CN108714027A - A kind of device and measurement method for measuring multi-electrode/scalp contact impedance in real time - Google Patents

Abstract

The invention discloses a device and a measurement method for real-time measurement of multi-channel electrode/scalp contact impedance, belonging to the field of bioelectrical impedance imaging technology and application. The device includes a host computer, a main control unit, an excitation current signal generation unit, a boundary voltage signal measurement unit, an excitation channel, a measurement channel and a contact impedance measurement module. The present invention introduces a remote reference electrode on the basis of the original imaging electrode. When measuring the contact impedance, multiple imaging electrodes are sequentially switched into the contact impedance measurement circuit through the electrode channel control unit, and real-time synchronous measurement can be realized through the electrode channel control unit. Boundary voltage data and contact impedance data. The method can accurately measure the information of the contact impedance in the following two dimensions: 1. The spatial consistency of the multi-channel contact impedance; 2. The temporal stability of the single-channel contact impedance. Compared with the existing measurement methods, the operation is simple and can be measured in real time.

Description

A device for real-time measurement of multi-channel electrode/scalp contact impedance and its measurement method

technical field

The invention belongs to the field of bioelectrical impedance imaging, in particular to a device and a measurement method for real-time measurement of multi-channel electrode/scalp contact impedance.

Background technique

Brain electrical impedance imaging technology is a new type of brain functional imaging technology, which has the advantages of non-invasive, non-radiation, fast imaging, and long-term continuous imaging. The basic principle is: use the electrodes pasted on the surface of the scalp to inject quantitative and safe AC current signals into the brain in sequence, measure the response voltage signals on each related electrode at the same time, and reconstruct the image that can respond to the brain through a specific image reconstruction algorithm. An image of the electrical impedance distribution inside the brain.

The brain electrical impedance imaging system includes multiple imaging electrodes, usually 16 electrodes or 32 electrodes, and all imaging electrodes are multiplexed between the two functions of applying current excitation and extracting response voltage signals. Compared with other simple bioelectrical signal extraction occasions (such as electroencephalograph, brain-computer interface, etc.), brain electrical impedance imaging is more complicated by the electrode/scalp contact impedance. In previous laboratory studies, electrodes were usually inserted into the scalp of anesthetized animals or fixed on the skull, which can effectively suppress the influence of scalp/contact impedance. However, in clinical applications, the electrodes are attached to the surface of the scalp, and the electrode/scalp contact impedance is relatively large, which is far greater than the transmission impedance of brain tissue. Scalp contact impedance fluctuates over time. Relevant studies have proved that when there is a large difference between the contact impedances of multiple imaging electrodes, measurement errors will be caused by affecting the performance of the signal acquisition system; during long-term continuous acquisition, when the contact impedance fluctuates greatly over time When , the imaging error is introduced in the differential imaging process.

At present, the research on the influence of electrode/scalp contact impedance in brain electrical impedance imaging mostly stays in the stage of simulation research, and there is a lack of means to obtain the spatial consistency and temporal stability of multi-channel contact impedance in real time.

Contents of the invention

The purpose of the present invention is to provide a device and method for real-time measurement of multi-channel electrode/scalp contact impedance. The device utilizes the data acquisition gap of electrical impedance imaging to quickly and real-time obtain contact impedance information corresponding to multiple imaging electrodes. It lays the foundation for the study and analysis of the influence of electrode/scalp contact impedance on imaging and the corresponding suppression method.

The present invention is achieved through the following technical solutions:

A device for real-time measurement of multi-channel electrode/scalp contact impedance disclosed by the present invention includes a host computer, a main control unit, an excitation current signal generation unit, a boundary voltage signal measurement unit, an excitation channel, a measurement channel and a contact impedance measurement module The contact impedance measurement module is composed of an electrode unit, an electrode channel control unit, a contact impedance signal measurement unit and an excitation current signal measurement unit; wherein:

The electrode unit includes a group of imaging electrodes and a reference electrode, the imaging electrodes are used to obtain electrical impedance imaging data, and the reference electrodes are used to assist in measuring the contact impedance of each imaging electrode;

The electrode channel control unit is used to realize the alternate operation of the two modes of boundary voltage measurement and contact impedance measurement by controlling the switching of the excitation channel and the measurement channel, and the time interval between the two modes is within 1s;

