Phase-variance Optical Coherence Tomography Research Articles

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.

Investigating the healing mechanisms of an angiogenesis-promoting topical treatment for diabetic wounds using multimodal microscopy.

Impaired skin wound healing is a significant comorbid condition of diabetes that is caused by poor microcirculation, among other factors. Studies have shown that angiogenesis, a critical step in the wound healing process in diabetic wounds, can be promoted under hypoxia. In this study, an angiogenesis-promoting topical treatment for diabetic wounds, which promotes angiogenesis by mimicking a hypoxic environment via inhibition of prolyl hydroxylase resulting in elevation or maintenance of hypoxia-inducible factor, was investigated utilizing a custom-built multimodal microscopy system equipped with phase-variance optical coherence tomography (PV-OCT) and fluorescence lifetime imaging microscopy (FLIM). PV-OCT was used to track the regeneration of the microvasculature network, and FLIM was used to assess the in vivo metabolic response of mouse epidermal keratinocytes to the treatment during healing. Results show a significant decrease in the fluorescence lifetime of intracellular reduced nicotinamide adenine dinucleotide, suggesting a hypoxic-like environment in the wounded skin, followed by a quantitative increase in blood vessel density assessed by PV-OCT. Insights gained in these studies could lead to new endpoints for evaluation of the efficacy and healing mechanisms of wound-healing drugs in a setting where delayed healing does not permit available methods for evaluation to take place.

Challenges and advantages in wide-field optical coherence tomography angiography imaging of the human retinal and choroidal vasculature at 1.7-MHz A-scan rate.

We present noninvasive, three-dimensional, depth-resolved imaging of human retinal and choroidal blood circulation with a swept-source optical coherence tomography (OCT) system at 1065-nm center wavelength. Motion contrast OCT imaging was performed with the phase-variance OCT angiography method. A Fourier-domain mode-locked light source was used to enable an imaging rate of 1.7MHz. We experimentally demonstrate the challenges and advantages of wide-field OCT angiography (OCTA). In the discussion, we consider acquisition time, scanning area, scanning density, and their influence on visualization of selected features of the retinal and choroidal vascular networks. The OCTA imaging was performed with a field of view of 16deg (5 mm×5 mm) and 30deg (9 mm×9 mm). Data were presented in en face projections generated from single volumes and in en face projection mosaics generated from up to 4 datasets. OCTA imaging at 1.7MHz A-scan rate was compared with results obtained from a commercial OCTA instrument and with conventional ophthalmic diagnostic methods: fundus photography, fluorescein, and indocyanine green angiography. Comparison of images obtained from all methods is demonstrated using the same eye of a healthy volunteer. For example, imaging of retinal pathology is presented in three cases of advanced age-related macular degeneration.

Protective Effect of Intravitreal Administration of Exosomes Derived from Mesenchymal Stem Cells on Retinal Ischemia

ABSTRACTPurpose: Exosomes derived from human mesenchymal stem cells (hMSCs) cultured under hypoxic conditions contain proteins and growth factors that promote angiogenesis. This study investigated the effect of intravitreal administration of these exosomes on retinal ischemia using a murine model.Methods: Oxygen-induced retinopathy (OIR) was induced by exposing one-week-old male C57BL/6J mice to 5 days of 75% hyperoxic conditioning, and returning to room air. After hyperoxic conditioning, the right eye of each mouse was injected intravitreally with 1 µl saline or exosomes derived from hMSCs and compared to control mice of the same age raised in room air without OIR injected intravitreally with saline. Two weeks post-injection, fluorescein angiography (FA) and phase-variance optical coherence tomography angiography (pvOCTA) were used to assess retinal perfusion. Retinal thickness was determined by OCT. The extent of retinal neovascularization was quantitated histologically by counting vascular nuclei on the retinal surface.Results: Among eyes with OIR, intravitreal exosome treatment partially preserved retinal vascular flow in vivo and reduced associated retinal thinning; retinal thickness on OCT was 111.1 ± 7.4µm with saline versus 132.1 ± 11.6µm with exosome, p < 0.001. Retinal neovascularization among OIR eyes was reduced with exosome treatment when compared to saline-treated eyes (7.75 ± 3.68 versus 2.68 ± 1.35 neovascular nuclei per section, p < 0.0001). No immunogenicity or ocular/systemic adverse effect was associated with intravitreal exosome treatment.Conclusions: Intravitreal administration of exosomes derived from hMSCs was well tolerated without immunosuppression and decreased the severity of retinal ischemia in this murine model. This appealing novel non-cellular therapeutic approach warrants further exploration.

