Methodology of H NMR Spectroscopy of the Human Brain at Very High Magnetic Fields

An ultrashort-echo-time stimulated echo-acquisition mode (STEAM) pulse sequence with interleaved outer volume suppression and VAPOR (variable power and optimized relaxation delays) water suppression was redesigned and optimized for human applications at 4 and 7 T, taking into account the specific requirements for spectroscopy at high magnetic fields and limitations of currently available hardware. In combination with automatic shimming, automated parameter adjustments and data processing, this method provided a user-friendly tool for routine (1)H nuclear magnetic resonance (NMR) spectroscopy of the human brain at very high magnetic fields. Effects of first- and second-order shimming, single-scan averaging, frequency and phase corrections, and eddy currents were described. LCModel analysis of an in vivo (1)H NMR spectrum measured from the human brain at 7 T allowed reliable quantification of more than fifteen metabolites noninvasively, illustrating the potential of high-field NMR spectroscopy. Examples of spectroscopic studies performed at 4 and 7 T demonstrated the high reproducibility of acquired spectra quality

Field mapping without reference scan using asymmetric echo-planar techniques

Improvements in B(o) mapping and shimming were achieved by measuring the static field information in multiple subsequent echoes generated by an asymmetric echo-planar readout gradient train. With careful compensation, eddy current effects were shown to affect the adjustment of the shim coils minimally. In addition to reducing the time required for field mapping by two-fold, the sensitivity was simultaneously optimized irrespective of the prevalent T2/* present, thereby minimizing the error of the static field measurement to below 0.1 Hz. With adiabatic low flip-angle excitation, the time required for field mapping was below 1 second. (C) 2000 Wiley-Liss, Inc

Altered Neurocircuitry in the Dopamine Transporter Knockout Mouse Brain

The plasma membrane transporters for the monoamine neurotransmitters dopamine, serotonin, and norepinephrine modulate the dynamics of these monoamine neurotransmitters. Thus, activity of these transporters has significant consequences for monoamine activity throughout the brain and for a number of neurological and psychiatric disorders. Gene knockout (KO) mice that reduce or eliminate expression of each of these monoamine transporters have provided a wealth of new information about the function of these proteins at molecular, physiological and behavioral levels. In the present work we use the unique properties of magnetic resonance imaging (MRI) to probe the effects of altered dopaminergic dynamics on meso-scale neuronal circuitry and overall brain morphology, since changes at these levels of organization might help to account for some of the extensive pharmacological and behavioral differences observed in dopamine transporter (DAT) KO mice. Despite the smaller size of these animals, voxel-wise statistical comparison of high resolution structural MR images indicated little morphological change as a consequence of DAT KO. Likewise, proton magnetic resonance spectra recorded in the striatum indicated no significant changes in detectable metabolite concentrations between DAT KO and wild-type (WT) mice. In contrast, alterations in the circuitry from the prefrontal cortex to the mesocortical limbic system, an important brain component intimately tied to function of mesolimbic/mesocortical dopamine reward pathways, were revealed by manganese-enhanced MRI (MEMRI). Analysis of co-registered MEMRI images taken over the 26 hours after introduction of Mn^(2+) into the prefrontal cortex indicated that DAT KO mice have a truncated Mn^(2+) distribution within this circuitry with little accumulation beyond the thalamus or contralateral to the injection site. By contrast, WT littermates exhibit Mn^(2+) transport into more posterior midbrain nuclei and contralateral mesolimbic structures at 26 hr post-injection. Thus, DAT KO mice appear, at this level of anatomic resolution, to have preserved cortico-striatal-thalamic connectivity but diminished robustness of reward-modulating circuitry distal to the thalamus. This is in contradistinction to the state of this circuitry in serotonin transporter KO mice where we observed more robust connectivity in more posterior brain regions using methods identical to those employed here

Extracellular-intracellular distribution of glucose and lactate in the rat brain assessed noninvasively by diffusion-weighted 1H nuclear magnetic resonance spectroscopy in vivo

To determine the distribution of cerebral glucose and lactate between the intracellular and the extracellular space of the rat brain in vivo, the diffusion characteristic of glucose and lactate was compared with that of metabolites known to be mainly intracellular (N-acetylaspartate, choline, creatine, glutamate, myo-inositol, and taurine) using a pulsed-field-gradient 1H nuclear magnetic resonance technique. The detection of a glucose signal at large diffusion weighting provided direct experimental evidence of intracellular glucose in the rat brain. At large diffusion weighting, the apparent diffusion coefficient (ADC) of glucose and lactate was similar to that of the intracellular metabolites such as N-acetylaspartate, creatine, and glutamate. At small diffusion weighting, the ADC of glucose and lactate was increased, which was explained by a decreased relative contribution of intracellular glucose to the total signal. The calculated extracellular volume fraction of glucose (0.19 ± 0.05) and lactate (0.17 ± 0.06) was consistent with a substantial fraction of glucose and lactate signals being intracellular. The findings were direct in vivo evidence that the largest concentration gradient of glucose is at the blood-brain barrier and that glucose is evenly distributed in the brain in vivo between the intracellular and extracellular space

Noninvasive measurements of [1-13C]glycogen concentrations and metabolism in rat brain in vivo

