Photoacoustic-and-OCT-based Functional Vascular And Neural Imaging Technology And Its Applications In The Visual System

Diseases of the visual system severely affects the life quality of patients by damaging their visual functions and even causing blindness in the patients.In the visual system,the blood vessels and the neural system form an integral functional unit to realize the visual function.Therefore,morphological and functional abnormalities in the vascular and neural system are key factors in the occurrence and progress of various visual system diseases.Non-invasive technologies capable of providing functional information of both the vasculature and the neural tissue in the visual system are therefore important for further investigating the pathogenesis of the visual system diseases,as well as developing more effective diagnosis methods and treatments to these diseases.Photoacoustic imaging is a novel biological imaging technology based on optical absorption.By applying the photoacoustic signals which are stimulated through laser and reflects the optical absorption properties of the biological tissue,photoacoustic imaging could realize high-contrast blood vessel imaging as well as noninvasive measurement of the blood oxygen saturation?SO₂?.On the other hand,as an optical imaging modality based on optical scattering,optical coherence tomography?OCT?has high sensitivity to the optical property changes in biological tissues caused by movements in the tissue or biological processes.Thus OCT could realize accurate blood flow imaging,as well as functional neural imaging based on detecting scattering changes as a result of the neural electrophysiology.Therefore,a combination of photoacoustic imaging with OCT could complement the advantages of each modality and form an ideal functional vascular-neural imaging technology.As a result,the purpose of this research is to develop optical-resolution photoacoustic microscopy?OR-PAM?and spectral-domain OCT?SD-OCT?imaging systems and solve key problems of photoacoustic-and-OCT-based functional vascular and neural imaging technology,which includes accomplishing high contrast structural vascular imaging and vascular tree extraction using OR-PAM,exploring accurate SO₂ measurement based on OR-PAM,realizing blood flow detection through OCT and detecting neural response by OCT.Based on this,by focusing on extracting functional information of vasculature and neural tissues in the visual system,this research implements a series of innovations in technology and applications.The research content is described as follows.OR-PAM provides higher contrast and accuracy for structural and SO₂-based functional imaging of blood vessels than scattering-based optical imaging technology.Thus,this research images the vasculature of cat primary visual cortex using OR-PAM.A vascular tree extraction algorithm is then developed to automatically extract independent and complete vascular trees,which are major functional units in the neural-vascular coupling mechanism.Based on the concept of blood vessel tracking,the proposed algorithm extracts complete vascular trees by utilizing a ray casting framework to realize functions such as vessel direction estimation,vessel branching identification and vessel crossover point identification.By virtue of high-contrast three-dimensional vascular imaging using OR-PAM,this research realizes the automatic vascular tree extraction from imaging results for the first time.On the basis of structural imaging of blood vessels,this research studies the factors which could influence the accuracy of OR-PAM SO₂ measurement through phantom studies,in vivo experiments and theoretical simulations.Utilizing two inks?red and blue?to mimic oxygenated and deoxygenated hemoglobin in the blood,the phantom experiment is conducted by OR-PAM imaging of microtubes injected with ink mixtures.The in vivo experiment is implemented to measure SO₂ of blood vessels in the nude mouse ear by OR-PAM.The theoretical simulation is accomplished through combining Monte Carlo optical simulation and photoacoustic signal generation to study SO₂ measurement.Based on these results,it is concluded that the major factor influencing the accuracy of SO₂ measurement is the organ of the photoacoustic signal.For the signal generated from the middle part of the blood vessel,the SO₂ can be accurately measured using the amplitude of the signal.However,the signal from the boundary part of the vessel cannot accurately reflect the optical absorption of blood in the vessel.In the end,the SO₂ time course in a pulse cycle on arteries of cat primary visual cortex is obtained in this research.Due to the effects of its inherent confocal filtering and coherence gating,OCT has higher sensitivity in blood flow imaging.The SD-OCT system in this research could measure the cyclic blood flow in retinal arteries through phase-resolved Doppler OCT technology.Building on this,a jump-scanning method is developed in this research for noninvasive measurement of retinal pulse wave velocity?PWV?in order to reflect vascular elasticity.With the pulse-wave-induced pulsatile arterial flow being used as the pulse shape,the jump-scanning method measures PWV by extracting the transit time of the pulse wave from an original OCT scanning site to another through a transient jump.Repetitive implementations for each jump distances are performed to reduce the errors caused by inherent changes in pulse cycles.The measured retinal arterial PWV of a young human subject with normal blood pressure was in the order of 20-30 mm/s,which was consistent with previous studies.As a comparison,PWV of 50 mm/s was measured for a young human subject with prehypertension,which was in accordance with the finding of strong association between retinal PWV and blood pressure.This is the first time that the arterial PWV was measured using SD-OCT.At last,based on its high sensitivity and three-dimensional detection of the inherent scattering changes caused by neural activity,for the first time,OCT is used to study the retina under transcorneal electrical stimulation?TES?.The results show that along with the application of the electrical stimulation,the scattering in the cat retina immediately changes.The scattering changes last during TES and the amplitude of the changes depends on the strengths of applied TES currents.Previous studies have proven that TES could induce response in primary visual cortex by stimulating the retina,but which neurons are stimulated in the retina and the properties of neural response of different retinal neurons remain unknown.Due to the three-dimensional OCT functional imaging,the scattering changes are detected in the whole retina and while both the positive and negative scattering changes are found in the retina,negative scattering changes are more prominent in the inner and outer retina,and the positive changes are stronger in the subretinal space.Possible origins of the observed scattering changes,such as TES-induced imaging artifacts,imaging quality degradation or structural changes in the retina,are ruled out in the experiments and it is concluded that the origin of the detected scattering changes is the change of retinal neural activity induced by TES.In summary,based on OR-PAM and OCT technology,this research realizes functional vascular-neural imaging to extract functional information of vasculature and neural tissues in the visual system.Building on this,this research supplies a new tool for neural-vascular coupling researches through the development of automatic vascular tree extraction algorithm,benefits OR-PAM-based SO₂ measurement by studying the factors which could influence the accuracy of measurement,delivers a new method for evaluating the elasticity of retinal blood vessels by proposing the Doppler-OCT-based jump-scanning method for PWV measurement,provides new knowledge of the stimulated retinal under TES and may aid the study of the physiological mechanisms underlying the therapeutic effects of TES using OCT.In the end,this research expands the application of photoacoustic-OCT-based functional vascular-neural imaging technology in the visual system and provides a foundation for realizing dual-modal photoacoustic and OCT functional vascular-neural imaging technology.