Magnetic resonance spectroscopy

It would be far beyond the scope of this chapter to explain magnetic resonance in depth. Several textbooks and overviews have been published that give an introduction into the basics of MRS (Gadian, 1982; Boesch, 1999, 2005; De Graaf, 1999).

Magnetic resonance became an established tool in diagnostic radiology and clinical research. Since it is so popular in medicine, it is often ignored that clinical magnetic resonance is just one of many applications of the 'nuclear magnetic resonance' (NMR) effect. In chemistry and biophysics, so-called 'high-resolution' NMR spectroscopy is one of the most important methods to study the structure of organic and inorganic substances. Today, the development and analysis of chemical compounds is unthinkable without NMR technology; even complicated three-dimensional structures of large biological molecules can be understood with the help of this potent method. In medicine, magnetic resonance imaging (MRI) is nowadays a particularly valuable and versatile instrument in diagnostic radiology (Glover and Herfkens, 1998), due to its detailed and accurate representation of in vivo anatomy and function. Magnetic resonance spectroscopy (MRS) combines the volume-selective data acquisition of MRI with the chemical information provided by NMR. Without reaching the sensitivity and resolution of high-resolution NMR, in situ MRS allows for a non-invasive examination of the chemical composition from selected volumes in humans (Howe et al., 1993; Boesch, 1999; De Graaf, 1999; Ross and Danielsen, 1999).

In principle, the above-mentioned applications of the NMR effect are all based on the fact that stable atomic nuclei have a magnetic moment and an angular momentum, called 'spin'. When a material is placed in an external magnetic field, these 'spins' begin to tumble in a precession. This motion enables the nuclei to absorb and emit electromagnetic waves with a frequency in the MHz range, depending on the strength of the magnetic field and the type of isotope. While MR images are generated in the majority of cases from the signals of hydrogen nuclei (1H) in water, MR signals can also be obtained from various other stable isotopes, including phosphorus (31P), carbon (13C), fluorine (19F), sodium (23Na) and others. Magnetic resonance spectroscopy makes particular use of 1H, 13P and 31C. Depending on the local chemical environment, different atoms in a molecule resonate at slightly different frequencies, resulting in the so-called 'chemical shift'. This chemical shift is resolved in spectroscopic applications (NMR and MRS), however it is neglected in standard imaging. It is measured in relative units (parts per million, ppm), and represents the x-axis of a spectrum. Figure 14.2 illustrates the spectrum of butyrate, a molecule that has three types of hydrogen atoms: an H-C-H group close to the carboxyl group C=O, a second H-C-H group in the middle and a CH3 group at the end of the molecule. The hydrogen atoms in each of these groups are identical and resonate at a specific position on the chemical shift axis. Since the area of the resonance peak under specific experimental conditions is proportional to the concentration of the chemical species, butyrate should show three single resonance lines with a 2 : 2 : 3 ratio of the areas. The reason why the lines in Figure 14.2 are further split into so-called 'multiplets' is a mutual influence of the nearest hydrogen atoms. This effect complicates the spectrum; however, it also helps to identify chemical species. In other words, spectroscopy (either NMR or MRS) uses frequency information of absorbed and emitted radio waves to identify different chemical compounds - the position of the signal in the

Butyrate

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