Digital holographic microscopy for red blood cell imaging

Digital holographic microscopy (DHM) can obtain biological parameters and morphological information of cells by reconstructing holograms, which is different from traditional optical microscopy. The DHM is a three-dimensional imaging technology which is effective, non-contact and non-destructive. With the developments of the image sensor and the computing technology, it has made significant progress in the field of living cells detection, especially for red blood cell. Compared with the technologies which are widely used in the field of cell imaging such as con-focal laser scanning microscopy, scanning near-field optical microscopy and optical coherence tomography, the DHM has the advantages including wide FOV and high-resolution to achieve higher imaging and quality. This paper introduces the principle of recording and reconstruction of digital holography, and then analyzes the performance of three reconstruction algorithms using the Fresnel method, the convolution method and the angular spectrum method. The Fresnel method is suitable for the sample size larger than the image sensor. Both the convolution method and the angular spectrum method have an optimal reconstruction distance. When the reconstruction distance is different from the optimal distance, the resolution of the reconstructed image will decrease, and the angular spectrum method is better than the convolution method in overall performance. The DHM system for RBC measurements mainly adopts the convolution algorithm or the angular spectrum algorithm to implement numerical reconstruction. The systems of the in-line DHM, the offaxis DHM and the optical tweezers combining with off-axis DHM are introduced. These techniques use algorithms including Rayleigh-Sommerfeld back-propagation, the sharpness quantification, the watershed segmentation, the numerical refocusing and the thermal fluctuation to determine the focal plane position and obtain the best reconstruction distance of the RBCs, and further detect the shape change of the RBCs and extract the information of high-resolution blood vessel shape and blood flow velocity. These techniques can even achieve the dynamic tracking and measure three-dimensional volume of RBCs in real-time which is helpful for pathological studies such as diabetes, cardiovascular disease and Parkinson's disease. With its unique non-contact and non-destructive characteristics, the DHM realizes real-time and quantitative detection that is difficult to achieve with traditional three-dimensional microscopic imaging technologies.