N.V. Anisimov, S.S. Batova, A.A. Samoylenko, E.I. Shalamova

Evolution of Larmour frequency of 0,5 Tesla MR scanner with a superconducting magnet within 14 years is discussed. Analysis is based on materials of a database of MRI studies. In the beginning of each MRI study automatic definition of operating frequency of a MR scanner is made. For this purpose, NMR signal from the tissues localized in the zone of sensitivity of the receiver coil is registered. Value of frequency F together with parameters of scanning and images is fixed in a database. Curves F(t) for different receiver coils (different zones of research - a head, neck, spine, abdomen, knee, etc) have been received. The curves display that measured value F depends on a water/fat proportion in investigated tissue because of chemical shift difference (3.5 ppm) of these components. From these curves one can see that in reality the adjustment of working frequency is made on a water signal only for the head, while for other body parts - on fat ones. However for all curves decay of frequency F is fixed. Time constant of decay is equal to about 125000 years. It may be explained by decay of polarizing magnetic field B. This field is generated by a current flowing through the solenoid coil made of a superconductor. Hence one can estimate electric resistance of this winding by measuring the rate of decay of F if inductance of the solenoid is known. To define specific resistance of a material of which it is made one must know wire-s diameter and length. These parameters one can estimate from geometrical proportions of the solenoid. For our model of magnet one can propose the top estimation of specific resistance of a superconductor made of niobium-titanic (Nb3Ti) cable - 1,4-10^?16 Ом*mm2/m. Other characteristics of curve F(t) are also analyzed - statistical features and presence of local «spikes». Some tendency to dispersion increase is explained by degradation of field B homogeneity, probably, connected with external factors. As regular decrease in frequency F is possible at the expense of aging of electronic components of a frequency synthesizer the estimation of influence of this factor is given. Possible influence on measurement F of disturbance of the magnetic field of the Earth is analyzed, in particular, in a context of global seismic activity. Importance of later factor is put into question. But there is an assumption that integration of the data about evolution of Larmour frequency from NMR equipment with different geographical localization can give a valuable material for geomagnetometry.