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Echo Planar Imaging, or EPI, Fast Imaging Techniques (MRI) - Essay Example

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By Course of Learning: Date: Article Critique Paper 1. Modules 3 to 5 discuss various methods to correct for magnetic susceptibility distortions, while maintaining a good EPI image. Consider the following clinical setting: You require a T2-weighted image which covers the whole of the brain, where the field of view and matrix size have been fixed at 23 cm x 23 cm, and 128 x 128 respectively…
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Echo Planar Imaging, or EPI, Fast Imaging Techniques (MRI)
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Download file to see previous pages Different results have been found from different body areas which have varying magnetic field strengths thus such body areas end up having varying processional water frequencies. Blood has been observed to have the highest susceptibility as it has iron contents as one of its components. Compositions based on water also have high susceptibility in comparison with air (McRobbie 2007) (a) List the parameters that could be changed to optimize the EPI image in this case, plus how you would change the parameter (i.e. increase or decrease). (2 marks) Slew rate and the receiver bandwidth Each of the listed parameters has a different way of changing it. For example, the case of slew rate increment, an overall reduction in echo spacing as well as reduction in geometric distortion is required for it to be altered. For receiver bandwidth to be increased, an overall reduction in echo spacing, signal to noise ratio and geometric distortion need to be lowered (Van Der Zwaag, at el. 2012, p. 129) In a normal scenario, MR equipments take their time until the trapezoid gradient gets to the flat top. When this time comes, the data points will then be sampled by the MR equipments in the direction of the frequency. This implies that no data acquisition takes place during the rise time. The rise time refers to the time when the gradient is not yet at its minimum or maximum amplitude. However, by employing ramp sampling method, the data points witnessed during the readout gradient switching can be acquired during the rise time. a) Two different features can be used while employing ramp sampling, and they include; 1. Having the geometric distortion minimized; ramp sampling can be used to reduce the flat top while at the same time keeping the Nx constant. This implies that the echo spacing can be decreased consequently minimizing geometric distortion. 2. Increasing resolution; by keeping the flat top at the same level, more Nx data points can be fitted during readout and thus increasing the overall resolution. b). Using Conjugate Synthesis; Conjugate synthesis is a symmetry property which means that only half of the raw data spaces in the whole MR can be acquired to come up with a Mr image that is complete. The most desirable means to achieving high resolution during a single shot EPI experiments is through having the readout duration on the ky to be as long as possible so as to have only the negative and positive kx values. c). Slew rate enhancement d). Echo train length reduction e) Making use of shimming to minimize distortion d). Employing parallel imaging to minimize echo spacing f) . Making use of Multi shot EPI so as to ensure that there are no cases of phase errors building up. g). Through increasing the TE which will in turn increase the transverse magnetization hence larger T2* and T2 (b) Discuss the chain of consequences if you optimize the image by increasing the receiver bandwidth. (4 marks) Receiver bandwidth generally describes how fast an MR signal can be digitized. In cases where the receiver bandwidth is generally higher, there is a corresponding faster digitization of the produced image. This is because the aspect of receiver bandwidth is inversely proportional to the time taken during the imaging process. The receiver bandwi ...Download file to see next pagesRead More
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Echo Planar Imaging, or EPI, Fast Imaging Techniques (MRI)
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Water proton amounts found in the inhomogeneity of the local field will result in off-resonance. The frequency changes of such spins vary in different regions and in different patients. However, its behavior will still resemble that of off-resonance fat. Several methods have been used in the correction of geometric distortions in MRI diffusion weighted normally resulting from static magnetic field inhomogeneties.
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