Proton spectroscopy allows the simultaneous quantification of a high number of metabolite concentrations termed the neurochemical profile. The spin echo full intensity acquired localization (SPECIAL) scheme with an echo time of 2.7 ms was used at 9.4T for excitation of a slab parallel to a home-built quadrature surface coil in conjunction with phase encoding in the two remaining spatial dimensions to yield an effective spatial resolution of 1.7 L. The absolute concentrations of at least 10 metabolites were calculated from the spectra of individual voxels using LCModel analysis. The calculated concentrations were used for constructing quantitative metabolic maps of the neurochemical profile in normal and pathological rat brain. Summation of individual spectra was used to assess the neurochemical profile of unique brain regions, such as corpus callosum, in rat for the first time. Following focal ischemia in rat pups, imaging the neurochemical profile indicated increased choline groups in the ischemic core and increased glutamine in the penumbra, which is proposed to reflect glutamate excitotoxicity. We conclude that it is feasible to achieve a sensitivity that is sufficient for quantitative mapping of the neurochemical profile at microliter spatial resolution. In the past decade we and others have established that proton MR localized spectroscopy with echo times on the order of 2 ms allows measuring metabolite concentrations in the brain (1-4). Approximately 18 metabolite concentrations that can be measured at 9.4T results in a neurochemical profile (2) consisting of putative markers implicated in myelination (phosphoethanolamine [PE] [Gln]). Short echo time proton spectroscopy enables us to quantify compounds with spin systems including strongly and/or weakly Jcoupled multiplets as well as singlet resonances without the need for correcting effects of T 2 relaxation times. Most studies to date have used single-voxel localized spectroscopy to obtain such neurochemical profiles only from one selected volume at a time.On the other hand, spectroscopic imaging can be used to obtain this information simultaneously from many voxels, amounting to a regional mapping of the neurochemical profile. It is also likely to yield important insight in diseases with complex regional distribution of metabolites such as focal ischemia or multiple sclerosis. Most studies based on spectroscopic imaging techniques have used long echo times, thereby limiting the obtained concentrations to prominent singlets of NAA, total creatine (tCr), and choline-containing compounds (Cho), and in some cases to a few J-coupled resonances, such as Lac and glutamate ϩ glutamine (Glx) (5,6) due to the additional signal loss incurred by J-modulation of strongly coupled spin systems.A number of practical implementations of short-echotime spectroscopic imaging in brain have been reported. In human studies short-echo-time measurements were used either for obtaining localized spectra and metabolite concentrations from specific regions of the brain (7-14) or fo...
Glycine is an amino acid present in mammalian brain, where it acts as an inhibitory and excitatory neurotransmitter. The two detectable protons of glycine give rise to a singlet at 3.55 ppm that overlaps with the more intense myo-inositol resonances, and its measurement has traditionally required specific editing efforts. The aim of the current study was to reduce the signal intensity of myo-inositol relative to that of glycine by exploiting the fast signal J-evolution of the myo-inositol spin system when using a single spin-echo localization method we recently introduced. Glycine was detected at TE ؍ 20 ms with an average Cramé r-Rao lower bound (CRLB) of 8.6% ؎ 1.5% in rat brain (N ؍ 5), at 9.4 T. The concentration of glycine was determined using LCModel analysis at 1.1 ؎ 0.1 mM, in good agreement with biochemical measurements previously reported. We conclude that at high magnetic fields, glycine can be measured at a relatively short echo time ( Key words: glycine; strongly coupled spin system; NMR spectroscopy; density matrix simulations; rat brain Glycine is an amino acid present in mammalian brain, where it acts as an inhibitory and excitatory neurotransmitter (1). Because of its critical role in N-methyl-D-aspartic acid (NMDA) transmission, altered levels of glycine are involved in a number of pathologies. In particular, elevated levels of glycine have been observed in the brain tissues of hyperglycinemia patients and in tumors (2,3). In a number of studies, glycine has been administered to schizophrenia patients to improve the NMDA receptor function, with the result of a significant decrease in negative and cognitive symptoms (4,5). Given its importance in mediating the action of the major excitatory neurotransmitter glutamate, a direct measurement of brain glycine concentration is desirable. The noninvasive measurement of glycine in brain is hampered by the fact that its twoproton singlet resonance at 3.55 ppm overlaps with the resonances of myo-inositol, which is present in the brain at much higher concentrations. Not surprisingly, glycine detection has only been reported in the human brain, using long TE when glycine was elevated or using additional editing techniques, such as TE-averaged point-resolved spectroscopy (PRESS) or 2D J-PRESS (6,7). Despite the fact that animal studies are typically performed at high field strengths (Ն7 T), where the substantial increase in SNR and spectral resolution could allow glycine detection without editing or TE-averaging, we are not aware of any such report to date.At very short TE (ϳ1-2 ms), multiplet resonances from coupled spin systems display virtually no dephasing induced by J-modulation. As TE increases, a loss of signal intensity occurs due to J-modulation. In particular, when using a single spin echo coherence generation at high field, myo-inositol undergoes rapid J-modulation due to its large J-coupling of ϳ10 Hz. In this study we sought to reduce the signal intensity of myo-inositol relative to that of glycine by exploiting its J-evolution when using a...
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