Device and Method for measuring electrical bioimpedance

Description

FIELD OF TECHNOLOGY

The invention belongs to the field of medical technology. More specifically, the invention relates to body-worn medical devices and their use for assessing of a patient's health status.

PRIOR ART

A multi-functional wrist-worn blood pressure monitoring device (US 2017/0340209, Apple Inc, published Nov. 30, 2017) is known, which has various sensors configured to non-invasively engage with the skin on the user's wrist to sense various physiological signals from the user's cardiovascular system. The various sensors comprise a pulse transit time (PTT) sensor, an ECG sensor, an electrical bioimpedance (EBI) sensor. The device uses a classic 4-electrode impedance measurement system for measuring the electrical bioimpedance of the body from the skin surface. The device comprises an excitation current generator, the first and second excitation current electrodes, the first and second voltage pickup electrodes and a voltage meter. The voltage drop caused by the current passing through the body impedance is picked up by the first voltage electrode and the second voltage electrode through the corresponding contact impedances of the first electrode and the second electrode. The disadvantage of the solution is that the contact impedances in the path of the current electrodes remain outside the measured body impedance according to the solution principle, but actually it remains undetermined which body area the excitation current passes through and therefore it remains undetermined which region of the body impedance we actually measure. To be noted that the contact resistances of the voltage electrodes with the measured body also causes uncertainty. The problem can be solved by using a voltage meter with a symmetrical differential input, but this is a complex and expensive solution. In addition, a measurement error inevitably occurs due to the presence of a common mode voltage on both the voltage electrodes, suppression of which is not perfect.

Known method and apparatus for measuring bioimpedance (U.S. Pat. No. 10,952,633, Samsung Electronics, published Mar. 23, 2023) uses a 4-electrode measurement circuit and comprises a first electrical signal generator configured to generate a first electrical signal to measure the bioimpedance of an object. The apparatus also includes a second, the compensation signal generator configured to generate a compensation signal to compensate the bio-signal measured with the help of the first electrical signal generator, and an amplifier configured to amplify the compensated bio-signal. In addition to the contact impedances of the four measurement electrodes, body impedances not included in the measurement chain are shown as potential sources of measurement errors. To eliminate the influence of the contact impedances of the electrodes of unknown size and the unknown body impedance components, the second generator with additional electrodes has been introduced, which generates an electrical compensation signal for measuring the reference impedance of a body part similar to the object, the corresponding voltage of which is subtracted from the voltage signal on the voltage electrodes. This compensates the influence of the contact impedances of the electrodes and the unknown body impedances on the measurement result. The disadvantage of the presented solution is the addition of electrodes and electronic components with additional electrical connections between them. The disadvantage is the increase in the complexity of the device and the fact that the efficiency of the compensation depends on poorly defined circumstances violating the success of the compensation effect.

Known are the methods for using wearable devices to monitor a patient's health status by measuring the electrical bioimpedance of their cardiovascular and pulmonary systems (Anand, G., et al 2021, Topical Review: Bioimpedance analysis as a tool for hemodynamic monitoring: overview, methods, and challenges. Physiol. Meas. DOI: 10.1088/1361-6579/abe80e), especially regarding measuring the impedance of the patient's wrist arteries. (Yu, Y., et al. The Investigation of Bio-impedance Analysis at a Wrist Phantom with Two Pulsatile Arteries. Cardiovasc. Eng. Technol., Oct. 17, 2023. DOI: 10.1007/s13239-023-00689-9).

