ISSN 1028-3358, Doklady Physics, 2013, Vol. 58, No. 8, pp. 323–326. © Pleiades Publishing, Ltd., 2013. Original Russian Text © I.V. Meglinski, V.V. Kal’chenko, Yu.L. Kuznetsov, B.I. Kuznik, V.V. Tuchin, 2013, published in Doklady Akademii Nauk, 2013, Vol. 451, No. 4, pp. 393–396. 323 The intense growth in the number of cardiovascular deceases, the increase in mortality from the deceases of the circulatory system [1], and the associated con- siderable economic damage caused by the loss of abil- ity to work and patients becoming invalids predeter- mine the development of new diagnostic methods and their incorporation into daily clinical practice. Laser Doppler flowmetry (LDF) has become the most wide- spread among other non-invasive optical methods of blood flow diagnostics [2]. The LDF principle is based on extraction of the component proportional to the motion velocity of the moving blood cells (the Dop- pler effect) from the detected signal caused by scatter- ing probing laser radiation in biological tissue. Due to the relatively low velocity of the red blood cells, the Doppler shift (DS) is rather small compared with the basic frequency of the probing optical radiation; therefore, lasers are extensively used in such measur- ing systems. Despite strong light scattering by biologi- cal tissues, the DS from the blood flow can be mea- sured with a rather high resolving ability against the background of the LDF signal in the absence of blood flow. It is assumed to be evident that the intensity fluc- tuations of the background LDF signal are caused exclusively by the Brownian motion of the macromol- ecules and the blood corpuscles [3, 4]. By analogy with the definition of the temperature, below which the normal vital functions of the organism are stopped, which is used in general biology, the above-mentioned low-level ischemic signals were called “the biological zero” [5]. Along with the LDF, researchers apply alternative methods using scattered laser radiation to detect blood flow, for example, diffusion-wave spectroscopy (DWS) [6]. The LDF and DWS methods are related to each other by the Fourier transform and in fact carry iden- tical information on the dynamics of the blood flow. The principle distinction is that low order scattering is used in the LDF to analyze the scattering radiation, while multiple scattering is used in the DWS, as a result of which, the method is sensitive to the spatial shifts of the light-scattering particles such as erythrocytes down to parts of nanometers [7, 8]. In addition, since the scattered laser radiation is detected in the DWS in the counting mode of single photons, this method is preferable when working with biological tissues by vir- tue of the power limitations (no larger than 30 mW), which are imposed on the laser radiation sources used in medical diagnostics. The magnitude measured in the DWS is the tempo- ral auto-correlation function (ACF) of intensity fluc- tuations g 2 (τ) [7–9]. For the Gaussian statistics, the ACF of the intensity fluctuations is associated with the first-order ACF (by the field) by the Siegert formula [7, 8]: , (1) where is the delay time; A = 〈i〉 2 is the square of the average value of the photocurrent or the base ACF line; and β is the dimensionless parameter (0 < β < 1), the so-called aperture function, which depends on the coherent properties of the probing laser radiation and τ= +β τ 2 2 1 () [1 ()] g A g τ Towards the Nature of Biological Zero in the Dynamic Light Scattering Diagnostic Modalities I. V. Meglinski a ,b , V. V. Kal’chenko c , Yu. L. Kuznetsov c , B. I. Kuznik d , and V. V. Tuchin b, e, f Presented by Academician I.A. Shcherbakov February 19, 2013 Received February 19, 2013 DOI: 10.1134/S102833581308003X a Departmet of Physics, University of Otago, PO Box 56, Dunedin, 9054 New Zealand b Scientific-Educational Institute of Optics and Biophotonics, Saratov State University, Saratov, 410012 Russia c The Weizmann Institute of Science, 234 Herzl Street, Rehovot, 76100 Israel d Chita State Medical Academy, ul. Gor’kogo 39A, Chita, 672090 Russia e Institute of Problems of Fine Mechanics and Control, Russian Academy of Sciences, ul. Rabochaya 24, Saratov, 410028 Russia f University of Oulu, PO Box 8000, Oulu, FI-90014 Finland e-mail: igor@physics.otago.ac.nz PHYSICS