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QUESTIONS
» Basic Physics

Where does the MRI signal come from? This section explores the basic physics of magnetic resonance imaging.

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Resonance

Answer this...

"Nuclear magnetic resonance" and "magnetic resonance imaging". What is the resonance?

The loud vibration / noise made by the MRI scanner.

The oscillation of the net magnetisation vector when it is out of alignment with he external magnetic field.

The rotation of individual nuclei when placed in an external magnetic field.

INCORRECT. The loud noises made by the MRI scanner are not resonance. (The noise arises from the interaction of the gradient magnetic fields with the static magnetic field: the gradient coils are subject to a twisting force each time they are switched on and off. This results in a mechanical vibration in the former supporting the coils (the former is a cylinder around which the coils are wound) and its mountings. They act as transducers and produce the loud sounds heard during an MRI scan.)

Try again.

net precession
Precession of the net magnetisation

CORRECT. During the MRI process, energy is put into the system to disturb the net magnetisation vector from its equilibrium position (pushing it out of alignment with the magnetic field). Since every single spin precesses, the net magnetic moment precesses around the field direction too. This precession is the resonance of NMR and MRI.

The precessing net magnetisation generates an oscillating magnetic field which is measured in appropriately placed signal detection equipment. For a particular magnetic field strength the (resonant) frequency of precession can be calculated via the Larmor equation.

INCORRECT. This answer is a bit tricky. Nuclei do not rotate; the direction of their magnetic moment, however, does precess around the external magnetic field direction. Had the answer stated "the precession of spins" or "the precession of the net magnetisation vector", it would have been correct.

Remember, spin is an intrinsic property of a particle—not actual rotation. Try again.

Further reading on this topic:
Books: MRI The Basics p33, MRI: Physical Principles & Sequence Design p2-4

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