Methods to Enhance Nuclear Magnetic Resonance Sensitivity at High Magnetic Field

MR is a well-established technique routinely used in preclinical and clinical studies. This thesis focussed on the improvement of sensitivity of different methods in MR.

Hyperpolarized 129Xe gas is used for studying pulmonary disease or cerebral perfusion. When hyperpolarization is conducted inside a polarizer, the achieved solid state polarization is currently limited to a few percent. One aim of this thesis was to counteract this bottleneck by investigating the effect of microwave frequency modulation for different spin systems. An increase in nuclear 129Xe spin polarization up to 14.5% was achieved compared to the polarization obtained without frequency modulation. Electron spin-lattice relaxation times T1S, as short as 5 ms, correlated directly with the gain in 129Xe nuclear polarization due to microwave frequency modulation, irrespective of the solvent or its deuteration. Simulations of the electron spin system confirmed that microwave frequency modulation could be an efficient method to considerably enhance nuclear spin polarization in short T1S matrices or in the presence of large linewidth radicals by promoting spectral diffusion.

DNP matrices can also be hyperpolarized through non-persistent radicals generated by UV-irradiation. They have the advantage of quenching upon dissolution, yielding a hyperpolarized solution with no need for radical filtration. Radical precursors researched up to date either yielded low 13C polarization, contained toxic matrix components or interfered with metabolic processes. Therefore, the two novel endogenously-occurring precursors, alpha-ketovalerate (akV) and alpha-ketobutyrate (akB) were investigated. Liquid state polarization of the metabolic substrate glucose increased to 16.3% using akB compared with 13.3% obtained with pyruvate. [1-13C]butyric acid was polarized to 12.1% for akV and 12.9% for akB and used for in vivo hyperpolarized cardiac MRS. Hyperpolarized [1-13C]butyrate metabolism in the heart revealed label incorporation into a wide range of metabolites. This study demonstrated the potential of using UV-induced radicals generated in the endogenously-occurring metabolites akV and akB as polarizing agents, enabling high polarization without requiring radical filtration for radical-free hyperpolarized MRI.

While MRI requires RF coils that generate high B1 homogeneity, MRS strongly depends on high sensitivity and potentially high B1 efficiency to allow for short-TE acquisitions. Two custom-made 1H coils resonating at 600 MHz, a single-channel saddle coil and an 8-leg quadrature birdcage coil, were investigated with respect to those criteria for applications at ultra-high field (14.1 T). The saddle coil yielded a 54% higher transmit field efficiency with a 20% higher SNR compared to the birdcage coil after full-FOV corrections. Using the saddle coil in glycoCEST imaging of a homogeneous phantom resulted in an MTR asymmetry coefficient of variation as small as 5.2% over an 11 mm diameter ROI. Overall, the saddle coil provided a good compromise between globally homogeneous excitation and high sensitivity. Finally, it was successfully used to map skeletal muscle glycogen content in vivo.

This thesis focused on improving MR sensitivity by means of adequate custom-made RF coils as well as DNP through the implementation of a method capitalizing on electronic spin properties and the development of novel endogenous precursors for in vivo hyperpolarized MRI.

Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals

Lionel Pierre Arn, Josefina Adriana Maria Bastiaansen, Andrea Capozzi, Rolf Gruetter, Alice Radaelli, Hikari Ananda Infinity Yoshihara, Claudia Christina Zanella

It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical-free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alpha-ketovalerate (alpha kV) and alpha-ketobutyrate (alpha kB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing alpha kV and alpha kB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of C-13-labeled substrates to the sample matrix altered the radical yield of the precursors. Using alpha kB increased the C-13-labeled glucose liquid-state polarization to 16.3% +/- 1.3% compared with 13.3% +/- 1.5% obtained with pyruvate, and 8.9% +/- 2.1% with alpha kV. For [1-C-13]butyric acid, polarization levels of 12.1% +/- 1.1% for alpha kV, 12.9% +/- 1.7% for alpha kB, 1.5% +/- 0.2% for OX063 and 18.7% +/- 0.7% for Finland trityl, were achieved. Hyperpolarized [1-C-13]butyrate metabolism in the heart revealed label incorporation into [1-C-13]acetylcarnitine, [1-C-13]acetoacetate, [1-C-13]butyrylcarnitine, [5-C-13]glutamate and [5-C-13]citrate. This study demonstrates the potential of alpha kV and alpha kB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications.

