RF properties of circular-ranging OCT signals.

Circular-ranging optical coherence tomography (CR-OCT) systems that use a time-stepped frequency comb source generate interference fringe signals that are more complex than those of a conventional swept-source OCT system. Here, we define a common terminology for describing these signals, and we develop a mathematical framework that relates the radio-frequency (RF) properties of these fringe signals to the parameters of the frequency comb source. With this framework, we highlight non-intuitive mechanisms whereby the design of the frequency comb source can affect imaging performance. We show, for example, that amplitude-pulsed time-stepped frequency comb sources have a sensitivity advantage over constant power time-stepped frequency comb sources. More broadly, this framework and associated terminology provide a foundation on which to design and optimize time-stepped frequency comb sources and systems.

Nanosecond SRS fiber amplifier for label-free near-infrared photoacoustic microscopy of lipids.

Near-infrared photoacoustics receives increasing interest as an intravital modality to sense key biomolecules. One of the most central types of biomolecules of interest are lipids as they constitute essential bio-hallmarks of cardiovascular and metabolic diseases and their in-vivo detection holds insightful information about disease progression and treatment monitoring. However, the full potential of near-infrared photoacoustic for high-resolution and high-sensitivity biomedical studies of lipids has so far not been exploited due a lack of appropriate excitation sources delivering short-pulses at high-repetition-rate, high-pulse-energy, and wavelength around 1200 nm. Here, we demonstrate a custom-built SRS fiber amplifier that provides optical excitations at 1192.8 nm, repetition rates of 200 kHz, pulse durations below 2 ns, and pulse energies beyond 5 µJ. We capitalize on the performance of our excitation source and show near-infrared photoacoustics resolving intrinsic lipid contrast in biomedically relevant specimens ranging from single cells to lipid-rich tissue with subcellular resolution.

Spectral- and Polarization-Dependent Scattering of Gold Nanobipyramids for Exogenous Contrast in Optical Coherence Tomography.

Polarization-sensitive optical coherence tomography (PS-OCT) reveals the subsurface microstructure of biological tissue and provides information regarding the polarization state of light backscattered from tissue. Complementing OCT's structural signal with molecular imaging requires strategies to simultaneously detect multiple exogenous contrast agents with high specificity in tissue. Specific detection of molecular probes enables the parallel visualization of physiological, cellular, and molecular processes. Here we demonstrate that, by combining PS-OCT and spectral contrast (SC)-OCT measurements, we can distinguish signatures of different gold nanobipyramids (GNBPs) in lymphatic vessels from the surrounding tissue and blood vessels in live mouse models. This technique could well be extended to other anisotropic nanoparticle-based OCT contrast agents and presents significant progress toward enabling OCT molecular imaging.

Multi-beam OCT imaging based on an integrated, free-space interferometer.

While it is a common practice to increase the speed of swept-source optical coherence tomography (OCT) systems by using a high-speed source, this approach may not always be optimal. Parallelization in the form of multiple imaging beams is an alternative approach, but scalable and low-loss multi-beam OCT architectures are needed to capitalize on its advantages. In this study, we demonstrate an eight-beam OCT system using an interferometer architecture comprising planar lightwave circuits (PLC) splitters, V-groove assemblies (VGA), and optical ribbon fibers. We achieved an excess loss and heterodyne efficiency on each channel that was close to that of single-beam systems. In vivo structural imaging of a human finger and OCT angiography imaging of a mouse ear was performed to demonstrate the imaging performance of the system. This work provides further evidence supporting multi-beam architectures as a viable strategy for increasing OCT imaging speed.

Resolving absolute depth in circular-ranging optical coherence tomography by using a degenerate frequency comb.

In Fourier-domain optical coherence tomography, an interference signal is generated that spans an RF bandwidth proportional to the product of three parameters: the imaging range, the imaging speed, and the inverse of the axial resolution. Circular-ranging optical coherence tomography (CR-OCT)architectures were introduced to ease long-range imaging by decoupling the imaging range from the signal RF bandwidth. As a consequence, present CR-OCT systems resolve the relative, but not the absolute, depth location of the scatterers. We introduce here a modified implementation of CR-OCT that uses a degenerate frequency comb source that allows recovery of absolute depth information while only minimally impacting the previously described RF bandwidth compression benefits of CR. We show that this degenerate frequency comb can be created by relatively simple modifications to existing frequency comb source designs, and we present absolute ranging capabilities through imaging studies and simulations.