The contact impedance signal measurement unit is used to process the contact impedance signal (response voltage signal between the imaging electrode and the reference electrode) and transmit it to the main control unit; processing refers to filtering, amplification, signal conditioning and other processing, and then A /D conversion;

The excitation current signal measurement unit is used to process the excitation current signal (the voltage signal at both ends of the sampling resistor) and transmit it to the main control unit; processing refers to processing such as filtering, amplification, and signal conditioning, and then performing A/D conversion;

The upper computer interacts with the main control unit through the data interface to send control instructions to the main control unit, and the main control unit uploads the collected data to the upper computer; the main control unit first generates an AC sinusoidal current signal by controlling the excitation current signal generation unit , and then set the excitation channel and the measurement channel through the electrode channel control unit, and finally complete the data acquisition by controlling the boundary voltage signal measurement unit, the excitation current signal measurement unit and the contact impedance signal measurement unit.

Preferably, the imaging electrode has both the functions of current excitation and voltage measurement, and is placed on the surface of the scalp to be measured, and is usually evenly distributed on a specific section of the brain;

Preferably, there are two alternatives for the reference electrode: one is a disposable medical monopolar needle electrode, and the needle adopts a beveled small-angle grinding structure; the other is a self-adhesive surface electrode, which is similar in structure to a disposable heart. Electric electrodes, but the Ag/AgCl coating area is much larger than ordinary ECG electrodes, and can be pasted on the top of the brain; the above two electrodes are purchased from regular medical device companies, and their safety is guaranteed.

Preferably, the electrode channel control unit includes a plurality of multi-channel analog switch devices for controlling switching of imaging electrode channels and a plurality of single-pole single-throw switch devices for controlling on/off of reference electrode channels.

Further preferably, four multiple-choice 1 analog switch devices are used to control multiple imaging electrodes, MUX1 and MUX2 are used to control the positive excitation channel and the negative excitation channel respectively, and MUX3 and MUX4 are respectively used to control the positive measurement channel and the negative measurement channel; Two single-pole single-throw switching devices SPST1 and SPST2 are used to control the reference electrode;

In the boundary voltage signal measurement mode, SPST1 and SPST2 are disconnected, the enable pins of MUX1, MUX2, MUX3 and MUX4 are set to high level, and the excitation channel and measurement channel are set by controlling the gate of MUX1, MUX2, MUX3 and MUX4 ;

In the contact impedance signal measurement mode, SPST1 and SPST2 are turned on, the enable pins of MUX1 and MUX3 are set to high level, and the enable pins of MUX2 and MUX4 are set to low level. By controlling the gate of MUX1 and MUX3, the setting Stimulus channel and measurement channel.

Preferably, the contact impedance signal measurement unit is composed of a filter circuit, a fixed gain signal amplification circuit, a programmable gain signal amplification circuit and an A/D conversion circuit, and is used to measure the voltage signal corresponding to the "two-electrode method"; the excitation The current signal measurement unit is composed of a sampling resistor, a filter circuit, a fixed gain signal amplifier circuit, a programmable gain signal amplifier circuit and an A/D conversion circuit, and is used to measure the current signal corresponding to the "two-electrode method".

Further preferably, the sampling resistor has high precision and low-temperature drift characteristics; the filter circuit is composed of a second-order active low-pass filter and a second-order active high-pass filter; a fixed-gain signal amplifier circuit is a first-stage amplifier circuit, It is composed of precision resistors and operational amplifiers; the programmable gain signal amplifier circuit is a second-stage amplifier circuit composed of programmable gain amplifier devices, which can dynamically adjust its gain range to meet the signal amplification requirements within a wide dynamic range; The A/D conversion circuit is composed of a high-speed and high-precision analog-to-digital converter and its peripheral circuit, and is used as a post-stage circuit of the signal measurement unit.

