Imaging peritoneal blood vessels through optical coherence tomography angiography for laparoscopic surgery

Laparoscopic surgery presents challenges in identifying blood vessels due to lack of tactile feedback. The image-guided laparoscopic surgical tool (IGLaST) integrated with optical coherence tomography (OCT) has potential for in vivo blood vessel imaging; however, distinguishing vessels from surrounding tissue remains a challenge. In this study, we propose utilizing an inter-A-line intensity differentiation-based OCT angiography (OCTA) to improve visualization of blood vessels. By evaluating a tissue phantom with varying flow speeds, we optimized the system's blood flow imaging capabilities in terms of minimum detectable flow and contrast-to-noise ratio. In vivo experiments on rat and porcine models, successfully visualized previously unidentified blood vessels and concealed blood flows beneath the 1 mm depth peritoneum. Qualitative comparison of various OCTA algorithms indicated that the intensity differentiation-based algorithm performed best for our application. We believe that implementing IGLaST with OCTA can enhance surgical outcomes and reduce procedure time in laparoscopic surgeries.

Direct Blood Cell Flow Imaging in Microvascular Networks

Blood flow dynamics in microvascular networks are intimately related to the health of tissues and organs. While numerous imaging modalities and techniques have been developed to assess blood flow dynamics for various applications, their utilization has been hampered by limited imaging speed and indirect quantification of blood flow dynamics. Here, direct blood cell flow imaging (DBFI) is demonstrated that provides visualization of individual motions of blood cells over a field of 0.71 mm × 1.42 mm with a time resolution of 0.69 ms (1450 frames s-1 ) without using any exogenous agents. DBFI enables precise dynamic analysis of blood cell flow velocities and fluxes in various vessels over a large field, from capillaries to arteries and veins, with unprecedented time resolution. Three exemplary applications of DBFI, quantification of blood flow dynamics of 3D vascular networks, analysis of heartbeat induced blood flow dynamics, and analysis of blood flow dynamics of neurovascular coupling, illustrate the potential of this new imaging technology.

Deep learning-based image enhancement in optical coherence tomography by exploiting interference fringe

Optical coherence tomography (OCT), an interferometric imaging technique, provides non-invasive, high-speed, high-sensitive volumetric biological imaging in vivo. However, systemic features inherent in the basic operating principle of OCT limit its imaging performance such as spatial resolution and signal-to-noise ratio. Here, we propose a deep learning-based OCT image enhancement framework that exploits raw interference fringes to achieve further enhancement from currently obtainable optimized images. The proposed framework for enhancing spatial resolution and reducing speckle noise in OCT images consists of two separate models: an A-scan-based network (NetA) and a B-scan-based network (NetB). NetA utilizes spectrograms obtained via short-time Fourier transform of raw interference fringes to enhance axial resolution of A-scans. NetB was introduced to enhance lateral resolution and reduce speckle noise in B-scan images. The individually trained networks were applied sequentially. We demonstrate the versatility and capability of the proposed framework by visually and quantitatively validating its robust performance. Comparative studies suggest that deep learning utilizing interference fringes can outperform the existing methods. Furthermore, we demonstrate the advantages of the proposed method by comparing our outcomes with multi-B-scan averaged images and contrast-adjusted images. We expect that the proposed framework will be a versatile technology that can improve functionality of OCT.

In vivo imaging of renal microvasculature in a murine ischemia-reperfusion injury model using optical coherence tomography angiography

Optical coherence tomography angiography (OCTA) provides three-dimensional structural and semiquantitative imaging of microvasculature in vivo. We developed an OCTA imaging protocol for a murine kidney ischemia-reperfusion injury (IRI) model to investigate the correlation between renal microvascular changes and ischemic damage. Mice were divided into mild and moderate IRI groups according to the duration of ischemia (10 and 35 mins, respectively). Each animal was imaged at baseline; during ischemia; and at 1, 15, 30, 45, and 60 mins after ischemia. Amplitude decorrelation OCTA images were constructed with 1.5-, 3.0-, and 5.8-ms interscan times, to calculate the semiquantitative flow index in the superficial (50-70 μm) and the deep (220-340 μm) capillaries of the renal cortex. The mild IRI group showed no significant flow index change in both the superfial and the deep layers. The moderate IRI group showed a significantly decreased flow index from 15 and 45 mins in the superficial and deep layers, respectively. Seven weeks after IRI induction, the moderate IRI group showed lower kidney function and higher collagen deposition than the mild IRI group. OCTA imaging of the murine IRI model revealed changes in superficial blood flow after ischemic injury. A more pronounced decrease in superficial blood flow than in deep blood flow was associated with sustained dysfunction after IRI. Further investigation on post-IRI renal microvascular response using OCTA may improve our understanding of the relationship between the degree of ischemic insult and kidney function.

Wide-Field Three-Dimensional Depth-Invariant Cellular-Resolution Imaging of the Human Retina

Three-dimensional (3D) cellular-resolution imaging of the living human retina over a large field of view will bring a great impact in clinical ophthalmology, potentially finding new biomarkers for early diagnosis and improving the pathophysiological understanding of ocular diseases. While hardware-based and computational adaptive optics (AO) optical coherence tomography (OCT) have been developed to achieve cellular-resolution retinal imaging, these approaches support limited 3D imaging fields, and their high cost and intrinsic hardware complexity limit their practical utility. Here, this work demonstrates 3D depth-invariant cellular-resolution imaging of the living human retina over a 3 × 3 mm field of view using the first intrinsically phase-stable multi-MHz retinal swept-source OCT and novel computational defocus and aberration correction methods. Single-acquisition imaging of photoreceptor cells, retinal nerve fiber layer, and retinal capillaries is presented across unprecedented imaging fields. By providing wide-field 3D cellular-resolution imaging in the human retina using a standard point-scan architecture routinely used in the clinic, this platform proposes a strategy for expanded utilization of high-resolution retinal imaging in both research and clinical settings.

Increased capillary stalling is associated with endothelial glycocalyx loss in subcortical vascular dementia

Proper regulation and patency of cerebral microcirculation are crucial for maintaining a healthy brain. Capillary stalling, i.e., the brief interruption of microcirculation has been observed in the normal brain and several diseases related to microcirculation. We hypothesized that endothelial glycocalyx, which is located on the luminal side of the vascular endothelium and involved in cell-to-cell interaction regulation in peripheral organs, is also related to cerebral capillary stalling. We measured capillary stalling and the cerebral endothelial glycocalyx (cEG) in male mice using in vivo optical coherence tomography angiography (OCT-A) and two-photon microscopy. Our findings revealed that some capillary segments were prone to capillary stalling and had less cEG. In addition, we demonstrated that the enzymatic degradation of the cEG increased the capillary stalling, mainly by leukocyte plugging. Further, we noted decreased cEG along with increased capillary stalling in a mouse model of subcortical vascular dementia (SVaD) with impaired cortical microcirculation. Moreover, gene expression related to cEG production or degradation changed in the SVaD model. These results indicate that cEG mediates capillary stalling and impacts cerebral blood flow and is involved in the pathogenesis of SVaD.

