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EPI7005 - Essay Example

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EPI7005_2 By Course of Learning: Date: EPI7005_2 Magnetic Susceptibility Tissues adjacent to each other yet having different magnetic susceptibilities result in local field disruption. Examples of such tissues include the bone-tissue interfaces that are located 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 Several methods have been used in the correction of geometric distortions in MRI diffusion weighted normally resulting from static magnetic field inhomogeneities. Such inhomogeneties may be as a result of 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 afield 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 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) 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 also 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 taken by the frequency gradient in sampling the signal, the less the distortion that 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, the less the blurring of the image. In EPI, minimizing the echo spacing also results in a reduction in the blurring as well as a reduction in geometric distortion. Figure 1. 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 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 spacing include: Ramp sampling Increasing the receiver bandwidth through achieving a stronger gradient Resolution reduction hence a reduction in the echoes train number Phase encoding collection in multiple sessions (segmented or multiple shot EPI) By using increased amplitudes or ...Download file to see next pagesRead More
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