In vivo burn scar assessment with speckle decorrelation and joint spectral and time domain optical coherence tomography.

Significance: Post-burn scars and scar contractures present significant challenges in burn injury management, necessitating accurate evaluation of the wound healing process to prevent or minimize complications. Non-invasive and accurate assessment of burn scar vascularity can offer valuable insights for evaluations of wound healing. Optical coherence tomography (OCT) and OCT angiography (OCTA) are promising imaging techniques that may enhance patient-centered care and satisfaction by providing detailed analyses of the healing process. Aim: Our study investigates the capabilities of OCT and OCTA for acquiring information on blood vessels in burn scars and evaluates the feasibility of utilizing this information to assess burn scars. Approach: Healthy skin and neighboring scar data from nine burn patients were obtained using OCT and processed with speckle decorrelation, Doppler OCT, and an enhanced technique based on joint spectral and time domain OCT. These methods facilitated the assessment of vascular structure and blood flow velocity in both healthy skin and scar tissues. Analyzing these parameters allowed for objective comparisons between normal skin and burn scars. Results: Our study found that blood vessel distribution in burn scars significantly differs from that in healthy skin. Burn scars exhibit increased vascularization, featuring less uniformity and lacking the intricate branching network found in healthy tissue. Specifically, the density of the vessels in burn scars is 67% higher than in healthy tissue, while axial flow velocity in burn scar vessels is 25% faster than in healthy tissue. Conclusions: Our research demonstrates the feasibility of OCT and OCTA as burn scar assessment tools. By implementing these technologies, we can distinguish between scar and healthy tissue based on its vascular structure, providing evidence of their practicality in evaluating burn scar severity and progression.

E2E-BPF microscope: extended depth-of-field microscopy using learning-based implementation of binary phase filter and image deconvolution.

Several image-based biomedical diagnoses require high-resolution imaging capabilities at large spatial scales. However, conventional microscopes exhibit an inherent trade-off between depth-of-field (DoF) and spatial resolution, and thus require objects to be refocused at each lateral location, which is time consuming. Here, we present a computational imaging platform, termed E2E-BPF microscope, which enables large-area, high-resolution imaging of large-scale objects without serial refocusing. This method involves a physics-incorporated, deep-learned design of binary phase filter (BPF) and jointly optimized deconvolution neural network, which altogether produces high-resolution, high-contrast images over extended depth ranges. We demonstrate the method through numerical simulations and experiments with fluorescently labeled beads, cells and tissue section, and present high-resolution imaging capability over a 15.5-fold larger DoF than the conventional microscope. Our method provides highly effective and scalable strategy for DoF-extended optical imaging system, and is expected to find numerous applications in rapid image-based diagnosis, optical vision, and metrology.

Ptychographic lens-less birefringence microscopy using a mask-modulated polarization image sensor.

Birefringence, an inherent characteristic of optically anisotropic materials, is widely utilized in various imaging applications ranging from material characterizations to clinical diagnosis. Polarized light microscopy enables high-resolution, high-contrast imaging of optically anisotropic specimens, but it is associated with mechanical rotations of polarizer/analyzer and relatively complex optical designs. Here, we present a form of lens-less polarization-sensitive microscopy capable of complex and birefringence imaging of transparent objects without an optical lens and any moving parts. Our method exploits an optical mask-modulated polarization image sensor and single-input-state LED illumination design to obtain complex and birefringence images of the object via ptychographic phase retrieval. Using a camera with a pixel size of 3.45 µm, the method achieves birefringence imaging with a half-pitch resolution of 2.46 µm over a 59.74 mm2 field-of-view, which corresponds to a space-bandwidth product of 9.9 megapixels. We demonstrate the high-resolution, large-area, phase and birefringence imaging capability of our method by presenting the phase and birefringence images of various anisotropic objects, including a monosodium urate crystal, and excised mouse eye and heart tissues.

Non-Invasive Retinal Blood Vessel Wall Measurements with Polarization-Sensitive Optical Coherence Tomography for Diabetes Assessment: A Quantitative Study.

