The unique viscoelastic properties of tissues throughout the human body can be utilized in a variety of clinical applications. Palpation techniques, for instance, enable surgeons to distinguish malignancies in tissue composition during surgical procedures. Additionally, imaging devices have begun utilizing the viscoelastic properties of tissue to delineate tumor margins. Vibroacoustography (VA), a non-invasive, high resolution imaging modality, has the ability to detect sub-millimeter differences in tissue composition. VA images tissue using a low frequency acoustic radiation force, which perturbs the target and causes an acoustic response that is dependent on the target’s viscoelastic properties. Given the unique properties specific to human and animal tissues, there are far-reaching clinical applications of VA. To date, however, a comprehensive model that relates viscoelasticity to VA tissue response has yet to be developed. Utilizing tissue-mimicking phantoms (TMPs) and fresh ex vivo tissues, a mechanical stress relaxation model was developed to compare the viscoelastic properties of known and unknown specimens. This approach was conducted using the Hertz theory of contact mechanics. Fresh hepatic tissue was obtained from porcine subjects (n=10), while gelatin and agar TMPs (n=12) were fabricated from organic extracts. Each specimen’s elastic modulus (E), long term shear modulus (η), and time constant (τ) were found to be unique. Additionally, each specimen’s stress relaxation profiles were analyzed using Weichert-Maxwell viscoelastic modeling, and retained high precision (R<sup>2</sup>>0.9) among all samples.
Vibroacoustography (VA) is an imaging technology that utilizes the acoustic response of tissues to a localized, low frequency radiation force to generate a spatially resolved, high contrast image. Previous studies have demonstrated the utility of VA for tissue identification and margin delineation in cancer tissues. However, the relationship between specimen viscoelasticity and vibroacoustic emission remains to be fully quantified. This work utilizes the effects of variable acoustic wave profiles on unique tissue-mimicking phantoms (TMPs) to maximize VA signal power according to tissue mechanical properties, particularly elasticity. A micro-indentation method was utilized to provide measurements of the elastic modulus for each biological replica. An inverse relationship was found between elastic modulus (E) and VA signal amplitude among homogeneous TMPs. Additionally, the difference frequency (Δf ) required to reach maximum VA signal correlated with specimen elastic modulus. Peak signal diminished with increasing Δf among the polyvinyl alcohol specimen, suggesting an inefficient vibroacoustic response by the specimen beyond a threshold of resonant Δf. Comparison of these measurements may provide additional information to improve tissue modeling, system characterization, as well as insights into the unique tissue composition of tumors in head and neck cancer patients.