Oybek Kholiqov

Interferometric Near-Infrared Spectroscopy (iNIRS) for determination of optical and dynamical properties of turbid media.

We introduce and implement interferometric near-infrared spectroscopy (iNIRS), which simultaneously extracts optical and dynamical properties of turbid media through analysis of a spectral interference fringe pattern. The spectral interference fringe pattern is measured using a Mach-Zehnder interferometer with a frequency-swept narrow linewidth laser. Fourier analysis of the detected signal is used to determine time-of-flight (TOF)-resolved intensity, which is then analyzed over time to yield TOF-resolved intensity autocorrelations. This approach enables quantification of optical properties, which is not possible in conventional, continuous-wave near-infrared spectroscopy (NIRS). Furthermore, iNIRS quantifies scatterer motion based on TOF-resolved autocorrelations, which is a feature inaccessible by well-established diffuse correlation spectroscopy (DCS) techniques. We prove this by determining TOF-resolved intensity and temporal autocorrelations for light transmitted through diffusive fluid phantoms with optical thicknesses of up to 55 reduced mean free paths (approximately 120 scattering events). The TOF-resolved intensity is used to determine optical properties with time-resolved diffusion theory, while the TOF-resolved intensity autocorrelations are used to determine dynamics with diffusing wave spectroscopy. iNIRS advances the capabilities of diffuse optical methods and is suitable for in vivo tissue characterization. Moreover, iNIRS combines NIRS and DCS capabilities into a single modality.

Reflectance-mode interferometric near-infrared spectroscopy quantifies brain absorption, scattering, and blood flow index in vivo

Interferometric near-infrared spectroscopy (iNIRS) is a new technique that measures time-of-flight- (TOF-) resolved autocorrelations in turbid media, enabling simultaneous estimation of optical and dynamical properties. Here, we demonstrate reflectance-mode iNIRS for noninvasive monitoring of a mouse brain in vivo. A method for more precise quantification with less static interference from superficial layers, based on separating static and dynamic components of the optical field autocorrelation, is presented. Absolute values of absorption, reduced scattering, and blood flow index (BFI) are measured, and changes in BFI and absorption are monitored during a hypercapnic challenge. Absorption changes from TOF-resolved iNIRS agree with absorption changes from continuous wave NIRS analysis, based on TOF-integrated light intensity changes, an effective path length, and the modified Beer-Lambert Law. Thus, iNIRS is a promising approach for quantitative and noninvasive monitoring of perfusion and optical properties in vivo.

Interferometric near-infrared spectroscopy directly quantifies optical field dynamics in turbid media

Sensing and imaging methods based on the dynamic scattering of coherent light (including laser speckle, laser Doppler, diffuse correlation spectroscopy, dynamic light scattering, and diffusing wave spectroscopy) quantify scatterer motion using light intensity fluctuations. The underlying optical field autocorrelation, rather than being measured directly, is typically inferred from the intensity autocorrelation through the Siegert relationship, assuming that the scattered field obeys Gaussian statistics. Here, we demonstrate interferometric near-infrared spectroscopy for measuring the time-of-flight (TOF) resolved field and intensity autocorrelations in turbid media. We find that the Siegert relationship breaks down for short TOFs due to static paths whose optical field does not decorrelate over experimental time scales. We also show that eliminating such paths by polarization gating restores the validity of the Siegert relationship. The unique capability of measuring optical field autocorrelations, as demonstrated here, enables the study of non-Gaussian and non-ergodic light scattering processes. Moreover, direct measurements of field autocorrelations are more efficient than indirect measurements based on intensity autocorrelations. Thus, optical field measurements may improve the quantiffcation of scatterer dynamics with coherent light.

