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Hao F. Zhang, Shuliang Jiao, Qing Wei, wei song; Multi-wavelength Photoacoustic Ophthalmoscopy for Retinal Oximetry. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2200. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To demonstrate the capability of photoacoustic ophthalmoscopy (PAOM) in measuring retinal hemoglobin oxygen saturation (sO2) based on multi-wavelength optical illumination and ultrasonic detection.
In PAOM, nanosecond laser pulses illuminate the retina at a 20-kHz pulse repetition rate and a high-sensitivity small-footprint ultrasonic transducer (center frequency: 35 MHz; bandwidth: 50%) detects the ultrasonic waves generated from retina as a result of optical absorption. By combing time-resolved recording of the laser-induced ultrasound with raster scanning of the illuminating light across the region of interest, a three-dimensional image was acquired within three seconds. To measure retinal sO2, a circular trajectory was scanned around the optic disk and three optical wavelengths (570 nm, 578 nm, and 588 nm) were used sequentially. Based on the molecular extinction coefficients differences between oxy-hemoglobin and deoxy-hemoglobin at the three optical wavelengths, sO2 in each retinal vessel scanned by the circular trajectory was calculated. The PAOM was also integrated with an optical coherence tomography (OCT), which provided optical alignment prior to PAOM imaging and complementary imaging contrast to PAOM. Doppler OCT was performed together with multi-wavelength PAOM to validate sO2 measurements.
Through circular scan around the optic disk, multi-wavelength PAOM acquired the sO2 levels in all the major retinal vessels and, therefore, retinal veins and arteries were separated according to the measured sO2 values, which agreed with other literature data. Doppler OCT scanned the same circular trajectory as multi-wavelength PAOM and the blood flow direction in every retinal vessel imaged by PAOM was obtained. As a result, retinal arteries and veins were also separated based on the fact that venous bloods flow inward and arterial bloods flow outward. The two independently achieved arterial-venous separations matched in every single vessel, which validated the PAOM measurements. We imaged several Sprague Dawley rats (body weight: ~ 500 g) and the results were consistent.
Being the only optical-absorption-based retinal imaging technology, PAOM hold promise in both fundamental study and clinical diagnosis of diseases, such as diabetic retinopathy, which manifest early pathological alterations in retinal oxygenation. In the future, by combining sO2 measurement with precise blood flow measurement by Doppler OCT, we can potentially acquire global or regional metabolic rate of oxygen in retina.
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