The closest analogue to the presented solution in terms of technical nature is a bioimpedance-based pulse waveform sensing system and method (US 2018/0078148, Mar. 22, 2018. Cohn, G. A., et al. Bioimpedance based pulse waveform sensing, Microsoft Technology Licensing, published Mar. 22, 2018), where the system comprises a pressure measuring sensor and an arterial pulse wave sensor based on bioimpedance measurement. The arterial pulse wave is determined by measuring the voltage difference between two voltage electrodes. The current generator in the bioimpedance measuring device generates an excitation current to the object through the current electrodes, and the measuring device detects and outputs the voltage difference created by the excitation current in the tissues of the wrist area, which is proportional to the bioimpedance between the voltage electrodes. A four-electrode circuit is used to measure the bioimpedance, where four short strip electrodes placed on the object-radial artery-perpendicularly. The disadvantage of this solution is that the electrical resistances between the biological object (artery) and the electrodes are on average about 100 times greater than the measured electrical bioimpedance Z and more than 1000 times greater than the average informative change in electrical bioimpedance. Measuring such a small impedance change against the background of large and only limitedly accurately compensated extraneous impedance components is difficult. Another disadvantage is the use of short strip electrodes placed perpendicularly to the measurement object, such as an artery, aorta, or other blood vessel, which, compared to the known point and round electrodes (U.S. Pat. No. 10,952,633, Samsung Electronics, published Mar. 23, 2021), does provide an opportunity to somewhat increase the sensitivity, but it is far from decisive improvement, because the voltage response between the two voltage electrodes remains very small when a pulse wave arrives in comparison with unstable contact impedances between the electrodes and the biological object. The reliability of the information obtained by measurement suffers significantly.

The primary and greatest disadvantage of the closest analogue solution is that the measured impedance Z=10-50Ω of a biological object is on average 100 times smaller than the contact resistance between the excitation current electrode and the body in the order of r1≈r2=1000-5000Ω, while the informative change of the measured impedance ΔZ=1-5Ω is only one thousandth of it. Measuring such small impedance changes against the background of large and still unstable contact resistances is very problematic.

Another major drawback concerns electrodes with configuration in which the four point electrodes or four short strip electrodes (two excitation current electrodes and two voltage pickup electrodes) are placed perpendicularly to the biological object, which do not allow to determine the trajectory of the excitation current i between the excitation electrodes with sufficient accuracy. This is a reason the impedance measurement result Z=V/i is not accurate neither stable, since it is not clear what part of the excitation current passes through the measured impedance Z and gives the measurement result in the form of a voltage drop V.

The closest method for measurement of the electrical bioimpedance is the procedure described in the US patent application (2018/0078148, Mar. 22, 2018. Cohn, G. A., et al. Bioimpedance based pulse waveform sensing, Microsoft Technology Licensing, Mar. 22, 2018), which deals with the recording of a blood pulse curve together with finding of parameters of this curve, such as the value of blood pressure and the characteristics of its change in real time from the results of electrical bioimpedance measurement of the wrist. The method is distinguished by the fact that instead of the usual four-point electrodes (2 electrodes for introducing the excitation current and 2 electrodes for voltage collection), short strip electrodes are used, which are placed on the wrist in a position perpendicular to both the radial and ulnar arteries. The perpendicular position of the electrodes allows to somewhat improve the signal/noise ratio compared to using of point electrodes, however, the level of the pulse signal remains weak, because the size of the non-informative and unstable contact resistances (r1, r2) between the electrode and the biological object exceeds the size of the measured bioimpedance Z many times and is up to thousands of times greater than the value of the informative variation ΔZ of the bioimpedance Z. This significantly limits the reliability and usability of the measurement method described above in clinical applications. An attempt has been made to solve the problem by improving the method (U.S. Pat. No. 10,952,633, Samsung Electronics, published Mar. 23, 2021), which consists in adding separate specific electrodes for measuring the resistances between the electrode and the body and then subtracting the obtained result from the original measurement results. This technique improves the accuracy of the initial measurement results, but does not lead to a decisive improvement, because the additional measurement performed with similar, but still different electrodes, and moreover, the bioimpedance of the same biological body measured, but from a different location, however. Therefore, subtracting the measurement results obtained using additional electrodes is not a sufficiently effective method for eliminating uncertainty.

The measurement method related to the closest technical solution has the following disadvantages.

The first disadvantage of the known measurement method is that it is not possible with the known electrodes to direct the entire generated excitation current i exactly through the region the impedance Z of which we want to measure. The path of the excitation current i affected by the large and poorly known contact resistances r1 and r2, through which the current electrodes are connected to the body part. As a result of the measurement, we do not know exactly what impedance region of the biological object we are measuring.

Another disadvantage of the known measurement method is the generation of a constant value excitation current throughout the measurement frequency range, while the safety standards allow the higher excitation currents at higher frequencies. Therefore, the power of excitation current cannot used to its maximum extent to achieve the best signal-to-noise ratio and measurement accuracy, especially at higher frequencies above 10 KHz.