Diabetes affects the structure of the blood vessel walls. Since the blood vessel walls are made of birefringent organized tissue, any change or damage to this organization can be evaluated using polarization-sensitive optical coherence tomography (PS-OCT). In this paper, we used PS-OCT along with the blood vessel wall birefringence index (BBI = thickness/birefringence2) to non-invasively assess the structural integrity of the human retinal blood vessel walls in patients with diabetes and compared the results to those of healthy subjects. PS-OCT measurements revealed that blood vessel walls of diabetic patients exhibit a much higher birefringence while having the same wall thickness and therefore lower BBI values. Applying BBI to diagnose diabetes demonstrated high accuracy (93%), sensitivity (93%) and specificity (93%). PS-OCT measurements can quantify small changes in the polarization properties of retinal vessel walls associated with diabetes, which provides researchers with a new imaging tool to determine the effects of exercise, medication, and alternative diets on the development of diabetes.

Untrained deep learning-based differential phase-contrast microscopy.

Quantitative differential phase-contrast (DPC) microscopy produces phase images of transparent objects based on a number of intensity images. To reconstruct the phase, in DPC microscopy, a linearized model for weakly scattering objects is considered; this limits the range of objects to be imaged, and requires additional measurements and complicated algorithms to correct for system aberrations. Here, we present a self-calibrated DPC microscope using an untrained neural network (UNN), which incorporates the nonlinear image formation model. Our method alleviates the restrictions on the object to be imaged and simultaneously reconstructs the complex object information and aberrations, without any training dataset. We demonstrate the viability of UNN-DPC microscopy through both numerical simulations and LED microscope-based experiments.

Full Stokes polarimetry using a monolithic off-axis polarizing interferometer and a 2D array sensor.

This paper describes a full Stokes polarimeter employing a monolithic off-axis polarizing interferometric module and a 2D array sensor. The proposed passive polarimeter provides a dynamic full Stokes vector measurement capability of around 30 Hz. As the proposed polarimeter employs no active devices and is operated by employing an imaging sensor, it has significant potential to become a highly compact polarization sensor for smartphone applications. To show the feasibility of the proposed passive dynamic polarimeter scheme, the full Stokes parameters of a quarter-wave plate are extracted and displayed on a Poincare sphere by varying the polarization state of the measured beam.

Polarization-sensitive intensity diffraction tomography.

Optical anisotropy, which is an intrinsic property of many materials, originates from the structural arrangement of molecular structures, and to date, various polarization-sensitive imaging (PSI) methods have been developed to investigate the nature of anisotropic materials. In particular, the recently developed tomographic PSI technologies enable the investigation of anisotropic materials through volumetric mappings of the anisotropy distribution of these materials. However, these reported methods mostly operate on a single scattering model, and are thus not suitable for three-dimensional (3D) PSI imaging of multiple scattering samples. Here, we present a novel reference-free 3D polarization-sensitive computational imaging technique-polarization-sensitive intensity diffraction tomography (PS-IDT)-that enables the reconstruction of 3D anisotropy distribution of both weakly and multiple scattering specimens from multiple intensity-only measurements. A 3D anisotropic object is illuminated by circularly polarized plane waves at various illumination angles to encode the isotropic and anisotropic structural information into 2D intensity information. These information are then recorded separately through two orthogonal analyzer states, and a 3D Jones matrix is iteratively reconstructed based on the vectorial multi-slice beam propagation model and gradient descent method. We demonstrate the 3D anisotropy imaging capabilities of PS-IDT by presenting 3D anisotropy maps of various samples, including potato starch granules and tardigrade.

Two concepts for ultra-high-resolution polarization-sensitive optical coherence tomography with a single camera.

