Faculty Bibliography

Glaucoma and other optic neuropathies are characterized by progressive dysfunction and loss of retinal ganglion cells and their axons. Given the high prevalence of glaucoma-related blindness and the availability of treatment options, improving the diagnosis and precise monitoring of progression in these conditions is paramount. Here we review recent progress in the development of novel biomarkers for glaucoma in the context of disease pathophysiology and we propose future steps for the field, including integration of exploratory biomarker outcomes into prospective therapeutic trials. We anticipate that, when validated, some of the novel glaucoma biomarkers discussed here will prove useful for clinical diagnosis and prediction of progression, as well as monitoring of clinical responses to standard and investigational therapies.

OBJECTIVE:Current intrapartum fetal monitoring technology is unable to provide physicians with an objective metric of fetal well-being, leading to degraded patient outcomes and increased litigation costs. Fetal oxygen saturation (SpO2) is a more suitable measure of fetal distress, but the inaccessibility of the fetus prior to birth makes this impossible to capture through current means. In this paper, we present a fully non-invasive, transabdominal fetal oximetry (TFO) system that provides in utero measures of fetal SpO2. METHODS:TFO is performed by placing a reflectance-mode optode on the maternal abdomen and sending photons into the body to investigate the underlying fetal tissue. The proposed TFO system design consists of a multi-detector optode, an embedded optode control system, and custom user-interface software. To evaluate the developed TFO system, we utilized an in utero hypoxic fetal lamb model and performed controlled desaturation experiments while capturing gold standard arterial blood gases (SaO2). RESULTS:Various degrees of fetal hypoxia were induced with true SaO2 values ranging between 10.5% and 66%. The non-invasive TFO system was able to accurately measure these fetal SpO2 values, supported by a root mean-squared error of 6.37% and strong measures of agreement with the gold standard. CONCLUSION:The results support the efficacy of the presented TFO system to non-invasively measure a wide-range of fetal SpO2 values and identify critical levels of fetal hypoxia. SIGNIFICANCE:TFO has the potential to improve fetal outcomes by providing obstetricians with a non-invasive measure of fetal oxygen saturation prior to delivery.

From the bipolar cells to higher brain visual centers, signals in the vertebrate visual system are transmitted along parallel on and off pathways. These two pathways are spatially segregated along the depth axis of the retina. Yet, to our knowledge, there is no way to directly assess this anatomical stratification in vivo. Here, employing ultrahigh resolution visible light Optical Coherence Tomography (OCT) imaging in humans, we report a stereotyped reflectivity pattern of the inner plexiform layer (IPL) that parallels IPL stratification. We characterize the topography of this reflectivity pattern non-invasively in a cohort of normal, young adult human subjects. This proposed correlate of IPL stratification is accessible through non-invasive ocular imaging in living humans. Topographic variations should be carefully considered when designing studies in development or diseases of the visual system.

Here we provide a counter-example to the conventional wisdom in biomedical optics that longer wavelengths aid deeper imaging in tissue. Specifically, we investigate visible light optical coherence tomography of Bruch's membrane (BM) in the non-pathologic eyes of humans and two mouse strains. Surprisingly, we find that shorter visible wavelengths improve the visualization of BM in pigmented eyes, where it is located behind a highly scattering layer of melanosomes in the retinal pigment epithelium (RPE). Monte Carlo simulations of radiative transport suggest that, while absorption and scattering are higher at shorter wavelengths, detected multiply scattered light from the RPE is preferentially attenuated relative to detected backscattered light from the BM.

Optical Coherence Tomography angiography (OCTA) is a widespread tool for depth-resolved imaging of chorioretinal vasculature with single microvessel resolution. To improve the clinical interpretation of OCTA, the conditions affecting visualization of microvessels must be defined. Here we inject a scattering plasma tracer (Intralipid) during OCTA imaging of the anesthetized rat eye. In the retina, we find that interlaminar (vertical) vessels that connect laminae have one-fourth to one-third the OCTA red blood cell to tracer (RBC-to-tracer) signal ratio of intralaminar (horizontal) vessels. This finding suggests that the OCTA signal from microvessels depends on angular orientation, making vertically-oriented vessels more difficult to visualize using intrinsic contrast alone. Clinicians should be aware of this potential artifact when interpreting OCTA.

