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Influence of dynamic elements in stable tissue phantoms on laser speckle decorrelation times

Buitink, Martin and Hekman, Rieks (2014) Influence of dynamic elements in stable tissue phantoms on laser speckle decorrelation times.

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Abstract:In this report the decorrelation of a speckle pattern created by the transmission of laser light through a stable tissue phantom with a scattering fluid flowing through a tube inside is discussed. After the introduction, theory is developed and adapted from literature in chapter 2 to describe this process. This is done along three lines. First, in section 2.1, an equation based on one-dimensional diffusion theory to calculate the amount of transmitted light that went through the tube in the sample is derived and discussed. As is shown in section 2.2, with this equation, the normalized autocorrelation function of the speckle pattern can be expressed in terms of the geometrical parameters of the sample and the intermediate scattering function as follows: where is the radius of the tube, is the radius of the illuminated area, is an instrumental factor ranging between 0 and 1 and is the intermediate scattering function. Two existing models are adapted in section 2.3 to find an expression for the intermediate scattering function . One is generally used in Laser Doppler Flowmetry (LDF) and includes both Brownian and translational movement. The other is based on Diffusing Wave Spectroscopy (DWS) and incorporates only Brownian motion. These models, which result in very distinctive autocorrelation functions, are compared and discussed in this same section. As a third and more basic theoretical approach, laminar flow of a fluid in a tube was considered in section 2.4. An equation was derived for the time scale of decorrelation effects caused by translation of the fluid: where is the wavelength of the light used and Q is the discharge. Though this equation gives no predictions about the shape or half times of the autocorrelation function, it gives an upper limit of the time scales on which decorrelation caused by translational motion of the fluid can take place. A final conclusion and discussion of the theoretical chapter 2 is given in section 2.5. Chapter 3 covers the experimental aspects of this research. In section 3.1, the predictions coming from the different models are discussed. Apart from the fact that the shapes of the autocorrelation functions from the LDF and DWS model differ, the time scales also differ a lot: for a 2.2 mm diameter tube, half times are in the range of 10-8 s for the LDF model, and in the range of 10-5 s for the DWS model. Meanwhile, the tube flow considerations give an upper limit to the time scales of decorrelation caused by fluid motion in the range of 10-4 s for the discharges considered in this report (excluding Q = 0). Section 3.2 explains the setup used in the experiments and gives relevant specifications (material parameters, camera specifications etc.). The experimental results are shown and discussed in the following section, 3.3. The experiments were done on two Delrin slabs, one with a 2.2 mm and the
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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