This presentation aims to provide insight into MR spectroscopy of humans and highlight the essential concepts of chemical shift, spectral dispersion associated with magnetic field strength, shimming, signal suppression, combination schemes for signal processing from phased array coils, sequence approach, and localization sequences.
Understanding:
MRS is a non-invasive technique that can be used to quantify compounds or metabolites that are present in high concentrations in a single voxel or multiple voxels within tissues. The technique is based on the same physical principles as MRI; however, the MRS measurement results in a spectrum where metabolite nuclei resonate in characteristic frequencies depending on their chemical and physical vicinity in a molecule. The surroundings change the local magnetic field experienced by the nuclei (nuclear shielding) and thus its resonance frequency. MRS has been demonstrated in vivo for different nuclei, including 1H, 31P, 13C, 15N, 19F and 23Na. However, clinical applications are mostly limited to 1H and 31P isotopes.
Briefly, in MRS, the nuclei of interest (e.g., 1H, 31P) are placed in a strong external magnetic field and the volume of interest is excited using broadband excitation radio frequency (RF) pulses. Subsequently, the response to the excitation RF pulses is recorded and Fourier transformed to obtain a spectrum consisting of peaks at specific resonant frequencies. The signal intensity and linewidth provide the "area" which can be used to quantitate the amount of the observed chemical. The most widely used MRS approach is 1H MRS since it can be executed with standard MR equipment and offers a large sensitivity advantage over other nuclei, by having not only the highest gyromagnetic ratio γ of non-radioactive nuclei but also a high natural abundance.