SUMMARY OF THE INVENTION

The aim of the invention is to develop a device and method for accurate measurement of electrical bioimpedance to increase the reliability of the results.

Known electrical bioimpedance measurement devices comprise a measuring device, excitation current electrodes and voltage pickup electrodes, electrical contact resistances (r1, r2) between the excitation current electrodes and the biological object, and a generator.

Known devices have disadvantages that limit their scope of application, especially in the field of medical diagnostics. To eliminate the disadvantages, the invention comprises the following improvements.

The following components with their interconnections have been additionally introduced into the electrical bioimpedance measurement device: an active electrode system, containing an operational amplifier and a current-impeding circuit, and an analog interface with inputs connected the outputs of three voltage signals-one of these is originating from the generator and two others (V1 and V2) from the active electrode system-which carry information about the bioimpedance Z of the biological object and its variation ΔZ. In the input of the analog interface there is a summator, which includes a first summing unit, a second summing unit and an amplifier, which separate the signal components corresponding to the impedance Z and its variation ΔZ. The summator is followed by a demodulator located in the analog interface, wherein there are two multipliers and a reference signal formator, and a low pass filter and a band pass filter, in which a signal carrying information about the impedance Z and a signal carrying its variation ΔZ, are demodulated separately but both synchronously with the generator voltage. One of the four voltage inputs of the measuring device is connected to the output of the operational amplifier for measuring the voltage V3 emanating from the excitation current electrode, and the other voltage input is connected to the voltage pickup electrode for measuring the voltage V1 emanating from it, whereas the remaining two voltage inputs of the measuring device are connected to the two outputs of the analog interface for measuring the demodulated output voltages DZ and DΔZ, which reflect the impedance Z and its variation ΔZ.

Supplementing the known devices with inventive elements enables to eliminate the following disadvantages.

1. Relatively high, unstable, and poorly defined contact resistances (r1, r2) between the excitation current electrodes and the biological object cause disturbing uncertainties up to the point of invalidating the measurement results.

This disadvantage is eliminated by introducing an active electrode system, which contains an inverting operational amplifier so that the excitation current electrodes are arranged in series through the electrode-body contact resistances r1 and r2 and the biological object in the negative feedback loop of the operational amplifier situating between the output and inverting input of the operational amplifier, which reduces the effect of the resistances r1 and r2 on the measurement result by a factor close to the gain of the operational amplifier without feedback. Since the average value of the gain factor of the operational amplifier is 100,000, the effective influence of the contact resistances r1 and r2 is only tens of milliohms (MΩ) from the informative variations within the range of ΔZ=1-5Ω. The misinformation effect of the contact resistances r1 and r2 becomes negligible.

2. The use of a constant excitation current i mostly used in the entire measurement frequency range does not enable the use of the permissible increase in the excitation current in living organisms at higher frequencies, because of which the possibility for increasing the measurement accuracy in higher operating frequency range remains unrealized.

This shortage eliminated by introducing a frequency dependent current-impeding circuit, switched between the generator's output and the inverting input of the operational amplifier, the frequency characteristics of the resistance R of which are as similar as possible to those of the impedance Z of the measured biological object. This enables us to achieve the highest permissible excitation current i value at all frequencies, but especially at high frequencies (f≥10 kHz). We obtain the best signal/noise ratio at all operating frequencies, which makes it possible to measure with the least measurement error in the entire measurement frequency range.

3. Point electrodes or short strip electrodes placed perpendicular to the biological object in the known devices do not allow us to determine the trajectory of the excitation current i between the excitation current electrodes with sufficient accuracy. The current dissipates in the biological object, which results in excitation current leakage. Due to the undetermined leakage current, the result of the impedance Z=V/i measurement is also not accurate or stable, since the size of the part measuring the excitation current is not exactly known. We obtain an inaccurate measurement result in the form of the voltage drop VZ.

