Intravascular optical coherence tomography [Invited]

Aging of deep venous thrombosis in-vivo using polarization sensitive optical coherence tomography

Deep venous thrombosis (DVT) is a medical condition with significant post-event morbidity and mortality coupled with limited treatment options. Treatment strategy and efficacy are highly dependent on the structural composition of the thrombus, which evolves over time from initial formation and is currently unevaluable with standard clinical testing. Here, we investigate the use of intravascular polarization-sensitive optical coherence tomography (PS-OCT) to assess thrombus morphology and composition in a rat DVT model in-vivo, including changes that occur over the thrombus aging process. PS-OCT measures tissue birefringence, which provides contrast for collagen and smooth muscle cells that are present in older, chronic clots. Thrombi in the inferior vena cava of two cohorts of rats were imaged in-vivo with intravascular PS-OCT at 24 hours (acute, nrats = 3, 73 cross-sections) or 28 days (chronic, nrats = 4, 41 cross-sections) after thrombus formation. Co-registered histology was labelled by an independent pathologist to establish ground-truth clot composition. Automated analysis of OCT cross-sectional images differentiated acute and chronic thrombi with 97.6% sensitivity and 98.6% specificity using a linear discriminant model comprised of both polarization and conventional OCT metrics. These results support PS-OCT as a highly sensitive imaging modality for the assessment of DVT composition to differentiate acute and chronic thrombi. Intravascular PS-OCT imaging could be integrated with advanced catheter-based treatment strategies and serve to guide therapeutic decision-making and deployment, by offering an accurate assessment of DVT patients in real time.

Large area robotically assisted optical coherence tomography (LARA-OCT)

We demonstrate large-area robotically assisted optical coherence tomography (LARA-OCT), utilizing a seven-degree-of-freedom robotic arm in conjunction with a 3.3 MHz swept-source OCT to raster scan samples of arbitrary shape. By combining multiple fields of view (FOV), LARA-OCT can probe a much larger area than conventional OCT. Also, nonplanar and curved surfaces like skin on arms and legs can be probed. The lenses in the LARA-OCT scanner with their normal FOV can have fewer aberrations and less complex optics compared to a single wide field design. This may be especially critical for high resolution scans. We directly use our fast MHz-OCT for tracking and stitching, making additional machine vision systems like cameras, positioning, tracking or navigation devices obsolete. This also eliminates the need for complex coordinate system registration between OCT and the machine vision system. We implemented a real time probe-to-surface control that maintains the probe alignment orthogonal to the sample by only using surface information from the OCT images. We present OCT data sets with volume sizes of 140 × 170 × 20 mm3, captured in 2.5 minutes.

Segmentation of anatomical layers and imaging artifacts in intravascular polarization sensitive optical coherence tomography using attending physician and boundary cardinality losses

Intravascular ultrasound and optical coherence tomography are widely available for assessing coronary stenoses and provide critical information to optimize percutaneous coronary intervention. Intravascular polarization-sensitive optical coherence tomography (PS-OCT) measures the polarization state of the light scattered by the vessel wall in addition to conventional cross-sectional images of subsurface microstructure. This affords reconstruction of tissue polarization properties and reveals improved contrast between the layers of the vessel wall along with insight into collagen and smooth muscle content. Here, we propose a convolutional neural network model, optimized using two new loss terms (Boundary Cardinality and Attending Physician), that takes advantage of the additional polarization contrast and classifies the lumen, intima, and media layers in addition to guidewire and plaque shadows. Our model segments the media boundaries through fibrotic plaques and continues to estimate the outer media boundary behind shadows of lipid-rich plaques. We demonstrate that our multi-class classification model outperforms existing methods that exclusively use conventional OCT data, predominantly segment the lumen, and consider subsurface layers at most in regions of minimal disease. Segmentation of all anatomical layers throughout diseased vessels may facilitate stent sizing and will enable automated characterization of plaque polarization properties for investigation of the natural history and significance of coronary atheromas.

Cross-attention learning enables real-time nonuniform rotational distortion correction in OCT

Nonuniform rotational distortion (NURD) correction is vital for endoscopic optical coherence tomography (OCT) imaging and its functional extensions, such as angiography and elastography. Current NURD correction methods require time-consuming feature tracking/registration or cross-correlation calculations and thus sacrifice temporal resolution. Here we propose a cross-attention learning method for the NURD correction in OCT. Our method is inspired by the recent success of the self-attention mechanism in natural language processing and computer vision. By leveraging its ability to model long-range dependencies, we can directly obtain the spatial correlation between OCT A-lines at any distance, thus accelerating the NURD correction. We develop an end-to-end stacked cross-attention network and design three types of optimization constraints. We compare our method with two traditional feature-based methods and a CNN-based method on two publicly-available endoscopic OCT datasets. We further verify the NURD correction performance of our method on 3D stent reconstruction using a home-built endoscopic OCT system. Our method achieves a ∼3 × speedup to real time (26 ± 3 fps), and superior correction performance.

Single-input polarization-sensitive optical coherence tomography through a catheter

Intravascular polarimetry with catheter-based polarization-sensitive optical coherence tomography (PS-OCT) complements the high-resolution structural tomograms of OCT with morphological contrast available through polarimetry. Its clinical translation has been complicated by the need for modification of conventional OCT hardware to enable polarimetric measurements. Here, we present a signal processing method to reconstruct the polarization properties of tissue from measurements with a single input polarization state, bypassing the need for modulation or multiplexing of input states. Our method relies on a polarization symmetry intrinsic to round-trip measurements and uses the residual spectral variation of the polarization states incident on the tissue to avoid measurement ambiguities. We demonstrate depth-resolved birefringence and optic axis orientation maps reconstructed from in-vivo data of human coronary arteries. We validate our method through comparison with conventional dual-input state measurements and find a mean cumulative retardance error of 13.2deg without observable bias. The 95% limit of agreement between depth-resolved birefringence is 2.80 · 10-4, which is less than the agreement between two repeat pullbacks of conventional PS-OCT (3.14 · 10-4), indicating that the two methods can be used interchangeably. The hardware simplification arising from using a single input state may be decisive in realizing the potential of polarimetric measurements for assessing coronary atherosclerosis in clinical practice.

