RU186685U1 - Sensor for measuring the volume distribution of the magnetic field - Google Patents

Abstract

Usage: to measure the volume distribution of the magnetic field. The essence of the utility model lies in the fact that the sensor for measuring the volume distribution of the magnetic field is a system of NMR sensors mounted on seven plates connected together, parallel, having a common center of symmetry, so that the system of NMR sensors forms a volume of uniformity of the magnetic field - a sphere in which 7 NMR sensors are located along the Z axis of the magnetic field, with one of them located in the center of the sphere, and the rest located on the surface of the sphere in a circle on each square otherwise, tuning capacitors are also fixed on the plates in an amount that coincides with the number of NMR sensors located on this plate, while the zenith angles between the NMR sensors that are on different plates are the same, the sphere diameter is 150 mm, the NMR sensors are made frameless, on the plates closer to the poles of the sphere, there are 9 NMR sensors each, on the remaining plates 13 each, electromagnetic screens are installed between adjacent, closely located NMR sensors or they are arranged orthogonally in relation to the rest. EFFECT: provision of the possibility of creating a sensor for measuring the volume distribution of a magnetic field positioned in a magnet with a gap of 200 mm and allowing measuring the volume distribution of a magnetic field with a strength of 0.4 T (operating frequency 17.5 MHz) for one to two minutes. 8 s.p. f-ly, 6 ill.

Description

The utility model relates to technical physics, namely to measuring techniques based on nuclear magnetic resonance (NMR). The claimed technical solution can be used in devices and NMR devices, as well as in other devices where it is required to obtain a magnetic field of the required configuration with high accuracy.

When the magnetic system is initially set up, specialized sensors are usually used to restore the spatial map of the magnetic field distribution by directly measuring the magnetic field strength. Typically, such sensors are an NMR meter of magnetic field strength and a device that allows you to position it at certain points in space. The procedure for measuring the magnetic field distribution map consists in sequentially measuring the magnetic field strength in certain places of the working area of the magnetic system with a single sensor. However, it requires a very large investment of time for measurements for NMR imaging, where significant volumes of the magnetic field are used.

A known sensor for measuring a spatial map of the field of a tomograph, which is an arc with seven point resonant circuits (sensors) located on it and a mechanical commutator. To correct the long-term drift of the main magnetic field, an alternate reading of the frequency of the detuning of the NMR signal from the contour to the circle and from the reference contour is used. Measurement of a field map using such an arc sensor takes at least 20 minutes and is highly dependent on the human factor [Biktimirov E.F., Fakhrutdinov A.R., Anashkin V.N. // Kazan Institute of Physics and Technology named after E.K. Zavoisky 2005. Yearbook. Kazan, Fikhtehpress. 2006. S. 164-166].

The closest technical solution to the claimed one is a sensor for measuring a three-dimensional map of a magnetic field, described in [Complex for measuring a three-dimensional map of a magnetic field / Biktimirov E.F., Fakhrutdinov A.R., Anashkin V.N. // Kazan Institute of Physics and Technology named after E.K. Zavoisky 2005. Yearbook. Kazan, PhysTechpress. 2006. S. 164-166]. Structurally, the sensor in this complex is a device of seven equidistant plates located, interconnected at the corners using brass spacers. 103 NMR sensors are rigidly fixed on the plates in such a way that 7 of them are located along the Z axis of the magnetic field (for a quick assessment of uniformity on the Z axis), the rest are located on the plates around the circumference of the homogeneity volume — a sphere of radius 300 mm.

The described device is made for a magnetic system that creates a field of 0.06 T (working frequency 2.5 MHz) in a sphere with a diameter of 300 mm and cannot be used for a small tomograph to measure a magnetic field of 0.4 T (working frequency 17.5 MHz), including because which cannot be positioned in a magnet with a gap of 200 mm, since the sensor has dimensions that exceed the gap of the magnet of a compact tomograph. In addition, in the device described in the prototype, frame inductors are used, which complicates the manufacture of inductors and the sensor as a whole. The presence of frames does not allow to achieve a significant duty cycle and increase the signal-to-noise ratio, which reduces the accuracy of measurements.