Resolving absolute depth in circular-ranging optical coherence tomography by using a degenerate frequency comb: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 371 (2020).OPLEDP0146-959210.1364/OL.379968.

Stable multi-megahertz circular-ranging optical coherence tomography at 1.3 µm.

In Fourier-domain optical coherence tomography (OCT), the finite bandwidth of the acquisition electronics constrains the depth range and speed of the system. Circular-ranging (CR) OCT methods use optical-domain compression to surpass this limit. However, the CR-OCT system architectures of prior reports were limited by poor stability and were confined to the 1.55 µm wavelength range. In this work, we describe a novel CR-OCT architecture that is free from these limitations. To ensure stable operation, temperature sensitive optical modules within the system were replaced; the kilometer-length fiber spools used in the stretched-pulse mode-locked (SPML) laser was eliminated in favor of a single 10 meter, continuously chirped fiber Bragg grating, and the interferometer's passive optical quadrature demodulation circuit was replaced by an active technique using a lithium niobate phase modulator. For improved imaging penetration in biological tissues, the system operating wavelength was shifted to a center wavelength of 1.29 µm by leveraging the wavelength flexibility intrinsic to CFBG-based dispersive fibers. These improvements were achieved while maintaining a broad (100 nm) optical bandwidth, a long 4 cm imaging range, and a high 7.6 MHz A-line rate. By enhancing stability, simplifying overall system design, and operating at 1.3 µm, this CR-OCT architecture will allow a broader exploration of CR-OCT in both medical and non-medical applications.

Intravascular Polarimetry in Patients With Coronary Artery Disease.

OBJECTIVES: The aims of this first-in-human pilot study of intravascular polarimetry were to investigate polarization properties of coronary plaques in patients and to examine the relationship of these features with established structural characteristics available to conventional optical frequency domain imaging (OFDI) and with clinical presentation. BACKGROUND: Polarization-sensitive OFDI measures birefringence and depolarization of tissue together with conventional cross-sectional optical frequency domain images of subsurface microstructure. METHODS: Thirty patients undergoing polarization-sensitive OFDI (acute coronary syndrome, n = 12; stable angina pectoris, n = 18) participated in this study. Three hundred forty-two cross-sectional images evenly distributed along all imaged coronary arteries were classified into 1 of 7 plaque categories according to conventional OFDI. Polarization features averaged over the entire intimal area of each cross section were compared among plaque types and with structural parameters. Furthermore, the polarization properties in cross sections (n = 244) of the fibrous caps of acute coronary syndrome and stable angina pectoris culprit lesions were assessed and compared with structural features using a generalized linear model. RESULTS: The median birefringence and depolarization showed statistically significant differences among plaque types (p < 0.001 for both, one-way analysis of variance). Depolarization differed significantly among individual plaque types (p < 0.05), except between normal arteries and fibrous plaques and between fibrofatty and fibrocalcified plaques. Caps of acute coronary syndrome lesions and ruptured caps exhibited lower birefringence than caps of stable angina pectoris lesions (p < 0.01). In addition to clinical presentation, cap birefringence was also associated with macrophage accumulation as assessed using normalized SD. CONCLUSIONS: Intravascular polarimetry provides quantitative metrics that help characterize coronary arterial tissues and may offer refined insight into coronary arterial atherosclerotic lesions in patients.

Quantitative depolarization measurements for fiber-based polarization-sensitive optical frequency domain imaging of the retinal pigment epithelium.

