in contact with the agar-based phantom held by the support.

Figure 2

Example of a signal recorded by the force sensor impacting the hammer (impact #i) with a 3% agar-based phantom mimicking soft tissues. The temporal indicator $\mathsf{\Delta }{t}_i$, where $t_{i,1}$ is the time of impact hammer and $t_{i,2}$ is the time of the first rebound of the punch on the hammer, and the impact force $IF$ are indicated. Here, $\mathsf{\Delta }{t}_i$ = 1.61 ms and $IF$ = 32.5 N.

Figure 3

Experimental set-up using MyotonPro device to characterize a 3% agar-based phantom mimicking soft tissues.

Figure 4

Schematic illustration of the custom vibration-based set-up performed on agar-based phantoms mimicking soft tissues to determine their Young’s moduli.

Figure 5

Illustration of the two experimental protocols for the estimation of the spatial resolution for the IBAM and MyotonPro. (a) Evaluation of the axial resolution for the two conditions “rigid on soft” and “soft on rigid” where the top layer thicknesses $h_{5\%}$ and $h_{2\%}$ (in bold on the figure) vary between 40 mm and 0 mm; (b) evaluation of the lateral resolution.

Figure 6

Variation in the indicators $\mathsf{\Delta }t$ and $S$ obtained with the IBAM and MyotonPro as a function of the sample diameter with a 3% agar mass concentration. The error bars correspond to the reproducibility of the measurements.

Figure 7

Variation in the indicators $\mathsf{\Delta }t$ and $S$ obtained with the IBAM and MyotonPro as a function of the sample length with a 3% agar mass concentration. The error bars correspond to the reproducibility of the measurements.

Figure 8

Variation in the indicators $\mathsf{\Delta }t$ and $S$ obtained with the IBAM and MyotonPro as a function of the top layer thickness $h_{5\%}$ for the “rigid on soft” configuration. The error bars correspond to the reproducibility of the measurements.

Figure 9

Variation in the indicators $\mathsf{\Delta }t$ and $S$ obtained with the IBAM and MyotonPro as a function of the top layer thickness $h_{2\%}$ for the “soft on rigid” configuration. The error bars correspond to the reproducibility of the measurements.

Figure 10

Variation in the indicators $\mathsf{\Delta }t$ and $S$ obtained with the IBAM and MyotonPro as a function of the measurement position $x$ on the upper surface of the bilayer samples. The error bars correspond to the reproducibility of the measurements. The color corresponds to the color indicated in Figure 5b.

Figure 11

Variation in the indicators $\mathsf{\Delta }t$ and $S$ obtained with the IBAM and MyotonPro as a function of the Young’s modulus $E$ of soft tissue phantoms obtained for agar mass concentrations varying between 1 and 5% using custom vibration-based set-up. The error bars correspond to the reproducibility of the measurements. The dashed line is the curve fitting with a power function of $\mathsf{\Delta }t$ as a function of $E$.

Figure 12

Comparison of the variation in the Young’s modulus of soft tissue phantoms $E$ as a function of the agar mass concentrations using various measurement methods [26,42,43,44]. The error bars correspond to the reproducibility of the measurements.