The posterior location of the dilator muscle induces anterior iris bowing during dilation, even in the absence of pupillary block.

PURPOSE: To examine the effect of the posterior location of the dilator on iris anterior curvature during dilation. METHODS: An in vivo human study, an ex vivo porcine experiment, and an in silico computational model were performed in parallel. Iris anterior curvature was measured in vivo before and after dilation by time-domain slit lamp optical coherence tomography (SL-OCT). All patients (n = 7) had undergone laser peripheral iridotomy to eliminate any pupillary block due to primary angle-closure glaucoma. In the ex vivo experiments, isolated porcine irides (n = 30) were secured at the periphery and immersed in an oxygenated Krebs-Ringer buffer. Dilation was induced pharmaceutically by the addition of 2.5% phenylephrine and 1% tropicamide. An in-house optical coherence tomography (OCT) system was used to obtain iris images before and after dilation. A finite element model was also developed based on typical geometry of the iris from the initial OCT image. The iris was modeled as a neo-Hookean solid, and the active muscle component was applied only to the region specified as the dilator. RESULTS: An increase in curvature and a decrease in chord length after dilation were observed in both experiments. In both the in vivo and ex vivo experiments, the curvature-to-chord length ratio increased significantly during dilation. Computer simulations agreed well with the experimental results only when the proper anatomic position of dilator was used. CONCLUSIONS: The posterior location of the dilator contributes to the anterior iris bowing via a nonpupillary block dependent mechanism.

Reconstructing micrometer-scale fiber pathways in the brain: multi-contrast optical coherence tomography based tractography.

Comprehensive understanding of connective neural pathways in the brain has put great challenges on the current imaging techniques, for which three-dimensional (3D) visualization of fiber tracts with high spatiotemporal resolution is desirable. Here we present optical imaging and tractography of rat brain ex-vivo using multi-contrast optical coherence tomography (MC-OCT), which is capable of simultaneously generating depth-resolved images of reflectivity, phase retardance, optic axis orientation and, for in-vivo studies, blood flow images. Using the birefringence property of myelin sheath, nerve fiber tracts as small as a few tens of micrometers can be resolved and neighboring fiber tracts with different orientations can be distinguished in cross-sectional optical slices, 2D en-face images and 3D volumetric images. Combinational contrast of MC-OCT images enables visualization of the spatial architecture and nerve fiber orientations in the brain with unprecedented detail. The results suggest that optical tractography, by virtue of its direct accessibility to nerve fibers, has the potential to validate diffusion magnetic resonance images and investigate structural connections in normal brain and neurological disorders. In addition, an endoscopic MC-OCT may be useful in neurosurgical interventions to aid in placement of deep brain stimulating electrodes.

Swept-source polarization-sensitive optical coherence tomography based on polarization-maintaining fiber.

We present a swept-source polarization-sensitive optical coherence tomography system based on a polarization-maintaining fiber interferometer. The system produces reflectivity and birefringence information along a depth profile with a single sweep of the optical spectrum. Unlike single-mode fiber systems, retardance and relative optical axis orientation images are calculated without compensation. The source is a 45 mW polygon-based swept-source centered at 1290 nm and tuned at a rate of 28 kHz. The interferometer consists of a single polarization-maintaining coupler that utilizes balanced detection for improved performance. Characterization data shows that this system yields accurate measurements with high sensitivity (106.2 dB) comparable to conventional setups. Images of biological tissues with high dynamic range are demonstrated.

Polarization-maintaining fiber based polarization-sensitive optical coherence tomography in spectral domain.

We demonstrate a polarization-sensitive spectral-domain optical coherence tomography system based on polarization-maintaining fiber technology. Using a single-line-scan camera, the system produces reflectivity and retardance information along a depth profile with a single measurement. The relative axis orientation is available as well. System design and characterization and images of a biological tissue are presented.

Measurement of Faraday rotation using phase-sensitive low-coherence interferometry.

We report a differential phase sensor capable of measuring Faraday rotation in reflection mode with a single measurement and a small field-depth factor. Based on a polarization-maintaining-fiber low-coherence interferometer, the sensor measures phase difference between two decorrelated and oppositely polarized circular states incident on a sample. Sensitivity of the sensor for the Faraday rotation is 0.31 arcmin, allowing applications on small volumes of liquids. The Verdet constants of various liquids, including clear and turbid samples, are measured at 857 nm.

Polarization-sensitive optical coherence tomography based on polarization-maintaining fibers and frequency multiplexing.

We report a novel polarization-maintaining fiber based optical coherence tomography for single detector imaging of tissue reflectivity and birefringence. A single depth scan yields quantitative birefringence information along the A-line accurately. Since the orthogonal polarization channels are frequency multiplexed, the polarization information is extracted by using digital band-pass filters. Here, we introduce the optical system and present the reflectivity and birefringence images of biological tissues with an axial resolution of 7.9 microm and SNR of 30 dB.