K. Twietmeyer

Optimization of Mueller matrix polarimeters in the presence of error sources

Methods are presented for optimizing the design of Mueller matrix polarimeters and and in particular selecting the retardances and orientation angles of polarization components to ensure accurate reconstruction of a samples Mueller matrix in the presence of error sources. Metrics related to the condition number and to the singular value decomposition are used to guide the design process for Mueller matrix polarimeters with the goal of specifying polarization elements, comparing polarimeter configurations, estimating polarimeter errors, and compensating for known error sources. The use of these metrics is illustrated with analyses of two example polarimeters: a dual rotating retarder polarimeter, and a dual variable retarder polarimeter.

Mueller matrix retinal imager with optimized polarization conditions.

A new Mueller matrix polarimeter was used to image the retinas of normal subjects. Light from a linearly polarized 780 nm laser was passed through a system of variable retarders and scanned across the retina. Light returned from the eye passed through a second system of retarders and a polarizing beamsplitter to two confocal detection channels. Optimization of the polarimetric data reduction matrix was via a condition number metric. The accuracy and repeatability of polarization parameter measurements were within +/- 5%. The magnitudes and orientations of retardance and diattenuation, plus depolarization, were measured over 15 degrees of retina for 15 normal eyes.

Retinal oximetry using intravitreal illumination.

Purpose: To demonstrate spectroscopic retinal oximetry measurements on arteries and veins in swine using intravitreal illumination. Retinal arterial and venous saturations are measured for a range of inspired O2 levels after pars plana vitrectomy. Methods: Pars plana vitrectomy and intravitreal manipulations were performed on two female American Yorkshire domestic swine. Light from a scanning monochromator was coupled into a fiberoptic intraocular illuminator inserted into the vitreous. The retinal vessels were illuminated obliquely, minimizing vessel glints. Multispectral images of the retinal vasculature were obtained as the swines arterial blood oxygen saturation was decreased from 100% to 67% in decrements of approximately 10%. Retinal vessel spectra were used to calculate oxygen saturation in selected arteries and veins. Arterial oxygen saturations were calibrated using blood gas analysis on blood drawn from a Swan-Ganz catheter placed in the femoral artery. Results: Oblique illumination of retinal vessels using an intravitreal fiberoptic illuminator provided a substantial reduction in the central vessel glint usually seen in fundus images, thus simplifying the analysis of spectral data. The vessel shadows were displaced from the vessel image simplifying the light paths in the eye. Using a full spectral analysis simplified by the light path reductions, we calculated retinal vessel saturations. The reduction of glint allowed for increased accuracy in measuring retinal vessel spectral optical density. Abnormally low retinal venous oxygen saturations were observed shortly after pars plana vitrectomy. Conclusions: Retinal oximetry using intravitreal illumination has been demonstrated. As a research tool, intravitreal illumination addresses several difficulties encountered when performing retinal oximetry with transcorneal illumination.

GDx -MM: An imaging Mueller matrix retinal polarimeter

An imaging polarimeter capable of measuring a complete Mueller matrix of the retina has been designed, built, calibrated, validated, and used on human subjects. Retardance, diattenuation, and depolarization images of normal retinas are presented.

Wavelength dependence of the apparent diameter of retinal blood vessels

Imaging of retinal blood vessels may assist in the diagnosis and monitoring of diseases such as glaucoma, diabetic retinopathy, and hypertension. However, close examination reveals that the contrast and apparent diameter of vessels are dependent on the wavelength of the illuminating light. In this study multispectral images of large arteries and veins within enucleated swine eyes are obtained with a modified fundus camera by use of intravitreal illumination. The diameters of selected vessels are measured as a function of wavelength by cross-sectional analysis. A fixed scale with spectrally independent dimension is placed above the retina to isolate the chromatic effects of the imaging system and eye. Significant apparent differences between arterial and venous diameters are found, with larger diameters observed at shorter wavelengths. These differences are due primarily to spectral absorption in the cylindrical blood column.

Condition Number as a Metric for the Effectiveness of Polarimetric Algorithms

Condition number is a standard metric used to quantify the effectiveness of polarimetric algorithms. In this work several definitions of condition number are compared, and their utility is illustrated by analyzing various polarimeter configurations.

Depolarization properties of the normal human fovea measured by the GDx-MM

A custom imaging Mueller matrix retinal polarimeter (the GDx-MM) is built. Mueller matrix images of normal human fovea were acquired with the GDx-MM over a 9° field at 780nm and have been analyzed for depolarization index and the variation of degree of polarization with incident polarization state. The degree of polarization (DoP) was often above 50% and varied in complex ways as a function of the incident polarization states. The depolarization properties around the macula loosely correlated with the retardance image. High spatial frequency depolarizing structures were evident throughout the fovea.

Depolarization properties of the normal human fovea

Depolarization data is provided for several normal retinas. Mueller matrix images of normal human fovea were acquired with a custom imaging Mueller matrix retinal polarimeter (the GDx-MM) over a 9° field at 780nm and have been analyzed for depolarization index and the variation of degree of polarization with incident polarization state. The degree of polarization (DoP) was often above 50% and varied in complex ways as a function of the incident polarization states. The depolarization properties around the macula loosely correlated with the retardance image. High spatial frequency depolarizing structures were evident throughout the fovea.