Magnetic resonance imaging

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Nuclear magnetic resonance imaging (MRI) is a variety of computed tomography. MR does not depend on ionising radiation, but uses similar mathematical and processing principles to CT to produce images from very different and complex radio frequency data. The data arises from relaxation of atoms with unpaired spin aligned by a magnetic field and subjected to radiofrequency radiation at different angles and polarisation. Understanding the physics is challenging as it does involve macro quantum mechanics.

It generally requires more acquisition time than CT, and cannot be used in patients with contraindications to exposure to high magnetic fields, such as those with permanent pacemakers. The most common form of MRI is proton resonance imaging, which is assumed to be the case unless qualified by terms such as 31P MRI.

The protons imaged are effectively those in water molecules, and as a result MRI can give more detailed structural information than CT. Additionally, unlike x-ray CT, it is not hampered by bony artefact. The combination of features gives excellent soft tissue definition and is particularly ideal for anatomical areas enclosed in bone, e.g. spinal cord, brain and contents of the pelvis.

This is illustrated by its ability to show the focal demyelination of multiple sclerosis. It can delineate inflammatory processes and most tumours better than CT. Techniques such as diffusion weighted imaging (DWI, which images the Brownian motion of water) can quickly demonstrate tissue ischaemia or tumour associated oedema and echo planar imaging (EPI) can freeze body motion to a 100 millisecond time frame. Magnetic resonance angiography is advancing rapidly with many competing techniques and potential contrast media of which gadolinium chelates are presently the most commonly used. Functional MR can look at biochemical processes.

Contents

Specific areas

Safety

  • Safety questionnaire - if ferromagnetic shrapnel or foreign bodies present (e.g. previous shrapnel, pace-makers, joint implants).
  • Avoid metal-containing patches as well as ECG gel electrodes and leads.[1]
  • IUCD may be safe depending on the make.[2] The Mirena® coil is made from polyethylene.[3]
  • Pacemakers and defibillators
    • There is now good evidence these can be managed safely at up to 1.5 Tesla magnetic field strength[4]
      • 0.4% will reset to back up mode
      • End of battery life might precipitate replacement from such a back up mode
      • Threshold can change either way with greater than a 50% increase in 1%
      • It would seem advisable to do MRIs dwhere the device can be checked before and after the procedure


Pregnancy

There is no indication of danger from MRI in pregnancy. Neither is there clear evidence of safety. MRI is discouraged in pregnancy and as with other interventions the question is the balance of risk and benefit compared with delay. The field strength may be relevant.

Contrast is, likewise, probably safe within reasonable interpretation.[5]

MRI image of a knee

References

  1. FDA Warning about electrode use in MRI
  2. Mühler M, Taupitz M. How safe is magnetic resonance imaging in patients with contraceptive implants?. Der Radiologe. 2006 Jul; 46(7):574-8.(Link to article – subscription may be required.)
  3. [1]
  4. ERROR: Download of reference details rejected. Click on the link - PMID:29281579
  5. Webb JA, Thomsen HS, Morcos SK. The use of iodinated and gadolinium contrast media during pregnancy and lactation. Eur Radiol 2005;15:1234-40

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