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MRES7004 - Essay Example

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Question One In MRI applications, the main magnetic field is homogeneous so the same Larmor frequency makes all spins resonate. Consequently one precessional frequency for the entire body is available for all spins. The strength of the magnetic field determines the Larmor frequency so to produce discernable data, gradient are applied in each direction…
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Download file to see previous pages The application of the sequence produces a FID which is read to get the gradient required. Multiple frequencies are produced as the read gradient is applied. The variation of frequencies is linearly connected. The total change of frequency experienced depends on the position within the gradient. After the FID is acquired, it is treated with a Fourier transform. This produces a spectrum that displays peaks corresponding to different frequencies. The sum total of all signal intensity values at one single place of observation become individual peaks. A one dimensional quantity is produced by the application of the read gradient as it is independent of time. (Weishaupt et al., 2006) Phase Direction Encoding A phase gradient is applied after applying a read gradient and slice selection. This is otherwise known as phase encoding and tends to increase the nuclei’s frequency such that it precesses at different angles that all match up with the Larmor frequency. The increase of frequency due to the application of a phase gradient directly impacts the total phase change displayed by nuclei. However there is a need to discern different nuclei which can be done by the application of Fourier transforms. (Westbrook et al., 2005) Question Two Using the Fourier transforms helps to convert the available data from the time domain to the frequency domain. This can then be utilised to form two dimensional or three dimensional images based on available data. Data is spatially encoded before becoming a part of the k space and so its position within the k space can be determined accordingly. Application of the first Fourier transforms aids in interpreting the data values that were encoded in the read direction. This is useful in identifying the frequency (alternatively signal intensity) within the plane selected for the application of the read gradient. This makes it simple to differentiate the positions within the k space’s horizontal trajectory. The data obtained in this way has its units changed from m-1 to m. Consequently only a one dimensional image is formed. (Woodward, 2001) Application of the second Fourier transform helps to differentiate various frequencies that were encoded along the phase direction after the application of a phase gradient. This transform separates all the values and lists them accordingly. The vertical k space trajectories are dealt with this transformation. The units again change from m-1 to m and the resulting image becomes two dimensional. (IMAIOS, 2009) The total k space contains data encoded from two directions that are the read and the phase directions. The read direction’s data is displayed as horizontal trajectories in the k space while the phase direction’s data is displayed as vertical trajectories in the k space. Fourier transforms aid in creating a complete two dimensional image of the concerned nuclear spin densities in relation to the slice positions. (Hashemi et al., 2004) Question Three Various experimental factors affect transverse spin coherence as well as the k space. These factors and their effects are listed below. Radio Frequency Pulse: A radio frequency pulse at 90o is utilised along with the chief magnetic field to produce magnetism such that the Z direction vector reorients itself into the X plane the Y plane. The magnetism produced is subsequently de-phased both in the X plane and the Y plane. This requires one more re-phasing at 180o. Read Gradient: Read ...Download file to see next pagesRead More
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These questions have been responded to in a certain chronological order as follow. Frequencies encoding gradients, as well as phase encoding gradients areboth essential part of the spatially encoded data. It is, thus, true that using the induced differences in the frequency and the phase of precession, makes the data amenable for analysis by the Fourier transform.
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