introduction to MRI

• basic magnetism

  • a magnetic field is a region of space in which magnetic forces are experienced
  • the strength is measured in teslas
  • direction is often represented by the letter B
• magnetic moments


  • a magnetic moment is a quantity that represents the direction and strength of a magnetic field
  • the stronger the magnetic field strength the magnetic moment
  • the direction is how it aligns with the magnetic field
• MRI magnet


  • requires a very strong magnetic field
  • this is achieved used a superconducting electromagnet
  • an electromagnet is configured using wire in the shape of a coil
• superconducting magnet


  • high resistance increases the temperature of the coil
  • cooler to a low temperature in order to operate
  • MRI scanner cooled with liquid helium
• radiofrequency coils


  • in order to produce an image the radio frequency pulse is administered to the patient
  • these are produced via the radio frequency transmitter coils
  • these coils are located within the main magnet
• receiver coils


  • receive the MRI signal
  • designed specifically for certain areas
  • need to be placed over the area to be imaged and as close to the patient as possible
• MRI active nuclei

  • atoms with an odd mass number are said to be MRI active nuclei
  • have a tendency to align their axis of rotation to an applied magnetic field
• hydrogen atoms


  • have a mass number of 1
  • contains a positively charged proton
  • gives a large magnetic moment
  • used in medical imaging as it is abundant in the human body
• alignment


  • without a magnetic field the magnetic moments are randomly orientated
  • when placed in a strong magnetic field the magnetic moments align with this field
  • some align parallel (low energy nuclei) and some anti parallel (high energy nuclei)
  • the parallel nuclei create a net magnetic vector (NMV)
• precession


  • each hydrogen nucleus is spinning on its axis
  • the magnetic field strength causes an additional spin - precession
  • the speed at which they spin is called the precession frequency