Magnetic Resonance Spectroscopy at ultra-high field in humans

Abstract

MR spectroscopy (MRS) has gained an important place in fundamental research as well as in clinical research. In vivo MR spectroscopy provides a unique window to measure the concentration of chemical compounds (metabolites) inside the human body without the need for a chirurgical intervention or puncture. It obviates the need for tissue excisions or biopsies and provides direct tissue biomarkers, rather than indirect parameters derived from blood, plasma or urine samples. MR signals from tissue volumes of less than 1 ml and metabolite concentrations below 1 mM can be detected. Therefore MR spectroscopy is applicable to study normal and pathological metabolism inside the human body. However, for many research purposes and especially for clinical practice, the sensitivity and specificity of the MRS measurement is still a limiting factor. The sensitivity is limited by the low concentrations in the body, which make it difficult to detect these signals above the noise. This puts a restriction on the spatial resolution that can be obtained and leads to long measurement times. Also the specificity of MR spectroscopy is often a limiting factor, as the spectral resolution is not much higher than the difference in chemical dispersion between some resonances. The overlap in the MR spectrum between different compounds makes it hard to separately detect metabolites. For example, the resonances of glutamate and glutamine show significant overlap, and are difficult to detect individually. The recent development of ultra-high field magnets for human use offers a platform where both the sensitivity and the specificity of MRS are substantially increased. The intrinsic signal-to-noise ratio (SNR) of the MRS measurements increases approximately linear with the static magnetic field strength. MR experiments at a field strength of 7 tesla therefore show a more than twofold increase in the intrinsic SNR, as compared to a 3T MR scanner. In addition, the chemical shift separation between resonances is increased, leading to a more reliable discrimination between metabolites in the MR spectrum. These two effects show potential for a next step in the evolution of in vivo MR spectroscopy. Before the increased sensitivity and specificity can be utilized in practice, several technical impediments will have to be overcome when performing MRS in the human body at ultra-high field. The most important challenges that ultra-high field MRS applications impose are the decreased homogeneity in the static magnetic field (B0) and the transmit field (B1), a demand for higher gradient strengths, altered relaxation times and an increased power deposition in the body. This theses describes several studies and new methods to counter these challenges, and shows how high quality MRS data can be acquired at ultra-high field in humans.