B0 magnetic field compensation device and compensation method for a magnetic resonance imaging device

Abstract

The application relates to a B0 magnetic field compensation device and method of a magnetic resonance imaging equipment, and belongs to the technical field of magnetic resonance. The device comprises a B0 magnetic field detection sensor, a B0 magnetic field compensation coil and a B0 magnetic field compensation circuit. After the B0 magnetic field compensation coil is powered through the compensation circuit, a compensation magnetic field can be generated in an imaging area, and the generated compensation magnetic field adopts a PID adjustment mode to offset the disturbance of low-frequency change magnetic fields such as electrical equipment, powered lines and changing geomagnetic fields in space to the B0 magnetic field of the imaging area of the magnetic resonance imaging equipment. The whole circuit structure is a feedback network, adopts closed-loop automatic control, monitors the low-frequency magnetic field fluctuation in real time, dynamically adjusts, minimizes the disturbance of the fluctuation interference of the external magnetic field to the static magnetic field in the magnetic resonance imaging area, and improves the stability of the B0 magnetic field.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of magnetic resonance, and relates to a B0 magnetic field compensation device and a compensation method of a magnetic resonance imaging equipment. BACKGROUND

[0002] As one of important medical imaging technologies, magnetic resonance imaging (MRI) is widely applied to scientific research, clinical medicine and many other fields due to its high spatial resolution and good spatial positioning which cannot be matched by other imaging technologies. At present, many local hospitals in China are equipped with magnetic resonance equipment, especially low-field magnetic resonance. Low-field magnetic resonance has low operation cost, and the imaging quality is increasingly improved. The development of high-level sequences is also increasingly mature, so that low-field magnetic resonance is rapidly popularized. However, due to the low field strength of low-field magnetic resonance, especially open magnetic resonance, compared with high-field magnetic resonance, it is more susceptible to external magnetic field interference, causing serious decline in image quality. For example, 50 Hz disturbance of power supply, low-frequency magnetic field disturbance, disturbance of external mechanical vibration and other electromagnetic interference can affect the stability of the main magnetic field. Magnetic resonance imaging benefits from a static magnetic field stable over time. The main magnetic field (or static magnetic field) of the magnetic resonance equipment is usually represented as a B0 magnetic field. In order to keep the static main magnetic field at a high stability and resist the disturbance influence brought by the external environment, a corresponding B0 magnetic field compensation device needs to be designed. SUMMARY

[0003] Therefore, the purpose of the application is to provide a B0 magnetic field compensation device and a compensation method of a magnetic resonance imaging equipment.

[0004] To achieve the above purpose, the application provides the following technical solutions.

[0005] A B0 magnetic field compensation device of a magnetic resonance imaging equipment, the device comprising:

[0006] A B0 magnetic field detection sensor, a plurality of which are arranged at any position in the main magnet cavity, for measuring the magnetic field changes in three dimensions of x direction, y direction and z direction in real time;

[0007] A B0 magnetic field compensation coil, a total of three pairs of which are arranged symmetrically on three opposite surfaces in the main magnet: an upper and lower distribution pair, a left and right distribution pair and a front and back distribution pair; the B0 magnetic field compensation coil is used to generate a specific magnetic field to offset the disturbance magnetic field;

[0008] The B0 magnetic field compensation circuit is used for controlling the B0 magnetic field detection sensor to measure the disturbance of the magnetic field; the B0 magnetic field compensation circuit comprises a preamplifier, a PID parameter dynamic adjustment circuit, an inverter and a Howland current source; the preamplifier is electrically connected with the B0 magnetic field detection sensor, amplifies and collects the voltage or current signal output by the B0 magnetic field detection sensor; the PID parameter dynamic adjustment circuit is electrically connected with the preamplifier, analyzes and calculates according to the collected voltage, and performs proportional integral differential dynamic adjustment; the inverter is electrically connected with the PID parameter dynamic adjustment circuit, and the voltage value after PID adjustment is obtained through the inverter to obtain a voltage signal with equal size and opposite phase; the Howland current source is electrically connected with the inverter, and the corresponding current value is obtained through the Howland current source.