Pericyte Loss Leads to Capillary Stalling Through Increased Leukocyte-Endothelial Cell Interaction in the Brain

The neurovascular unit is a functional unit composed of neurons, glial cells, pericytes, and endothelial cells which sustain brain activity. While pericyte is a key component of the neurovascular unit, its role in cerebral blood flow regulation remains elusive. Recently, capillary stalling, which means the transient interruption of microcirculation in capillaries, has been shown to have an outsized impact on microcirculatory changes in several neurological diseases. In this study, we investigated capillary stalling and its possible causes, such as the cerebral endothelial glycocalyx and leukocyte adhesion molecules after depleting pericytes postnatally in mice. Moreover, we investigated hypoxia and gliosis as consequences of capillary stalling. Although there were no differences in the capillary structure and RBC flow, longitudinal optical coherence tomography angiography showed an increased number of stalled segments in capillaries after pericyte loss. Furthermore, the extent of the cerebral endothelial glycocalyx was decreased with increased expression of leukocyte adhesion molecules, suggesting enhanced interaction between leukocytes and endothelial cells. Finally, pericyte loss induced cerebral hypoxia and gliosis. Cumulatively, the results suggest that pericyte loss induces capillary stalling through increased interaction between leukocytes and endothelial cells in the brain.

Comprehensive Assessment of High-Risk Plaques by Dual-Modal Imaging Catheter in Coronary Artery

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Fluorescence lifetime imaging (FLIm) allows label-free biochemical visualization of atheromas; however, it remains unknown whether FLIm can characterize high-risk plaque features in coronary arteries in a beating heart. Also, implementation of a novel analytic methodology is required for multispectral FLIm because it yields massive biochemical readouts.

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This study first demonstrated a simultaneous structural and biochemical assessment of high-risk plaques in the beating swine coronary arteries using a fully integrated optical coherence tomography-FLIm and a 2.9-F low-profile dual-modal catheter.

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Biochemical components of atherosclerotic plaques, including lipids, macrophages, lipids+macrophages, and fibrotic tissues, had unique fluorescence lifetime signatures that were clearly distinguishable using multispectral FLIm.

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Machine learning framework was successfully integrated with multispectral FLIm and enabled an automated, quantitative imaging of multiple key components associated with plaque destabilization.

Coronary plaque destabilization involves alterations in microstructure and biochemical composition; however, no imaging approach allows such comprehensive characterization. Herein, the authors demonstrated a simultaneous microstructural and biochemical assessment of high-risk plaques in the coronary arteries in a beating heart using a fully integrated optical coherence tomography and fluorescence lifetime imaging (FLIm). It was found that plaque components such as lipids, macrophages, lipids+macrophages, and fibrotic tissues had unique fluorescence lifetime signatures that were distinguishable using multispectral FLIm. Because FLIm yielded massive biochemical readouts, the authors incorporated machine learning framework into FLIm, and ultimately, their approach enabled an automated, quantitative imaging of multiple key components relevant for plaque destabilization.

Macrophage targeted theranostic strategy for accurate detection and rapid stabilization of the inflamed high-risk plaque

Rationale: Inflammation plays a pivotal role in the pathogenesis of the acute coronary syndrome. Detecting plaques with high inflammatory activity and specifically treating those lesions can be crucial to prevent life-threatening cardiovascular events. Methods: Here, we developed a macrophage mannose receptor (MMR)-targeted theranostic nanodrug (mannose-polyethylene glycol-glycol chitosan-deoxycholic acid-cyanine 7-lobeglitazone; MMR-Lobe-Cy) designed to identify inflammatory activity as well as to deliver peroxisome proliferator-activated gamma (PPARγ) agonist, lobeglitazone, specifically to high-risk plaques based on the high mannose receptor specificity. The MMR-Lobe-Cy was intravenously injected into balloon-injured atheromatous rabbits and serial in vivo optical coherence tomography (OCT)-near-infrared fluorescence (NIRF) structural-molecular imaging was performed. Results: One week after MMR-Lobe-Cy administration, the inflammatory NIRF signals in the plaques notably decreased compared to the baseline whereas the signals in saline controls even increased over time. In accordance with in vivo imaging findings, ex vivo NIRF signals on fluorescence reflectance imaging (FRI) and plaque inflammation by immunostainings significantly decreased compared to oral lobeglitazone group or saline controls. The anti-inflammatory effect of MMR-Lobe-Cy was mediated by inhibition of TLR4/NF-κB pathway. Furthermore, acute resolution of inflammation altered the inflamed plaque into a stable phenotype with less macrophages and collagen-rich matrix. Conclusion: Macrophage targeted PPARγ activator labeled with NIRF rapidly stabilized the inflamed plaques in coronary sized artery, which could be quantitatively assessed using intravascular OCT-NIRF imaging. This novel theranostic approach provides a promising theranostic strategy for high-risk coronary plaques.

Robust and easy-to-operate stretched-pulse mode-locked wavelength-swept laser with an all-polarization-maintaining fiber cavity for 10 MHz A-line rate optical coherence tomography

We demonstrate robust and easy-to-operate stretched-pulse mode-locked laser (SPML) architectures using all-polarization-maintaining fiber laser cavities. Because of the polarization-maintaining construction, the laser performance is unaffected by mechanical perturbation on the cavity fibers. The lasers automatically initiate linear-in-wavenumber sweeps across 100 nm centered at 1290 nm with a 10 MHz repetition rate. OCT imaging with a sensitivity of 98 dB and a single-sided 6 dB coherence length of 2.5 mm is demonstrated. OCT angiography of a mouse brain that visualized three-dimensional cerebral microvasculature over a field of 1.5mm×1.5mm (398 A-lines × 380 B-scans) at a rate of 5.26 volumes per second is also presented. The robust all-PMF SPML lasers are a turnkey, high-performance source for ultrahigh-speed OCT imaging.

Visualization of two-dimensional transverse blood flow direction using optical coherence tomography angiography

SIGNIFICANCE

Evaluation of vessel patency and blood flow direction is important in various medical situations, including diagnosis and monitoring of ischemic diseases, and image-guided vascular surgeries. While optical coherence tomography angiography (OCTA) is the most widely used functional extension of optical coherence tomography that visualizes three-dimensional vasculature, inability to provide information of blood flow direction is one of its limitations.

AIM

We demonstrate two-dimensional (2D) transverse blood flow direction imaging in en face OCTA.

