CN116008871A - Precision calibration method for magnetocardiograph probe - Google Patents
Precision calibration method for magnetocardiograph probe Download PDFInfo
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- CN116008871A CN116008871A CN202211313413.6A CN202211313413A CN116008871A CN 116008871 A CN116008871 A CN 116008871A CN 202211313413 A CN202211313413 A CN 202211313413A CN 116008871 A CN116008871 A CN 116008871A
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Abstract
The invention discloses a precision calibration method for a magnetocardiogram instrument probe, which aims to reduce the influence of the difference of SQUID in the manufacturing process on subsequent magnetic measurement data and ensure that the measurement precision of each probe of the magnetocardiogram instrument is at a consistent level. The method can effectively improve the measurement accuracy of the magnetocardiogram signal and the accuracy of the subsequent magnetocardiogram inversion.
Description
Technical Field
The invention relates to a magnetocardiograph, in particular to a precision calibration method for a magnetocardiograph probe.
Background
Superconducting quantum interferometers (superconducting quantum interference device, SQUID) can detect 1FT, i.e. 10, due to their ultra-high sensitivity -15 The weak magnetic field of tesla can be used for detecting weak biological magnetic signals such as cerebral magnetism, cardiac magnetism and the like. A Magnetocardiography (MCG) system made of SQUID can detect weak magnetic field of heart of human body in a non-radiative, non-contact and non-invasive manner.
In the case of an Electrocardiogram (ECG), both the magnetocardiogram and the electrocardiograph are derived from ionic currents due to heart activity, but since the ECG requires contact with the human body, the received signals are affected by the human body structure and the conductive medium. And the magnetocardiogram signal generated by the ion current can be transmitted to the outside of the body to form spatial distribution, and the signal is little influenced by the conductivity change. The MCG performs isomagnetic map (Magnetic Field Maps, MFM) reconstruction, current density vector distribution and inverse solution of a dipole model by analyzing the spatial configuration of ion current in the heart, so that the situation of heart activity can be reflected, timely diagnosis can be made in early stage of heart diseases, and the method has important significance for screening early stage heart diseases. Various clinical studies have shown that MCG is more sensitive to myocardial ischemia at rest than ECG.
In the multi-channel MCG system, since the difference in the fabrication of the probes in the hardware system causes errors in the measurement accuracy of the fabricated sensors, it is necessary to precisely calibrate the magnetic field-voltage transmission coefficient (V CAL /B CAL ) So as to ensure the accuracy of the magnetocardiogram signals measured by the combined MCG system and the accuracy of the subsequent magnetocardiogram inversion.
The existing calibration methods at present comprise an electrified coil calibration method, a PCB coil calibration method and a Helmholtz coil calibration method. The electrified coil calibration method is proposed by Ornelas P H team in 2003, generates a magnetic field to the SQUID gradiometer by arranging a long wire or a round coil, and calculates the magnitude of the magnetic field generated at the SQUID according to the Biot-Savart theorem. However, the method is limited by the calculation of the coil moving distance and flux values, and is only suitable for the calibration of a single-channel SQUID gradiometer; the PCB coil calibration method was proposed by Zhang Yongsheng in 2014. The team uses the high symmetry of the gradiometer to take the voltage response of the selected channel as an absolute calibration reference value, and compares the calibration results of other channels with the reference value to obtain a relative calibration coefficient. However, the opaque closed dewar increases the alignment difficulty of the gradiometer and the calibration coil, resulting in divergence of the calibration result; the method for calibrating the Helmholtz coils simultaneously calibrates a plurality of SQUID by using a method for generating a uniform magnetic field by using the three-dimensional Helmholtz coils, and the method does not need to accurately position a multichannel gradiometer in the Dewar, so that the calibration result has small error. However, the helmholtz coil occupies too much space, which is disadvantageous for the integration of MCG systems.
