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Blood is a body fluid that circulates through the body through a network of vessels to support the vital functions (e.g., nutrition, oxygenation, immunity) of all organs. Blood is a two-phase suspension: (1) the liquid plasma, an aqueous solution of organic molecules, proteins, and salt, and (2) solid corpuscles such as normal and abnormal cells, their microparticles, and aggregates. Normal blood consists of erythrocytes [red blood cells (RBCs)], leukocytes [white blood cells (WBCs)], and platelets. Blood may also carry a variety of abnormal cells (e.g., circulating tumor cells [CTCs], bacteria, leukemic cells), and viruses and, thus, disseminate a disease from one organ to another. Owing to the overwhelming majority of RBCs, their red color, and their ability to absorb light at specific wavelengths, blood vessels and blood flow can be visualized and detected without labeling in a whole body. This advantage of blood has been extensively used to develop in vivo assays for diagnosis of blood vessels and blood flow using different modifications of optical (e.g., transmission and fluorescent microscopies; multiphoton microscopy) and laser-based (e.g., Doppler, laser speckle, and photoacoustic methods) techniques. Laser speckle and Doppler effect based methods addressed to blood flow dynamics are based on measurements of the correlation and spectral characteristics of fluctuations of scattered light at the diffraction region of the focused Gaussian beam and their application to measurements of blood or lymph flow in a single vessel. On one hand, this scattering phenomenon is treated as inhomogeneous dynamic biospeckles or, with some conditions, speckled biospeckles by means of the interpretation of speckle fluctuations. On the other hand, the same phenomenon is discussed with relation to the spectral broadening of Doppler shifts by means of the interpretation of the Doppler effect. This means that there are different possible approaches that are used to interpret the above-mentioned scattering phenomenon. Therefore, to consider the relation of the Doppler effect and the speckle fluctuations is meaningful for understanding the phenomenon from different points of view. In this section, we briefly discuss the interrelation between the Doppler and speckle techniques for general cases of measurements of blood flows in a single vessel.
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