Sequences

Sequences

A variety of acronyms describe MRI sequences. Each manufacturer adds proprietary names to already extensive list of generic sequences. When looked from the point of view of technique behind image generation, one way to categorize MR sequences would be the following:

  • Spin Echo
    • Conventional Spin Echo
    • Fast or Turbo Spin Echo
  • Gradient Echo
  • Inversion Recovery
  • Phase Contrast
  • Echo Planar Imaging

Another way of thinking about MR sequences is to classify what tissue properties were are trying to highlight in an image. For example:

  • T1 weighted
  • T2 weighted
  • Proton density
  • Flow within a vessel
  • Diffusion
  • Anisotropy
  • Oxygenation

Here, the lines between tissues and cells and molecules start to become blurry. On a diffusion tensor image of white matter tracts, are we imaging tissue or axons or molecular diffusion? The answer depends on the level of detail and resolution of the image. Today, we are not quite at the individual axon level yet, but it is the combined diffusion properties of many molecules that result in an image of corticospinal tract or inferior longitudinal fasciculus.

Various parameters of MR sequences can affect the final quality of the acquired image. These include:

  • TR
  • TE
  • Bandwidth
  • Slice thickness
  • NEX
  • Post-processing filters