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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 Selection

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How is slice selection achieved?

An RF pulse is directed at the section of interest in the patient.

The imaging slice location is moved to the centre of the magnet bore; signal is returned from the centre of the magnet.

A magnetic field gradient is temporarily applied.

INCORRECT. RF pulses used for slice selection are not directed to particular parts of patient. Any RF pulse used for slice selection causes an oscillating magnetic field to occur throughout the scanner bore. We can limit magnetisations will respond (resonate) to within a section of the patient, however. This requires that we vary the Larmor frequency throughout the patient. How might we do this?

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INCORRECT. If there is time, the imaging slice is moved to the isocentre where the magnetic field is most homogenous. And by employing slice selection, a signal will be emitted from only that section of the patient in the scanner. But how does slice selection work? If an RF pulse is applied on its own, signal will be returned from the whole patient.

Try again.

Slice selection method

CORRECT. A gradient magnetic field is applied at the same time as the RF pulse.

If we change the Larmor frequency in a linear fashion across the patient from head to foot, then a range of Larmor frequencies will exist in that direction (the z-axis). This means we can "pick out" the section which we want to excite by choosing the right frequency range of RF excitation pulse. The section which contains Larmor frequencies which match the frequencies of the oscillating magnetic field will respond. An MRI signal will be generated only from that section of the patient. This is slice selection.

Usually the slice selection gradient is applied in the z-axis—the head-foot direction in the scanner. But because a gradient magnetic field may be applied in any orientation, slices may be acquired at literally any angle or orientation in the patient. This is one strength of MRI.

Slice selection pulse sequence

Note that the thickness of the slice can be changed by varying the steepness of the magnetic field gradient, or by changing the transmitted RF pulse bandwidth. Note also that the RF pulse and the magnetic field gradient have to applied together. This process may be depicted in a pulse sequence timing diagram.

The shape of the RF excitation pulse in time is not a square (on/off) shape. This is because to excite a discrete range of frequencies (a slice) a sinc shape pulse is used which can be seen by calculating sin(x)/x.

Further reading on this topic:
Books: Q&A in MRI p75, MRI The Basics p97, MRI From Picture to Proton p112
Online: Basics of MRI

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