Nathaniel J. Kemp

Dehydration mechanism of optical clearing in tissue

Previous studies identified various mechanisms of light scattering reduction in tissue induced by chemical agents. Our results suggest that dehydration is an important mechanism of optical clearing in collagenous and cellular tissue. Photographic and optical coherence tomography images indicate that air-immersed skin and tendon specimens become similarly transparent to glycerol-immersed specimens. Transmission electron microscopy images reveal that dehydration causes individual scattering particles such as collagen fibrils and organelles to become more densely packed, but does not significantly alter size. A heuristic particle-interaction model predicts that the scattering particle volume fraction increase can contribute substantially to optical clearing in collagenous and cellular tissue.

Depth-resolved birefringence imaging of the primate retinal nerve fiber layer using polarization-sensitive OCT

Imaging the optical phase retardation per unit depth (OPR/UD) in the retinal nerve fiber layer (RNFL) may aid in glaucoma diagnosis. Polarization Sensitive Optical Coherence Tomography (PSOCT) was used to record in vivo high-resolution images of the RNFL in two cynomologous monkeys. The depth variation in the Stokes vector of reflected light was used to calculate the OPR/UD as a function of RNFL position. OPR/UD decreased from 35 degree(s)/100 micrometers near the optic nerve to 5 degree(s)/100 micrometers at a location 600 micrometers superior to the optic nerve. Variation of OPR/UD in the RNFL with retinal position demonstrates a change in birefringence for different densities of ganglion cell axons. PSOCT may be useful for noninvasive determination of RNFL thickness and fiber density.

Birefringence and nerve fiber orientation in healthy in vivo primate RNFL with enhanced polarization-sensitive optical coherence tomography (EPS-OCT)

Form-birefringent properties of the retinal nerve fiber layer (RNFL) have become increasingly important as investigators strive to provide an improved methodology for glaucoma diagnosis. Techniques such as scanning laser polarimetry (SLP) and polarization-sensitive optical coherence tomography (PS-OCT) are two approaches which directly assess RNFL neurotubules, the sub-cellular structures responsible for form-birefringence and axoplasmic transport in retinal ganglion cell axons. We present a novel algorithm for enhancing the sensitivity of PS-OCT. Enhanced polarization-sensitive OCT (EPS-OCT) is capable of detecting small transformations in polarization typically experienced by light propagating through the thin and weakly birefringent primate RNFL. We report birefringence and nerve fiber orientation measurements for the peripapillary region in healthy in vivo primate RNFL and discuss the implications of the enhanced-sensitivity approach on noninvasive quantification of form-birefringence in glaucoma diagnostics.ABSTRACT Form-birefringent properties of the retinal nerve fiber layer (RNFL) have become increasingly important asinvestigators strive to provide an improved methodology for glaucoma diagnosis. Techniques such as scanning laserpolarimetry (SLP) and polarization-sensitive optical coherence tomography (PS-OCT) are two approaches whichdirectly assess RNFL neurotubules, the sub-cellular structures responsible for form-birefringence and axoplasmictransport in retinal ganglion cell axons. We present a novel algorithm for enhancing the sensitivity of PS-OCT.Enhanced polarization-sensitive OCT (EPS-OCT) is capable of detecting small transformations in polarization typicallyexperienced by light propagating through the thin and weakly birefringent primate RNFL. We report birefringence andnerve fiber orientation measurements for the peripapillary region in healthy in vivo primate RNFL and discuss theimplications of the enhanced-sensitivity approach on noninvasive quantification of form-birefringence in glaucomadiagnostics.Keywords: Glaucoma, retinal nerve fiber layer, birefringence, retardation, axis orientation, polarization, opticalcoherence tomography, neurotubules

Applications of metrology for optical coherence tomography

Optical coherence tomography (OCT) is a novel technique that has recently emerged as a promising biomedical imaging modality in ophthalmology, cardiology, oncology and a number of other medical disciplines. Basic contrast mechanism for OCT is gradient in refractive index of tissue constituents. Additional contrast mechanisms include Doppler, polarization and phase-sensitive imaging modalities. Applications of metrology include high-resolution spectrum analyzers for spectral domain OCT instrumentation development and characterization of refractive index of tissue constituents.