© Greg Brown
vision@adelaide.on.net

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

• When to expect trouble with fat suppression
• Getting better spectral fat suppression (Fatsat) results with array or surface coils
• How to do a MAP-Shim
• How to check the centre frequency is tuned to water
• How to make a CF Check Tool
• Further reading

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

• about 40 cm from the magnet iso-centre in any direction
• near significant air tissue interfaces such as around the shoulders, skull base, nasopharynx and breasts.

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: -

• Set up the sequence and switch to the Sequence Change platform
• Tick the Bodyscout box, select Load
• Watch the screen to confirm that a shim adjustment is performed successfully

(if not you must do a manual MAP-Shim)
• Untick the Bodyscout box and confirm the array segments selected.
• Start the sequence with Measure, not Start

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

• Set up the sequence and switch to the Sequence Change platform
• Tick the Bodyscout box
• Select Adjust. At the Adjustment platform select MAP-Shim
• Select Start and look at the numeric readout in the upper right half of the platform. You need a hash mark (#) next to each shim channel selected (X, Y, Z plus any selected in the lower right of the screen). Be persistent. It can take 30 - 50 attempts to get shim convergence.
• The message "Shim Adjusted" will appear in the bottom right line of the platform
• Select Apply, then Exit to return to the adjustment platform.
• Confirm the Centre Frequency (CF) is still adjusted to the water signal (see Confirming CF), then select Change to return to the Protocol Change platform.
• Untick the Bodyscout box and confirm the array segments.
• Start the sequence with Measure (not Start).

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.

• From the Adjustment platform, select Frequency
• Click Start, repeat until the message "Frequency Adjusted" appears at the bottom right of the platform.
• Place the CF check tool on the screen lining up the marks with the end of the frequency scale.
• Ensure that the Water peak (the left peak) is aligned with the CF mark. If it isn't, repeat the MAP-Shim and frequency adjustments or use STIR or WE sequences to reduce fat signal.
• Select Exit, then Change

Making a Centre Frequency Check tool

The Easy Way

• Print this page (select page 3 in your printer dialogue) and photocopy the image below on to X-ray film or an overhead transparency sheet.

The Hard Way

• Call up the Map-Shim platform
• Fix a piece of paper to the screen and mark the endpoints of the frequency scale/
• Fold the paper to find the point midway between the scale ends.
• Use that gauge to place the end and centre marks on a small piece of X-ray film

Remember

• When the field is well shimmed the water peaks and fat peaks will be separate.
• The Water peak appears to the left of the Fat peak (higher frequency)
• The Water peak is usually higher than the Fat peak, but that depends on how much fat is within the field of view, and on the coil sensitivity pattern.

Fat Suppression Technique Notes. Greg Brown 1997

Fat Suppression Presentation Slides Greg Brown 1997

Numaris Reference Guide B.14

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