Advanced metabolite mapping at ultra-high field using 1H-MRS, 1H-MRSI and macromolecules: applications in a rat model of type C hepatic encephalopathy

Magnetic Resonance Spectroscopy (MRS) is the only technique capable of measuring a large number of metabolites simultaneously in vivo. Ultra-high magnetic fields (UHF) combined with ultra-short echo time (TE) sequences allow the detection of high-quality 1H MR spectra and the quantification of 20 different metabolites in the brain (markers of energy metabolism, osmoregulation etc.).In vivo brain localized 1H MR spectra at short TEs contain the contribution of mobile macromolecules (MM). Reliable detection and fitting of MM are crucial for accurate quantification. Higher spectral resolution at UHF led to increased interest in using a parametrized MM spectrum and flexible spline baselines to address unpredicted spectroscopic components.In this thesis the MM spectra (from the rat brain at 9.4T) were characterized using an improved methodological approach for their post-processing, fitting and quantification. This method provided an efficient tool for parametrization of the MM spectrum into individual components and estimation of their T2app relaxation times. An extensive assessment on how the MM spectrum and spline baseline stiffness affect the metabolite and MM quantification is also reported Type C hepatic encephalopathy (HE) is a complication of chronic liver disease (CLD). Children and adults respond differently to CLD and its related toxic accumulation of molecules (i.e. ammonium (NH4+), glutamine (Gln)). Children with CLD may grow up with significant neurocognitive deficits even after liver transplantation. Despite considerable advances in understanding the pathogenesis of type C HE, the exact metabolic mechanisms and their regional variations are not fully understood.The advantages of UHF short TE 1H MRS were used herein to describe the regional distribution of metabolites in the developing and adult brain using the bile duct ligated model (BDL) of type C HE (adult and postnatal day 21 rats). Three brain regions were assessed (hippocampus, cerebellum and striatum) pointing towards cerebellum as a region with the heaviest burden of Gln and unique metabolic response. Changes in cell morphology were followed longitudinally and related to the metabolic alterations. Elevated oxidative stress is reported using electron paramagnetic resonance, together with the decreased antioxidants (1H MRS) emphasizing its important role in HE. The brain regional measurements confirmed the higher susceptibility of developing brain to the disease and the increased vulnerability of cerebellum. Finally, the beneficial effect of Cr supplementation on the neurometabolic profile is described using 1H MRS and 31P MRS in CLD pups (BDL at postnatal day 15) suggesting that an appropriate treatment may have significant public health impact.MRSI is a powerful tool to non-invasively and spatially map the brain regional distribution of metabolites in vivo. While MRSI in human brain is increasingly used, preclinical MRSI is not widely applied mainly due to the small rodent brain, long acquisition times and low signal to noise ratio. The implementation of a novel approach: free induction decay (FID) MRSI on the 14.1T preclinical scanner is described herein. This method offers a fast and robust data acquisition with high spatial resolution resulting in high quality spectroscopic maps. Finally, preliminary assessment of the effect of two noise reduction techniques (MP-PCA and TGV reconstruction) on the spectra from preclinical MRSI datasets is briefly presented.

Brain metabolism during Chronic Hepatic Encephalopathy studied by in vivo 1H and 31P MRS

