Non-destructive biomechanical testing of tissues using a vibrational optical coherence tomography setup
Creators: name: Shah, Ruchit
Publisher: name: No Publisher Supplied
Year: 2018

description: The mechanical properties of tissues and implants contribute to their physiological functionality, and have been vast areas of research in fields such as Dermatology, Cardiology, Neurology, Ophthalmology, Orthopedics and Urology. Several tests and modalities have been used to measure the mechanical properties of tissues and implants including tensile, compressive, shear, and bending in one or more axial directions. However, majority of these tests are destructive, rendering the material unusable post-testing; whereas most of the non-destructive imaging modalities such as magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR) and ultrasound are costly and have lower spatial resolution. Optical coherence tomography (OCT) is an optical imaging modality that provides high-resolution images and has been extended to optical coherence elastography (OCE) in various embodiments to compute the mechanical properties of biological tissues. The application of elastography in computing biomechanical properties of tissues such as elastic moduli generally excludes the viscosity component of the tissues. The computation of viscoelastic properties using OCE is still in a nascent stage but has a promising future.  The primary goal of this dissertation was to develop and engineer a vibrational OCT system that performs nondestructive computation of viscoelastic properties of monophasic and composite biological tissues and implants. The underlying hypotheses of this dissertation are: i) the modulus of a tissue computed by using its resonant frequency will be comparable to its elastic modulus obtained from the uniaxial tensile testing method; ii) the resonant frequency and thereby the computed modulus will vary with strain; iii) tissues will demonstrate elastic and viscoelastic properties when pulse-vibrated at resonant and non-resonant frequencies, respectively; and iv) the moduli obtained from the vibrational OCT setup will differ between healthy and diseased tissues.  To validate the efficacy of vibrational OCT system to measure elastic modulus of mono- and multi-phasic biological tissues, a benchtop OCT system with an external speaker was set up to vibrate clamped samples with a continuous sinusoidal force at different strains. The resonant frequency of the samples was identified by observing the speaker-induced tissue displacements over a frequency range of 50 to 1000Hz, and their physical dimensions were used to determine their elastic moduli using the vibrational setup. These moduli were almost identical to those obtained using the gold standard method of uniaxial tensile testing. Multiphasic samples (bovine cartilage and pigskin) exhibited multiple resonant peaks.  In order to compute viscoelastic properties of tissues, the calibrated vibrational OCT setup was modified to generate a burst of 3 cycles of sinusoidal input waves. The samples demonstrated response similar to that of an underdamped system. The half-power bandwidth method was utilized to determine the loss modulus percent of the samples at different strains. The results showed that the viscous component of the dermis is strongest under 100Hz and that the materials exhibit almost purely elastic response at resonant frequencies.  The vibrational OCT setup was also used to characterize/ differentiate chorionic plate tissues from human placentas belonging to early term, normal and abnormal pregnancies. Each group showed distinct biomechanical properties reflecting altered tissue compositions across time and disease state.