10.4.6 Magnetic Resonance Imaging

What is MR Scanning?

• Magnetic resonance (MR) scanning, or magnetic resonance imaging (MRI) is an imaging technique which takes cross-sectional images of a patient's body
• The basic principle is that by exposing the patient to a magnetic field, hydrogen nuclei in the body respond, and the location of these responses can be determined
  • This information is then used to show structures in the body (but not how much they are functioning)

• This makes MR scanning a powerful tool for imaging organs and locating masses, such as tumours, in the body

Cross-Sectional MR Image of the Brain

MRI produces several cross-sectional images that can be used in combination to show the extent of structures throughout the patient's body.

Nuclear spin & precession

• Hydrogen nuclei (protons) possess a property known as spin
  • When a charge moves, it generates a magnetic field
  • Therefore, the spin of a proton generates a very small magnetic field around it
  • This magnetic field has an associated magnetic moment

• Protons have two possible spin states, they can either be in a spin up state or a spin down state
• In the absence of an applied magnetic field:
  • Both spin states of a proton have the same energy
  • Equal numbers of protons occupy one of the two states
  • Therefore, the magnetic moments (the magnetic fields produced by the protons) cancel out

• However, when a magnetic field is applied:
  • A difference in the energy arises between the two spin states
  • Most protons occupy the lower energy level state
  • Therefore, there is a net magnetic moment which can be detected
• The two energy states depend on the direction of the proton's magnetic moment:
  • When the magnetic moment is parallel to the applied magnetic field - this is the lower energy level
  • When the magnetic moment is antiparallel to the applied magnetic field - this is the higher energy level

• Another effect of the applied magnetic field on a proton is precession
• The spinning protons precess about the direction of the applied field

Precession of a Hydrogen Nucleus

The spin axis of a hydrogen nucleus (proton) precesses around the direction of the applied magnetic (like a spinning top toy on a table)

How does MR Scanning Work?

• MR machines operate on the basis of nuclear magnetic resonance (NMR), which is

When a proton absorbs a photon of exactly the energy required to flip its spin from a lower energy state (spin up) to a higher energy state (spin down)

• The tissues in the human body contain more hydrogen nuclei (protons) than any other element
  • Therefore, if all their magnetic fields could be aligned, then nuclear magnetic resonance can be observed

• The patient lies along the axis of a large solenoid, which generates a very strong uniform magnetic field
• When the uniform field is applied, the magnetic moments of the nuclei align with the applied field
  • The spinning hydrogen nuclei begin to precess about the direction of the applied magnetic field

• A pulse of electromagnetic radiation in the radio-frequency (RF) range is emitted which changes the alignment of the spins of the hydrogen nuclei
  • This is an excited state for the hydrogen nuclei

Hydrogen nucleus absorbing an RF Photon

The nucleus in a lower energy spin state (aligned with B) absorbs an RF photon with the exact energy to excite it to the higher energy spin state (aligned against B)

• The hydrogen nuclei then de-excite, realigning with the external field
  • In this process, they emit photons with the same RF
  • These photons are detected by a ring of detectors

Hydrogen nucleus emitting an RF Photon

 The hydrogen proton emits an RF photon as it relaxes to the lower energy spin state

• Another set of coils generates the gradient field
  • For a given cross-section of the body, this slightly varies the magnetic flux density, B, at different positions
  • This means the hydrogen nuclei emit RF photons with frequencies that depend on the position of the hydrogen protons
• This information is fed into a computer, which then identifies the density of hydrogen nuclei at each position for a cross-section

Advantages & Disadvantages of MRI Scanning

• The main advantages of an MRI scan are:
  • It is non-ionising and non-invasive
  • It produces extremely high-resolution images
  • It can diagnose very small differences between cells e.g. cancerous cells

• The main disadvantages of an MRI scan are:
  • It is a time-consuming procedure which can be uncomfortable for patients
  • It is very expensive