Role of plaque imaging for identification of vulnerable patients beyond the stage of myocardial ischemia

Chronic coronary syndrome (CCS) is a progressive disease, which often first manifests as acute coronary syndrome (ACS). Imaging modalities are clinically useful in making decisions about the management of patients with CCS. Accumulating evidence has demonstrated that myocardial ischemia is a surrogate marker for CCS management; however, its ability to predict cardiovascular death or nonfatal myocardial infarction is limited. Herein, we present a review that highlights the latest knowledge available on coronary syndromes and discuss the role and limitations of imaging modalities in the diagnosis and management of patients with coronary artery disease. This review covers the essential aspects of the role of imaging in assessing myocardial ischemia and coronary plaque burden and composition. Furthermore, recent clinical trials on lipid-lowering and anti-inflammatory therapies have been discussed. Additionally, it provides a comprehensive overview of intracoronary and noninvasive cardiovascular imaging modalities and an understanding of ACS and CCS, with a focus on histopathology and pathophysiology.

Endoscopic imaging of white matter fiber tracts using polarization-sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PSOCT) has been shown to image and delineate white matter fibers in a label-free manner by revealing optical birefringence within the myelin sheath using a microscope setup. In this proof-of-concept study, we adapt recent advancements in endoscopic PSOCT to perform depth-resolved imaging of white matter structures deep inside intact porcine brain tissue ex-vivo, through a small, rotational fiber probe. The probe geometry is comparable to microelectrodes currently used in neurosurgical interventions. The presented imaging system is mobile, robust, and uses biologically safe levels of optical radiation making it well suited for clinical translation. In neurosurgery, where accuracy is imperative, endoscopic PSOCT through a narrow-gauge fiber probe could provide intra-operative feedback on the location of critical white matter structures.

Intravascular optical coherence elastography

Optical coherence elastography (OCE), a functional extension of optical coherence tomography (OCT), visualizes tissue strain to deduce the tissue's biomechanical properties. In this study, we demonstrate intravascular OCE using a 1.1 mm motorized catheter and a 1.6 MHz Fourier domain mode-locked OCT system. We induced an intraluminal pressure change by varying the infusion rate from the proximal end of the catheter. We analysed the pixel-matched phase change between two different frames to yield the radial strain. Imaging experiments were carried out in a phantom and in human coronary arteries in vitro. At an imaging speed of 3019 frames/s, we were able to capture the dynamic strain. Stiff inclusions in the phantom and calcification in atherosclerotic plaques are associated with low strain values and can be distinguished from the surrounding soft material, which exhibits elevated strain. For the first time, circumferential intravascular OCE images are provided side by side with conventional OCT images, simultaneously mapping both the tissue structure and stiffness.

Automatic bifurcation detection utilizing pullback characteristics of bifurcation in intravascular optical coherence tomography

Bifurcation detection in coronary arteries is significant since it influences the treatment strategy selection and optimization. Bifurcations are also reliable landmarks for image registration. Intravascular optical coherence tomography (IVOCT) is a high-resolution imaging modality that is very useful in percutaneous coronary intervention stenting optimization. We present a bifurcation identification method utilizing pullback characteristics for IVOCT, which can effectively identify the bifurcations with a small size. The longitudinal view of the pullback will appear as an outward discontinuity in the bifurcation area. By detecting this discontinuity, bifurcation can be identified with high accuracy. We also use the normal vectors method to extract the ostium of bifurcation. We compare the proposed method with the widely-used distance transformation method by clinical 5302 IVOCT images from 22 pullbacks. The average metrics of true positive rate (TPR), true negative rate (TNR), positive predictive value (PPV), and negative predictive value (NPV) for the proposed method are 86.97%, 98.50%, 85.56%, and 98.67%, respectively. TPR, PPV, and NPV by the proposed method are improved by 40.24%, 9.31%, 3.90%, and TNR is on par compared with the distance transformation method. Especially in the small bifurcation identification, TPR of the proposed method is 64.71% higher than the distance transformation method with a bifurcation area ratio less than 0.2.

Current research and future prospects of IVOCT imaging-based detection of the vascular lumen and vulnerable plaque

Intravascular optical coherence tomography (IVOCT) is an imaging method that has developed rapidly in recent years and is useful in coronary atherosclerosis diagnosis. It is widely used in the assessment of vulnerable plaque. This review summarizes the main research methods used in recent years for blood vessel lumen boundary detection and segmentation and vulnerable plaque segmentation and classification. This article aims to comprehensively and systematically introduce the research progress on internal tissues of blood vessels based on IVOCT images. The characteristics and advantages of various methods have been summarized to provide theoretical ideas and methods for the reference of relevant researchers and scholars.

Bismuth Selenide Nanostructured Clusters as Optical Coherence Tomography Contrast Agents: Beyond Gold-Based Particles

Optical coherence tomography (OCT) is an imaging technique currently used in clinical practice to obtain optical biopsies of different biological tissues in a minimally invasive way. Among the contrast agents proposed to increase the efficacy of this imaging method, gold nanoshells (GNSs) are the best performing ones. However, their preparation is generally time-consuming, and they are intrinsically costly to produce. Herein, we propose a more affordable alternative to these contrast agents: Bi₂Se₃ nanostructured clusters with a desert rose-like morphology prepared via a microwave-assisted method. The structures are prepared in a matter of minutes, feature strong near-infrared extinction properties, and are biocompatible. They also boast a photon-to-heat conversion efficiency of close to 50%, making them good candidates as photothermal therapy agents. In vitro studies evidence the prowess of Bi₂Se₃ clusters as OCT contrast agents and prove that their performance is comparable to that of GNSs.