The present invention also discloses a measuring method based on the above-mentioned measuring device, comprising the following steps:

1) Arrangement of electrodes

A group of imaging electrodes is placed and fixed according to the conventional method of brain electrical resistance imaging, and then a reference electrode is placed at a position approximately the same as the distance between each imaging electrode;

2) Boundary voltage data collection

Through the electrode channel control unit, the reference electrode channel is disconnected, and several imaging electrodes in a group are sequentially switched to complete a frame of boundary voltage data acquisition, and the boundary piezoelectric data is used for imaging, which is imaging data;

3) Contact impedance data and excitation current data acquisition

Contact impedance data acquisition: through the electrode channel control unit, the reference electrode channel is turned on, and the imaging electrode channels are selected sequentially; using the two-electrode method, an excitation current signal is injected between the reference electrode and the selected imaging electrode, and the two electrodes are measured. The voltage signal between the electrodes is denoted as U _i , where i=1, 2,..., N is the electrode number, and N is the number of imaging electrodes;

Excitation current data collection: By measuring the voltage signal on the sampling resistor, the excitation current signal is indirectly measured, which is recorded as U _sampling ;

4) Repeat step 2) and step 3) to enter the next frame of data acquisition cycle until the host computer sends a "terminate acquisition" command.

Preferably, when arranging the reference electrode in step 1), it is selected according to the needs. If a disposable medical monopolar needle electrode is used, it will be inserted under the scalp after disinfection and fixed. If a disposable customized reference electrode is used, it will be placed on the scalp surface and fixed;

Preferably, in step 3), the actual excitation current when the imaging electrode i is gated is:

I _i =U _sampling /R _sampling ;

The impedance value measured by the two-electrode method is:

Z _i =U _i /I _i =U _i /(U _sample /R _sample )=U _i *R _sample /U _sample ;

The essence of Z _i is the sum of the contact impedance Zc _i of the measured imaging electrode, the transmission impedance Zt _i between the imaging electrode and the reference electrode, and the contact impedance Zcf of the reference electrode, namely:

Z _i =Zc _i +Zt _i +Zcf;

Define ΔZ _m,n as follows:

Among them, m,n=1,2,...,N and m≠n; Zc _m -Zc _n is the difference between the contact impedance of imaging electrode m and imaging electrode n, recorded as ΔZc _m,n ; Zt _m -Zt _n is the difference in transmission impedance between the imaging electrode m and imaging electrode n and the reference electrode, denoted as ΔZt _m,n ; Zcf _m -Zcf _n is the fluctuation error of the reference electrode contact impedance during two measurements, denoted as ΔZcf _{m , n} .

Preferably, among the two reference electrodes selected in the present invention, one of them can penetrate under the scalp, and the other has a larger effective contact area, which can ensure that Zcf<<Zc _i and ΔZcf _m,n ≈0, while referring to The distance between the electrode and each imaging electrode is approximately the same, which can ensure that the difference between each Zt _i is small, that is, satisfy: ΔZt _m,n << ΔZc _m,n ;

Then: ΔZc _m,n = Zc _m -Zc _n ≈ Z _m -Z _n ;

Define ΔZ _i (t1,t2) as:

Among them, i=1,2,...,N; t1, t2 are two moments; Zc _i (t1)-Zc _i (t2) is the change of the i-th imaging electrode contact impedance between the two moments of t1 and t2 value, denoted as ΔZc _i (t1, t2); Zt _i (t1)-Zt _i (t2) is the change value of the i-th imaging electrode contact impedance between t1 and t2, denoted as ΔZt _i (t1, t2 ); ΔZcf _i (t1, t2) is the change value of the contact impedance of the reference electrode between the two moments of t1 and t2;

The transmission impedance is affected by the position of the electrode and the change of the intracranial impedance. Compared with the contact impedance, its time stability is higher, that is, ΔZt _i (t1,t2)≈0; the reference electrode contact impedance achieves time stability through manual intervention, that is, ΔZcf _i (t1,t2)≈0.

Therefore, we can measure the contact impedance information in two dimensions by this method, that is, the spatial consistency index ΔZc _m,n of the contact impedance of multiple imaging electrodes and the time stability index ΔZc _i of the contact impedance of each imaging electrode (t1,t2).

