Techniques aimed at the non-invasive characterization of soft tissues according to elastic properties are rapidly evolving. Virtual touch-based elastographic methods including acoustic radiation force imaging (ARFI) and optical elastography measure the peak axial displacement (PD) and time-to-peak-displacement (TTP) of tissue in response to a localized force. These measurements have been used clinically to differentiate tissues, albeit with mixed results. However, to date, the reason has not been fully understood. In this study, we apply a novel modeling approach to explore the mechanistic link between simplistic displacement measurements and tissue viscoelasticity in the application of virtual touch-based elastographic methods to staging chronic liver disease (CLD). To our knowledge, such a study has not been reported in the literature. Specifically, a numerical screening study was first conducted to identify factors that most strongly determine PD and TTP. Response surface experimental designs were then applied to these factors to produce meta-models of expected PD and TTP probability density functions (PDFs) as functions of identified factors. Results from the screening study suggest that both PD and TTP measurements are primarily influenced by three factors: the initial Young’s modulus of the tissue, the first viscoelastic Prony series time constant, and pre-compression ap- plied during acquisition. To investigate the implications of these results, stochastic inputs for these three factors associated were used to determine a robust response surface. The identified response surface methodology can be used to determine optimal cutoff values for PD and TTP that could be used in order to stage chronic liver disease.
A characteristic sign of aging skin is loss of firmness. Current bioinstruments to measure the influence of formulations on skin mechanics are limited in sensitivity and have high operator variability. To address these issues an optical elastography system was developed. Regions of the upper inner bicep were treated with various commercial formulations including wrinkle reducers, firming films, and moisturizers. These regions along with adjacent untreated areas that served as internal controls were imaged with a custom designed optical elastography system. The elastography system employed a polarized 50 mW 532 nm cw laser as an illumination source, a CCD camera imaging the skin at 160 Hz frame rate and a polarization analyzer aligned parallel to the incident beam. The skin was mechanically loaded using compressed air reduced in pressure and modulated using a proportional valve to provide a 1 Hz sinusoidally varying pressure to the skin with a peak force of 0.15 N. Subjects ranging in ages from 19 to 60 years old were recruited with IRB approval. Displacement and strain encoded elastograms were generated simultaneously for the treated and untreated areas. The ratio of the strain response in the two regions was calculated to quantify the relative effect of the skin agents. Significant differences were found in the strain response to the imposed loads between treated and control areas in all age groups and genders. Optical elastography systems such as the one prototyped in this study may prove to be useful for the cosmetics industry for assessing product efficacy.
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