Advice request on achieving recommended linewidths

Hello MRS community,

I am new to MRS and have the bad data to prove it!

System Specifications:

  • 9.4T Varian/ Agilent magnet
  • 100 G/cm SGRAD 88/55/HD/S gradient
  • 40mm Millipede quadrature volume RF coil (single channel proton)
  • VnmrJ 4.2a software
  • PRESS pulse sequence

Questions/ Problem:

Currently I am having difficulty shimming an in vivo subject (mouse model). My linewidths on a 3x3x3 mm^3 voxel are 30-60Hz, which is clearly not acceptable based on the consensus publications. I can see “peaks” at around 2.0 and 3.0 ppm but nothing like the results in a typical journal article. For shimming I use the GE3DSHIM for 1&z2 voxel shimming. Other issues are FASTMAP fails to run on my current system and I cannot shim using shimming protocols at 1&2 or higher order. At higher orders the XY and YZ shims max out and are not saved. Do you have any suggestions for breaking past this 30Hz barrier and achieving the recommended linewidths?

More questions to come, but I figured I would start with the most critical problem first. Thanks in advance.

Hi Evan,

I’m not a mouse person myself, but let me tag @JamieNear and @mdehghani61 who might be able to give you some pointers where to start.


Hi Evan,

Congratulations on your first bad dataset. It’s a great place to start, as it only gets better. (And then sometimes worse again haha )

Have you tried manual shimming? I normally wouldn’t recommend this, but the Varian system has a great interface for manual shimming (better than any of the other vendors imo). Also I’m not familiar with the automated shim tools on Varian. A side benefit of the manual shimming approach is that it’s also a good way to check if your shim coils are actually working. If you change the current in each coil, it should have a noticeable effect on the line shape. If not, then that would explain why none of the automated shimming tools are working.

If the shim coils are working, then you can adjust them manually and see how well you can shim, or you can go back and have another try with the automated methods.

What part of the brain are you trying to shim? There are some brain areas that are just really hard to shim.

Best wishes,

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cudalbu_testshims.pdf (2.0 MB)

hi Evan,

just to follow on Jamie’s email - you can find attached the pdf explaining how to manually test your shims on Varian
have you tried to shim a phantom?
we use Vnmrj 3.2 in our lab and I have no experience with Vnmrj 4.2 - question here - are you sure that FASTMAP is working here. What are the parameters that you are using?
i also asked Ivan Tkac to take a look at your post
keep us posted on the future tests you are doing

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Hi Evans,
Could it be that FASTMAP is not calibrated on your system? on VnmrJ, you typically need to generate the shim calibrate table based on phantom measurement!

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