APPROACH

A series of triangular beam scans for the fast axis was implemented in the horizontal direction for the first volume scan and in the vertical direction for the following volume scan, and the inter A-line OCTA was performed for the blood flow direction imaging while the stepwise pattern was used for each slow axis scan. The decorrelation differences between the forward and the backward inter A-line OCTA were calculated for the horizontal and the vertical fast axis scans, and the ratio of the horizontal and the vertical decorrelation differences was utilized to show the 2D transverse flow direction information.

RESULTS

OCTA flow direction imaging was verified using flow phantoms with various flow orientations and speeds, and we identified the flow speed range relative to the scan speed for reliable flow direction measurement. We demonstrated the visualization of 2D transverse blood flow orientations in mouse brain vascular networks in vivo.

CONCLUSIONS

The proposed OCTA imaging technique that provides information of 2D transverse flow direction can be utilized in various clinical applications and preclinical studies.

High-speed optical coherence tomography angiography for the measurement of stimulus-induced retrograde vasodilation of cerebral pial arteries in awake mice

Significance: Having a clear understanding of functional hyperemia is crucial for functional brain imaging and neurological disease research. Vasodilation induced by sensory stimulus propagates from the arterioles to the upstream pial arteries in a retrograde fashion. As retrograde vasodilation occurs briefly in the early stage of functional hyperemia, an imaging technique with a high temporal resolution is required for its measurement. Aim: We aimed to present an imaging method to measure stimulus-induced retrograde vasodilation in awake animals. Approach: An imaging method based on optical coherence tomography angiography, which enables a high-speed and label-free vessel diameter measurement, was developed and applied for the investigation. Results: The propagation speed of retrograde vasodilation of pial artery was measured in awake mice. Other characteristics of functional hyperemia such as temporal profile and amplitude of the vascular response were also investigated. Conclusions: Our results provide detailed information of stimulus-induced hemodynamic response in the brain of awake mice and suggest the potential utility of our imaging method for the study of functional hyperemia in normal and diseased brain.

9.4 MHz A-line rate optical coherence tomography at 1300 nm using a wavelength-swept laser based on stretched-pulse active mode-locking

In optical coherence tomography (OCT), high-speed systems based at 1300 nm are among the most broadly used. Here, we present 9.4 MHz A-line rate OCT system at 1300 nm. A wavelength-swept laser based on stretched-pulse active mode locking (SPML) provides a continuous and linear-in-wavenumber sweep from 1240 nm to 1340 nm, and the OCT system using this light source provides a sensitivity of 98 dB and a single-sided 6-dB roll-off depth of 2.5 mm. We present new capabilities of the 9.4 MHz SPML-OCT system in three microscopy applications. First, we demonstrate high quality OCTA imaging at a rate of 1.3 volumes/s. Second, by utilizing its inherent phase stable characteristics, we present wide dynamic range en face Doppler OCT imaging with multiple time intervals ranging from 0.25 ms to 2.0 ms at a rate of 0.53 volumes/s. Third, we demonstrate video-rate 4D microscopic imaging of a beating Xenopus embryo heart at a rate of 30 volumes/s. This high-speed and high-performance OCT system centered at 1300 nm suggests that it can be one of the most promising high-speed OCT platforms enabling a wide range of new scientific research, industrial, and clinical applications at speeds of 10 MHz.

Dll4 Suppresses Transcytosis for Arterial Blood-Retinal Barrier Homeostasis

RATIONALE

Central nervous system has low vascular permeability by organizing tight junction (TJ) and limiting endothelial transcytosis. While TJ has long been considered to be responsible for vascular barrier in central nervous system, suppressed transcytosis in endothelial cells is now emerging as a complementary mechanism. Whether transcytosis regulation is independent of TJ and its dysregulation dominantly causes diseases associated with edema remain elusive. Dll4 signaling is important for various vascular contexts, but its role in the maintenance of vascular barrier in central nervous system remains unknown.

OBJECTIVE

To find a TJ-independent regulatory mechanism selective for transcytosis and identify its dysregulation as a cause of pathological leakage.

METHODS AND RESULTS

We studied transcytosis in the adult mouse retina with low vascular permeability and employed a hypertension-induced retinal edema model for its pathological implication. Both antibody-based and genetic inactivation of Dll4 or Notch1 induce hyperpermeability by increasing transcytosis without junctional destabilization in arterial endothelial cells, leading to nonhemorrhagic leakage predominantly in the superficial retinal layer. Endothelial Sox17 deletion represses Dll4 in retinal arteries, phenocopying Dll4 blocking-driven vascular leakage. Ang II (angiotensin II)-induced hypertension represses arterial Sox17 and Dll4, followed by transcytosis-driven retinal edema, which is rescued by a gain of Notch activity. Transcriptomic profiling of retinal endothelial cells suggests that Dll4 blocking activates SREBP1 (sterol regulatory element-binding protein 1)-mediated lipogenic transcription and enriches gene sets favorable for caveolae formation. Profiling also predicts the activation of VEGF (vascular endothelial growth factor) signaling by Dll4 blockade. Inhibition of SREBP1 or VEGF-VEGFR2 (VEGF receptor 2) signaling attenuates both Dll4 blockade-driven and hypertension-induced retinal leakage.

CONCLUSIONS

In the retina, Sox17-Dll4-SREBP1 signaling axis controls transcytosis independently of TJ in superficial arteries among heterogeneous regulations for the whole vessels. Uncontrolled transcytosis via dysregulated Dll4 underlies pathological leakage in hypertensive retina and could be a therapeutic target for treating hypertension-associated retinal edema.

Quantitative hemodynamic analysis of cerebral blood flow and neurovascular coupling using optical coherence tomography angiography

Functional hyperemia in the rat cortex was investigated using high-speed optical coherence tomography (OCT) angiography and Doppler OCT. OCT angiography (OCTA) was performed to image the hemodynamic stimulus-response over a wide field of view. Temporal changes in vessel diameters in different vessel compartments, which were determined as the diameters of erythrocyte flows in OCT angiograms, were measured in order to monitor localized hemodynamic changes. Our results showed that the dilation of arterioles at the site of activation was accompanied by the dilation of upstream arteries. Relatively negligible dilation was observed in veins. An increase in the OCTA signal was observed during stimulus in multiple capillaries, which may imply that capillary blood flow increases as a result of the expanded arterial blood volume. These results agree with previous observations using two-photon laser scanning microscopy (TPLSM). Doppler OCT was performed to quantitatively measure stimulus-induced blood flow response in pial arteries. The measurement showed small but clear hemodynamic response in upstream arteries with diameters exceeding 100 μm. Our results demonstrate the potential of OCTA and Doppler OCT for the investigation of neurovascular coupling in small animal models.

