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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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Slice Location

Answer this...

How is the slice location varied?

The slice to be imaged is moved to the centre of the scanner bore—the isocentre.

The carrier frequency of the RF excitation pulse may be changed.

CORRECT. Actually, it could be either answer. If there's time between imaging different slices, we can simply move the patient-table so that the section of interest within the patient is at the isocentre. This is preferable because placing the section of interest in the part of the main magnetic field which is most homogenous will give us better images.

Slice selection method

CORRECT. Actually, it could be either answer. The carrier frequency of the transmitted RF pulse determines which spins along the patient will resonate (because they have a matching Larmor frequency). If multiple slices are to be acquired in quick sequence, the carrier frequency can be set to determine the location of the imaging slice in the patient.

Further reading on this topic:
Books: MRI From Picture to Proton p115

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