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Evolution of working frequency of the MR scanner with a superconducting magnet is analysed. The materials of MRI database studies received for 14 years are used for this purpose. Statistical features of the data and specificity of body of interest are marked. From the evolutionary curve the field decay time is defined as 125000 years. This value is used for calculation of resistance of winding of magnet and an estimation of the top value of specific conductivity of a superconducting material (1,4*10-16Ом*mm2/mThe possible reasons of local "spikes" on the evolution curve are considered: technogenic factors, variations of the magnetic field of the Earth connected with global seismic activity.