Magnetic Resonance Imaging

Precession

If magnetization vector placed in the XY plane...

it precesses about B_o at n_o.

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T₂Processes

Magnetization placed in the XY plane dephases.

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M_XY = M_XYo e^-t/T2

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Spin-spin relaxation time (T₂)

A time constant describing the return to equilibrium of M_XY.

The time to reduce the transverse magnetization by a factor of e.

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T₂ < = T₁

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Any transverse magnetization behaves the same way.

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Contributions to the decay of transverse magnetization

1. molecular interactions (said to lead to a pure pure T₂ molecular effect)
   
2. variations in B_o (said to lead to an inhomogeneous T₂ effect
   1/T₂* = 1/T₂ + 1/T_2inhomo.

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Rotating Frame of Reference

An X'Y'Z coordinate system rotating about the Z axis at n_o

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Motions in the Rotating Frame.

Z motion

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M_XY at n_o

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M_XY > n_o

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M_XY < n_o

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Many n

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