Magnetic resonance imaging (MRI)

Medical Physics | AQA A-Level Physics

MRI exploits the fact that hydrogen nuclei (protons) have a property called nuclear spinAn intrinsic angular momentum possessed by protons and neutrons. In a magnetic field, a spinning proton behaves like a tiny bar magnet and can align parallel or antiparallel to the external field.. In a strong external magnetic field, protons precessThe wobbling motion of a spinning proton's axis around the direction of an external magnetic field, similar to a spinning top wobbling around the vertical. The frequency of precession is called the Larmor frequency. (wobble) around the field direction at a characteristic frequency called the Larmor frequency.
Protons can exist in two spin states: parallel (lower energy) or antiparallel (higher energy) to the external field. There is a slight excess in the parallel state.
A radio-frequency (RF) pulse at exactly the Larmor frequency is applied. This is nuclear magnetic resonanceThe absorption of electromagnetic radiation by nuclei in a magnetic field when the frequency of the radiation matches the precession (Larmor) frequency. This causes protons to flip from the parallel to the antiparallel spin state. (NMR): protons absorb the RF photon and flip from parallel to antiparallel.
When the RF pulse is switched off, the protons relax back to the lower energy state and emit RF photons. These emitted signals are detected by receiver coils.
A gradient fieldA magnetic field that varies in strength across the body. It causes protons at different positions to precess at slightly different frequencies, allowing the MRI computer to determine the location of each signal. is superimposed on the main field so that the magnetic field strength varies across the body. This means protons at different positions precess at different frequencies, allowing the computer to determine where each signal comes from and build a spatial image.
Different tissues have different hydrogen densities and relaxation times, giving excellent soft-tissue contrast.
Advantages: non-ionising (no X-rays), excellent soft-tissue contrast, can image in any plane.
Disadvantages: very expensive, time-consuming (scans take 15-90 minutes), noisy, cannot be used with metallic implants, and claustrophobic for some patients.