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Proton magnetic resonance spectroscopy of human brain : T1 and T2 relaxation and absolute concentrations of metabolites in patients and healthy volunteers Brief, Elana Esther

Abstract

Absolute concentrations have been measured of the brain metabolites choline, creatine and N-Acetyl-Aspartate using non-invasive proton (1H) Magnetic Resonance Spectroscopy (MRS) in vivo. The accuracy of 1H-MRS has been improved as a result of better metabolite T1 and T2 relaxation time measurements and a novel technique for measuring brain tissue water concentration. The precision of MRS has been measured for all major brain metabolites including choline CHO, the methyl group of creatine CRE, the methylene group of creatine (CR), glutamine (GLU), glutamate (GLN), myo-inositol (INS) and N-Acetyl-Aspartate NAA. T1 and T2 have been measured more precisely and accurately than previously reported studies because of improvements made to the saturation recovery and decay experiments. Contrary to previous results, metabolite T1 relaxation was found to differ for all metabolites between and within three regions of normal human brain. In addition, T2 relaxation of CHO and NAA have been found to be shorter than previously reported. The T1 and T2 times measured in previous studies are explained in terms of short-comings in their experimental design. Brain tissue water content has been measured using a T2 relaxation imaging sequence and normalizing to an external standard. This determination of water content is a vast improvement over previous experiments which either quoted literature values for water content for pure tissues, or made erroneous assumptions regarding the magnetic resonance visibility of the protons associated with water. The concentrations of CHO and NAA exhibited a regional dependence in the parietal white and occipital grey regions examined whereas CRE did not. The concentrations measured here were found to be within the range of the concentrations quoted in the literature, however the slight differences can be explained in terms of the improved water content measurements and the T2 corrections. Clinical applications of the T1 and absolute concentration studies are discussed. In particular, the T1 times of NAA and CHO within multiple sclerosis lesions were reduced compared the the corresponding metabolites in healthy white matter. This may help explain confusing published observations such as the reversible decrease of NAA in MS lesions. Three separate studies are introduced which investigate the brain metabolite concentrations in patients with traumatic brain injury, patients with phenylketonuria and patients with chronic fatigue syndrome. This work represents the development and improvement of a new clinical tool which can be used in collaboration with radiologists, neurologists, endocrinologists and other specialists to investigate neurologic disease.

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