RESUMEN

Glucose is the primary fuel for the brain; its metabolism is linked with cerebral function. Different magnetic resonance spectroscopy (MRS) techniques are available to assess glucose metabolism, providing complementary information. Our first aim was to investigate the difference between hyperpolarized 13C-glucose MRS and non-hyperpolarized 2H-glucose MRS to interrogate cerebral glycolysis. Isoflurane anesthesia is commonly employed in preclinical MRS, but it affects cerebral hemodynamics and functional connectivity. A combination of low doses of isoflurane and medetomidine is routinely used in rodent fMRI and shows similar functional connectivity, as in awake animals. As glucose metabolism is tightly linked to neuronal activity, our second aim was to assess the impact of these two anesthetic conditions on the cerebral metabolism of glucose. Brain metabolism of hyperpolarized 13C-glucose and 2H-glucose was monitored in two groups of mice in a 9.4 T MRI system. We found that the very different duration and temporal resolution of the two techniques enable highlighting the different aspects in glucose metabolism. We demonstrate (by numerical simulations) that hyperpolarized 13C-glucose reports on de novo lactate synthesis and is sensitive to CMRGlc. We show that variations in cerebral glucose metabolism, under different anesthesia, are reflected differently in hyperpolarized and non-hyperpolarized X-nuclei glucose MRS.

RESUMEN

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.

Asunto(s)

RESUMEN

The high molecular weight and low concentration of brain glycogen render its noninvasive quantification challenging. Therefore, the precision increase of the quantification by localized (13) C MR at 9.4 to 14.1 T was investigated. Signal-to-noise ratio increased by 66%, slightly offset by a T(1) increase of 332 ± 15 to 521 ± 34 ms. Isotopic enrichment after long-term (13) C 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 T(1) 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.

Asunto(s)

RESUMEN

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. (15)N MRS is an alternative approach to (13)C MRS. Incorporation of labeled (15)N from ammonia in cerebral glutamine allows to measure several metabolic reactions related to nitrogen metabolism, including the glutamate-glutamine cycle. To measure (15)N incorporation into the position 5N of glutamine and position 2N of glutamate and glutamine, we developed a novel (15)N pulse sequence to simultaneously detect, for the first time, [5-(15)N]Gln and [2-(15)N]Gln+Glu in vivo in the rat brain. In addition, we also measured for the first time in the same experiment localized (1)H spectra for a direct measurement of the net glutamine accumulation. Mathematical modeling of (1)H and (15)N 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.

Asunto(s)

RESUMEN

Among numerous magnetic resonance imaging (MRI) techniques, perfusion MRI provides insight into the passage of blood through the brain's 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 9.4 T. 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 ± 26 mL/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. (1)H MRS and diffusion MRI etc, in studying murine brains.

Asunto(s)