A full quantitative evaluation of the depolarization of light may serve to assess concentrations of depolarizing particles in the retinal pigment epithelium and to investigate their role in retinal diseases in the human eye. Optical coherence tomography and optical frequency domain imaging use spatial incoherent averaging to compute depolarization. Depolarization depends on accurate measurements of the polarization states at the receiver but also on the polarization state incident upon and within the tissue. Neglecting this dependence can result in artifacts and renders depolarization measurements vulnerable to birefringence in the system and in the sample. In this work, we discuss the challenges associated with using a single input polarization state and traditional depolarization metrics such as the degree-of-polarization and depolarization power. We demonstrate quantitative depolarization measurements based on Jones vector synthesis and polar decomposition using fiber-based polarization-sensitive optical frequency domain imaging of the retinal pigment epithelium in a human eye.

Extended Coherence Length and Depth Ranging Using a Fourier-Domain Mode-Locked Frequency Comb and Circular Interferometric Ranging.

Fourier domain mode-locking (FDML) has been a popular laser design for high speed optical frequency domain imaging (OFDI) but the achievable coherence length, and therefore imaging range, has been limited. The narrow instantaneous linewidth of a frequency comb (FC) FDML laser could provide an attractive platform for high speed as well as long range OFDI. Unfortunately, aliasing artifacts arising from signals beyond the principle measurement depth of the free spectral range have prohibited the use of a FC FDML for imaging so far. To make the enhanced coherence length of FC FDML laser available, methods to manage such artifacts are required. Recently, coherent circular ranging has been demonstrated that uses frequency combs for imaging in much reduced RF bandwidths. Here, we revisit circular ranging as a tool of making the long coherence length of an FDML frequency comb laser as well as its use for tissue imaging accessible. Using an acousto-optic frequency shifter (AOFS), we describe an active method to mitigate signal aliasing that is both stable and wavelength independent. We show that an FC FDML laser offers an order of magnitude improved coherence length compared to traditional FDML laser designs without requiring precise dispersion engineering. We discuss design parameters of a frequency stepping laser resonator as well as aliasing from a frequency comb and AOFS in OFDI with numerical simulations. The use of circular ranging additionally reduced acquisition bandwidths 15-fold compared with traditional OFDI methods. The FC FDML/AOFS design offers a convenient platform for long range and high speed imaging as well as exploring signal and image processing methods in circular ranging.

Repeatability Assessment of Intravascular Polarimetry in Patients.

Intravascular polarimetry with polarization sensitive optical frequency domain imaging (PS-OFDI) measures polarization properties of the vessel wall and offers characterization of coronary atherosclerotic lesions beyond the cross-sectional image of arterial microstructure available to conventional OFDI. A previous study of intravascular polarimetry in cadaveric human coronary arteries found that tissue birefringence and depolarization provide valuable insight into key features of atherosclerotic plaques. In addition to various tissue components, catheter and sample motion can also influence the polarization of near infrared light as used by PS-OFDI. This paper aimed to evaluate the robustness and repeatability of imaging tissue birefringence and depolarization in a clinical setting. 30 patients scheduled for percutaneous coronary intervention at the Erasmus Medical Center underwent repeated PS-OFDI pullback imaging, using commercial imaging catheters in combination with a custom-built PS-OFDI console. We identified 274 matching cross sections among the repeat pullbacks to evaluate the reproducibility of the conventional backscatter intensity, the birefringence, and the depolarization signals at each spatial location across the vessel wall. Bland-Altman analysis revealed best agreement for the birefringence measurements, followed by backscatter intensity, and depolarization, when limiting the analysis to areas of meaningful birefringence. Pearson correlation analysis confirmed highest correlation for birefringence (0.86), preceding backscatter intensity (0.83), and depolarization (0.78). Our results demonstrate that intravascular polarimetry generates robust maps of tissue birefringence and depolarization in a clinical setting. This outcome motivates the use of intravascular polarimetry for future clinical studies that investigate polarization properties of arterial atherosclerosis.

High-speed optical coherence tomography by circular interferometric ranging.

