Cross-vendor harmonisation: alpha and kappa

One aim we have to overcome differences in GABA concentration measurements across vendor.  Currently there are two main ways to quantify GABA signal:

1. As an integral ratio to Creatine in the 'off' spectrum.
2. As a concentration in institutional units (i.u.), relative to the unsuppressed water signal.

The main advantage of method 2 is that differences between scanners (mentioned below) can be accommodated to give values that can be compared across studies and across scanners.

The concentration c_G of GABA in i.u. can be expressed as:

where subscript G and w refer to water and GABA and where factors R account for T1 and T2 relaxation, kappa is the editing efficiency of GABA signal (typically around 50%) and alpha is the signal fraction of macromolecular signal.  In this framework, only two things change specifically by vendor* - the editing efficiency and the MM signal fraction and appropriate correction factors should result in cross-vendor comparable i.u. values.

The editing efficiency will depend on multiple factors including the slice-selective refocusing pulse spatial profile and bandwidth.  Finite bandwidth refocusing leads to a loss of edited signal efficiency related to the chemical-shift displacement (see e.g. our 2007 paper).

The MM signal fraction will depend on editing pulse selectivity. Although editing pulses are placed at 1.9 ppm in the GABA-editing experiment, they partially invert MM spins at 1.77 ppm, which are coupled to MM signals at 3 ppm, leading to a significant MM contribution to the edited signal.  The degree of this inversion at 1.7 ppm will depend on the inversion pulse shape and length, with length largely being limited by the ~70 ms echo time preferred for edited experiments and the B1.

So here is the plan - measure kappa for each MEGA-PRESS implementation and somehow get a handle on differences in alpha.  



* ..but we mustn't forget that the R terms vary by TE and TR which may vary site-to-site.