Spectroscopy of Cancer

Abstract

Nuclear magnetic resonance (NMR) was initially described as a physical phenomenon and initially was developed and used exclusively for chemical and structural analyses [1, 2]. Nowadays, the most commonly used NMR methodologies in biomedicine are based on studying the physical properties of hydrogen nuclei (protons) in tissue water (which is commonly known as magnetic resonance imaging, MRI) followed by proton NMR spectroscopy (or MRS) on other endogenous metabolites and, less frequently, other “magnetically active” nuclei such as 31P, 13C, 19F, and 23Na. The strong external magnetic field aligns the magnetic moments of protons (with spins −½ and ½) parallel and antiparallel to the magnetic field. The population of spins in the parallel orientation (which are of a slightly lower energy level) is larger than in the antiparallel orientation as described by the Boltzmann distribution. This results in a net “magnetization” in the sample aligned with the direction of the external magnetic field. When a radiofrequency (rf) pulse is applied to the sample, the magnetization orientation is changed, and after the pulse is gone, the system relaxes to its original status. Hydrogen nuclei in different tissues have different relaxation properties, which can be detected by rf MR receivers. MR tissue relaxation characteristics following an excitation rf pulse reveal information about the concentration, mobility, and chemical bonding of hydrogen and, less frequently, other tissue elements. The strongest NMR signals in a living system arise from hydrogens in water and fat protons due to their metabolic abundance (water compromises up to 66 % of the body weight). Other endogenous and exogenous metabolites also give signals on 1H-NMR but with much weaker signal-to-noise ratios due to lower concentrations. A significant number of endogenous metabolite signals are usually obscured by the water signal (both in vivo and in vitro), and their detection requires adequate techniques for water suppression (and often lipid or macromolecule suppression).