Review on Laser Technology in Intravascular Imaging and Treatment

Blood vessels are one of the most essential organs, which nourish all tissues in our body. Once there are intravascular plaques or vascular occlusion, other organs and circulatory systems will not work properly. Therefore, it is necessary to detect abnormal blood vessels by intravascular imaging technologies for subsequent vascular treatment. The emergence of lasers and fiber optics promotes the development of intravascular imaging and treatment. Laser imaging techniques can obtain deep vascular images owing to light scattering and absorption properties. Moreover, photothermal and photomechanical effects of laser make it possible to treat vascular diseases accurately. In this review, we present the research progress and applications of laser techniques in intravascular imaging and treatment. Firstly, we introduce intravascular optical coherent tomography and intravascular photoacoustic imaging, which can obtain various information of plaques. Multimodal intravascular imaging techniques provide more information about intravascular plaques, which have an essential influence on intravascular imaging. Secondly, two laser techniques including laser angioplasty and endovenous laser ablation are discussed for the treatment of arterial and venous diseases, respectively. Finally, the outlook of laser techniques in blood vessels, as well as the integration of laser imaging and treatment are prospected in the section of discussions.

Transient-mode photothermal optical coherence tomography

Photothermal optical coherence tomography (PT-OCT) is an emerging extension of OCT, which forms images based on both scattering and absorption of light. The speed of PT-OCT, however, has been limited by the necessity for lock-in detection with extensive temporal sampling of the sample's PT response. Here, we demonstrate transient-mode PT-OCT (TM-PT-OCT), which increases the effective A-line rate by orders of magnitude from 10-100 Hz to 1.5-7.5 kHz, by interrogating the sample's transient thermal response to a single diode laser pulse. Functional imaging of moving samples with TM-PT-OCT at video rate is also presented. This significant improvement in imaging speed is expected to open the door for downstream integration of PT-OCT in clinical systems for in vivo imaging.

Automatic lumen segmentation using uniqueness of vascular connected region for intravascular optical coherence tomography

We present an automatic lumen segmentation method using uniqueness of connected region for intravascular optical coherence tomography (IVOCT), which can effectively remove the effect on lumen segmentation caused by blood artifacts. Utilizing the uniqueness of vascular wall on A-lines, we detect the A-lines shared by multiple connected regions, identify connected regions generated by blood artifacts using traversal comparison of connected regions' location, shared ratio and area ratio and then remove all artifacts. We compare these three methods by 216 challenging images with severe blood artifacts selected from clinical 1076 IVOCT images. The metrics of the proposed method are evaluated including Dice index, Jaccard index and accuracy of 94.57%, 90.12%, 98.02%. Compared with automatic lumen segmentation based on the previous morphological feature method and widely used dynamic programming method, the metrics of the proposed method are significantly enhanced, especially in challenging images with severe blood artifacts.

Intravascular imaging of coronary artery: Bridging the gap between clinical needs and technical advances

Coronary artery disease is the leading cause of mortality worldwide. Almost seven million deaths are reported each year due to coronary disease. Coronary artery events in the adult are primarily due to atherosclerosis with seventy-five percent of the related mortality caused by plaque rupture. Despite significant progress made to improve intravascular imaging of coronary arteries, there is still a large gap between clinical needs and technical developments. The goal of this review is to identify the gap elements between clinical knowledge and recent advances in the domain of medical image analysis. Efficient image analysis computational models should be designed with respect to the exact clinical needs, and detailed features of the tissues under review. In this review, we discuss the detailed clinical features of the intracoronary plaques for mathematical and biomedical researchers. We emphasize the importance of integrating this clinical knowledge validated by clinicians to investigate the potentially effective models for proper features efficiency in the scope of leveraging the state-of-the-art of coronary image analyses.

A review of low-cost and portable optical coherence tomography

Optical coherence tomography (OCT) is a powerful optical imaging technique capable of visualizing the internal structure of biological tissues at near cellular resolution. For years, OCT has been regarded as the standard of care in ophthalmology, acting as an invaluable tool for the assessment of retinal pathology. However, the costly nature of most current commercial OCT systems has limited its general accessibility, especially in low-resource environments. It is therefore timely to review the development of low-cost OCT systems as a route for applying this technology to population-scale disease screening. Low-cost, portable and easy to use OCT systems will be essential to facilitate widespread use at point of care settings while ensuring that they offer the necessary imaging performances needed for clinical detection of retinal pathology. The development of low-cost OCT also offers the potential to enable application in fields outside ophthalmology by lowering the barrier to entry. In this paper, we review the current development and applications of low-cost, portable and handheld OCT in both translational and research settings. Design and cost-reduction techniques are described for general low-cost OCT systems, including considerations regarding spectrometer-based detection, scanning optics, system control, signal processing, and the role of 3D printing technology. Lastly, a review of clinical applications enabled by low-cost OCT is presented, along with a detailed discussion of current limitations and outlook.

A deep learning-based model for characterization of atherosclerotic plaque in coronary arteries using optical coherence tomography images

PURPOSE

Coronary artery events are mainly associated with atherosclerosis in adult population, which is recognized as accumulation of plaques in arterial wall tissues. Optical Coherence Tomography (OCT) is a light-based imaging system used in cardiology to analyze intracoronary tissue layers and pathological formations including plaque accumulation. This state-of-the-art catheter-based imaging system provides intracoronary cross-sectional images with high resolution of 10-15 µm. But interpretation of the acquired images is operator dependent, which is not only very time-consuming but also highly error prone from one observer to another. An automatic and accurate coronary plaque tagging using OCT image post-processing can contribute to wide adoption of the OCT system and reducing the diagnostic error rate.

METHOD

In this study, we propose a combination of spatial pyramid pooling module with dilated convolutions for semantic segmentation to extract atherosclerotic tissues regardless of their types and training a sparse auto-encoder to reconstruct the input features and enlarge the training data as well as plaque type characterization in OCT images.

RESULTS

The results demonstrate high precision of the proposed model with reduced computational complexity, which can be appropriate for real-time analysis of OCT images. At each step of the work, measured accuracy, sensitivity, specificity of more than 93% demonstrate high performance of the model.

CONCLUSION

The main focus of this study is atherosclerotic tissue characterization using OCT imaging. This contributes to wide adoption of the OCT imaging system by providing clinicians with a fully automatic interpretation of various atherosclerotic tissues. Future studies will be focused on analyzing atherosclerotic vulnerable plaques, those coronary plaques which are prone to rupture.

