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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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Frequency Encoding Timing

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When is the frequency encoding gradient applied?

During the recording of the MRI signal.

Before recording the MRI signal.

During the phase encoding gradient.


The frequency encoding gradient (Gf) is applied during signal measurement

CORRECT. The frequency encoding gradient causes the precession of net magnetisations within the slice to be position dependent in that direction, whilst the signal is being recorded.

INCORRECT. If the frequency encoding gradient is not applied at the same time as measuring the MRI signal, the signals from the different columns in the imaging slice will all have the same frequency. We cannot, therefore, use the Fourier transform to separate them out.

Try again.

INCORRECT. If the frequency encoding gradient is not applied at the same time as measuring the MRI signal, the signals from the different columns in the imaging slice will all have the same frequency. We cannot, therefore, use the Fourier transform to separate them out.

Since phase encoding must be completed before the signal can be recorded, the phase encoding and frequency encoding gradients cannot coincide.

Try again.

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

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