A precision calibration method for a magnetocardiograph probe

Abstract

This invention discloses a method for calibrating the accuracy of magnetocardiogram (MCG) probes. To reduce the impact of variations in SQUID manufacturing processes on subsequent magnetic field measurement data and ensure consistent measurement accuracy across all MCG probes, this invention provides a calibration disk design for MCG probes, along with a method for calibrating the accuracy of these probes. The calibration disk includes a main body and a base. A matrix of excitation coils is arranged between the main body and the base. A signal source interface is located around the circumference of the main body. The main body and the base are interlocked and bonded together using epoxy resin. This method effectively improves the measurement accuracy of magnetocardiogram (MCG) signals and the accuracy of subsequent MCG inversion.

Description

Technical Field

[0001] This invention relates to magnetocardiographs, and particularly to a method for calibrating the accuracy of magnetocardiograph probes. Background Technology

[0002] The superconducting quantum interference device (SQUID) can detect 1 FT, or 10⁻⁶ FT, due to its extremely high sensitivity. -15 Tesla's weak magnetic field can therefore be used to detect weak biomagnetic signals such as brain magnetoencephalography (BME) and heart magnetoencephalography (MCG). A magnetocardiography (MCG) system made with SQUID can detect the weak magnetic field of the human heart in a radiation-free, non-contact, and non-invasive manner.

[0003] Compared to an electrocardiogram (ECG), both magnetic resonance imaging (MRCI) and electrocardiogram (ECG) signals originate from ion currents generated by cardiac activity. However, because ECG requires contact with the human body, the received signal is affected by the body's structure and conductive medium. In contrast, the magnetic resonance imaging (MRCI) signal generated by ion currents propagates outside the body, forming a spatial distribution, and is less affected by changes in conductance. MCG, by analyzing the spatial configuration of ion currents in the heart to reconstruct magnetic field maps (MFM), current density vector distribution, and inversely solve the dipole model, can reflect the state of cardiac activity, enabling timely diagnosis of early heart disease and playing a significant role in early screening for heart disease. Various clinical studies have shown that MCG is more sensitive than ECG to myocardial ischemia at rest.

[0004] In multi-channel MCG systems, variations in probe fabrication during the hardware system lead to measurement accuracy errors in the individual sensors. Therefore, it is necessary to accurately calibrate the magnetic field-voltage transfer coefficient (V0) of each SQUID. CAL / B CAL This is to ensure the accuracy of the magnetic field signals measured by the combined MCG system and the accuracy of subsequent magnetic field inversion.

[0005] Currently available calibration methods include the energized coil calibration method, the PCB coil calibration method, and the Helmholtz coil calibration method. The energized coil calibration method, proposed in 2003 by Ornelas PH's team, generates a magnetic field for the SQUID gradiometer by setting a long wire or circular coil, and calculates the magnitude of the magnetic field at the SQUID using the Biot-Savart theorem. However, this method is limited by the coil movement distance and flux calculation, and is only applicable to the calibration of single-channel SQUID gradiometers. The PCB coil calibration method was proposed by Zhang Yongsheng in 2014. This team utilizes the high symmetry of the gradiometer, using the voltage response of a selected channel as the absolute calibration reference value, and comparing the calibration results of other channels with the reference value to obtain relative calibration coefficients. However, the opaque, enclosed Dewar flask increases the difficulty of aligning the gradiometer and calibration coil, leading to divergent calibration results. The Helmholtz coil calibration method uses a three-dimensional Helmholtz coil to generate a uniform magnetic field to simultaneously calibrate multiple SQUIDs. This method does not require precise positioning of the multi-channel gradiometers inside the Dewar flask, resulting in small calibration errors. However, Helmholtz coils occupy too much space, which is not conducive to the integration of MCG systems.

[0006] Maxwell's equations combine time-varying electric and magnetic fields, and the volume current generated by ion activity within cardiomyocytes can also form a weak magnetic field. MCG can reconstruct cardiac pathological information by inverting the cardiac magnetic signal. The performance of MCG depends on the quality of the detected cardiac magnetic signal. Typical values ​​for cardiac magnetic signals are tens to hundreds of pT (10⁻¹⁰). -12T The environmental noise is very strong, such as the typical intensity of the Earth's magnetic field, which is 30–50 μT (10). -6T Urban environmental noise can reach hundreds of nT (10). -9T In order to extract extremely weak magnetic field signals from noisy magnetic fields, effective noise suppression techniques are required.

