Conventional methods of spectroscopic Optical Coherence Tomography (OCT) determine depth-resolved spectra. Here, we present a spectroscopic method of assessing hemoglobin in OCT which, rather than determine a depth-resolved spectrum, determines a depth-resolved autocorrelation function. This complex-valued autocorrelation function is then fit with a model that incorporates the spectral absorption characteristics of different chromophores present in tissue. The proposed method does not use windowed Fourier transforms of the OCT data, and is well-suited for assessing chromophores in dynamic scattering environments such as blood vessels. The new autocorrelation spectroscopy method is compared against the conventional windowed Fourier transform method in the retina.
Bruch’s membrane (BM) is a pentalaminar structure that mediates transport between the retinal pigment epithelium (RPE) and choriocapillaris. With near-infrared Optical Coherence Tomography (OCT), it has been challenging to visualize, let alone quantify, BM non-invasively in non-pathologic eyes. First, we show that shorter wavelength visible light OCT consistently delineates BM better than longer wavelength visible light OCT in pigmented human subjects, independent of axial resolution. Second, we develop a physical model of RPE and BM reflectivity to explain this finding. Third, we employ this model to devise a morphometric algorithm to more accurately map BM thickness in the normal macula.
Measuring the point spread function (PSF) rolloff is a time-consuming part of spectral/Fourier domain Optical Coherence Tomography (SD-OCT) system characterization. Here, we introduce a method that predicts the PSF (sensitivity and axial resolution) rolloff without an interferometer. Instead, the method analyzes correlations of incoherent excess noise from the light source across the detector array to characterize the spectrometer response. We demonstrate this technique using supercontinuum and superluminescent diode sources, showing that a noise time course of just 0.5 seconds predicts axial imaging performance, as confirmed by meticulous measurements using a Michelson interferometer. This approach promises to facilitate SD-OCT system development.
Significance: Visible light optical coherence tomography (OCT) is emerging for spectroscopic and ultrahigh resolution imaging, but challenges remain. Depth-dependent dispersion limits retinal image quality and current correction approaches are cumbersome. Inconsistent group refractive indices during image reconstruction also limit reproducibility.
Aim: To introduce and evaluate water wavenumber calibration (WWC), which corrects depth-dependent dispersion and provides an accurate depth axis in water.
Approach: Enabled by a visible light OCT spectrometer configuration with a 3- to 4-dB sensitivity roll-off over 1 mm in air across a 90-nm bandwidth, we determine the spectral phase of a 1-mm water cell, an affine function of water wavenumber. Via WWC, we reconstruct visible light OCT human retinal images with 1.3-μm depth resolution in water.
Results: Images clearly reveal Bruch’s membrane, inner plexiform layer lamination, and a thin nerve fiber layer in the temporal parafovea. WWC halves the processing time, while achieving the same image definition as an assumption-free gold standard approach, suggesting that water wavenumber is a suitable proxy for tissue wavenumber. WWC also provides a depth axis in water without explicitly assuming a group refractive index.
Conclusions: WWC is a simple method that helps to realize the full potential of visible light OCT.
Spectral resolution is a crucial parameter in spectral / Fourier domain Optical Coherence Tomography (OCT). The sensitivity roll-off is determined by the spectral resolution, while depth-dependent axial resolution changes are caused by spectral resolution variations with wavelength. Currently, spectral resolution assessment is performed using either a narrow linewidth light source or broadband interferometry. Although commonly used, these methods require either additional components or time-consuming procedures. Here, we present a simple method to directly measure the spectral resolution at all wavelengths based on excess noise correlations. We apply this method to a visible light OCT system and validate it against interferometry.
Bruch’s membrane (BM) and retinal pigment epithelium (RPE) changes, thought to be the earliest signs of impending atrophy in dry age-related macular degeneration (AMD), are subtle and challenging to detect non-invasively. Here, we demonstrate imaging and quantification of BM and RPE thickness in the in vivo human eye using achromatized visible light optical coherence tomography (OCT) in normal human subjects. Our consistent visualization of BM and agreement of thickness values with ranges from prior histological studies is attributed to the increased axial resolution, and potentially also to the enhanced contrast of BM in the visible light range.
