Magnetic Resonance Imaging

Phase Encoding Gradient

A magnetic field gradient used to encode the spatial location of a spins.

Assume there are 3 locations on the x-axis with spin.
Magnetization vectors from the 3 locations after a 90^o pulse.

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When a gradient is turned on: n = g ( B_o + x G_x) = n_o + g x G_x

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When the gradient is turned off.

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FT Tomographic Imaging

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Timing Diagram for the Sequence

Parts:
    90^o slice selective pulse
    slice selection gradient
    phase encoding gradient
    frequency encoding gradient
    signal
    TR = repetition time

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The sequence is repeated and the magnitude of the phase encoding gradient is changed.

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Possible combinations of the slice, phase, and frequency encoding gradients for 3 orthogonal imaging planes.

	Gradient
Slice Plane	Slice	Phase	Frequency
XY	Z	X or Y	Y or X
XZ	Y	X or Z	Z or X
YZ	X	Y or Z	Z or Y

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Spin vectors for the sequence

The imaged object.

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The location of the plane.

Z = Dn / g G_s

where:
    Dn = frequency offset from n_o ( i.e. n - n_o )
    G_s = magnitude of slice selection gradient
    g = gyromagnetic ratio

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A 3x3 subset of net magnetization vectors in this plane.

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Vectors precess.

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Application of the phase encoding gradient.

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Phase encoding gradient turned off.

The phase of a vector was set by the phase encoding gradient pulse.

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Application of the frequency encoding gradient pulse.

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Copyright © 2000 J.P. Hornak.
All Rights Reserved.