Optimizing MMsup MEGA sLASER Parameters on 3T Siemens

Data Acquisition
1

Hello! We are currently optimizing the MEGA-sLASER sequence with an interleaved water reference from the CMRR package to collect macromolecule-suppressed GABA from infants in a medial visual cortical seed on a Siemens 3T Prisma (software VE11C). While we have been able to consult the current literature for how to add macromolecule suppression, use an interleaved water reference, and use sLASER for localization, we had a few outstanding questions about how some of these choices interact in terms of our parameters. Parameters: TR=2000ms, TE=80ms, averages=96 for each editing pulse, editing pulse frequencies: 1.5ppm & 1.9ppm, editing pulse BW=80 Hz, delta frequency=-1.7ppm, voxel=3cm3, applying 2 saturation bands with 30mm thickness, 5mm pos offset, -3.4ppm delta frequency

  1. The default VAPOR delay pulses are: listed as 150ms, 100ms, 146ms, 105ms, 106ms, 68ms, 74ms, 28ms with a flip angle of 80 deg. However, we see that the expert consensus paper (Tkáč et al., 2021) recommends: 160 – 110 – 132 – 115 – 112 – 71 – 88 – 24. We also only have one choice for the flip angle, but the consensus paper recommends α – α – 1.78α – α – 1.59α – α – 1.78α – 1.86α. Do these expert consensus paper recommendations apply for macromolecule suppression and the use of OVS?
  2. Would adding GOIA refocusing pulses to further reduce chemical shift displacement offer any benefit in this context? If yes, would they interact with our other parameters in any way we should be mindful of?
  3. When we were using the Siemens WIP MEGA-PRESS sequence, the recommendation for water suppression bandwidth was 50Hz. Seeing as this is a different sequence and the lowest number we can use given our other parameters is 58Hz, would this suffice or should we change something else to get back to a possible 50hz setting?
  4. With an excite pulse duration of 3200us, to maintain a 180 deg flip angle for our refocusing pulse, we need to increase the refocus duration to 7260us. However, previous sequences we have seen have a refocus pulse duration of 5600us (which would require a 161 deg flip angle). Which parameter should we prioritize?
  5. We have not been able to find much information about OVS placements in prior literature. Would you agree that the following placements are adequate?
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Happy to provide any more info if requested. Thank you in advance for any clarifications!

2

Hi,

1 - I don’t see why not. Changing VAPOR and the OVS might cause a slight change in the MT effect, but it’s a general concern, not specific to MM-suppressed GABA.

2 - You mean Using, not Adding, right? I can’t say about CSDE, don’t know their BW. But I suppose they might help with outer-volume echoes. For the interaction with other parameters see reply 4.

3 - 50 Hz is pretty narrow. Even 100 Hz is < ~0.4 ppm each side around the water, so I would not fight for the smallest value here.

4 - You should have a duration long enough to achieve 180 degrees. I would consider going for a safer option allowing 200 deg, if it works out with the other parameters.

Cheers,
Andy