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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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Gradient Echo Pulse Sequence

Do this...

Draw a gradient echo pulse sequence diagram.

Reveal the diagram.

A "gradient echo" pulse sequence.

The gradient echo pulse sequence. A small amount of magnetisation is rotated into the xy plane by a smaller RF pulse (lower flip angle). This means that (i) the signal measured will be smaller, but (ii) less time must be allowed to pass before we can take the next measurement (shorter TR). This reduces the scan time to a reasonable duration.

This basic pulse sequence is a mainstay of MRI, and the majority of advanced pulse sequences are based on a gradient echo. It is repeated for many different levels of phase encoding gradient (e.g. 256 times), until the k-space matrix is full. A 2D Fourier transform is performed on this data set, from which a single MRI image results.

Further reading on this topic:
Books: MRI From Picture to Proton p107, MRI The Basics p244
Online: Basics of MRI, MR-TIP

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