Existing three-dimensional optical imaging methods excel in controlled environments but are difficult to deploy over large, irregular and dynamic fields. This has limited imaging in areas such as material inspection and medicine. To better address these applications, we developed methods in optical coherence tomography (OCT) to efficiently interrogate sparse scattering fields, i.e., those in which most locations (voxels) do not generate meaningful signal. Frequency comb sources are used to superimpose reflected signals from equispaced locations through optical subsampling. This results in circular ranging, and reduces the number of measurements required to interrogate large volumetric fields. As a result, signal acquisition barriers that have limited speed and field in OCT are avoided. With a new ultrafast, time-stretched frequency comb laser design operating with 7.6 MHz to 18.9 MHz repetition rates, we achieved imaging of multi-cm3 fields at up to 7.5 volumes per second.

Optic axis mapping with catheter-based polarization-sensitive optical coherence tomography.

Birefringence offers an intrinsic contrast mechanism related to the microstructure and arrangement of fibrillary tissue components. Here we present a reconstruction strategy to recover not only the scalar amount of birefringence but also its optic axis orientation as a function of depth in tissue from measurements with catheter-based polarization sensitive optical coherence tomography. A polarization symmetry constraint, intrinsic to imaging in the backscatter direction, facilitates the required compensation for wavelength-dependent transmission through system elements, the rotating catheter, and overlying tissue layers. Applied to intravascular imaging of coronary atherosclerosis in human patients, the optic axis affords refined interpretation of plaque architecture.

Coronary Plaque Microstructure and Composition Modify Optical Polarization: A New Endogenous Contrast Mechanism for Optical Frequency Domain Imaging.

OBJECTIVES: This study aimed to evaluate whether polarimetry, performed using a modified optical frequency domain imaging (OFDI) system, can improve the assessment of histological features relevant to characterizing human coronary atherosclerosis. BACKGROUND: The microscopic structure and organization of the arterial wall influence the polarization of the infrared light used by OFDI. Modification of the OFDI apparatus, along with recently developed image reconstruction methods, permits polarimetric measurements simultaneously with conventional OFDI cross-sectional imaging through standard intravascular imaging catheters. METHODS: The main coronary arteries of 5 cadaveric human hearts were imaged with an OFDI system capable of providing polarimetric assessment. Cross-sectional views of tissue birefringence, measured in refractive index units, and depolarization, expressed as the ratio of depolarized signal to total intensity, were reconstructed, together with conventional OFDI images. Following imaging, the vessels underwent histological evaluation to enable interpretation of the observed polarization features of individual tissue components. RESULTS: Birefringence in fibrous tissue was significantly higher than in intimal tissue with minimal abnormality (0.44 × 10-3 vs. 0.33 × 10-3; p < 0.0001). Birefringence was highest in the tunica media (p < 0.0001), consistent with its high smooth muscle cell content, cells known to associate with birefringence. In fibrous areas, birefringence showed fine spatial features and close correspondence with the histological appearance of collagen. In contrast, necrotic cores and regions rich in lipid elicited significant depolarization (p < 0.0001). Depolarization was also evident in locations of cholesterol crystals and macrophages. CONCLUSIONS: Intravascular measurements of birefringence and depolarization can be obtained using conventional OFDI catheters in conjunction with a modified console and signal processing algorithms. Polarimetric measurements enhance conventional OFDI by providing additional information related to the tissue composition and offer quantitative metrics enabling characterization of plaque features.

Depolarization signatures map gold nanorods within biological tissue.

Owing to their electromagnetic properties, tunability and biocompatibility, gold nanorods (GNRs) are being investigated as multifunctional probes for a range of biomedical applications. However, detection beyond the reach of traditional fluorescence and two-photon approaches and quantitation of their concentration in biological tissue remain challenging tasks in microscopy. Here we show how the size and aspect ratio that impart GNRs with their plasmonic properties also make them a source of entropy. We report on how depolarization can be exploited as a strategy to visualize GNR diffusion and distribution in biologically relevant scenarios ex vivo, in vitro and in vivo. We identify a deterministic relation between depolarization and nanoparticle concentration. As a result, some of the most stringent experimental conditions can be relaxed, and susceptibility to artefacts is reduced, enabling microscopic and macroscopic applications.

Extended bandwidth wavelength swept laser source for high resolution optical frequency domain imaging.

