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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 Gradient Timing

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Should the slice-selection gradient be applied before, during or after the RF excitation pulse?

Before the RF excitation.

During the RF excitation.

After the RF excitation.

INCORRECT. If the RF excitation pulse is applied without a simultaneous magnetic field gradient, spin isochromats from the whole patient will be put into a state of resonance. A slice will not be selected.

Try again.


Slice selection

CORRECT. The slice selection gradient must be switched on when the RF pulse is applied. Additionally, no orthogonal gradients can be applied at the this time; slice slection must be completed before subsequent spatial encoding magnetic field gradients can be applied.

Applying a magnetic field gradient causes a variation of the Larmor frequency along the patient because the magnetic field varies along the patient. Without the gradient, all of the spin isochromats in the patient will respond to the same Lamor frequency. With the gradient, a small range of Larmor frequencies may be chosen (the RF pulse bandwidth) which corresponds to a particular section (a slice) of the patient.

After slice selection has occurred, the frequency of precession (and the frequency of the signal, therefore) returns to the Larmor frequency which corresponds with the main magnetic field. Only spin isochromats which were excited—in the slice—are emitting the signal (or "echo").

INCORRECT. If the RF excitation pulse is applied without a simultaneous magnetic field gradient, spin isochromats from the whole patient will be put into a state of resonance. A slice will not be selected..

Try again.

Further reading on this topic:
Books: Q&A in MRI p75, MRI The Basics p97, MRI From Picture to Proton p112
Online: Basics of MRI

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