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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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Fourier Transform

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What does the Fourier transform do to a signal waveform?

It amplifies the signal.

It encodes the signal into frequencies.

It separates out the frequencies which contribute to the signal.

INCORRECT. It doesn't.

Try again.

INCORRECT. Encoding of the signals from different parts of an image slice into different frequencies is achieved by turning on a magnetic field gradient during signal measurement (the frequency encoding gradient). Once we have measured the signal (which comes as a mix of all the signals from the whole slice), we need a method of separating out the original signals according to their frequencies.

Try again.

The action of the Fourier transform

CORRECT. The Fourier transform can separate the frequencies which contribute to the signal which is emitted from the image slice. Crucially, it also tells us the amplitude of those waves, which will correspond to signal intensity levels in an image.

Further reading on this topic:
Books: Q&A in MRI p72, MRI The Basics p90, MRI From Picture to Proton p346
Online: Basics of MRI, e-MRI 1, 2

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