WILEY2021

Development and implementation of ¹H magnetic resonance techniques for an improved in vivo assessment of models of cerebral metabolic disorders

Mélanie Craveiro

Nuclear magnetic resonance (NMR) is a physical phenomenon that is widely used in the biomedical field due to its non-invasive and non-destructive properties, which make it an optimal tool for the in vivo investigation of living organs such as the brain. This thesis focused on the development of 1H magnetic resonance (MR) techniques at ultra-high magnetic field strength to improve the measurement and quantification of the 1H MR signal in the rodent brain, and to accurately assess the alterations of the metabolite and water concentrations in several cerebral metabolic disorders. In rodents, 1H MR spectroscopy (MRS) allows the assessment of the concentration of up to 20 metabolites that take part in many cerebral metabolic processes such as neurotransmission, cell energy metabolism, cell growth and osmosis. 1H MRS thus provides a unique tool to non-invasively investigate the cerebral metabolism in healthy and pathological conditions in vivo. However, it is essential for reliable quantification that systematic errors and overlap of the measured metabolite signals are minimized. To that aim, the impact of a potential additional short T2 relaxation time component, which might affect the glutamine quantification at long echo times, was assessed in this thesis. The J-difference editing technique MEGA-SPECIAL was optimized to obtain the unequivocal detection of the glutamine signal at moderate echo time. As a control, the glutamine concentration obtained with this method was then compared to short echo time 1H MR spectroscopy measurements. Since the two measurements of the glutamine concentration at short and moderate echo time did not result in significant differences, this study concluded that there is a low probability of an effect of a short T2 relaxation time component on the glutamine concentration measurement. Since a reliable quantification of 1H spectra partly relies on the accurate assessment of the overlapping macromolecule contribution to the metabolites, an optimized method for the post-processing of the measured macromolecule signal was developed to ensure an accurate assessment of its contribution. This method was applied to investigate potential regional differences in the mouse brain macromolecule signals that may affect metabolite quantification when not taken into account, as well as for the assessment of macromolecule alterations in a human-glioma mouse model. No regional macro- molecule variation was found to significantly affect the metabolite quantification in the healthy mouse brain, which supports the common use of a general macromolecule spec- trum for healthy rodent brain 1H spectra quantification. However, several alterations of the macromolecule spectrum, some of which were reported for the first time, were observed in glioma tissues, and their accurate assessment was shown to be necessary for reliable metabolite quantification. Localized 1H MR spectroscopy techniques that are designed to acquire the1H MR signal from a 3D volume can also be combined with MR imaging (MRI) to acquire information on the spatial distribution of the individual metabolites, which is often of interest when heterogeneous tissues are investigated (such as in pathological conditions). This MR spectroscopic imaging (MRSI) technique is however limited in its conventional implementation by the long measurement time that is necessary to acquire the spatial information. A fast MRSI technique, spectroscopic RARE (spRARE), was therefore implemented at high magnetic field strength in this thesis in order to benefit from the advantage of high magnetic fields in addition to the rapid spatial encoding scheme offered by spRARE. Several optimizations of the metabolite signal detection were implemented in spRARE, before the efficiency of the technique was validated at 9.4 T through a comparison of the acquired in vitro metabolite signals with numerical simulations using the density matrix formalism. The in vivo quantification of the water content is also of interest as it offers perspec- tives for investigating cerebral disease progression that is associated with brain water content alterations. In addition, when the water signal is used as an internal reference for metabolite concentration quantification in MRS, such knowledge can also contribute to a more reliable quantification. A method that is based on the multi-spin-echo MRI technique was therefore developed for the absolute quantification of the water content in the rodent brain, with the aim of assessing alterations of its content in pathological conditions. It was then applied on a bile-duct-ligation rat model of chronic hepatic encephalopathy (HE) to assess the brain water content changes associated with the disease as well as its metabolic alterations. The absence of significant brain water content changes detected in the investigated HE rat model, which was supported by post-mortem brain water content measurements using gravimetry and the dry/wet weight-ratio technique, potentially opens the way for investigating cerebral metabolic processes in chronic HE in absence of edema formation. Localized 1H MRS and conventional MRSI were also applied for the investigation of rodent models of brain disorders in order to assess the metabolic profile associated with those disorders. These studies for the first time allowed the assessment of GABA as a potential early Parkinson’s disease marker, while they also provided new insights into the role of the PPAR-β brain receptor in the early neuroprotective effects following ischemia.

