Physics of MRI
Magnetic resonance imaging (MRI) relies on the properties of spin to obtain images. Protons, neutrons and electrons all spin around a central axis. If combined in equal numbers within a nucleus, protons and neutrons will balance out and lead to a zero spin nucleus. Unbalanced nuclei, such as hydrogen, create a small magnetic field. This small magnetic field is known as a magnetic moment. In the normal situation, these tiny magnetic moments oppose each other to generate a neutral magnetic field. These magnetic moments may be influenced by an external magnetic field.
Application of external magnetic field
An MRI device applies a strong magnetic field to an area (usually 1.5 - 3 Tesla (T)). When hydrogen atoms are placed within this field, they precess around the direction of the field. Precession is akin to a spinning top. The precession of nuclei will tend to cancel out, leading to nil net magnetic field.
Importantly, external magnetic fields are not homogenous which makes using them for spatial positioning inaccurate.
Application of electromagnetic radiation
If electromagnetic radiation is applied to the precessing nulcei at a particular frequency (the Larmor frequency), the nuclei will shift to align in a different direction. Instead of the random precession seen with an external field, the nuclei will spin in harmony and are said to be 'in phase'.
T1 and T2 relaxation
When the electromagnetic field is turned off, the nuclei will return to their original precession around the external magnetic field. This involves two processes:
- T1 relaxation refers to the restoration of magnetic moments towards their original direction.
- T2 relaxation refers to the loss of phase in nuclear precession after the electromagnetic field is removed.
T1 and T2 times in water are similar, but as the nuclei are restrained differently in various tissue differences start to emerge. In general, as nuclei become more constrained T1 times tend to increase and T2 times tend to fall. This allows very good differentiation between different soft tissues (and even between different parts of the same soft tissue such as the brain).
Use of MRI
MRI can be used for treatment planning but suffers from poor spatial accuracy due to magnetic field inhomogeneity. It is commonly fused with CT images which have excellent spatial accuracy.
- Superb contrast between different soft tissues
- Higher resolution than CT
- No ionising radiation
- 3D data acquisition
- May be fused with CT scans
- Machines are more expensive than CT imagers
- Small magnet bore prevents scanning in treatment position (especially breast treatments)
- More artefacts than CT
- Some patients are unable to have CT due to presence of sensitive equipment (eg. pacemakers, cerebral aneurysm clips).
- Other implantable devices (hip replacements, dental fillings etc) are not damaged by the magnetic field but may cause localised artefact.
- Magnetic field inhomogeneity leads to inaccuracy in determining the position of volumes