Compared with the prior art, the present invention has the following beneficial technical effects:

The device for real-time measurement of multi-channel electrode/scalp contact impedance disclosed by the present invention is provided with a contact impedance measurement module, which is composed of an electrode unit, an electrode channel control unit, a contact impedance signal measurement unit and an excitation current signal measurement unit. Wherein, the electrode unit includes a group of imaging electrodes and a reference electrode. The present invention introduces a remote reference electrode on the basis of the original imaging electrode. When measuring the contact impedance, multiple imaging electrodes are sequentially switched into the contact impedance measurement circuit through the electrode channel control unit, and real-time synchronous measurement can be realized through the electrode channel control unit. Boundary voltage data and contact impedance data. In the present invention, the contact impedance signal measurement unit, the excitation current signal measurement unit and the boundary voltage signal measurement unit are independent of each other to meet the high-precision measurement of different amplitudes and different types of signals.

The real-time measurement method for multi-channel electrode/scalp contact impedance in brain electrical impedance imaging disclosed by the present invention adopts the "two-electrode method" measurement principle, and uses the excitation current signal measurement unit and the contact impedance signal measurement unit to measure each imaging The actual injection current signal and response voltage signal between the electrode and the reference electrode, and the impedance value between each imaging electrode and the reference electrode is calculated; thus the spatial consistency and temporal stability information of the multi-channel contact impedance is analyzed. The method can accurately measure the information of the contact impedance in the following two dimensions: 1. The spatial consistency of the multi-channel contact impedance; 2. The temporal stability of the single-channel contact impedance. Compared with the existing measurement methods, it is easy to operate, can be measured in real time, and has higher accuracy in obtaining information in the above two dimensions, laying a foundation for studying the influence of contact impedance on brain electrical impedance imaging and its suppression methods.

Description of drawings

FIG. 1 is a schematic diagram of a brain electrical impedance imaging data measurement mode;

Fig. 2 is the schematic diagram of contact impedance data acquisition method;

Fig. 3 is a schematic diagram of the placement method of the imaging electrode and the reference electrode;

Fig. 4 is a structural block diagram of brain electric impedance imaging hardware system;

5 is a schematic structural diagram of an electrode channel control unit;

Figure 6 shows the spatial consistency analysis results of the 16-way electrode/scalp contact impedance (the 16-way contact impedance is relatively uniform); among them, (a) is the measured value (Ω) of the 16-way contact impedance; (b) is the relative difference of the 16-way contact impedance (%);

Figure 7 shows the spatial consistency analysis results of 16-way electrode/scalp contact impedance (the contact impedance of No. 6 imaging electrode is abnormal); among them, (a) is the measured value of 16-way contact impedance (Ω); Impedance relative difference (%);

Figure 8 shows the analysis results of the time stability of the 16-way electrode/scalp contact impedance (the contact impedance of imaging electrode No. ); (b) is the relative change (%) of the 16-way contact impedance between t1 and t2.

Detailed ways

The present invention will be further described in detail below in conjunction with specific embodiments, which are explanations of the present invention rather than limitations.

The purpose of this embodiment is to realize the real-time measurement of the contact impedance of 16 imaging electrodes. For the 16-electrode brain electrical impedance imaging system, the data acquisition mode of "contralateral excitation, adjacent measurement" was adopted. The system has a total of 16 excitation electrode pairs, and each excitation electrode pair corresponds to 16 measurement electrode pairs, so a complete data collection includes 256 measurements, that is, a frame of data includes 256 measurement values. Referring to Fig. 1 , Fig. 1 shows: the excitation current is injected into the cranium through the excitation electrode pair 1-9, and the response voltage signal on the measurement electrode pair 3-4 is measured. Due to the contact resistance between the imaging electrode and the scalp, that is, Zc ₁ , Zc ₃ , Zc ₄ and Zc ₉ in the figure. Therefore, both the excitation signal loop and the measurement signal loop are affected by the contact impedance.

1. Introduction to measurement principle

The principle of the method for real-time measurement of multi-channel electrode/scalp contact impedance in a brain electrical impedance imaging system provided by the present invention is: use the two-electrode method to measure the impedance value between each imaging electrode and the reference electrode in real time, and accordingly The contact impedance information of multiple imaging electrodes is obtained by analysis, as shown in Fig. 2 .

The 16 imaging electrodes in this embodiment are placed on the surface of the scalp and evenly distributed on a certain section of the brain; the reference electrode is a self-adhesive surface electrode, which is placed on the top of the brain, and the distance from the 16 imaging electrodes is approximately the same ,As shown in Figure 3.

