Fundus Angiography Based On Full-field Time-domain Optical Coherence Tomography

The progress of most common ophthalmic diseases such as diabetic retinopathy and age-related macular degeneration are irreversible,and the severe cases may lead to blindness.Early detection and intervention is the main means of reducing blindness due to illness.Many studies have shown that the abnormalities of the vasculature occur earlier than the changes of tissue structures.Therefore,early detection of diseases can be achieved by monitoring the vasculature system.Fluorescein angiography and indocyanine green angiography are the current gold standards for the diagnosis of retinal and choroidal vascular diseases,but both of them require long time for preparing and could cause adverse reactions because of the dye injection.Optical coherence tomography angiography(OCTA)is a novel angiography technology,which is based on optical coherence tomography(OCT)technology,and does not require dye injection but based on motion contrast mechanism to obtain the vasculature information.It has the advantages of non-invasive,high resolution,fast and sectioning ability,and can provide the same effect as the current gold standards.OCTA has shown the great application prospects in ophthalmic diseases evaluation.Therefore,this project proposed to study OCTA technology.Vasculature shows a network-like distribution on the lateral section,and rich information can be observed on it,such as the presence,density,disconnection and blood perfusion.While on the longitudinal section,the vasculature presents discontinuous and irregular,and thus few observable information.Therefore,the observation of the vasculature is mainly on the lateral section.However,most of the existing OCTA systems are based on Fourier-domain OCT,which needs to go through the process "longitudinal 1D-longitudinal 2D-3D-lateral 2D" to obtain the vasculature,and thus has some problems such as complex process,large amount of data,and difficult to real-time.Full-field time-domain(FF-TD)OCT technology,which adopts 2D illumination and 2D detection,can directly obtain the lateral 2D image and thus perfectly match the vasculature distribution.Therefore,this project proposes to develop a fundus angiography based on FF-TD-OCT technology,which can perform high-resolution imaging to observe the microvasculature,and the results are more accurate because of no relative motion between the different pixels of the images.In this study,FF-TD-OCTA theory and angiography algorithms have been studied,an experimental system of fundus angiography has been designed and built,and its feasibility has also been verified by experiments,providing a reference for further research of human eye OCTA.The main works could be summarized as below:1.The theory of OCTA was analyzed,a FF-TD-OCTA experimental system was developed,and the selection basis of each parameter and the description of system design were given.Theoretical axial resolution of 4.5μm and lateral resolution of 1μm in tissue can been obtained.2.The synchronous control principle of hardware was studied.A method of hardware external triggering combined with software setting was proposed to realize the synchronization among signal acquisition,axial scanning of reference mirror and phase modulation.The logic and method of implementing synchronous control were introduced in detail,and finally the feasibility of this method was verified by imaging experiments.3.The correlation mapping algorithm was studied.A method of using structural similarity to calculate correlation coefficient was proposed to obtain the difference between two adjacent frames of images and thereby extract the vasculature information.The method was verified by experiments.4.Relevant experiments were implemented on the developed system.Coin-stripe and a blood vessel phantom were imaged,and different layers of coin-stripe and clear contour of blood vessel were observed,proving that the system has the sectioning ability and can been used for angiography.