Visible light optical coherence microscopy imaging of the mouse cortex with femtoliter volume resolution

Most flying-spot Optical Coherence Tomography (OCT) and Optical Coherence Microscopy (OCM) systems use a symmetric confocal geometry, where the detection path retraces the illumination path starting from and ending with the spatial mode of a single mode optical fiber. Here, we describe a visible light OCM instrument that breaks this symmetry to improve transverse resolution without sacrificing collection efficiency in scattering tissue. This was achieved by overfilling a 0.3 numerical aperture (NA) water immersion objective on the illumination path, while maintaining a conventional Gaussian mode detection path (1/e2 intensity diameter ~0.82 Airy disks), enabling ~1.1 μm full-width at half-maximum (FWHM) transverse resolution. At the same time, a ~0.9 μm FWHM axial resolution in tissue, achieved by a broadband visible light source, enabled femtoliter volume resolution. We characterized this instrument according to paraxial coherent microscopy theory, and then used it to image the meningeal layers, intravascular red blood cell-free layer, and myelinated axons in the mouse neocortex in vivo through the thinned skull. Finally, by introducing a 0.8 NA water immersion objective, we improved the lateral resolution to 0.44 μm FWHM, which provided a volumetric resolution of ~0.2 fL, revealing cell bodies in cortical layer I of the mouse brain with OCM for the first time.

Highly parallel, interferometric diffusing wave spectroscopy for monitoring cerebral blood flow dynamics

Light-scattering methods are widely used in soft matter physics and biomedical optics to probe dynamics in turbid media, such as diffusion in colloids or blood flow in biological tissue. These methods typically rely on fluctuations of coherent light intensity, and therefore cannot accommodate more than a few modes per detector. This limitation has hindered efforts to measure deep tissue blood flow with high speed, since weak diffuse light fluxes, together with low single-mode fiber throughput, result in low photon count rates. To solve this, we introduce multimode fiber (MMF) interferometry to the field of diffuse optics. In doing so, we transform a standard complementary metal-oxide-semiconductor (CMOS) camera into a sensitive detector array for weak light fluxes that probe deep in biological tissue. Specifically, we build a novel CMOS-based, multimode interferometric diffusing wave spectroscopy (iDWS) system and show that it can measure ∼20 speckles simultaneously near the shot noise limit, acting essentially as ∼20 independent photon-counting channels. We develop a matrix formalism, based on MMF mode field solutions and detector geometry, to predict both coherence and speckle number in iDWS. After validation in liquid phantoms, we demonstrate iDWS pulsatile blood flow measurements at 2.5 cm source-detector separation in the adult human brain in vivo. By achieving highly sensitive and parallel measurements of coherent light fluctuations with a CMOS camera, this work promises to enhance performance and reduce cost of diffuse optical instruments.

Interferometric near-infrared spectroscopy (iNIRS) at short source-detector separations (Conference Presentation)

Interferometric near-infrared spectroscopy (iNIRS) is a recently introduced time-of-flight- (TOF-) resolved sensing method for quantifying optical and dynamical properties of turbid media non-invasively. iNIRS measures the interference spectrum of light traversing a turbid medium using a rapidly tunable, or frequency swept, light source. While the modality was successfully demonstrated in vivo in the nude mouse brain for monitoring absorption, reduced scattering, and blood flow index, translation towards human measurements requires improving light collection efficiency. Particularly, interrogating cortical tissue beneath the adult human scalp and skull remains challenging due to the limited core size and throughput of the single mode fiber currently used for detection. To tackle this problem, we implement a short to null source-detector separation geometry setup to significantly improve the number of detected diffuse photons. We discuss both hardware and post-processing improvements to isolate the desired diffuse signal from the large, non-diffuse and specular signals. Furthermore, key improvements in the iNIRS optical system, including higher TOF resolution (22-60 ps), optimized dynamic range (36-47 dB), faster sweep rate (50-500 kHz), and a technique for combining the forward and backward sweeps to double the effective optical field autocorrelation sampling rate, are presented. These allow for more precise and quantitative extraction of in vivo optical properties and TOF-resolved dynamics at long path lengths. Collectively, these key advances in the technology pave the way for translating iNIRS towards non-invasive, real-time, and quantitative measurements of oxygen metabolism and blood perfusion in deep human tissues.

