This electron shielding causes the observed resonance frequency of the atoms to slightly different and therefore identifiable with MRS. This is possible because the electron cloud surrounding different chemical compounds shields the resonant atoms of spectroscopic interest to varying degrees depending on the specific compound and the specific position in the compound. Instead of using the frequency information (provided by the read-out or frequency gradient) to provide spatial or positional information, the frequency information is used to identify different chemical compounds. The important difference between an imaging sequence and a spectroscopy sequence is that for spectroscopy, a read-out gradient is not used during the time the RF coil is receiving the signal from the person or object being examined. Many sequences used for imaging can be used for spectroscopy also (such as the spin echo sequence). The simplest sequence consists of a 90 degree radiofrequency (RF) pulse, without any gradients, with reception of the signal by the RF coil immediately after the single RF pulse. Magnetic resonance spectroscopy (MRS) is performed with a variety of pulse sequences. If water suppression is not successful then a general slope to the baseline can be demonstrated, changing the relative heights of peaks. Water suppression is therefore part of any MRS sequence, either via inversion recovery or chemical shift selective (CHESS) techniques. If raw signal was processed then the spectra would be dominated by water, which would make all other spectra invisible.
Phosphorus-31 (P-31) is typically used to look at the ratio of adenosine triphosphate (ATP) to phosphocreatine and other metabolites, and can be used to assess the energy charge of the cell. MR spectra can be acquired from any "NMR-active" nucleus, which is a nucleus possessing non-zero spin: protons, carbon-13 and phosphorus-31 are the most commonly encountered, and in clinical practice essentially only proton spectra (which enable the resolution of metabolite profiles in vivo) are encountered. The technique is identical to that of nuclear magnetic resonance (NMR) as used in analytical chemistry, but the community commonly refers to in vivo NMR as MRS to avoid confusion (and, arguably, the word "nuclear"). This results in slightly different resonant frequencies, which in turn return a slightly different signal. The basic principle that enables MR spectroscopy (MRS) is that the distribution of electrons within an atom cause nuclei in different molecules to experience a slightly different magnetic field.
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