Photothermal optical lock-in optical coherence tomography for in vivo imaging

Artificial neural network for enhancing signal-to-noise ratio and contrast in photothermal optical coherence tomography

Optical coherence tomography (OCT) is a medical imaging method that generates micron-resolution 3D volumetric images of tissues in-vivo. Photothermal (PT)-OCT is a functional extension of OCT with the potential to provide depth-resolved molecular information complementary to the OCT structural images. PT-OCT typically requires long acquisition times to measure small fluctuations in the OCT phase signal. Here, we use machine learning with a neural network to infer the amplitude of the photothermal phase modulation from a short signal trace, trained in a supervised fashion with the ground truth signal obtained by conventional reconstruction of the PT-OCT signal from a longer acquisition trace. Results from phantom and tissue studies show that the developed network improves signal to noise ratio (SNR) and contrast, enabling PT-OCT imaging with short acquisition times and without any hardware modification to the PT-OCT system. The developed network removes one of the key barriers in translation of PT-OCT (i.e., long acquisition time) to the clinic.

An OCT-A Analysis of the Importance of Intermediate Capillary Plexus in Diabetic Retinopathy: A Brief Review

Optical coherence tomography-angiography is a technique that allows us to non-invasively study in vivo the different retinal vascular networks. This allows a deeper understanding of retinal capillary anatomy and function, in addition to the pathophysiologic changes encountered in diverse diseases. The four retinal capillary layers have different anatomies and functions, implying distinct adaptation and roles in the course of the diseases. Diabetic retinopathy is the leading cause of blindness in working-age adults. Several studies have evaluated how each retinal capillary layer is specifically affected according to the stage of the disease. Unfortunately, too few studies have considered the intermediate capillary plexus as a separate layer, as it has often been incorporated in another layer. In this review, we shed light on the potential role the intermediate capillary plexus plays in the physiopathology of diabetic retinal disease as well as its potential use in grading diabetic retinopathy and its clinical added value in estimating the disease prognosis.

Photothermal optical coherence tomography for 3D live cell detection and mapping

Imaging cells in their 3D environment with molecular specificity is important to cell biology study. Widely used microscopy techniques, such as confocal microscopy, have limited imaging depth when probing cells in optically scattering media. Optical coherence tomography (OCT) can provide millimeter-level depth for imaging of highly scattering media but lacks the contrast to distinguish cells from extracellular matrix or to distinguish between different types of cells. Photothermal OCT (PT-OCT) is a promising technique to obtain molecular contrast at the imaging scale of OCT. Here, we report PT-OCT imaging of live, nanoparticle-labeled cells in 3D. In particular, we demonstrate detection and mapping of single cell in 3D without causing call death, and show the feasibility of 3D cell mapping through optical scattering media. This work presents live cell detection and mapping at an imaging scale that complements the major microscopy techniques, which is potentially useful to study cells in their 3D native or culture environment.

Label-free photothermal optical coherence microscopy to locate desired regions of interest in multiphoton imaging of volumetric specimens

Biochip-based research is currently evolving into a three-dimensional and large-scale basis similar to the in vivo microenvironment. For the long-term live and high-resolution imaging in these specimens, nonlinear microscopy capable of label-free and multiscale imaging is becoming increasingly important. Combination with non-destructive contrast imaging will be useful for effectively locating regions of interest (ROI) in large specimens and consequently minimizing photodamage. In this study, a label-free photothermal optical coherence microscopy (OCM) serves as a new approach to locate the desired ROI within biological samples which are under investigation by multiphoton microscopy (MPM). The weak photothermal perturbation in sample by the MPM laser with reduced power was detected at the endogenous photothermal particles within the ROI using the highly sensitive phase-differentiated photothermal (PD-PT) OCM. By monitoring the temporal change of the photothermal response signal of the PD-PT OCM, the hotspot generated within the sample focused by the MPM laser was located on the ROI. Combined with automated sample movement in the x-y axis, the focal plane of MPM could be effectively navigated to the desired portion of a volumetric sample for high-resolution targeted MPM imaging. We demonstrated the feasibility of the proposed method in second harmonic generation microscopy using two phantom samples and a biological sample, a fixed insect on microscope slide, with dimensions of 4 mm wide, 4 mm long, and 1 mm thick.

