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### QUESTIONS» Basic Physics

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

# Nuclear Spin

## What is nuclear spin?

Rotation of a nucleon.

Intrinsic angular momentum.

Both of the above.

INCORRECT. Spin is not rotation of a particle. It is a highly abstract concept, which is only truly defined with quantum mechanical equations (not given here). However, we sometimes still think about spin as if it were actual rotation, because we want to grasp this concept. The concept of a particle rotating around its own axis is helpful, but it is intellectually sterile; for example, at absolute zero temperature when all motion ceases, a particle still has "spin".

Remember, an elementary particle with spin is not actually rotating. Try again.

CORRECT. Spin is a quantum mechanical intrinsic property of elementary particles. It is very difficult to imagine this property, and the notion of actual rotation can be somewhat helpful. However, it is wise to separate this notion of a spinning particle from the quantum mechanical property we call "spin".

Although spin is a form of angular momentum, an elementary particle with spin does not mean it is rotating; particles with spin simply have spin. For example, although an electron has mass, it is indicated to be a "point particle", occupying no volume of space at all. How can we imagine an electron rotating? Diagrams and explanations of spin and its consequences can help, but we must be careful not to confuse quantum mechanical (quantized angular momentum) and classical (rotating particle) explanations of MRI.

INCORRECT. Using classical mechanics, we do think of nucleons as spinning particles. However, spin is not rotation. It is an intrinsic property of a particle. This is hard (or impossible) to grasp.

Beware: mixing up the classical (rotating particle) and quantum mechanical (quantized angular momentum) explanations of MRI can cause confusion. Try again.

Books: Spin Dynamics
Online: Hyperphysics, Wikipedia