Quantifying time-of-flight-resolved optical field dynamics in turbid media with interferometric near-infrared spectroscopy (iNIRS) (Conference Presentation)

Sensing and imaging methods based on the dynamic scattering of coherent light, including laser speckle, laser Doppler, and diffuse correlation spectroscopy quantify scatterer motion using light intensity (speckle) fluctuations. The underlying optical field autocorrelation (OFA), rather than being measured directly, is typically inferred from the intensity autocorrelation (IA) through the Siegert relationship, by assuming that the scattered field obeys Gaussian statistics. In this work, we demonstrate interferometric near-infrared spectroscopy (iNIRS) for measurement of time-of-flight (TOF) resolved field and intensity autocorrelations in fluid tissue phantoms and in vivo. In phantoms, we find a breakdown of the Siegert relationship for short times-of-flight due to a contribution from static paths whose optical field does not decorrelate over experimental time scales, and demonstrate that eliminating such paths by polarization gating restores the validity of the Siegert relationship. Inspired by these results, we developed a method, called correlation gating, for separating the OFA into static and dynamic components. Correlation gating enables more precise quantification of tissue dynamics. To prove this, we show that iNIRS and correlation gating can be applied to measure cerebral hemodynamics of the nude mouse in vivo using dynamically scattered (ergodic) paths and not static (non-ergodic) paths, which may not be impacted by blood. More generally, correlation gating, in conjunction with TOF resolution, enables more precise separation of diffuse and non-diffusive contributions to OFA than is possible with TOF resolution alone. Finally, we show that direct measurements of OFA are statistically more efficient than indirect measurements based on IA.

Interferometric near-infrared spectroscopy (iNIRS): performance tradeoffs and optimization

Interferometric near-infrared spectroscopy (iNIRS) is a time-of-flight- (TOF-) resolved sensing modality for determining optical and dynamical properties of a turbid medium. iNIRS achieves this by measuring the interference spectrum of light traversing the medium with a rapidly tunable, or frequency-swept, light source. Thus, iNIRS system performance critically depends on the source and detection apparatus. Using a current-tuned 855 nm distributed feedback laser as the source, we experimentally characterize iNIRS system parameters, including speed, sensitivity, dynamic range, TOF resolution, and TOF range. We also employ a novel Mach-Zehnder interferometer variant with a multi-pass loop to monitor the laser instantaneous linewidth and TOF range at high tuning speeds. We identify and investigate tradeoffs between parameters, with the goal of optimizing performance. We also demonstrate a technique to combine forward and backward sweeps to double the effective speed. Combining these advances, we present in vivo TPSFs and autocorrelations from the mouse brain with TOF resolutions of 22-60 ps, 36-47 dB peak-sidelobe dynamic range, 4-10 μs autocorrelation lag time resolution, a TOF range of nanoseconds or more, and nearly shot noise limited sensitivity.

Interferometric near-infrared spectroscopy(Conference Presentation)

We introduce and implement interferometric near-infrared spectroscopy (iNIRS), which simultaneously extracts the optical and dynamic properties of turbid media from the analysis of the spectral interference fringe pattern. The spectral interference fringe pattern is measured using a Mach-Zehnder interferometer with a frequency swept narrow bandwidth light source such that the temporal intensity autocorrelations can be determined for all photon path lengths. This approach enables time-of-flight (TOF) resolved measurement of scatterer motion, which is a feature inaccessible in well-established diffuse correlation spectroscopy techniques. We prove this by analyzing intensity correlations of the light transmitted through diffusive fluid phantoms with photon random walks of up to 55 (approximately 110 scattering events) using laser sweep rates on the order of 100kHz. Thus, the results we present here advance diffuse optical methods by enabling simultaneous determination of depth-resolved optical properties and dynamics in highly scattering samples.