Magnetic resonance gradient eddy current measuring device

Abstract

The invention discloses a magnetic resonance gradient eddy current measuring device, and relates to the technical field of magnetic resonance imaging. Comprising an eddy current coil support, the eddy current coil support is provided with two eddy current coil units in each axis direction of a three-dimensional space axis, and each eddy current coil unit comprises a coaxial coil, a feed panel, a pre-amplifier and a small signal ball; the coaxial line coils are annular and are electrically connected with the feed panel, a signal small ball is fixed in the center position of each coaxial line coil, magnetic resonance signals of the signal small balls are transmitted to the pre-amplifier through the feed panel to be processed and finally output to a computer end to be subjected to eddy current calculation and parameter compensation, and the eddy current of the magnetic resonance system is corrected. According to the invention, the self-shielding effect of the coaxial coil is utilized to realize the characteristic of high isolation between the eddy current coil units, an eddy current compensation parameter is designed according to an eddy current measurement result to obtain a good eddy current correction result, the corrected system is good in imaging, and the eddy current of the system is greatly reduced.

Description

Technical Field

[0001] The invention relates to the technical field of magnetic resonance imaging, in particular to a magnetic resonance gradient eddy current measuring device. Background Art

[0002] Magnetic resonance imaging is widely used in many fields. Its imaging technology is the result of the combined action of the main magnetic field, gradient field and radio frequency. The gradient magnetic field can realize the spatial positioning of the magnetic resonance signal in the main magnetic field, making magnetic resonance imaging possible. The gradient magnetic field is generated by applying current to the gradient coil. When the rapidly switching current passes through the gradient coil, it will cause the surrounding metal parts to produce eddy current effects. The presence of eddy currents changes the actual gradient field in the imaging area into a synthesis of the excitation gradient magnetic field and the eddy current-generated magnetic field, resulting in a slower rising edge and a longer falling edge. Using such a gradient field for encoding will deteriorate the linearity of the spatial positioning of the sample, causing distortion and artifacts in the images obtained by the system, which seriously affects the imaging quality of the magnetic resonance image. Therefore, in order for the MRI system to obtain high-quality images, the eddy current problem must be solved.

[0003] At present, there are three main methods to eliminate the influence of eddy current on magnetic resonance images: anti-eddy current plate, self-shielding coil and gradient waveform pre-emphasis: The anti-eddy current plate is an additional component made of high resistivity material, placed between the gradient coil and the magnet, which reduces eddy currents through high magnetic permeability; The self-shielding coil is a set of coils with opposite current directions added outside the gradient coil to offset the gradient field strength in the magnetic pole side area to reduce eddy currents. Both methods achieve the purpose of eliminating the influence of eddy currents by adding additional components to the magnetic resonance, but they will also introduce other problems. The obvious one is that it will increase the thickness of the gradient coil part, occupy more magnet space, resulting in a reduction in the imaging area or an increase in the magnet manufacturing cost to maintain the imaging area. The anti-eddy current plate will also introduce additional residual magnetism due to its high magnetic permeability. The self-shielding coil will cause a significant temperature rise, which will directly affect the magnetic resonance frequency offset and cause image quality problems. The gradient waveform pre-emphasis method compensates for the attenuation of high-frequency components during transmission by enhancing the high-frequency components of the signal at the signal transmitting end, i.e., pre-emphasis technology, thereby reducing the impact of eddy currents on MRI image quality. It includes collecting echo signals of a preset sampling pulse sequence, performing data processing on the echo signals using a preset algorithm, calculating the eddy current component parameters, and generating a pre-emphasis waveform based on these parameters, performing eddy current compensation on the magnetic resonance imaging gradient pulse, and obtaining the expected target gradient waveform. The pre-emphasis waveform is obtained based on the eddy current characteristics of the system, so how to accurately measure the eddy current of the magnetic resonance system is a very critical issue; The Chinese invention patent with publication number CN112462310A discloses a device and method for gradient eddy current detection and compensation based on an inductor coil. The device detects gradient eddy currents by using an inductor coil method implemented on a PCB. The actual gradient magnetic field is obtained by digitally integrating the differential voltage of the signal collected by the ADC board in the digital domain. The measured value of the gradient magnetic field is then compared with the theoretical value to calculate the size of the "overcurrent" that requires gradient waveform pre-emphasis.

