RU2619430C2 - Magnetic resonance imaging (mri) - Google Patents

Abstract

FIELD: medical equipment.

SUBSTANCE: invention relates to medical equipment, namely to means of magnetic resonance imaging. MRI comprises main coil placed in magnet cavity, capable to operate as transmitting or receiving-transmitting one, receiving coil and additional coil located near investigated object, capable to operate as transmitting or receiving-transmitting one, or shorted at ends, system of coils’ switching, including switch, automatic switch, adder and selector and receiver and transmitter. Main and additional coils are connected via appropriate cables to switch connected via automatic switch with transmitter and made with possibility of disconnection of main coil with simultaneous connection of additional coil. Receiving coil is connected to one input of adder, other input of which is connected to automatic switch with possibility of obtaining signal from additional coil via switch, while adder output is connected to selector input, having three positions – first position for connection with automatic switch of main coil connection in receiving mode, second position for connection of adder, third position for connecting receiving coil, selector output is connected to receiver.

EFFECT: invention usage allows to reduce and optimize distribution of power for excitation of NMR signal.

4 cl, 5 dwg

Description

FIELD OF THE INVENTION

The invention is intended for use in biology and medicine, allows you to receive images depicting the internal structure of various objects, including animals and humans, using nuclear magnetic resonance (NMR) methods. It is of particular importance for the study of small objects, which may be in demand, for example, in medical studies of small areas of the human body, pediatrics. In addition, it can be useful in biological studies of small laboratory animals, MRI microscopy, local NMR spectroscopy.

State of the art

The NMR method involves the excitation of spins in a constant magnetic field, using the RF field created by the transmitting coil, and receiving the response of the spins to the receiving coil. Therefore, in MRI, much attention is paid to the optimization and improvement of these coils. The term “coil” in MRI is used to denote a radio engineering device, where the main element is an inductor acting as an antenna, which can be receiving or transmitting.

In the first case, it implies the reception of a nuclear induction signal from nuclear magnetic moments precessing in a constant magnetic field. Synchronously precessing spins form a magnetic flux that passes through the turns of the inductor, and due to the fact that it changes in time (with the precession frequency), the induction emf is induced at the ends of this coil (Faraday law).

Further in the text, the term “coil ends” refers specifically to the ends of the inductance coil, which is part of the coil as a radio engineering device.

In the second case, we mean the transmission of a radio frequency (RF) magnetic field to excite this precession. If you place an inductor through an alternating current near nuclear magnetic moments placed in a constant magnetic field, this current forms an alternating magnetic field near this coil, which can act on nuclear spins.

Depending on what functions the coil performs as a radio engineering device, it can be receiving or transmitting. Typically, the structure of the coil as a radio engineering device, in addition to the inductor itself, includes one more element - a capacitor, which determines together with this coil the resonance properties of the device - the ability to amplify the current circulating in a closed circuit made up of these two electrically connected elements.

Since the precession is excited by applying an RF pulse, and registering the NMR signal after the end of this pulse, the same coil can be used to transmit RF power and to receive the NMR signal by switching it from the transmitter to the receiver. Such a coil is called transceiver.

Typically, in medical device-oriented tomographs, the NMR signals are excited by a transmitting coil built into the magnet cavity, the main coil, and signals are received from one of the receiving coils included with the tomograph. Each of the coils of this kit is adapted (due to the design and method of fixation) for the study of a specific organ - head, neck, knee, etc. The adaptation of the coil as a radio engineering device, first of all, involves the adaptation of its main element - the inductor acting as a receiving antenna - the correlation of its dimensions with the dimensions of the scanned object or the size of the scan zone.

For example, the Toshiba Vantage 1.5 T MP scanner:

(http://www.toshibamedicalsystems.com/tmd/english/products/mri/titan3r_function.html#р01).

Siemens tomographs - Magnetom 1.5 T (http://www.healthcare.siemens.com/magnetic-resonance-imaging), have the same equipment.

as well as Bruker Tomikon S50:

(Acquisition and Processing of NMR Images and Spectra Paravision (Part 1 and 2) // User's Guide and Instruction Manual, Bruker Medizintechnik, Doc. No .: T4J-1137, 1996, 948 pp.), See also (http: / /pdf.reestrsi.ru/file/17691-98.pdf).

