106 research outputs found
Measurement of brain lactate during visual stimulation using a long TE semiâLASER sequence at 7 T
Estimation of metabolic changes during neuronal activation represents a challenge for in vivo MRS, especially for metabolites with low concentration and signal overlap, such as lactate. In this work, we aimed to evaluate the feasibility of detecting lactate during brain activation using a long urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0001 (144 ms) semiâLASER sequence at 7 T. urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0002 spectra were acquired on healthy volunteers ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0003) during a paradigm with 15 min of visual stimulation. Outerâvolume signals were further attenuated by the use of saturation slabs, and macromolecular signals in the vicinity of the inverted lactate peak were individually fitted with simulated Lorentzian peaks. All spectra were free of artefacts and highly reproducible across subjects. Lactate was accurately quantified with an average CramĂ©râRao lower bound of 8%. Statistically significant ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0004, oneâtailed urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0005âtest) increases in lactate ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-000610%) and glutamate ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-00073%) levels during stimulation were detected in the visual cortex. Lactate and glutamate changes were consistent with previous measurements. We demonstrated that quantification of a clear and nonâcontaminated lactate peak obtained with a long TE sequence has the potential of improving the accuracy of functional MRS studies targeting nonâoxidative reaction pathways
Diffusion-weighted SPECIAL improves the detection of J-coupled metabolites at ultra-high magnetic field
A new sequence for single-voxel diffusion-weighted 1H MRS (DWS), named
DW-SPECIAL, is proposed to improve the detection and subsequent estimation of
the diffusion properties of strongly J-coupled metabolites. It combines the
semi-adiabatic SPECIAL sequence with a stimulated echo (STE) diffusion block.
Acquisitions with DW-SPECIAL and STE-LASER, the current gold-standard for
rodent DWS experiments at high fields, were performed at 14.1T on phantoms and
in vivo on the rat brain. The apparent diffusion coefficient and intra-stick
diffusivity (Callaghan's model) were fitted and compared between the sequences
for glutamate, glutamine (Gln), myo-inositol, taurine, total N-acetylaspartate,
total choline, total creatine and the macromolecules. The shorter echo time
achieved with DW-SPECIAL (18 ms against 33 ms with STE-LASER) substantially
limited the metabolites' signal loss caused by J-evolution. In addition,
DW-SPECIAL preserved the main advantages of STE-LASER: absence of cross-terms,
diffusion time during a STE and limited sensitivity to B1 inhomogeneities. In
vivo, compared to STE-LASER, DW-SPECIAL yielded the same spectral quality and
reduced the Cramer Rao Lower Bounds (CRLB) for J-coupled metabolites,
irrespective of the b-value. DW-SPECIAL also reduced the standard deviation of
the metabolites' diffusion estimates based on individual animal fitting without
loss of accuracy compared to the fit on the averaged decay. We conclude that
due to its reduced echo time, DW-SPECIAL can serve as an alternative to
STE-LASER when strongly J-coupled metabolites like Gln are investigated,
thereby extending the range of accessible metabolites in the context of DWS
acquisitions.Comment: Submitted to Magnetic Resonance in Medecin
Noise-reduction techniques for 1H-FID-MRSI at 14.1T: Monte-Carlo validation & in vivo application
Proton magnetic resonance spectroscopic imaging (1H-MRSI) is a powerful tool
that enables the multidimensional non-invasive mapping of the neurochemical
profile at high-resolution over the entire brain. The constant demand for
higher spatial resolution in 1H-MRSI led to increased interest in
post-processing-based denoising methods aimed at reducing noise variance. The
aim of the present study was to implement two noise-reduction techniques, the
Marchenko-Pastur principal component analysis (MP-PCA) based denoising and the
low-rank total generalized variation (LR-TGV) reconstruction, and to test their
potential and impact on preclinical 14.1T fast in vivo 1H-FID-MRSI datasets.
Since there is no known ground truth for in vivo metabolite maps, additional
evaluations of the performance of both noise-reduction strategies were
conducted using Monte-Carlo simulations. Results showed that both denoising
techniques increased the apparent signal-to-noise ratio SNR while preserving
noise properties in each spectrum for both in vivo and Monte-Carlo datasets.
Relative metabolite concentrations were not significantly altered by either
methods and brain regional differences were preserved in both synthetic and in
vivo datasets. Increased precision of metabolite estimates was observed for the
two methods, with inconsistencies noted on lower concentrated metabolites. Our
study provided a framework on how to evaluate the performance of MP-PCA and
LR-TGV methods for preclinical 1H-FID MRSI data at 14.1T. While gains in
apparent SNR and precision were observed, concentration estimations ought to be
treated with care especially for low-concentrated metabolites.Comment: Brayan Alves and Dunja Simicic are joint first authors. Currently in
revision for NMR in Biomedicin
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Hyperpolarized 13C-glucose magnetic resonance highlights reduced aerobic glycolysis in vivo in infiltrative glioblastoma.
