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