Detection of pigment epithelial detachment vascularization in age-related macular degeneration using phase-variance OCT angiography.

PurposeTo demonstrate the use of phase-variance optical coherence tomography (PV-OCT) angiography for detection of pigment epithelial detachment (PED) vascularization in age-related macular degeneration (AMD).Patients and methodsPatients with PEDs and exudative AMD were evaluated by the Retina Services at the University of California, Davis, and the University of California, San Francisco. Each subject underwent fluorescein angiography and structural optical coherence tomography (OCT). Phase-variance OCT analysis was used to create angiographic images of the retinal and choroidal vasculature. PV-OCT-generated B-scans were superimposed on structural OCT B-scans to allow easy identification of perfused vascular structures.ResultsThree patients with vascularized PEDs were imaged with PV-OCT, and each was found to have a vascular signal extending from the choroid into the hyperreflective substance of the PED. Two patients who had no evidence of PED vascularization on fluorescein angiography did not have vascular signals within their PEDs on PV-OCT.ConclusionStructural OCT and PV-OCT images can be combined to create composite B-scans that offer high-resolution views of the retinal tissue along with dynamic vascular visualization. This technique offers a fast, noninvasive method for detecting vascularization of PEDs in AMD and may aid in the early detection of neovascular disease.

Age-related macular degeneration: clinical findings, histopathology and imaging techniques.

Age-related macular degeneration (AMD) is the most common cause of blindness among people over age 55 years in industrialized countries. Known major risk factors for AMD include: age >55 years, history of smoking, white race, and mutations in various components of the complement system. Early AMD is characterized by the presence of drusen and pigmentary abnormalities. Late AMD is associated with central visual loss and is characterized by the presence of choroidal neovascularization and/or geographic atrophy. Early AMD is associated with a number of biochemical abnormalities including oxidative damage to retinal pigment epithelium (RPE) cells, complement deposition in the RPE-Bruch's membrane-choriocapillaris complex, lipidization of Bruch's membrane, and extracellular matrix abnormalities (e.g. collagen crosslinking, advanced glycation end product formation). Antiangiogenic drugs block the vascular leakage associated with choroidal new vessels, thus reducing retinal edema and stabilizing or restoring vision. At this time, there are no proven effective treatments for the nonexudative complications of AMD. Modern ocular imaging technologies (including spectral domain and phase variance optical coherence tomography, short- and long-wavelength fundus autofluorescence, adaptive optics-scanning laser ophthalmoscopy, and near-infrared reflectance) enable one to follow changes in the RPE, photoreceptors, and choriocapillaris quantitatively as the disease progresses. In addition, one can quantitatively assess the volume of drusen and areas of atrophy. These data, when correlated with the known histopathology of AMD, may provide useful measures of treatment efficacy that are likely to be more sensitive and reproducible than conventional end points such as visual acuity and rate of enlargement of geographic atrophy. As a result, these imaging technologies may be valuable in assessing the effects of cell-based therapy for patients with AMD.

