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Shireen Khattak, Neeru Gupta, Clinton Hupple, Yeni H Yucel; Melanin Distribution in Intact Human and Minipig Eyes Detected by Photoacoustic Imaging. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):896.
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© ARVO (1962-2015); The Authors (2016-present)
Melanin is an abundant endogenous chromophore in the iris, ciliary body and choroid of the eye, altered in many diseases such as macular degeneration and eye tumors. Here we attempt to evaluate melanin distribution in intact eyes using a novel photoacoustic imaging tool.
Following St. Michael’s Hospital Research Ethics Board approval, we imaged whole eyes by photoacoustic imaging (MSOT inVision128, iThera Medical Inc., Germany), constructed with a Nd:YAG laser (1064/532 nm) with 8 ns pulse length and 10 Hz pulse frequency coupled to optical parameter oscillator. Five minipig eyes and three human eyes (Human Eye Biobank for Research) fixed in 10% formalin were embedded in 1.5% agar and placed in a clear plastic bag with distilled water in the imaging chamber. Images were taken in 300µm steps with wavelengths from 680-980 nm in 5 nm steps, and 4 averages per scan. Spectral unmixing was performed using a linear regression algorithm with spectra for deoxygenated and oxygenated hemoglobin and for melanin (Fig. 1A), providing images of entire eye sections. Known concentrations of synthetic melanin in 1.5% agar were used to determine the relationship between concentration of melanin and signal intensity.
In all human eyes, melanin was detected by photoacoustic imaging in the iris, ciliary body and choroid (Fig. 2A, melanin in green). In all Yucatan minipig eyes, in addition to the iris, ciliary body, and choroid, melanin was observed in optic nerve head (Fig. 2B, melanin in green). Histological examination of the eyes was used to confirm the location of melanin rich eye tissues. A linear relationship was found between the concentration of synthetic melanin and photoacoustic signal intensity (R2 = 0.99, Fig. 1B).
Melanin distribution was reliably detected by photoacoustic imaging in whole intact human and minipig eyes. The linear relation between melanin and photoacoustic signal intensity supports the potential use for melanin quantification in eye specimens. Photoacoustic imaging may be useful for qualitative and quantitative assessment of the enucleated eye with uveal melanoma, including extraocular extension, and other eye diseases.
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