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» Basic Physics

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

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Initial Effect of a Magnetic Field

Answer this...

What happens to spins placed in a magnetic field, before a net magnetisation forms?

They precess around the field direction.

They align parallel and antiparallel with the field.

Both of the above.


lab frame
The direction of the external magnetic field is in the head-foot direction in an MRI scanner. However, in MRI diagrams this is the z-axis and the frame of reference is turned.

CORRECT. From classical perspective, when spins are placed in a magnetic field they precess around the direction of the field, like a spinning tops in a gravitational field. They do not align with or against the direction of the field at first; they simply start precessing at whatever angle to the field they happen to be in. All nuclear magnetic moments cancel out because they point in any and all directions in 3D space (an isotropic distribution). Initially, there is no net magnetisation in a sample.

(Then thermal energy destroys this isotropy, allowing a net magnetisation vector to form.)

INCORRECT. That spins align parallel or antiparallel with the field direction is one classical description of the formation of the net magnetisation vector. What happens before this?

Try again.

INCORRECT. Spins have not changed their alignment with the field direction before formation of the net magnetisation vector.

Try again.

INCORRECT. Something happens!

Try again.

Further reading on this topic:
Books: MRI The Basics p26, Spin Dynamics p29
Online: Hyperphysics

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