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MRI sequences (basic)

An MRI sequence is a number of radiofrequency pulses and gradients that result in a set of images with particular appearance. This article presents a simplified approach to recognising and thinking about common MRI sequences, but does not concern itself with the particulars of each sequences. 

For a more complete and accurate discussion please refer to MRI pulse sequences

Overview

The simplest way to think about the multitude of sequences available on modern scanners is to divide them according to the dominant influence on the appearance of tissues. This leads to a division of all sequences into T1 weighted, T2 weighted, Diffusion weighted, Flow sensitive and 'miscellaneous'. A number of 'optional add-ons' can also be considered, such as fat or fluid attenuation, or contrast enhanced. This leads to a broad categorisation as follows: 

  • T1
    • fat suppressed
    • gadolinium enhanced
  • T2
    • fat suppressed
    • fluid attenuated
    • susceptibility sensitive
  • diffusion weighted
  • flow sensitive
    • MR angiography
    • MR venography
    • CSF flow studies
  • miscellaneous
    • MR spectroscopy
    • MR perfusion
    • functional MRI
    • tractography

Terminology

In MRI we refer to the shade of grey of tissues or fluid with the word intensity, leading to the following absolute terms:

  • high signal intensity = white
  • intermediate signal intensity = grey
  • low signal intensity = black

Often we refer to the appearance by relative terms:

  • hyperintense = brighter than the thing we are comparing it to
  • isointense = same brightness as the thing we are comparing it to
  • hypointense = darker than the thing we are comparing it to

Annoyingly these relative terms are used without reference to the tissue being used as the comparison. In some instances this does not lead to any problems; for example, a hyperintense lesion in the middle of the liver is clearly hyperintense compared to the surrounding liver parenchyma. In many other situations however use of relative terms leads to potential confusion. Imagine a lesion within the ventricles of the brain described as "hypointense". Does this denote a lesion darker than CSF or than the adjacent brain? 

As such it is preferable to either use absolute terminology or, if using relative terms, to acknowledge the comparison tissue e.g. "the lesion is hyperintense to the adjacent spleen". 

NB: the word density is for CT, and there are few better ways to show yourself as an MRI noob than by making this mistake. 

T1 weighted sequences

T1 weighted sequences are part of almost all MRI protocols and are best thought of as the most 'anatomical' of images, resulting in images that most closely approximate the appearances of tissues macroscopically, although even this is a gross simplification. 

The dominant signal intensities of different tissues are:

  • fluid (e.g. urine, CSF): hypointense (black)
  • muscle: intermediate (grey)
  • fat: hyperintense (white)
  • brain
    • grey matter: intermediate (grey)
    • white matter: high signal (white-ish)

T2 weighted sequences

T2 weighted sequences are part of almost all MRI protocols.  Without modification the dominant signal intensities of different tissues are:

  • fluid (e.g. urine, CSF): hyperintense (white)
  • muscle: intermediate (grey) 
  • fat: hyperintense (white)

Diffusion weighted sequences

T1 weighted sequences are part of almost all MRI protocols and are best thought of as the most 'anatomical' of images, resulting in images that most closely approximate the appearances of tissues macroscopically, although even this is a gross simplification. 

The dominant signal intensities of different tissues are:

  • fluid (e.g. urine, CSF)
  • muscle
  • fat

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