Time Constrained PRESS & MEGA PRESS acquisition from same region

Data Acquisition
2

Hi Anushree,

Some thoughts (similar to the ones I’ve shared in this thread yesterday) below.

  • Most importantly: You don’t need a separate TE = 68 ms PRESS scan if you’re already running a TE = 68 MEGA-PRESS scan. The MEGA-PRESS edit-OFF will be identical to the TE = 68 ms PRESS scan (assuming that you put the edit-OFF editing pulse far away from anything, like at 8 or 9 ppm). Getting rid of this separate scan will free up transients that you can spend on the MEGA-PRESS. I don’t understand why this particular scan is the only one that is targeting a (massively smaller) volume. This would quite likely not get enough SNR to be viable (SNR drops linearly with measurement volume).
  • Speaking of MEGA-PRESS: Get as many MEGA-PRESS transients and as large a voxel as you can. 144 transients is awfully short.
  • I would probably try and keep TR consistent across all scans.
  • I would also keep the voxel sizes consistent across all scans.
  • The reason is that you can use the same short-TE water reference scan for quantification for both the short-TE PRESS and the medium-TE MEGA-PRESS acquisitions.
  • You can probably reduce dummy/start-up scans to 2 instead of 4. Saves a few seconds.
  • You don’t have to set up separate water reference scans. Philips can acquire these for you automatically. Just set Postproc → Spectral Correction to yes for all of your scans (MEGA-PRESS and short-TE PRESS) and specify the number of transients in the next line. These will be exported as _ref.sdat/spar.
  • You definitely don’t need 32 transients for your water reference data. 1-4 will absolutely suffice.
  • If you have VAPOR as a water suppression option, go for that one. If not, go for any other option (e.g., MOIST) than excitation.

Here’s what I’d set up for each region (left DLPFC 32 x 27 x 32 mm3; left TPJ 30 x 30 x 30 mm3) if I had 34 minutes to burn:

  • Short-TE PRESS (TR/TE = 2000/30 ms)
    • 64 transients
    • 1 water reference transient
    • total duration: 2 sec * (64 + 1 + 2) = 134 sec (2:14 mins)
  • MEGA-PRESS (TR/TE = 2000/68 ms)
    • 320 transients
    • 1 water reference transient
      • ADDENDUM: I can’t quite recall how the product MEGA sequence does it, but I believe it can do an interleaved water frequency update similar to what we built a few years ago. I can probably find that out (tagging @sganji in the hope he might see it)
    • total duration: 2 sec * (320 + 1 + 2) = 646 sec (10:46 mins)
      • slightly longer if you increase the number of water reference transients

So per region you’ll be clocking in at around 13 mins pure measurement time. You’ll want to allow for proper preparation in each region (calibrations, shimming, water suppression optimization etc. take about a minute to a minute and a half). And you always, always, want an extra few minutes in your protocol if things go wrong and you need to restart a scan (say, you get a bunch of lipids in your signal and you need to reposition your voxel. Absolutely common in dlPFC).

This protocol would leave you with very good quality data and give you everything you need and, most importantly, it would not put you under the intense pressure of not being allowed the tiniest failure. (This can make all the difference between OK-ish data and really excellent data).

Cheers,
Georg

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