[0009] Optionally, the preamplifier is composed of a resistor and a differential amplifier in series; the detection sensor detects a low-frequency magnetic field signal in space, outputs a current, obtains a voltage through the resistor, and amplifies and collects the voltage or current signal output by the B0 magnetic field detection sensor through the differential amplification circuit.

[0010] Optionally, the PID parameter dynamic adjustment circuit is composed of an analog-to-digital converter ADC, a microprocessor, a digital-to-analog converter DAC, a resistor and a gain amplifier; the voltage detected by the B0 magnetic field detection sensor is output after being amplified by the preamplifier; the output voltage is amplified by the gain amplifier and the analog-to-digital converter; the microprocessor is sent the voltage signal collected by the analog-to-digital converter; the microprocessor executes a PID adjustment program, controls the digital-to-analog converter to output a direct current according to the size of the collected voltage, controls the voltage amplification ratio of the gain amplifier, and performs integral differential proportional dynamic adjustment, so that the voltage signal u(k) detected by the B0 magnetic field detection sensor is 0, and the B0 magnetic field compensation effect is obtained.

[0011] Optionally, the inverter is composed of a resistor and a reverse amplifier in parallel, and is electrically connected with the PID parameter dynamic adjustment circuit; the voltage value after PID adjustment is obtained through the inverter to obtain a voltage signal with equal size and opposite phase.

[0012] Optionally, the Howland current source is composed of two groups of amplification circuits; the reverse voltage is input into the Howland current source circuit to obtain a corresponding current value, and then output to the B0 magnetic field compensation coil.

[0013] Optionally, the B0 magnetic field detection sensor is one or more of a Hall effect sensor, a magnetic resonance sensor, a fluxgate sensor, a magnetoresistance sensor and a coil.

[0014] A B0 magnetic field compensation method of a magnetic resonance imaging device based on the device, the method comprising the following steps:

[0015] S1: A plurality of B0 magnetic field detection sensors are arranged at any position in the main magnet cavity to measure the changes in the x, y and z directions of the magnetic field at the corresponding positions;

[0016] S2: After the B0 magnetic field detection sensor detects the low-frequency magnetic field signal, a voltage signal is obtained, which is amplified by a preamplifier and collected and output to the PID parameter dynamic adjustment circuit;

[0017] S3: After receiving the preamplifier signal, the PID parameter dynamic adjustment circuit analyzes and calculates the voltage signal amplitude, frequency and phase, sets the PID algorithm formula, and performs proportional, integral and differential dynamic adjustment to real-time adjust the output control of the logarithmic analog converter voltage value;

[0018] S4: The adjusted signal passes through an inverter to obtain a voltage with equal size and opposite phase;

[0019] S5: The reverse voltage passes through the Howland current source circuit to obtain the corresponding current;

[0020] S6: The current drives the B0 magnetic field compensation coil to generate a low-frequency magnetic field signal opposite to the detected signal, thereby canceling the low-frequency magnetic interference in space;

[0021] S7: The B0 magnetic field detection sensor detects the magnetic field in the imaging area after the low-frequency interference is canceled;

[0022] S8: Repeat steps S2-S6, after each magnetic field cancellation, the B0 magnetic field detection sensor repeatedly detects, according to the detection result, applies the corresponding current to cancel the fluctuating magnetic field, until the voltage signal collected by the analog-to-digital converter tends to zero, and the best compensation effect is obtained.

