Optical Coherence Tomography Angiography

As the only visible vessels in vivo directly,the ocular circulation not only maintains the normal function of the eyes,but also reflects the healthy condition of the eyes,and even the healthy condition of the whole body as it being a part of the cerebrovascular.Therefore,it is very important to imaging the ocular vessels with high resolution and high sensitivity in clinics.As the golden method in clinics,Fluorescence Angiography(FA)and Indocyanine Green Angiography(ICGA)can visualize the fine ocular vessels with dye contrast agent.However,it may have adverse effects such as nausea,vomiting and even anaphylaxis after the dye injection,which limits the further applications of the FA and ICGA in some degree.Doppler ultrasound and laser speckle contrast imaging may not visualize the blood flow well because of their poor resolutions,either.And optical coherence tomography which has been applied widely in the clinical diagnosis,especially in the diagnosis and treatments of the eye diseases,is an alternative imaging method to visualize the blood flow label-freely due to its high resolution and noninvasive advantages,and its functional imaging of optical coherence angiography as many groups demonstrated.Among the current optical coherence angiography methods,they operate at a high scanning speed(above 50 kHz)OCT system to acquire a fine blood vasculature,while the widely used clinical OCT systems have a low scanning speed(generally below 40kHz)which cannot imaging the blood flow well after applying the current optical angiography methods limited by the low scanning speed.Therefore,to solve the above problem,this study is to explore an OCT method to label-freely visualize the blood flow with a low scanning speed OCT system,which may be suitable for the clinical OCT system and provide more functional information for the diagnosis of the diseases in clinics.Firstly,this paper optimized the OCT signals to improve the image quality.It is inevitable to induce the noises into the OCT signals during the imaging process,which will contaminate the OCT images seriously.To reduce these noises especially the speckle noises inducing by the high scattering nature of the Bio-tissue,this paper explored a method which combined the correlation analysis of wavelet coefficients and threshold processing to denoise the OCT images.Compared with other denoising methods,the wavelet-based method has a better performance in denoising and improving the quality of the OCT images,which contributes more to the optical coherence angiography.Secondly,by analyzing the current optical coherence angiography methods,this paper studied the application of Doppler based angiography and Speckle based angiography methods respectively on the low scanning speed OCT system,finding that the current methods may not suitable for the clinical OCT system and cannot acquire the fine vessel image.Therefore,according to the above study results,this paper developed a method combining the phase information relating to the velocity of blood flow directly and the amplitude information with high sensitivity and high signal-noise ratio and applying the histogram analysis on the phase information to filter blood flow dynamically,which is suitable for OCT imaging with a low scanning speed.As the application of the histogram analysis on the phase information,this method results more precisely in detecting blood flow compared with the empirical threshold method to separate blood flow.The results showed that this method had a good performance in imaging retinal and choroidal vessels with a low scanning speed,and also in detecting the micro-vessels effectively,which demonstrated the feasibility being implemented on the clinical OCT system.