MEGA-PRESS Study Design Help

ZanettaK1 · February 18, 2025, 6:22pm

Dear Experts,

I have been piloting the Siemens (Scanner: Prisma; Software version: VE11C) MEGAPRESS (859G WIP) for a few months now using Osprey for processing. Recently, I decided to run the sequence in the PCC to check that all my sequence parameters are correct and that I get the desired result. I believe acquisition in the PCC looks as expected, but its the acquisition in the mPFC or any frontal region (including ACC) that’s giving me an issue.

For my most recent data I also tried the three different fitting algorithms (Default:RobSpecReg RestrSpecReg and, ProbSpecReg) and I believe the default fits best.

In the power point linked here I have included details regarding each acquisition, voxel localization, chemical shift PDFs (only for the RobSpecReg as the chemical shift PDFs for the other fittings were very similar), and the metab fit plots for each fit. The metab fit plots are generated using eddy correction with the water reference scan. I have also attached the entire acquisition protocol. (note: the last two acquisitions are both localized to the mPFC, I just mistakenly left the “PCC” naming during the scan.)

The KC Edit acquisition was one used by other researchers in my department, so I just wanted to test it out to see if I get good results.

For SVS Edit 1 & 2, I went off of another post on MRS hub to optimize the parameters. I think these two acquisitions are quite comparable. Ideally I would use a shorter TR and # of averages in my final acquisition as I need to decrease my scan time as much as possible.

SVS Edit 3 is exactly like SVS Edit 2 but just localized in the mPFC. This is where I struggle the most and don’t know what to do in order to improve the fit. The chemical shift is of poor quality, the data is quite noisy.

I know there are a few other things that could be contributing to this noisy data (shim, motion). I did an interactive shim and got it down to 20.4 Hz in the mPFC. Admittedly, the other shims were below 14 Hz for the PCC/OCC, but it was simply not possible to go down to that value in the mPFC. This was also the last picture that I took, so perhaps there is some motion artifact here but I am not sure. Right before this image I acquired two PRESS sequences also in the mPFC and the data came out very clean.

I am unsure how to proceed. Ultimately, I need to localize in the frontal region and I wonder what else I can do to improve the data? Any and all suggestions would be highly appreciated. THANK YOU!

Regards,
Zanetta

Human_01272025.pdf (114.1 KB)

admin · February 18, 2025, 7:08pm

Hi Zanetta,

A couple thoughts:

Anything prefrontal/frontal tends to be more difficult to shim. What is your shim procedure before you go to the interactive shim? Some folks have recommended doing the regular GRE or brain shim three times in a row. If you have access to the CMRR spectro package, consider the FASTMAP/FASTESTMAP modules, which can give good results in tricky regions.
Shim is one thing, but another huge problem in your mPFC spectrum is an awful lot of the spurious echoes (high-frequency oscillations) that you see in your mPFC spectrum; they tend to occur a lot in tricky-to-shim regions. One thing you can experiment with to ameliorate that is the order and orientations of the slice-selective gradients (I believe that on the Siemens this is in Routine → Orientation, and I can’t recall the name of the other option, but it allows to try different orders - send a screenshot of the exam card tabs and I can take another look). See this thread and this paper.
Your SVS Edit 1 and 2 protocols in the OCC look good to me and have approximately what I consider good SNR (we typically tell people to measure for 10 mins in a 27-ml voxel at TR = 2 sec).
I would advise against going lower on TR (introduces T1 weighting to all of your signals), voxel size, or the number of transients. Yes, I know everyone wants to keep their scans as short as possible and measure in the smallest possible volume, but in order to measure GABA+ reliably, you can’t beat the laws of physics/SNR. I’d say your signals look just about acceptable in the KC protocol, but this operates the very lower end of what I usually recommend for SNR (the KC protocol is already at just about 40% of that size and 80% of that measurement duration, so your SNR is probably down to about 30% of what I’d let people start with). My advice is usually that it’s better to sacrifice one region of interest but at least get interpretable data rather than a bunch of noise.