In the invention, the above-mentioned drawback is eliminated by using long strip electrodes, constituting an electrode system surrounding the biological object containing the measured bioimpedance Z in such a way that on one side of it there are placed one excitation current electrode and one voltage pickup electrode constituting one pair of electrodes, and on the other side there is another voltage pickup electrode and another excitation current electrode constitutes another pair of electrodes, whereby the voltage pickup electrodes have typically a wider grip than the dissipation of excitation current. There are specific electrode configurations, selection of which depends on the certain shape of the biological object, surrounded with electrodes to measure the bioimpedance Z in both a straight line and curved manner, capturing it in either an open or closed form. As a result, we achieve the minimal current leakage, which enables the entire generated excitation current i to pass through the measured bioimpedance Z, ensuring thereby the best measurement accuracy, especially for the informative variation ΔZ of biological object's bioimpedance Z.

4. A very troublesome drawback of the known solutions is the problematics connected with measuring the informative variation ΔZ constituting only a few hundredths or even thousandths of the total impedance Z. Uncertainties can cause large measurement errors in the presence of disturbances and noise, the measurement results may even prove to be unsuitable due to an excessively large measurement errors (error ≥30%).

To eliminate the drawback, a summator introduced into the input of the analog interface. It comprises the first and second summing units separating the voltage components VZ and VΔZ corresponding to the impedance Z and its variation ΔZ. The demodulator following the summator, detects synchronously with the generator voltage separately the voltage VZ corresponding to the bioimpedance Z and the amplified voltage signal VΔZ carrying information about its variation thereby. As a result, the accuracy and informativeness of the measurement results, especially of variation ΔZ improved.

5. The deficiency of the known solutions manifested in the fact that the value of the contact resistances r1 and r2 exceeds the value of the measured bioimpedance Z. Because of that, the measurement can lead to a nonlinear operation of the active electrode system and cause overloading. As a result, the errors can distort the measurement results completely up to the point of being fully unsuitable.

In the invention, the problem solved by means of control measurements. The first voltage pickup electrode connected to a measuring device, the voltage V1 measured there must remain below the level V1≤permissible to avoid fatal measurement errors. Secondly, the excitation current electrode located at the output of the operational amplifier connected directly to the measuring device, the measurement result V3 of which must remain below the level V3≤permissible to avoid nonlinear distortions of signals. If the given condition are not met, the correctness of attachment the electrode onto the body is to be checked, or the too high magnitude of the excitation current must be reduced. This way the formation of nonlinear distortions is avoided, and the large measurement errors are prevented.

The method of measuring electrical bioimpedance comprises the following steps:

- placing the excitation current electrodes and voltage pickup electrodes on a biological object, generating, and directing the electrical excitation current through the biological object via the excitation current electrodes;
- modulating the voltage generated in the biological object by the excitation current directed through the electrical bioimpedance Z of the biological object and recording the voltage modulated by the electrical bioimpedance Z on the voltage pickup electrodes;
- demodulating and measuring the voltages taken from the voltage pickup electrodes and deriving the value of the electrical bioimpedance Z=V/i by dividing the voltage drop V measured on the voltage pickup electrodes by the excitation current i directed through the biological object.

LIST OF FIGURES

FIG. 1. The structure of the electrical bioimpedance measurement device and the connections between its main components—the active electrode system (AES), the analog interface (AI), the generator (G) and the measuring device (MD).

FIG. 2. The active electrode system (AES) connected with the analog interface (AI) and the generator (G).

FIG. 3. The electrical circuit diagram of the active electrode system (AES) connected with the generator (G).

FIG. 4. The excitation current flow and voltage distribution between the electrodes.

FIG. 5. The electric diagram of the current-impeding circuit (R) comprising the resistor r3.

FIG. 6. The electric diagram of the current-impeding circuit (R) comprising the parallel resistor rp1 connected in parallel with the resistor r3 through the capacitance C1.

FIG. 7. The electric diagram of the current-impeding circuit (R), which comprises a parallel resistor rp1 connected in parallel with the resistor r3 through the capacitance C1 and a parallel resistor rp2 connected through the capacitance C2.

FIG. 8. Structural diagram of the analog interface (AI).

FIG. 9. Structural diagram of the summator (Σ1) containing in the analog interface (AI).

FIG. 10. Structural diagram of demodulator (DEM) containing in the analog interface (AI).

FIG. 11. An active electrode system (AES), where open electrodes are in parallel to the curvature area of the object.

FIG. 12. An active electrode system (AES), where open electrodes are located opposite each other in arc-shape form to the area of the object.