Oxygen-Induced Retinopathy and Choroidopathy: In Vivo Longitudinal Observation of Vascular Changes Using OCTA

Purpose

The purpose of this study was to assess the retinal and choroidal vasculatures of an oxygen-induced retinopathy (OIR) rat model using optical coherence tomography angiography (OCTA) as well as to verify the performance of OCTA for visualizing in vivo vascular alterations, longitudinally and quantitatively.

Methods

To induce OIR, Sprague Dawley rat pups were incubated in an 80% oxygen chamber from postnatal day 1 (P1) to P11 and returned to room air. OCTA imaging was performed in six eyes at P15, P18, P21, and P24. All eyes were imaged with ex vivo retinal flat mount immunofluorescence microscopy for comparison with OCTA. The areas of the neovascular tufts, retinal vessel tortuosities and diameters, and vessel densities of different retinal and choroidal layers were quantified.

Results

The neovascular tufts were observed in two OIR eyes. The tuft areas decreased spontaneously from P18 to P24. The increase in arterial tortuosity and venous dilation were observed in the OIR eyes at P15 and P18. The retardation of vascular developments was observed in the deep vascular plexus and the choroidal layer in the OIR group while the superficial vascular plexus did not show developmental delay.

Conclusions

This study demonstrates an application of OCTA for quantitative and longitudinal studies on in vivo vascular alterations, including neovascular tufts, increase in arterial tortuosity, venous dilation, and developmental delay in the OIR rat model.

Tie2 activation promotes choriocapillary regeneration for alleviating neovascular age-related macular degeneration

Choriocapillary loss is a major cause of neovascular age-related macular degeneration (NV-AMD). Although vascular endothelial growth factor (VEGF) blockade for NV-AMD has shown beneficial outcomes, unmet medical needs for patients refractory or tachyphylactic to anti-VEGF therapy exist. In addition, the treatment could exacerbate choriocapillary rarefaction, necessitating advanced treatment for fundamental recovery from NV-AMD. In this study, Tie2 activation by angiopoietin-2-binding and Tie2-activating antibody (ABTAA) presents a therapeutic strategy for NV-AMD. Conditional Tie2 deletion impeded choriocapillary maintenance, rendering eyes susceptible to NV-AMD development. Moreover, in a NV-AMD mouse model, ABTAA not only suppressed choroidal neovascularization (CNV) and vascular leakage but also regenerated the choriocapillaris and relieved hypoxia. Conversely, VEGF blockade degenerated the choriocapillaris and exacerbated hypoxia, although it suppressed CNV and vascular leakage. Together, we establish that angiopoietin-Tie2 signaling is critical for choriocapillary maintenance and that ABTAA represents an alternative, combinative therapeutic strategy for NV-AMD by alleviating anti-VEGF adverse effects.

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.

Macrophage-Mediated Exocytosis of Elongated Nanoparticles Improves Hepatic Excretion and Cancer Phototherapy

The introduction of nanoparticle-mediated delivery and therapy has revolutionized cancer treatment approaches. However, there has been limited success in clinical trials because current approaches have not simultaneously satisfied therapeutic efficacy and biosafety criteria to an adequate degree. Here, we employ efficient macrophage-mediated exocytosis of elongated nanoparticles to facilitate their localization in tumor cells for cancer therapy and their transport to hepatocytes for hepatobiliary excretion. In vitro studies show that PEGylated high-aspect ratio gold nanoparticles exit macrophages more rapidly and remain in tumor cells longer, compared with low-aspect ratio and spherical nanoparticles. In tumors, high-aspect ratio nanoparticles tend to stay in tumor cells and escape from tumor-associated macrophages when they are taken up by those cells. In the liver, high-aspect ratio nanoparticles cleared by Kupffer cells mostly take the hepatobiliary excretion pathway through efficient Kupffer cell-hepatocyte transfer. Furthermore, we demonstrate that time-dependent localization of elongated gold nanoparticles toward tumor cells in tumor tissues enhances the overall phototherapeutic outcome. Engineering nanoparticles to modulate their exocytosis provides a new approach to improve cancer nanomedicine and pave the way toward clinical translation.

Wide dynamic range high-speed three-dimensional quantitative OCT angiography with a hybrid-beam scan

We demonstrate a novel hybrid-beam scanning-based quantitative optical coherence tomography angiography (OCTA) that provides high-speed wide dynamic range blood flow speed imaging. The hybrid-beam scanning scheme enables multiple OCTA image acquisitions with a wide range of multiple time intervals simultaneously providing wide dynamic range blood flow speed imaging independent of the blood vessel orientation, which was quantified over a speed range of 0.6∼104  mm/s through the blood flow phantom experiments. A fully automated high-speed hybrid-beam scanning-based quantitative OCTA system demonstrates visualization of blood flow speeds in various vessels from the main arteries to capillaries in a 4  mm×4  mm area (1024 A-lines × 512 B-scans) in vivo in 20 s, showing its potential as a useful imaging tool for various biomedical applications.

Multispectral analog-mean-delay fluorescence lifetime imaging combined with optical coherence tomography

The pathophysiological progression of chronic diseases, including atherosclerosis and cancer, is closely related to compositional changes in biological tissues containing endogenous fluorophores such as collagen, elastin, and NADH, which exhibit strong autofluorescence under ultraviolet excitation. Fluorescence lifetime imaging (FLIm) provides robust detection of the compositional changes by measuring fluorescence lifetime, which is an inherent property of a fluorophore. In this paper, we present a dual-modality system combining a multispectral analog-mean-delay (AMD) FLIm and a high-speed swept-source optical coherence tomography (OCT) to simultaneously visualize the cross-sectional morphology and biochemical compositional information of a biological tissue. Experiments using standard fluorescent solutions showed that the fluorescence lifetime could be measured with a precision of less than 40 psec using the multispectral AMD-FLIm without averaging. In addition, we performed ex vivo imaging on rabbit iliac normal-looking and atherosclerotic specimens to demonstrate the feasibility of the combined FLIm-OCT system for atherosclerosis imaging. We expect that the combined FLIm-OCT will be a promising next-generation imaging technique for diagnosing atherosclerosis and cancer due to the advantages of the proposed label-free high-precision multispectral lifetime measurement.

Therapeutic Effects of Targeted PPARɣ Activation on Inflamed High-Risk Plaques Assessed by Serial Optical Imaging In Vivo

Rationale: Atherosclerotic plaque is a chronic inflammatory disorder involving lipid accumulation within arterial walls. In particular, macrophages mediate plaque progression and rupture. While PPARγ agonist is known to have favorable pleiotropic effects on atherogenesis, its clinical application has been very limited due to undesirable systemic effects. We hypothesized that the specific delivery of a PPARγ agonist to inflamed plaques could reduce plaque burden and inflammation without systemic adverse effects.