Two designs with a multiplexed superluminescent diode for ultra-high-resolution spectral-domain polarization-sensitive optical coherence tomography (UHR-PS-OCT) are introduced. In the first design, a Wollaston prism separates orthogonal polarization states next to each other on one linescan camera; the other design uses a beam displacer to separate orthogonal states onto two lines of a linescan camera with multiple rows of detectors. The coherence lengths measured with the two systems were 3.6 µm and 2.9 µm (n=1.38), respectively. Measurements were collected from the fovea of a healthy subject, a healthy subject with astigmatism, and a patient with central serous retinopathy (CSR). A single volumetric scan provides double pass retardance induced by the retinal nerve fiber layer birefringence (RNFL) and its birefringence, the cumulative double pass retardance induced by the Henle fiber layer, and the retardance that is induced by the retinal pigment epithelium-Bruch's membrane complex. The high axial resolution in UHR-PS-OCT is particularly helpful for the measurements of thin retinal tissue, such as the RNFL in the fovea, where birefringence values of around 1°/µm were found. Tilting of the retina due to a CSR or by off centering the imaging beam in the pupil causes an artificial increase in the double pass retardance induced by the RNFL and Henle fiber layer.

Massively parallel direct writing of nanoapertures using multi-optical probes and super-resolution near-fields.

Laser direct-writing enables micro and nanoscale patterning, and is thus widely used for cutting-edge research and industrial applications. Various nanolithography methods, such as near-field, plasmonic, and scanning-probe lithography, are gaining increasing attention because they enable fabrication of high-resolution nanopatterns that are much smaller than the wavelength of light. However, conventional methods are limited by low throughput and scalability, and tend to use electron beams or focused-ion beams to create nanostructures. In this study, we developed a procedure for massively parallel direct writing of nanoapertures using a multi-optical probe system and super-resolution near-fields. A glass micro-Fresnel zone plate array, which is an ultra-precision far-field optical system, was designed and fabricated as the multi-optical probe system. As a chalcogenide phase-change material (PCM), multiple layers of Sb65Se35 were used to generate the super-resolution near-field effect. A nanoaperture was fabricated through direct laser writing on a large-area (200 × 200 mm2) multi-layered PCM. A photoresist nanopattern was fabricated on an 8-inch wafer via near-field nanolithography using the developed nanoaperture and an i-line commercial exposure system. Unlike other methods, this technique allows high-throughput large-area nanolithography and overcomes the gap-control issue between the probe array and the patterning surface.

Engineering pupil function for optical adversarial attacks.

Adversarial attacks inject imperceptible noise to images to deteriorate the performance of deep image classification models. However, most of the existing studies consider attacks in the digital (pixel) domain where an image acquired by an image sensor with sampling and quantization is recorded. This paper, for the first time, introduces a scheme for optical adversarial attack, which physically alters the light field information arriving at the image sensor so that the classification model yields misclassification. We modulate the phase of the light in the Fourier domain using a spatial light modulator placed in the photographic system. The operative parameters of the modulator for adversarial attack are obtained by gradient-based optimization to maximize cross-entropy and minimize distortion. Experiments based on both simulation and a real optical system demonstrate the feasibility of the proposed optical attack. We show that our attack can conceal perturbations in the image more effectively than the existing pixel-domain attack. It is also verified that the proposed attack is completely different from common optical aberrations such as spherical aberration, defocus, and astigmatism in terms of both perturbation patterns and classification results.

Dynamic spectroscopic imaging ellipsometry.

A dynamic spectroscopic imaging ellipsometer (DSIE) employing a monolithic polarizing interferometer is described. The proposed DSIE system can provide spatio-spectral ellipsometric phase map data Δ(λ, x) dynamically at a speed of 30 Hz. We demonstrate the ultrafast mapping capability of the spectroscopic ellipsometer by measuring a patterned 8-inch full wafer with a spatial resolution of less than 50 × 50 µm2 in an hour.

Optical dosimeter for selective retinal therapy based on multi-port fiber-optic interferometry.

Selective retinal therapy (SRT) employs a micro-second short-pulse lasers to induce localized destruction of the targeted retinal structures with a pulse duration and power aimed at minimal damage to other healthy retinal cells. SRT has demonstrated a great promise in the treatment of retinal diseases, but pulse energy thresholds for effective SRT procedures should be determined precisely and in real time, as the thresholds could vary with disease status and patients. In this study, we present the use of a multi-port fiber-based interferometer (MFI) for highly sensitive real-time SRT monitoring. We exploit distinct phase differences among the fiber ports in the MFI to quantitatively measure localized fluctuations of complex-valued information during the SRT procedure. We evaluate several metrics that can be computed from the full complex-valued information and demonstrate that the complex contour integration is highly sensitive and most correlative to pulse energies, acoustic outputs, and cell deaths. The validity of our method was demonstrated on excised porcine retinas, with a sensitivity and specificity of 0.92 and 0.88, respectively, as compared with the results from a cell viability assay.