For your Osprey analysis, the three algorithm choices you mention only pertain to the spectral alignment, not the modeling. Based on the initial three KC plots, RobSpecReg appears to be much better in aligning than RestrSpecReg, which gives massive subtraction artefacts in the GABA region.

You will definitely want to include modeling of the co-edited MM resonance at 3 ppm. This is more thoroughly explained in the documentation - we should probably add an example job file for TWIX data as well (you’re the second person this week that built from the example file for un-edited data and missed this crucial part), but you can find a good template in the Philips MEGA example. This will substantially improve the modeling of the 3-ppm signal compared to what you have now.

Specifically, add:

% Add macromolecule and lipid basis functions to the fit?
opts.fit.fitMM              = 1;                % OPTIONS:    - 0 (no)
                                                %             - 1 (yes, default)

% How do you want to model the co-edited macromolecules at 3 ppm for GABA-edited MRS?
opts.fit.coMM3              = '3to2MM';      % OPTIONS:    - {'3to2MM'} (default)
                                                %             - {'3to2MMsoft'}
                                                %             - {'1to1GABA'}
                                                %             - {'1to1GABAsoft'}
                                                %             - {'freeGauss'}
                                                %             - {'fixedGauss'}
                                                %             - {'none'}

opts.fit.FWHMcoMM3          = 14;

You will then need to report the composite of GABA+macromolecules (GABA+, which is provided by OspreyQuantify), because GABA alone cannot be reliably estimated from a MEGA-PRESS experiment.

HTH,
Georg

admin · February 18, 2025, 7:13pm

I just looked at your protocol PDF and found a few things worth fixing:

It was correct in some protocols but not others, but to be clear: For your water reference scans, you need to set the delta frequency (Sequence → Common) to 0 ppm to ensure that the water signal is co-localized with the metabolite scans (reverting the chemical shift displacement error).
You don’t need 16 water reference averages. One would be enough, 2 is good, 4 is plenty. Saves a couple second.
We’ve had very mixed experiences with the RFA feature (interleaved frequency adjustment), which some of our collaborators have reported to do more harm than good. If your scanner doesn’t drift that hard and/or you’re not running a bunch of gradient-heavy scans (e.g. DTI) in the half hour before your GABA MRS, you are probably fine turning that off.

ZanettaK1 · February 21, 2025, 6:53pm

Hi Georg,

This is exceptional, thank you so much for the detailed suggestions I am beyond grateful! Its been quite the journey getting this project off the ground.

To respond to some of your points:

For my shimming procedure I do the auto GRE shim 2x with a shim box larger than my volume (but not close to the edges of the brain etc.). I apply the autoshim, decrease the shim box size to match that of my VOI and then do an intershim where I manually change the first order shims to see if I can knock down the FWHM a few Hz. This has gotten me the best results thus far on my machine. I will look into the CMRR spectro package.
In terms of the orientation, it is set to Coronal and greyed out so I can’t seem to change it. Screenshots of the exam card are attached.

Ok it seems that sticking to a TR 2000 is the best option with voxel size 27 mm iso.
I did in fact miss those lines of code. I added them and repeated the analysis. Results are here. However, should the resulting plots be labeled with “Gaba+” and not just “Gaba”? And my concern is that it quantifies a Gaba concentration of 0, even in the two svs protocols in the PCC. Attached is an example job file.
jobTwix_invivo_megapress_859G_mPFC_MMRob_EXAMPLE.m (19.1 KB)
Noted on the # of averages for the water ref scan as well as the delta frequency under Sequence → Common, it seems I missed it for the KC edit.
So you suggest turning RFA off for all sequences? So far I have only turned it off for the water ref scan to achieve “RF OFF” water suppression. Otherwise when RFA is turned on, the only options for water suppression are “water sat.” or “weak water suppression.” This is a mostly clinical scanner so I am unsure if there are heavy duty scans run frequently on the machine. I will double check - thank you for the tip.

Thank you again for your prompt and detailed reply!
All the best,
Zanetta