The inner plexiform layer (IPL) of the retina comprises extremely thin sublaminae with connections between bipolar cells, amacrine cells, and ganglion cells. So far, observations of IPL lamination in near-infrared Optical Coherence Tomography (OCT) images have been anecdotal. Visible light OCT theoretically provides higher axial resolution than near-infrared OCT for a given wavelength bandwidth. Imaging of the human retina with ultrahigh resolution visible light OCT and longitudinal chromatic aberration correction was recently shown, with a focus on the outer retina. Here, we demonstrate in vivo imaging of lamination in the inner plexiform layer using achromatized visible light Optical Coherence Tomography (OCT). To further improve the achievable axial resolution and contrast, we incorporate a grating light valve spatial light modulator (GLV-SLM) spectral shaping stage into our setup. The GLV-SLM rapidly and dynamically shapes the source spectrum to either reduce sidelobes in the axial point spread function, improve axial resolution by reducing the width of the axial point spread function, or switch between red light alignment mode and white light acquisition mode. In vivo retinal OCT images acquired from human subjects show that the IPL consists of 3 hyper-reflective bands and 2 hypo-reflective bands, corresponding well with the standard anatomical division of the IPL into 5 layers. Strategies to improve contrast of the subtle bands representing the IPL sublaminae are investigated. Possible explanations for the ability of visible light OCT to visualize IPL sublaminae, based only on backscattering or backreflection contrast, and implications for glaucoma progression monitoring, are discussed.
Interferometric near-infrared spectroscopy (iNIRS) is a time-of-flight- (TOF-) resolved sensing method for direct and simultaneous quantification of tissue optical properties (absorption and reduced scattering) and dynamics (blood flow index) in vivo with a single modality. The technique has previously been validated in Intralipid phantoms, and applied to continuously and non-invasively monitor optical properties and blood flow index in the brains of head-fixed, anesthetized mice. A demonstration of robust iNIRS measurements in human tissues with motion would support the viability of iNIRS for clinical applications. Here, we perform non-contact iNIRS in human tissues. We show that phase drift caused by involuntary motion during acquisition significantly distorts the optical field autocorrelation, particularly at early TOFs. To solve this issue, we present a novel numerical phase drift correction method to isolate field dynamics due to just red blood cell motion within the sample. Upon correction, TOF-resolved autocorrelations exhibit exponential decay behavior, whether acquired from Intralipid, the human forearm, or the human forehead. We confirm the link between bulk motion artifacts and phase drift by simultaneous, co-registered iNIRS and Optical Coherence Tomography measurements. By applying conventional, time-resolved diffusion theory and diffusing wave spectroscopy theory, we quantify optical properties and time-of-flight-resolved dynamics in Intralipid, the human forearm, and the human brain. Finally, we explore strategies for increased photon collection through parallelization of iNIRS, to probe greater depths in the human brain. This work conclusively shows that diffuse optical measurements of field dynamics are possible, even in the presence of motion artifacts.
Over the past 5 years, visible light Optical Coherence Tomography (OCT) has emerged as a promising technique for ultrahigh resolution microstructural imaging and depth-resolved imaging of chromophores. In the retina, visible light OCT can simultaneously induce and observe retinal changes during the phototransduction cascade, including bleaching-related absorption changes, as well as intrinsic scattering, cell swelling, and possible longer-term changes in retinal chromophores. Here we investigate outer retinal reflectance changes during visible light OCT in mice to better understand the contributions of these various signals.
All experiments were performed on pigmented (C57BL/6J) and albino (BALB/c) mice in an initially dark-adapted state. There were no consistent reflectance changes in any layers including and proximal to the External Limiting Membrane (ELM). However, reflectance increased in the inner segment / outer segment (IS / OS) junction and outer segments tips (OST) of both strains. Layers distal to the photoreceptors such as the Retinal Pigment Epithelium (RPE), Bruch’s membrane (BM), and choroid showed a consistent increase in pigmented mice and showed no significant change in albino mice. Though our results are qualitatively well-explained by results from photopigment bleaching and intrinsic optical signal experiments in the literature, the time scale of some of the changes observed in our study is too long, which could indicate either signals with a different physiological origin or the need for a more precise model to describe imaging and stimulation using the same beam profile.