| Photoacoustic (PA) imaging is an emerging technology that is capableof non-invasively measuring the optical absorption properties in biologicaltissues. Some tissue chromophores, such as hemoglobin and melanin, haverapid conversion of optical irradiation to heat, and thus an initial pressureincrease can be produced due to the temperature rise. As a result, thebroadband ultrasounds are generated, which can be detected for the imagereconstruction of the optically absorbing tissue molecules. The combinationof optical excitation and acoustic detection in PA imaging technologyenables the typical advantages of high contrast and high spatial resolution.In order to investigate the optical absorption properties of the retina,photoacoustic ophthalmoscopy (PAOM) was developed based onphotoacoustic effect. By studying the optical absorption and opticalscattering from biological tissues, the excitation light of PAOM wasoptimiazed to be532-nm laser wavelength,2-ns laser duration,60nJ laserintensity per pulse, and24kHz laser pulse repetition rates. T o overcome thelimitation of completed eye anatomy, a two-dimensional galvanometer wasintroduced for raster scanning the retina, and an unfocused transducer w asemployed to detect photoacoustic waves. PAOM demonstrated multipleadvantages including high transverse and axial resolutions of about20μm,fast imaging acquisition, and good imaging contrast. In vivo volumetricretinal images were provided by detecting laser induced acoustic waves fromthe retina, where retinal vessels, choroidal vasculatures and melanin ofretinal pigmented epethium (RPE) were clearly visualized. PAOM took about2.7seconds to acquire three-dimensional retinal image.To measure the flow velocity, laser-scanning Doppler photoacousticmicroscopy based on temporal correlation was proposed. Doppler angle, theangle of the flow with respect to the detection axis, was estimated byrecording the distances from the multiple illumination points along flowsuspension to the ultrasonic transducer. By extracting the time shift of twoconsecutive PA waves caused by the moving microshere through opticalfocus region, the flow speed and flow direction were identified incombination with the sound speed and the time interval of two pulsedlasers. The flow velocity range was estimated from3.66to124.6mm/s byimaging blood phatom. Moreover, the structural imaging and flow mapping can be acquired simultaneously, and the flow velocity profile was alsomapped along the flow cross section. The reported method has potential forthe blood flow mapping of the retina in the future.Multimodal ophthalmological imaging technology was established forcomprehensively investigating the retina, which integrated PAOM, opticalcoherence tomography (OCT), and fluorescence ophthalmic imaging (suchas fluorescein angiography and autofluorescence). Based on the LQ vLvR eyeimages, the multimodal retinal imaging platform was able to providemultiple optical contrasts including optical absorption, optical scattering,and fluorescence. Since hemoglobin and melanin has strong opticalabsorption, PAOM visualized the blood vessels and RPE melanin in theretina at high contrast, but the other retinal layers were invisible due totheir low optical absortion. Optical-scatterring-based OCT resolved the fineretinal layers because of its high axial resolution. By recording thefluorescence photons of contrast agents or fluorescent molecule labeledtissues, fluorescein angiography (FA) and autofluorescence (AF)demonstrated the capability of imaging the retinal vascular network andretinal green fluorescent protein expressed retinal tissue. These retinalimages acquired by the integrated PAOM, OCT and FA/AF werecomplementary, which provided better understanding on the retinalphysiological status. Compared to the single technology alone, multimodalophthalmological imaging can achieve more comprehensive evaluation ofthe retina; therefore, it is an invaluable tool in fundamental investigationand clinical diagnosis of major retinal disorders. |
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