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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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Slice Selection

Answer this...

When a slice is selected, what happens in the rest of the patient?

Nothing.

The Larmor frequencies change.

Rotation of magnetisation by the RF excitation pulse.

INCORRECT. Almost correct. Ostensibly nothing happens, because we don't get a signal back from the rest of the patient.

Try again.

CORRECT. During slice selection, the slice selection gradient acts on the whole length of the scanner bore. This means that the Larmor frequencies of hydrogen protons vary from one end bore to the other. This allows us to choose a particular section to excite (rotate magnetisations into the xy-plane) by using an oscillating magnetic fields of certain frequencies (an RF pulse with a certain bandwidth).

INCORRECT. Rotation of magnetisation by the RF excitation pulse is exactly what doesn't happen in the rest of the patient.

Try again.

Further reading on this topic:
Books: MRI From Picture to Proton p116, Q&A in MRI p75, MRI The Basics p100
Online: Medcyclopaedia, Basics of MRI

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