[0004] However, the inductor coil proposed in the invention adopts a multi-winding planar PCB structure, and uses multiple coils to collect signals at the same time. The distance between each other is limited, and the magnetic field directions of the coils are the same, so there is mutual interference. The collected signals will introduce eddy currents caused by other coil units, resulting in inaccurate eddy current field measurements. In addition, the device has complex structural parts, and uses screws, guide rails and other structures for assembly in many places. It is also fixed to the bed with screws, which makes it inconvenient to disassemble and the process is complicated, which is not conducive to the rapid measurement of compensating eddy currents. To this end, we propose a magnetic resonance gradient eddy current measurement device. Summary of the invention

[0005] The purpose of the present invention is to provide a magnetic resonance gradient eddy current measuring device, which is structurally designed in combination with a magnetic resonance system, uses a coaxial line to make a coil unit, and uses the self-shielding effect of its inner and outer conductors to reduce mutual interference between coils, enhance isolation, and improve eddy current measurement accuracy. At the same time, the device is designed as a product, and the coil, feed board, preamplifier, signal ball, etc. are firmly fixed on an integrated bracket. The overall structure is simple, easy to use, and easy to move.

[0006] The present invention can be implemented by the following technical scheme: a magnetic resonance gradient eddy current measuring device, comprising an eddy current coil bracket positioned and matched with a magnetic resonance bed, the eddy current coil bracket is located in each axis direction of the three-dimensional space axis and two eddy current coil units are respectively arranged, each eddy current coil unit comprises a coaxial line coil, a feed board, a preamplifier and a signal ball; The coaxial line coil is ring-shaped and electrically connected to the feed board. A signal ball is fixed at the center of each coaxial line coil. The magnetic resonance signal of the received signal ball is transmitted to the preamplifier for processing through the LC resonant matching circuit on the feed board, and finally output to the computer through the back-end receiving link for eddy current calculation and compensation parameter design, so as to correct the eddy current parameters of the magnetic resonance system.

[0007] A further technical improvement of the present invention is that the two eddy current coil units in each axial direction are symmetrically distributed about the same center, and the distance between the two eddy current coil units in each axial direction is equal; in the working state, the symmetry center coincides with the magnet center of the magnetic resonance system.

[0008] A further technical improvement of the present invention is that the coaxial line coil includes an inner conductor, a dielectric layer and an outer conductor, the inner conductor is provided with an opening, and both ends of the opening are electrically connected to the feed board, an outer opening is provided on the side of the outer conductor away from the inner conductor opening, and whether a capacitor is connected to the outer opening is determined based on whether the circuit at the feed board end can reach the target frequency.

[0009] The further technical improvement of the present invention is that the outer conductor is kept grounded, a dielectric layer is placed between the outer conductor and the inner conductor, the outer conductor and the inner conductor form a flat plate capacitor and are connected to the LC resonant matching circuit connected to the inner conductor.

[0010] The further technical improvement of the present invention is that a protective cover is provided on the outer cover of the eddy current coil bracket, screw holes are set at the four corners of the protective cover and it is fixed to the bottom plate of the eddy current coil bracket through fixing nuts; a wire hole is opened on the side wall of the protective cover to facilitate the passage of the coaxial output cable.

[0011] A further technical improvement of the present invention is that the bottom of the eddy current coil bracket is set as a convex arc surface, and a strip-shaped positioning protrusion is provided in the middle of the convex arc surface. The setting of the convex arc surface and the positioning protrusion facilitates positioning and matching with the magnetic resonance examination bed.

[0012] A further technical improvement of the present invention is that it also includes a placement base for placing the eddy current coil bracket, the top of the placement base is set to a concave arc surface that matches the convex arc surface, and a positioning groove that matches the positioning protrusion is opened in the middle position of the concave arc surface. The placement base is used to place the device when it is in a non-working state.

