Detection of changes in quadrupolar peaks by FFC-MRI in skeletal muscle

The concentration of immobilised and soluble protein in skeletal muscle can potentially change during hydration-dehydration, hypertrophy-atrophy or as a result of pathological changes such as oedema or muscle injury and regeneration. The gold standard to measure the concentrations of overall or individual proteins is a muscle biopsy followed by a protein assay or by a Western blot against individual proteins. Muscle biopsy, however, is invasive which limits its applicability. The aim of our study is to develop a non-invasive technique to measure the concentration of immobilised protein using Fast Field-Cycling MRI (FFC MRI). Fast Field Cycling (FFC) is a technique that emerged some decades ago [1] as a means to measure changes in relaxation rates at different magnetic fields. This proved useful for many applications, from characterisation of polymers to liquid crystals. However, it is still rarely applied to MRI because of technical challenges. The experiment here uses one of the FFC MRI scanners that have been developed in our laboratory [2]. This FFC MRI scanner has the capability to change its magnetic field between 0 and 120 mT, allowing us to perform image acquisition with T1-dispersion contrast. One of the numerous uses of field cycling is to detect and measure the coupling between the water protons and the quadrupolar moment of immobilised N [3]. This is of special interest in living tissues since proteins are rich in nitrogen and water. Moreover, since it has been shown that the quadrupolar signal recorded by FFC MRI is proportional to the concentration of immobile proteins [4], this technique allows the possibility of measuring the quantity of quadrupole-interacting protein in a tissue, thus making it a good candidate for the detection of protein changes in response to physical training.