The size of the zone irradiated by the radio frequency (RF) field is determined by the diameter of the main coil. Since the cavity of the magnet must be spacious enough to accommodate an adult (at least 60 cm in diameter), its size also determines the diameter of the transmitting coil. With such dimensions, it is possible to provide the necessary parameters of a typical scanning pulse sequence only when using a high power transmitter - more than 1 kW. Accordingly, increased power requirements are imposed on switching and protective elements. If this coil needs to be adapted for scanning nuclei other than protons, then the transmitter power must be increased by (γ _p / γ _x ) ² times, where γ _p and γ _x are the gyromagnetic ratios of the protons and the nuclei under study, respectively.

The main disadvantage of tomographs with a single transmitting coil is that when working with small objects or scanning a small zone, the method of generating an RF field is irrational, since the size of the irradiated zone becomes larger than the zone of interest. This creates problems in the study of infants, as well as in the presence of metal implants in the patient's body. Small patients run the risk of getting excessive RF load, and the implants can heat up under the influence of the RF field, and heating can occur far from the scanned area.

Currently, a number of MRI models are being produced in which this problem is partially solved, for example: Achieva 3 T from Philips

(http://www.healthcare.philips.com/main/products/mri/systems/achieva).

In this design, in addition to the receiving coils connected via a flexible cable (from the set of receiving coils included in the kit) of a small size, which are adapted for a specific organ, the user is offered similar ones, but allowing them to be used as transceivers. This allows you to more efficiently use RF power and reduce the RF load on the patient. However, the cost of such coils is significantly higher than purely receiving. It is possible to optimize costs if the kit included a coil that could work as a transmitter together with at least one of the set of receivers. This refers to the possibility of placing it next to the receiving coil.

The disadvantage of this and other known MRI is that such an opportunity is not provided for the user. Therefore, the technical resources of the tomograph are used irrationally. Because of this, the user either incurs significant costs by acquiring a complete set of transceiver coils, or is limited by the available coils, but then the RF transmitter does not work in the optimal mode when scanning for all organs.

In addition, the existing MRI does not pay attention to local suppression of the RF field when scanning a large area. This is especially true if there are metal implants in the patient’s body that can heat up under the influence of an RF field.

The closest in design to the claimed object is an MRI Tomikon S50 firm "Bruker" (http://pdf.reestrsi.ru/file/17691-98.pdf), containing the main coil, rigidly fixed in the cavity of the magnet, with the ability to work as transmitting or receiving and transmitting, and a receiving coil, which has the ability to be placed inside the magnet cavity near the object under study, a switching system that controls the connection of the coils to the receiver and transmitter, as well as the opening of the coils.

Proposed as a prototype, MRI is a medical diagnostic device designed to obtain images of various human organs in plane-parallel sections in three main sections (axial, sagittal and coronary) and in oblique sections with an angle of inclination of up to 45 degrees. The tomograph allows you to visualize the internal structures of various human organs - the head, neck, spine, abdominal cavity, pelvis, mammary glands, shoulder, knee, ankle joints, hands, and other fragments of the body.

Its main disadvantage is that the RF power used to excite the NMR signal is wasted irrationally, especially when examining small objects or scanning small sections. The RF field is created by the flow of current through the main coil, which has a large internal diameter of 60 cm, determined by the size of the magnet cavity. At the same time, the proprietary configuration of this tomograph does not allow replacing the main coil with an alternative one or turning it off in order to connect an additional coil. As a result, a transmitter with a power of 2 kW is used to implement typical scanning pulse sequences, and powerful switching semiconductor elements, pin diodes, are used to close and open the main coil when working together with the receiver. Large voltages (hundreds of volts) accompanying the RF power supplied to the main coil determine the requirements (design, dimensions, etc.) for capacitors and supply cables, the capacitors are located outside the magnet zone in a separate massive block, and the cables connecting the coil with a switching system, very heavy, have a large thickness of 2 cm and are not adapted for repeated manipulations associated with their movements and bends.

Disclosure of invention

The objective of the invention is to develop a new design of MRI, including investigation of small objects or scanning of small areas while optimizing the consumption of RF power.

The technical result of the proposed MRI is to expand the range of objects under study - optimizing the scanning conditions for objects of different sizes on the same tomograph, as well as the rational use of power to excite the NMR signal (the possibility of its reduction and optimal spatial distribution) in the study of small objects, which is especially relevant in medical studies of infants, in biological studies of small laboratory animals, MRI microscopy, local NMR spectroscopy copies, and the presence in the patient's body metal implants which can be heated by the action of an RF field.

