Title : Photonics and vibrational analysis together make a new tool to noninvasively study the mechanical properties of soft tissues
Tissue stiffness is recognized as an important predictor of tissue homeostasis and the onset of disease. As tissues age, they become stiffer and lose their flexibility leading to diseases such as osteoarthritis and peripheral vascular diseases. Some workers believe that increased tissue stiffness causes upregulation of mechanotransduction leading to increased risk of cancer and resistance to chemotherapeutic drugs. Therefore, it is very important to be able to accurately measure tissue stiffness in terms of an elastic modulus to follow changes in tissue properties that are coincidental with altered tissue structure and function. We have developed a new technique that uses audible sound and infrared light to measure the resonant frequency of tissue components based on determination of the frequency at which the maximum component tissue displacement occurs. Since soft tissues are highly viscoelastic the resonant frequency needs to be measured from the displacement in-phase with the acoustic force. The result of vibrational optical coherence tomography measurements (VOCT) is a series of vibrational peaks that represent cellular, collagenous, blood vessels and other components of the tissues. Each component of the tissue has a characteristic resonant frequency that can be converted into a tissue elastic modulus using an empirical equation found by comparing VOCT results to calibration results from tensile stress-strain curves.
Results of VOCT studies on human skin, skin cancers, ocular tissues, muscles, nerves, tendons, and fibrotic tissues indicate that cells, collagen fibers, blood vessels, and other tissue components have resonant frequencies and moduli that are similar in different tissues. The moduli of these tissue components are also similar from person to person while the distribution of these components may vary in health and disease. Increased tissue stiffness is associated with fibrosis that is associated with skin cancers and changes observed in tissue components in the corneal disease of keratoconus. Fibrosis associated with skin cancers can be differentiated from fibrosis associated with tissue changes associate with inflammation. Fibrosis affects the tissue’s ability to store, transmit and dissipate elastic energy. In addition to measurements of the elastic modulus, VOCT can be used to estimate the loss modulus due to tissue viscosity. Soft tissues contain 50% or more of water leading to changes in tissue volume that occur during mechanical loading. The rearrangement of fluid during articular cartilage loading plays an important role in energy dissipation in the joint space and for this reason energy storage, transmission, and dissipation are important concepts that need to be considered to evaluate the health of any tissue. In vessel wall and ocular tissues, applied energy is stored, transmitted, and dissipated away from the site of direct loading which protects these tissues from premature mechanical failure. Tissue stiffening that occurs with age decreases the ability of these soft tissues to store, transmit, or dissipate energy leading to premature mechanical energy.
Audience Take Away Notes :
- What is VOCT and how is it useful?
- How does tissue stiffness affect the disease process and resistance to treatments?
- What are the resonant frequencies and moduli of normal tissue components?
- How do the resonant frequencies and moduli of tissue components change during disease?
- Why is soft tissue viscoelasticity such an important property of tissues?