The structural block diagram of the brain electrical impedance imaging hardware system composed of the multi-channel electrode/scalp contact impedance real-time measurement module provided by the present invention is shown in FIG. 4 . The system consists of host computer, data interface, main control unit, excitation current signal generation unit, boundary voltage signal measurement unit, contact impedance signal measurement unit, excitation current signal measurement unit, electrode channel control unit and electrode unit. Wherein, each unit included in the dotted line frame constitutes a 16-way electrode/scalp contact impedance measurement module.

The working principle of the brain electrical impedance imaging system is: the upper computer sends control instructions to the main control unit through the data interface. After the main control unit receives the data acquisition instruction, it first generates the AC sinusoidal current signal with the set frequency and amplitude by controlling the excitation current signal generation unit; then sets the excitation channel and the measurement channel through the electrode channel control unit; finally controls the boundary voltage The signal measurement unit, the contact impedance signal measurement unit and the excitation current signal measurement unit complete data collection. The main control unit uploads the collected data to the upper computer through the data interface. Among them, the boundary voltage signal measurement unit is used to measure the boundary voltage signal; the contact impedance signal measurement unit and the excitation current signal measurement unit are used to measure the voltage signal and current signal corresponding to the "two-electrode method", and then the contact impedance value is obtained through calculation.

In this embodiment, the excitation current signal during contact impedance measurement and the excitation current signal during boundary voltage signal measurement are generated by the same excitation current signal generation unit. Its working principle is as follows: firstly, digital frequency synthesis technology is adopted, and the sinusoidal AC voltage signal of the required frequency and amplitude is generated by the DAC device. After filtering and amplification processing, the voltage-controlled current source is converted into a sinusoidal AC current signal. The electrical amplitude range of the current signal is 10-2500uA, and the frequency range is 1k-200kHz. In this embodiment, the current amplitude is 1250uA, and the frequency is 50kHz.

In this embodiment, the specific scheme of controlling the excitation channel and the measurement channel by the electrode channel control unit is shown in FIG. 5 . Four 16-to-1 analog switch devices are used to control 16 imaging electrodes, MUX1 and MUX2 respectively control the positive and negative excitation channels, and MUX3 and MUX4 respectively control the positive and negative measurement channels. Two SPST switching devices (SPST1 and SPST2) are used to control the reference electrode. In the boundary voltage data acquisition mode, SPST1 and SPST2 are disconnected, the enable pins of MUX1~MUX4 are set to high level, and the excitation channel and measurement channel are set by controlling the gate of MUX1~MUX4; in the contact impedance measurement mode, SPST1 and SPST2 is turned on, the enable pins of MUX1 and MUX3 are set to high level, and the enable pins of MUX2 and MUX4 are set to low level. By controlling the gating of MUX1 and MUX3, the excitation channel and measurement channel are set.

2. Analysis of measured contact impedance data

In this embodiment, the impedance value measured by the two-electrode method is:

Z _i =Zc _i +Zt _i +Zcf

Wherein, i=1,2,...,16, Zc _i is the contact impedance of the imaging electrode i, Zt _i is the transfer impedance between the imaging electrode i and the reference electrode, Zcf is the contact impedance of the reference electrode.

(1) Qualitative analysis of 16-way electrode/scalp contact impedance

The reference electrode adopted in this embodiment is a monopolar needle electrode, which can penetrate under the scalp, so the contact impedance of the reference electrode is much smaller than that of the imaging electrode;

In addition, studies have reported that the transfer impedance of brain tissue is much smaller than the contact impedance of imaging electrodes; namely:

Zcf<<Zc _i , Zt _i <<Zc _i

Therefore, Z _i ≈ Zc _i can be used to qualitatively evaluate the contact impedance of the 16 imaging electrodes.

Referring to Fig. 6(a) and Fig. 7(a), the measured values of the 16-way contact impedance are displayed using the radar chart method. The 16 contact impedance values in Figure 6(a) are similar, indicating that the contact conditions of the 16 electrodes are close. In Figure 7(a), it can be observed that the sixth measured value is significantly greater than the other values, indicating that the contact impedance of No. 6 imaging electrode is abnormally large. It can be preliminarily inferred that the contact condition of No. 6 imaging electrode is not good.

