Medical Physics : MRI

Question 1

What is nuclear magnetism?

Answer 1

Many atomic nuclei have intrinsic angular momentum, known as spin.

The spinning nucleus produces a magnetic field, and the nucleus has a magnetic moment, mu.

Question 2

What are the conditions for a nucleus to have a magnetic moment?

Answer 2

An odd number of protons or neutrons.

Question 3

How are the magnetic moment and angular momentum related.

Answer 3

They are proportional - the constant of proportionality is the magnetogyric ratio, gamma.

Question 4

What is the magnetogyric ratio?

Answer 4

The constant of proportionality that relates the magnetic moment to the angular momentum

Question 5

What conditions are there for angular momentum?

Answer 5

It is quantised. For H-1 it can only take the value of +- hbar/2

Question 6

Describe Equilibrium Magnetism?

Answer 6

It is energetically favourable for the nuclei to come to equilibrium pointing parallel to the field.

But as the differemce in energy between the two states is small, random thermal fluctuations mean that at normal temperatures there are almost as many anti-parallel as there are parallel to the field

However as there is a small excess in the parallel state which gives rise to a small net magnetic moment which is alighned with the field