In vivo pump‐probe optical coherence tomography imaging in Xenopus laevis

Currently, optical coherence tomography (OCT), is not capable of obtaining molecular information often crucial for identification of disease. To enable molecular imaging with OCT, we have further developed a technique that harnesses transient changes in light absorption in the sample to garner molecular information. A Fourier-domain Pump-Probe OCT (PPOCT) system utilizing a 532 nm pump and 830 nm probe has been developed for imaging hemoglobin. Methylene blue, a biological dye with well-know photophysics, was used to characterize the system before investigating the origin of the hemoglobin PPOCT signal. The first in vivo PPOCT images were recorded of the vasculature in Xenopus laevis. The technique was shown to work equally well in flowing and nonflowing vessels. Furthermore, PPOCT was compared with other OCT extensions which require flow, such as Doppler OCT and phase-variance OCT. PPOCT was shown to better delineate tortuous vessels, where nodes often restrict Doppler and phase-variance reconstruction.

Phase-Variance Optical Coherence Tomography: A Technique for Noninvasive Angiography

Phase-variance optical coherence tomography (PV-OCT) provides volumetric imaging of the retinal vasculature without the need for intravenous injection of a fluorophore. We compare images from PV-OCT and fluorescein angiography (FA) for normal individuals and patients with age-related macular degeneration (AMD) and diabetic retinopathy.This is an evaluation of a diagnostic technology.Four patients underwent comparative retinovascular imaging using FA and PV-OCT. Imaging was performed on 1 normal individual, 1 patient with dry AMD, 1 patient with exudative AMD, and 1 patient with nonproliferative diabetic retinopathy.Fluorescein angiography imaging was performed using a Topcon Corp (Tokyo, Japan) (TRC-50IX) camera with a resolution of 1280 (H) × 1024 (V) pixels. The PV-OCT images were generated by software data processing of the entire cross-sectional image from consecutively acquired B-scans. Bulk axial motion was calculated and corrected for each transverse location, reducing the phase noise introduced from eye motion. Phase variance was calculated through the variance of the motion-corrected phase changes acquired within multiple B-scans at the same position. Repeating these calculations over the entire volumetric scan produced a 3-dimensional PV-OCT representation of the vasculature.Feasibility of rendering retinal and choroidal microvasculature using PV-OCT was compared qualitatively with FA, the current gold standard for retinovascular imaging.Phase-variance OCT noninvasively rendered a 2-dimensional depth color-coded vasculature map of the retinal and choroidal vasculature. The choriocapillaris was imaged with better resolution of microvascular detail using PV-OCT. Areas of geographic atrophy and choroidal neovascularization imaged by FA were depicted by PV-OCT. Regions of capillary nonperfusion from diabetic retinopathy were shown by both imaging techniques; there was not complete correspondence between microaneurysms shown on FA and PV-OCT images.Phase-variance OCT yields high-resolution imaging of the retinal and choroidal microvasculature that compares favorably with FA.

Noninvasive, 3D imaging of retinal and choroidal vasculature using phase-variance optical coherence tomography

Noninvasive, 3D imaging of retinal and choroidal vasculature using phase-variance optical coherence tomography

Optical imaging of the chorioretinal vasculature in the living human eye

Detailed visualization of microvascular changes in the human retina is clinically limited by the capabilities of angiography imaging, a 2D fundus photograph that requires an intravenous injection of fluorescent dye. Whereas current angiography methods enable visualization of some retinal capillary detail, they do not adequately reveal the choriocapillaris or other microvascular features beneath the retina. We have developed a noninvasive microvascular imaging technique called phase-variance optical coherence tomography (pvOCT), which identifies vasculature three dimensionally through analysis of data acquired with OCT systems. The pvOCT imaging method is not only capable of generating capillary perfusion maps for the retina, but it can also use the 3D capabilities to segment the data in depth to isolate vasculature in different layers of the retina and choroid. This paper demonstrates some of the capabilities of pvOCT imaging of the anterior layers of choroidal vasculature of a healthy normal eye as well as of eyes with geographic atrophy (GA) secondary to age-related macular degeneration. The pvOCT data presented permit digital segmentation to produce 2D depth-resolved images of the retinal vasculature, the choriocapillaris, and the vessels in Sattler's and Haller's layers. Comparisons are presented between en face projections of pvOCT data within the superficial choroid and clinical angiography images for regions of GA. Abnormalities and vascular dropout observed within the choriocapillaris for pvOCT are compared with regional GA progression. The capability of pvOCT imaging of the microvasculature of the choriocapillaris and the anterior choroidal vasculature has the potential to become a unique tool to evaluate therapies and understand the underlying mechanisms of age-related macular degeneration progression.