Improving the axial resolution by providing wider bandwidth wavelength swept lasers remains an important issue for optical frequency domain imaging (OFDI). Here, we demonstrate a wide tuning range, all-fiber wavelength swept laser at a center wavelength of 1250 nm by combining two ring cavities that share a single Fabry-Perot tunable filter. The two cavities contain semiconductor optical amplifiers with central wavelengths of 1190 nm and 1292 nm, respectively. To avoid disturbing interference effects in the overlapping spectral region, we modulated the amplifiers in order to obtain consecutive wavelength sweeps in the two spectral regions. The two sweeps were fused together in post-processing to achieve a total scanning range of 223 nm, corresponding to 3.3 µm axial resolution in air. We confirm improved image quality and reduced speckle size in tomograms of swine esophagus ex vivo, and human skin and nailbed in vivo.

Four-Dimensional Imaging of Cardiac Trabeculae Contracting In Vitro Using Gated OCT.

Cardiac trabeculae are widely used as experimental muscle preparations for studying heart muscle. However, their geometry (diameter, length, and shape) can vary not only among samples, but also within a sample, leading to inaccuracies in estimating their stress production, volumetric energy output, and/or oxygen consumption. Hence, it is desirable to have a system that can accurately image each trabecula in vitro during an experiment. To this end, we constructed an optical coherence tomography system and implemented a gated imaging procedure to image actively contracting trabeculae and reconstruct their time-varying geometry. By imaging a single cross section while monitoring the developed force, we found that gated stimulation of the muscle was sufficiently repeatable to allow us to reconstruct multiple contractions to form a four-dimensional representation of a single muscle contraction cycle. The complete muscle was imaged at various lengths and the cross-sectional area along the muscle was quantified during the contraction cycle. The variation of cross-sectional area along the length during a contraction tended to increase as the muscle was contracting, and this increase was greater at longer muscle lengths. To our knowledge, this is the first system that is able to measure the geometric change of cardiac trabeculae in vitro during a contraction, allowing cross-sectional stress and other volume-dependent parameters to be estimated with greater accuracy.

Degree of polarization (uniformity) and depolarization index: unambiguous depolarization contrast for optical coherence tomography.

The degree of polarization (uniformity) has attracted increased interest as a functional contrast in optical coherence tomography (OCT). However, its computation from a single polarization state suggests an ambiguity that is strongly dependent on a sample's orientation. We here propose an improved metric to present depolarization with respect to the optical system rather than the propagating field. Using numerical simulations and optical frequency domain imaging, we evaluate the conventional DOP(U) for different polarization states and compare its performance with the unambiguous depolarization index.

Single input state, single-mode fiber-based polarization-sensitive optical frequency domain imaging by eigenpolarization referencing.

Fiber-based polarization-sensitive optical frequency domain imaging is more challenging than free-space implementations. Using multiple input states, fiber-based systems provide sample birefringence information with the benefit of a flexible sample arm but come at the cost of increased system and acquisition complexity, and either reduce acquisition speed or require increased acquisition bandwidth. Here we show that with the calibration of a single polarization state, fiber-based configurations can approach the conceptual simplicity of traditional free-space configurations. We remotely control the polarization state of the light incident at the sample using the eigenpolarization states of a wave plate as a reference, and determine the Jones matrix of the output fiber. We demonstrate this method for polarization-sensitive imaging of biological samples.

Optical coherence tomography imaging of cardiac trabeculae.

An integrated instrument is being developed to study live cardiac trabeculae, which is capable of stimulating the muscle under controlled conditions while measuring the heat production, force, and sarcomere length distribution. To improve the accuracy of estimation of stress, strain, and volumetric heat production, the geometry of the muscle must be known. A spectral domain optical coherence tomography system (SD-OCT) has been constructed and calibrated to image the trabecula mounted inside the instrument. This system was mounted above the muscle chamber and a series of equally-spaced cross-sectional images were obtained. These were processed using a workflow developed to extract cross-sectional area and volume. The initial results have demonstrated the feasibility of using OCT to capture the overall geometry of cardiac trabecula mounted in the instrument. Further work will be directed to improve the image quality for larger samples and apply meshing algorithms to the acquired data.