[0023] Optionally, in S3, the integral and differential dynamic adjustment formula is:

[0024] Δu(k)=L p ×[e(k)-e(k-1)]+L i ×e(k)+L d ×[e(k)-2e(k-1)+e(k-2)] (1)

[0025] u(k)=u(k-1)+△u(k) (2)

[0026] Where Δu(k) represents the voltage offset value of the B0 magnetic field compensation device, L P is the proportional parameter, L i is the integral parameter, and L dFor the differential parameter, k represents the sampling number, u(k) represents the voltage value obtained by the kth sampling, u(k-1) represents the voltage value obtained by the previous sampling of k, e is the deviation of the process quantity target value and the actual value in the PID adjustment process, e(k) is the difference between the current voltage value collected and the set voltage value, e(k-1) is the previous e(k) value, k-1 represents the previous sampling of k, e(k-2) is the previous e(k-1) value, k-2 represents the previous sampling of k-1, and the final PID adjustment time is determined by the finally measured voltage value; if u(k)>0, the proportional integral differential coefficient continues to be adjusted until△u(k) is equal to 0, and then the proportional integral differential coefficient is suitable; the microprocessor executes the PID adjustment program according to the adjusted proportional integral differential coefficient, dynamically adjusts, and maintains the best compensation effect of the B0 magnetic field.

[0027] Optionally, the circuit structures of S1-S5 adopt a PID adjustment mode, and the parameters of the voltage are dynamically adjusted through the information fed back by the B0 magnetic field detection sensor until the B0 magnetic field detection sensor cannot detect the voltage signal, and the fluctuation interference of the low-frequency magnetic field on the B0 magnetic field is minimized.

[0028] The beneficial effects of the present application are that:

[0029] The compensation magnetic field generated by the present application adopts a PID adjustment mode to offset the disturbance of the low-frequency change magnetic field such as electrical equipment, energized lines and changing geomagnetic field in the space on the B0 magnetic field of the imaging region of the magnetic resonance imaging device, the entire circuit structure is a feedback network, adopts closed-loop automatic control, monitors the low-frequency magnetic field fluctuation in real time, dynamically adjusts, minimizes the disturbance of the fluctuation interference of the external magnetic field on the static magnetic field in the magnetic resonance imaging region, and improves the stability of the B0 magnetic field.

[0030] Other advantages, objects, and features of the present application will be apparent to those skilled in the art from the following specification, and will be learned from the practice of the present application. The objects and other advantages of the present application can be achieved and obtained by the following specification. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to make the objects, technical solutions and advantages of the present application clearer, the preferred detailed description of the present application will be combined with the drawings to make the objects, technical solutions and advantages of the present application clearer, wherein:

[0032] Figure 1 It is the layout diagram of the B0 magnetic field compensation coil in the present application;

[0033] Figure 2 It is the structural schematic diagram of the device of the present application;

[0034] Figure 3This is the circuit schematic diagram of the present invention;

[0035] Figure 4 This is a circuit structure diagram of an embodiment of the invention. Detailed Implementation

[0036] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0037] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0038] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0039] The magnitude of the static main magnetic field B0 in magnetic resonance imaging fluctuates with changes in the environment. For example, mechanical vibration, electromagnetic interference (including static and alternating magnetic fields), and temperature changes can affect the stability of the main magnetic field. To maintain high stability of the static main magnetic field and resist the influence of disturbances from the external environment, a corresponding B0 magnetic field compensation device needs to be designed.

[0040] like Figure 1 As shown, a B0 magnetic field compensation device for a magnetic resonance imaging (MRI) device includes:

[0041] The B0 magnetic field compensation device consists of three parts: a B0 magnetic field detection sensor, a B0 magnetic field compensation coil, and a B0 magnetic field compensation circuit.

[0042] The B0 magnetic field detection sensor is used to measure the size of the B0 magnetic field in real time to track the disturbance. The B0 magnetic field compensation coil is used to generate a specific magnetic field to offset the disturbance of the B0 magnetic field. The B0 magnetic field compensation circuit is used to control the B0 magnetic field detection sensor to measure the disturbance of the magnetic field.

[0043] The B0 magnetic field compensation coil has three pairs, one pair distributed up and down, one pair distributed left and right, and one pair distributed front and back. The three coil pairs are improved Helmholtz coil pairs.