Polarization properties of retinal blood vessel walls measured with polarization sensitive optical coherence tomography.

A new method based on polarization-sensitive optical coherence tomography (PS-OCT) is introduced to determine the polarization properties of human retinal vessel walls, in vivo. Measurements were obtained near the optic nerve head of three healthy human subjects. The double pass phase retardation per unit depth (DPPR/UD), which is proportional to the birefringence, is higher in artery walls, presumably because of the presence of muscle tissue. Measurements in surrounding retinal nerve fiber layer tissue yielded lower DPPR/UD values, suggesting that the retinal vessel wall tissue near the optic nerve is not covered by retinal nerve fiber layer tissue (0.43°/µm vs. 0.77°/µm, respectively). Measurements were obtained from multiple artery-vein pairs, to quantify the different polarization properties. Measurements were taken along a section of the vessel wall, with changes in DPPR/UD up to 15%, while the vessel wall thickness remained relatively constant. A stationary scan pattern was applied to determine the influence of involuntary eye motion on the measurement, which was significant. Measurements were also analyzed by two examiners, with high inter-observer agreement. The measurement repeatability was determined with measurements that were acquired during multiple visits. An improvement in accuracy can be achieved with an ultra-broad-bandwidth PS-OCT system since it will provide more data points in-depth, which reduces the influence of discretization and helps to facilitate better fitting of the birefringence data.

Polarization-sensitive differential phase-contrast microscopy.

We present a novel, to the best of our knowledge, form of polarization microscopy capable of producing quantitative optic-axis and phase retardation maps of transparent and anisotropic materials. The proposed method operates on differential phase-contrast (DPC) microscopy that produces a phase image of a thin specimen using multi-axis intensity measurements. For polarization-sensitive imaging, patterned illumination light is circularly polarized to illuminate a specimen. The light transmitted through a specimen is split into two orthogonal polarization states and measured by an image sensor. Subsequent DPC computation based on the illumination patterns, acquired images, and the imaging model enables the retrieval of polarization-dependent quantitative phase images, which are utilized to reconstruct the orientation and retardation of the specimen. We demonstrate the validity of the proposed method by measuring the optic-axis and phase retardation maps of calibrated and various anisotropic samples.

Light sheet fluorescence microscopy using axi-symmetric binary phase filters.

Light sheet fluorescence microscopy (LSFM) has become an indispensable tool in biomedical studies owing to its depth-sectioning capability and low photo-bleaching. The axial resolution in LSFM is determined mainly by the thickness of the illumination sheet, and a high numerical-aperture lens is thus preferred in the illumination to increase the axial resolution. However, a rapid divergence of the illumination beam limits the effective field-of-view (FoV), that provides high-resolution images. Several strategies have been demonstrated for FoV enhancement, which involve the use of Bessel or Airy beams, for example. However, the generation of these beams requires complicated optical setup or phase filters with continuous phase distributions, which are difficult to manufacture. In contrast, a binary phase filter (BPF) comprising concentric rings with 0 or π phases produces a response similar to its continuous original and is easy to realize. Here, we present a novel form of LSFM that integrates BPFs derived from two representative axi-symmetric aberrations, including phase axicon and spherical aberrations, to improve the imaging performance. We demonstrate that these BPFs significantly increase the FoV, and those derived from axicon generate self-reconstructing beams, which are highly desirable in imaging through scattering specimens. We validate its high-contrast imaging capability over extended FoV by presenting three-dimensional images of microspheres, imaginal disc of Drosophila larva, and Arabidopsis.

Single-pixel, single-input-state polarization-sensitive wavefront imaging.

