Low-coherence enhanced backscattering beyond diffusion

In vivo two-dimensional quantitative imaging of skin and cutaneous microcirculation with perturbative spatial frequency domain imaging (p-SFDI)

We introduce perturbative spatial frequency domain imaging (p-SFDI) for fast two-dimensional (2D) mapping of the optical properties and physiological characteristics of skin and cutaneous microcirculation using spatially modulated visible light. Compared to the traditional methods for recovering 2D maps through a pixel-by-pixel inversion, p-SFDI significantly shortens parameter retrieval time, largely avoids the random fitting errors caused by measurement noise, and enhances the image reconstruction quality. The efficacy of p-SFDI is demonstrated by in vivo imaging forearm of one healthy subject, recovering the 2D spatial distribution of cutaneous hemoglobin concentration, oxygen saturation, scattering properties, the melanin content, and the epidermal thickness over a large field of view. Furthermore, the temporal and spatial variations in physiological parameters under the forearm reactive hyperemia protocol are revealed, showing its applications in monitoring temporal and spatial dynamics.

Handheld spatial frequency domain imager for noninvasive Sjögren's syndrome labial salivary gland biopsy

A labial salivary gland biopsy (LSGB) plays an essential role in diagnosing Sjögren's syndrome (SS), but its clinical application is limited due to its invasiveness. Here, we present a handheld single snapshot multiple-frequency demodulation-spatial frequency domain imaging (SSMD-SFDI) device for a rapid optical biopsy of labial salivary glands noninvasively. The structural and physiological parameters of lower lip mucosa were obtained from the light reflectance of the layered oral mucosa. The recovered parameters were found to correlate strongly with the progression of SS. In our pilot study on 15 healthy subjects and 183 SS patients, a support vector machine (SVM) classifier using the measured parameters distinguished healthy subjects, LSGB I, II, III, and IV patients in sequence with AUCs of 0.979, 0.898, 0.906, and 0.978, respectively. Critical structural and physiological alterations in the mucosa due to SS were further identified and used to assess its risk using an explainable neural network. The handheld spatial frequency domain imager may serve as a valuable label-free and noninvasive tool for early diagnosing and surveying SS.

Dynamic microcirculation PIPE model for functional neuroimaging, non-neuroimaging, and coherent hemodynamics spectroscopy: blood volume and flow velocity variations, and vascular autoregulation

We present a dynamic microcirculation PIPE model for functional neuroimaging, non-neuroimaging, and coherent hemodynamics spectroscopy. The temporal evolution of the concentration and oxygen saturation of hemoglobin in tissue, comprised of the contributions from the arterioles, capillaries, and venules of microvasculature, is determined by time-resolved hemodynamic and metabolic variations in blood volume, flow velocity, and oxygen consumption with a fluid mechanics treatment. Key parameters regarding microcirculation can be assessed, including the effective blood transit times through the capillaries and the venules, and the rate constant of oxygen release from hemoglobin to tissue. The vascular autoregulation can further be quantified from the relationship between the resolved blood volume and flow velocity variations. The PIPE model shows excellent agreement with the experimental cerebral and cutaneous coherent hemodynamics spectroscopy (CHS) and fMRI-BOLD data. It further identifies the impaired cerebral autoregulation distinctively in hemodialysis patients compared to healthy subjects measured by CHS. This new dynamic microcirculation PIPE model provides a valuable tool for brain and other functional studies with hemodynamic-based techniques. It is instrumental in recovering physiological parameters from analyzing and interpreting the signals measured by hemodynamic-based neuroimaging and non-neuroimaging techniques such as functional near-infrared spectroscopy (fNIRS) and functional magnetic resonance imaging (fMRI) in response to brain activation, physiological challenges, or physical maneuvers.