Multi-spectral intravascular photoacoustic/ultrasound/optical coherence tomography tri-modality system with a fully-integrated 0.9-mm full field-of-view catheter for plaque vulnerability imaging

Myocardial infarctions are most often caused by the so-called vulnerable plaques, usually featured as non-obstructive lesions with a lipid-rich necrotic core, thin-cap fibroatheroma, and large plaque size. The identification and quantification of these characteristics are the keys to evaluate plaque vulnerability. However, single modality intravascular methods, such as intravascular ultrasound, optical coherence tomography and photoacoustic, can hardly achieve all the comprehensive information to satisfy clinical needs. In this paper, for the first time, we developed a novel multi-spectral intravascular tri-modality (MS-IVTM) imaging system, which can perform 360° continuous rotation and pull-backing with a 0.9-mm miniature catheter and achieve simultaneous acquisition of both morphological characteristics and pathological compositions. Intravascular tri-modality imaging demonstrates the ability of our MS-IVTM system to provide macroscopic and microscopic structural information of the vessel wall, with identity and quantification of lipids with multi-wavelength excitation. This study offers clinicians and researchers a novel imaging tool to facilitate the accurate diagnosis of vulnerable atherosclerotic plaques. It also has the potential of clinical translations to help better identify and evaluate high-risk plaques during coronary interventions.

A Miniaturized Platform for Multiplexed Drug Response Imaging in Live Tumors

Simple Summary

We have developed an implantable microdevice that is placed into a live tumor, and can directly image how effective various chemotherapy drugs are at inducing cell death, without having to remove or process the tumor tissue. Currently drug optimization is performed by assessing tumor shrinkage after treating a patient with systemic doses of a chemotherapy agent; this only evaluates a single treatment at a time and typically takes weeks-months before an optimal treatment strategy is found (if found at all) for a specific patient. In contrast, using the technology presented here, a personalized cancer treatment strategy can potentially be optimized and tailored to a specific patient’s tumor characteristics within several hours, without requiring surgical tissue removal or prolonged trials of potentially ineffective chemotherapies.

Abstract

By observing the activity of anti-cancer agents directly in tumors, there is potential to greatly expand our understanding of drug response and develop more personalized cancer treatments. Implantable microdevices (IMD) have been recently developed to deliver microdoses of chemotherapeutic agents locally into confined regions of live tumors; the tissue can be subsequently removed and analyzed to evaluate drug response. This method has the potential to rapidly screen multiple drugs, but requires surgical tissue removal and only evaluates drug response at a single timepoint when the tissue is excised. Here, we describe a “lab-in-a-tumor” implantable microdevice (LIT-IMD) platform to image cell-death drug response within a live tumor, without requiring surgical resection or tissue processing. The LIT-IMD is inserted into a live tumor and delivers multiple drug microdoses into spatially discrete locations. In parallel, it locally delivers microdose levels of a fluorescent cell-death assay, which diffuses into drug-exposed tissues and accumulates at sites of cell death. An integrated miniaturized fluorescence imaging probe images each region to evaluate drug-induced cell death. We demonstrate ability to evaluate multi-drug response over 8 h using murine tumor models and show correlation with gold-standard conventional fluorescence microscopy and histopathology. This is the first demonstration of a fully integrated platform for evaluating multiple chemotherapy responses in situ. This approach could enable a more complete understanding of drug activity in live tumors, and could expand the utility of drug-response measurements to a wide range of settings where surgery is not feasible.

Diagnosing colorectal abnormalities using scattering coefficient maps acquired from optical coherence tomography

Optical coherence tomography (OCT) has shown potential in differentiating normal colonic mucosa from neoplasia. In this study of 33 fresh human colon specimens, we report the first use of texture features and computer vision-based imaging features acquired from en face scattering coefficient maps to characterize colorectal tissue. En face scattering coefficient maps were generated automatically using a new fast integral imaging algorithm. From these maps, a gray-level cooccurrence matrix algorithm was used to extract texture features, and a scale-invariant feature transform algorithm was used to derive novel computer vision-based features. In total, 25 features were obtained, and the importance of each feature in diagnosis was evaluated using a random forest model. Two classifiers were assessed on two different classification tasks. A support vector machine model was found to be optimal for distinguishing normal from abnormal tissue, with 94.7% sensitivity and 94.0% specificity, while a random forest model performed optimally in further differentiating abnormal tissues (i.e., cancerous tissue and adenomatous polyp) with 86.9% sensitivity and 85.0% specificity. These results demonstrated the potential of using OCT to aid the diagnosis of human colorectal disease.

Effects of lipid composition on photothermal optical coherence tomography signals

SIGNIFICANCE

Photothermal optical coherence tomography (PT-OCT) has the promise to offer structural images coregistered with chemical composition information, which can offer a significant impact in early detection of diseases such as atherosclerosis.

AIM

We take the first step in understanding the relation between PT-OCT signals and the endogenous tissue composition by considering the interplay between the opto-thermo-physical properties of tissue as a function of its lipid composition and the ensuing effects on the PT-OCT signals.

APPROACH

Multiparameter theoretical estimates for PT-OCT signal as a function of composition in a two-component lipid-water model are derived and discussed. Experimental data from various concentrations of lipid in the form of droplets and injections under bovine cardiac muscle align with theoretical predictions.

RESULTS

Theoretical and experimental results suggest that the variations of heat capacity and mass density with tissue composition significantly contribute to the amount of optical path length difference measured by OCT phase.

CONCLUSION

PT-OCT has the potential to offer key insights into the chemical composition of the subsurface lipid pools in tissue; however, the interpretation of results needs to be carried out by keeping the nonlinear interplay between the tissue of opto-thermo-physical properties and PT-OCT signals in mind.

Differential phase imaging in full-field optical coherence microscopy using a short multimode fiber probe

We demonstrate phase imaging that reduces the common phase noise in full-field optical coherence microscopy using a short multimode fiber (SMMF) probe. Using a cover glass, phase images of the SMMF and sample surfaces were measured simultaneously. Subtracting the phase of the SMMF surface as a reference, the phase drifts in the sample region are reduced. The axial and lateral resolutions were 2.3 µm and <4.4µm, respectively. The standard deviation of the time variation in the phase decreased from 14.3 deg to 9.2 deg and was reduced by 64% when in contact with the polymer film at the SMMF. In quantitative evaluations, the measured phases closely correspond to the phases changed by a piezoelectric device.