Another technical advantage of the proposed MRI is the ability to weaken the RF field only in the local area. This is relevant in the study of objects whose dimensions are commensurate with the dimensions of the magnet cavity, and the suppression of the RF field must be ensured only in the localization zone of the implant.

To achieve these technical results, a magnetic resonance imaging scanner (MRI) is proposed, which contains a main coil rigidly fixed in the magnet cavity, which can operate as a transmitting, or transmitting and receiving coil, which can be placed inside the magnet cavity near the object under study, the switching system that controls the connection of the coils to the receiver and transmitter, as well as the opening of the coils, according to the invention, is equipped with the ability to be placed inside the polo ti magnet near the object under study additional coil having an opportunity to operate in either transmission or transceiver, or short-circuited at the ends, and the switching system also has the ability to manage connection to an additional coil transmitter and receiver, as well as its opening.

Due to the equipment of an MRI that can be placed inside the magnet cavity near the object under study, an additional coil that can operate either transmitting, receiving-transmitting, or shorted at the ends, and the switching system can also control the connection of the additional coil to the receiver and transmitter, as well as by opening it during scanning, it is possible to work with less RF power, more flexibly distribute the technical resources of MRI - select the most adapted to For the studied studied configuration objects for receiving and transmitting coils, apply scanning with local suppression of the RF field. The possibility of placing any coil near the object under study, first of all, concerns its main component - the inductor. This is what determines the requirements for its design and size.

The possibility of achieving new technical effects is explained by the following.

1. Since it is assumed that the additional coil can be placed inside the cavity of the magnet, this means that its size is smaller than the main coil. Therefore, when an additional coil operates as a transmitter or a transmitter / receiver, the RF field generated from this coil is localized in an area whose dimensions are determined by the dimensions of this coil, i.e. less than the main one. Since the additional coil is smaller than the main one, it is able to be placed closer to the object under study than the main one. Due to this, in the area of the object under study, a stronger RF field can be obtained from the additional coil than from the main coil. Therefore, if objects are investigated whose dimensions are comparable with the dimensions of the additional coil, its use is preferable in comparison with the main coil, since lower currents can be used to generate RF fields. Accordingly, use a less powerful transmitter and not so high voltage, and therefore more compact elements that accompany the transfer of RF power to the transmitting coil, capacitors, cables, etc. If we compare the operation of the main and additional coils as transceivers, then due to the possibility of closer placement of the additional coil relative to the object under study, a stronger NMR signal can be obtained from it compared to the main one. As a result, when researching small objects, the use of an additional coil instead of the main one allows you to increase sensitivity, reduce RF power, and prevent RF power from entering an area that is not of interest for scanning. As a result, MRI, equipped with not only the main, but also an additional coil, has the ability to work equally effectively for objects of different sizes.

2. When placing an additional coil in the magnet cavity inside it, local amplification or attenuation of the RF field generated by the main coil is possible. The effect is explained as follows. During a pulsed action on the nuclear magnetization, the main coil generates an RF field in the magnet cavity, which changes according to the law: B = B ₁ cosωt. If the lines of force of this field cross the plane of the coil with an area S of an additional coil, then at the ends of this coil the induction emf V is induced according to the formula:

V = -dF / dt,

where Φ = BS (Faraday law).

If the ends of this coil having inductance L and ohmic resistance R are closed, then current flows through the coil. From this current, a secondary field B ₂ arises, which can be calculated by the formula:

B ₂ = LI / S

It is added to the primary field B ₁ , as a result of which a field B ₃ = B ₁ + B ₂ = B ₁ + LI / S is formed in the center of the additional coil.

To calculate the current I, we use the methods practiced for calculating radio circuits, where to take into account the response to the harmonic effect of currents and voltages on circuits containing inductors and capacitances, we introduce the concept of reactance Z, namely, for inductance Z = jωL, and for capacitance - Z = -j / ωC. These expressions contain the imaginary unit j, symbolizing a phase shift of 90 degrees between the alternating voltage applied to this element and the current flowing through this element. Accordingly, -j corresponds to a phase shift of -90 degrees. To further simplify the calculation of radio circuits, the harmonic function cos (ωt + ф) is replaced by the exponent - exp (jωt + ф)).