FIG. 13. An active electrode system (AES), where electrodes form a sector of concentrical location.

FIG. 14. An Active electrode system (AES) consisting of closed electrodes located concentrically on the object (the letter d represents the inner diameter of the electrode (5).

FIG. 15. The circuit diagram of the active electrode system (AES) with concentric electrodes, where the outer excitation current electrode 6 is connected to the output of the operational amplifier 9 and the innermost excitation current electrode 5, which has been reduced to a surface electrode (d→0), is connected to the inverting input of the operational amplifier 4.

FIG. 16. The circuit diagram of the active electrode system (AES) with concentric electrodes, but different from a circle, where the innermost excitation current electrode 5 forms a line electrode that is connected to the inverting input of the operational amplifier.

FIG. 17. Fragment of the excitation current electrode 6 and the voltage pickup electrode 8 running in parallel to the curvature area of the biological object (see FIG. 11), which are formed from serially connected point electrodes.

EMBODIMENTS OF THE INVENTION

The device for measuring electrical bioimpedance in FIG. 1 consists of an active electrode system (AES) 1, an analog interface (AI) 2, a measuring device (MD) 3 and a generator (G) 4, whereby the active electrode system (AES) 1 (FIG. 1, FIG. 2) comprises two excitation current electrodes (5, 6) and two voltage pickup electrodes (7, 8) and an operational amplifier (OA) 9, a current-impeding circuit (R) 10, contact resistances r1 and r2 and a measured biological object 100 with a bioimpedance Z.

The electrodes 5, 6, 7, 8 (FIG. 2) are grouped as a pair (5, 7) of one excitation current electrode 5 and one voltage pickup electrode 7, and as a pair (6,8) of another excitation current electrode 6 and another voltage pickup electrode 8, which on the biological object 100 are arranged as strip electrodes so that on one side of the measured bioimpedance Z there is one pair of strip electrodes (5, 7) and on the other side there is another pair of strip electrodes (6, 8), see FIG. 2 and FIGS. 11 to 16, whereby the electrodes (5-8) may, if necessary, be formed as closed contours (FIGS. 14 to 16), which are designed to surround the measured impedance Z of the biological object 100. FIG. 17 shows an example where the strip electrodes presented as a series of connected point electrodes.

The simplest variant (FIG. 5) of the current-impeding circuits 10 (FIGS. 5 to 7) comprises one resistor (r3) 21, the more complex basic version (FIG. 6) comprises, in addition to the resistor (r3) 21, a parallel resistor (rp1) 22 and a capacitor (C1) 23, the most complex third version (FIG. 7) comprises in addition to the resistor (r3) 21, the parallel resistor (rp1) 22 and the capacitor (C1) 23, also another parallel resistor (rp2) 24 and a second capacitor (C2) 25.

The analog interface (AI) 2 in the FIG. 1, FIG. 2 and FIGS. 8 to 10, comprises a summator (Σ1) 11 and a demodulator (DEM) 12 (FIG. 8), wherein the summator (Σ1) 11 consists of a first summing unit (S1) 13 and a second summing unit (S2) 14, and an amplifier 15 (FIG. 9), the demodulator (DEM) 12 consists of the first multiplier (X1) 16, the second multiplier (X2) 17, and the formator (F), low-pass filter (LPF) 19 and bandpass filter (BPF) 20 (FIG. 10).

The active electrode system (AES) 1 (FIG. 1, FIG. 2) is connected to the biological object 100 through the contact resistances r1 and r2 of excitation current i electrodes (5, 6) of the bioimpedance Z to be measured, whereby one excitation current electrode 6 is connected to the output of operational amplifier 9 and the other excitation current electrode 5 is connected to the inverting input (−) of the same operational amplifier 9, the non-inverting input (+) of which is connected to the common ground of the device with potential V=0 (FIG. 2, FIG. 3).

The voltages V1 and V2 taken from the measured impedance Z by the voltage pickup electrodes (7, 8), see FIGS. 2 to 4.

The current-impeding circuit 10 (FIGS. 1 to 3) is connected to the inverting input (−) of the operational amplifier (OA) 9, through which this inverting (−) input is connected to the output of the generator (G) 4, which is also connected to the input of the analog interface (AI) 2.