[0013] A further technical improvement of the present invention is that when performing measurement work, the isolation between any two coaxial line coils is above 30db.

[0014] Compared with the prior art, the present invention has the following beneficial effects: The present invention designs a coaxial line coil as a gradient eddy current coil for signal acquisition, tests the crosstalk between different coaxial line coils to quantify the coupling signal between eddy current coil units, realizes its high isolation characteristic, and designs eddy current compensation parameters according to the eddy current measurement results to obtain good eddy current correction results. The DWI high gradient switching rate sequence verifies that the corrected system has good imaging and greatly reduces the system eddy current; the verification results show that the eddy current measured by the device is accurate, true and effective; In addition, the entire device is designed with a magnetic resonance magnet and a bed, which is easy to assemble and can achieve rapid positioning. It has a compact and simple structure, is easy to move and operate, and tends to be commercialized. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to facilitate understanding by those skilled in the art, the present invention is further described below with reference to the accompanying drawings.

[0016] Figure 1 It is a schematic diagram of the overall internal perspective structure of the present invention; Figure 2 It is a schematic diagram of the external structure of the present invention; Figure 3 It is a cross-sectional schematic diagram of the eddy current coil support of the present invention; Figure 4 This is a schematic diagram of the fixed connection structure of the signal ball of the present invention; Figure 5 It is a schematic diagram of the structure of the placement base of the present invention; Figure 6 It is a schematic diagram of the connection of the eddy current coil unit of the present invention; Figure 7 LC resonance matching circuit diagram of the present invention; Figure 8 This is a physical diagram of the present invention.

[0017] In the figure: 1. eddy current coil bracket; 2. protective cover; 3. eddy current coil unit; 4. placement base; 101. ring groove; 102. accommodating hole; 103. convex arc surface; 104. positioning protrusion; 105. fixing buckle; 201. wire hole; 301. coaxial line coil; 302. feeding board; 303. signal ball; 304. preamplifier; 401. concave arc surface; 402. positioning groove; 3011. outer conductor; 3012. dielectric layer; 3013. inner conductor. DETAILED DESCRIPTION

[0018] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments.

[0019] See also Figure 1-3 As shown, a magnetic resonance gradient eddy current measuring device comprises an eddy current coil support 1 and a protective cover 2 arranged outside the eddy current coil support 1, screw holes are arranged at four corners of the protective cover 2, and the protective cover 2 can be tightly connected with screws fixed at four corners of the bottom plate of the eddy current coil support 1 through fixing nuts to form a whole; Six eddy current coil units 3 are arranged on the eddy current coil bracket 1, and every two eddy current coil units 3 are symmetrically distributed in the three directions of the X, Y and Z axes based on the central axis, and the center distance between the two eddy current coil units 3 on each axis is equal. In this embodiment, the center distance is set to 200 mm; Furthermore, the eddy current coil support 1 is divided into three layers, wherein two groups (two in each group) of eddy current coil units 3 in the X and Z axis directions are cross-arranged in the middle layer, the Y axis coincides with the central axis, and the two eddy current coil units 3 in the Y axis direction are respectively located at the center of the upper layer and the lower layer. It should be noted that, from the orientation setting of the magnetic resonance system, the head and foot direction is set as the Z axis, the left and right is the X axis, and the top and bottom is the Y axis; Each eddy current coil unit 3 is composed of a coaxial coil 301, a feed board 302, a preamplifier 304 and a signal ball 303. Among them, the coaxial coil 301 is directly embedded in the annular groove 101 opened at the corresponding position of the eddy current coil bracket 1 and fixed. A rectangular groove is provided on one side of the annular groove 101, and a feed board 302 is embedded in the rectangular groove. The coaxial coil 301 is electrically connected to the feed board 302. A receiving hole 102 is opened at the center of the annular groove 101. The signal ball 303 is fixed in the receiving hole 102. In this embodiment, the diameter of the signal ball 303 is set to 20mm; a plurality of preamplifiers 304 are arranged at the lower layer of the eddy current coil unit 3. Each feed board 302 passes through the protective cover 2 through a coaxial cable and outputs a signal to the rear-end receiving link. The side wall of the protective cover 2 is provided with a wire hole 201 for the coaxial cable to pass through.