The technical problem, which is claimed by the claimed utility model, is the creation of a sensor for measuring the volume distribution of the magnetic field, positioned in a magnet with a gap of 200 mm and allowing to measure the volume distribution of the magnetic field of 0.4 T (operating frequency 17.5 MHz), for one two minutes.

The technical problem cannot be solved by simply reducing the size of the prototype, since this raises the problem of radio-frequency isolation of nearby circuits tuned to the same frequency.

The technical result consists in the constructive device of the inventive sensor for measuring the volume distribution of the magnetic field, providing the possibility of its positioning in the magnet with a gap of 200 mm, the absence of mutual influence of nearby contours and increasing the accuracy of measuring the volume distribution of the magnetic field.

The problem is solved, and the technical result is achieved by a new sensor for measuring the volume distribution of the magnetic field, which is a system of NMR sensors mounted on seven plates connected together, parallel, having a common center of symmetry, so that the system of NMR sensors forms a volume of magnetic uniformity field bounded by a sphere in which 7 NMR sensors are located along the Z axis of the magnetic field, one of them being in the center of the sphere, and the rest are located on the surface of the sphere s circumferentially on each plate, the plates also are fixed tuning capacitors, in an amount which coincides with the number of the plate located on NMR sensors. A feature of the inventive sensor is that the diameter of the sphere is 150 mm, on the plates located closer to the poles of the sphere, there are 9 NMR sensors, on the other plates - 13, closely adjacent neighboring sensors are separated by electromagnetic screens or orthogonal to for the other, in the absence of mutual influence, the NMR sensors are frameless to increase the duty cycle and signal to noise ratio to improve measurement accuracy.

In FIG. 1 presents a photographic image of the claimed device.

The sensor for measuring the volume distribution of the magnetic field is 83 NMR measuring sensors 1, located as follows: 76 NMR sensors 1 are located on a sphere with a diameter of 150 mm, limiting the volume of magnetic field uniformity with a strength of 0.4 T (operating frequency 17.5 MHz), and another 7 NMR sensors 1 are located along the axis of the magnetic field (Z axis), including one sensor located in the center of the sphere. This sensor is installed in the isocenter of the magnetic system when the sensor is positioned before measurements.

Structurally, such an arrangement of NMR sensors 1 is achieved by the fact that all NMR sensors 1 are rigidly fixed on seven interconnected plane-parallel plates 2: on two plates 2 located closer to the poles of the sphere on the surface of which the NMR sensors 1 are located, - " external "plates (marked as 2A), - each contains 9 NMR sensors 1, on the other five -" internal "plates 2 (marked as 2B) - 13 NMR sensors 1, and plates 2 are located so that the zenith angles between measurement points (NMR sensor s), which are located on different plates, are identical, as shown schematically in FIG. 2.

The plates 2 are made integral and of the same size from a non-magnetic material, for example, from a foil-coated fiberglass, and are a rectangle with a length of sides lying in the range of 180-205 mm. The plates 2 are rigidly interconnected using parts of a non-magnetic material, for example, using brass spacers (not indicated).

Figure 3 schematically shows the "inner" plate 2B with the 13th NMR sensors 1 mounted thereon, and figure 4 shows its photographic image.

As the figures illustrate, 12 NMR sensors 1 are located on the plate 2, forming a circle belonging to the sphere of uniformity of the magnetic field. The symmetry axis of 10 (labeled 1A) of the 12 indicated NMR sensors are parallel to each other, and 2 (labeled 1B) are located orthogonally with respect to them. The 13th NMR sensor 1 (1B) is located in the center of the circle and is one of 7 NMR sensors 1 located on the axis of the magnetic field (Z axis).

Each of the NMR sensors 1 is a resonant circuit, consisting of an inductor in which an ampoule with a resonant substance is placed, and a capacitor assembled in the form of a capacitive divider to match the equivalent circuit resistance and input resistance of the circuits connected to the circuit. Unlike the prototype, the NMR sensors 1 do not have frames, which simplifies the manufacture of inductors and the sensor as a whole and gives an advantage compared to the prototype in terms of increasing the duty cycle, and hence increasing the signal-to-noise ratio.

