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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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RF pulses

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What is an RF pulse?

A magnetic field oscillating at radio frequency.

A magnetic field gradient.

A burst of radio waves.



Top: thermal equilibrium; bottom: immediately after a 90° RF pulse (laboratory frame of reference shown).

CORRECT. An RF pulse is a magnetic field, the direction of which is oscillating at the Larmor frequency.

Recall that when the patient is moved into the MRI scanner, the first thing which happens is the precession of spins around the direction of the external magnetic field (the z-axis). Similarly, if another magnetic field is temporarily switched on in a different direction (in the direction of the x- or y-axis, say) precession will occur around the direction of that magnetic field also. If the second applied magnetic field is static, the resultant movement of the net magnetisation of a spin isochromat will be a complicated motion due to precession from the two static fields. However, if the second magnetic field which is temporarily applied is oscillating with the frequency of precession of the precessing spins, a simple rotation of the net magnetisation vector results. (Rotation of magnetisation into the xy-plane is a "90° pulse". The dephasing of the components of magnetisation in the xy plane starts to occur straight away, as does the regrowth of magnetisation in the z-direction.)

This oscillating magnetic field at the Larmor frequency is switched on for a very small amount of time (a few milliseconds) to achieve such a rotation. This magnetic field is called an RF pulse; it is short (a burst or pulse) and the Larmor frequency for MRI is in the radio frequency range (tens of MHz). This process is sometimes called RF excitation of the spin system. Different amounts of rotation can be achieved by applying the oscillating magnetic field for different durations.

INCORRECT. Magnetic field gradients are static, whereas RF pulses are oscillating magnetic fields.

Try again.

INCORRECT. Radio waves are not used in MRI (though they are present). The MRI process does not involve transmission and reception of radio waves, but magnetic induction from oscillating magnetic fields. The oscillating magnetic fields are from (i) the RF excitation pulse and (ii) the precession of net magnetisation in the xy plane. You can read more about this in Hoult, "The magnetic resonance myth of radio waves", Concepts in Magnetic Resonance 1989;1:1-5.

Try again.

Further reading on this topic:
Books: MRI From Picture to Proton p38, MRI The Basics p32
Online: Basics of MRI

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