[0020] Specifically, Figure 3-4 As shown, the longitudinal section of the receiving hole 102 is set to a truncated cone structure that is wide at the top and narrow at the bottom. The signal ball 303 is fixed in the receiving hole 102 by the fixing buckle 105 arranged around it, and the fixing buckle 105 is fixed to the eddy current coil bracket 1 by the screw made of ABS material; The circuit connection of the eddy current coil unit 3 is as follows: Figure 6 As shown, the coaxial line coil 301 is a coil body, which is divided into three parts: an inner conductor 3013, a dielectric layer 3012 and an outer conductor 3011; wherein, an opening is provided on a side of the outer conductor 3011 away from the connection end with the feed board 302, and whether a capacitor is connected at the opening is determined according to whether the circuit composition at the feed board 302 end can be adjusted to the set target frequency: if the feed board 302 end can be adjusted to the target frequency, it is unnecessary to connect a capacitor at the opening, otherwise a capacitor is connected at the opening to achieve the effect of lowering the resonant frequency; in addition, the outer conductor 3011 is kept grounded as a whole, a dielectric layer 3012 is spaced between the outer conductor 3011 and the inner conductor 3013, and the outer conductor 3011 and the inner conductor 3013 form a flat plate capacitor and are connected to the LC resonant matching circuit connected to the inner conductor 3013.

[0021] The inner conductor 3013 is also provided with an opening on the side away from the opening of the outer conductor 3011, and both ends of the opening are connected to the feed board 302 to form an LC resonant matching circuit, through which the received magnetic resonance signal is transmitted to the preamplifier 304 for pre-amplification and preliminary filtering, and finally output to the back-end receiving link transmission processing. The coaxial line structure limits the current flow inside the coil when it is working, and the outer conductor 3011 participates in the composition of the coil circuit and also plays a shielding function, which can effectively reduce the coupling with other coils and reduce the signal crosstalk between coil units.

[0022] like Figure 7 As shown, the LC resonant matching circuit is mainly composed of a resonant circuit, a matching circuit and a detuning circuit. In this circuit structure, the resonant circuit is composed of the main body of the coaxial coil 301, the frequency modulation capacitor at the opening of the outer conductor 3011 and the feeding end capacitor to form an LC resonant circuit. The matching circuit is connected to the resonant circuit and output to the source end; the detuning circuit is composed of a single diode and an inductor.

[0023] More, such as Figure 3 , Figure 5 As shown, the device is also provided with a placement base 4 for daily use when not in use. Since the eddy current coil bracket 1 needs to cooperate with the magnetic resonance examination bed when in use, its bottom is set to match the convex arc surface 103 of the bed, and a strip-shaped positioning protrusion 104 is provided in the middle position of the convex arc surface 103; therefore, the top of the placement base 4 is set to a concave arc surface 401, and a positioning groove 402 is provided in the middle position of the arc surface. The convex arc surface 103 at the bottom of the eddy current coil bracket 1 is adapted to the concave arc surface 401 on the placement base 4 to ensure stability during placement, and the positioning protrusion 104 cooperates with the positioning groove 402 to position the eddy current coil bracket 1 on the placement base 4.

[0024] The basic principle of the eddy current measurement device involved in the present invention is: fitting the gradient eddy current field of the magnetic resonance system based on the phase information of the magnetic resonance signals collected at different positions in space. The sequence used for eddy current measurement includes three parts: test gradient, radio frequency excitation and FID data acquisition: after applying the test gradient, after a delay time, radio frequency pulse excitation is applied, and then the FID signal is immediately collected for subsequent eddy current calculation. Since the eddy current decays exponentially with time, it is necessary to set multiple delay times during scanning, collect data at different time points, and finally fit the eddy current attenuation function according to the eddy current change curve, and set compensation according to the fitting result. In actual magnetic resonance imaging, eddy currents will introduce an additional spatially varying magnetic field. When only first-order eddy current compensation is considered, the gradient field changes linearly in space. At this time, the precession frequency at different positions for: in, for The gyromagnetic ratio of protons is 42.58MHz / T. is the magnetic resonance system field strength, is the eddy current field strength.