Between adjacent parallel-mounted NMR sensors 1A, electromagnetic shields 3 (8 pieces) made of well-conducting non-magnetic metal, for example, copper foil foil fiberglass, are rigidly fixed.

The use of electromagnetic screens 3 and the orthogonal arrangement of the NMR sensors 1B with respect to the NMR sensors 1A where the use of screens is difficult, solves the problem of radio-frequency isolation of nearby contours.

At the edges of the plates 2, tuning capacitors 4 are rigidly fixed, used to fine tune the loops to the resonant frequency. The tuning capacitors 4 are made of non-magnetic material, in an amount coinciding with the number of 2 NMR sensors 1-13 located on this plate on the "internal" plates 2, 9 - on the "external" ones. The location of the tuning capacitors 4 at the edges of the plates 2 allows you to configure the NMR sensors 1 directly in the gap of the magnet, which increases the usability of the inventive device.

The figure 5 presents a diagram of the "external" plate 2A with 9 NMR sensors installed on it 1. The symmetry axes of the 8 NMR sensors (1A) are parallel to each other, forming a circle belonging to the sphere of uniformity of the magnetic field. Between adjacent NMR sensors 1A are electromagnetic screens 3 (6 pieces). In the center of the circle formed by the NMR sensors 1A, orthogonally with respect to the NMR sensors 1A, there is one NMR sensor 1B lying on the Z axis of the magnetic field.

The operation of the inventive device is illustrated by a specific example of the implementation of the inventive sensor for measuring the volume distribution of a magnetic field with a strength of 0.4 T (operating frequency 17.5 MHz) of a small-sized tomograph TMP-0.4-KPTI, created in KPTI them. E.K. Zavoysky KazSC RAS.

The sensor is a rectangular parallelepiped with sides length-width-height = 205-180-140 mm (the height is indicated without taking into account the NMR sensors fixed on the plates) and is placed in the magnet gap TMP-0.4-KPTI 200 mm. The sensor consists of seven rigidly interconnected parallel to each other and having a common center of symmetry of the plates. The plates are made whole of foil fiberglass, the same size (length-width-height (taking into account the installed elements) = 205-180-16 / 20/24 mm) and interconnected using brass spacers at the four corners of the plates so that the anti-aircraft the angles between the measurement points (NMR sensors), which are on different plates, are the same, as shown schematically in FIG. 2. The distance between the plates is: 18 mm between the outer and the next after the outer inner plate (between the first and second, seventh and sixth), 22 mm between the next after the outer inner plate and the next inner plate (between the second and third, sixth and fifth) , 26 mm between the last indicated and middle plate (between the third and fourth, fifth and fourth).

On the “external” plates located closer to the poles of the sphere (homogeneity volume), on the surface of which there are NMR sensors, there are 9 NMR sensors, on the other five - “internal” plates — 13 NMR sensors, the sensors are located on the plates as described previously and shown in figures 3-5. Thus, 76 of 83 NMR sensors are located on a sphere with a diameter of 150 mm, which limits the volume of magnetic field uniformity, and another 7 NMR sensors are located on the axis of the magnetic field (Z axis).

The NMR sensor is a resonant circuit consisting of an inductor made of a copper wire that holds well in shape (inductance of the order of 1 μH) and a capacitor assembled in the form of a capacitive divider to match the equivalent circuit resistance and input resistance of the circuits connected to the circuit. A sealed ampoule of dielectric material is placed in the inductor, completely filled with a resonant substance - a solution of copper sulfate.

Between parallel NMR sensors, rectangular electromagnetic screens made of a non-magnetic material - foil-coated fiberglass are rigidly fixed. The number and size of the screens should ensure the absence of radio frequency communication of nearby contours; in the particular sensor described, the length of the screens located on the "internal" plates (8 pieces) is 70 mm, on the "external" (6 pieces) - 100 mm for two located on the edges of the screens, 47 mm for the remaining four. The height of the screens depends on the distance between the plates (1-2 mm less).