Maxwell's equations combine time-varying electric and magnetic fields, and the volumetric current generated by ionic activity in the myocardial cells can also form weak magnetic fields. The MCG can reconstruct the heart pathology information after inverting the magnetocardiogram signals. The performance of MCG depends on the quality of the detected magnetocardiogram signal. Typical values for magnetocardiographic signals are tens to one hundred pT (10 -12T ) While ambient field noise is quite intense, e.g. the typical intensity of the earth's magnetic field is 30-50. Mu.T (10 -6T ) Urban environmental field noise can reach hundreds nT (10) -9T ). In order to extract extremely weak magnetocardiogram signals from the noise-suppressing magnetic field, an effective noise suppression technique is required.
Common noise suppression techniques include: magnetic shielding room, construction of large uniform magnetic field, gradiometer and signal processing technology. Among them, the magnetic shielding room made of permalloy with high conductivity can effectively shield the low frequency magnetic field, but the disadvantages of high cost and frequent remanence severely limit the feasibility of general application. The signal processing technology has to cooperate with the magnetic flux acquisition part to carry out the post-processing of the magnetocardiogram data due to the limitation of the algorithm. Therefore, gradiometers are commonly used in magnetocardiography to effectively improve signal-to-noise ratio in an unshielded environment.
Disclosure of Invention
The invention aims at: in order to reduce the influence of the difference of the SQUID in the manufacturing process on the subsequent magnetic measurement data and enable the measurement precision of each probe of the magnetocardiogram instrument to be in a consistent level, the invention provides a set of calibration disc designs related to the magnetocardiogram instrument probes and a precision calibration method for the magnetocardiogram instrument probes by using the disc, and can effectively improve the measurement precision of magnetocardiogram signals and the accuracy of subsequent magnetocardiogram inversion.
The technical scheme of the invention is as follows:
a precision calibration method for a magnetocardiograph probe comprises the following steps:
s1, connecting a signal source interface of an accuracy calibration disc with a controller of a magnetocardiogram instrument, wherein the controller uses an N bit analog-digital converter and a signal range delta V;
s2, calculating a magnetic field of an excitation coil, wherein according to the Biot-Savart law, a single horizontally placed circular coil generates a magnetic field Bz of a Z-direction component at (x, y, l) with the following size:
s3, obtaining a calibration magnetic field B output by the coil from the magnetic field Bz CAL ；
S4, transmission coefficient K of analog-digital converter ADC The method comprises the following steps:
s5, output voltage V of equipment CAL Is that
Wherein N is CAL Is a calibration coefficient;
s6, the magnetic flux-voltage transmission coefficient G is
Preferably, the precision calibration disc comprises a disc main body and a chassis, a plurality of excitation coils arranged in a matrix are arranged between the disc main body and the chassis, a signal source interface is arranged on the circumference of the disc main body, and the disc main body and the chassis are buckled and bonded by using epoxy resin glue.
Preferably, in the precision calibration process, a second-order gradiometer is also adopted to shield far-field noise.
Preferably, the second-order gradiometer is completely immersed in a Dewar filled with liquid helium, and the liquid helium is in a temperature range of 0-300K.
Preferably, in step S1, an energized coil model is built, current excitation is applied, an x-Z plane is selected as a reference plane, and the magnetic flux density is increased at a position closer to the exciting coil on the Z axis; to maximize the magnetic flux through the second order gradiometer pick-up coil, the excitation coil of the precision calibration disk is placed as close as possible to the bottom of the Dewar; when a single SQUID sensor is calibrated, a rectangular wave signal with a duty ratio smaller than 50% and a certain amplitude is applied to the exciting coil, and when the amplitude of the rectangular wave output by the channel is maximum, the second-order gradiometer is positioned right above the exciting coil.
Preferably, in order to prevent electromagnetic interference between coils, a dial switch is used for controlling the work of the calibration coils; the precision calibration disc is tightly attached to the bottom of the Dewar, all excitation coils are controlled to be in a working state, the angle of the precision calibration disc is finely adjusted, and if the amplitude of the multi-channel output signal is the maximum, each excitation coil and the second-order gradiometer are coaxially placed;
then controlling only one exciting coil to work, starting the corresponding channel automatic calibration button, and calculating a calibration coefficient N by software CAL1 Automatically writing the value into a corresponding channel, and completing the channel calibration; and sequentially executing the steps until all the channel calibration coefficients are obtained.
