Diffusion weighted (DW) Magnetic Resonance Imaging MRI Essay

Diffusion weighted (DW) Magnetic Resonance Imaging MRI - Essay Example The exploration of diffusion in MRI includes the way MRI is sensitized to diffusion; the effects of spins, both stationary and moving spins; how diffusion-weighted images are affected by physiological motion; the how of the achievement of the various diffusion weightings; and information on diffusion and how those are measured (Hagmann et al. 2006; Mori and Barker 1999, pp. 102-106; Tonarelli 2012; Parker 2004, pp. S176-S178; Everdingen et al. 1998; Barker 1999; Maas 2005; Le Bihan et al. 2006; Yablonskiy et al. 2003; Koh and Collins 2007; Le Bihan 2011; Basser and Jones 2002; Battal et al. 2012; De Foer 2010; Luypaert et al. 2001; Williams et al. 1992; Topgaard 2006) II. Background on Diffusion Diffusion on the molecular level is said to be the result of natural Brownian movement, where molecules randomly move through the diffusion medium because of the agitation caused by thermal characteristics of the medium. In all the displacement of the molecules comes up to zero by mean figure s, but over time, there are positive probabilities associated with the non-zero movement of a molecule, so that over time, a molecule is said to probably have moved from an initial position at an earlier time. Here the time elapsed corresponds to a correlation with the distance moved, where different fluids acting as diffusion mediums determine the distance as characterized by the diffusion constant for that liquid type. There is a difference between the freely diffusing movement of water molecules, meanwhile, to the diffusion of liquids in the tissues of human beings, so that in human tissues one talks of an ADC, or an apparent diffusion coefficient, to be differentiated from the free diffusion coefficients of liquids outside of human bodies, such as those used to characterize water in containers at certain temperatures. On the other hand, for human tissues, various considerations further come into play, such as differences in the mobility of different fluids in different parts of the body and in different parts of a particular organ, such as the human brain. Boundary conditions also differ for liquids found in different body parts. All these affect the coefficient of diffusion in various ways, with the general observation that the ADC is generally smaller in comparison to the free diffusion coefficients of liquids like water outside of the human body (Luypaert et al. 2001; Roberts and Rowley 2003). Going into diffusion types, meanwhile, there are two, one being isotropic diffusion and the other being anisotropic diffusion. In isotropic diffusion, the rate of diffusion is the same in all directions, and so the resulting diffusion distribution is spherical. In anisotropic diffusion, the diffusion rate depends on where the diffusion is oriented, and there is uneven diffusion in different directions. The distance of the diffusion is orientation-dependent, in other words, and the diffusion distribution is characterized by an ellipsoid (Module 1 2013). III. MRI an d Diffusion A. How MRI is Sensitized to Diffusion In a hypothetical case, the typical distribution of displacement of water molecules in such a container is said to follow a bell curve, with majority of the water molecules able to travel only for short distances from their initial location, whereas a few of the water molecules are able to be displaced at further distances from average. For a given initial temperature of the water, moreover,