Single snapshot spatial frequency domain imaging for risk stratification of diabetes and diabetic foot

Diabetic foot is one of the major complications of diabetes. In this work, a real-time Single Snapshot Multiple-frequency Demodulation (SSMD) - Spatial Frequency Domain Imaging (SFDI) system was used to image the forefoot of healthy volunteers, diabetes, and diabetic foot patients. A layered skin model was used to obtain the 2D maps of optical and physiological parameters, including cutaneous hemoglobin concentration, oxygen saturation, scattering properties, melanin content, and epidermal thickness, from every single snapshot. We observed a strong correlation between the measured optical and physiological parameters and the degree of diabetes. The cutaneous hemoglobin concentration, oxygen saturation, and epidermal thickness decrease, whereas the melanin content increases with the progress of diabetes. The melanin content further increases, and the reduced scattering coefficient and scattering power are lower for diabetic foot patients than those of both healthy and diabetic subjects. High accuracies (AUC) of 97.2% (distinguishing the diabetic foot patients among all subjects), 95.2% (separating healthy subjects from the diabetes patients), and 87.8% (classifying mild vs severe diabetes), respectively, are achieved in binary classifications in sequence using the SSMD-SFDI system, demonstrating its applicability to risk stratification of diabetes and diabetic foot. The prognostic value of the SSMD-SFDI system in the prediction of the occurrence of the diabetic foot and other applications in monitoring tissue microcirculation and peripheral vascular disease are also addressed.

In vivo real-time imaging of cutaneous hemoglobin concentration, oxygen saturation, scattering properties, melanin content, and epidermal thickness with visible spatially modulated light

We present the real-time single snapshot multiple frequency demodulation - spatial frequency domain imaging (SSMD-SFDI) platform implemented with a visible digital mirror device that is capable of imaging and monitoring dynamic turbid medium and processes over a large field of view. One challenge in quantitative imaging of biological tissue such as the skin is the complex structure rendering techniques based on homogeneous medium models to fail. To address this difficulty we have also developed a novel method that maps the layered structure to a homogeneous medium for spatial frequency domain imaging. The varying penetration depth of spatially modulated light on its wavelength and modulation frequency is used to resolve the layered structure. The efficacy of the real-time SSMD-SFDI platform and this two-layer model is demonstrated by imaging forearms of 6 healthy subjects under the reactive hyperemia protocol. The results show that our approach not only successfully decouples light absorption by melanin from that by hemoglobin and yields accurate determination of cutaneous hemoglobin concentration and oxygen saturation, but also provides reliable estimation of the scattering properties, the melanin content and the epidermal thickness in real time. Potential applications of our system in imaging skin physiological and functional states, cancer screening, and microcirculation monitoring are discussed at the end.

Diagnosis of the phase function of random media from light reflectance

Light reflectance has been widely used to diagnose random media in both in situ and in vivo applications. The quantification of the phase function of the medium from reflectance measurements, however, remains elusive due to the lack of an explicit connection between the light reflectance profile and the phase function. Here we first present an analytical model for reflectance of scattered light at an arbitrary source-detector separation by forward-peaked scattering media such as biological tissue and cells. The model incorporates the improved small-angle scattering approximation (SAA) to radiative transfer for sub-diffusive light reflectance and expresses the dependence of the light reflectance on the phase function of the scattering medium in a closed form. A spreading length scale, l_Θ, is found to characterise subdiffusive light reflectance at the high spatial frequency (close separation) limit. After validation by Monte Carlo simulations, we then demonstrate the application of the model in accurate determination of the complete set of optical properties and the phase function of a turbid medium from the profile of subdiffusive and diffusive light reflectance.

Dynamic coherent backscattering mirror

The phase of multiply scattered light has recently attracted considerable interest. Coherent backscattering is a striking phenomenon of multiple scattered light in which the coherence of light survives multiple scattering in a random medium and is observable in the direction space as an enhancement of the intensity of backscattered light within a cone around the retroreflection direction. Reciprocity also leads to enhancement of backscattering light in the spatial space. The random medium behaves as a reciprocity mirror which robustly converts a diverging incident beam into a converging backscattering one focusing at a conjugate spot in space. Here we first analyze theoretically this coherent backscattering mirror (CBM) phenomenon and then demonstrate the capability of CBM compensating and correcting both static and dynamic phase distortions occurring along the optical path. CBM may offer novel approaches for high speed dynamic phase corrections in optical systems and find applications in sensing and navigation.