With these designations, the magnitude of the current through the coil, closed at the ends, will be I = V / Z. If only inductance is used as the load, then Z = jωL + R, and I = -B ₁ S (jω) / (jωL + R).

Then we have:

If the resistance R can be neglected in comparison with the value of ωL, then B ₁ vanishes. Thus, inside the auxiliary coil, at which the ends of the inductor are shorted, a complete suppression of the external RF field is possible. By positioning this coil near the zone in which the implant is located, the RF field in this zone can be significantly attenuated.

It can be noted that if the additional coil is not short-circuited but connected to a capacitor with a capacitance C, then inside it a significant amplification of the RF field is possible. This can be seen by analyzing the expression for the current, taking into account the fact that the load for the EMF source is not only the inductance and ohmic resistance, but also the capacitance. Then, in the expression for the current, instead of ωL, substitute (ωL-1 / ωC). The greatest effect is achieved at ωL = 1 / ωС, which corresponds to the resonance tuning of the additional coil. In this case, an increase in the RF field by approximately Q times is possible, where Q = ωL / R is the quality factor of the coil. This is a useful effect if an NMR signal is used as the RF field. Therefore, when registering the signal, the additional coil acts as a closed resonant circuit. However, this effect is undesirable when applying an RF pulse from the main coil, since it can cause excessive RF load in the local area. Therefore, when operating an MRI in modes using the main coil, either an additional coil is opened or shorted.

Brief Description of the Drawings

The invention is illustrated by drawings. In FIG. 1 shows the basic elements of an MRI; in FIG. 2 is a connection diagram of all coils with a receiver and a transmitter using a switching system, as well as switching inside the coils, in FIG. 3-5, materials of a test experiment are presented, showing the possibility of using an additional coil for local attenuation of the RF field. In FIG. 3 is a photograph of an additional coil located adjacent to a test object containing an implant simulator, and FIG. Figures 4 and 5 show the tomograms of the object - in the open state of the coil and with a shorted inductance. An MRI contains a main coil 1, rigidly fixed in the cavity of a magnet 2 made of superconducting, having the ability to work as a transmitter, or transceiver and receiver coil 3, which has the ability to be placed inside the cavity of magnet 2 near the test object placed on a mobile platform 4, the system switching 5, controlling the connection of coils 1, 3 to the receiver 6 and transmitter 7, as well as the opening of the coils 1,3. The MRI is equipped with an additional coil 8 that can be placed inside the cavity of the magnet 2 near the test object 4, which can operate either transmitting, receiving and transmitting, or shorted at the ends, and the switching system 5 is also able to control the connection of the additional coil 8 to receiver 6 and the transmitter 7, as well as its opening.

Superconducting magnet 2 is made in the form of a current solenoid immersed in a capacity of 1200 l filled with liquid helium. The induction of the static field of magnet 3 in its working part within a sphere with a diameter of 40 cm is 0.5 T. The main 1, receiving 3 and additional coils 8 are installed inside the cavity of magnet 2. Inside the cavity of magnet 2 with a diameter of 60 cm there is a mobile platform with the object under study 4. Next to the magnet 2 is a switching system 5, one RF input of which is supplied via cable from the transmitter 7, and to another input - cables from the main 1, receiving 3 and an additional 8 coils.

From the output of the switching system, an NMR signal is sent to the input of receiver 6 by cable. The supply and control voltages are also suitable for the switching system. The room in which magnet 2 is located is shielded with copper plates to minimize the effect of external RF radiation on the operation of the MRI. Electronic components are installed in a separate room - a processor that generates tomograph control signals, a transmitter 7 and a receiver 6, to which an NMR signal from a receiving coil 3 is received through a switching system 5, followed by its digitization and recording in electronic form. The tomograph control signals - RF and gradient pulses are fed to power amplifiers and further to the tomograph. The signal from the receiver 6 enters another separate room where the control unit 9 is located. It is based on a computer controlled by the operator. Using this computer, instructions are sent to the processor and signal data is received. On the same computer that the operator controls, the data is processed - MRI images are reconstructed and their post-processing is processed - scaling, contrast control, volumetric processing, archiving by writing to disk or other media, and issuing images on film or in print.

