Delta Frequency Setting for Water Reference Scan in Siemens MEGA-PRESS SVS for GABA

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1

Hi everyone,

I’m using the Siemens MEGA-PRESS SVS editing sequence to measure GABA. For the water reference scan, I’m a bit unsure about the correct setting for the delta frequency (under Sequence Common). Should it be set to 0 ppm or -2.8 ppm?

Would the Edit Mirror Frequency (e.g., 7.5ppm or 4.7ppm) affect this decision? Please let me know if there are any other factors or considerations I should be aware of for this decision.

Thanks in advance for your help!

2

Hi Timo,
I have no experience with the Siemens MEGA-PRESS pulse sequence (only CMRR). The delta frequency (under Sequence → Common) should only determine the transmitter frequency of your slice selective rf pulses, not the MEGA puls). This parameter is relative to the water frequency → 0 ppm corresponds to the water frequency at 4.7ppm and -2.8 ppm ^3CH_2 (1.9ppm) of GABA. So, with this parameter you determine the localization of your voxel and for the water reference scan your water should have the same localization where your metabolite spectrum is from → so set it to 0ppm.
Due to the MEGA pulse, I don’t believe that MT effects are relevant as they could be with OVS, for example. But the MEGA pulse should be placed in such a way that it has no direct effect on water (not 4.7ppm or in this region).

Best,
Heiner

3

Hi Heiner,

Thanks for the thoughtful response. Unfortunately, it seems that the original scan in our data was acquired with the water reference scan delta frequency mistakenly set to -2.8 ppm instead of the correct 0 ppm. We now understand how this could impact the voxel localization and metabolite quantification.

Thanks again for your insight, and we’ll keep this in mind for future scans!

Best,
Timo