Glioblastoma (GBM) is the most aggressive brain tumor type in adults. GBM is heterogeneous, with a compact core lesion surrounded by an invasive tumor front. This front is highly relevant for tumor recurrence but is generally non-detectable using standard imaging techniques. Recent studies demonstrated distinct metabolic profiles of the invasive phenotype in GBM. Magnetic resonance (MR) of hyperpolarized 13C-labeled probes is a rapidly advancing field that provides real-time metabolic information. Here, we applied hyperpolarized 13C-glucose MR to mouse GBM models. Compared to controls, the amount of lactate produced from hyperpolarized glucose was higher in the compact GBM model, consistent with the accepted "Warburg effect". However, the opposite response was observed in models reflecting the invasive zone, with less lactate produced than in controls, implying a reduction in aerobic glycolysis. These striking differences could be used to map the metabolic heterogeneity in GBM and to visualize the infiltrative front of GBM
The Gaia satellite: a tool for Emission Line Stars and Hot Stars
The Gaia satellite will be launched at the end of 2011. It will observe at
least 1 billion stars, and among them several million emission line stars and
hot stars. Gaia will provide parallaxes for each star and spectra for stars
till V magnitude equal to 17. After a general description of Gaia, we present
the codes and methods, which are currently developed by our team. They will
provide automatically the astrophysical parameters and spectral classification
for the hot and emission line stars in the Milky Way and other close Local
Group galaxies such as the Magellanic Clouds.Comment: SF2A2008, session GAIA, invited tal
Hyperpolarized <sup>13</sup>C Magnetic Resonance Spectroscopy Reveals the Rate-Limiting Role of the Blood-Brain Barrier in the Cerebral Uptake and Metabolism of l-Lactate in Vivo.
The dynamics of l-lactate transport across the blood-brain barrier (BBB) and its cerebral metabolism are still subject to debate. We studied lactate uptake and intracellular metabolism in the mouse brain using hyperpolarized <sup>13</sup> C magnetic resonance spectroscopy (MRS). Following the intravenous injection of hyperpolarized [1- <sup>13</sup> C]lactate, we observed that the distribution of the <sup>13</sup> C label between lactate and pyruvate, which has been shown to be representative of their pool size ratio, is different in NMRI and C57BL/6 mice, the latter exhibiting a higher level of cerebral lactate dehydrogenase A ( Ldha) expression. On the basis of this observation, and an additional set of experiments showing that the cerebral conversion of [1- <sup>13</sup> C]lactate to [1- <sup>13</sup> C]pyruvate increases after exposing the brain to ultrasound irradiation that reversibly opens the BBB, we concluded that lactate transport is rate-limited by the BBB, with a 30% increase in lactate uptake after its disruption. It was also deduced from these results that hyperpolarized <sup>13</sup> C MRS can be used to detect a variation in cerebral lactate uptake of <40 nmol in a healthy brain during an in vivo experiment lasting only 75 s, opening new opportunities to study the role of lactate in brain metabolism
Fast high-resolution metabolite mapping in the rat brain using 1H-FID-MRSI at 14.1T
Magnetic resonance spectroscopic imaging (MRSI) enables the simultaneous
non-invasive acquisition of MR spectra from multiple spatial locations inside
the brain. While 1H-MRSI is increasingly used in the human brain, it is not yet
widely applied in the preclinical settings, mostly because of difficulties
specifically related to very small nominal voxel size in the rodent brain and
low concentration of brain metabolites, resulting in low signal-to-noise ratio
SNR.
In this context, we implemented a free induction decay 1H-MRSI sequence
(1H-FID-MRSI) in the rat brain at 14.1T. We combined the advantages of
1H-FID-MRSI with the ultra-high magnetic field to achieve higher SNR, coverage
and spatial resolution in the rodent brain, and developed a custom dedicated
processing pipeline with a graphical user interface: MRS4Brain toolbox.
LCModel fit, using the simulated metabolite basis-set and in-vivo measured
MM, provided reliable fits for the data at acquisition delays of 1.3 and 0.94
ms. The resulting Cram\'er-Rao lower bounds were sufficiently low (<40%) for
eight metabolites of interest, leading to highly reproducible metabolic maps.
Similar spectral quality and metabolic maps were obtained between 1 and 2
averages, with slightly better contrast and brain coverage due to increased SNR
in the latter case. Furthermore, the obtained metabolic maps were accurate
enough to confirm the previously known brain regional distribution of some
metabolites. The acquisitions proved high repeatability over time.
We demonstrated that the increased SNR and spectral resolution at 14.1T can
be translated into high spatial resolution in 1H-FID-MRSI of the rat brain in
13 minutes, using the sequence and processing pipeline described herein.
High-resolution 1H-FID-MRSI at 14.1T provided reproducible and high-quality
metabolic mapping of brain metabolites with significantly reduced technical
limitations.Comment: Dunja Simicic and Brayan Alves are joint first author
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