[0007] Common noise suppression techniques include magnetic shielding chambers, constructing large, uniform magnetic fields, gradiometers, and signal processing. Among these, magnetic shielding chambers made of high-conductivity permalloy can effectively shield low-frequency magnetic fields, but their high cost and frequent residual magnetism severely limit their widespread application. Signal processing techniques, due to limitations in their algorithms, must be used in conjunction with the magnetic flux acquisition section for post-processing of magnetocardiogram (MCC) data. Therefore, MCCs commonly use gradiometers in unshielded environments to effectively improve the signal-to-noise ratio. Summary of the Invention

[0008] The purpose of this invention is to reduce the impact of differences in SQUID manufacturing process on subsequent magnetic measurement data and to ensure that the measurement accuracy of each probe of the magnetocardiograph is at a consistent level. This invention provides a calibration disk design for magnetocardiograph probes and a method for calibrating the accuracy of the probes using this disk, which can effectively improve the measurement accuracy of magnetic signal and the accuracy of subsequent magnetic inversion.

[0009] The technical solution of this invention is:

[0010] A method for calibrating the accuracy of a magnetocardiograph probe, comprising:

[0011] S1. Connect the signal source interface of the precision calibration disk to the controller of the magnetocardiograph. The controller uses an N-bit analog-to-digital converter with a signal range of ΔV.

[0012] S2. Calculate the magnetic field of the excitation coil. According to Biot-Savart's law, the magnitude of the magnetic field Bz in the Z direction component produced by a horizontally placed single circular coil at (x, y, l) is:

[0013]

[0014] S3. Obtain the calibration magnetic field B output by the coil from the magnetic field Bz. CAL ;

[0015] S4, Analog-to-Digital Converter Transmission Coefficient K ADC for:

[0016]

[0017] S5, Equipment output voltage V CAL for

[0018]

[0019] Where N CAL For calibration coefficients;

[0020] S6, the flux-voltage transfer coefficient G is

[0021]

[0022] Preferably, the precision calibration disk includes a disk body and a base, with a plurality of excitation coils arranged in a matrix between the disk body and the base, a signal source interface being provided around the circumference of the disk body, and the disk body and the base being interlocked and bonded together using epoxy resin adhesive.

[0023] Preferably, during the accuracy calibration process, a second-order gradiometer is also used to shield far-field noise.

[0024] Preferably, the second-order gradient meter is completely immersed in a Dewar filled with liquid helium, the liquid helium being in the temperature range of 0–300 K.

[0025] Preferably, in step S1, an energized coil model is established and a current excitation is applied. The xz plane is selected as the reference plane, and the magnetic flux density is greater the closer the position is to the excitation coil on the Z-axis. In order to maximize the magnetic flux through the second-order gradiometer pickup coil, the excitation coil of the accuracy calibration disk is placed as close as possible to the bottom of the Dewar. When calibrating a single SQUID sensor, a rectangular wave signal with a duty cycle of less than 50% and a certain amplitude is applied to the excitation coil. When the amplitude of the rectangular wave output by the channel is the largest, the second-order gradiometer is located directly above the excitation coil.

[0026] Preferably, to prevent electromagnetic interference between coils, a DIP switch is used to control the operation of the calibration coil; the accuracy calibration disk is placed close to the bottom of the Dewar, all excitation coils are controlled to be in working state, the angle of the accuracy calibration disk is finely adjusted, and if the amplitude of the multi-channel output signal is at its maximum, each excitation coil is placed coaxially with the second-order gradient meter.

[0027] Then, control only one excitation coil to work, activate the corresponding channel's automatic calibration button, and the software calculates the calibration coefficient N. CAL1 The value is then automatically written to the corresponding channel, and the calibration of that channel is complete. Repeat the above steps in sequence until the calibration coefficients for all channels are obtained.

[0028] Preferably, after obtaining the calibration coefficients for all channels, the difference verification begins:

[0029] The accuracy of the calibration is evaluated by the difference in the maximum amplitude of the sampling points of each channel; the second-order gradiometer based on the SQUID sensor converts the collected magnetic flux change into an electrical signal, which is then converted into a digital signal by an analog-to-digital converter. The recorded sampling points can reflect the original information of the corresponding excitation source; the MCG multi-channel differences derived from the sampling points can further verify the reliability of the system calibration.

[0030] Preferably, the specific verification steps include:

[0031] After completing the calibration, record the maximum value A of each channel sampling point. MCGi Determine channel A of N MCGi The maximum point A in the middle max and minimum point A min ; Calculate multi-channel differential DE i :

[0032]

[0033] When DE i When the value is ≥5%, the MCG multi-channel variability is too large, and recalibration is required until DE is reached. i If the value is less than 5%, the MCG system can function normally.

[0034] The advantages of this invention are:

[0035] To reduce the impact of variations in SQUID manufacturing on subsequent magnetic measurement data and ensure consistent measurement accuracy across all magnetocardiogram (MCG) probes, this invention provides a calibration disk for MCG probes and a method for calibrating the accuracy of the MCG probes using this disk. This method can effectively improve the measurement accuracy of magnetic signal and the accuracy of subsequent magnetic inversion. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0037] Figure 1 A schematic diagram of the main body of the precision calibration disk;

[0038] Figure 2 A schematic diagram of the base for calibrating the disc for precision.