Veronika Rackayová

Magnetic resonance spectroscopy (MRS) is a powerful tool increasingly used in biomedical research and also in clinical practice. This thesis focused on the improvements of in vivo 31P MRS and 31P magnetization transfer methods at high magnetic field (9.4T). Then, in vivo 31P MRS was combined with 1H MRS to longitudinally study neurometabolic changes during chronic hepatic encephalopathy (CHE) in adult and developing brain using a well-recognized animal model of cholestatic chronic liver disease and HE type C â bile duct ligated (BDL) rats. CHE is neuro-psychiatric disorder caused by chronic liver disease (CLD), with not well-understood molecular mechanisms, inducing important and sometimes lethal brain changes. In a next step, potential protective treatments (probiotics and high creatine diet) were tested. To the best of our knowledge such a detailed approach was not yet published. Firstly, we showed by in vivo 1H MRS in hippocampus that adult BDL rats suffered from increase in brain glutamine, decrease in metabolites involved in osmoregulation including creatine, decrease in neurotransmitters and ascorbate and we were able to show the importance of early metabolic changes compared to late changes thanks to longitudinal measurements. 31P MRS study in larger brain volume found only mild perturbation of energy metabolism such as non-significant trend in ATP decrease in adult BDL rats. Secondly, studies in young BDL rats at two developmental stages by 1H MRS in the hippocampus revealed similar changes that the one observed in adult brain during CHE. However almost all the neurometabolic changes were more profound or appeared earlier in the evolution of the disease in young BDL rats. A new surface 1H-31P double-tuned coil was built and several optimizations and development were done to improve the spectral resolution, SNR and localization, including implementation of 1H-31P NOE enhancement and 1H decoupling in static 31P MRS, optimization of localization method and also development of an optimal protocol for 31P magnetization transfer. 31P MRS study in p21 BDL rats was performed with an improved 31P MRS protocol and revealed a decreased creatine kinase rate constant 2 weeks after BDL surgery and significant changes in metabolites involved in high-phosphate metabolism, in phospholipid metabolism and also cellular redox state 6 weeks after BDL. In the last part of this thesis, some therapeutic approaches were tested. A probiotic treatment delayed some neurometabolic changes present in CHE in adult BDL rats and improved motor activity. Based on previously shown devastating effects of decreased brain creatine during brain development, we proposed the high creatine diet as a possible treatment for young BDL rats. We demonstrated that oral administration of creatine partially restored brain creatine levels and also had a positive effect on other neurometabolic changes present in CHE. In conclusion, the work done in this thesis revealed for the first time a large spectrum of neurometabolic alterations present in adult and young BDL rats suffering from CLD and CHE, together with the time evolution of these changes during the disease progression, using in vivo longitudinal 1H MRS, improved 31P MRS and 31P magnetization transfer protocols developed herein. In addition, we brought some insight on the positive effects of two innovative treatments on the progression of CHE in adult and young BDL rats.