Functional relationships of the main components of MRI are shown in FIG. 2. The arrows indicate the direction of the RF signal, which can be either an RF pulse from the transmitter 7, or an induction signal from precessing nuclei entering the receiver 6. The transmitter 7, receiver 6 and switching system 5 are controlled through the control unit 9, which receives user instructions via the interface (not shown in the diagram). From the output of the receiver, the signals are sent to a computer that processes the signals and outputs data to electronic devices for temporary storage and archiving (RAM, hard disk), visualization (display), documentation (film or printout), other information resources (Internet, etc. .). This computer and associated peripherals are not shown in the diagram.

According to the invention, the principle of operation of MRI is as follows. The RF pulse from the transmitter 7 through the communication system 5 is fed to the main coil 1, rigidly fixed in the magnet cavity, which can operate as either transmitting or receiving and transmitting through cable 10. This RF pulse generates nuclear magnetization. Since the voltage transferred from the transmitter to the main coil is hundreds of volts, a cable with thick insulation is used, and therefore very hard. The transition of this coil from the state of the transmitter to the receiver is carried out using an automatic switch 11. It is controlled by an electronic circuit that monitors the arrival of an RF pulse from the transmitter. As a result, during the action of the RF pulse from the transmitter 7, the switch 11 is in the upper position, and at the end it is in the lower position.

Excited magnetic moments give an NMR signal. The NMR signal from the receiving coil 3 is fed to another input of the switching system 5, after which it is able to go to the input of the receiver 6. The realization of this possibility is determined by a pre-programmed selector 12. It is an electronic switch whose task is to send the NMR signal to receiver 6 or from the main coil 1 (upper position of selector switch 12), or from receiving 3 (lower position of this switch).

The size of the receiving coil 3 is significantly smaller than the size of the cavity. And since the signals with which it deals are low-power (NMR signals do not exceed hundreds of millivolts), it is connected to switching system 5 using cable 13 with a small layer of insulation, and therefore flexible. This makes it possible to accommodate the receiving coil 3 near the object under study, including the random placement of the latter inside the cavity of magnet 2.

According to the invention, an additional coil 8 is connected, which, like the main 1, can operate either as a transmitter or as a transmitter / receiver. At the same time, its dimensions are significantly smaller than the dimensions of the cavity, which makes it possible to work with less RF power. The latter circumstance reduces the requirements for breakdown qualities to the elements accompanying this power - isolation capacitors, cables, etc. Therefore, the additional coil is connected to the switching system 5 in the same way as the receiving coil, using a conventional flexible RF cable 14. This makes it possible to be placed in the same way as the receiving 3 near the object under study with the arbitrary placement of the latter inside the magnet 2 cavity .

In the proposed MRI, it is possible to disconnect the main coil 1 with the simultaneous connection of an additional 8. This function is performed by the switch 15. To ensure the summation of the signals from the receiving 3 and the additional 8 coils, a separate input of the selector 12 is used, to which the output of the adder 16 is connected, to one of which a signal is received from the receiving coil 3, and to the other input through the switch 15 and the automatic switch 11 is the signal from the additional coil 8. Then, in the middle position of the selector 12, the receiver t is the sum of the signals from the receiving 3 and additional 8 coils. This allows you to increase the sensitivity of MRI compared with the usual registration scheme.

All coils have the same functional structure and differ only in circuit solutions that take into account:

1) the magnitude of the voltages that can come from the transmitter 7. Since the signal from the transmitter 7 does not arrive at the receiving coil 3, these solutions can be arbitrary. For example, it is possible to use elements of electronic control of the circuit capacity - varicaps, which is unacceptable for coils designed to operate as transmitting or receiving and transmitting;

2) the possibility of shorting the coil. This function in the invention is provided only for an additional coil.

Each coil (1, 3, 8), from the point of view of radio engineering, is an oscillating circuit composed of inductance (17, 18, 19) and capacitance (20, 21, 22). Typically, the circuit is closed - the inductance is electrically connected to the capacitance, and if an alternating voltage is applied to the circuit (from the transmitter or induction EMF is induced), then the current circulates in the circuit, the value of which is maximum at known relations between the capacitance, inductance and frequency of the voltage change. The invention provides for the opening of these circuits using switches (23, 24, 25) controlled from the switching system, for which signals (26, 27, 28) from the switching system are connected to the coils. Opening is equivalent to disconnecting the circuit. This is used when working with MRI in dual-coil modes, one of which works only as a receiving one, to prevent currents giving secondary RF fields in the coil circuits, which greatly complicates the overall picture of the distribution of RF fields, and ultimately complicates the interpretation MRI images. When MRI is operating in the indicated modes, the loops are connected and disconnected simultaneously - when the RF pulse is applied, the receiving coil is turned off, and when the NMR signal is registered, the receiving coil is connected, but the transmitting one is turned off.

