Quantitation of Hyperspectral Autofluorescence (AF) from Human Retinal Pigment Epithelium (RPE) Ex Vivo

C. Nabati, A. Johri, R. Post, Paul Sajda, T. Ach, C. Curcio, T. Smith

Purpose
Quantify the hyperspectral AF signal from RPE/Bruch’s membrane (BrM) flat mounts.

Methods
Hyperspectral AF images (hypercubes) were captured from 66, 40X fields in 11 RPE/BrM flat mounts from human donor eyes using techniques described in detail in the abstract submitted by K. Agarwal. Briefly, for each 40X field, the hypercube has the two spatial dimensions of the field, and at each spatial point the photon counts recorded at each wavelength, hence the third or spectral dimension. For reproducible quantification of these data, exposure times were calibrated so that photon counts per spectral channel fell within the 12-bit linear range of the detector and then were offset by the dark current. Scaled counts-per-second were determined by exposure time (Eqn. 1) and calibrated to a standard fluorescent reference (courtesy of F Delori) to correct for any variation in power of the excitation light, yielding quantified hypercubes with units of photon counts per second at each point and wavelength.

Results
The Root Mean Square(RMS ) difference of quantified hypercubes from repeat imaging of the same location was within the noise level (dark current) of the Nuance detector, establishing reproducibility. Separation of RPE signal from BrM (Fig. 2) and further mathematical analyses of the hypercubes (see abstract of A. Johri) therefore extracted reliable quantitative RPE lipofuscin spectra for individual constituents and their corresponding spatial co-localizations.

Conclusions
Hyperspectral AF images of human RPE flatmounts may be reliably quantified for use as a surrogates for measurement of abundant of lipofuscin components. Such quantitative information can help guide the analysis of RPE physiology and biochemistry.

Accepted 1 April 2014
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