An improved double quantum coherence (DQC) filter for the selective in vivo detection of glutathione (GSH) in the human brain at 1.5 Tesla is presented. The goal was to minimize contamination of the DQC-filtered GSH signal at 2.9 ppm with contributions arising from GABA. The modification consists of tailoring the frequency response of the read pulse, which converts DQC into anti-phase single quantum coherence in such a way that the GABA  and ␥ resonances at 3.0 and 1.9 ppm, respectively, remain unaffected. The feasibility of selective in vivo detection of L-␥-glutamyl-L-cysteinyl-glycine (GSH) in the human brain at 1.5 T by means of double quantum coherence (DQC) filtering in combination with point-resolved spectroscopy (PRESS) volume selection was demonstrated recently (1). The target of the filter is the cysteinyl compound of GSH, which forms an ABX spin system. The focus is on the strongly coupled methylene group (AB part) with its nearly equivalent -protons resonating at 2.87 and 2.94 ppm, respectively. Neighbors in the spectral region of the cysteinyl AB resonances are the creatine methyl singlet at 3.03 ppm, the ␥-aminobutyric acid (GABA) ␥-methylene triplet at 3.01 ppm and the aspartate  b multiplet at 2.82 ppm.The DQC filter sequence incorporates the filtering part into the PRESS sequence (2), which enables spatial localization of the detected signal. The filtering section consists of two nonslice-selective ( /2)-pulses (c.f. Fig. 2
in Ref. 1).The first one creates DQC and after phase encoding by means of the filtering gradient G 1 , the second ( /2)-pulse converts DQC back into single-quantum antiphase magnetization. After being decoded by filtering gradient G 2 , the latter becomes observable during acquisition.If the second pulse of the filtering ( /2)pulses is made frequency-selective such that it only affects the X-part of cysteinyl, but not its AB part, the conversion of DQC of the form 2A ϩ1 X ϩ1 , 2B ϩ1 X ϩ1 , 4A ϩ1 B 0 X ϩ1 and 4A 0 B ϩ1 X ϩ1 becomes twice as effective (whereas DQC of the form 2A ϩ1 B ϩ1 is not converted into single-quantum antiphase magnetization). This results in a signal increase of 66 % (1). In the first implementation of the sequence (1), the task of the frequency selective pulse was fulfilled by an 11-binomial pulse. It consists of a ( /4) and a (-/4) block pulse that are spaced by t s ϭ 1/2⌬ , with ⌬ ϭ X -AB being the chemical shift difference (in Hz) between the X and the AB part of cysteinyl and the carrier frequency of the pulses being set to AB . The frequency response of this pulse is periodic, with a maximum at X and a zero passage at AB .As demonstrated, the double-quantum coherence filtering technique provides excellent discrimination between the cysteinyl compound of GSH and creatine (1). Signal from the strongly coupled ABX-spin system of aspartate leaks through the filter but may be separated from the GSH contribution due to the difference in chemical shift. However, contamination of the DQC-filtered GSH signal arises from GABA, as the GABA -protons (reson...