[0044] The B0 magnetic field detection sensor is a combination of multiple sensors such as Hall effect sensor, magnetic resonance sensor, magnetic flux gate sensor, magnetic resistance sensor, coil, etc. (for example, Hall effect sensor + magnetic resonance sensor, magnetic resonance sensor + magnetic flux gate sensor, Hall effect sensor + magnetic resonance sensor + magnetic flux gate sensor, etc.), to improve detection sensitivity and accuracy.

[0045] As Figure 2 The B0 magnetic field compensation circuit is used to control the B0 magnetic field detection sensor to measure the disturbance of the magnetic field. The B0 magnetic field compensation circuit includes a preamplifier, a PID parameter dynamic adjustment circuit, an inverter, and a Howland current source. The preamplifier is electrically connected to the B0 magnetic field detection sensor, amplifies and collects the voltage or current signal output by the B0 magnetic field detection sensor; the PID parameter dynamic adjustment circuit is electrically connected to the preamplifier, and the PID parameter dynamic adjustment circuit analyzes and calculates according to the collected voltage size, and performs proportional integral differential dynamic adjustment; the inverter is electrically connected to the PID parameter dynamic adjustment circuit, and the voltage value after PID adjustment is obtained through the inverter to obtain a voltage signal with equal size and opposite phase; the Howland current source is electrically connected to the inverter, and the inverted voltage is obtained through the Howland current source to obtain a corresponding current value.

[0046] Specifically, the B0 magnetic field compensation method is that the voltage detected by the B0 magnetic field detection sensor is output after being amplified by a preamplifier, the output voltage is amplified by a gain amplifier and an analog-to-digital converter, the voltage signal collected by the analog-to-digital converter is sent to a microprocessor, the microprocessor executes a PID adjustment program, controls a digital-to-analog converter to output a direct current according to the collected voltage, controls the voltage amplification ratio of the gain amplifier, and performs integral and differential proportional dynamic adjustment, and the adjusted voltage value is output to an inverter, and a voltage with equal size and opposite phase is obtained by the reverse amplification circuit of the inverter, the reverse voltage passes through a Howland current source circuit to obtain a corresponding current value, and finally the current flows through the B0 magnetic field compensation coil, the B0 magnetic field compensation coil generates a low-frequency magnetic field, and the phase of the low-frequency magnetic field is just opposite to that of the detected low-frequency magnetic field, so as to offset the low-frequency magnetic field interference. In the embodiment, the B0 magnetic field compensation circuit is a closed-loop automatic feedback network, which monitors the low-frequency magnetic field fluctuation in real time and dynamically adjusts, and the specific method is as follows:

[0047] As shown in Figure 1 , four B0 magnetic field detection sensors are arranged at any positions in the main magnet cavity, and the changes of the magnetic field in three dimensions (x direction, y direction and z direction) at the corresponding positions are measured, as shown in Figures 2 to 4 , the B0 magnetic field detection sensor detects the low-frequency magnetic field signal to obtain a voltage signal, the voltage signal is amplified by a preamplifier, and the voltage or current signal output by the B0 magnetic field detection sensor is collected and output to a PID parameter dynamic adjustment circuit, the PID parameter dynamic adjustment circuit analyzes and calculates the voltage signal amplitude, frequency and phase after receiving the preamplifier signal, sets a PID algorithm formula, performs proportional integral differential dynamic adjustment, and adjusts the output control of the analog-to-digital converter voltage value in real time. The integral differential dynamic adjustment formula is:

[0048] Δu(k)=L p ×[e(k)-e(k-1)]+L i ×e(k)+L d ×[e(k)-2e(k-1)+e(k-2)] (1)

[0049] u(k)=u(k-1)+△u(k) (2)