Preferably, after all channel calibration coefficients are obtained, the differential verification is started:
the calibration accuracy is evaluated through the difference of the maximum amplitude of the sampling points of each channel; the second-order gradiometer based on the SQUID sensor converts the acquired magnetic flux variation into an electric signal, the electric signal is obtained after passing through the analog-to-digital converter, and the recorded sampling points can reflect the original information of the corresponding excitation source; the MCG multichannel difference deduced from the sampling points can further verify the reliability of system calibration.
Preferably, the specific verification step includes:
recording the maximum value A of sampling points of each channel on the basis of completing calibration MCGi Determination of N channel A MCGi Maximum point A of (2) max And minimum point A min The method comprises the steps of carrying out a first treatment on the surface of the Calculation of multichannel Difference DE i ：
When DE i When the difference is more than or equal to 5%, the MCG multichannel has overlarge difference and needs to be calibrated again until DE i And less than 5 percent, and the MCG system can work normally.
The invention has the advantages that:
in order to reduce the influence of the difference of the SQUID in the manufacturing process on the subsequent magnetic measurement data and enable the measurement precision of each probe of the magnetocardiography to be at a consistent level, the invention provides a set of calibration discs related to the magnetocardiography probes and a precision calibration method for the magnetocardiography probes by using the discs, and the measurement precision of magnetocardiography signals and the accuracy of subsequent magnetocardiography inversion can be effectively improved.
Drawings
The invention is further described below with reference to the accompanying drawings and examples:
FIG. 1 is a schematic diagram of a disk body of a precision calibration disk;
FIG. 2 is a schematic diagram of a chassis of a precision calibration disk;
FIG. 3 is a schematic view of a connection between a disk body and a chassis;
FIG. 4 is a schematic diagram of a second order gradiometer;
fig. 5 is a graph of calibration results.
Detailed Description
Because the SQUID manufacturing process produces signals output by the SQUID probe with a certain gap, a calibration disc is needed to calibrate the multichannel probe of the magnetocardiography instrument, so that the output signal data can be kept at a certain data precision. The existing calibration methods at present comprise an electrified coil calibration method, a PCB coil calibration method and a Helmholtz coil calibration method; the calibration scheme has the advantages that: the invention has smaller volume, can calibrate a plurality of SQUID probes at the same time, and is easy to align with the MCG end probe. The technical scheme includes that the magnetocardiogram instrument calibration disc design and calibration step design are included, and the method further includes difference verification.
1. Design of precision calibration disc
As shown in fig. 1-3, the precision calibration disc comprises a disc main body 1 and a chassis 4, a plurality of excitation coils 2 arranged in a matrix are arranged between the disc main body 1 and the chassis 4, a signal source interface 3 is arranged on the circumference of the disc main body 1, the disc main body 1 and the chassis 4 are buckled and bonded by using epoxy resin glue, and then the disc main body 1 and the chassis 4 are fixed on the signal source interface 3 by using a connector 5.
2. Precision calibration method for magnetocardiograph probe
SQUID is the flux measuring device with the highest sensitivity so far. When quantitative measurements are made using SQUID-based second order axial gradiometers, it is important to calibrate the system correctly. The principle is a method of correlating the output of the magnetic flux sensor with the external magnetic field value of a known reference system. The Low DC SQUID magnetic flux sensor marking method is different from a sensor working at normal temperature due to the application of the superconducting quantum interference principle. SQUID is calibrated using low-to-normal temperature, and to reach the operating temperature zone, the SQUID must be immersed in a dewar filled with liquid helium.
According to the law of Bi-Ossary, the magnetic field of a single horizontally placed circular coil in the Z axis is as large as
The distribution of the magnetic field in the Z direction over the coil of the circular coil current calculated from the above is shown in fig. 4.
When the single SQUID sensor is calibrated, a rectangular wave signal with a duty ratio smaller than 50% and a certain amplitude is applied to the small coil, and when the amplitude of the rectangular wave output by the channel is maximum, the second-order gradiometer is positioned right above the excitation coil.
