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T. Q. Duong, H. Cheng; Dynamic Imaging of Oxygenation, Blood Volume, and Blood Flow Changes in the Retina. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4952.
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© ARVO (1962-2015); The Authors (2016-present)
While coupling of blood flow, oxygenation and blood volume in the brain has been well described, experimental evidence of such coupling in the retina was reported only recently. Retinal vessels have been reported to vasoconstrict and vasodilate more strongly than cerebral vessels in response to physiologic stimuli. In this study, we developed laser speckle imaging (LSI) and intrinsic optical imaging (IOI) to study the dynamic changes of total hemoglobin concentration, oxygenation and blood flow in the rat retina. We used 100% O2 and 5% CO2 challenges, known to vasoconstrict and vasodilate, respectively, to investigate its vascular coupling.
Male rats (300-400g, n=6) were imaged under ~1% isoflurane. 100% O2 and 5% CO2 (in air) challenges were used with air as baseline. A combined LSI and IOI system was developed by modifying a commercial video imaging system. Images were acquired at 25Hz and 7µm resolution using LSI with a diode laser (780nm) and IOI with four wavelengths (546, 605, 630 and 700nm) in an interleaved fashion. Blood flow (BF) maps were computed (Cheng, Phys Med Biol 2004). Deoxyhemoglobin (Hb), oxyhemoglobin (HbO2) and total hemoglobin (HbT, blood volume) maps were computed using a modified Lambert-Beer law with differential path-length factors for diffeent wavelengths (Kohl, Phys Med Biol 2000). Vessel types (eg, venules and arterioles) were identified based on oximetric images.
Following 100% O2 or 5% CO2, LSI maps showed heterogeneous BF changes and IOI maps showed heterogeneous blood oxygenation and blood volume changes, with the largest changes generally found in readily identified blood vessels. In response to 100% O2, arterioles vasoconstricted 10-15% in diameters and BF decreased 20±3%. In contrast, large veins showed no vasoconstriction and BF remained unchanged (2±2%). HbO2 increased, Hb and HbT decreased. In response to 5% CO2, arterioles vasodilated 13-18%, BF increased 23±5%. In contrast, large veins showed no vasodilation and BF remained unchanged (3±2%). Hb decreased, HbO2 and HbT increased.
Combined LSI and IOI provided important physiological parameters, offering a means to study vascular coupling in the retina. BF, Hb, HbO2 and HbT were modulated by 100% O2 and 5% CO2 as expected. Future studies will measure changes in different vessel diameters, and to study visual stimulations and retinal diseases. Our long-term goal is to use LSI and IOI to cross-validate our recent functional MRI findings which reported differential oxygenation responses in the retinal and choroidal vascular layers (Cheng, PNAS 2006).
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