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STIR Imaging

Page 5.2.17 · Imaging Physics for Knee Specialists · imaging page

The robust way to suppress fat, which is what the post-operative knee is imaged with.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

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STIR Imaging

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For Medical Students

What It Shows

Short tau inversion recovery suppresses fat by using its short recovery time rather than its resonant frequency. 5.2.16 covers fat suppression.

Because it does not depend on field uniformity, it keeps working where the field is distorted. 5.2.12 covers the basic principles.

It matters because metal distorts the field, so this is the sequence that still suppresses fat next to a prosthesis or a fixation device. 5.5.3 covers the post-operative protocol.

How It Is Acquired

An inversion pulse is applied and the image is acquired when the fat signal passes through zero. 5.2.12 covers the basic principles.

Anything with a similar recovery time is suppressed too, which is why the suppression is less selective than the frequency based method. 5.2.16 covers fat suppression.

The signal to noise is lower, so the images look coarser than a frequency suppressed equivalent. 5.5.1 covers the standard protocol.

It takes longer, which is why it is used where it is needed rather than everywhere. 5.5.3 covers the post-operative protocol.

It cannot be combined with intravenous contrast in the usual way, because enhancing tissue may also be suppressed. 5.2.21 covers advanced magnetic resonance techniques.

Key Learning Points

  • Fat is suppressed by its recovery time rather than its frequency.
  • It does not depend on field uniformity, so it works near metal.
  • The suppression is less selective and the images are coarser.
  • It takes longer than frequency based suppression.
  • It does not combine with contrast in the usual way.

MRCS Level

How It Is Acquired

In a viva this is the sequence to name when asked how to image a knee with metal in it.

The expected points are the inversion pulse, the loss of selectivity, the lower signal to noise, the time cost, and the contrast limitation.

Say that it is chosen for robustness rather than for quality. 5.2.16 covers fat suppression.

Say that it is the sequence for a post-operative marrow question. 5.5.3 covers the post-operative protocol.

Say that it is less useful with contrast. 5.2.21 covers advanced magnetic resonance techniques.

How Good It Is

This sequence is more reliable and less detailed than frequency based fat suppression, which is the whole trade.

Its best contribution is the metal bearing knee, where it is the only reliable way to assess marrow. 5.17.8 covers total knee replacement imaging.

It is limited by the time cost, the coarser images and the loss of selectivity. 5.2.16 covers fat suppression.

No figures appear for accuracy on this page, under decision 028, and no inversion time appears either: that value is field strength and scanner specific and this environment cannot check published figures.

Key Learning Points

  • More reliable and less detailed than frequency based suppression.
  • The only reliable marrow assessment in a metal bearing knee.
  • Chosen for robustness rather than for quality.
  • The costs are time, detail and selectivity.
  • Less useful with intravenous contrast.

FRCS Level

How Good It Is

At decision level this sequence is why a post-operative marrow question is answerable at all.

Peri-prosthetic marrow oedema in a painful replacement is a real question and needs reliable suppression to assess. 5.17.8 covers total knee replacement imaging.

Suspected infection next to metal is the clearest indication, because the finding sought is marrow signal change. 5.14.7 covers septic arthritis.

Fixation after a cruciate reconstruction distorts the field locally, so a tunnel or graft question benefits from it. 5.7.12 covers tunnel assessment.

The choice is made at protocol level rather than by the reporter, so it belongs in the request. 5.20.1 covers structured reporting.

Where It Misleads

The commonest error is accepting a frequency suppressed study near metal, where the suppression has failed. 5.2.16 covers fat suppression.

The coarser images are read as poor quality rather than as the expected appearance. 5.5.1 covers the standard protocol.

Suppression of a structure with a similar recovery time is read as absence. 5.2.12 covers the basic principles.

A contrast enhanced study on this sequence underestimates enhancement. 5.2.21 covers advanced magnetic resonance techniques.

The sequence is omitted from a post-operative protocol and the marrow question goes unanswered. 5.5.3 covers the post-operative protocol.

Key Learning Points

  • A frequency suppressed study near metal has failed suppression.
  • The coarser appearance is expected rather than poor quality.
  • Suppression of a similar tissue is read as absence.
  • Enhancement is underestimated on this sequence.
  • Omitting it leaves a post-operative marrow question unanswered.

Fellowship Level

Where It Misleads

At fellowship level the suppression choice is a protocol decision that decides whether a post-operative study can answer its question, and the request rarely specifies it.

A painful arthroplasty study acquired on the standard knee protocol will have failed suppression around the implant, which is where the answer was. 5.17.8 covers total knee replacement imaging.

The distinction between peri-prosthetic infection and aseptic change rests on marrow and soft tissue signal, so the sequence choice is the diagnostic decision. 5.14.7 covers septic arthritis.

Metal artefact reduction techniques are advancing and are unevenly available, so two departments answer the same question with different confidence. 5.2.19 covers artefacts.

Nothing in the usual request names the implant, so the department cannot select the protocol even when it has it. 5.20.1 covers structured reporting.

The practical conclusion is that a post-operative knee request should name what is in the knee, which is the cheapest improvement available in this chapter. 5.5.3 covers the post-operative protocol.

Key Learning Points

  • A standard protocol fails suppression exactly where the answer is.
  • The sequence choice is the diagnostic decision in a painful arthroplasty.
  • Metal artefact reduction is advancing and unevenly available.
  • The request rarely names the implant, so the protocol cannot be selected.
  • Naming the implant on the request is the cheapest improvement here.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 028 rather than an omission: the comparative standing of the modalities is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page. Any measurement here, including a sequence parameter, an angle or a millimetre, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

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