At the edges of the plates, tuning capacitors with a nominal capacitance of 10-120 pF are rigidly fixed, in an amount that coincides with the number of NMR sensors located on this plate.

NMR sensors previously, before collecting the sensor to measure the volume distribution of the magnetic field, are tuned to the resonant frequency on each plate separately using tuning capacitors by changing the capacitance of the capacitor by rotating the capacitor rotor. During the assembly of the inventive device, as well as during its placement in the magnet gap, the resonant frequency of some NMR sensors may slightly change. In this case, the resonance frequency of the NMR sensors is tuned using the tuning capacitors. The location of the tuning capacitors at the edges of the plates allows you to configure the NMR sensors directly in the gap of the magnet, increasing the usability of the inventive device.

To automate the process of measuring the map of the distribution of the magnetic field using an automated system, schematically depicted in figure 6, similar to that described in the prototype. The work of the complex is described in [Fakhrutdinov A.R., Fattakhov Y.V., Shagalov V.A., Khabipov R.Sh., Bayazitov A.A. Hardware-software complex for measuring the volumetric map of the magnetic field. The collection of materials of the II All-Russian scientific-practical conference "Scientific instrumentation - the current state and development prospects." Kazan, June 4-7, 2018, S. 294-295]. Each of the NMR sensors of the sensor is connected to the multiplexer board, designed to connect the desired NMR sensor in the sensor at a certain point in time, and connected to the control board, which is connected to the computer and allows you to control the sensor through the computer. The multiplexer board is also connected to a radio spectrometer, which in turn is connected to a computer. The personal computer that controls the tomograph acts as a coordinating device for the complex.

A software module has been developed for the complex that implements a spatial field map. It forms a sequence of switched sensors, in accordance with which the currently needed sensor is connected to the receiving path of the tomograph, after which, using the hardware and software of the tomograph, an NMR signal is generated and recorded and the frequency offset for the given sensor is calculated. The driver managing the complex, in addition to communication functions, also binds the sensor numbers to spatial coordinates, which is necessary for forming a field map.

When the software module is operating, the following modes are available to the operator: “1”, “2”, “3”.

In the “1” mode, any sensor out of 83 can be selected in the parameters window of the tomography radio spectrometer control program. Pulse sequence parameters are also set such as the amplitude and duration of the radio frequency pulse, the number of points of digitization of the decay signal of free induction (SSI) and the interval between them, value cutoff frequencies of the low-pass filter, the number of retries (accumulations), retry time between scans, etc. After starting the program with the Start button in the scan window, the computer controlling the tomograph sends the corresponding command to the microcontroller of the complex control board. In this case, the multiplexer board, upon command from the control board, connects the sensor sensor selected by the operator. The pulse program begins to work in stand-alone mode, and the operator can observe the SSI signal at the corresponding point in the space of the magnet gap. This mode can be used either for operational control of the magnetic field at a selected point, or for adjusting the operating frequency of the tomograph to the magnetic field in the magnet isocenter (sensor "0") so that readings at other points in space are made relative to this point. The tomography software allows you to determine the frequency of the detuning of the SSI signal and, if necessary, write this value to the corresponding file.

In the "2" mode, the software module operates as follows. After starting the pulse program with the preset parameters, the computer controlling the tomograph sends a command to connect the “0” sensor to the control board, the pulse program starts its work, the detuning frequency is determined by the SSI signals and the values are written to the corresponding file. When the number of scans equal to those set by the operator is executed, the multiplexer board switches to the next sensor upon command from the computer to the control board. The impulse program continues its work. The numbers of sensors located on the Z axis of the magnetic field are predefined in the text of the pulse sequence before compiling the program. Upon completion of the pulse program, a file is generated containing the coordinates of the sensors and the corresponding average values of the frequency detuning from the resonance value.