EPFL2013

Biophysical Basis of the Diffusion-Weighted Magnetic Resonance Signal in the Rat Brain

Nicolas Kunz

Nuclear magnetic resonance (NMR) can be used in-vivo in a vast array of applications, such as anatomical imaging (magnetic resonance imaging, MRI), localized chemical composition characterization (magnetic resonance spectroscopy, MRS), cellular structure assessment (diffusion tensor imaging, DTI) and cerebral activity mapping (functional imaging, fMRI) for the most important one. This thesis focused on the development of diffusion NMR spectroscopy and imaging methods at ultra-high magnetic field, with the aim of a better characterization of the diffusion mechanism in-vivo. DTI measures the water molecule displacement due to the thermal agitation in the sample. In cellular tissue, the molecules are restrained in compartments delimited by the cell membranes, which mean that DTI can provide information on the cerebral cellular microstructure. DTI is thus widely used to investigate cerebral disorders such as brain ischemia, trauma, and tumors, as well as the structural changes occurring during brain development and normal aging. However, the presence of water molecules in both the intra- and extra-cellular compartments may bias the assessed structural information. On the other hand, the metabolites, measured by 1H-MRS, are mostly localized in the intracellular compartment, which may provide a more precise insight of the cellular structure. With the availability of high magnetic field systems, 1H-MRS allows the measurement of 15 – 20 metabolites in-vivo in localized region in the order of &icro;l in the rodent brain. These metabolites are involved in cell energy management, neurotransmission, neuro-protection, neuronal development and membrane growth, and thus provide a rather complete status of the brain neurochemical profile. In this context, a diffusion tensor spectroscopy pulse sequence (DT-MRS) was implemented and used to investigate the metabolite diffusion properties in specific brain regions in-vivo at ultra-high magnetic field 14.1 T to benefit from the increased sensitivity and spectral resolution at high B0. The diffusion tensor of 5 metabolites were reconstructed in the corpus-callosum, CC, (NAA, Tau, Glu, Ins and Mac) and 8 in the cortex, Cx, (with in addition Cr, PCr, GABA and GSH). Metabolites apparent diffusion coefficients were significantly higher in the CC (0.115 – 0.153 µm2/ms) than in the Cx (0.087 – 0.107 µm2/ms) and so was the fractional anisotropy (0.51 – 0.62 and 0.34 – 0.51 respectively). In addition, the metabolite preferential diffusion directions showed an excellent agreement with the known rat brain structure. The aforementioned DW-MRS pulse sequence was then combined with an inversion recovery module, allowing direct assessment of the macromolecule (MM) signal, which overlaps with metabolites resonances. An inaccurate estimation of the MM baseline may result in a significant bias during the quantification process. At ultra-high magnetic field, the MM spectral shape becomes more complex, and therefore, the previously reported experimental techniques would require post-processing to remove residual metabolite signals. In this study, several metabolite contaminations remaining in the metabolite nulled spectrum were successfully identified and attributed to creatine, myo-inositol, taurine, choline, N-acetylaspartate, glutamine and glutamate. The macromolecule baseline assessed by DW-MRS resulted in a twofold reduction of the metabolite residuals with minimal effect on the macromolecule signal (i.e. < 8% signal reduction). Although at high magnetic field spectral resolution and sensitivity are improved, metabolite diffusion measurements remain very time consuming and challenging due to the low metabolite concentrations in-vivo (1 – 15 mM) compared to water (44 M). Therefore, a fast and robust EPI-based DTI pulse sequence was developed, implemented and validated to investigate the cerebral architecture in-vivo at 14.1 T through water diffusion. Combined with the use of a purpose-built quadrature surface coil and adiabatic RF-pulses, it resulted in images with a full coverage of the rat brain and a total acquisition time of one hour. The rapid signal loss arising from the use of a surface coil for the MRI acquisitions, which was previously in this thesis overcome experimentally, was also addressed in collaboration with the Signal Processing Core of CIBM, by a direct correction applied on reconstructed magnitude images. Based on a low-pass frequency mode; the developed method showed better accuracy in enhancing image intensity compared to standard approaches. In view of a future application of the presented diffusion NMR techniques, investigating structural changes during brain development, a study was performed to characterize different intra-uterine growth retardation (IUGR) models by 1H-MRS. Dexamethasone-treated animals (100 µg/kg/day induced from the day 15 of gestation) showed more severe alterations in cerebral energy metabolism and neurotransmitter action at postnatal day 7 and 21 than the reference IUGR model, caloric restriction (30% food intake of ad libitum fed dams from day 1 of gestation). In a continuation of the IUGR study, the exposure to the estrogen-like molecule (i.e. bisphenol-A, BPA), was investigated. Here, 1H-MRS showed a significant increase of the Glu/Asp ratio in the hippocampus of the BPA-treated group at 20 days old, indicating a possible deviation in the malate-aspartate shuttle. Finally, the diverse NMR techniques used and developed during this thesis have required the implementation of several data processing tools, which were centralized in a framework written in Matlab (Mathworks, Inc). Two principal modules were build: 1. to reconstruct DTI experiments data and 2. to facilitate and standardize the 1H-MRS spectra quantification by LCModel. The proposed NMR data processing framework was used to process most of the data presented in this thesis and has demonstrated good performance. In addition, this framework was tested and used by five external users to analyze for data-analysis in ongoing studies.

EPFL2010