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Magnetic Resonance Imaging

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Magnetic Resonance Imaging
Photo by National Cancer Institute / Unsplash

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MRI is magic.

I refuse to be told otherwise.

However that doesn't get you any points in your FRCA exam, so there are some factual sciencey bits you need to know. Here is our really simple explanation that will get you through a viva, and hopefully makes a bit of sense of well.

There's loads of hydrogen in the body.

Your body has lots of water in it, hopefully, and water has two hydrogen atoms in every molecule. So if we're trying to make a map of the body, then it makes sense to use hydrogen as the atom that we 'measure'.

Hydrogen atoms have a thing called 'spin'.

They're not actually spinning like a top. Or maybe they are, we've never actually seen them properly. But let's say they are.

If you put an atom that has 'spin' in a strong magnetic field, it will align with the magnetic field, like iron filings next to a magnet.

Cool.

Protons with spin, in a magnetic field, can absorb and emit energy.

This is the important bit. If all of your hydrogen ions are in alignment with a magnetic field, and you fire a pulse of electromagnetic radiation at them (at a very specific frequency), they'll absorb this energy and deflect out of alignment.

Kind of like flicking a spinning top and making it wobble.

Very quickly afterwards, they'll then flip back to being aligned with the magnetic field again, and release this energy, again as electromagnetic radiation.

Now you just need a sensor.

So you've fired a pulse of radiation at a human body that's inside an enormously powerful magnet. So all of the aligned hydrogen atoms have absorbed the energy, and then emitted it again in different directions.

If you can put a bunch of sensors around the body, and tell the sensors what type of signal to expect and when, then they can work out where that impulse of energy came from, (a bit like a GPS satellite) and therefore where that hydrogen atom is.

Do this a whole bunch of times very quickly and you can build a map of all the hydrogen atoms in the body.

That's an MRI scan.

Sort of.

A bit more science

  • Static magnetic field generated by scanner
  • Interacts with tiny magnetic fields around individual atoms that have a magnetic dipole moment which allows them to orient themselves at an angle to the external field
  • These nuclei will also precess (wobble)around the direction of the magnetic field when subjected to another force
  • They do this at a frequency determined by the field strength
  • The precession state (antiparallel), and aligned state (parallel), have different energy levels (parallel is lower energy)
  • By oscillating the magnetic field, more nuclei can be recruited from the parallel state to the antiparallel state
  • When the nuclei relax back into the parallel state, they emit energy at the same frequency
  • This can be detected by receiving coils in the scanner
  • The scanner fires ever so slightly different frequency pulses at different times, meaning the received signals can be spatially localised

Tell me about the helium

  • Helium is used as a superconductor to maintain the static magnetic field
  • This is around 1000 litres of liquid helium, a couple of degrees above absolute zero
  • If this somehow escapes, or there is a field shut down and it warms up, then the helium suddenly evaporates
  • Unless it is vented out of the room very quickly, it will rapidly expand and fill the room with a dangerously hypoxic helium gas mixture
  • The pressure can also hold doors shut and prevent escape

Hence the MRI suites have oxygen sensors and pressure equalisation mechanisms.

What are the benefits of MRI over CT?

  • Avoids ionising radiation
  • Increased contrast resolution (better detail of soft tissues)
  • Greater sensitivity for identifying pathology

What are the hazards of an MRI?

  • Very strong magnetic field
  • Loud noise
  • Radiofrequency heating of metal components
  • Risk of helium escaping and hypoxic environment

What implanted objects can harm a patient in an MRI scanner?

  • Pacemaker
  • Defibrillator
  • Cochlear implant
  • Nerve stimulator
  • Joint replacement
  • Schrapnel or metal debris, particularly any metal in the eye

Who can't have an MRI scan?

Most people are absolutely fine, especially with modern scanners and modern implants (pacemakers etc), however contraindications include:

  • Ferromagnetic schrapnel in eye
  • Recent surgery with ferromagentic clips or implants (cochlear implants)
  • Intra-aortic balloon pump
  • Some aortic stents and ventricular assist devices
  • Neurostimulator and programmable hydrocephalus shunts
  • Implanted heart valves and stents

References and Further reading