The current-impeding circuit (R) 10 comprises a resistor r3 (FIG. 5), to which, in a basic variant (FIG. 6), the parallel resistor rp1 with serially switched capacitor C1 are connected in parallel, and, in the most complex case (FIG. 7), another parallel resistor rp2 with serially switched capacitor C2 are connected parallelly.

The voltage pickup electrodes 7 and 8 are connected to the inputs (FIG. 8) of the summator (Σ1) 11 located in the analog interface (AI) 2 in FIG. 2, whereby the voltage pickup electrode 7 has a connection with its inverting input (−) with voltage V1 and the voltage pickup electrode 8 relates to its non-inverting input (+) with voltage V2 (FIG. 8). The output of the generator 4 with voltage VG (FIG. 1, FIG. 2 and FIG. 8) is connected to the second inverting input (−−) of the summator 11. The output from the excitation current electrode 6 with a voltage V3 is connected directly to the input of the measuring device (MD) 3, see FIG. 1 and FIG. 2.

The inverting input (−) of the first summing unit (S1) 13 (FIG. 9) situating in the summator (Σ1) 11 (FIG. 8), represents also the inverting input (−) of the summator 11, which is connected to the voltage pickup electrode 7 with the voltage V1, and the non-inverting input (+) of the summing unit (S1) 13, being also the non-inverting input (+) of the summator 11, is connected to the voltage pickup electrode 8 with voltage V2 (FIG. 2, FIG. 4 and FIG. 8). The output of the first summing unit (S1) 13 with voltage VZ is connected to the non-inverting input (+) of the second summing unit (S2) 14 as well as to one input of the demodulator (DEM) 12 (FIG. 8), whereas the output of the generator 4 with voltage VG is connected to the inverting input (−−) of the second summing unit (S2) 14. The output of the second summing unit (S2) 14 is connected to the input of the amplifier 15, the output of which with voltage VΔZ is connected to the second input of the demodulator DEM (12) located in the analog interface (AI) 2 (FIG. 8), to the reference input of which the output of the generator 4 with voltage VG is connected. The two outputs of the demodulator (DEM) 12 with demodulated voltages DZ and DΔZ are connected to the two inputs of the measuring device (MD) 3, the output from the voltage pickup electrode 7 with voltage V1 is connected to the separate input of the measuring device (MD) 3 through the analog interface (AI) 2.

Operation of the device. The alternating excitation current i from the generator (G) 4 is directed through the current-impeding circuit (R) 10 to the inverting input (−) and through the negative feedback of the operational amplifier 9, while passing through the impedance Z of the biological object 100 and following through the contact resistances r1 and r2 of the excitation current electrodes 5 and 6 to the output of the operational amplifier 9 (FIG. 2, FIG. 3). The current-impeding circuit 10 imitates the measurable bioimpedance Z of the biological object 100 in terms of the properties and parameters of its resistance R. Due to the high gain of the operational amplifier 9, its inverting input (−) and the excitation current electrode 5 are connected to receive the virtual ground potential V≈0 (FIGS. 2 to 4). Let us note that in different applications, especially in medical diagnostics, the most informative is the variation ΔZ≈(1-10) Ω of the full bioimpedance Z=Z0+ΔZ (FIG. 3), whereas the variation ΔZ consists only 1/1000 to 1/100 of the base value Z0≈1000Ω.