EPFL2018

In Vivo Magnetic Resonance Spectroscopy (MRS) of the Activated Human Brain at 7T

Benoît Schaller

The principal aim of this thesis was to investigate human brain energy metabolism during physiological activation using proton (1H) magnetic resonance spectroscopy (MRS) at 7 Tesla (T). High magnetic fields (≥ 7 T) provide several advantages in human brain imaging and spectroscopy. In particular, signal-to-noise ratio (SNR) and chemical shift dispersion increase at higher magnetic fields, allowing for improved accuracy and precision of metabolite quantification. To take profit of the benefits of MRS at high field, one needs to provide an advanced methodology including hardware, pulse sequences experimental setup and data analysis. Each of these aspects was thoroughly explored in this thesis. In the field of functional studies, functional MRS (fMRS) has recently been used as a research tool to investigate the neuroenergetic and metabolic basis of physiologic brain activation. It provides direct insight into brain metabolism by non-invasively determining concentrations of metabolites. This is critical for the basic understanding of overall brain function, and potentially of the pathogenesis of many neurodegenerative diseases. In general, an optimized MR system and accurate acquisition methodology are required for high sensitivity and reliable metabolite quantification. The metabolites of interest for fMRS studies (e.g. lactate, glucose) are present in low concentration and metabolite changes are small under activation. Hence, studies of metabolite changes, in particular lactate, have led to inconsistent reports in the literature over the last decades. To address these challenges, it was essential to develop a robust MR protocol for the quantitative measurement of metabolite changes. A fMRS study was performed to investigate metabolite changes during visual stimulation using the enhanced sensitivity of the SPin ECho full Intensity Acquired Localized (SPECIAL) sequence. Small but significant increases of lactate (19 ± 4 %, P < 0.05) and glutamate (4 ± 1 %, P < 0.001) were observed using a small number of subjects (n = 6). With the exception of glucose (12 ± 5 %, P < 0.001), no other significant metabolite concentration changes beyond experimental error were observed. Based on this successful fMRS study, we further investigated brain energy metabolism. A subsequent fMRS study was performed to determine metabolite changes occurring during motor activation in the human brain. This second study demonstrated that increases in lactate (17 ± 5 %, P < 0.001) and glutamate (2 ± 1 %, P < 0.005) during motor stimulation were small, but similar to those observed during visual stimulation. These metabolite changes were further analyzed, and they supported the hypothesis of an increase in the change of cerebral metabolic rate of oxygen, ∆CMRO2, that is transiently lower than that of glucose, ∆CMRGlc, during the first one to two minutes of stimulation. Finally, we hypothesized that the observed glutamate and lactate increases were a general manifestation of the blood-oxygenation level dependent effect. The accuracy and the reliability of metabolite quantification is particularly challenging at short echo times due to the presence of broad underlying resonances overlapping with those of metabolite. Such resonances arise from macromolecules, which are characterized by short T1 and T2 relaxation times. Two studies were performed at 3 T and 7 T to investigate the characteristics of the macromolecule signal. An important finding was that the mathematical approximation (spline baseline) was a sufficient estimation of the macromolecule contribution to 1H spectra at 3 T compared to the experimentally measured macromolecule signal for healthy subjects. As a caveat, small, but significant, differences in the quantification may need to be taken into account when comparing metabolite concentrations obtained with these two different approaches, such as for glutamate and for γ-amino-butyric, which were more reliably quantified when using the measured macromolecule baseline. In addition, a study on the tissue-specificity of the macromolecule signal in the healthy human occipital lobe at 7 T reported that a general average macromolecule signal ensured reliable metabolite quantification. In general, these studies helped to set up a robust and reliable methodology in order to investigate human brain metabolism with improved accuracy. Despite their great potential for MRS, high main magnetic fields also compose challenges. In particular, the wavelength of the proton spin resonance becomes comparable to the size of a human head (at 7 T, λ = 11 cm). The phase variation across the brain produces signal addition (constructive interference, central bright spot) and signal cancellation (destructive interference, signal drop). Transceive arrays have been developed to address this issue. In this study, we designed, tested, and built an eight-channel transceive array, which was used to acquire images of the human head at 7 T (297 MHz). The design aimed for a low mutual coupling between the microstrip elements (> -15 dB), a relatively large penetration depth in the loading sample, reduced radiation losses and load-invariant tuning and matching. Electromagnetic simulations were run using Microwave Studio (CST, Darmstadt, Germany) for design optimization. The model was further demonstrated with the construction of the transceive array. Finally, the resulting array coil was shown to be handling, stable and also suited for techniques in which the transmission signals from all channels with different amplitudes and phases are combined. To conclude, this thesis provided complementary and new metabolic information about cerebral energy metabolism during physiological activation. Furthermore, the macromolecule studies and the building of the transceive array allowed to improve the accuracy and sensitivity of NMR measurements.

EPFL2013

(1)H and (31)P magnetic resonance spectroscopy in a rat model of chronic hepatic encephalopathy: in vivo longitudinal measurements of brain energy metabolism

Corina Mihaela Berset, Cristina Ramona Cudalbu, Bernard Lanz, Veronika Rackayová

Chronic liver disease (CLD) leads to a spectrum of neuropsychiatric disorders named hepatic encephalopathy (HE). Even though brain energy metabolism is believed to be altered in chronic HE, few studies have explored energy metabolism in CLD-induced HE, and their findings were inconsistent. The aim of this study was to characterize for the first time in vivo and longitudinally brain metabolic changes in a rat model of CLD-induced HE with a focus on energy metabolism, using the methodological advantages of high field proton and phosphorus Magnetic Resonance Spectroscopy ((1)H- and (31)P-MRS). Wistar rats were bile duct ligated (BDL) and studied before BDL and at post-operative weeks 4 and 8. Glutamine increased linearly over time (+146 %) together with plasma ammonium (+159 %). As a compensatory effect, other brain osmolytes decreased: myo-inositol (-36 %), followed by total choline and creatine. A decrease in the neurotransmitters glutamate (-17 %) and aspartate (-28 %) was measured only at week 8, while no significant changes were observed for lactate and phosphocreatine. Among the other energy metabolites measured by (31)P-MRS, we observed a non-significant decrease in ATP together with a significant decrease in ADP (-28 %), but only at week 8 after ligation. Finally, brain glutamine showed the strongest correlations with changes in other brain metabolites, indicating its importance in type C HE. In conclusion, mild alterations in some metabolites involved in energy metabolism were observed but only at the end stage of the disease when edema and neurological changes are already present. Therefore, our data indicate that impaired energy metabolism is not one of the major causes of early HE symptoms in the established model of type C HE.

Springer Verlag2016