When the nine-channel MCG system is calibrated, calibration errors are increased each time the corresponding channel is searched by using the exciting coil. Although the second order gradiometer is placed in an opaque dewar, the dewar bottom structure limits the individual channel locations. The SQUID sensor may be calibrated by means of an excitation coil set fixed within a calibration disk. The team secures nine excitation coil sets to the calibration disk in a 3 x 3 square matrixIn this way, nine second-order gradiometer positions can be found out at one time. In addition, in order to prevent electromagnetic interference between coils, a dial switch is used for controlling the operation of the calibration coils. The calibration disk is tightly attached to the bottom of the Dewar, the dial switch No.10 is turned ON, the other switches are turned OFF (at the moment, all nine excitation coils are in working states), the angle of the calibration disk is finely adjusted, and if the amplitude of the output signal of the nine channels is the largest, all the excitation coils and the second-order gradiometer are coaxially arranged. The dial switch No.1 is turned ON, the other switches are turned OFF (only one coil works at the moment), an automatic calibration button of the channel is started, and software calculates a calibration coefficient N CAL1 And automatically writing the value into the first channel, and completing the calibration of the first channel. And sequentially executing the steps until nine-channel calibration coefficients are obtained.
The transmission coefficients are calculated according to the following equation:
wherein a calibration magnetic field B is input CAL Output voltage V CAL Calculated according to the following formula.
K ADC The coefficients are transmitted for the analog-to-digital converter. Using a 16-bit analog to digital converter, the input voltage range ± 10V, i.e. the signal range Δv=20v, is given by the following formula ADC ：
3. Differential verification
Due to the mechanical unbalance of the second-order gradiometer in each channel and the difference between the back-end circuits, the penholder adopts the method to absolutely calibrate the SQUID sensor. The accuracy of the calibration can be evaluated through the difference of the maximum amplitude values of the nine-channel sampling points. Second-order gradiometer based on SQUID sensorThe collected magnetic flux variation is converted into an electric signal, a digital signal is obtained after the electric signal passes through an analog-to-digital converter, and the recorded sampling points can reflect the original information of the corresponding excitation source. The MCG nine-channel difference deduced from the sampling points can further verify the reliability of system calibration. The specific verification steps are as follows: recording the maximum value A of sampling points of each channel on the basis of completing calibration MCGi Determination of nine channels A MCGi Maximum point A of (2) max And minimum point A min . Nine-channel differential DE can be obtained by calculation by using a formula i ：
When DE i When the difference of MCG nine channels is more than or equal to 5%, calibrating again until DE i And less than 5 percent, and the MCG system can work normally.
Table 2 calibration coefficients for each channel
After the nine-channel MCG is calibrated for the first time by using the calibration disc, the DE is obtained through the difference verification 1 29.4% (> 5%) and recalibration is required. DE after secondary calibration 1 4.4%, each channel has small difference and can work normally. The graph is a comparison graph of the two calibration results, analysis shows that the calibration result trend of each channel has consistency, the maximum amplitude of sampling points of other channels except the channel six is larger than 1000, and the transmission coefficient is larger than 1.526. This shows that the SQUID performance of channel six is weaker than the other channels due to fabrication variations. FIG. 5 is a graph of calibration results, and Table 2 shows the coefficients obtained after the nine-channel secondary calibration, wherein the transmission coefficient of each channel is 1.41-1.84 mV/pT.
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 the same according to the content of the present invention, and are not intended to limit the scope of the present invention. All modifications made according to the spirit of the main technical proposal of the invention should be covered in the protection scope of the invention.