In the “3” mode, the program module works similar to the “1” mode with the difference that every ten sensors switch to the “0” sensor in order to measure and compensate for the possible field drift during the measurement time, which is 2-3 minutes in depending on the number of accumulations and the set retry time. Each time, the software module analyzes the frequency values for the zero sensor and corrects the frequencies for ten points between these values according to a linear law. Upon completion of the pulse program, the corresponding file is also generated in which the maximum value of the magnetic field inhomogeneity is calculated by the formula:

where ƒ ₀ is the frequency in the isocenter of the magnet.

Before measurements, the sensor is placed in the gap of the tomograph magnet in such a way that the NMR sensor located in the center of the volume of magnetic field uniformity - in the middle of the middle (fourth) plate (sensor 0) - is installed in the isocenter of the magnetic system.

The hardware-software complex is launched in the “1” mode in order to adjust the operating frequency of the tomograph to the magnitude of the magnetic field at a point with coordinates (0; 0; 0). The frequency of the SSI beats at this point is almost zero, taking into account a small temperature drift. Next, the complex is launched in the "2" mode. In this mode, the sensors located on the axis are switched sequentially, the results of measuring the beat frequency at these points are recorded in the corresponding file. To correct the inhomogeneity of the magnetic field of the first order (linear gradients), correction currents excited in the gradient coils of the tomograph can be used.

Thus, a sensor was created for a small-sized tomograph, which allows it to be positioned in a magnet with a gap of 200 mm, and which makes it possible to quickly - within one to two minutes - and rather accurately evaluate the uniformity of the magnetic field with a strength of 0.4 T (operating frequency 17.5 MHz) and calculate the amplitudes of the spherical components of the field, which is necessary at the stages of installing the magnet during commissioning, when checking gradient fields and during operation of the tomograph to quickly correct field inhomogeneity. The absence of frames of NMR sensors provides an increase in the duty cycle and signal-to-noise ratio, which leads to an increase in measurement accuracy compared to the prototype.

Claims (9)

1. A sensor for measuring the volume distribution of the magnetic field, which is a system of NMR sensors mounted on seven plates connected together, parallel to each other, having a common center of symmetry, so that the system of NMR sensors forms a volume of uniformity of the magnetic field — a sphere in which 7 NMR sensors are located along the Z axis of the magnetic field, one of which is located in the center of the sphere, and the rest are located on the surface of the sphere in a circle on each plate, the plates are also fixed capacitors, in an amount that coincides with the number of NMR sensors located on a given plate, characterized in that the zenith angles between the NMR sensors that are on different plates are the same, the sphere diameter is 150 mm, the NMR sensors are frameless on the plates located closer to the poles of the sphere, there are 9 NMR sensors, 13 on the other plates, electromagnetic shields are installed between adjacent closely located NMR sensors, or they are located orthogonally with respect to the others. 2. The sensor according to claim 1, characterized in that all its parts are made of non-magnetic material. 3. The sensor according to claim 1, characterized in that on the plates located closer to the poles of the sphere, 8 NMR sensors are arranged in parallel, forming a circle belonging to the sphere of uniformity of the magnetic field, and electromagnetic screens are installed between them, and 1 NMR sensor is located in the center of the circle is orthogonal to the rest. 4. The sensor according to claim 1, characterized in that on the plates, in addition to the spheres located closer to the poles, 12 NMR sensors are located on a circle belonging to the magnetic field uniformity sphere, 10 of them are parallel and electromagnetic screens are installed between them, 2 are located orthogonally with respect to 10, and 1 NMR sensor is located in the center of the circle. 5. The sensor according to claim 1, characterized in that the NMR sensor is a resonant circuit, consisting of an inductor in which an ampoule with a resonant substance is placed, and a capacitor. 6. The sensor according to claim 5, characterized in that the inductor is made of a copper wire that holds well in shape and has an inductance of the order of 1 μH. 7. The sensor according to claim 5, characterized in that the capacitor is assembled in the form of a capacitive divider to match the equivalent circuit resistance and input resistance of the circuits connected to the circuit. 8. The sensor according to claim 5, characterized in that the resonant substance is a solution of copper sulfate. 9. The sensor according to claim 1, characterized in that the tuning capacitors are installed at the edges of the plates.

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