MRI


Physics

The MRI (magnetic resonance imaging) machine generates an extremely strong magnetic field and pulses of radiofrequency energy, which align hydrogen nuclei in tissues and body water. The subsequent loss of alignment with time produces the MRI signal.

There are two main MRI sequences:

  • T1: Water is dark - better for anatomy (soft tissue structures)
  • T2: Water is bright - better for pathology (inflammation, oedema)

T1 MRI brain (sagittal plane)

T1 MRI

 

T2 MRI brain (sagittal plane)

T2 MRI

 

Other commonly used types are:

  • DWI (diffusion weighted imaging): Diffusion restriction is bright
    • useful for ischaemic strokes, abscesses, most tumours
  • FLAIR (fluid attenuated inversion recovery): Like T2, but Water is dark
    • useful for multiple sclerosis (periventricular lesions)
  • STIR (short tau inversion recovery): Like T2, but fat is dark
    • useful for oedema in tissues, perianal abscesses
  • MRA (magnetic resonance angiography): Vessels are bright
    • useful for AVMs, aneurysms (can be done with or without contrast!)

Contrast

Gadolinium is a metal-based contrast given IV.

It can rarely cause nephrogenic systemic fibrosis (similar to scleroderma) in patients with renal failure.

Avoid in patients with eGFR <30.


Contraindications

Always go through the MRI checklist!

Metallic foreign bodies, especially from previous eye trauma, can cause serious damage if they move during the scan.
If unsure of their presence, exclude with an X-ray.

Most modern implants (pacemakers, stents, joint replacements) are MRI safe, but they must always be checked to ensure compatibility. They may, however, cause a black void artefact on the MRI image.

The MRI machine is very noisy and cramped. Some patients may not be able to tolerate it if they are claustrophobic or unable to lie still.

Monitoring leads can heat up excessively during the scan, causing burns to the patient.

Loose ferromagnetic objects can turn into projectiles if inadvertently brought into the room. The machine is not turned off in between scans! Take a look at the videos below to see some of the potential dangers of MRI scanning.

 






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