Editing Glutathione

Over the last year, we have been expanding our horizons to editing things that aren't GABA.  Probably the next most popular editing target is glutathione (GSH), so I thought I'd post some thoughts about how to take a GABA scan and modify it to edit GSH.

Editing Frequencies

To address the most obvious change first, the editing-on frequency changes from 1.9 ppm for GABA to 4.56 ppm for GSH (based on shifts from Govindaraju et al.).  The editing-off frequency is less critical, but putting it at 8 ppm seems safe.

Echo time

The least obvious issue to resolve is echo time, which we recently wrote a paper about (Chan et al. 2017, the subject of a recent MRM piece). To cut a long story short, it doesn't make a lot of difference whether you stick with medium-TE (~70 ms) or go longer (~120 ms), although the longer TE is slightly better (and certainly expected to be better).  Longer TE also has secondary advantages, like accommodating longer more selective editing pulses or longer, higher-bandwidth refocusing pulses.  But beware, some editing implementations e.g. the Siemens WIP, don't move the editing pulses when you increase TE, so it is impossible to edit well at longer TEs.  


Another important consideration, if you are planning a GSH editing study, is that it is now possible to edit GABA and GSH in the same scan without substantially impacting SNR (Saleh et al. 2016).  So if you're planning to edit GSH, why not edit GABA too?


Gannet performs post-processing frequency-and-phase correction of data to minimize subtraction artefacts due to motion and scanner drift.  For simple GSH editing, the 'NAA' alignment works pretty well. ('SpecReg' fails due to editing pulses impacting water). For HERMES, we have a new Gannet version in progress.