Neurotransmitter measurements in the human brain at 3 Tesla and 7 Tesla

Abstract

Magnetic Resonance Spectroscopy (MRS) is a powerful technique that can potentially be used to measure metabolite concentrations in the brain non-invasively. However, the available signal-to-noise ratios (SNR) of MR spectra are limited because metabolites are present in low (millimolar or less) concentrations, and different resonances overlap in frequency. These challenges have led to the use of specialized pulse sequences and increasingly higher static magnetic field strengths. The emergence of higher fields presents opportunities for making more precise measurements of metabolites and for acquisitions from smaller volumes or in faster times. First, this thesis addresses the need to re-evaluate the optimal measurement and sequence parameters to be used at ultra-high field (7 Tesla) for detecting specific brain metabolites. By calculating theoretical Cramer-Rao Lower Bound (CRLB) values for a set of 17 metabolites that are above or near the detection threshold, the optimal PRESS sequence timings for acquisitions have been designed. The advantage of this approach is that calculation of CRLBs is versatile and may be used to evaluate optimal parameters for other pulse sequences. Second, this thesis includes a quantitative comparison of measurements of the neurotransmitter GABA made using MEGA-PRESS between 3 Tesla and 7 Tesla, and defines the bandwidth requirements for making measurements with higher precision at higher fields. Third, the thesis also evaluates and demonstrates the potential of novel event-related functional spectroscopic acquisitions and investigates changes in metabolite profiles as a function of neuronal activity with relatively high temporal resolution. The transient changes in metabolites that occur following short periods of stimulation of visual cortex have been quantified.