Methods: Herein, we newly developed a macrophage mannose receptor (MMR)-targeted biocompatible nanocarrier loaded with lobeglitazone (MMR-Lobe), which is able to specifically activate PPARγ pathways within inflamed high-risk plaques, and investigated its anti-atherogenic and anti-inflammatory effects both in in vitro and in vivo experiments.

Results: MMR-Lobe had a high affinity to macrophage foam cells, and it could efficiently promote cholesterol efflux via LXRα-, ABCA1, and ABCG1 dependent pathways, and inhibit plaque protease expression. Using in vivo serial optical imaging of carotid artery, MMR-Lobe markedly reduced both plaque burden and inflammation in atherogenic mice without undesirable systemic effects. Comprehensive analysis of en face aorta by ex vivo imaging and immunostaining well corroborated the in vivo findings.

Conclusion: MMR-Lobe was able to activate PPARγ pathways within high-risk plaques and effectively reduce both plaque burden and inflammation. This novel targetable PPARγ activation in macrophages could be a promising therapeutic strategy for high-risk plaques.

Comparison of Accuracy of One-Use Methods for Calculating Fractional Flow Reserve by Intravascular Optical Coherence Tomography to That Determined by the Pressure-Wire Method

Although the identification of the hemodynamic significance of coronary lesions becomes important for revascularization strategy, the potential role of 3-dimensional high-resolution intracoronary optical coherence tomography (OCT) for predicting functional significance of coronary lesions remains unclear. We assessed the diagnostic performance of 2 computational approaches for deriving fractional flow reserve (FFR) from intravascular OCT images. We developed 2 methods to derive FFR-OCT by AFD (FFR-OCT_AFD) and FFR-OCT by CFD (FFR-OCT_CFD). Among 217 eligible patients between 2011 and 2014, 104 were included for data analysis (9 for derivation, 95 for validation). Luminal geometries from 3-dimensional OCT were used for both FFR-OCT_AFD and FFR-OCT_CFD calculations. The analytical fluid dynamics method calculated FFR from the blood flow resistance estimated using Poiseuille's law. For computational fluid dynamics, we numerically solved the Navier-Stokes equation in a steady-state flow with the distal porous media model for the capillary vessels. We examined the diagnostic performance of FFR-OCT_AFD and FFR-OCT_CFD compared with the pressure-wire measured FFR. The accuracy, sensitivity, specificity, PPV, and NPV were 86%, 65%, 94%, 81%, and 88% for FFR-OCT_AFD and 86%, 73%, 91%, 76%, and 90% for FFR-OCT_CFD. The area under the curve of the receiver-operating characteristic curve was 0.88 for FFR-OCT_AFD and 0.86 for FFR-OCT_CFD. FFR-OCT_AFD and FFR-OCT_CFD showed a strong linear correlation with the measured FFR (r = 0.631; p <0.001, r = 0.655; p <0.001, respectively). FFR derived from high-resolution volumetric OCT images showed high diagnostic performance for the detection of coronary ischemia. In conclusion, OCT-derived FFR may be useful for guiding the management of coronary artery disease.

Imaging Laser-Induced Choroidal Neovascularization in the Rodent Retina Using Optical Coherence Tomography Angiography

PURPOSE

The purpose of this study was to evaluate the performance of optical coherence tomography angiography (OCTA) in visualizing laser-induced choroidal neovascularization (CNV) in the rodent retina.

METHODS

Choroidal neovascularization was induced via laser photocoagulation in 2 male Brown Norway rats and 2 male C57BL/6 mice. For qualitative comparison, the animals were imaged in vivo with OCTA, indocyanine green angiography (ICGA), and fluorescein angiography (FA), and ex vivo with immunofluorescence confocal microscopy, 14 days post laser photocoagulation without anti-vascular endothelial growth factor (anti-VEGF) intervention. For longitudinal quantitative analysis, CNV was induced in 6 additional male C57BL/6 mice. Three mice intravitreally received an anti-VEGF agent and the remaining 3 mice phosphate buffered saline (PBS) vehicle 7 days post laser photocoagulation. These animals were imaged using OCTA 6, 14, and 21 days post laser photocoagulation. The area and volume of the laser-induced CNV lesions were measured longitudinally.

RESULTS

In both mice and rats, OCTA qualitatively showed high correlation with FA, ICGA, and immunofluorescence imaging. Unlike FA and ICGA, which does not show the microvasculature due to dye leakage, OCTA visualized the CNV microvasculature with resolution and contrast comparable to immunofluorescence images. Longitudinal imaging enabled normalization of the CNV area and volume, reducing inherent variation in the CNV size. By using only 3 mice in each group, statistically significant differences (P < 0.01) in the CNV area and volume could be demonstrated.

CONCLUSIONS

Optical coherence tomography angiography enables noninvasive visualization of the laser-induced CNV microvasculature in the rodent retina with high resolution and tissue-lumen contrast, providing quantifiable in vivo measurements for longitudinal analysis.

Intravascular optical coherence tomography [Invited]

Shortly after the first demonstration of optical coherence tomography for imaging the microstructure of the human eye, work began on developing systems and catheters suitable for intravascular imaging in order to diagnose and investigate atherosclerosis and potentially to monitor therapy. This review covers the driving considerations of the clinical application and its constraints, the major engineering milestones that enabled the current, high-performance commercial imaging systems, the key studies that laid the groundwork for image interpretation, and the clinical research that traces intravascular optical coherence tomography (OCT) from early human pilot studies to current clinical trials.

A Study on CYP2C19 and CYP2D6 Polymorphic Effects on Pharmacokinetics and Pharmacodynamics of Amitriptyline in Healthy Koreans

We performed a double-blinded, genotype-based stratification study to explore the pharmacokinetics and pharmacodynamics of amitriptyline according to CYP2C19 and CYP2D6 genotype in Korean subjects. Twenty-four healthy adults were grouped by genotype of CYP2C19 and CYP2D6. After a single dose of 25 mg of amitriptyline, blood samples were collected and anticholinergic effects were measured. The extent of N-demethylation of amitriptyline significantly decreased in subjects carrying two nonfunctional alleles of CYP2C19. The extent of hydroxylation of amitriptyline or nortriptyline was significantly reduced in subjects carrying two CYP2D6 decreased functional alleles compared with those with no or one decreased functional allele. The overall metabolic pathway of amitriptyline was more likely to be dominated by CYP2C19 than CYP2D6. The gene variations of CYP2C19 and CYP2D6 did not change the pharmacodynamic effect. The findings of this study will provide useful information on individualized drug treatment with amitriptyline considering both CYP2D6 and CYP2C19 gene variations.