Intravascular Polarimetry: Clinical Translation and Future Applications of Catheter-Based Polarization Sensitive Optical Frequency Domain Imaging

Optical coherence tomography (OCT) and optical frequency domain imaging (OFDI) visualize the coronary artery wall and plaque morphology in great detail. The advent of these high-resolution intracoronary imaging modalities has propelled our understanding of coronary atherosclerosis and provided enhanced guidance for percutaneous coronary intervention. Yet, the lack of contrast between distinct tissue types and plaque compositions impedes further elucidation of the complex mechanisms that contribute to acute coronary syndrome (ACS) and hinders the prospective identification of plaques susceptible to rupture. Intravascular polarimetry with polarization-sensitive OFDI measures polarization properties of the coronary arterial wall using conventional intravascular imaging catheters. The quantitative polarization metrics display notable image contrast between several relevant coronary plaque microstructures that are difficult to identify with conventional OCT and OFDI. Tissues rich in collagen and smooth muscle cells exhibit birefringence, while lipid and macrophages cause depolarization. In this review, we describe the basic principles of intravascular polarimetry, discuss the interpretation of the polarization signatures, and outline promising avenues for future research and clinical implications.

Endovascular optical coherence tomography imaging in cerebrovascular disease

Endovascular optical coherence tomography (OCT) is the highest resolution imaging modality currently available with spatial resolution of 10 µm. Although originally developed for interventional cardiology, the ability to visualize the luminal environment and anatomy, along with the stent-vessel interaction could be of great utility for various cerebrovascular diseases, and the adoption of endovascular OCT imaging in the evolving field of interventional neuroradiology seems instinctive. The purpose of this study is to conduct a systematic review of the literature regarding applications of endovascular OCT in the diagnosis and treatment of cerebrovascular diseases. In addition, the authors report their institutional experience with the use of OCT in carotid atherosclerotic disease, cerebral aneurysms, and acute ischemic stroke. A systematic review of the literature was undertaken. Peer-reviewed articles were collected through MEDLINE, Embase, Scopus, and Cochrane Central Register of Controlled Trials (CENTRAL) searches through March 2020. A total of 34 studies with 598 patients were included in the qualitative synthesis. These include 23 studies of carotid atherosclerotic disease, 7 studies of cerebral aneurysms, and 4 studies of non-aneurysmal posterior circulation pathology. OCT imaging was feasible in 94% of patients with 0.6% complication rate. Endovascular OCT appears to be safe and feasible, allowing clinicians to visualize stent-vessel interactions, aneurysmal healing, and vulnerable atherosclerotic plaque features. OCT carries great promise, however additional investigations are needed before any imposing statement can be made about the role of OCT in cerebrovascular imaging.

A neurovascular high-frequency optical coherence tomography system enables in situ cerebrovascular volumetric microscopy

Intravascular imaging has emerged as a valuable tool for the treatment of coronary and peripheral artery disease; however, no solution is available for safe and reliable use in the tortuous vascular anatomy of the brain. Endovascular treatment of stroke is delivered under image guidance with insufficient resolution to adequately assess underlying arterial pathology and therapeutic devices. High-resolution imaging, enabling surgeons to visualize cerebral arteries' microstructure and micron-level features of neurovascular devices, would have a profound impact in the research, diagnosis, and treatment of cerebrovascular diseases. Here, we present a neurovascular high-frequency optical coherence tomography (HF-OCT) system, including an imaging console and an endoscopic probe designed to rapidly acquire volumetric microscopy data at a resolution approaching 10 microns in tortuous cerebrovascular anatomies. Using a combination of in vitro, ex vivo, and in vivo models, the feasibility of HF-OCT for cerebrovascular imaging was demonstrated.

Ultrathin monolithic 3D printed optical coherence tomography endoscopy for preclinical and clinical use

Preclinical and clinical diagnostics increasingly rely on techniques to visualize internal organs at high resolution via endoscopes. Miniaturized endoscopic probes are necessary for imaging small luminal or delicate organs without causing trauma to tissue. However, current fabrication methods limit the imaging performance of highly miniaturized probes, restricting their widespread application. To overcome this limitation, we developed a novel ultrathin probe fabrication technique that utilizes 3D microprinting to reliably create side-facing freeform micro-optics (<130 µm diameter) on single-mode fibers. Using this technique, we built a fully functional ultrathin aberration-corrected optical coherence tomography probe. This is the smallest freeform 3D imaging probe yet reported, with a diameter of 0.457 mm, including the catheter sheath. We demonstrated image quality and mechanical flexibility by imaging atherosclerotic human and mouse arteries. The ability to provide microstructural information with the smallest optical coherence tomography catheter opens a gateway for novel minimally invasive applications in disease.

Spectral Domain Optical Coherence Tomography Imaging Performance Improvement Based on Field Curvature Aberration-Corrected Spectrometer

We designed and fabricated a telecentric f-theta imaging lens (TFL) to improve the imaging performance of spectral domain optical coherence tomography (SD-OCT). By tailoring the field curvature aberration of the TFL, the flattened focal surface was well matched to the detector plane. Simulation results showed that the spot in the focal plane fitted well within a single pixel and the modulation transfer function at high spatial frequencies showed higher values compared with those of an achromatic doublet imaging lens, which are commonly used in SD-OCT spectrometers. The spectrometer using the TFL had an axial resolution of 7.8 μm, which was similar to the theoretical value of 6.2 μm. The spectrometer was constructed so that the achromatic doublet lens was replaced by the TFL. As a result, the SD-OCT imaging depth was improved by 13% (1.85 mm) on a 10 dB basis in the roll-off curve and showed better sensitivity at the same depth. The SD-OCT images of a multi-layered tape and a human palm proved that the TFL was able to achieve deeper imaging depth and better contrast. This feature was seen very clearly in the depth profile of the image. SD-OCT imaging performance can be improved simply by changing the spectrometer’s imaging lens. By optimizing the imaging lens, deeper SD-OCT imaging can be achieved with improved sensitivity.

