Determination of the Zones of Action of Alternating Magnetic Field on the Patient's Body

Abstract

Alternating magnetic fields (AMF) are widely used in modern medical practice [1, 3, 5-12, 14, 15, 1721, 23]. Magnetic fields (MF) used for physiotherapy vary considerably: from constant fields and alternating fields with a frequency of 50 Hz to pulsed fields with pulse duration of 0.2-2 msec and magnetic induction of 1.5 T [14, 15]. Many questions concerning the mechanisms of physiological action of magnetic field still remain open. This makes it difficult to state exact requirements for medical equipment and methods for MF treatment. Constant magnetic field acts only on moving electric charges. In tissues and plasma of the human body charges are carried by very slow ions. The action of a constant magnetic field on such ions is negligible. Magnetic field also acts on spin moments of atoms. The mechanism of this action is well studied. However, on the biological level this action is also very weak [2, 9, 20, 23]. Presumably, the biological effect of magnetic field is caused by currents induced in tissues rather than by direct action. The biological action of electric currents is well studied. They are used both for stimulation and physiotherapy. Currents exert an effect on the state and metabolism of cells. Within the range of electric field strength E > 0.2-2 V/cm (EMF U > 220 mV for typical linear size of cell structure) depolarization of cell membranes (including excitation of nerve pulses) is initiated. Within the range of electric field strength from 0.02 to 0.2 V/cm, natural depolarization is improved, and natural metabolic processes are activated and synchronized. Electric fields with significantly smaller strength presumably have no biological effect, or this effect is manifested only upon a very long exposure. Leaving aside the difficult problem of assessment of complete biological effect of MF, we consider in this work only the biological effect of electric currents induced by MF. It should be noted that induced currents cause heating of tissues, which also has a biological effect [23]. The effect of MP on the domain structure of water is also of interest because water is the main solvent of plasma ions. However, it should be taken into account that the time of relaxation of domain boundaries in water decreases sharply with increasing temperature. At body temperature this relaxation time is very small. The currents induced by AMF are calculated from the magnetic induction B(xyz) and the rate of changes in the induction dB/dt. It is rather difficult to assess the induction B(xyz) inside a certain organ. Special software was developed for this purpose (see Appendix). The magnetic field configuration is taken to be known, whereas the rate of field change is taken to be uniquely determined by the pulse duration T. It is required to determine the distribution of field strength E(xyz) and electric currents in a patient’s body from the known magnetic field B(xyz). Faraday law is used to determine EMF and currents induced in a closed circuit L [13, 16, 22]: