We developed a quantitative optical coherence elastography (qOCE) system for nonlinear mechanical characterization of
biological tissues. The fiber-optic probe of the qOCE system had an integrated Fabry-Perot force sensor. To perform
mechanical characterization, the tissue was compressed uniaxially by the fiber-optic probe of the qOCE system. Using the
optical coherence tomography (OCT) signal detected by a spectral domain OCT engine, we were able to simultaneously
quantify the force exerted to the tissue and the displacement of tissue. The quantification of the force was critical for
accurate assessment of the elastic behavior of tissue, because most biological tissues have nonlinear elastic behavior. We
performed qOCE characterization on tissue mimicking phantoms and biological tissues. Our results demonstrated the
capability of the qOCE system for linear and nonlinear assessment of tissue elasticity.
Optical coherence tomography (OCT) signal can provide microscopic characterization of biological tissue and assist clinical decision making in real-time. However, raw OCT data is noisy and complicated. It is challenging to extract information that is directly related to the pathological status of tissue through visual inspection on huge volume of OCT signal streaming from the high speed OCT engine. Therefore, it is critical to discover concise, comprehensible information from massive OCT data through novel strategies for signal analysis. In this study, we perform Shannon entropy analysis on OCT signal for automatic tissue characterization, which can be applied in intraoperative tumor margin delineation for surgical excision of cancer. The principle of this technique is based on the fact that normal tissue is usually more structured with higher entropy value, compared to pathological tissue such as cancer tissue. In this study, we develop high-speed software based on graphic processing units (GPU) for real-time entropy analysis of OCT signal.
Optical coherence tomography (OCT) is a versatile imaging technique and has great potential in tissue characterization for breast cancer diagnosis and surgical guidance. In addition to structural difference, cancerous breast tissue is usually stiffer compared to normal adipose breast tissue. However, previous studies on compression optical coherence elastography (OCE) are qualitative rather than quantitative. It is challenging to identify the cancerous status of tissue based on qualitative OCE results obtained from different measurement sessions or from different patients. Therefore, it is critical to develop technique that integrates structural imaging and force sensing, for quantitative elasticity characterization of breast tissue. In this work, we demonstrate a quantitative OCE (qOCE) microsurgery device which simultaneously quantifies force exerted to tissue and measures the resultant tissue deformation. The qOCE system is based on a spectral domain OCT engine operated at 1300 nm and a probe with an integrated Febry-Perot (FP) interferometric cavity at its distal end. The FP cavity is formed by the cleaved end of the lead-in fiber and the end surface of a GRIN lens which allows light to incident into tissue for structural imaging. The force exerted to tissue is quantified by the change of FP cavity length which is interrogated by a fiber-optic common-paths phase resolved OCT system with sub-nanometer sensitivity. Simultaneously, image of the tissue structure is acquired from photons returned from tissue through the GRIN lens. Tissue deformation is obtained through Doppler analysis. Tissue elasticity can be quantified by comparing the force exerted and tissue deformation.
Optical imaging based on time-stretch process has recently been proven as a powerful tool for delivering ultra-high
frame rate (< 1MHz) which is not achievable by the conventional image sensors. Together with the capability of optical
image amplification for overcoming the trade-off between detection sensitivity and speed, this new imaging modality is
particularly valuable in high-throughput biomedical diagnostic practice, e.g. imaging flow cytometry. The ultra-high
frame rate in time-stretch imaging is attained by two key enabling elements: dispersive fiber providing the time-stretch
process via group-velocity-dispersion (GVD), and electronic digitizer. It is well-known that many biophotonic
applications favor the spectral window of ~1μm. However, reasonably high GVD (< 0.1 ns/nm) in this range can only be
achieved by using specialty single-mode fiber (SMF) at 1μm. Moreover, the ultrafast detection has to rely on the state-of-
the-art digitizer with significantly wide-bandwidth and high sampling rate (e.g. <10 GHz, <40 GS/s). These stringent
requirements imply the prohibitively high-cost of the system and hinder its practical use in biomedical diagnostics. We
here demonstrate two cost-effective approaches for realizing time-stretch confocal microscopy at 1μm: (i) using the
standard telecommunication SMF (e.g. SMF28) to act as a few-mode fiber (FMF) at 1μm for the time-stretch process,
and (ii) implementing the pixel super-resolution (SR) algorithm to restore the high-resolution (HR) image when using a
lower-bandwidth digitizer. By using a FMF (with a GVD of ~ 0.15ns/nm) and a modified pixel-SR algorithm, we can
achieve time-stretch confocal microscopy at 1μm with cellular resolution (~ 3μm) at a frame rate 1 MHz.
Ultra-broadband supercontinuum (SC) at 1-μm wavelength is regarded as diagnostics window in bio-photonics due to its
large penetration depth in tissues and less Rayleigh scattering. Dispersive Fourier transform (DFT) is an important
technique to realize the high-speed, ultra-fast and high-throughput spectroscopy. Thus, a stable light source with good
temporal stability plays an important role in the bio-imaging and spectroscopy applications. We here demonstrate
stabilized and enhanced SC generation at 1 μm by a minute continuous-wave (CW) triggering scheme. By introducing a
weak CW (~200,000 times weaker than the pump), a significant broadening in the SC bandwidth and an improvement in
the temporal stability can be obtained. Over 8 dB gain is achieved in both blue and red edges and the SC spectrum can
span from 900 nm to over 1300 nm with the CW trigger. We present the CW-triggered SC capability of enabling highspeed
spectroscopy based on DFT at 1 μm. In regards to the performance of DFT, the wavelength-time mapping
fluctuation reduced by 50% which is an indication of the improvement of the temporal stability. This triggering scheme
allows, for the first time, 1-μm DFT at a spectral acquisition rate of 20 MHz with good temporal stability - paving the
way toward realizing practical real-time, ultrafast biomedical spectroscopy and imaging.
Dispersive Fourier Transform (DFT) is a powerful technique for real-time and high-speed spectroscopy. In DFT, the
spectral information of an optical pulse is mapped into time using group velocity dispersion (GVD) in the dispersive
fibers with an ultrafast real-time spectral acquisition rate (>10 MHz). Typically, multi-mode fiber (MMF) is not
recommended for performing DFT because the modal dispersion, which occurs simultaneously with GVD, introduces
the ambiguity in the wavelength-to-time mapping during DFT. Nevertheless, we here demonstrate that a clear
wavelength-to-time mapping in DFT can be achieved by using the few-mode fibers (FMFs) which, instead of having
hundreds of propagation modes, support only a few modes. FMF-based DFT becomes appealing when it operates at the
shorter wavelengths e.g. 1-μm range, a favorable spectral window for biomedical diagnostics, where low-cost single
mode fibers (SMFs) and high-performance dispersion-engineered fibers are not readily available for DFT. By employing
the telecommunication SMFs (e.g. SMF28), which are in effect FMFs in the 1-μm range as their cut-off wavelength is
~1260 nm, we observe that a 3nm wide spectrum can be clearly mapped into time with a GVD as high as -72ps/nm and a
loss of 5 dB/km at a spectral acquisition rate of 20 MHz. Moreover, its larger core size than the high-cost 1-μm SMFs
renders FMFs to exhibit less nonlinearity, especially high-power amplification is implemented during DFT to enhance
the detection sensitivity without compromising the speed. Hence, FMF-based DFT represents a cost-effective approach
to realize high-speed DFT-based spectroscopy particularly in the biomedical diagnostics spectral window.
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