Concept for Markerless 6D Tracking Employing Volumetric Optical Coherence Tomography

Optical tracking systems are widely used, for example, to navigate medical interventions. Typically, they require the presence of known geometrical structures, the placement of artificial markers, or a prominent texture on the target’s surface. In this work, we propose a 6D tracking approach employing volumetric optical coherence tomography (OCT) images. OCT has a micrometer-scale resolution and employs near-infrared light to penetrate few millimeters into, for example, tissue. Thereby, it provides sub-surface information which we use to track arbitrary targets, even with poorly structured surfaces, without requiring markers. Our proposed system can shift the OCT’s field-of-view in space and uses an adaptive correlation filter to estimate the motion at multiple locations on the target. This allows one to estimate the target’s position and orientation. We show that our approach is able to track translational motion with root-mean-squared errors below 0.25 mm and in-plane rotations with errors below 0.3°. For out-of-plane rotations, our prototypical system can achieve errors around 0.6°.

Dual-side view optical coherence tomography for thickness measurement on opaque materials

Optical coherence tomography (OCT), as an optical interferometric imaging technique, has found wide applications in various fields. In principle, OCT is well suited for imaging layered structures, and thus, one of the typical applications is thickness measurement. However, due to the limited imaging depth resulting from light attenuation, thickness measurement by OCT is limited to non-opaque materials. In this study, we developed a novel (to the best of our knowledge) dual-side view OCT (DSV-OCT) system for thickness measurement on opaque materials. The dual-side view was achieved on a conventional swept source OCT platform by creating two symmetrical sampling arms. This allows us to image both sides of the material simultaneously and produce the surface contours of the two sides in a single C scan. Finally, the thickness of the opaque material can be calculated from the two surface contours above. We demonstrated that our DSV-OCT technique can measure the thickness of opaque material with an accuracy of about 3 µm.

18F-FDG uptake velocity but not uptake level is associated with progression of carotid plaque

OBJECTIVES

The objective of this study was to evaluate whether baseline ¹⁸F-fluorodeoxyglucose (FDG) uptake is associated with carotid plaque progression.

METHODS

A total of 156 subjects with carotid plaque were enrolled and underwent carotid magnetic resonance imaging (MRI) (at baseline and the 12-month follow-up) and positron emission tomography-computed tomography (PET-CT) (baseline). Carotid plaque progression was evaluated by two indices (the incidence of plaque progression and percentage of plaque increase) with three-dimensional (3D) imaging, while the ¹⁸F-FDG uptake was evaluated by the ¹⁸F-FDG uptake levels and ¹⁸F-FDG uptake velocity. The association between plaque progression and ¹⁸F-FDG uptake was investigated by the trend test and multivariate logistic regression analysis.

RESULTS

Of the 156 subjects, 80 (51.3%) showed carotid plaque progression during the 12-month follow-up. Firstly, no association was found between ¹⁸F-FDG uptake levels and plaque progression. Secondly, significant differences in the incidence of plaque progression were observed among the groups with different uptake velocities, showing a significant decreasing trend ranging from high to intermediate to low (p = 0.002, trend test). After adjusting for covariates, an adequate prediction of the ¹⁸F-FDG uptake velocity for the incidence of plaque progression was revealed (OR = 0.682, p < 0.05). In addition, no association was found between the ¹⁸F-FDG uptake velocity and the percentage of plaque increase in the subjects with plaque progression (p = 0.757, trend test).

CONCLUSIONS

Our findings suggest ¹⁸F-FDG uptake velocity is independently associated with the incidence of carotid plaque progression. Additionally, the ¹⁸F-FDG uptake velocity, as another important parameter of PET-CT, warrants further study in future clinical research.

KEY POINTS

• The¹⁸F-FDG uptake levels were not associated with the carotid plaque progression. • The¹⁸F-FDG uptake velocity could predict the incidence of carotid plaque progression. • The¹⁸F-FDG uptake velocity with related factors warrants more attention in future clinical research.

Characterization of the human myocardium by optical coherence tomography

Imaging of cardiac tissue structure plays a critical role in the treatment and understanding of cardiovascular disease. Optical coherence tomography (OCT) offers the potential to provide valuable, high-resolution imaging of cardiac tissue. However, there is a lack of comprehensive OCT imaging data of the human heart, which could improve identification of structural substrates underlying cardiac abnormalities. The objective of this study was to provide qualitative and quantitative analysis of OCT image features throughout the human heart. Fifty human hearts were acquired, and tissues from all chambers were imaged with OCT. Histology was obtained to verify tissue composition. Statistical differences between OCT image features corresponding to different tissue types and chambers were estimated using analysis of variance. OCT imaging provided features that were able to distinguish structures such as thickened collagen, as well as adipose tissue and fibrotic myocardium. Statistically significant differences were found between atria and ventricles in attenuation coefficient, and between adipose and all other tissue types. This study provides an overview of OCT image features throughout the human heart, which can be used for guiding future applications such as OCT-integrated catheter-based treatments or ex vivo investigation of structural substrates.

Intravascular Polarimetry for Tissue Characterization of Coronary Atherosclerosis

The microscopic tissue structure and organization influence the polarization of light. Intravascular polarimetry leverages this compelling intrinsic contrast mechanism by using polarization-sensitive optical frequency domain imaging to measure the polarization properties of the coronary arterial wall. Tissues rich in collagen and smooth muscle cells appear birefringent, while the presence of lipid causes depolarization, offering quantitative metrics related to the presence of important components of coronary atherosclerosis. Here, we review the basic principle, the interpretation of polarization signatures, and first clinical investigations of intravascular polarimetry and discuss how this extension of contemporary intravascular imaging may advance our knowledge and improve clinical practice in the future.

Broadband, freeform focusing micro-optics for a side-viewing imaging catheter

Successful implementation of a catheter-based imaging system relies on the integration of high-performance miniaturized distal end optics. Typically, compensation of chromatic dispersion, as well as astigmatism introduced by the device's sheath, can be addressed only by combining multiple optical elements, adversely impacting size and manufacturability. Here, we present a 300×300×800  μm³ monolithic optic that provides high optical performances over an extended wavelength range (near UV-visible-IR) with minimal chromatic aberrations. The design of the optic, fully optimized using standard optical simulation tools, provides the ability to freely determine aperture and working distance. Manufacturing is cost effective and suited for prototyping and production alike. The experimental characterization of the optic demonstrates a good match with simulation results and performances well suited to both optical coherence tomography and fluorescence imaging, thus paving the way for high-performance multimodal endoscopy systems.