[0025] During scanning, two eddy current coil units 3 on a certain axis respectively collect FID signals from the signal ball 303 at two corresponding spatial positions. According to the phase change in the signal, the precession frequency can be calculated, and combined with the position information of the signal ball 303, the magnitude of the eddy current field strength can be calculated: is the phase extracted from the FID signal at the corresponding position, is the sampling period, The distance between the two signal balls 303 on the same axis is set to 200 mm in this embodiment. Therefore, the eddy current measuring device proposed in the present invention can be used to obtain the eddy current field strength of the magnetic resonance system according to the above principle, and then the compensation parameters are designed according to the eddy current curve to correct the eddy current of the system, and then the measurement is repeated, and the process is iterated until the eddy current influence is eliminated.

[0026] Based on the above principle, the steps of using the device of the present invention to perform measurement are: Step 1: Device positioning The eddy current measuring device is taken out from the placing base 4 and placed on the magnetic resonance examination bed to complete the positioning and matching. The eddy current measuring device includes an eddy current coil bracket 1 and a protective cover 2; The laser of the magnetic resonance system is used to locate the center of the device. By designing the height of the eddy current coil bracket 1, the center position of the middle layer can be at the center height position of the magnetic resonance system magnet. After positioning, automatic bed entry can ensure that the device is exactly in the center of the magnet. The two eddy current coil units 3 on each axis are symmetrical about the center of the magnet, and the positioning is completed.

[0027] Step 2: Signal confirmation First, use the positioning sequence scan to confirm that the positioning in step one is correct. If there is a deviation, make appropriate fine adjustments; then observe whether the MR images of each signal ball 303 are normal, and then use the FID sequence to verify that the signal curves of each eddy current coil unit 3 are normal.

[0028] Step 3: Crosstalk test Compared with the PCB eddy current coils used in the document mentioned in the background technology, the six coils of the three axes are easily interfered with each other due to the limited spacing, and the phase information of the corresponding spatial position is obtained by the MRI signal to calculate the eddy current, which requires the isolation between the coils to be as high as possible; the device of the present invention overcomes this problem through the design of the coaxial line coil 301, and the coupling signal between the eddy current coil units 3 is quantified by testing the crosstalk between different coaxial line coils 301, and its high isolation characteristics are verified, specifically: (1) Signal acquisition: First, place the signal ball 303 only in a single eddy current coil, and leave the other units unloaded. Scan and collect FID signals, and quantitatively evaluate the severity of the crosstalk based on the size of the collected signals.

[0029] (2) Crosstalk calculation: The signal obtained by the channel where the signal ball 303 is placed is taken as the main signal, and the signals obtained by the other unloaded channels are taken as the crosstalk signals. The time domain signal is converted into the frequency domain signal through fast Fourier transform, and the frequency components in each channel are analyzed. The total power within the spectrum range of the signal is calculated, and the power of the channel where the signal ball 303 is placed is the signal power. , the channel power of the no-signal ball 303 is the crosstalk power ; Calculate the signal to crosstalk power ratio using the signal power and crosstalk power : ; (3) Result analysis: Replace the channel where the signal ball 303 is located in turn, and repeat the above two steps (1) and (2) until the crosstalk results between the six channels are obtained. After research, it is found that the isolation between channels should be >30dB for effective eddy current measurement.

[0030] By comparing the traditional copper conductor (copper wire, copper bar, etc.) eddy current coil and the coaxial line coil 301 proposed in the present invention, it is found that the isolation between the coaxial line coils 301 is above 30dB when the distance between the coils is the same, while the isolation of the traditional coils on the same axis can reach 30dB, and the isolation between adjacent coils is only about 20dB, or even lower. Therefore, the traditional coil will bring additional interference due to the coupling between the coils, resulting in interference in the collected MR signal and inaccurate eddy current measurement results.

