EPI7005 Essay Example | Topics and Well Written Essays - 4000 words

EPI7005 - Essay Example slightly(prenominal) methods puddle been used in the correction of geometric distortions in MRI diffusion weighted usually 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 commonly 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 strait. 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 embarrass the use of a fi led map. However, such methods of applying field maps are not reliable and accurate thusly do not result in reliable EPI alignment with structural images that correspond to it. The T2-weighted structural image onslaught 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 embroil A. Reducing the cadence of phone train length The longer the length of the echo train, the more time is required by the swag to gather chassis 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 try the signal, the less the distortion that will be witnessed as shown in figure 1. The head for sampling is normally during the flat readout gradient portion unless ther e is ramp sampling. In FSE, the less the second sight or the echo spacing, the less the blurring of the image. In EPI, minimizing the echo spacing also results in a lessening 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 period 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 in force(p) 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 alike the sinuses, the geometric distortion can appear with a larger shots number. As the shots number increases beyond a given point, the distortion r eduction 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 bigeminal sessions (segmented or multiple shot EPI) By using increased amplitudes or