Comparison of optical coherence tomography angiography and narrow-band imaging using a bimodal endoscope

We present coregistered images of tissue vasculature that allow a direct comparison between the performance of narrow-band imaging (NBI) and optical coherence tomography angiography (OCTA). Images were generated with a bimodal endomicroscope having a size of 15  ×  2.4  ×  3.3  ³   ( l  ,  w  ,  h  )   that combines two imaging channels. The white light imaging channel was used to perform NBI, the current gold standard for endoscopic visualization of vessels. The second channel allowed the simultaneous acquisition of optical coherence tomography (OCT) and OCTA images, enabling a three-dimensional (3-D) visualization of morphological as well as functional tissue information. In order to obtain 3-D OCT images scanning of the light-transmitting fiber was implemented by a small piezoelectric tube. A field of view of ∼1.1  mm was achieved for both modalities. Under the assumption that OCTA can address current limitations of NBI, their performance was studied and compared during in vivo experiments. The preliminary results show the potential of OCT regarding an improved visualization and localization of vessel beds, which can be beneficial for diagnosis of pathological conditions.

Long-range common-path spectral domain optical coherence tomography

Fiber-based common-path spectral domain optical coherence tomography (SD-OCT) is compact and polarization insensitive, which is usually used in endoscopic biomedical imaging. In this study, we investigate a method to extend the working distance of a common-path SD-OCT system. Common-path OCT light, which consisting of sample and reference light signal, is directed into a free space optical interferometer. The OCT light is split spatially into two beam segments by a wavefront-splitting mirror, and the two parallel beams interfere noncollinearly in the interferometer. Distance between the end of the probing fiber, which serves as the reference plane of our OCT system, and the OCT sample is about 140 mm. The OCT performance is demonstrated by imaging biological samples. The proposed method can be used to develop polarization insensitive OCT probe for biomedical imaging applications.

Intravascular optical coherence tomography to validate finite-element simulation of angioplasty balloon inflation

Concrete methods are lacking to examine angioplasty simulation results. For the first time, we explored the application of intravascular optical coherence tomography (IVOCT) to experimentally validate results obtained from finite-element simulation of angioplasty balloon deployment. In order to simulate each experimental scenario, IVOCT images were used to create initial geometrical models for the balloon and the phantoms. The study comprised three scenarios. The first scenario involved experimentally monitoring as well as simulating free expansion of the balloon. The second scenario involved experimentally monitoring as well as simulating balloon inflation inside three artery phantoms with different mechanical properties. The third scenario involved experimentally monitoring as well as simulating balloon unfolding and inflation inside a multilayer phantom. Using the first scenario, a constitutive model was developed for the balloon and was tuned to fit the IVOCT balloon inflation monitoring results. This model was used to simulate the balloon's response in simulations involving phantoms corresponding to the second and third scenarios. Diameter values were calculated both from images and simulation results. These values were then compared for each scenario. The obtained results highlight the potentials of IVOCT monitoring to validate simulation procedures.

Current and Emerging Preclinical Approaches for Imaging-Based Characterization of Atherosclerosis

Atherosclerotic plaques can remain quiescent for years, but become life threatening upon rupture or disruption, initiating clot formation in the vessel lumen and causing acute myocardial infarction and ischemic stroke. Whether and how a plaque ruptures is determined by its macroscopic structure and microscopic composition. Rupture-prone plaques usually consist of a thin fibrous cap with few smooth muscle cells, a large lipid core, a dense infiltrate of inflammatory cells, and neovessels. Such lesions, termed high-risk plaques, can remain asymptomatic until the thrombotic event. Various imaging technologies currently allow visualization of morphological and biological characteristics of high-risk atherosclerotic plaques. Conventional protocols are often complex and lack specificity for high-risk plaque. Conversely, new imaging approaches are emerging which may overcome these limitations. Validation of these novel imaging techniques in preclinical models of atherosclerosis is essential for effective translational to clinical practice. Imaging the vessel wall, as well as its biological milieu in small animal models, is challenging because the vessel wall is a small structure that undergoes continuous movements imposed by the cardiac cycle as it is adjacent to circulating blood. The focus of this paper is to provide a state-of-the-art review on techniques currently available for preclinical imaging of atherosclerosis in small animal models and to discuss the advantages and limitations of each approach.

Catheter-Based Polarization Sensitive Optical Coherence Tomography Using Similar Mueller Matrix Method

OBJECTIVE

Research of catheter-based polarization sensitive optical coherence tomography (PS-OCT) is a challenging field. In this paper, we present a new polarization determination method, similar Mueller matrix (SMM) method, for a catheter-based PS-OCT system using a standard clinical catheter probe with an outer diameter of 0.9 mm.

METHODS

The SMM method can remove the diattenuation and depolarization compositions by polar decomposition. By constructing the similarity between the measured Mueller matrices and sample matrices, the phase retardance of the sample can be determined from the trace of the measured matrices.

RESULTS

In the experiments, we find that images processed by the SMM method without any averaging or phase correction have a better polarization contrast of multiple biological tissues than those by the Jones matrix based method. We also preliminarily achieve phase retardance imaging of the ex vivo porcine cardiac blood vessel.

CONCLUSION

Compared with the Jones matrix based method, the presented SMM method can provide a more robust birefringence imaging of biological tissues under low signal-to-noise ratio, depolarization, diattenuation, and phase instability.

SIGNIFICANCE

The SMM method has a potential to become a widely accepted polarization determination method for catheter-based PS-OCT.

