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H. Cheng, Y. Yan, T. Q. Duong; Laser Speckle Imaging of Blood-Flow Responses to Visual Stimulation in the Rat Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4228.
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© ARVO (1962-2015); The Authors (2016-present)
While visually evoked functional imaging in the brain based on hemodynamic parameters has been well described, similar studies in the retina have only been reported recently using: i) laser Doppler flowmetry which detects blood flow (BF) changes at a single point, ii) intrinsic optical imaging which detects reflectance or oximetric changes, and iii) optical coherence tomography which detects changes in optical scatterings. In this study, we developed and applied laser speckle imaging (LSI) to investigate BF responses to visual stimulation in the rat retina at high spatiotemporal resolution. This approach provides unique physiologically relevant information and could complement existing retinal imaging techniques.
Three male SD rats were anesthetized with 1% isoflurane, paralyzed and mechanically ventilated. LSI was acquired at 25Hz at 4µm with an image area of 2mm using near infrared light (785nm). Visual stimuli used a single flash (full field) of green light (550nm). These two light sources are coupled and transmitted to the rat eye. BF percent-change maps and time courses were calculated [Cheng & Duong, Optics Let 2007]. Blood vessel diameters were also analyzed.
LSI reliably detected robust BF responses to a single flash in the rat retina. The spatial and temporal dynamics of the BF responses were visualized. BF changes started at regions away from the optic nerve head (ONH) and ended at the ONH, indicative of the draining effects as expected. Percent-change LSI maps at discrete time points showed heterogeneous BF increases. The largest changes were generally found in the arterioles, smaller changes in regions void of obvious surface vessels, and the smallest changes in the ONH. BF of the whole retina excluding the ONH averaged 25±4% increase (P<0.05). BF in the ONH averaged 10±5% increase (P<0.05). These results are in general agreement with those reported by laser Doppler flowmetry and intrinsic optical imaging. Arterioles dilated 10-15% in diameters. In contrast, large veins did not shown significant diameter changes.
This study demonstrates that LSI is highly sensitive and can detect robust BF responses to a single flash in the retina at high spatial and temporal resolution, allowing dynamic characterization of the visual responses. This approach may prove useful to study retinal physiology. Future studies will integrate LSI with intrinsic optical imaging to visualize oximetry and blood volume changes, and apply this integrated approach to study focal visual stimuli and retinal diseases.
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