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Hao F. Zhang, Carmen A. Puliafito, Shuliang Jiao; Imaging RPE Melanin Optical Absorption Using Photoacoustic Ophthalmoscopy. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5873.
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© ARVO (1962-2015); The Authors (2016-present)
To demonstrate the capability of photoacoustic ophthalmoscopy (PAOM) in imaging melanin in the retinal pigment epithelium (RPE) by measuring optical absorption property of the RPE.
A multimodal imaging system, which seamlessly fused PAOM and spectral-domain optical coherence tomography (SD-OCT), was built to image the retina of small animals in vivo. PAOM illuminates the retina with nanosecond laser pulses at 578 nm. After the optical energy is absorbed by, for example, the RPE, ultrasonic waves are generated and are detected by a high-sensitivity small-footprint ultrasonic transducer (center frequency: 35 MHz; bandwidth: 50%) to form an image. Hence, PAOM is provides direct measure of the optical absorption of the RPE, which dominantly from melanin. The SD-OCT (center wavelength: 840 nm) provides guidance for alignment and complementary retinal anatomy to the PAOM. The lateral resolution of the PAOM and SD-OCT is ~20 µm in rat eyes; the axial resolutions of the SD-OCT and PAOM are 6 µm and 23 µm, respectively.
Both albino and pigmented rat eyes were imaged in vivo. In the pigmented rat eyes, high-resolution images of the RPE melanin distribution were acquired by PAOM with high contrast. In the albino rat eyes, RPE layer becomes invisible in PAOM images due to the absence of melanin, which resulted in the visualization of the choroidal vasculature network. In comparison, SD-OCT images showed no observable differences in the imaged RPE between pigmented and albino rat eyes.
Imaging optical absorption provides the most direct measure of RPE melanin, which has the potential to quantify RPE melanin content with a much improved specificity comparing with autofluorescence and optical polarization based imaging technologies. As the only available optical-absorption-based ophthalmic imaging technology, PAOM guided by OCT holds promise in both fundamental study and clinical diagnosis of diseases correlated with melanin-related RPE dysfunctions, such as age-related macular degeneration.
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