Comprehensive intravascular imaging of atherosclerotic plaque in vivo using optical coherence tomography and fluorescence lifetime imaging

Comprehensive imaging of both the structural and biochemical characteristics of atherosclerotic plaque is essential for the diagnosis and study of coronary artery disease because both a plaque's morphology and its biochemical composition affect the level of risk it poses. Optical coherence tomography (OCT) and fluorescence lifetime imaging (FLIm) are promising optical imaging methods for characterizing coronary artery plaques morphologically and biochemically, respectively. In this study, we present a hybrid intravascular imaging device, including a custom-built OCT/FLIm system, a hybrid optical rotary joint, and an imaging catheter, to visualize the structure and biochemical composition of the plaque in an atherosclerotic rabbit artery in vivo. Especially, the autofluorescence lifetime of the endogenous tissue molecules can be used to characterize the biochemical composition; thus no exogenous contrast agent is required. Also, the physical properties of the imaging catheter and the imaging procedures are similar to those already used clinically, facilitating rapid translation into clinical use. This new intravascular imaging catheter can open up new opportunities for clinicians and researchers to investigate and diagnose coronary artery disease by simultaneously providing tissue microstructure and biochemical composition data in vivo without the use of exogenous contrast agent.

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

Histogram-based standardization of intravascular optical coherence tomography images acquired from different imaging systems

PURPOSE

Intravascular optical coherence tomography (OCT) is widely used for analysis of the coronary artery disease. Its high spatial resolution allows for visualization of arterial tissue components in detail. There are different OCT systems on the market, each of which produces data characterized by its own intensity range and distribution. These differences should be taken into account for the development of image processing algorithms. In order to overcome this difference in the intensity range and distribution, we developed a framework for matching intensities based on the exact histogram matching technique.

METHODS

In our method, the key step for using the exact histogram matching is to determine the target histogram. For this, we proposed two schemes: a global scheme that uses a single histogram as the target histogram for all the pullbacks, and a local scheme that selects for each single image a target histogram from a predefined database. These two schemes are compared on a unique dataset containing pairs of pullbacks that were acquired shortly after each other with systems from two vendors, St. Jude and Terumo. Pullbacks were aligned according to anatomical landmarks, and a database of matched histogram pairs was created. A leave-one-out cross validation was used to compare performance of the two schemes. The matching accuracy was evaluated by comparing: (a) histograms using Euclidean (dx2 ) and Kolmogorov-Smirnov (dKS ) distances, and (b) median intensity level within anatomical regions of interest.

RESULTS

Leave-one-out validation indicated that both matching schemes yield comparably high accuracies across the entire validation dataset. The local scheme outperforms the global scheme with marginally lower dissimilarities at both histogram level and intensity level. High visual similarity was observed when comparing the matched images to their aligned counterparts.

CONCLUSION

Both local and global schemes are robust and produce accurate intensity matching. While local scheme performs marginally better than the global scheme, it requires a predefined histogram dataset and is more time consuming. Thus, for offline standardization of the images, the local scheme should be preferred for being more accurate. For online standardization or when another system is involved, the global scheme can be used as a simple and nearly-as-accurate alternative.

Blood Coagulation Following an Acute Ischemic Stroke

Objective. Hemostasis is a complex physiological process that stops bleeding at the site of a vascular injury. Although the majority of vascular accidents are ischemic, the role of hypercoagulable state and stroke needs further investigation. Materials and Methods. Fresh whole blood was taken from 61 acute ischemic stroke patients and compared to 18 healthy subjects and investigated with optical coherence tomography imaging after initiating coagulation. We used an OCT1300SS system (Thorlabs) and did 3D scans. We then processed the images with ImageJ. For each image mean, integrated density, skewness and kurtosis of gray values were analyzed. Results. Mean gray value and integrated intensity of sampled data showed an intrinsic difference detected with OCT. This difference was further confirmed by the data distribution analysis. Conclusions. Results suggest, that normal blood coagulation, is not a random reaction while in the case of stroke patients, the relatively symmetrical distribution of gray values brings coagulation closer randomized process.

Passively Driven Probe Based on Miniaturized Propeller for Intravascular Optical Coherence Tomography

Optical coherent tomography (OCT) has enabled clinical applications ranging from ophthalmology to cardiology that revolutionized in vivo medical diagnostics in the last few decades, and a variety of endoscopic probes have been developed in order to meet the needs of various endoscopic OCT imaging. We propose a passive driven intravascular optical coherent tomography (IV-OCT) probe in this paper. Instead of using any electrically driven scanning device, the probe makes use of the kinetic energy of the fluid that flushes away the blood during the intravascular optical coherence tomography imaging. The probe converts it into the rotational kinetic energy of the propeller, and the rotation of the rectangular prism mounted on the propeller shaft enables the scanning of the beam. The probe is low cost, and enables unobstructed stable circumferential scanning over 360 deg. The experimental results show that the probe scanning speed can exceed 100 rotations per second (rps). Spectral-domain OCT imaging of a phantom and porcine cardiac artery are demonstrated with axial resolution of 13.6 μm, lateral resolution of 22 μm, and sensitivity of 101.7 dB. We present technically the passively driven IV-OCT probe in full detail and discuss how to optimize the probe in further.

Dimension reduction technique using a multilayered descriptor for high-precision classification of ovarian cancer tissue using optical coherence tomography: a feasibility study

Optical coherence tomography (OCT) yields microscopic volumetric images representing tissue structures based on the contrast provided by elastic light scattering. Multipatient studies using OCT for detection of tissue abnormalities can lead to large datasets making quantitative and unbiased assessment of classification algorithms performance difficult without the availability of automated analytical schemes. We present a mathematical descriptor reducing the dimensionality of a classifier's input data, while preserving essential volumetric features from reconstructed three-dimensional optical volumes. This descriptor is used as the input of classification algorithms allowing a detailed exploration of the features space leading to optimal and reliable classification models based on support vector machine techniques. Using imaging dataset of paraffin-embedded tissue samples from 38 ovarian cancer patients, we report accuracies for cancer detection [Formula: see text] for binary classification between healthy fallopian tube and ovarian samples containing cancer cells. Furthermore, multiples classes of statistical models are presented demonstrating [Formula: see text] accuracy for the detection of high-grade serous, endometroid, and clear cells cancers. The classification approach reduces the computational complexity and needed resources to achieve highly accurate classification, making it possible to contemplate other applications, including intraoperative surgical guidance, as well as other depth sectioning techniques for fresh tissue imaging.

Introduction to the feature issue on the 25 year anniversary of optical coherence tomography

The guest editors introduce a feature issue commemorating the 25th anniversary of optical coherence tomography.