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EPI7005_2 - Essay Example

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EPI7005_2 By Course of Learning: Date: Magnetic Susceptibility Tissues adjacent to each other yet having different magnetic susceptibilities lead to the disruption of the local field. Examples of these tissues include the interfaces involving bones and tissues that are found in the sinus and in the lungs like the maxillary, mastoid, and the ethmoid sinuses…
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Download file to see previous pages Such inhomogeneties may be a result in some imperfection within the magnet or magnetic susceptibility spatial variation of the object that is being imaged (Christ, 2000, p. 78). The spatial variation is normally referred to as susceptibility artifacts. EPI applied in most of protocols used in diffusion weighted acquisition uses a homogenous static field. Such a field does not normally hold for MRI head. The distortion that comes out of this is significant enough to even exceed ten millimeters. Such artifacts come in the way of accurate diffusion images alignment with structural MRI and are considered obstacles to the head MRI joint analysis of structure and connectivity. Some of the methods used for correction of magnetic susceptibility distortions include the use of a filed map. However, such methods of applying field maps are not reliable and accurate; hence, they do not result in reliable EPI alignment with structural images that correspond to it. The T2-weighted structural image approach that relies on EPI baseline image and still gives account of the susceptibility artifacts is the recommended approach (Edelman and Schmitt, 1994, p. 610). How accurate the reconstruction of the image is depends on the spin’s resonance frequency (rf) which is formed normally through a spatial linear gradient superposition and an external filed that is homogenous and strong. The above conditions have to meet failure to which the anatomical structures relative positions will end up being misrepresented in the MRI image reconstruction. A good example of this is a case in which a spin is presented to a magnetic field that is not the same as the expected value resulting from the magnetic susceptibility. In this case, the geometric distortions process will be witnessed from the MRI image (Reinsberg, Doran, Charles-Edwards, and Leach, 2005). When the spin is increased, it leads to the production of MRI signals whose intensity is relatively low while areas having high magnetic susceptibility experiencing total signal loss (Zhou and Gullapalli, 2006). Question 1A Several parameters have to be reduced in order to optimize EPI image, and they include: A. Reducing the time of echo train length: the longer the length of the echo train, the more time is required by the spin to gather phase error and the greater the distortion witnessed. One of the ways of minimizing the total time of train length is by minimizing the echo spacing. The less the time is taken by the frequency gradient in sampling the signal, the less the distortion will be witnessed as shown in figure 1. The period for sampling is normally during the flat readout gradient portion unless there is ramp sampling. In FSE, the less the ESP or the echo spacing is, the less the blurring of the image gets. In EPI, minimizing the echo spacing also results in a reduction in the blurring and geometric distortion. Figure 2. The sampling time and the read encoding gradient during normal gradient performance (McMahon, 2012) As the segmentation degree increases, the echo number acquired per rf excitation decreases. This in turn minimizes the geometric distortion as the phase error time of accumulation is reduced. Figure 3 below demonstrates the reduction in ...Download file to see next pagesRead More
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...the geometric distortion as the phase error time of accumulation is reduced. Figure 2 below demonstrates a reduction in effective inter-echo spacing (McMahon, 2012). Gradients are from two varying excitations, interleaved and offset with respect to each other. Figure 3 shows geometric distortion reduction as the shot number increases. In areas with low inherent homogeneity like the sinuses, the geometric distortion can appear with a larger shots number. As the shots number increases beyond a given point, the distortion reduction becomes unnoticeable. Figure 5; demonstration of the reduction of geometric distortion as the number of shots is increased (McMahon, 2012) B. Reducing echo spacing; methods used to shorten echo...
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