[0031] Step 4: Eddy current measurement After the performance verification of the device of the present invention, all signal balls 303 are assembled, and the protective cover 2 is added, positioning is completed, and the eddy current measurement sequence is scanned. The eddy current compensation parameters are designed according to the eddy current measurement results to obtain good eddy current correction results. The DWI high gradient switching rate sequence verifies that the corrected system has good imaging and greatly reduces the system eddy current, indicating that the eddy current measured by the device is accurate, true, and effective.

[0032] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technical personnel in this field can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A magnetic resonance gradient eddy current measuring device, characterized in that: It comprises an eddy current coil support (1) that is positioned and matched with a magnetic resonance bed, the eddy current coil support (1) being located in the direction of each axis of a three-dimensional space axis and having two eddy current coil units (3) respectively arranged thereon, each eddy current coil unit (3) comprising a coaxial line coil (301), a feed board (302), a preamplifier (304) and a signal ball (303); The coaxial line coil (301) is annular and electrically connected to the feed board (302); a signal ball (303) is fixed at the center of each coaxial line coil (301); a magnetic resonance signal received from the signal ball (303) is transmitted to a preamplifier (304) for processing via an LC resonant matching balance circuit on the feed board (302); and finally output to a computer via a back-end receiving link for eddy current calculation and compensation parameter design, thereby correcting the eddy current parameters of the magnetic resonance system.

2. A magnetic resonance gradient eddy current measuring device according to claim 1, characterized in that: The two eddy current coil units (3) in each axial direction are symmetrically distributed about the same center, and the two eddy current coil units (3) in each axial direction are equidistant from each other; in a working state, the symmetry center coincides with the magnet center of the magnetic resonance system.

3. A magnetic resonance gradient eddy current measuring device according to claim 1, characterized in that: The coaxial line coil (301) comprises an inner conductor (3013), a dielectric layer (3012) and an outer conductor (3011); the inner conductor (3013) is provided with an opening, and both ends of the opening are electrically connected to the feed board (302); an outer opening is provided on a side of the outer conductor (3011) away from the opening of the inner conductor (3013); whether a capacitor is connected to the outer opening is determined based on whether a circuit at the feed board end can reach a target frequency.

4. A magnetic resonance gradient eddy current measuring device according to claim 3, characterized in that: The outer conductor (3011) is kept grounded, a dielectric layer (3012) is spaced between the outer conductor (3011) and the inner conductor (3013), and the outer conductor (3011) and the inner conductor (3013) form a flat plate capacitor and are connected to an LC resonant matching circuit connected to the inner conductor (3013).

5. The magnetic resonance gradient eddy current measuring device according to claim 1, characterized in that: The outer cover of the eddy current coil support (1) is provided with a protective cover (2), screw holes are provided at four corners of the protective cover (2) and the protective cover (2) is fixed to the bottom plate of the eddy current coil support (1) via fixing nuts; and a wire hole (201) is provided on the side wall of the protective cover (2) for facilitating the passage of a coaxial output cable.

6. The magnetic resonance gradient eddy current measuring device according to claim 1, characterized in that: The bottom of the eddy current coil support (1) is provided with a convex arc surface (103), and a strip-shaped positioning protrusion (104) is provided in the middle of the convex arc surface (103). The arrangement of the convex arc surface (103) and the positioning protrusion (104) facilitates positioning and matching with the magnetic resonance examination bed.

7. The magnetic resonance gradient eddy current measuring device according to claim 6, characterized in that: It also includes a placement base (4) for placing the eddy current coil bracket (1), wherein the top of the placement base (4) is arranged as a concave arc surface (401) adapted to the convex arc surface (103), and a positioning groove (402) is provided in the middle of the concave arc surface (401) for positioning with the positioning protrusion (104). The placement base (4) is used to place the device when it is in a non-working state.

8. The magnetic resonance gradient eddy current measuring device according to claim 1, characterized in that: When performing measurement work, the isolation between any two coaxial line coils (301) is above 30db.

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