Yves Pilloud

Cerebral glutamine metabolism under hyperammonemia determined in vivo by localized 1H and 15N NMR spectroscopy

Brain glutamine synthetase (GS) is an integral part of the glutamate—glutamine cycle and occurs in the glial compartment. In vivo Magnetic Resonance Spectroscopy (MRS) allows noninvasive measurements of the concentrations and synthesis rates of metabolites. 15N MRS is an alternative approach to 13C MRS. Incorporation of labeled 15N from ammonia in cerebral glutamine allows to measure several metabolic reactions related to nitrogen metabolism, including the glutamate—glutamine cycle. To measure 15N incorporation into the position 5N of glutamine and position 2N of glutamate and glutamine, we developed a novel 15N pulse sequence to simultaneously detect, for the first time, [5-15N]Gln and [2-15N]Gln + Glu in vivo in the rat brain. In addition, we also measured for the first time in the same experiment localized 1H spectra for a direct measurement of the net glutamine accumulation. Mathematical modeling of 1H and 15N MRS data allowed to reduce the number of assumptions and provided reliable determination of GS (0.30 ± 0.050 μmol/g per minute), apparent neurotransmission (0.26 ± 0.030 μmol/g per minute), glutamate dehydrogenase (0.029 ± 0.002 μmol/g per minute), and net glutamine accumulation (0.033 ± 0.001 μmol/g per minute). These results showed an increase of GS and net glutamine accumulation under hyperammonemia, supporting the concept of their implication in cerebral ammonia detoxification.

Fluorine-19 magnetic resonance angiography of the mouse.

PURPOSE To implement and characterize a fluorine-19 ((19)F) magnetic resonance imaging (MRI) technique and to test the hypothesis that the (19)F MRI signal in steady state after intravenous injection of a perfluoro-15-crown-5 ether (PCE) emulsion may be exploited for angiography in a pre-clinical in vivo animal study. MATERIALS AND METHODS In vitro at 9.4T, the detection limit of the PCE emulsion at a scan time of 10 min/slice was determined, after which the T(1) and T(2) of PCE in venous blood were measured. Permission from the local animal use committee was obtained for all animal experiments. 12 µl/g of PCE emulsion was intravenously injected in 11 mice. Gradient echo (1)H and (19)F images were obtained at identical anatomical levels. Signal-to-noise (SNR) and contrast-to-noise (CNR) ratios were determined for 33 vessels in both the (19)F and (1)H images, which was followed by vessel tracking to determine the vessel conspicuity for both modalities. RESULTS In vitro, the detection limit was ∼400 µM, while the (19)F T(1) and T(2) were 1350±40 and 25±2 ms. The (19)F MR angiograms selectively visualized the vasculature (and the liver parenchyma over time) while precisely coregistering with the (1)H images. Due to the lower SNR of (19)F compared to (1)H (17±8 vs. 83±49, p<0.001), the (19)F CNR was also lower at 15±8 vs. 52±35 (p<0.001). Vessel tracking demonstrated a significantly higher vessel sharpness in the (19)F images (66±11 vs. 56±12, p = 0.002). CONCLUSION (19)F magnetic resonance angiography of intravenously administered perfluorocarbon emulsions is feasible for a selective and exclusive visualization of the vasculature in vivo.

A comparison of in vivo 13C MR brain glycogen quantification at 9.4 and 14.1 T

The high molecular weight and low concentration of brain glycogen render its noninvasive quantification challenging. Therefore, the precision increase of the quantification by localized 13C MR at 9.4 to 14.1 T was investigated. Signal‐to‐noise ratio increased by 66%, slightly offset by a T1 increase of 332 ± 15 to 521 ± 34 ms. Isotopic enrichment after long‐term 13C administration was comparable (∼40%) as was the nominal linewidth of glycogen C1 (∼50 Hz). Among the factors that contributed to the 66% observed increase in signal‐to‐noise ratio, the T1 relaxation time impacted the effective signal‐to‐noise ratio by only 10% at a repetition time = 1 s. The signal‐to‐noise ratio increase together with the larger spectral dispersion at 14.1 T resulted in a better defined baseline, which allowed for more accurate fitting. Quantified glycogen concentrations were 5.8 ± 0.9 mM at 9.4 T and 6.0 ± 0.4 mM at 14.1 T; the decreased standard deviation demonstrates the compounded effect of increased magnetization and improved baseline on the precision of glycogen quantification. Magn Reson Med, 2011.

Continuous arterial spin labeling of mouse cerebral blood flow using an actively-detuned two-coil system at 9.4T

Among numerous magnetic resonance imaging (MRI) techniques, perfusion MRI provides insight into the passage of blood through the brains vascular network non-invasively. Studying disease models and transgenic mice would intrinsically help understanding the underlying brain functions, cerebrovascular disease and brain disorders. This study evaluates the feasibility of performing continuous arterial spin labeling (CASL) on all cranial arteries for mapping murine cerebral blood flow at 9.4T. We showed that with an active-detuned two-coil system, a labeling efficiency of 0.82±0.03 was achieved with minimal magnetization transfer residuals in brain. The resulting cerebral blood flow of healthy mouse was 99±26mL/100g/min, in excellent agreement with other techniques. In conclusion, high magnetic fields deliver high sensitivity and allowing not only CASL but also other MR techniques, i.e. 1H MRS and diffusion MRI etc, in studying murine brains.