Three-dimensional opto-thermo-mechanical model for predicting photo-thermal optical coherence tomography responses in multilayer geometries

Photothermal optical coherence tomography (PT-OCT) is a functional extension of OCT with the ability to generate qualitative maps of molecular absorptions co-registered with the micron-resolution structural tomograms of OCT. Obtaining refined insight into chemical information from PT-OCT images, however, requires solid understanding of the multifactorial physics behind generation of PT-OCT signals and their dependence on system and sample parameters. Such understanding is needed to decouple the various physical effects involved in the PT-OCT signal to obtain more accurate insight into sample composition. In this work, we propose an analytical model that considers the opto-thermo-mechanical properties of multi-layered samples in 3-D space, eliminating several assumptions that have been limiting previous PT-OCT models. In parametric studies, the model results are compared with experimental signals to investigate the effect of sample and system parameters on the acquired signals. The proposed model and the presented findings open the door for: 1) better understanding of the effects of system parameters and tissue opto-thermo-mechanical properties on experimental signals; 2) informed optimization of experimentation strategies based on sample and system parameters; 3) guidance of downstream signal processing for predicting tissue molecular composition.

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.

Investigation of Gd2O3: Er3+/Yb3+ Upconversion Nanoparticles (UCNPs) as a Multi-model Contrast Agent for Functional Optical Coherence Tomography (fOCT)

Currently, upconversion nanoparticles (UCNPs) implanted as a contrast agent for optical coherence tomography (OCT) system due to its biocompatibility, anti-stock emission, narrow emission bandwidth non-photobleaching effects etc., but it was not used as multi model imaging probe. We synthesized multimodal imaging probe having upconversion property along with paramagnetic property and used as dual contrast agents for Photothermal Optical Coherence Tomography (PTOCT) and Magnetomotive Optical Coherence Tomography (MMOCT). The synthesized Gd₂O₃:Er³⁺/Yb³⁺ UCNPs shows the bright yellow upconversion emission, biocompatibility with hydrophilic property. A custom built SSOCT setup modified for PTOCT and MMOCT imaging along with custom MATLAB algorithm for signal extraction. A dynamic study was performed with synthesized UCNPs as an imaging probe and functional OCT system for targeted imaging. This shows the utility of the Gd₂O₃:Er³⁺/Yb³⁺ UCNPs as molecular probe for targeted imaging applications.

Ex-vivo molecular imaging with upconversion nanoparticles (UCNPs) using photo thermal optical coherence tomography (PTOCT)

We demonstrate a photothermal optical coherence tomography (PTOCT) system, with upconversion nanoparticles (UCNPs) as a molecular probe. We synthesized hydrophilic, biocompatible upconversion nanoparticles (UCNPs) using hydrothermal synthesis. We developed the PTOCT system along with the signal processing tool and applied this technique on animal tissue phantom for targeted imaging. The 'lock-in detection' of the amplitude modulated photothermal beam (980 nm), which used to excite the UCNPs was the backbone of the signal processing algorithm. The signal processing was further established in different aspects. As an application part, the diffusion dynamics of the UCNPs was performed inside the tissue to study molecular movement and subsequent changes in tissue properties. A comparison of photothermal optical coherence tomography (PTOCT) with phase variance optical coherence tomography (PVOCT) for targeted molecular imaging also presented.