(2) Spatial consistency analysis of 16-way electrode/scalp contact impedance

Spatial consistency refers to the difference between the 16 contact impedances at a certain moment.

First, compare the 16 measured values to find the minimum value, the corresponding electrode number is a, and define ΔZ _i,a as follows:

ΔZ _i,a =Z _i -Z _a

＝(Zc _i +Zt _i +Zcf _i )-(Zc _a +Zt _a +Zcf _a )

＝(Zc _i -Zc _a )+(Zt _i -Zt _a )+(Zcf _i -Zcf _a )

Wherein, i=1, 2, . . . , 16. Zc _i -Zc _a is the difference in contact impedance between imaging electrode i and imaging electrode a, recorded as ΔZc _i,a ; Zt _i -Zt _a is the difference in transmission impedance between imaging electrode i and imaging electrode a and the reference electrode , denoted as ΔZt _i,a ; Zcf _i -Zcf _a is the fluctuation error of the contact impedance of the reference electrode during two measurements, denoted as ΔZcf _i,a .

Since ΔZcf _i,a ≈0 and ΔZt _i,a <<ΔZc _i,a , then,

ΔZc _i,a ＝Zc _i -Zc _a ≈Z _i -Z _a

Then the spatial consistency coefficient of the 16-way contact impedance can be expressed as:

Referring to Fig. 6(b) and Fig. 7(b), corresponding to Fig. 6(a) and Fig. 7(a) respectively, it shows the spatial consistency of the 16-way contact impedance, that is, the relative difference. In Figure 6(b), the 16 values are similar, indicating that the contact conditions of the 16 electrodes are close; while in Figure 7(a), it can be observed that the sixth value is significantly larger than the other values, indicating that the contact impedance of imaging electrode No. 6 is abnormally large , It can also be preliminarily inferred that the contact condition of the No. 6 imaging electrode is not good.

(3) Time stability analysis of 16-way electrode/scalp contact impedance

Time stability refers to the relative change rate of the 16-way contact impedance over time at two moments. Firstly, the measured values of 16 contact impedances are displayed in the same radar chart at two moments, as shown in Fig. 8(a). It can be observed that the eighth value has the largest variation between the two time points, and it can be inferred that the temporal stability of the No. 8 imaging electrode is significantly worse than that of other imaging electrodes.

In order to quantitatively analyze the relative change of the 16-way contact impedance over time, ΔZ _i (t1, t2) is defined as:

ΔZ _i (t1,t2)=Z _i (t1)-Z _i (t2)

＝[Zc _i (t1)-Zc _i (t2)]+[Zt _i (t1)-Zt _i (t2)]+ΔZcf _i (t1,t2)

Wherein, i=1, 2, . . . , 16. Zc _i (t1)-Zc _i (t2) is the variation of the contact impedance of imaging electrode i between the two moments t1 and t2, which is recorded as ΔZc _i (t1, t2); Zt _i (t1)-Zt _i (t2) is the variation of the contact impedance of the imaging electrode i between the two moments t1 and t2, recorded as ΔZt _i (t1, t2); ΔZcf _i (t1, t2) is the variation of the contact impedance of the reference electrode between the two moments t1 and t2 .

Since ΔZt _i (t1,t2)≈0 and ΔZcf _i (t1,t2)≈0, then,

_ΔZci (t1,t2)= _Zci (t1) _-Zci (t2) _≈Zi (t1) _-Zi (t2).

Then the time stability coefficient of the 16-way contact impedance can be expressed as:

Referring to FIG. 8( b ), corresponding to FIG. 8( b ), it shows the time stability of the 16-way contact impedance, that is, the relative change between two moments t1 and t2. It can be observed that the 8th value is the largest, and it can be inferred that the temporal stability of No. 8 imaging electrode is significantly worse than that of other imaging electrodes.

From this, we can obtain two-dimensional contact impedance information by analyzing the data, that is, the spatial consistency index α of 16 contact impedances and the temporal stability index β of the contact impedance of each imaging electrode.

