Research Of Contrast Angiography Algorithm In Optical Coherence Tomography

Optical coherence tomography(OCT)is a new type of noninvasive optical imaging technology developed in the early 1990s.As a detection technique,the key advantages of OCT include high-resolution,high-speed,high-sensitivity,non-invasive,horizontal resolution and vertical resolution independent of each other in vivo.With the development of light sources and detection techniques,high-speed Fourier domain OCT systems can not only achieve structural imaging but also perform functional imaging.The applications of OCT extended largely in different fields,such as ophthalmology,diabetic retinopathy(DR),glaucoma,age-related macular degeneration(AMD)and other diseases.Optical coherence tomography angiography(OCTA)is increasingly becoming useful and important imaging techniques in clinic,because OCT system can achieve in vivo microangiography without the contrast agents.In recent years,numerous OCT angiography algorithms have been proposed,including angiography based on the phase of OCT signal,angiography based on the magnitude of OCT signal,angiography based on both the magnitude and phase of OCT signal.In this paper,firstly,the theory of laser speckle contrast algorithm was analyzed in detail,and the laser speckle contrast algorithm was introduced into OCT angiography;An improved laser speckle contrast algorithm was proposed to suppress the noise of the lines generated by laser speckle contrast algorithm;Using the improved contrast image to mark the mean image,an optical coherence tomography contrast angiography(OCTCA)algorithm was proposed to further improve the signal-to-noise ratio of angiographic images,and provided new way for obtaining micro angiographic images.The method is adopted to perform motion artifact correction on the structural images acquired by OCT,consecutive frames of the same location were repeatedly scanned and matched;the matched frames were averaged to enhance the intensity of blood flow signals;then use the improved speckle contrast image to obtain the information of the dynamic blood flow;finally,the vasculature of tissue was generated by masking the averaged image with the improved speckle contrast image.We found that OCTCA can exclude the information of static tissue while preserving the information of the dynamic blood flow.Secondly,in order to verify the reliability and validity of the proposed algorithm,we performed blood flow phantoms experiments and animal experiments.In the flow phantoms experiments,the blood imitator was injected into a rubber tube(inner diameter 300 ?m,outer diameter 500 ?m)using a syringe pump,and an imitative angiographic image was obtained by OCTCA algorithm,the intensity of pipe was obviously weakened,the stronger reflected signal on the pipe wall was basically eliminated,the noise had been significantly suppressed,and the fluid region can be seen clearly;In the animal experiments,OCTCA algorithm was used to perform microcirculation angiography on nude mouse ear capillaries,the noise of the laser speckle contrast angiography image was suppressed,the image signal-to-noise ratio was improved,the blood flow of a nude mouse ear was imaged to obtain clear capillary blood flow images.Thirdly,we performed in vivo experiments on the human fundus tissue.Due to some involuntary movements of the subjects during the detection process,motion corrections were required.The angiography of the human fundus tissue was performed using OCTCA algorithm to obtain a clear microcirculation angiography image.At the same time,in order to view the blood flow information between different retinal layers,segmented the layers of the retina with the method based on gray features,performed noise reduction preprocessing with filtering and matched OCT image with morphological operations.According to the literature reports,we selected three OCT angiography algorithms with excellent performance for evaluation by use of the same images.The results showed that the algorithm proposed in this paper can obtain clearly blood flow images.The OCTCA provided the best performance in terms of vessel connectivity,image contrast and SNR.Finally,this paper extends the application of optical coherence tomography contrast angiography to the evaluation of mouse lower limb ischemia model.By comparing the results of optical coherence tomography angiography before and after the arterial artery ligation,the results show that optical coherence tomography angiography can observe the angiogenesis of the lower limbs of mice without trauma.In conclusion,the improved speckle contrast optical coherence tomography angiography algorithm eliminated static tissue information while preserved blood flow information.Through flow phantoms and in vivo experiments,the results showed that OCTCA obtained clear fundus microvascular images and could be used for the diagnosis of diseases.