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Artefact minimized spectral editing at 7 T : quick and accurate in-vivo detection of GABA

Introduction: Spectral editing techniques are used in MRS to improve the detection of low concentrated metabolites like GABA [1], of which signals are overlapped with resonances of other highly concentrated compounds, such as Creatine and macromolecules. However, these methods rely on subtraction of two in vivo MR spectra and, therefore, are prone to artefacts. MRS at high field benefits from higher SNR and chemical shift dispersion, improving spectral editing comparing to low field. On the other hand, due to increasing chemical shift displacement error (CSDE) at high field, accuracy in voxel localization is hampered. Because of this, the observed signal in localized editing techniques is originating from four compartments that apart from misalignment of the voxel, reduces the editing efficiency substantially. Recently, a semi-LASER localization sequence was shown to enable H-MRS of the human brain at 7T with minimal CSDE [2]. This sequence can be used for spectral editing, however, its four adiabatic inversion pulses limit the duration of the editing pulses within fixed echo time. Fortunately, increased chemical shift dispersion at high fields allows the use of selective editing pulses with shorter duration than at lower fields. Here, we implemented and performed an accurate and efficient spectral editing of the 3 ppm GABA resonance in-vivo in the human brain at 7T, utilizing two editing pulses in a semi-LASER localization sequence. High accuracy of this editing technique enabled an estimation of T2 relaxation time of the 3 ppm GABA resonance based on two edited MR spectra obtained at TE of 74 and 222 ms. Methods: Two double-banded inversion MEGA pulses (10 ms) were created with MatPulse (Matlab) (Fig. 1A). The left inversion band is intended for water suppression and the right one for either 1) decoupling of 3.0 and 1.9 ppm GABA resonances (MEGA1) or 2) minimization of the co-edited macromolecule resonance by affecting the resonance at 1.7 ppm in the same manner (MEGA2) [3]. Semi-LASER voxel localization was used with four 5 ms adiabatic refocusing pulses at an echo time of 74 ms, which is the optimal TE for the 3ppm GABA resonance in this sequence (based on the simulations) (Fig. 1B). The switch between MEGA1 and MEGA2 pulses was performed in an interleaved manner to minimize artefacts due to phase disturbances between every acquisition. In vivo measurements were performed with a 7T MR scanner interfaced to a volume transmit head coil and an integrated 16 channel receiver array. GABA editing with the MEGA-sLASER technique was performed in a phantom (10 mM GABA) with three different echo times to demonstrate that a small extension of echo time has a negligible effect (less than 2% signal loss) on the edited MR spectra as predicted from sin(π·TE·J) time dependence of the antiphase term 4IxS1zS2z (Fig. 2A), (TR/TE = 4100/72,74,78 ms, NSA 1, voxel 20/20/20 mm ). In the in-vivo measurement MEGA-