Phase-sensitive lock-in detection for high-contrast mid-infrared photothermal imaging with sub-diffraction limited resolution

Imaging of the phase output of a lock-in amplifier in mid-infrared photothermal vibrational microscopy is demonstrated for the first time in combination with nonlinear demodulation. In general, thermal blurring and heat transport phenomena contribute to the resolution and sensitivity of mid-infrared photothermal imaging. For heterogeneous samples with multiple absorbing features, if imaged in a spectral regime of comparable absorption with their embedding medium, it is demonstrated that differentiation with high contrast is achieved in complementary imaging of the phase signal obtained from a lock-in amplifier compared to standard imaging of the photothermal amplitude signal. Specifically, by investigating the relative contribution of the out-of-phase lock-in signal, information based on changes in the rate of heat transport can be extracted, and inhomogeneities in the thermal diffusion properties across the sample plane can be mapped with high sensitivity and sub-diffraction limited resolution. Under these imaging conditions, wavenumber regimes can be identified in which the thermal diffusion contributions are minimized and an enhancement of the spatial resolution beyond the diffraction limited spot size of the probe beam in the corresponding phase images is achieved. By combining relative diffusive phase imaging with nonlinear demodulation at the second harmonic, it is demonstrated that 1-μm-size melamine beads embedded in a thin layer of 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal can be detected with a 1.3-μm spatial full-width at half-maximum (FWHM) resolution. Thus, imaging with a resolving power that exceeds the probe diffraction limited spot size by a factor of 2.5 is presented, which paves the route towards super-resolution, label-free imaging in the mid-infrared.

Coherently broadened, high-repetition-rate laser for stimulated Raman scattering-spectroscopic optical coherence tomography

We present a novel light source specifically tailored for stimulated Raman scattering-spectroscopic optical coherence tomography (SRS-SOCT), which is, to the best of our knowledge, a novel molecular imaging method that combines the molecular sensitivity of SRS with the spatial and spectral multiplexing capabilities of SOCT. The novel laser consists of an 8 W, 450 fs Yb:KGW oscillator, with a repetition rate of 40 MHz, which delivers the Stokes beam for SRS-SOCT and also pumps and amplifies an optical parametric oscillator (OPO). The output of the amplified OPO is then frequency doubled and coherently broadened using a custom-made tapered fiber that generates bandwidth pulses >40  nm, compressible to <50  fs, with the average power over 150 mW, near the shot-noise limit above 250 kHz. The broadened and compressed pulse simultaneously serves as the pump beam and SOCT light source for SRS-SOCT. This light source is assessed for SRS-SOCT, and its implications for other imaging methods are discussed.

Imaging Melanin Distribution in the Zebrafish Retina Using Photothermal Optical Coherence Tomography

Purpose

To demonstrate and validate that photothermal optical coherence tomography (PT-OCT) can image melanin in the retinal pigment epithelium (RPE) and can observe light-driven melanosome translocation in the zebrafish retina.

Methods

A commercial spectral domain OCT system was modified to perform both OCT and PT-OCT. Four adult tyrosinase-mosaic zebrafish with varying levels of melanin expression across their retinas were imaged, and the PT-OCT signal for pigmented and nonpigmented regions were compared. Wild-type dark-adapted (n = 11 fish) and light-adapted (n = 10 fish) zebrafish were also imaged with OCT and PT-OCT. Longitudinal reflectivity and absorption profiles were generated from B-scans to compare the melanin distribution between the two groups.

Results

A significant increase in PT-OCT signal (P < 0.0001, Student's t-test) was observed in pigmented regions of interest (ROI) compared to nonpigmented ROIs in the tyrosinase-mosaic zebrafish, which confirms the PT-OCT signal is specific to melanin in the eye. A significant increase in PT-OCT signal intensity (P < 0.0001, Student's t-test) was also detected in the light-adapted wild-type zebrafish group compared to the dark-adapted group. Additionally, light-adapted zebrafish display more distinct melanin banding patterns than do dark-adapted zebrafish in PT-OCT B-scans.

Conclusions

PT-OCT can detect different levels of melanin absorption and characterize pigment distribution in the zebrafish retina, including intracellular changes due to light-driven melanosome translocation within the RPE.

Translational Relevance

PT-OCT could quantify changes in pigmentation that occur in retinal diseases. The functional information provided by PT-OCT may also enable a better understanding of the anatomical features within conventional OCT images.