The above-mentioned embodiments are preferred examples for realizing the present invention, and the present invention is not limited to the above-mentioned embodiments. Any non-essential additions, replacements, or simple setting and adjustment of the judgment threshold made by those skilled in the art based on the technical features of the technical solutions of the present invention fall within the scope of protection of the present invention.

Claims (10)

1. A device for real-time measurement of multi-channel electrode/scalp contact impedance, characterized in that it includes a host computer, a main control unit, an excitation current signal generation unit, a boundary voltage signal measurement unit, an excitation channel, a measurement channel and contact impedance measurement module; The contact impedance measurement module is composed of an electrode unit, an electrode channel control unit, a contact impedance signal measurement unit and an excitation current signal measurement unit; wherein: The electrode unit includes a group of imaging electrodes and a reference electrode, the imaging electrodes are used to obtain electrical impedance imaging data, and the reference electrodes are used to assist in measuring the contact impedance of each imaging electrode; The electrode channel control unit is used to realize the alternate operation of the two modes of boundary voltage measurement and contact impedance measurement by controlling the switching of the excitation channel and the measurement channel; The contact impedance signal measurement unit is used to process the response voltage signal between the imaging electrode and the reference electrode and transmit it to the main control unit; The excitation current signal measurement unit is used to process the voltage signal at both ends of the sampling resistor and transmit it to the main control unit; The upper computer interacts with the main control unit through the data interface to send control instructions to the main control unit, and the main control unit uploads the collected data to the upper computer; the main control unit first generates an AC sinusoidal current signal by controlling the excitation current signal generation unit , and then set the excitation channel and the measurement channel through the electrode channel control unit, and finally complete the data acquisition by controlling the boundary voltage signal measurement unit, the excitation current signal measurement unit and the contact impedance signal measurement unit. 2. The device for real-time measurement of multi-channel electrode/scalp contact impedance according to claim 1, characterized in that the imaging electrodes have both current excitation and voltage measurement functions, and a group of imaging electrodes are all attached to the scalp of the subject to be measured surface. 3. The device for real-time measurement of multi-channel electrode/scalp contact impedance according to claim 1, wherein the reference electrode is a disposable medical monopolar needle electrode or a self-adhesive surface electrode. 4. The device for measuring multi-channel electrode/scalp contact impedance in real time according to claim 1, wherein the electrode channel control unit includes a plurality of multi-channel analog switch devices and multiple channels for controlling the switching of imaging electrode channels. A single-pole single-throw switching device that controls the on/off of the reference electrode channel. 5. The device for real-time measurement of multi-channel electrode/scalp contact impedance according to claim 4, characterized in that four multiple-choice 1 analog switch devices are used to control a plurality of imaging electrodes, and MUX1 and MUX2 are used to control the positive direction respectively. The excitation channel and the negative excitation channel, MUX3 and MUX4 respectively control the positive measurement channel and the negative measurement channel; two single-pole single-throw switching devices SPST1 and SPST2 are used to control the reference electrode; In the boundary voltage signal measurement mode, SPST1 and SPST2 are disconnected, the enable pins of MUX1, MUX2, MUX3 and MUX4 are set to high level, and the excitation channel and measurement channel are set by controlling the gate of MUX1, MUX2, MUX3 and MUX4 ; In the contact impedance signal measurement mode, SPST1 and SPST2 are turned on, the enable pins of MUX1 and MUX3 are set to high level, and the enable pins of MUX2 and MUX4 are set to low level. By controlling the gate of MUX1 and MUX3, the setting Stimulus channel and measurement channel. 6. The device for measuring multi-channel electrode/scalp contact impedance in real time according to claim 1, characterized in that the contact impedance signal measurement unit is composed of filter circuit, fixed gain signal amplifier circuit, program-controlled gain signal amplifier circuit and A /D conversion circuit, used to measure the voltage signal corresponding to the two-electrode method; The excitation current signal measurement unit is composed of a sampling resistor, a filter circuit, a fixed gain signal amplifier circuit, a program-controlled gain signal amplifier circuit and an A/D conversion circuit, and is used to measure the voltage signal corresponding to the two-electrode method. 