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What is the phase encoding gradient?
The phase encoding gradient (GPE) intervenes for a limited time period. While it is applied, it modifies the spin resonance frequencies, inducing dephasing, which persists after the gradient is interrupted. This results in all the protons precessing in the same frequency but in different phases.
When is phase encoding gradient turned on?
Once the RF pulse is complete the slice selection gradient is turned off and a phase encoding gradient is turned on.
What is frequency encoding gradient MRI?
Frequency-encoding of spatial position is accomplished through the use of supplemental magnetic field gradients. Gradients mildly distort the main magnetic field in a predictable pattern, causing the resonance frequency to vary as a function of position.
What does the frequency encoding gradient do?
The final step in spatial encoding consists in applying a frequency encoding gradient, when the signal is received, in the last direction (horizontal in our example). This modifies the Larmor frequencies in the horizontal direction throughout the time it is applied.
How do gradient coils work in MRI?
Gradients are simply loops of wire or thin conductive sheets on a cylindrical shell that lies just inside the bore of an MRI Scanner. When an electrical current passes through these coils, the result is a secondary magnetic field. This gradient field distorts the main magnetic field in a slight but predictable pattern.
When is a dephasing frequency encoding gradient applied?
A dephasing frequency encoding gradient is applied at the same time as the phase encoding gradient so as to cause the spins to be in phase at the center of the acquisition period. This gradient is negative in sign from that of the frequency encoding gradient turned on during the acquisition of the signal.
When does phase encoding start in MRI shark?
The phase encoding gradient is activated after our slice selection gradient. A pulse sequence diagram will continue after this point until that selected slice is filled with signal. Our pulse sequence diagram needs to be run as many times as we need lines of K space to be filled.
When to apply phase encoding gradient in spin echo?
Spin-Echo Imaging. As in the previous imaging sequences, the phase encoding gradient is varied in 128 or 256 steps between G m and -G m. The phase encoding gradient could be applied after the 180 o pulse, however if we want to minimize the TE period the pulse is applied between the 90 o and 180 o RF pulses.
How is the phase of a gradient pulse determined?
The phase being determined by the duration and magnitude of the phase encoding gradient pulse. Once the phase encoding gradient pulse is turned off a frequency encoding gradient pulse is turned on. In this example the frequency encoding gradient is in the -Y direction.