Raman Spectroscopy of Ocular Tissue - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 297

eye cups, these tissue properties provide essentially an optically thin film with
minimal self-absorption for both the excitation and Raman scattered light, if
properly excited in the long-wavelength shoulder of the absorption. Second,
since Raman scattering uses only the backscattered, single-path Raman re-
sponse from the lutein- and zeaxanthin-containing MP layers, as indicated in
Fig 12.8, and since these layers are located anteriorly in the optical pathway
through the retina, absorption and fluorescence effects originating from other
chromophores, such as rhodopsin in the photoreceptor layer, and melanin and
lipofuscin in the retinal pigment epithelial layer, RPE, respectively, can be
ignored or subtracted from the Raman spectra.
Initial “proof of principle” studies of ocular carotenoid RRS employed
a laboratory-grade high-resolution Raman spectrometer and flat-mounted
human cadaver retinas and eyecups. In Fig. 12.10, typical Raman spec-
tra are shown for an excised eyecup, in which the excitation laser was
aimed at the center of the macula, the fovea, and at two locations with in-
creased eccentricities. In all cases, RRS spectra are obtained that display the
Fig. 12.10. Raman spectra of excised human eyecup, obtained from tissue locations
in the central fovea ( trace a ), the parafovea ( trace b ), and the peripheral macula
( trace c ). Macular pigment carotenoid peaks are obtained at 1159 and 1524 cm − 1
with good signal-to-noise ratio, and decreasing strengths (factor of ∼ 30) when the
excitation beam is moved from the center of the macula ( trace a ) toward the pe-
ripheral retina ( trace c )