M.V. Gulyaev
Ph. D. (Phys.-Math.), Senior Research Scientist, Faculty of fundamental medicine, M.V. Lomonosov Moscow State University
E-mail: gulyaev@physics.msu.ru
O.S. Pavlova
Post-graduate Student, Engineer-Laboratory Assistant, Faculty of fundamental medicine, M.V. Lomonosov Moscow State University
E-mail: ofleurp@mail.ru
D.V. Volkov
Student, Faculty of Physics, M.V. Lomonosov Moscow State University
E-mail: mdanf1@gmail.com
N.V. Anisimov
Dr.Sc.(Phys.-Math.), Senior Research Scientist, Faculty of fundamental medicine, M.V. Lomonosov Moscow State University
E-mail: anisimovnv@mail.ru
Yu. A. Pirogov
Dr.Sc. (Phys.-Math.), Professor, Faculty of Physics, M.V. Lomonosov Moscow State University
E-mail: yupi937@gmail.com

Magnetic resonance imaging (MRI) is one of the most popular introscopy method of the medical diagnostic, which gives information about internal structures of an organism as sliced tomographic images. The quality of these images depends on two main factors: the spatial resolution and the signal-to-noise ratio (SNR). Both of them are linked – an increase in the spatial resolution leads to decrease in the intensity of the NMR signal and, consequently, in the SNR. There are some methods to solve this problem. The major one is increasing the sensitivity of radiofrequency (RF) coils. For this purpose, surface coils can be useful; they give high NMR signal near coil turns. However, their sensitivity decreases with the distance from the plane of the coils that hampers to visualize internal structures located deep inside a body. Given problem can be solved if the coil is implanted close to particular organ. These devices having a high sensitivity near themselves and being implanted in a body are called implantable coils. They all can be divided into two groups: wired and wireless.

The first one has a coaxial RF cable which connects the electronic scheme of the coil with a preamplifier of MRI system. These coils can be receiving or transmit/receive coils, and the NMR signal from organs induced in such coil goes to the preamplifier. Most MRI studies with wired implantable coils refer to endoscopic MRI. The main their application is the visualization of vessels. They are a twisted wire in a various geometric forms, tuned to the required resonance frequency (usually hydrogen nuclei) by discrete capacitors. These coils have to be a needle-shaped and flexible enough to be placed in a catheter. Such signal amplification due to implantable coils turned out useful for MR microscopy which allows to visualize single cells. In this direction, the solenoidal microcoils are usually used. Their design injures surrounding tissues much less than other types of implantable coils. The solenoidal coils can be used for vessel imaging (MRI angiography) and even for diagnostic and monitoring epilepsy or breast cancer. In the present review, we considered theoretical foundations and equations applied for developments of similar coils. However, the main disadvantage of wired coils is associated with the local heating of the RF cable, especially in high magnetic fields. Additionally, when the coils are implanted deep into the body, the conductive pathway must goes to the outside that injures surrounding tissues.

Another type of implantable coils relates to wireless coils, which do not have RF cables. They connect to external transmit/receive coil by the effect of the mutual inductance. Depending on the study, various types of inductive coupling wireless implantable coils are used in MRI. The important direction in the application of wireless implantable coils relates to the visualization of the spine of laboratory animals. Due to the axial symmetry of the spine, these coils are usually designed in the form of the letter V. Besides, wireless implantable coils can be used to monitor the process of cell culture implantation into the body. In this case, the loop-gap (one-turn circuit with a gap) coils are used. The main problem of wireless implantable coils is the complications in resonance tuning these coils. When wireless implantable coils are coating with an insulator or injecting into a body, the resonance frequency shift occurs. It causes an additional inductance. The change of matching is associated also with the appearance of additional resistance from the insulator and surrounding tissues. When designing implantable coils, it is important to take into account their rigidity and size, since they press on the surrounding tissue and can lead to various damages or changes in the physiological functions of this tissue. The solution of this problem is to use flexible coils made with micromolding on thin dielectric substrates. The design of these coils is based on the principle of TLR (transmission line resonator). They consist of two conducting circuits, separated by a layer of dielectric (for example, Teflon). Thus, such coil represents an oscillatory circuit, which has self-resonant frequency. The thickness of the dielectric for TLR coils can reach 100 μm, and the diameter of the coil can be only 5-6 mm. This review also contains description of the theoretical foundations and equations for the development of such coils.

This work is supported by Russian Science Foundation grant No.17-79-10448.

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