N.V. Anisimov1, Yu.A. Pirogov2

1,2 Lomonosov Moscow State University (Moscow, Russia)

1 anisimovnv@mail.ru, 2 yupi937@gmail.com

MRI experiments in a field of 0.5 T are described, demonstrating an increase in the NMR signal inside a solenoid immersed in water compared to the signal outside it. The effect is explained by the fact that the solenoid in water becomes a wireless resonant circuit with a frequency of 21 MHz, which coincides with the Larmor frequency for protons. In air, the circuit has only concentrated inductance and interturn capacitance. The latter is small and comparable to the parasitic capacitances of the environment. Because of this, the frequency of the circuit is very high, and its resonant resistance is unacceptably high. In water, the value of the interturn capacitance increases 80 times (by the value of the dielectric constant of water), which determines the value of the capacitance necessary for a resonant circuit with a frequency of 21 MHz. In this work, the solenoid is used as a wireless coil, inductively coupled to a wired one, which makes it possible to amplify the NMR signal in a small area determined by the dimensions of the wireless coil. As a result, images were obtained demonstrating the effect of a significant (up to 4 times) amplification of the NMR signal inside the solenoid compared to the signal outside it. The effect depends on the electrical conductivity of the water, and therefore appears only in clean (distilled) water. Otherwise, instead of increasing the signal inside the solenoid, it may decrease. Calculations of the parameters that determine the basic properties of a circuit based on a solenoid immersed in water are presented - the value of the distributed (interturn) capacitance, the mutual induction between the circuit and the wire coil, and the electrical conductivity of water. Good agreement between experimental and calculated data is noted. The possibility of studying objects immersed in water is being considered. In relation to wildlife objects, the problem of a possible decrease in the effectiveness of the method due to an increase in the electrical conductivity of water due to the presence of sodium ions involved in cellular processes is noted, and it is proposed to solve this problem by forcing the influx of distilled water into the area of interest.

Anisimov N.V., Pirogov Yu.A. Resonant properties of a solenoid immersed in water. Electromagnetic waves and electronic systems. 2024. V. 29. № 2. P. 44−54. DOI: https://doi.org/10.18127/j15604128-202402-05 (in Russian)

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