Procedure for quantitative1H magnetic resonance spectroscopy and tissue characterization of human brain tissue based on the use of quantitative magnetic resonance imaging

Introduction: In this thesis, one of the major objectives was to implement a method for (absolute) quantitative magnetic resonance spectroscopy (qMRS) of the human brain, intended for clinical use. The implemented method was based on standard spatially selective MRS sequences. The tissue water was used as an internal reference, which was calibrated using whole brain quantitative magnetic resonance imaging (qMRI). The second objective was to apply the method in clinical neuroimaging investigation, of different disease processes in the human brain.Materials and Methods: In total, 158 subjects were included and 507 MRS measurements (330 in white matter and 177 in the thalamus) were acquired.In a cross-sectional study of multiple sclerosis (MS), 35 ‘clinically definite MS’ (CDMS) patients were included, of which 15 were atypical CDMS patients with a very low number of white matter lesions (two or fewer), and 20 were typical CDMS patients with white matter lesions (three or more) were included. The metabolite concentrations in normal appearing white matter (NAWM) and the thalamus were assessed using the qMRS method developed in this thesis, and the brain parenchymal fraction (BPF) was calculated from the qMRI data. A cohort of 27 CDMS patients were then treated with Natalizumab and examined both at baseline, and after one year of treatment. Both qMRS and CSF samples for the purpose of assessing intrathecal inflammation were obtained. In addition, the frontal deep white matter (FDWM) and the thalamus were investigated in 20 idiopathic normal pressure hydrocephalus (iNPH) patients using qMRS. Finally, the left thalamus of 14 Kleine-Levin Syndrome (KLS) patients were examined using both qMRS and functional MRI (fMRI) of neurological activation of the left thalamus during a working memory test. Moreover, 63 healthy subjects were included as controls for this work.Results: A quantitative MRS method based on water referencing was successfully developed, implemented, and evaluated at 1.5 T. Both healthy subjects and MS patients showed a positive correlation between the concentrations of total Creatine (tCr) and myo Inositol (mIns) and age, and also a negative correlation with BPF were observed. Glutamate and Glutamine (Glx) levels were elevated for all MS patient groups compared to healthy controls. In contrast, lower concentrations of total N-acetyl aspartate and N-acetyl aspartate glutamate (tNA) and higher mIns concentrations in NAWM were only observed in MS patients that had developed white matter lesions. Moreover, the change in concentrations of tCr and total Choline (tCho) in MS patients during Natalizumab-treatment were positively correlated with markers of intrathecal inflammation. The iNPH patients had lower tNA and N-acetyl aspartate (NAA) concentrations in the thalamus compared to the controls. In addition, the NAA concentrations in the left thalamus were inversely correlated to the fMRI activation in the left thalamus during the working memory test in KLS patients.Discussion: The calculated calibration factors were in good agreement with the results found in the literature, indicating that the calibration factors were accurate.The observed elevated Glx concentration in MS could be due to increased concentrations of glutamate (Glu), which is neurotoxic at high concentrations, thus the elevated Glx could be linked to the clinically observed neurodegeneration in MS both in patients that have developed lesions and in atypical patients that do not develop any (or extremely few) lesions.Both tCr and mIns can be used as glia markers, thus the correlations of tCr and mIns concentrations with both age and BPF indicates that the local glia cell density, or tissue fraction, increases with age and atrophy. Moreover, the higher mIns concentrations in the NAWM of MS patients with a substantial white matter lesion load indicate that the glia tissue amount in NAWM is increased in MS patients that develop lesions. NAA is neuronal-specific, thus the lower tNA concentrations indicate that the neurone concentration is lower in the NAWM of MS patients that develop MS lesions. The lack of correlation between tNA with age and BPF in combination with the presence of correlation between tCr and mIns with both age and BPF, might be explained using a model for neurodegeneration. In which, there is a higher neurone loss compared to the glia loss. However, the lost tissue is compensated by compression of the tissue, which keeps the density of neurones more or less constant and the density of glia increased.The low concentration levels of the neuronal marker NAA in the thalamus of the iNPH patients indicates that the basal ganglia-thalamic-subcortical frontal circuits are damage or at least strongly modulated in the thalamus.The correlation between strong activation in left thalamus during a working memory test with the neuronal marker NAA indicate that the KLS patients that have low neuronal concentration also needed to utilise the working memory circuitry more heavily in order to perform the task as healthy subjects.Conclusion: It is possible to use qMRI for accurate and robust determination of qMRS in clinical practice, even at 1.5 T field strength. The tGlx concentration may be an important marker for pathology in the nonlesional white matter of MS-patients. The increased glia and loss of neurones in the NAWM are associated with the formation of white matter lesions.