Single cardiac cycle three-dimensional intracoronary optical coherence tomography

While high-speed intracoronary optical coherence tomography (OCT) provides three-dimensional (3D) visualization of coronary arteries in vivo, imaging speeds remain insufficient to avoid motion artifacts induced by heartbeat, limiting the clinical utility of OCT. In this paper, we demonstrate development of a high-speed intracoronary OCT system (frame rate: 500 frames/s, pullback speed: 100 mm/s) along with prospective electrocardiogram (ECG) triggering technology, which enabled volumetric imaging of long coronary segments within a single cardiac cycle (70 mm pullback in 0.7 s) with minimal cardiac motion artifact. This technology permitted detailed visualization of 3D architecture of the coronary arterial wall of a swine in vivo and fine structure of the implanted stent.

Characterization of lipid-rich plaques using spectroscopic optical coherence tomography

Intravascular optical coherence tomography (IV-OCT) is a high-resolution imaging method used to visualize the internal structures of walls of coronary arteries in vivo. However, accurate characterization of atherosclerotic plaques with gray-scale IV-OCT images is often limited by various intrinsic artifacts. In this study, we present an algorithm for characterizing lipid-rich plaques with a spectroscopic OCT technique based on a Gaussian center of mass (GCOM) metric. The GCOM metric, which reflects the absorbance properties of lipids, was validated using a lipid phantom. In addition, the proposed characterization method was successfully demonstrated in vivo using an atherosclerotic rabbit model and was found to have a sensitivity and specificity of 94.3% and 76.7% for lipid classification, respectively.

Abstract 225: Anti-atherosclerotic Effects of Plaque Macrophage Targetable Nanoparticle-mediated PPAR-γ Agonist Delivery in Atherogenic Mice

Introduction: Atherosclerosis is a process of lipid accumulation and inflammation. In particular, plaque macrophages are associated with fibrous cap destabilization and rupture. We hypothesized that the specific delivery of a PPAR-γ agonist to inflamed plaques via nanoprobe (NP) targeting macrophage mannose receptors could reduce plaque burden and inflammation.

Methods and Results: Macrophage mannose receptor targetable nanoprobe (MMR-NP) was fabricated by chemically conjugating thiolated glycol chitosan with mannose-PEG-maleimide, followed by incorporating PPAR-γ agonist (lobeglitazone) into this nanoscale delivery system (MMR-Lobe). For in vivo monitoring of therapeutic response, near-infrared fluorescence NP was prepared by conjugation of Cy5.5 (ext/emi 675/694 nm) with MMR-NP. In cellular uptake study, the MMR-NP showed high affinity to mannose receptors on macrophages. MMR-Lobe attenuated LPS-induced inflammatory cytokines such as TNF-α, IL-6, and MMP-9 in RAW264.7 cells. Additionally, MMR-Lobe increased expression of ABCA1, ABCG1 and LXR-α, known as PPAR-γ regulator genes involved in cholesterol efflux, in RAW264.7 cells. Using a customized high-resolution intravital fluorescence microscope, the in vivo serial imaging of carotid atheroma in apoE-/- mice injected with MMR-Lobe (7 mg/kg, twice weekly for 4 weeks) revealed a significant decrease in plaque burden and inflammation (Figure) as compared to baseline, or non-treated controls, or p.o. Lobe treated mice (p<0.01 respectively, 6 mice in each group). En face imaging, FM, and immunostainings corroborated the in vivo findings.

Conclusions: MMR-Lobe was able to selectively target atheroma macrophages, and effectively reduce both plaque burden and inflammation as assessed by serial intravital optical imaging. We suggest that nanoprobe-mediated PPAR-γ agonist delivery targeting plaque macrophages holds a promising theranostic approach for high-risk atheromata.

Intravascular optical imaging of high-risk plaques in vivo by targeting macrophage mannose receptors

Macrophages mediate atheroma expansion and disruption, and denote high-risk arterial plaques. Therefore, they are substantially gaining importance as a diagnostic imaging target for the detection of rupture-prone plaques. Here, we developed an injectable near-infrared fluorescence (NIRF) probe by chemically conjugating thiolated glycol chitosan with cholesteryl chloroformate, NIRF dye (cyanine 5.5 or 7), and maleimide-polyethylene glycol-mannose as mannose receptor binding ligands to specifically target a subset of macrophages abundant in high-risk plaques. This probe showed high affinity to mannose receptors, low toxicity, and allowed the direct visualization of plaque macrophages in murine carotid atheroma. After the scale-up of the MMR-NIRF probe, the administration of the probe facilitated in vivo intravascular imaging of plaque inflammation in coronary-sized vessels of atheromatous rabbits using a custom-built dual-modal optical coherence tomography (OCT)-NIRF catheter-based imaging system. This novel imaging approach represents a potential imaging strategy enabling the identification of high-risk plaques in vivo and holds promise for future clinical implications.

Intracoronary dual-modal optical coherence tomography-near-infrared fluorescence structural-molecular imaging with a clinical dose of indocyanine green for the assessment of high-risk plaques and stent-associated inflammation in a beating coronary artery

AIMS

Inflammation plays essential role in development of plaque disruption and coronary stent-associated complications. This study aimed to examine whether intracoronary dual-modal optical coherence tomography (OCT)-near-infrared fluorescence (NIRF) structural-molecular imaging with indocyanine green (ICG) can estimate inflammation in swine coronary artery.

METHODS AND RESULTS

After administration of clinically approved NIRF-enhancing ICG (2.0 mg/kg) or saline, rapid coronary imaging (20 mm/s pullback speed) using a fully integrated OCT-NIRF catheter was safely performed in 12 atheromatous Yucatan minipigs and in 7 drug-eluting stent (DES)-implanted Yorkshire pigs. Stronger NIRF activity was identified in OCT-proven high-risk plaque compared to normal or saline-injected controls (P = 0.0016), which was validated on ex vivo fluorescence reflectance imaging. In vivo plaque target-to-background ratio (pTBR) was much higher in inflamed lipid-rich plaque compared to fibrous plaque (P < 0.0001). In vivo and ex vivo peak pTBRs correlated significantly (P < 0.0022). In vitro cellular ICG uptake and histological validations corroborated the OCT-NIRF findings in vivo. Indocyanine green colocalization with macrophages and lipids of human plaques was confirmed with autopsy atheroma specimens. Two weeks after DES deployment, OCT-NIRF imaging detected strong NIRF signals along stent struts, which was significantly higher than baseline (P = 0.0156). Histologically, NIRF signals in peri-strut tissue co-localized well with macrophages.

CONCLUSION

The OCT-NIRF imaging with a clinical dose of ICG was feasible to accurately assess plaque inflammation and DES-related inflammation in a beating coronary artery. This highly translatable dual-modal molecular-structural imaging strategy could be relevant for clinical intracoronary estimation of high-risk plaques and DES biology.

