Flexible MEGA editing scheme with asymmetric adiabatic pulses applied for T2 measurement of lactate in human brain

Abstract

Purpose: A flexible MEGA editing scheme which decouples the editing efficiency from TE is proposed and the utility of asymmetric adiabatic pulses for this new technique is explored. It is demonstrated that the method enables robust $T_2$ measurement of lactate in healthy human brain.

Methods: The proposed variation of the MEGA scheme applies editing pulses in both acquired spectra, ensuring that the difference in J-evolution of the target resonance leads to maximal signal yield in the difference spectrum for arbitrary TE. A MEGA-sLASER sequence is augmented with asymmetric adiabatic editing pulses for enhanced flexibility and immunity to $B_1^{+}$ miscalibration and inhomogeneities. The technique is validated and optimized for flexible lactate editing via a simple analytical model, numerical simulations and in vitro experiments. The $T_2$ relaxation constant of lactate is determined in vivo via multiple-TE measurements with the proposed method and a dedicated postprocessing and quantification approach.

Results: Asymmetric adiabatic editing pulses improve robustness and facilitate efficient J-editing in sequences or protocols with strong timing constraints. Single voxel measurements using the proposed MEGA scheme in the occipital cortex of six healthy subjects yield a relaxation constant of $T_2=171\pm 19$ ms for the methyl resonance of lactate at a field strength of 3T.

Conclusions: The proposed MEGA editing scheme allows for novel kinds of J-editing experiments and promises to be an asset to robust $T_2$ measurement of lactate and potentially other J-coupled metabolites in vivo.

Keywords: ${\mathrm{T}}_2$; J-editing; MEGA; MRS; adiabatic editing; human brain; lactate.