Better Shim Adjustments & Fat Suppression

© Greg Brown

This note explains how to do get good fat suppression when using the array or surface coils on the Siemens VISION.
It includes

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When to Expect Trouble with Fat Suppression

Sequences stored with the Shim Adj. box ticked will perform a Global shim or MAP-shim automatically if this has not been done before for this patient. The automatic MAP-shim will only perform a specific number of attempts (iterations) to achieve a well shimmed field. If these attempts are unsuccessful you will see the message "Gradients could not reach convergence" and other action will be needed to get good spectral fat suppression (Fatsat).

Fatsat will fail wherever there is more than 1.75 ppm variation in field homogeneity.

This commonly occurs

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Getting Better FATSAT Results with Array or Surface Coils

Use the body coil to receive when doing the shim adjustment. Remember the system will only perform one shim adjust. If the system has already shimmed the patient you must use the manual method.

Automatic Shim with body coil

The simplest way to do this is: -

Manual MAP-Shim with Body Coil

A more thorough approach is to do a manual MAP-Shim with the body coil.

The process is detailed in sections B.14 of the Numaris Reference Guide.
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Why does it help?

Because these coils have spatially varying sensitivity they can be swamped by strong signals from fat near the coil. This gives distorted information for the MAP-Shim to work with and increases the likelihood of reading the fat rather than the water signal. The frequency adjustment routine tries to adjust the centre frequency of the MR system to the resonant frequency of water. This is done by analysing the frequency curve of a single slice, and assuming that the water causes the maximum peak amplitude. The method will be tricked when there is a low ratio of water to fat signal, or where there is a lot of fat close to a surface coil. 

Confirming the Centre Frequency (CF)

The CF is the frequency that the scanner is tuned to. With poor shimming or when fat is close to the coil during initial frequency adjustments the system may select the fat peak instead of the water peak, resulting in poor images when using spectral fat suppression.

The CF must be confirmed whenever the A20 (Z2) channel is used during manual MAP-Shim, and should be done after any MAP-Shim to ensure good results.

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Making a Centre Frequency Check tool

The Easy Way

CT test tool

The Hard Way


Further Reading

Fat Suppression Technique Notes. Greg Brown 1997

Fat Suppression Presentation Slides Greg Brown 1997

Numaris Reference Guide B.14

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