GPU-accelerated framework for intracoronary optical coherence tomography imaging at the push of a button

Frequency domain optical coherence tomography (FD-OCT) has become one of the important clinical tools for intracoronary imaging to diagnose and monitor coronary artery disease, which has been one of the leading causes of death. To help more accurate diagnosis and monitoring of the disease, many researchers have recently worked on visualization of various coronary microscopic features including stent struts by constructing three-dimensional (3D) volumetric rendering from series of cross-sectional intracoronary FD-OCT images. In this paper, we present the first, to our knowledge, "push-of-a-button" graphics processing unit (GPU)-accelerated framework for intracoronary OCT imaging. Our framework visualizes 3D microstructures of the vessel wall with stent struts from raw binary OCT data acquired by the system digitizer as one seamless process. The framework reports the state-of-the-art performance; from raw OCT data, it takes 4.7 seconds to provide 3D visualization of a 5-cm-long coronary artery (of size 1600 samples x 1024 A-lines x 260 frames) with stent struts and detection of malapposition automatically at the single push of a button.

Multi-functional angiographic OFDI using frequency-multiplexed dual-beam illumination

Detection of blood flow inside the tissue sample can be achieved by measuring the local change of complex signal over time in angiographic optical coherence tomography (OCT). In conventional angiographic OCT, the transverse displacement of the imaging beam during the time interval between a pair of OCT signal measurements must be significantly reduced to minimize the noise due to the beam scanning-induced phase decorrelation at the expense of the imaging speed. Recent introduction of dual-beam scan method either using polarization encoding or two identical imaging systems in spectral-domain (SD) OCT scheme shows potential for high-sensitivity vasculature imaging without suffering from spurious phase noise caused by the beam scanning-induced spatial decorrelation. In this paper, we present multi-functional angiographic optical frequency domain imaging (OFDI) using frequency-multiplexed dual-beam illumination. This frequency multiplexing scheme, utilizing unique features of OFDI, provides spatially separated dual imaging beams occupying distinct electrical frequency bands that can be demultiplexed in the frequency domain processing. We demonstrate the 3D multi-functional imaging of the normal mouse skin in the dorsal skin fold chamber visualizing distinct layer structures from the intensity imaging, information about mechanical integrity from the polarization-sensitive imaging, and depth-resolved microvasculature from the angiographic imaging that are simultaneously acquired and automatically co-registered.

All-fiber wavelength swept ring laser based on Fabry-Perot filter for optical frequency domain imaging

Innovations in laser engineering have yielded several novel configurations for high repetition rate, broad sweep range, and long coherence length wavelength swept lasers. Although these lasers have enabled high performance frequency-domain optical coherence tomography, they are typically complicated and costly and many require access to proprietary materials or devices. Here, we demonstrate a simplified ring resonator configuration that is straightforward to construct from readily available materials at a low total cost. It was enabled by an insight regarding the significance of isolation against bidirectional operation and by configuring the sweep range of the intracavity filter to exceed its free spectral range. The design can easily be optimized to meet a range of operating specifications while yielding robust and stable performance. As an example, we demonstrate 240 kHz operation with 125 nm sweep range and >70 mW of average output power and demonstrate high quality frequency domain OCT imaging. The complete component list and directions for assembly of the laser are posted on-line at www.octresearch.org.

Fully integrated high-speed intravascular optical coherence tomography/near-infrared fluorescence structural/molecular imaging in vivo using a clinically available near-infrared fluorescence-emitting indocyanine green to detect inflamed lipid-rich atheromata in coronary-sized vessels

BACKGROUND

Lipid-rich inflamed coronary plaques are prone to rupture. The purpose of this study was to assess lipid-rich inflamed plaques in vivo using fully integrated high-speed optical coherence tomography (OCT)/near-infrared fluorescence (NIRF) molecular imaging with a Food and Drug Administration-approved indocyanine green (ICG).

METHODS AND RESULTS

An integrated high-speed intravascular OCT/NIRF imaging catheter and a dual-modal OCT/NIRF system were constructed based on a clinical OCT platform. For imaging lipid-rich inflamed plaques, the Food and Drug Administration-approved NIRF-emitting ICG (2.25 mg/kg) or saline was injected intravenously into rabbit models with experimental atheromata induced by balloon injury and 12- to 14-week high-cholesterol diets. Twenty minutes after injection, in vivo OCT/NIRF imaging of the infrarenal aorta and iliac arteries was acquired only under contrast flushing through catheter (pullback speed up to ≤20 mm/s). NIRF signals were strongly detected in the OCT-visualized atheromata of the ICG-injected rabbits. The in vivo NIRF target-to-background ratio was significantly larger in the ICG-injected rabbits than in the saline-injected controls (P<0.01). Ex vivo peak plaque target-to-background ratios were significantly higher in ICG-injected rabbits than in controls (P<0.01) on fluorescence reflectance imaging, which correlated well with the in vivo target-to-background ratios (P<0.01; r=0.85) without significant bias (0.41). Cellular ICG uptake, correlative fluorescence microscopy, and histopathology also corroborated the in vivo imaging findings.

CONCLUSIONS

Integrated OCT/NIRF structural/molecular imaging with a Food and Drug Administration -approved ICG accurately identified lipid-rich inflamed atheromata in coronary-sized vessels. This highly translatable dual-modal imaging approach could enhance our capabilities to detect high-risk coronary plaques.

Polarization-sensitive OFDI using polarization-multiplexed wavelength-swept laser

We demonstrate a novel polarization-sensitive optical frequency domain imaging system employing passive polarization multiplexing. Simple modification of a fiber delay line in the wavelength-swept light source enables illumination with two perpendicular polarizations that are required for determination of the Stokes vector components of the light reflected from each depth of the tissue. This simple all-passive approach provides a robust and low-cost solution for PS imaging replacing relatively complex conventional schemes such as polarization modulation or frequency-encoded polarization multiplexing.

Dual-wavelength band spectroscopic optical frequency domain imaging using plasmon-resonant scattering in metallic nanoparticles

We demonstrate a dual-wavelength band optical frequency domain imaging (OFDI) system that provides high-resolution spectroscopic imaging with metallic nanoparticles as exogenous contrast agents. The local increase of the OFDI signal by elastic scattering from two different custom-fabricated nonspherical nanoparticles resonant at each wavelength band of the system was successfully detected, and we were able to distinguish and visualize the location of each of the nanoparticles in a scattering sample and in biological tissue.

