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» Image Creation

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

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Frequency Encoding Gradient

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What does the frequency encoding gradient do?

It moves net magnetisations into the xy-plane.

It reads out the MRI signal.

It causes a range of Larmor frequencies to exist.

INCORRECT. This is the action of a RF excitation pulse (a magnetic field oscillating at the Larmor frequency).

Try again.

INCORRECT. The MRI signal has been oscillating away as soon as the RF excitation pulse was applied. "Reading out" a signal is simply a case of recording the voltage in an appropriately placed receiver coil. With or without the frequency encoding gradient, a signal can be recorded. However, the frequency encoding gradient is applied during the recording of the MRI signal. Why is a this? What does this accomplish?

Try again.

The frequency encoding magnetic field gradient effects a change in the Larmor frequency in one direction (left-right in this case).

CORRECT. The frequency encoding gradient is a static gradient field, just like the slice selection magnetic field gradient. It does the same thing; it causes range of Larmor frequencies to exist in the direction in which it is applied (according to the Larmor equation).

By causing this range of frequencies to exist, we can use the Fourier transform to separate them out after we measure an MRI signal (which is a mix of all signals from a slice).

Further reading on this topic:
Books: Q&A in MRI p79, MRI The Basics p108, MRI From Picture to Proton p122
Online: e-MRI

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