Using a specific 13C NMR localization method, 13C label incorporation into the glycogen C1 resonance was measured while infusing [1- 13C]glucose in intact rats. The maximal concentration of [1-13C]glycogen was 5.1 ± 0.6 μmol g-1 (mean ± SE, n = 8). During the first 60 rain of acute hyperglycemia, the rate of 13C label incorporation (synthase flux) was 2.3 ± 0.7 μmol g-1 h-1 (mean ± SE, n = 9 rats), which was higher (p 0.05 for correlation). The results implied that net glycogen synthesis of ~3 μmol g-1 had occurred, similar to previous reports. When infusing unlabeled glucose before [1-13C]glucose in three studies, the rate of glycogen C1 accumulation was 0.46 ± 0.08 μmol g-1 h-1. The results suggest that steady-state glycogen turnover rates during hyperglycemia are ~1% of glucose consumption

A localization method for the measurement of fast relaxing 13C NMR signals in humans at high magnetic fields

Nuclear magnetic resonance (NMR) signals with short T1 and T2, such as the 13C signal of glycogen, are difficult to localize in three dimensions without major signal loss. A pulse sequence that accomplishes the spatial localization of 1H-decoupled 13C NMR signals on a whole-body scanner within the Food and Drug Administration guidelines for specific absorption rates was designed. The method uses an optimized three-dimensional outer volume suppression scheme combined with one-dimensional image-selected in vivo spectroscopy and surface coil detection. The localization performance of the sequence was validated at 4 T with double chambered phantoms and 13C magnetic resonance imaging. Localized 13C spectra were acquired from human brain and muscle. © Springer-Verlag 2005

Quantification of vitamin C in the rat brain in vivo using short echo-time 1H MRS

Both ultrashort echo-time STEAM and MEGA-PRESS-edited spectroscopy were used to validate noninvasive quantification of vitamin C (ascorbate) in the developing rat brain, where changes in ascorbate concentration have been reported. Despite strong overlap with resonances from glutamine, glutamate, glutathione, and macromolecules, reliable quantification of ascorbate (Cramer-Rao lower bounds < 0.2 μmol/g) by LC-Model analysis of STEAM (TE = 2 ms) spectra was possible at 9.4 T. Ascorbate concentrations quantified from the STEAEM spectra were in very good agreement with concentrations calculated from fully resolved ascorbate resonances in MEGA-PRESS-edited spectra measured from identical volumes of interest. Ascorbate concentrations measured using STEAM decreased with increasing postnatal rat age, in agreement with published brain ascorbate concentrations measured in vitro using high-performance liquid chromatography (HPLC). © 2006 Wiley-Liss, Inc

Oligodendrocytes Do Not Export NAA-Derived Aspartate In Vitro.

Oligodendroglial cells are known to de-acetylate the N-acetylaspartate (NAA) synthesized and released by neurons and use it for lipid synthesis. However, the role of NAA regarding their intermediary metabolism remains poorly understood. Two hypotheses were proposed regarding the fate of aspartate after being released by de-acetylation: (1) aspartate is metabolized in the mitochondria of oligodendrocyte lineage cells; (2) aspartate is released to the medium. We report here that aspartoacylase mRNA expression increases when primary rat oligodendrocyte progenitor cells (OPCs) differentiate into mature cells in culture. Moreover, characterising metabolic functions of acetyl coenzyme A and aspartate from NAA catabolism in mature oligodendrocyte cultures after 5 days using isotope-labelled glucose after 5-days of differentiation we found evidence of extensive NAA metabolism. Incubation with [1,6-13C]glucose followed by gas chromatography-mass spectrometry and high performance liquid chromatography analyses of cell extracts and media in the presence and absence of NAA established that the acetate moiety produced by hydrolysis of NAA does not enter mitochondrial metabolism in the form of acetyl coenzyme A. We also resolved the controversy concerning the possible release of aspartate to the medium: aspartate is not released to the medium by oligodendrocytes in amounts detectable by our methods. Therefore we propose that: aspartate released from NAA joins the cytosolic aspartate pool rapidly and takes part in the malate-aspartate shuttle, which transports reducing equivalents from glycolysis into the mitochondria for ATP production and enters the tricarboxylic acid cycle at a slow rate.This work was supported by grants from the UK Multiple Sclerosis Society and from Qatar Foundation. The work was further supported by core funding from the Wellcome Trust and MRC to the Wellcome Trust-Medical Research Council Cambridge Stem Cell Institute. The authors acknowledge the excellent technical support in GC-MS and HPLC analysis from Lars Evje (NTNU, Norway).This is the final version of the article. It first appeared from Springer at http://dx.doi.org/10.1007/s11064-016-1985-y

In vivo 1H NMR spectroscopy of the human brain at high magnetic fields: Metabolite quantification at 4T vs. 7T

A comprehensive comparative study of metabolite quantification from the human brain was performed on the same 10 subjects at 4T and 7T using MR scanners with identical consoles, the same type of RF coils, and identical pulse sequences and data analysis. Signal-to-noise ratio (SNR) was increased by a factor of 2 at 7T relative to 4T in a volume of interest selected in the occipital cortex using half-volume quadrature radio frequency (RF) coils. Spectral linewidth was increased by 50% at 7T, which resulted in a 14% increase in spectral resolution at 7T relative to 4T. Seventeen brain metabolites were reliably quantified at both field strengths. Metabolite quantification at 7T was less sensitive to reduced SNR than at 4T. The precision of metabolite quantification and detectability of weakly represented metabolites were substantially increased at 7T relative to 4T. Because of the increased spectral resolution at 7T, only one-half of the SNR of a 4T spectrum was required to obtain the same quantification precision. The Cramé r-Rao lower bounds (CRLB), a measure of quantification precision, of several metabolites were lower at both field strengths than the intersubject variation in metabolite concentrations, which resulted in a strong correlation between metabolite concentrations of individual subjects measured at 4T and 7T. © 2009 Wiley-Liss, Inc