Photothermal optical coherence tomography of indocyanine green in ex vivo eyes

Indocyanine green (ICG) is routinely used during surgery to stain the inner limiting membrane (ILM) and provide contrast on white light surgical microscopy. While translation of optical coherence tomography (OCT) for intraoperative imaging during ophthalmic surgery has enhanced visualization, the ILM remains difficult to distinguish from underlying retinal structures and ICG does not provide additional OCT contrast. We present photothermal OCT (PT-OCT) for high-specificity detection of ICG on retinal OCT images. We demonstrate our technique by performing an ILM peel in ex vivo eyes using low ICG concentrations and laser powers. These results establish the feasibility of PT-OCT for intraoperative guidance during retinal surgery.

Cross-correlation photothermal optical coherence tomography with high effective resolution

We developed a cross-correlation photothermal optical coherence tomography (CC-PTOCT) system for photothermal imaging with high lateral and axial resolution. The CC-PTOCT system consists of a phase-sensitive OCT system, a modulated pumping laser, and a digital cross-correlator. The pumping laser was used to induce the photothermal effect in the sample, causing a slight phase modulation of the OCT signals. A spatial phase differentiation method was employed to reduce phase accumulation. The noise brought by the phase differentiation method and the strong background noise were suppressed efficiently by the cross-correlator, which was utilized to extract the photothermal signals from the modulated signals. Combining the cross-correlation technique with spatial phase differentiation can improve both lateral and axial resolution of the PTOCT imaging system. Clear photothermal images of blood capillaries of a mouse ear in vivo were successfully obtained with high lateral and axial resolution. The experimental results demonstrated that this system can enhance the effective transverse resolution, effective depth resolution, and contrast of the PTOCT image effectively, aiding the ongoing development of the accurate 3D functional imaging.

Thermo-elastic optical coherence tomography

The absorption of nanosecond laser pulses induces rapid thermo-elastic deformation in tissue. A sub-micrometer scale displacement occurs within a few microseconds after the pulse arrival. In this Letter, we investigate the laser-induced thermo-elastic deformation using a 1.5 MHz phase-sensitive optical coherence tomography (OCT) system. A displacement image can be reconstructed, which enables a new modality of phase-sensitive OCT, called thermo-elastic OCT. An analysis of the results shows that the optical absorption is a dominating factor for the displacement. Thermo-elastic OCT is capable of visualizing inclusions that do not appear on the structural OCT image, providing additional tissue type information.

In vivo photothermal optical coherence tomography of endogenous and exogenous contrast agents in the eye

Optical coherence tomography (OCT) has become a standard-of-care in retinal imaging. OCT allows non-invasive imaging of the tissue structure but lacks specificity to contrast agents that could be used for in vivo molecular imaging. Photothermal OCT (PT-OCT) is a functional OCT-based technique that has been developed to detect absorbers in a sample. We demonstrate in vivo PT-OCT in the eye for the first time on both endogenous (melanin) and exogenous (gold nanorods) absorbers. Pigmented mice and albino mice (n = 6 eyes) were used to isolate the photothermal signal from the melanin in the retina. Pigmented mice with laser-induced choroidal neovascularization lesions (n = 7 eyes) were also imaged after a systemic injection of gold nanorods to observe their passive accumulation in the retina. This experiment demonstrates the feasibility of PT-OCT to image the distribution of both endogenous and exogenous absorbers in the mouse retina.

