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Echo-planar Imaging in Magnetic Resonance Imaging - Essay Example

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EPI in MRI Introduction MRI, or magnetic resonance imaging, fabricates information for a picture from a sequence of separate samples of signals. Echo planar imaging, or EPI, is a method that can form an absolute image from a single data sample. This provides significant advantages, as far as the speed factor is concerned (Cohen, 2000, p.2)…
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Echo-planar Imaging in Magnetic Resonance Imaging
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Download file to see previous pages However, EPI can be very intolerant and the system needs careful selection of parameters. Echo-planar imaging is the fastest and most supple means of MR imaging nowadays. It offers substantial autonomy in the assortment of the parameters needed for contrast and resolution. However, according to the process, the system of formation of the images works close to its confines of performance depending on amplitude of the gradients and the number of times it rises, the stability in the structure and general stature of the noise formed and, thus, proves to be a difficult method. Even so, the advantages of EPI in “functional neuroimaging” have increased its demand and provide technological development (Cohen, 2000, p.15). The present study focuses on the concept of EPI in MRI and discusses the method, its working, the issues of image ghosting, its disadvantages, its sensitivity, as well as clinical benefits. BLIP EPI Method Echo-planar imaging (EPI) is skilled to considerably cut down the times of magnetic resonance (MR) imaging. It allows getting hold of representations within a timeframe of only 20–100 msec. This particular resolution of time enables successful elimination of motion-related relics. Consequently, it becomes possible to achieve imaging of rapidly changing physiologic processes. In order to understand the basic principles and working of the method, the understanding of k space theory is necessary. K space is a realistic medium of data where the MR imaging is in a digitized form and represents the picture before “Fourier transform analysis”. All points in k space contain data from all locations within an MR image. The Fourier transform of k space is the image” (Poustchi-Amin et al, 2001, pp.767-779). While studying the working of echo-planar imaging, it is helpful to put the usual spin-echo (SE) imaging side by side. In pulse progression of a SE, a single line of the data of representation, which is actually a single line in k space or a single stride of encoding a phase, is composed in every time of repetition (TR) period. Then the series of the pulse is continued for numerous times of TR in anticipation of all the steps of encoding the phase, collection and filling the k space. As a result, the time of imaging becomes equal to the creation of the TR, as well as the number of phase-encoding steps (Poustchi-Amin et al, 2001, pp.767-779). As compared to Fast Spin Echo (FSE), the gradient refocused echo in EPI adds a single line in the area of k-space. The direction in which line is read is altered by the positive and negative read gradients. A shift in k-space occurs through the presence of the phase blip that occurs between the echoes. This is the method known as blip EPI. The spins are excited once in the process of EPI and the pulses between the echoes do not involve any 180 degrees RF. If there is a spin echo EPI sequence, a large positive phase may be used that encodes the gradient if it does not reach the 180° degree pulse. Gradients are used by EPI, having both a negative and positive polarity, such that both odd and even echoes may be produced. The contrast in EPI can be restricted by varying the preliminary preparation of the echo from a spin echo to a gradient echo, or by the use of inversion recovery (Module 1, pp.7-16). In the process of echo-planar imaging, several lines of information related to the picture are achieved following a ...Download file to see next pagesRead More
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