Quantitative proton spectroscopic imaging of the neurochemical profile in rat brain with microliter resolution at ultra‐short echo times

Abstract

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...

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