Dynamic optical coherence elastography (OCE) tracks mechanical wave propagation in the subsurface region of tissue to map its shear modulus. For bulk shear waves, the lateral resolution of the reconstructed modulus map (i.e., elastographic resolution) can approach that for OCT, typically a few tens of microns. However, skin, cornea and many other tissues are layered or bounded leading to the formation of guided mechanical waves. We performed numerical simulations and acoustic micro-tapping experiments to show that in bounded media, the elastographic resolution cannot reach the OCT structural resolution and is mainly defined by the thickness of the bounded tissue layer.
Recent advances in dynamic OCE have resulted in tools that can generate/track sub-mm wavelength mechanical waves in tissue. However, reconstructing material elasticity from measured wavefields needs an appropriate model accounting for tissue anisotropy, structure and geometry. We assume that tissues consisting of collagen fibers can be locally described with a model of a nearly incompressible transverse isotropic (NITI) medium using three elastic parameters to describe shear and tensile behavior. Examples of NITI media are discussed and the problem of inversion of moduli from bulk shear, Rayleigh and guided waves is considered.
Reconstructive skin surgeries drive the clinical need for non-contact objective measurements of skin elasticity. Here we demonstrate that all three of skin’s elastic constants (in-plane and out-of-plane shear moduli and an additional modulus defining skin’s tensile anisotropy) and the orientation of collagen fibers in dermis can be determined from Rayleigh wave anisotropy in-plane with acoustic micro-tapping (AuT) OCE. A nearly-incompressible transverse isotropic (NITI) model was used to reconstruct skin’s moduli from OCE measurements in human forearm in vivo for five healthy volunteers. Co-registered polarization-sensitive (PS-) OCT shows that optical and mechanical axes are co-aligned at measured sites.
KEYWORDS: Light emitting diodes, Photoacoustic imaging, 3D image processing, Imaging arrays, Deep tissue imaging, Tissue optics, Ultrasonography, Tumors, Switches, Signal to noise ratio
Low cost and high-power LED’s are replacing highly expensive and bulky Lasers in photoacoustic imaging (PAI) technology. LEDs bring the opportunity to image samples at a higher repetition rate (in kHz) compared to the conventional Q switched lasers (typically upto 20 Hz) and thus offer high frame rate and averaging power while maintain real-time imaging capability. In order to make a complete switch from Laser to LEDs, a proper understanding of this newly emerging class of LED arrays is important, for e.g., beam divergence, beam focus, coherence, time-dependent power and wavelength stability, penetration depth and sensitivity, etc. We performed characterization and optimization experiments on different tissue mimicking phantoms with various optical scattering and absorbing properties using a commercially available AcousticX PAI system with customized 3D printed holders for the LED array. The axial and lateral positions of LED arrays with respect to the acoustic probe to image deep lesions upto 1.5 cm were optimized. We also report the sensitivity of the system and Signal-to-noise-ratio (SNR) dependency of the LED arrays’ axial and lateral positions. Finally, we also demonstrate the adaptability of our custom designed, 3D printed variable angle and distance holder to perform deep tissue ultrasound and photoacoustic imaging probe on phantoms of with absorbers at different depth and in subcutaneous tumors and compare it with traditional laser based photoacoustic imaging systems.
Photodynamic therapy (PDT), a spatially localized phototoxic therapy that involves irradiation of a photosensitizer (PS) with specific wavelengths of light, has shown exceptional promise in impacting cancer treatment outcomes, particularly oral cancer. To reduce PDT outcome variability, attempts toward image-guided personalized PDT are being pursued by monitoring PS uptake either via fluorescence or photoacoustic imaging (PAI), a nonionizing modality dependent on optical absorption properties of the tissue. PAI-guided PDT requires a near-infrared contrast agent for deep tissue imaging with minimal photobleaching effect. We evaluate the impact of PDT agent, benzoporphyrin derivative (BPD), on PAI agent indocyanine green (ICG) and vice versa, given that they have different optical absorption properties and singlet oxygen quantum yields for PDT. Specifically, we demonstrate in two oral squamous cell carcinoma lines (FaDu and SCC4) that ICG has minimal effect on BPD PDT efficacy when irradiated with either a continuous or pulsed laser. Furthermore, the impact of BPD on ICG photodegradation was monitored with PAI in tissue-mimicking phantoms. These studies inform us that the combination of BPD and ICG can be utilized for PAI-guided PDT. However, researchers need to consider the photodegradation effects of ICG in the presence of BPD when designing their drug delivery strategies for PAI-guided PDT.
The recently introduced photothermoelectric (PTE) effect is proposed for investigating the dynamic thermal parameters of condensed matter samples. The front detection configuration, together with the thermal-wave resonator cavity (TWRC) method as scanning procedure, was used to measure the value of thermal effusivity. The back configuration, together with the TWRC technique and/or the chopping frequency of the incident radiation as scanning parameter, leads to the direct measurement of thermal diffusivity. Recent applications to solid (materials with different values of thermal parameters) and liquid (nanofluids) samples are summarized. The ferro-paraelectric phase transition of triglycine-sulphate (TGS) was also detected. The performances of the PTE method have been compared with those of the photopyroelectric (PPE) technique.
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