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MR technology

In-vivo magnetic resonance spectroscopy (MRS) is a special technique connected to magnetic resonance imaging (MRI). MRS is a non-invasive, non-ionising technique that studies metabolic changes in different tissues such as skeletal muscle. The technique thus exploits the magnetic properties of certain nuclei. 1H, 31P and 13C are some of the most common isotopes studied. In a magnetic field and when an electromagnetic pulse is applied, the isotope nuclei absorb energy from the pulse and radiate it back out. This phenomenon is due to the intrinsic property of the nuclei: as stable isotopes with an odd number of protons and/or neutrons, they possess a magnetic moment or spin, whereas isotopes with an even number do not show a magnetic moment and have zero spin. The MR signal is proportional to the applied magnetic field.

Phosphorus 31-magnetic resonance spectroscopy (31P MRS), for example, provides an insight into the mitochondrial function of muscle cells. In the case of phosphorus-31, the isotope is stimulated with a specific frequency of 25.8 MHz within a magnetic field of 1.5 Tesla. This can be achieved using a conventional clinical magnetic resonance tomograph (MRT) capable of recording spectroscopy measurements.

With a magnetic field strength between 1.5 and 3 Tesla. changes in phosphocreatine (PCr), inorganic phosphate (Pi), phosphomonoester and phosphodiester as well as the three phosphates of adenosine triphosphate (ATP), α - β - and γ ATP can be displayed.

However, α - and γ – positions of ATP show an overlap with the signal from phosphates of ADP and nicotinamide-adenine dinukleotide (NAD+/NADH). The β position of ATP is not superimposed by other phosphates and can therefore be used for ATP quantification (Stubbs 1996). To measure metabolite changes in the loaded muscle, a high temporal resolution of 4 to 20 seconds should be used in order to capture metabolic alterations especially at the onset of exercise (Schocke 2008). Different methods can be used to quantify metabolites. One option is to employ an external standard, which is used within the measuring arrangement. Phenylphosophonic acid [10 mM] is a typical example of an external standard that is measured together with the metabolites in the muscle. Because of the known concentration of the external standard, the concentrations of the remaining metabolites can be measured. However, one problem associated with this method is that the external standard is outside the human body and is therefore only partly comparable with physiologic metabolites within the body. Another method uses ATP as a so-called internal standard. The ATP concentration in the resting muscle shows only low variability between different people and is around 8.2 mmol/l. The advantage is that ATP occurs intracellularly within the body and is subject to the same changes as the metabolites under scrutiny. This is the method predominantly used for quantification (Kemp 2007).

Relaxation times and quantification of metabolites
One problem is that relaxation times differ for the various metabolites observed and may vary with the magnetic field strength. In order to quantify metabolites, partial saturation, which stems from the different relaxation times of the metabolites, must be compensated for. With a fully saturated spectrum, which is normally measured with a repetition time of 30 seconds, the different relaxation times can be overlooked. In order to assess the dynamic changes in the muscle during loading, a temporal resolution higher than 30 s is required for measuring purposes. As different metabolites have a distinct relaxation time, the metabolic changes expressed as the area under the curve of the integrals cannot be compared to each other. The area is no longer consistent with the actual concentrations and must be corrected. A fully relaxed spectrum with a time resolution of 30 s is therefore acquired in the resting state of the muscle, before the onset of dynamic exercise. This spectrum can then be used to correct the relations between the various metabolites. However, metabolite concentrations can be also corrected computationally at a constant repetition time over the known relaxation times (Meyerspeer 2003).