[0039] Figure 3 This is a schematic diagram of the connection between the main body of the disc and the chassis;

[0040] Figure 4 This is a schematic diagram of a second-order gradient meter.

[0041] Figure 5 The calibration results are shown in the figure. Detailed Implementation

[0042] Due to the manufacturing process of SQUID probes, the output signals of the produced SQUID probes may vary. Therefore, a calibration disk is needed to calibrate the multi-channel probes of the magnetocardiograph (MCG) to ensure that the output signal data maintains a certain level of accuracy. Currently available calibration methods include energized coil calibration, PCB coil calibration, and Helmholtz coil calibration. The advantages of this calibration scheme are: the invention is small in size, can calibrate multiple SQUID probes simultaneously, and is easily aligned with the MCG probe. The technical solution includes the design of the MCG calibration disk and the design of the calibration steps, as well as differential verification.

[0043] I. Design of the Precision Calibration Disc

[0044] like Figure 1-3 As shown, the precision calibration disk includes a disk body 1 and a base 4. A plurality of excitation coils 2 arranged in a matrix are arranged between the disk body 1 and the base 4. A signal source interface 3 is arranged around the circumference of the disk body 1. The disk body 1 and the base 4 are interlocked and bonded together with epoxy resin. Then, a connector 5 is used on the signal source interface 3 to fix the disk body 1 and the base 4.

[0045] II. Accuracy Calibration Method for Magnetocardiogram Probes

[0046] SQUID is the most sensitive flux measurement device to date. Proper system calibration is crucial when using a SQUID-based second-order axial gradiometer for quantitative measurements. The principle involves relating the flux sensor's output to the external magnetic field value of a known reference system. Due to the application of superconducting quantum interference principles, the calibration method for Low DC SQUID flux sensors differs from that of sensors operating at room temperature. SQUID employs cryogenic-room temperature calibration; to reach its operating temperature range, the SQUID must be immersed in a Dewar filled with liquid helium.

[0047] According to the Biot-Ossafari law, the magnitude of the magnetic field of a horizontally placed single circular coil along the Z-axis is:

[0048]

[0049]

[0050] The distribution of the magnetic field in the Z direction above the coil, calculated from the above formula, is as follows: Figure 4 As shown.

[0051] When calibrating a single SQUID sensor, a rectangular wave signal with a duty cycle of less than 50% and a certain amplitude is applied to the small coil. When the amplitude of the rectangular wave output by the channel is at its maximum, the second-order gradient meter is located directly above the excitation coil.

[0052] When calibrating a nine-channel MCG system, repeatedly using excitation coils to locate the corresponding channel increases calibration error. Although the second-order gradiometer is placed in an opaque Dewar, the bottom structure of the Dewar restricts the position of each channel. The SQUID sensor can be calibrated by fixing the excitation coil group inside a calibration disk. Our team fixed nine excitation coil groups in a 3×3 array inside the calibration disk, allowing for simultaneous accurate location of all nine second-order gradiometers. Furthermore, to prevent electromagnetic interference between coils, DIP switches are used to control the operation of the calibration coils. With the calibration disk firmly against the bottom of the Dewar, DIP switch No. 10 is turned ON, and the remaining switches are turned OFF (at this point, all nine excitation coils are active). The angle of the calibration disk is finely adjusted; if the output signal amplitude of all nine channels is at its maximum, then each excitation coil is placed coaxially with the second-order gradiometer. DIP switch No. 1 is turned ON, and the remaining switches are turned OFF (at this point, only coil number one is active). The automatic calibration button for channel one is activated, and the software calculates the calibration coefficient N. CAL1 The value is then automatically written to channel one, completing the calibration of that channel. Repeat these steps sequentially until the calibration coefficients for all nine channels are obtained.

[0053] Calculate the transmission coefficient using the following formula:

[0054]

[0055] The input calibration magnetic field B CAL =200pT, output voltage V CAL It is calculated using the following formula.

[0056]

[0057] K ADC Let K be the transfer coefficient of the analog-to-digital converter. Using a 16-bit analog-to-digital converter with an input voltage range of ±10V (i.e., a signal range ΔV = 20V), K is obtained from the following formula. ADC :

[0058]

[0059] III. Difference Verification

[0060] Due to the mechanical imbalance of the second-order gradiometers in each channel and the differences between the back-end circuits, the above method was used to perform absolute calibration of the SQUID sensor. The accuracy of the calibration can be evaluated by the difference in the maximum amplitude of the nine channel sampling points. The second-order gradiometer based on the SQUID sensor converts the acquired magnetic flux change into an electrical signal, which is then converted into a digital signal by an analog-to-digital converter. The recorded sampling points can reflect the original information of the corresponding excitation source. The differences in the nine MCG channels derived from the sampling points can further verify the reliability of the system calibration. Specific verification steps: After completing the calibration, record the maximum value A of the sampling points of each channel. MCGi Determine the nine-channel A MCGi The maximum point A in the middle max and minimum point A min The nine-channel differential DE can be calculated using the formula. i :

[0061]

[0062] When DE i When the variability of the nine MCG channels is ≥5%, recalibration is required until DE is reached. i If the value is less than 5%, the MCG system can function normally.