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.

Diffusing wave spectroscopy (DWS) and diffuse correlation spectroscopy (DCS) can assess blood flow index (BFI) of biological tissue with multiply scattered light. Though the main biological function of red blood cells (RBCs) is advection, in DWS/DCS, RBCs are assumed to undergo Brownian motion. To explain this discrepancy, we critically examine the cumulant approximation, a major assumption in DWS/DCS. We present a precise criterion for validity of the cumulant approximation, and in realistic tissue models, identify conditions that invalidate it. We show that, in physiologically relevant scenarios, the first cumulant term for random flow and second cumulant term for Brownian motion alone can cancel each other. In such circumstances, assuming pure Brownian motion of RBCs and the first cumulant approximation, a routine practice in DWS/DCS of BFI, can yield good agreement with data, but only because errors due to two incorrect assumptions cancel out. We conclude that correctly assessing random flow from scattered light dynamics requires going beyond the cumulant approximation and propose a more accurate model to do so.

Studies of flow-metabolism coupling often presume that microvessel architecture is a surrogate for blood flow. To test this assumption, we introduce an in vivo Dynamic Contrast Optical Coherence Tomography (DyC-OCT) method to quantify layer-resolved microvascular blood flow and volume across the full depth of the mouse neocortex, where the angioarchitecture has been previously described. First, we cross-validate average DyC-OCT cortical flow against conventional Doppler OCT flow. Next, with laminar DyC-OCT, we discover that layer 4 consistently exhibits the highest microvascular blood flow, approximately two-fold higher than the outer cortical layers. While flow differences between layers are well-explained by microvascular volume and density, flow differences between subjects are better explained by transit time. Finally, from layer-resolved tracer enhancement, we also infer that microvascular hematocrit increases in deep cortical layers, consistent with predictions of plasma skimming. Altogether, our results show that while the cortical blood supply derives mainly from the pial surface, laminar hemodynamics ensure that the energetic needs of individual cortical layers are met. The laminar trends reported here provide data that links predictions based on the cortical angioarchitecture to cerebrovascular physiology in vivo.

In biological tissue, longer near-infrared wavelengths generally experience less scattering and more water absorption. Here we demonstrate an optical coherence tomography (OCT) system centered at 2.1 microns, whose bandwidth falls in the 2.2 micron water absorption optical window, for in vivo imaging of the rodent brain. We show in vivo that at 2.1 microns, the OCT signal is actually attenuated less in cranial bone than at 1.3 microns, and is also less susceptible to multiple scattering tails. We also show that the 2.2 micron window enables direct spectroscopic OCT assessment of tissue water content. We conclude that with further optimization, 2.2 micron OCT will have advantages in low-water-content tissue such as bone, as well as applications where extensive averaging is possible to compensate absorption losses.

Visible light optical coherence tomography (OCT) theoretically provides finer axial resolution than near-infrared OCT for a given wavelength bandwidth. To realize this potential in the human retina in vivo, the unique technical challenges of visible light OCT must be addressed. We introduce three advances to further the performance of visible light OCT in the human retina. First, we incorporate a grating light valve spatial light modulator (GLV-SLM) spectral shaping stage to modify the source spectrum. This enables comfortable subject alignment with a red light spectrum, and image acquisition with a broad "white light" spectrum, shaped to minimize sidelobes. Second, we develop a novel, Fourier transform-free, software axial motion tracking algorithm with fast, magnetically actuated stage to maintain near-optimal axial resolution and sensitivity in the presence of eye motion. Third, we implement spatially dependent numerical dispersion compensation for the first time in the human eye in vivo. In vivo human retinal OCT images clearly show that the inner plexiform layer consists of 3 hyper-reflective bands and 2 hypo-reflective bands, corresponding with the standard anatomical division of the IPL. Wavelength-dependent images of the outer retina suggest that, beyond merely improving the axial resolution, shorter wavelength visible light may also provide unique advantages for visualizing Bruch's membrane.