Net magnetization vector

water droplets on glass
  • Two theories – classical and quantum theory

Classical theory

  • Based on the direction of magnetic moments
  • Parallel alignment: magnetic moments align parallel to external magnetic field
  • Anti-parallel alignment: magnetic moment align opposite to external magnetic field
  • At room temperature, more moments are aligned parallel and as a result, there is a net magnetization vector parallel to the external magnetic field

Quantum theory

  • Based on energy states of MR active nuclei
  • MR active nuclei on entering an external magnetic field show changes in energy states. Only two outcomes possible, a high energy or a low energy state
  • Spin-up nuclei have low energy, do not oppose the external field, aligns its magnetic moments parallel to the external field
  • Spin-down nucleis have higher energy, oppose the external field, aligns its magnetic moments anti-parallel to the external field
  • Difference between the high and low energy states is proportional to the strength of the external magnetic field. Higher the external field strength, there is a proportional increase in low energy state nuclei and hence more magnetic moments align in favor of the external field. They always align at an angle to the external magnetic field