Claims (8)
1. The precision calibration method for the magnetocardiograph probe is characterized by comprising the following steps of:
s1, connecting a signal source interface of an accuracy calibration disc with a controller of a magnetocardiogram instrument, wherein the controller uses an N bit analog-digital converter and a signal range delta V;
s2, calculating a magnetic field of an excitation coil, wherein according to the Biot-Savart law, a single horizontally placed circular coil generates a magnetic field Bz of a Z-direction component at (x, y, l) with the following size:
s3, obtaining a calibration magnetic field B output by the coil from the magnetic field Bz CAL ；
S4, transmission coefficient K of analog-digital converter ADC The method comprises the following steps:
s5, output voltage V of equipment CAL Is that
Wherein N is CAL Is a calibration coefficient;
s6, the magnetic flux-voltage transmission coefficient G is
2. The precision calibration method for the magnetocardiography probe according to claim 1, wherein the precision calibration disc comprises a disc main body and a chassis, a plurality of excitation coils arranged in a matrix are arranged between the disc main body and the chassis, a signal source interface is arranged on the circumference of the disc main body, and the disc main body and the chassis are buckled and bonded by using epoxy resin glue.
3. The method for calibrating the precision of the magnetocardiography probe according to claim 1, wherein a second-order gradiometer is also adopted to shield far-field noise in the precision calibration process.
4. The method for calibrating the precision of the magnetocardiography probe according to claim 3, wherein the second-order gradiometer is completely immersed in dewar filled with liquid helium, and the liquid helium is in a temperature range of 0-300K.
5. The method for calibrating the precision of a magnetocardiography probe according to claim 4, wherein in the step S1, an energized coil model is established, current excitation is applied, an x-Z plane is selected as a reference plane, and the magnetic flux density is increased at a position closer to an excitation coil on a Z axis; to maximize the magnetic flux through the second order gradiometer pick-up coil, the excitation coil of the precision calibration disk is placed as close as possible to the bottom of the Dewar; when a single SQUID sensor is calibrated, a rectangular wave signal with a duty ratio smaller than 50% and a certain amplitude is applied to the exciting coil, and when the amplitude of the rectangular wave output by the channel is maximum, the second-order gradiometer is positioned right above the exciting coil.
6. The method for calibrating the precision of the magnetocardiography probe according to claim 5, wherein the calibration coil is controlled to work by a dial switch in order to prevent electromagnetic interference between coils; the precision calibration disc is tightly attached to the bottom of the Dewar, all excitation coils are controlled to be in a working state, the angle of the precision calibration disc is finely adjusted, and if the amplitude of the multi-channel output signal is the maximum, each excitation coil and the second-order gradiometer are coaxially placed;
then controlling only one exciting coil to work, starting the corresponding channel automatic calibration button, and calculating a calibration coefficient N by software CAL1 Automatically writing the value into a corresponding channel, and completing the channel calibration; and sequentially executing the steps until all the channel calibration coefficients are obtained.
7. The method for calibrating the precision of the magnetocardiography probe according to claim 6, wherein the difference verification is started after all channel calibration coefficients are obtained:
the calibration accuracy is evaluated through the difference of the maximum amplitude of the sampling points of each channel; the second-order gradiometer based on the SQUID sensor converts the acquired magnetic flux variation into an electric signal, the electric signal is obtained after passing through the analog-to-digital converter, and the recorded sampling points can reflect the original information of the corresponding excitation source; the MCG multichannel difference deduced from the sampling points can further verify the reliability of system calibration.
8. The method for calibrating the accuracy of the magnetocardiograph probe according to claim 7, wherein the specific verification step comprises:
recording the maximum value A of sampling points of each channel on the basis of completing calibration MCGi Determination of N channel A MCGi Maximum point A of (2) max And minimum point A min The method comprises the steps of carrying out a first treatment on the surface of the Calculation of multichannel Difference DE i ：
When DE i When the difference is more than or equal to 5%, the MCG multichannel has overlarge difference and needs to be calibrated again until DE i And less than 5 percent, and the MCG system can work normally.
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| CN117724028A (en) * | 2024-01-16 | 2024-03-19 | 哈尔滨工业大学 | Calibration system and calibration method for three-dimensional multi-point magnetic probe |
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| CN117724028A (en) * | 2024-01-16 | 2024-03-19 | 哈尔滨工业大学 | Calibration system and calibration method for three-dimensional multi-point magnetic probe |
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