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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

Intensity 

When describing most MRI sequences 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. 

Diffusion

When describing diffusion weighted sequences, we also use the term intensity but additionally we use the words restricted diffusion and facilitated diffusion to denote whether water can move around less easily (restricted) or more easily (facilitated) than expected for that tissue. Again many use these words as if they are absolute terms and this leads to confusion (more on this issue here). 

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): low signal intensity (black)
  • muscle: intermediate signal intensity (grey)
  • fat: high signal intensity (white)
  • brain
    • grey matter: intermediate signal intensity (grey)
    • white matter: hyperintense compared to grey matter (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): high signal intensity (white)
  • muscle: intermediate signal intensity (grey) 
  • fat: high signal intensity (white)
  • brain
    • grey matter: intermediate signal intensity (grey)
    • white matter: hypointense compared to grey matter (dark-ish)

Diffusion weighted sequences

Diffusion weighted imaging assess the ease with which water molecules move around within a tissue (mostly representing fluid within the extracellular space) and give insights into cellularity (e.g. tumours), cell swelling (e.g. ischaemia) and oedema. 

The dominant signal intensities of different tissues are:

  • fluid (e.g. urine, CSF): no restriction to diffusion
  • soft tissues (muscle, solid organs, brain): intermediate diffusion
  • fat: little signal due to paucity of water

Typically you will find three sets of images when diffusion weighted imaging is performed: DWI, ADC and B=0 images. 

DWI

When we say "DWI" we usually are referring to what is better terms an isotropic T2 weighted map as it represents the combination of actual diffusion values and T2 signal.

It is a relatively low resolution image the following appearance: 

  • grey matter: intermediate signal intensity (grey)
  • white matter: slightly hypointense compared to grey matter
  • CSF: low signal (black)
  • fat: little signal due to paucity of water
  • other soft tissues: intermediate signal intensity (grey)

Acute pathology (ischaemic stroke, cellular tumour, pus) usually appears as increased signal denoting restricted diffusion. However (and importantly) because there is a component of the image derived from T2 signal some tissues that are bright on T2 will appear bright on DWI images without there being and abnormal restricted diffusion. This phenomenon is known as T2 shine through

ADC

Apparent diffusion coefficient maps (ADC) are images representing the actual diffusion values of the tissue without T2 effects. They are therefore much more useful, and objective measures of diffusion values can be obtained, however they are much less pretty to look at. They appear basically as grey-scale inverted DWI images. 

They are relatively low resolution image the following appearance: 

  • grey matter: intermediate signal intensity (grey)
  • white matter: slightly hyperintense compared to grey matter
  • CSF: high signal (white)
  • fat: little signal due to paucity of water
  • other soft tissues: intermediate signal intensity (grey)

Acute pathology (ischaemic stroke, cellular tumour, pus) usually appears as decreased signal denoting restricted diffusion. 

B=0

If you see these, don't worry. They are only used to calculate ADC values. They are essentially T2 weighted images with a bit of susceptibility effects. 

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