Quantifying Micro-mechanical Properties of Soft Biological Tissues with Scanning Acoustic Microscopy
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Quantifying Micro-mechanical Properties of Soft Biological Tissues with Scanning Acoustic Microscopy Xuegen Zhao1, Steven Wilkinson1, Riaz Akhtar1,2, Michael J Sherratt2, Rachel E B Watson3 and Brian Derby1 1
School of Materials, the University of Manchester, Manchester, M1 7HS, United Kingdom School of Biomedicine, Faculty of Medical and Human Sciences, The University of Manchester, Manchester, M13 9PT, United Kingdom 3 Dermatological Sciences Research Group, Faculty of Medical and Human Sciences, The University of Manchester, Manchester, M13 9PT, United Kingdom 2
ABSTRACT In this study we have established a new approach to more accurately map acoustic wave speed (which is a measure of stiffness) within soft biological tissues at micrometer length scales using scanning acoustic microscopy. By using thin (5 μm thick) histological sections of human skin and porcine cartilage, this method exploits the phase information preserved in the interference between acoustic waves reflected from the substrate surface as well as internal reflections from the acoustic lens. A stack of images were taken with the focus point of acoustic lens positioned at or above the substrate surface, and processed pixel by pixel using custom software developed with LABVIEW and IMAQ (National Instruments) to extract phase information. Scanning parameters, such as acoustic wave frequency and gate position were optimized to get reasonable phase and lateral resolution. The contribution from substrate inclination or uneven scanning surface was removed prior to further processing. The wave attenuation was also obtained from these images. INTRODUCTION Age or disease related changes in the mechanical properties of dynamic tissues such as lungs, skin, blood vessels and heart, have a profound impact on human morbidity and mortality. Global demographic shifts towards an ageing population will therefore further exacerbate the incidence of mechanical failures within the cardio-respiratory and musculoskeletal systems. However, detailed knowledge of the micro-mechanical properties of both healthy and diseased soft tissues is currently unavailable. Although acoustic methods have been used extensively to determine the bulk mechanical properties of biological tissues, little is known about their mechanical properties at microscopic length scales. Scanning acoustic microscopy (SAM) [1] uses ultra-high frequency sound vibrations to image materials, with the contrast related to the stiffness, density, thickness and attenuation of the sample. As the acoustic beam is formed by converging propagating waves, the size of the focal spot is limited by diffraction. The use of higher frequency excitation provides higher spatial resolution but reduces signal level dramatically due to attenuation. When operated at 1GHz, the spatial resolution of the SAM is about 1 m, which is considerably better than what is currently achievable through nanoindentation [2] and makes SAM potentially a high spatial resolution tool for the study of the micromechanical properties of biological t
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