In this Letter, we describe a single-pixel polarization-sensitive imaging technique, capable of generating the birefringence map of a thin specimen by using single-pixel detectors. Spatially modulated light is circularly polarized to illuminate the specimen. The transmitted light through the specimen is then focused via a lens and measured by position-sensitive detectors in two orthogonal polarization channels. The measurement of the irradiance and centroid position of the optical focus and subsequent computations enable the production of polarization-dependent wavefront maps, which can then be utilized to reconstruct sample birefringence information. We demonstrate the feasibility of our method by measuring distribution of optic-axis orientation and phase retardation of various birefringent samples.

Direct replication of a glass micro Fresnel zone plate array by laser irradiation using an infrared transmissive mold.

It is not yet possible to fabricate micrometer-scale, glass optical components with nanometer-scale precision. Glass thermal imprinting enhances production efficiency. However, dimensional changes caused by shrinkage are inevitable because of phase transitions. Replication is very difficult when high-level pitch precision is essential. We used an infrared-transparent silicon mold and a CO2 laser to perform replica-type, thermal surface texturing at the nanoscale level; we analyzed a glass Fresnel zone plate array to this end. The Fresnel zone plate array was 10 × 10 mm2 in area and featured a 20 × 20 array. The individual Fresnel zone plate diameter was 500 µm and had 21 rings of minimum linewidth 2.9 µm and height 737 nm.

Electrically focus-tuneable ultrathin lens for high-resolution square subpixels.

Owing to the tremendous demands for high-resolution pixel-scale thin lenses in displays, we developed a graphene-based ultrathin square subpixel lens (USSL) capable of electrically tuneable focusing (ETF) with a performance competitive with that of a typical mechanical refractive lens. The fringe field due to a voltage bias in the graphene proves that our ETF-USSL can focus light onto a single point regardless of the wavelength of the visible light-by controlling the carriers at the Dirac point using radially patterned graphene layers, the focal length of the planar structure can be adjusted without changing the curvature or position of the lens. A high focusing efficiency of over 60% at a visible wavelength of 405 nm was achieved with a lens thickness of <13 nm, and a change of 19.42% in the focal length with a 9% increase in transmission was exhibited under a driving voltage. This design is first presented as an ETF-USSL that can be controlled in pixel units of flat panel displays for visible light. It can be easily applied as an add-on to high resolution, slim displays and provides a new direction for the application of multifunctional autostereoscopic displays.

A Portable Smartphone-linked Device for Direct, Rapid and Chemical-Free Hemoglobin Assay.

We describe the development and clinical evaluation of an automated smartphone-linked sensor capable of chemical-free, quantitative measurement of hemoglobin concentration ([Hb]) in whole blood samples. We have demonstrated that our sensor could analyze an unprocessed blood specimen with a mean processing time of <8 s and provided the [Hb] results with ~99% accuracy against a reference hematology analyzer with coefficient of variation (CV) of 1.21% measured at [Hb] = 11.2 g/dL. Its diagnostic capability for anemia was evaluated by measuring [Hb] of 142 clinical blood specimens and comparing the results with those from an automated hematology analyzer (ADVIA 2120i, Siemens AG, Germany) and a portable hemoglobinomteter (Hb201+, Hemocue, Sweden). The sensor yielded comparable sensitivities and specificities of 87.50% and 100.00% for males, and 94.44% and 100.00% for females, respectively, for anemic detection. The results suggested that our optical sensor based on the intrinsic photothermal response of Hb molecules and advances in consumer electronics, particularly smartphone capabilities, enables a direct, chemical-free [Hb] assay accessible to people in both developed and developing countries.

Recent advances in high-sensitivity detection methods for paper-based lateral-flow assay.

Paper-based lateral-flow assays (LFAs) have achieved considerable commercial success and continue to have a significant impact on medical diagnostics and environmental monitoring. Conventional LFAs are typically performed by examining the color changes in the test bands by naked eye. However, for critical biochemical markers that are present in extremely small amounts in the clinical specimens, this readout method is not quantitative, and does not provide sufficient sensitivity or suitable detection limit for a reliable assay. Diverse technologies for high-sensitivity LFA detection have been developed and commercialization efforts are underway. In this review, we aim to provide a critical and objective overview of the recent progress in high-sensitivity LFA detection technologies, which involve the exploitation of the physical and chemical responses of transducing particles. The features and biomedical applications of the technologies, along with future prospects and challenges, are also discussed.