Near Infrared Fluorescence Imaging in Nano-Therapeutics and Photo-Thermal Evaluation

The unresolved and paramount challenge in bio-imaging and targeted therapy is to clearly define and demarcate the physical margins of tumor tissue. The ability to outline the healthy vital tissues to be carefully navigated with transection while an intraoperative surgery procedure is performed sets up a necessary and under-researched goal. To achieve the aforementioned objectives, there is a need to optimize design considerations in order to not only obtain an effective imaging agent but to also achieve attributes like favorable water solubility, biocompatibility, high molecular brightness, and a tissue specific targeting approach. The emergence of near infra-red fluorescence (NIRF) light for tissue scale imaging owes to the provision of highly specific images of the target organ. The special characteristics of near infra-red window such as minimal auto-fluorescence, low light scattering, and absorption of biomolecules in tissue converge to form an attractive modality for cancer imaging. Imparting molecular fluorescence as an exogenous contrast agent is the most beneficial attribute of NIRF light as a clinical imaging technology. Additionally, many such agents also display therapeutic potentials as photo-thermal agents, thus meeting the dual purpose of imaging and therapy. Here, we primarily discuss molecular imaging and therapeutic potentials of two such classes of materials, i.e., inorganic NIR dyes and metallic gold nanoparticle based materials.

Quantitative depth-resolved microcirculation imaging with optical coherence tomography angiography (Part Ι): Blood flow velocity imaging

The research goal of the microvascular network imaging with OCT angiography is to achieve depth-resolved blood flow and vessel imaging in vivo in the clinical management of patents. In this review, we review the main phenomena that have been explored in OCT to image the blood flow velocity vector and the vessels of the microcirculation within living tissues. Parameters that limit the accurate measurements of blood flow velocity are then considered. Finally, initial clinical diagnosis applications and future developments of OCT flow images are discussed.

Depth-resolved analytical model and correction algorithm for photothermal optical coherence tomography

Photothermal OCT (PT-OCT) is an emerging molecular imaging technique that occupies a spatial imaging regime between microscopy and whole body imaging. PT-OCT would benefit from a theoretical model to optimize imaging parameters and test image processing algorithms. We propose the first analytical PT-OCT model to replicate an experimental A-scan in homogeneous and layered samples. We also propose the PT-CLEAN algorithm to reduce phase-accumulation and shadowing, two artifacts found in PT-OCT images, and demonstrate it on phantoms and in vivo mouse tumors.

Magnetic and Plasmonic Contrast Agents in Optical Coherence Tomography

Optical coherence tomography (OCT) has gained widespread application for many biomedical applications, yet the traditional array of contrast agents used in incoherent imaging modalities do not provide contrast in OCT. Owing to the high biocompatibility of iron oxides and noble metals, magnetic and plasmonic nanoparticles, respectively, have been developed as OCT contrast agents to enable a range of biological and pre-clinical studies. Here we provide a review of these developments within the past decade, including an overview of the physical contrast mechanisms and classes of OCT system hardware addons needed for magnetic and plasmonic nanoparticle contrast. A comparison of the wide variety of nanoparticle systems is also presented, where the figures of merit depend strongly upon the choice of biological application.

Contrast-enhanced optical coherence tomography with picomolar sensitivity for functional in vivo imaging

Optical Coherence Tomography (OCT) enables real-time imaging of living tissues at cell-scale resolution over millimeters in three dimensions. Despite these advantages, functional biological studies with OCT have been limited by a lack of exogenous contrast agents that can be distinguished from tissue. Here we report an approach to functional OCT imaging that implements custom algorithms to spectrally identify unique contrast agents: large gold nanorods (LGNRs). LGNRs exhibit 110-fold greater spectral signal per particle than conventional GNRs, which enables detection of individual LGNRs in water and concentrations as low as 250 pM in the circulation of living mice. This translates to ~40 particles per imaging voxel in vivo. Unlike previous implementations of OCT spectral detection, the methods described herein adaptively compensate for depth and processing artifacts on a per sample basis. Collectively, these methods enable high-quality noninvasive contrast-enhanced imaging of OCT in living subjects, including detection of tumor microvasculature at twice the depth achievable with conventional OCT. Additionally, multiplexed detection of spectrally-distinct LGNRs was demonstrated to observe discrete patterns of lymphatic drainage and identify individual lymphangions and lymphatic valve functional states. These capabilities provide a powerful platform for molecular imaging and characterization of tissue noninvasively at cellular resolution, called MOZART.