High frame-rate intravascular optical frequency-domain imaging in vivo

Intravascular optical frequency-domain imaging (OFDI), a second-generation optical coherence tomography (OCT) technology, enables imaging of the three-dimensional (3D) microstructure of the vessel wall following a short and nonocclusive clear liquid flush. Although 3D vascular visualization provides a greater appreciation of the vessel wall and intraluminal structures, a longitudinal imaging pitch that is several times bigger than the optical imaging resolution of the system has limited true high-resolution 3D imaging, mainly due to the slow scanning speed of previous imaging catheters. Here, we demonstrate high frame-rate intravascular OFDI in vivo, acquiring images at a rate of 350 frames per second. A custom-built, high-speed, and high-precision fiber-optic rotary junction provided uniform and high-speed beam scanning through a custom-made imaging catheter with an outer diameter of 0.87 mm. A 47-mm-long rabbit aorta was imaged in 3.7 seconds after a short contrast agent flush. The longitudinal imaging pitch was 34 μm, comparable to the transverse imaging resolution of the system. Three-dimensional volume-rendering showed greatly enhanced visualization of tissue microstructure and stent struts relative to what is provided by conventional intravascular imaging speeds.

Spectral binning for mitigation of polarization mode dispersion artifacts in catheter-based optical frequency domain imaging

Polarization mode dispersion (PMD) has been recognized as a significant barrier to sensitive and reproducible birefringence measurements with fiber-based, polarization-sensitive optical coherence tomography systems. Here, we present a signal processing strategy that reconstructs the local retardation robustly in the presence of system PMD. The algorithm uses a spectral binning approach to limit the detrimental impact of system PMD and benefits from the final averaging of the PMD-corrected retardation vectors of the spectral bins. The algorithm was validated with numerical simulations and experimental measurements of a rubber phantom. When applied to the imaging of human cadaveric coronary arteries, the algorithm was found to yield a substantial improvement in the reconstructed birefringence maps.

Artifacts in polarization-sensitive optical coherence tomography caused by polarization mode dispersion

Polarization mode dispersion (PMD) severely degrades images of biological tissue measured with polarization-sensitive optical coherence tomography. It adds a bias to the local retardation value that can be spatially confined, resulting in regions of seemingly high sample birefringence that are purely artificial. Here, we demonstrate and analyze this effect, both experimentally and with numerical simulations, and show that artifacts can be avoided by limiting the system PMD to less than the system axial resolution. Even then, spatial averaging over a dimension larger than that characteristic of speckle is required to remove a PMD-induced bias of the local retardation values.

Complex wavefront shaping for optimal depth-selective focusing in optical coherence tomography

We report on an approach to exploit multiple light scattering by shaping the incident wavefront in optical coherence tomography (OCT). Most of the reflected signal from biological tissue consists of multiply scattered light, which is regarded as noise in OCT. A digital mirror device (DMD) is utilized to shape the incident wavefront such that the maximal energy is focused at a specific depth in a highly scattering sample using a coherence-gated reflectance signal as feedback. The proof-of-concept experiment demonstrates that this approach enhances depth-selective focusing in the presence of optical inhomogeneity, and thus extends the penetration depth in spectral domain-OCT (SD-OCT).

Numerical compensation of system polarization mode dispersion in polarization-sensitive optical coherence tomography

Polarization mode dispersion (PMD), which can be induced by circulators or even moderate lengths of optical fiber, is known to be a dominant source of instrumentation noise in fiber-based PS-OCT systems. In this paper we propose a novel PMD compensation method that measures system PMD using three fixed calibration signals, numerically corrects for these instrument effects and reconstructs an improved sample image. Using a frequency multiplexed PS-OFDI setup, we validate the proposed method by comparing birefringence noise in images of intralipid, muscle, and tendon with and without PMD compensation.

>400 kHz repetition rate wavelength-swept laser and application to high-speed optical frequency domain imaging

We demonstrate a high-speed wavelength-swept laser with a tuning range of 104 nm (1228-1332 nm) and a repetition rate of 403 kHz. The design of the laser utilizes a high-finesse polygon-based wavelength-scanning filter and a short-length unidirectional ring resonator. Optical frequency domain imaging of the human skin in vivo is presented using this laser, and the system shows sensitivity of higher than 98 dB with single-side ranging depth of 1.7 mm over 4 dB sensitivity roll-off.

Performance of reduced bit-depth acquisition for optical frequency domain imaging

High-speed optical frequency domain imaging (OFDI) has enabled practical wide-field microscopic imaging in the biological laboratory and clinical medicine. The imaging speed of OFDI, and therefore the field of view, of current systems is limited by the rate at which data can be digitized and archived rather than the system sensitivity or laser performance. One solution to this bottleneck is to natively digitize OFDI signals at reduced bit depths, e.g., at 8-bit depth rather than the conventional 12-14 bit depth, thereby reducing overall bandwidth. However, the implications of reduced bit-depth acquisition on image quality have not been studied. In this paper, we use simulations and empirical studies to evaluate the effects of reduced depth acquisition on OFDI image quality. We show that image acquisition at 8-bit depth allows high system sensitivity with only a minimal drop in the signal-to-noise ratio compared to higher bit-depth systems. Images of a human coronary artery acquired in vivo at 8-bit depth are presented and compared with images at higher bit-depth acquisition.

Single-detector polarization-sensitive optical frequency domain imaging using high-speed intra A-line polarization modulation

We demonstrate a novel high-speed polarization-sensitive optical frequency domain imaging system employing high-speed polarization modulation. Rapid and continuous polarization modulation of light prior to illumination of the sample is accomplished by shifting the frequency of one polarization eigenstate by an amount equal to one quarter of the digitization sampling frequency. This approach enables polarization-sensitive imaging with a single detection channel and overcomes artifacts that may arise from temporal variations of the birefringence in fiber-optic imaging probes and spatial variation of birefringence in the sample.

High-speed polarization sensitive optical frequency domain imaging with frequency multiplexing

Polarization sensitive optical coherence tomography (PS-OCT) provides a cross-sectional image of birefringence in biological samples that is complementary in many applications to the standard reflectance-based image. Recent ex vivo studies have demonstrated that birefringence mapping enables the characterization of collagen and smooth muscle concentration and distribution in vascular tissues. Instruments capable of applying these measurements percutaneously in vivo may provide new insights into coronary atherosclerosis and acute myocardial infarction. We have developed a polarization sensitive optical frequency domain imaging (PS-OFDI) system that enables high-speed intravascular birefringence imaging through a fiber-optic catheter. The novel design of this system utilizes frequency multiplexing to simultaneously measure reflectance of two incident polarization states, overcoming concerns regarding temporal variations of the catheter fiber birefringence and spatial variations in the birefringence of the sample. We demonstrate circular cross-sectional birefringence imaging of a human coronary artery ex vivo through a flexible fiber-optic catheter with an A-line rate of 62 kHz and a ranging depth of 6.2 mm.