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QUESTIONS
» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

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Excitation

Answer this...

What is excitation?

The induction of a signal in appropriately placed signal detection equipment.

Drinking too much coffee so that you go all shaky.

The process of moving the net magnetisation away from the z-axis.

INCORRECT. Although the process of excitation involves magnetic induction, excitation does not refer to the induction of a voltage in detection equipment.

Try again.

INCORRECT. Hmmm. That's not excitation, so much as overstimulation.

Cut down your coffee intake, and try again.

CORRECT. Excitation is simply another name for moving the net magnetisation vector (or part of it) of spin isochromats into the xy-plane. This happens by magnetic induction; the RF magnetic field induces a rotation of the net magnetisation of the sample.

The vector component Mxy precesses (and decays), creating an oscillating magnetic field and inducing a signal in our detection equipment.

You may read "excitation of the spin system"; this simply means an RF pulse has been applied. To excite only a single section—or slice—of the patient, we use another magnetic field: A gradient magnetic field.

Further reading on this topic:
Books: MRI From Picture to Proton p38, MRI The Basics p32
Online: Basics of MRI

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