[0050] Where Δu(k) represents the voltage offset value of the B0 magnetic field compensation device, L P is a proportional parameter, L i is an integral parameter, and L dFor the differential parameter, k represents the sampling number, u(k) represents the voltage obtained by the kth sampling, e is the deviation between the target value and the actual value in the PID adjustment process, e(k) is the difference between the current voltage value and the set voltage value, e(k-1) is the previous e(k) value, k-1 represents the previous sampling of k, e(k-2) is the previous e(k-1) value, k-2 represents the previous sampling of k-1, and the final PID adjustment time is determined by the finally measured voltage offset value. If △u(k)>0, the proportional integral differential coefficient continues to be adjusted until △u(k) is equal to 0, and then the adjusted proportional integral differential coefficient is applicable. The microprocessor executes the PID adjustment program according to the adjusted proportional integral differential coefficient and dynamically adjusts. The adjusted voltage signal passes through the inverter to obtain a voltage with equal size and opposite phase, and the reverse voltage passes through the Howland current source circuit to obtain the corresponding current. The current drives the B0 magnetic field compensation coil to generate a low-frequency magnetic field signal opposite to the detected signal, thereby canceling the low-frequency magnetic interference in the space. The B0 magnetic field detection sensor detects the magnetic field after the low-frequency interference is canceled in the imaging area again. After each magnetic field cancellation, the B0 magnetic field detection sensor detects again, checks according to the detection result, and applies the corresponding current to cancel the fluctuating magnetic field until the voltage signal collected by the analog-to-digital converter approaches zero, and the best compensation effect is obtained.

[0051] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions, and they should be covered in the scope of the claims of the present application.

Claims

1. A B0 magnetic field compensation device for a magnetic resonance imaging (MRI) device, characterized in that: The device includes: There are several B0 magnetic field detection sensors, which are used to measure the changes in magnetic field in three dimensions: x, y and z, in real time, and are set at any position inside the cavity of the main magnet. The B0 magnetic field compensation coil consists of three pairs: a vertical pair, a horizontal pair, and a front-to-back pair. These pairs are symmetrically arranged on three opposite sides of the main magnet. The B0 magnetic field compensation coil is used to generate a specific magnetic field to counteract the disturbance magnetic field. The B0 magnetic field compensation circuit is used to control disturbances in the magnetic field measured by the B0 magnetic field detection sensor. The B0 magnetic field compensation circuit includes a preamplifier, a PID parameter dynamic adjustment circuit, an inverter, and a Howland current source. The preamplifier is electrically connected to the B0 magnetic field detection sensor, amplifying and acquiring the voltage or current signal output by the sensor. The PID parameter dynamic adjustment circuit is electrically connected to the preamplifier, analyzing and calculating based on the acquired voltage magnitude, and performing proportional-integral-derivative dynamic adjustment. The inverter is electrically connected to the PID parameter dynamic adjustment circuit, passing the voltage value adjusted by the PID through the inverter to obtain a voltage signal of equal magnitude but opposite phase. The Howland current source is electrically connected to the inverter, and the inverted voltage is passed through the Howland current source to obtain the corresponding current value. The preamplifier consists of a resistor and a differential amplifier connected in series. The detection sensor detects a low-frequency magnetic field signal in the space and outputs a current. The current is passed through the resistor to obtain a voltage. The voltage is then amplified and acquired by the differential amplifier circuit. The inverter consists of a resistor and an inverting amplifier connected in parallel, and is electrically connected to a PID parameter dynamic adjustment circuit. The voltage value adjusted by the PID is passed through the inverter to obtain a voltage signal of equal magnitude but opposite phase. The Howland current source consists of two sets of amplifier circuits. The reverse voltage passes through the Howland current source circuit to obtain the corresponding current value, which is then output to the B0 magnetic field compensation coil.

