In this respect, Nuclei that are accelerated by the effect of the gradient tend to move faster than their precessional path as opposed to the case in which the phase encoding gradient is switched on. On the contrary, for nuclei that are slowed down, the reverse is true. It is the degree of the steepness in the phase encoding gradient slopethat detects the amount of phase shift between the two different points throughout the axis of the gradient. In essence, it implies that a steep gradient creates a large shift in the phase between these two points along the gradient axis. However, the shallow gradient create smaller shift between these two points (Kaut-Roth et al, 2005). Response to Question 2: This part deals with the Nyquist’s theorem. This is a theorem that is used for determining accurately the speed of digitizing frequency of the sine curve. For this to apply, the process of sampling the signal should always be equal or larger than double the signal frequency. When such happens, more points acquired results in better defined frequency. Dwell time is experessed mathematically as: Dwell time = 1/ (2?sweep width) (MRES7004, 2012). Dwell time = 1/(2?20,000) = 2.5 microseconds While acquisition time is, on the other hand, expressed mathematically as: Acquisition time = dwell time ?no. of data points Acquisition time = 0.64 miliseconds. Response to Question 3 This question is based on analyzing diagram of five lines have five processes. The 90 degrees RF pulse (excitation pulse) causes the longitudinal magnetization vector to move along the Z axis into the x-y plane (transverse plane). Due to the magnetic field inhomogeneity...
Frequency encoding gradient helps locate signals along the long axis of the part that is imaged.In the event that the slice is selected, the signal that originates from the slice should always be positioned on both axis of the image.In respect to this, the signal position along the axis of the gradient can be determined from its frequency. This is achieved through establishing a difference in the frequency either linearly fashion or through a shift in the signal along the gradient axis, in which case the frequency encoding gradient is turning on.In the course of acquiring the signal it is often the frequency encoding gradient that is turned on. This is explains why it is often referred to asa readout gradient. As has been established through research, the degree of the steepness of the frequency encoding gradient slope often determines the field of view of the anatomy that undergoes scanning and it is known to be the last period of spatially encoding the signal.
For the case of the Phase encoding gradient in the diagram, the gradient is to be applied immediately prior to the application of the 180 degrees RF pulse. Whenever it is applied, the net magnetization phase (lose coherence) leading to signal reduction. The phase gradient is altered in every repetition time (TR), following the use of rephrasing gradients for filling different lines in the K space with data. Using the high phase encoding gradient causes a decrease in the signal intensity.