[0063] Table 2 Calibration coefficients for each channel

[0064]

[0065] After the initial calibration of the nine-channel MCG using a calibration disk, the DE1 was found to be 29.4% (>5%), requiring recalibration. After the second calibration, the DE1 was 4.4%, with minimal differences between channels, indicating normal operation. The figure shows a comparison of the two calibration results. Analysis reveals a consistent trend in the calibration results for each channel; except for channel six, the maximum amplitude of the sampling points for all other channels is greater than 1000, and the transmission coefficient is greater than 1.526. This indicates that due to manufacturing deviations, the SQUID performance of channel six is ​​weaker than that of the other channels. Figure 5 The calibration results are shown in Table 2, which shows the coefficients obtained after the second calibration of the nine channels. The transmission coefficients of each channel range from 1.41 to 1.84 mV / pT.

[0066] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All modifications made according to the spirit and essence of the main technical solution of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for calibrating the accuracy of a magnetocardiograph probe, characterized in that, include: S1. Connect the signal source interface of the precision calibration disk to the controller of the magnetocardiograph. The controller uses an n-bit analog-to-digital converter with a signal range of ΔV. S2. Calculate the magnetic field of the excitation coil. According to Biot-Savart's law, the magnitude of the magnetic field Bz in the Z direction component produced by a horizontally placed single circular coil at (x, y, l) is: ； S3, calibration magnetic field B resulting from the magnetic field Bz from which the coil output CAL ; S4, Analog-to-Digital Converter Transmission Coefficient K ADC for: ； S5, Equipment output voltage V CAL for ； in For calibration coefficients; S6, the flux-voltage transfer coefficient G is... ； During the accuracy calibration process, a second-order gradiometer is also used to shield far-field noise; The second-order gradient meter is completely immersed in a Dewar filled with liquid helium, which is in the temperature range of 0~300K. In step S1, a current-carrying coil model is established and current excitation is applied. The xz plane is selected as the reference plane. The magnetic flux density is greater closer to the excitation coil on the Z-axis. When calibrating a single SQUID sensor, a rectangular wave signal with a duty cycle of less than 50% and a certain amplitude is applied to the excitation coil. When the amplitude of the rectangular wave output by the channel is the largest, the second-order gradient meter is located directly above the excitation coil. To prevent electromagnetic interference between coils, a DIP switch is used to control the operation of the calibration coils; the accuracy calibration disk is placed close to the bottom of the Dewar, all excitation coils are controlled to be in working state, and the angle of the accuracy calibration disk is finely adjusted. If the amplitude of the multi-channel output signal is at its maximum, each excitation coil is placed coaxially with the second-order gradient meter. Then, control only one excitation coil to work, activate the corresponding channel's automatic calibration button, and the software calculates the calibration coefficient. The value is then automatically written to the corresponding channel, and the channel calibration is complete. Calculate the calibration coefficient as described above. The steps are executed sequentially until all channel calibration coefficients are obtained.

2. The accuracy calibration method for a magnetocardiogram probe according to claim 1, characterized in that, The precision calibration disk includes a disk body and a base. A plurality of excitation coils are arranged in a matrix between the disk body and the base. A signal source interface is provided around the circumference of the disk body. The disk body and the base are interlocked and bonded together using epoxy resin adhesive.

3. The accuracy calibration method for a magnetocardiogram probe according to claim 1, characterized in that, After obtaining the calibration coefficients for all channels, begin the difference verification: The accuracy of the calibration is evaluated by the difference in the maximum amplitude of the sampling points of each channel; the second-order gradiometer based on the SQUID sensor converts the collected magnetic flux change into an electrical signal, which is then converted into a digital signal by an analog-to-digital converter. The recorded sampling points can reflect the original information of the corresponding excitation source; the MCG multi-channel differences derived from the sampling points can further verify the reliability of the system calibration.

4. The accuracy calibration method for a magnetocardiogram probe according to claim 3, characterized in that, The specific verification steps include: After completing the calibration, record the maximum value of each channel sampling point. Determine the N channels Midpoint and minimum point ; Calculate multi-channel differences : = 100％ when When the variability of MCG multi-channels is ≥5%, recalibration is required until... If the value is less than 5%, the MCG system can function normally.

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