Review of speckle and phase variance optical coherence tomography to visualize microvascular networks

High-resolution mapping of microvasculature has been applied to diverse body systems, including the retinal and choroidal vasculature, cardiac vasculature, the central nervous system, and various tumor models. Many imaging techniques have been developed to address specific research questions, and each has its own merits and drawbacks. Understanding, optimization, and proper implementation of these imaging techniques can significantly improve the data obtained along the spectrum of unique research projects to obtain diagnostic clinical information. We describe the recently developed algorithms and applications of two general classes of microvascular imaging techniques: speckle-variance and phase-variance optical coherence tomography (OCT). We compare and contrast their performance with Doppler OCT and optical microangiography. In addition, we highlight ongoing work in the development of variance-based techniques to further refine the characterization of microvascular networks.

Noninvasive imaging of the foveal avascular zone with high-speed, phase-variance optical coherence tomography.

To demonstrate the application of phase-variance optical coherence tomography (pvOCT) for contrast agent-free in vivo imaging of volumetric retinal microcirculation in the human foveal region and for extraction of foveal avascular zone dimensions. A custom-built, high-speed Fourier-domain OCT retinal imaging system was used to image retinas of two healthy subjects and eight diabetic patients. Through the acquisition of multiple B-scans for each scan location, phase differences between consecutive scans were extracted and used for phase-variance contrast, identifying motion signals from within blood vessels and capillaries. The en face projection view of the inner retinal layers segmented out from volumetric pvOCT data sets allowed visualization of a perfusion network with the foveal avascular zone (FAZ). In addition, the authors presented 2D retinal perfusion maps with pseudo color-coded depth positions of capillaries. Retinal vascular imaging with pvOCT provides accurate measurements of the FAZ area and its morphology and a volumetric perfusion map of microcapillaries. In this study using two images from each fundus fluorescein angiography (FA) and pvOCT, the measured average areas of the FAZ from two healthy subjects were below 0.22 mm(2), and each of eight diabetic patients had an enlarged FAZ area, larger than 0.22 mm(2). Moreover, the FAZ areas demonstrated a significant correlation (r = 0.91) between measurements from FA and pvOCT. The high-speed pvOCT allows contrast agent-free visualization of capillary networks in the human foveal region that is analogous to fundus FA imaging. This could allow for noninvasive diagnosis and progression monitoring of diabetic retinopathy in clinical settings.

In vivo volumetric imaging of human retinal circulation with phase-variance optical coherence tomography

We present in vivo volumetric images of human retinal micro-circulation using Fourier-domain optical coherence tomography (Fd-OCT) with the phase-variance based motion contrast method. Currently fundus fluorescein angiography (FA) is the standard technique in clinical settings for visualizing blood circulation of the retina. High contrast imaging of retinal vasculature is achieved by injection of a fluorescein dye into the systemic circulation. We previously reported phase-variance optical coherence tomography (pvOCT) as an alternative and non-invasive technique to image human retinal capillaries. In contrast to FA, pvOCT allows not only noninvasive visualization of a two-dimensional retinal perfusion map but also volumetric morphology of retinal microvasculature with high sensitivity. In this paper we report high-speed acquisition at 125 kHz A-scans with pvOCT to reduce motion artifacts and increase the scanning area when compared with previous reports. Two scanning schemes with different sampling densities and scanning areas are evaluated to find optimal parameters for high acquisition speed in vivo imaging. In order to evaluate this technique, we compare pvOCT capillary imaging at 3x3 mm2 and 1.5x1.5 mm2 with fundus FA for a normal human subject. Additionally, a volumetric view of retinal capillaries and a stitched image acquired with ten 3x3 mm2 pvOCT sub-volumes are presented. Visualization of retinal vasculature with pvOCT has potential for diagnosis of retinal vascular diseases.