Coherent backscattering cone shape depends on the beam size

Coherent backscattering (CBS) is a beautiful physical phenomenon that takes place in a highly scattering medium, which has potential application in noninvasive optical property measurement. The current model that explains the CBS cone shape, however, assumes the incoming beam diameter is infinitely large compared to the transport length. In this paper, we evaluate the effect of a finite scalar light illumination area on the CBS cone, both theoretically and experimentally. The quantitative relationship between laser beam size and the CBS cone shape is established by using two different finite beam models (uniform top hat and Gaussian distribution). A series of experimental data with varying beam diameters is obtained for comparison with the theory. Our study shows the CBS cone shape begins to show distortion when beam size becomes submillimeter, and this effect should not be ignored in general. In biological tissue where a normal large beam CBS cone is too narrow for detection, this small beam CBS may be more advantageous for more accurate and higher resolution tissue characterization.

Effect of external field on coherent backscattering in nematics

For a one-elastic-constant model of nematic liquid crystal the optical theorem is shown to produce an explicit relationship between the scattering length of extraordinary wave mode and magnetic coherence length. The Monte Carlo simulation of coherent backscattering is performed accounting for the long-range orientational fluctuations and scattering length anisotropy; the coherent backscattering peak is shown to change quite weakly while the magnetic field varies several orders.

Depth-resolved measurement of mucosal microvascular blood content using low-coherence enhanced backscattering spectroscopy

Low-coherence enhanced backscattering (LEBS) spectroscopy is a light scattering technique which uses partial spatial coherence broadband illumination to interrogate the optical properties at sub-diffusion length scales. In this work, we present a post-processing technique which isolates the hemoglobin concentration at different depths within a sample using a single spectroscopic LEBS measurement with a fixed spatial coherence of illumination. We verify the method with scattering (spectralon reflectance standard and polystyrene microspheres) and absorbing (hemoglobin) phantoms. We then demonstrate the relevance of this method for quantifying hemoglobin content as a function of depth within biological tissue using the azoxymethane treated animal model of colorectal cancer.

Enhancement factor in low-coherence enhanced backscattering and its applications for characterizing experimental skin carcinogenesis

We experimentally study potential mechanisms by which the enhancement factor in low-coherence enhanced backscattering (LEBS) can probe subtle variations in radial intensity distribution in weakly scattering media. We use enhanced backscattering of light by implementing either (1) low spatial coherence illumination or (2) multiple spatially independent detections using a microlens array under spatially coherent illumination. We show that the enhancement factor in these configurations is a measure of the integrated intensity within the localized coherence or detection area, which can exhibit strong dependence on small perturbations in scattering properties. To further evaluate the utility of the LEBS enhancement factor, we use a well-established animal model of cutaneous two-stage chemical carcinogenesis. In this pilot study, we demonstrate that the LEBS enhancement factor can be substantially altered at a stage of preneoplasia. Our animal result supports the idea that early carcinogenesis can cause subtle alterations in the scattering properties that can be captured by the LEBS enhancement factor. Thus, the LEBS enhancement factor has the potential as an easily measurable biomarker in skin carcinogenesis.

Characterization of light transport in scattering media at sub-diffusion length scales with Low-coherence Enhanced Backscattering

Low-coherence enhanced backscattering (LEBS) is a technique that has recently shown promise for tissue characterization and the detection of early pre-cancer. Although several Monte Carlo models of LEBS have been described, these models have not been accurate enough to predict all of the experimentally observed LEBS features. We present an appropriate Monte Carlo model to simulate LEBS peak properties from polystyrene microsphere suspensions in water. Results show that the choice of the phase function greatly impacts the accuracy of the simulation when the transport mean free path (ls*) is much greater than the spatial coherence length (L(SC)). When ls* < L(SC), a diffusion approximation based model of LEBS is sufficiently accurate. We also use the Monte Carlo model to validate that LEBS can be used to measure the radial scattering probability distribution (radial point spread function), p(r), at small length scales and demonstrate LEBS measurements of p(r) from biological tissue. In particular, we show that pre-cancerous and benign mucosal tissues have different small length scale light transport properties.