7. The device for real-time measurement of multi-channel electrode/scalp contact impedance according to claim 6, wherein the filter circuit is composed of a second-order active low-pass filter and a second-order active high-pass filter ; The fixed gain signal amplification circuit is a first-stage amplification circuit, which is composed of a precision resistor and an operational amplifier; the programmable gain signal amplification circuit is a second-stage amplification circuit, which is composed of a programmable gain amplification device and can be dynamically adjusted The gain range satisfies the requirement of signal amplification within a wide dynamic range; the A/D conversion circuit is composed of an analog-to-digital converter and its peripheral circuit, and is used as a post-stage circuit of the signal measurement unit. 8. A measurement method based on the device for real-time measurement of multi-channel electrode/scalp contact impedance according to any one of claims 1 to 7, characterized in that it comprises the following steps: 1) Arrangement of electrodes A group of imaging electrodes is placed and fixed according to the conventional method of brain electrical resistance imaging, and then a reference electrode is placed at a position approximately the same as the distance between each imaging electrode; 2) Boundary voltage data acquisition Through the electrode channel control unit, the reference electrode channel is disconnected, and several imaging electrodes in a group are sequentially switched to complete a frame of boundary voltage data acquisition, and the boundary piezoelectric data is used for imaging, which is imaging data; 3) Contact impedance data and excitation current data acquisition Contact impedance data acquisition: through the electrode channel control unit, the reference electrode channel is turned on, and the imaging electrode channels are selected sequentially; using the two-electrode method, an excitation current signal is injected between the reference electrode and the selected imaging electrode, and the two electrodes are measured. The voltage signal between the electrodes is denoted as U _i , where i=1, 2,..., N is the electrode number, and N is the number of imaging electrodes; Excitation current data collection: By measuring the voltage signal on the sampling resistor, the excitation current signal is indirectly measured, which is recorded as U _sampling ; 4) Repeat step 2) and step 3) to enter the next frame of data acquisition cycle until the host computer issues a command to terminate the acquisition. 9. measuring method according to claim 8, is characterized in that, step 3) in, the actual excitation current when imaging electrode i gate is: I _i =U _sampling /R _sampling ; The impedance value measured by the two-electrode method is: Z _i =U _i /I _i =U _i /(U _sample /R _sample )=U _i *R _sample /U _sample ; The essence of Z _i is the sum of the contact impedance Zc _i of the measured imaging electrode, the transmission impedance Zt _i between the imaging electrode and the reference electrode, and the contact impedance Zcf of the reference electrode, namely: Z _i =Zc _i +Zt _i +Zcf; Define ΔZ _m,n as follows: Among them, m,n=1,2,...,N and m≠n; Zc _m -Zc _n is the difference between the contact impedance of imaging electrode m and imaging electrode n, recorded as ΔZc _m,n ; Zt _m -Zt _n is the difference in transmission impedance between the imaging electrode m and imaging electrode n and the reference electrode, denoted as ΔZt _m,n ; Zcf _m -Zcf _n is the fluctuation error of the reference electrode contact impedance during two measurements, denoted as ΔZcf _{m , n} . 10. The measurement method according to claim 9, wherein the effective contact area of the reference electrode can ensure that Zcf<<Zc _i and ΔZcf _m,n ≈0, while the distance between the reference electrode and each imaging electrode is approximately the same , which can ensure that the difference between each Zt _i is small, that is, satisfy: ΔZt _m,n << ΔZc _m,n ; Then: ΔZc _m,n = Zc _m -Zc _n ≈ Z _m -Z _n ; Define ΔZ _i (t1,t2) as: Among them, i=1,2,...,N; t1, t2 are two moments; Zc _i (t1)-Zc _i (t2) is the change of the i-th imaging electrode contact impedance between the two moments of t1 and t2 value, denoted as ΔZc _i (t1, t2); Zt _i (t1)-Zt _i (t2) is the change value of the i-th imaging electrode contact impedance between t1 and t2, denoted as ΔZt _i (t1, t2 ); ΔZcf _i (t1, t2) is the change value of the contact impedance of the reference electrode between the two moments of t1 and t2; The transmission impedance is affected by the position of the electrode and the change of the intracranial impedance. Compared with the contact impedance, its time stability is higher, that is, ΔZt _i (t1,t2)≈0; the reference electrode contact impedance achieves time stability through manual intervention, that is, ΔZcf _i (t1,t2)≈0.