Since the proposed MRI provides for the use of an additional coil not only for excitation and registration of the NMR signal, but also for their use in scanning mode, in which it is planned to attenuate the RF field in the location zone of this coil, it is provided that the ends of this coil are closed using switch 29. Its switching does not require high speed, therefore, unlike other switches controlling the coils, there is no need to use a switching system for its operation.

An example of the use of the proposed MRI

In FIG. Figure 3 demonstrates the use of an additional coil to attenuate the RF field in the localization zone of the implant. As a scanned object, a cylindrical container 30 filled with water was used. At the end of the cylinder, a metal coin 31 of non-ferromagnetic material is attached as a simulator of an implant. An additional coil 8, having the shape of a ring 32 connected to a block in which the tuning capacitance and switching elements 33 are located, is located next to the scanned object. The coil is leaned against the end of the cylinder, with the center of the ring positioned near the implant. The object together with the coil was placed in an MRI, with which a cross-sectional scan of the object was carried out in the direction perpendicular to the axis of the cylinder, which, in turn, was oriented in the direction of the lines of force of the magnetic field generated by the main coil. Scanning was carried out using the main coil as a transceiver, and the gradient echo technique with a small tipping angle was used to detect changes in the MRI signal associated with the magnitude of the RF field and not other side factors (the effect of relaxation, etc.). ) The results of scanning by the method are shown in tomograms - FIG. 4, 5. In the first case, the turns of the auxiliary coil were open, and in the second, closed.

On the tomograms you can see a spot from the "implant". It can be seen that the area of the attenuated signal coincides with the area of overlapping of the object by the applied ring. Moreover, this effect occurs only with a short circuit of the coil of an additional coil. When the coil is open, its presence next to the scanned object does not appear on the tomogram.

An example is implemented on a Tomikon S50 MRI scanner from Bruker. As an additional coil, a proprietary coil was used, which was originally designed to study the shoulder joint and worked only as a receiving one. To adapt it to the functions of the additional coil, the following actions were taken:

1. To work as a transmitting or receiving and transmitting, the varicaps used as tuning tanks have been removed. Instead, tanks are installed, the basis of which are coaxially located movable metal plates. The capacity value depends on the degree of overlap of the plates. Unlike varicaps, a significant voltage from the transmitter can be applied to such a capacity - hundreds of volts.

2. To ensure shorting of the ends of the coil, short wires are soldered to the ends of the annular wire, which the user could close by ordinary twisting.

When using an additional coil to attenuate the RF field, the direction of the magnetic field lines of the main coil for each specific MRI model should be taken into account - the axis of the additional coil should be parallel to them.

Claims (4)

1. Magnetic resonance imager containing installed in the cavity of the magnet rigidly fixed main coil made with the ability to work as transmitting or receiving and transmitting, and placed near the object under study, the receiving coil and an additional coil made with the ability to work as transmitting or receiving - transmitting, or shorted at the ends, coil switching system, including a switch, an automatic switch, an adder and a selector, and a receiver and transmitter, while the main and additional Extra coils are connected via appropriate cables to a switch connected via an automatic switch with a transmitter and configured to disconnect the main coil while connecting an additional coil, the receiving coil is connected to one input of the adder, the other input of which is connected to the automatic switch with the possibility of receiving a signal from the switch additional coil, and the output of the adder is connected to the input of the selector having three positions - the first position for soy line with an automatic switch for connecting the main coil in the receiving mode, the second position for connecting the adder, the third position for connecting the receiving coil, the output of the selector is connected to the receiver. 2. The tomograph according to claim 1, characterized in that the cable for connecting the receiving and additional coils is made flexible and radio frequency. 3. The tomograph according to claim 1, characterized in that each of the coils is made in the form of an oscillatory circuit, including an inductance, a capacitance and a circuit disconnect switch, configured to control the switching system. 4. The tomograph according to claim 3, characterized in that the additional coil is equipped with a switch for closing the ends of the coil.

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