The contact resistances r1 and r2 exceed the impedance change ΔZ value by hundreds of times, but the electrical scheme in FIG. 3 and the mathematical relationships presented in FIG. 4 enable to calculate the values of the contact resistances r1 and r2 found from the voltages V1, V2, and V3 of the electrodes 6, 7 and 8 (FIG. 2), and then eliminate their influence on the bioimpedance Z measurement results in the analog interface 2 (FIG. 8, FIG. 9), in which the summator (Σ1) 11 subtracts the voltages V2 and V1 of the voltage pickup electrodes 7 and 8 (FIG. 9). As a result, we obtain the voltage VZ corresponding to the measured impedance Z. The subtraction operation is performed in the summing unit (S1) 13 of the summator (Σ1) 11. The variation ΔZ of the bioimpedance Z is obtained by the summing unit (S2) 14, where the voltage VG of the generator 4, proportional to the constant component Z₀of the impedance Z, is subtracted from the voltage VZ. The tiny variation ΔZ obtained because of the subtracting operation, is amplified at the output of the summing unit (S2) 14 by the amplifier 15 giving the output signal in the form of the information-carrying voltage VΔZ. The voltages VZ and VΔZ are detected in the demodulator 12 by means of the first multiplier (X1) 16 and the second multiplier (X2) 17 synchronously with the voltage signal VG of the generator 4, which is converted into the reference signal in the shaping formator (F) 18 (FIG. 10). The shaping consists in changing the waveform and shifting the phase for obtain the precise synchronization. The detected signal DZ, which corresponds to the impedance Z, is filtered out using low pass filter (LPF) 19 at the output of the first multiplier 16. The other detected signal DΔZ corresponding to the variation part ΔZ of the impedance Z, is filtered out by the bandpass filter (BPF) 20 from the output of the second multiplier 17.

The detected voltages DZ and DΔZ measured in the measuring device (MD) 3, represent the values of the bioimpedance Z and its variation ΔZ in digital form (FIG. 1). The voltages V1 and V3, are also measured and evaluated in the measuring device (MD) 3 and the values of the contact resistances r1 and r2 are calculated and the fulfillment of the conditions for the correctness of the measurement process (r1≤permissible) and (r2≤permissible) are checked to avoid signal distortions causing measurement errors in the analogue interface 2.

Advantages of the Invention in Terms of the Device

1. Introducing the inverting operational amplifier (OA) 9 for creating the active electrode system (AES) 1, makes possible to use a virtual ground with a potential very close to zero (V≈0) for one of the excitation current electrodes 5, which is practically equal to the common ground potential of the device (V=0), FIGS. 2 to 4. The structure of the device is simplified, energy consumption reduced, the reliability and measurement accuracy improved.

2. Measurement of the voltage V1=i·r1=(VG/R)·r1 from the voltage pickup electrode 7 (FIGS. 1 to 4) at the excitation current i=VG/R, gives the contact resistance r1=V1/i=R(VG/V1). Checking r1≤permissible, ensuring the work of operational amplifier 9 in linear mode. Avoidance of nonlinear distortions ensures measurement accuracy.

3. The possibility of measuring the voltage V2=i(r1+Z) of the voltage pickup electrode 8 (FIGS. 2 to 4) at the excitation current i=VG/R, gives the sum of resistances r1+Z=R(V2/VG), from which the exact value for the impedance Z is acquired as Z=R[(V2−V1)/VG] regardless of the value of r1 if the requirement r1≤permissible is followed.

4. A decisive reduction of the effect of contact resistance r2 is ensured by deep negative feedback from the output of the operational amplifier (OA) 9 through the contact resistance 12 of the excitation current electrode 6, the measured bioimpedance Z of the biological object 100, the contact resistance r1, and the excitation current electrode 5 to its inverting input (−), see FIGS. 2 to 4. In the case of a large amplification factor K≥10⁵of the operational amplifiers makes negligible the effect of the contact resistance r2 on the measurement accuracy, see FIG. 2 and FIG. 3.

5. The possibility of measuring the voltages V2 and V3 provides the value of the contact resistance r2=(V3−V2)·(R/VG), which is necessary for the calculating and eliminating residual errors in the measured bioimpedance Z (FIGS. 2 to 4) for having smaller values of the gain factor K of the operational amplifier 9, for example, 10³≤K≤10⁵. Moreover, checking the condition r2≤permissible ensures the work of operational amplifier 9 in the linear mode and avoids the errors from nonlinearities.

6. The use of the generator (G) 4 as a voltage source enables to generate the excitation current (i=VG/R) within a wide range of operating frequencies f by the aid of a simple electronic circuit. The use of current generators known from widespread practice of impedance measurements, requires complicated solutions that does not ensure distortion-free operation in a wide frequency range from f≥100 kHz.

7. Using the current-impeding circuit 10 making the excitation current i dependable inversely to the frequency dependance of the measurable impedance Z, it means that the product (ix Z) is close to a constant in a wide range of measurement frequencies, for example, in the range f=10 Hz to 10 MHz. This solution ensures the best measurement accuracy in conditions of noise and interference.