2. The B0 magnetic field compensation device for a magnetic resonance imaging apparatus according to claim 1, characterized in that: The PID parameter dynamic adjustment circuit consists of an analog-to-digital converter (ADC), a microprocessor, a digital-to-analog converter (DAC), resistors, and a gain amplifier. The voltage detected by the B0 magnetic field sensor is amplified by a preamplifier and then output. The output voltage passes through a gain amplifier and the ADC. The ADC acquires the voltage signal and sends it to the microprocessor. The microprocessor executes the PID adjustment program, controlling the ADC to output DC based on the acquired voltage magnitude, thereby controlling the voltage amplification ratio of the gain amplifier and performing integral-derivative-proportional dynamic adjustment to make the voltage signal u(k) detected by the B0 magnetic field sensor equal to 0, thus achieving the B0 magnetic field compensation effect.

3. The B0 magnetic field compensation device for a magnetic resonance imaging apparatus according to claim 1, characterized in that: The B0 magnetic field detection sensor is one or more of the following: Hall effect sensor, magnetic resonance sensor, fluxgate sensor, magnetoresistive sensor, and coil.

4. A method for compensating the B0 magnetic field of a magnetic resonance imaging device based on the apparatus of any one of claims 1 or 2, characterized in that: The method includes the following steps: S1: Place several B0 magnetic field detection sensors at arbitrary positions inside the cavity of the main magnet to measure the changes in the magnetic field in the three dimensions of x, y and z at the corresponding positions. S2: After the B0 magnetic field sensor detects the low-frequency magnetic field signal, it obtains a voltage signal. This voltage signal is amplified by the preamplifier, and the voltage or current signal output by the B0 magnetic field sensor is collected and output to the PID parameter dynamic adjustment circuit. S3: After receiving the signal from the preamplifier, the PID parameter dynamic adjustment circuit analyzes and calculates the amplitude, frequency, and phase of the voltage signal, sets the integral-derivative dynamic adjustment PID formula, performs proportional-integral-derivative dynamic adjustment, and adjusts the output control of the digital-to-analog converter voltage value in real time. S4: The regulated signal passes through an inverter to obtain a voltage of equal magnitude but opposite phase; S5: The reverse voltage passes through the Howland current source circuit to obtain the corresponding current; S6: The current drives the B0 magnetic field compensation coil to generate a low-frequency magnetic field signal that is opposite to the detected signal, thereby canceling low-frequency magnetic interference in the space. S7: The B0 magnetic field detection sensor re-detects the magnetic field within the imaging area after canceling low-frequency interference; S8: Repeat steps S2 to S6. After each magnetic field cancellation, the B0 magnetic field detection sensor will repeat the detection again. Based on the detection results, apply the corresponding current to cancel the fluctuating magnetic field until the voltage signal collected by the analog-to-digital converter approaches zero to obtain the best compensation effect. In S3, the integral-derivative dynamic adjustment PID formula is: (1) (2) in This indicates the voltage offset value of the B0 magnetic field compensation device. For proportional parameters, For integration parameters, For differential parameters, k Indicates the number of samples. Indicates the first k The voltage value obtained from the second sampling. express k The voltage value obtained from the previous sampling, e This refers to the deviation between the target value and the actual value of the process variable during PID control. The difference between the current voltage value and the set voltage value. For the previous one value, k -1 indicates k The previous sampling, For the previous one value, k -2 indicates k The final PID adjustment time is determined by the final measured voltage value, based on the previous sample of -1. If the coefficients are greater than 0, continue adjusting the proportional, integral, and derivative coefficients until... If the value is 0, the adjusted proportional-integral-derivative (PI) coefficients are applicable. The microprocessor executes the PID control program according to the adjusted PI coefficients to dynamically adjust and maintain the best compensation effect of the B0 magnetic field. The circuit structure of S1 to S5 adopts PID regulation, which dynamically adjusts the voltage parameters based on the information fed back by the B0 magnetic field detection sensor until the B0 magnetic field detection sensor can no longer detect the voltage signal, thus minimizing the impact of low-frequency magnetic field fluctuations on the B0 magnetic field.

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