80 ms > 68 ms

With a strong sense of the robustness and reproducibility of GABA-edited MRS, we need a new experiment like a hole in the head. But the main limitation of standard 68-ms-TE acquisitions (and the main barrier to clear and precise interpretation of results) is the co-editing of macromolecular signal.  It is also an easy (and fair) complaint of reviewers that observed effects may be driven by the MM signal. So maybe there is room for improvement.

There are two ways to address MM: acquire a metabolite-nulled scan (as done in the original Rothman and Mescher papers); or use Henry's symmetric editing scheme.  Metabolite nulling has two main issues, adding noise to the GABA signal and invovling the subtraction of two scans that cannot be acquired in an interleaved fashion. The elegant symmetric editing scheme is not widely applied at 3T because of constraints on the length,  and therefore selectivity, of editing pulses that can be shoe-horned into a 68 ms TE. 

The slice-selective PRESS scan on which MEGA-PRESS is based typically has a minimum TE of 30-35 ms, due to limited peak B1 (~ 14 uT), the need for shaped RF pulses with 'rectangular' spatial profile and the inclusion of gradient pulses. Based on such a sequence, it is typical to use editing pulses of duration 14 ms for GABA editing in a TE of 68 ms.  14 ms pulses are insufficiently selective for symmetric editing, and suppression of MM results in suppression of GABA signal as well.

However, simply by increasing the TE to 80 ms (and there is surprisingly little downside to doing so), 20 ms editing pulses can be accommodated which allow symmetric suppression of MM. Bob is, as they say, your mother's brother.  Unfortunately now we need to spend a couple of years establishing the reproducibility and robustness of the 80 ms sequence... harumph.

(For the record, I should say that some Siemens scanners can do PRESS at much shorter TEs, being capable of 20 uT body coil RF.  On these scanners there is more room for longer editing pulses in a 68 ms acquisition and all this is irrelevant.)

All this is explained better, and more fully, in our forthcoming MRM paper.  Watch this space

Simulations

Following an interesting chat at ISMRM, I was put onto NMRSCOPEB - a simulation package within jMRUI.  It is incredibly easy to install and start working with and after a half day of work, I have managed to modify their PRESS sequence to have editing pulses in it and produce the following...

Small steps, but a great new (to me) tool.

NMRSCOPEB outputs data in matlab format and works through simple code files and a great table-based GUI. It also has a bunch of spin-systems already saved (as per Govindaraju et al. for GABA). I am happy to share the sequence files with anyone interested.

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Less Glowing Update
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Following this up, I started trying to do simulations of spatial positions within the voxel (which is needed to get the right simulated spectrum.  There appears to be a bug in how NMRSCOPEB handles spatial information, so I would strongly recommend not using NMRSCOPEB for simulation of spectra for makeing basis sets etc, where spatial information is crucial. As a quick look-see is seems to be OK.