ABSTRACTThe need to quantify brain glutathione (GSH) accurately by J‐difference spectroscopy has stimulated assessment of the TE effects on GSH edited signals at the popular field strength 3 T. We performed multiple‐TE J‐difference MRS at two sites to evaluate the GSH T2 relaxation and TE dependence of the GSH signal resolution. Two 10‐ms spectrally selective Gaussian editing RF pulses were implemented in 3 T MEGA‐PRESS sequences at two sites having different vendors. The sequences were optimized, with numerical and phantom analyses, for editing of the GSH 2.95 ppm resonance. The timings of the editing pulses within the sequences were tailored for high‐amplitude GSH signal production for a TE range of 58–160 ms. In vivo human brain data were collected at five TEs (58, 70, 88, 116, and 150 ms) from five subjects at each site. Following LCModel analysis of difference and edit‐off spectra independently between the sites, metabolite T2 values were estimated with mono‐exponential regression of the signal estimates. Simulations and phantom data indicated that the MEGA‐edited GSH peak amplitude was progressively larger with increasing TE up to 125–140 ms and the maximum amplitude was 2‐ to 2.5‐fold greater than the amplitude at TE of 58 ms. For in vivo data, the edited GSH peak was the largest at TE of 88 ms among the five TEs. Brain GSH T2 was measured as 88 ± 11 ms from 10 subjects, with no significant difference between the sites. The LCModel‐returned correlation coefficient between GSH and co‐edited N‐acetylaspartate (NAA) multiplet was significantly smaller at short TEs than at long TEs. Our data suggest that MEGA‐edited GSH signal undergoes extensive attenuation with increasing TE due to the fast T2 relaxation, and the edited GSH signal can be well resolved at short TEs with small interferences from adjacent co‐edited NAA multiplet.

PURPOSE:Application of highly selective editing RF pulses provides a means of minimizing co-editing of contaminants in J-difference MRS (MEGA), but it causes reduction in editing yield. We examined the flip angles (FAs) of narrow-band editing pulses to maximize the lactate edited signal with minimal co-editing of threonine. METHODS:The effect of editing-pulse FA on the editing performance was examined, with numerical and phantom analyses, for bandwidths of 17.6-300 Hz in MEGA-PRESS editing of lactate at 3T. The FA and envelope of 46 ms Gaussian editing pulses were tailored to maximize the lactate edited signal at 1.3 ppm and minimize co-editing of threonine. The optimized editing-pulse FA MEGA scheme was tested in brain tumor patients. RESULTS:Simulation and phantom data indicated that the optimum FA of MEGA editing pulses is progressively larger than 180° as the editing-pulse bandwidth decreases. For 46 ms long 17.6 Hz bandwidth Gaussian pulses and other given sequence parameters, the lactate edited signal was maximum at the first and second editing-pulse FAs of 241° and 249°, respectively. The edit-on and difference-edited lactate peak areas of the optimized FA MEGA were greater by 43% and 25% compared to the 180°-FA MEGA, respectively. In-vivo data confirmed the simulation and phantom results. The lesions of the brain tumor patients showed elevated lactate and physiological levels of threonine. CONCLUSION:The lactate MEGA editing yield is significantly increased with editing-pulse FA much larger than 180° when the editing-pulse bandwidth is comparable to the lactate quartet frequency width.

Sparse-spokes pulses are 2D slice-selective pulses that effectively mitigate inhomogeneities in the transmitted RF field and reduce unwanted RF artifacts in MR images. Here we consider the practical design of such pulses for high-field MRI and demonstrate limitations of the technique. We analyze the performance of pulses considering input noise as well as other effects such as saturation and T2∗ relaxation. We discuss in detail the correspondence between the reduction of RF inhomogeneities and the fidelity of the input parameters, such as the transmit B1+ field map and combined phase of the main B0 field and eddy-currents. Results include simulations, utilizing 7T field maps acquired in phantoms and in-vivo, as well as in-vivo experiments. The necessary performance of system hardware components to achieve significant improvements is described.