8. The measurement accuracy of the impedance Z and its variation ΔZ does not depend on the stability of voltage VG of the generator 4, because the same excitation current i=VG/R flows both through the impedance Z and the current-impeding circuit 10 with the resistance R. We obtain the dependence on the voltage ratio, Z=R (VZ/VG), but not on the magnitude of the excitation current i. We avoid the measurement errors due to instability of the voltage VG of the generator 4.

9. The long strip electrodes (5-8) following the area of the measured impedance Z of the biological object 100 with a wider pickup area of voltage electrodes (7,8) than the scattering range of the excitation current i from current electrodes (5, 6), minimizes the leakage of the excitation current outside the area of the measured impedance Z. Minimizing poorly defined current leakages up to their elimination ensures the best measurement accuracy.

10. The separation of the voltage VΔZ corresponding to the variation ΔZ of the impedance Z (FIG. 8, FIG. 9) and a separate demodulation of it (FIG. 10) provides high accuracy measurement for the demodulated voltage DΔZ and for the determination of the corresponding impedance variations ΔZ in the measuring device (MS) 3 (FIG. 1).

The method for measuring electrical bioimpedance.

For measuring the electrical bioimpedance, an active electrode system 1 corresponding to the configuration of the biological object 100 is selected and placed on the body grasping the contours of the biological object 100 in parallel with it, whereby the excitation current electrodes 5, 6 grasp the contours externally with respect to the voltage pickup electrodes 7, 8, where the position of the voltage pickup electrodes 7, 8 determines the contours on the biological object 100 to be measured (FIGS. 2 to 4 and FIGS. 11 to 17). In the case of absence of the electrode system corresponding to the required configuration, the electrodes created using a blank electrode system from strip electrodes so that the necessary configuration formed from the strips (FIGS. 11 to 16). An effective example would be the use of a matrix of point electrodes, in which the strip electrode paths created (FIG. 17) using, for example, mechanical processing or programmable electrical connecting.

Preferred is the formation of strip electrodes through the electronic programming of the electrode matrix, whereby there are two ways to create the strip electrode paths: a) from the ready-made connections of the point electrodes interrupting (burning out) the excessive ones; b) connecting only these point electrodes that are required for creating the strip electrodes by closing the needed connections by the aid of electrical pulses. The connections and interruptions created similarly to the programming of matrix memories.

After formation of the electrodes, placing the excitation current electrodes 5, 6 and the voltage collection electrodes 7, 8 on the biological object 100 following the required contours takes place, and connecting the electrodes 5, 6 and 7, 8 to the inverting input and output of the operational amplifier 9 located in the active electrode system 1 will be done. The placing of electrodes also involves connecting the active electrode system 1 to the analog interface 2 and the generator (G) 4.

The generator (G) 4 is set to the required frequency f and the voltage VG of it is regulated to a suitable level for generating the excitation current i=VG/R using the current-impeding circuit 10 (FIG. 2, FIG. 3 and FIGS. 5 to 7) and directing the excitation current i via the excitation current electrodes 5, 6 through the electrical bioimpedance Z of the biological object 100, which is switched into the negative feedback circuit of the operational amplifier 9 between the excitation current electrodes 5, 6 (FIG. 2, FIG. 3). When setting the excitation current, the established restrictions on the strength of the electric current in living biological subjects (Medical Devices IEC 60601-1-Electrical Safety EN 60601-1) must considered.

The configuration of the current-impeding circuit (R) 10 (FIGS. 5 to 7) and the values of the components 21-25 included therein (resistors r3, rp1 and rp2 and capacitances C1 and C2, are chosen so that the total electrical resistance R of the current-impeding circuit 10 and its dependence on the operating frequency f, would be as similar as possible to the impedance Z of the biological object being measured at the frequency Z(f).

The voltage VZ=i·Z created by the excitation current i flowing through the electrical bioimpedance Z of the biological object 100 recorded in the form of the difference between the voltages V1 and V2 on the voltage pickup electrodes 7, 8 (FIG. 8, FIG. 9). The voltages V1 and V2 processed in the analog interface 2 together with the output voltage VG of the generator 4 to obtain the voltage modulation VΔZ=[(V2−V1)−VG] corresponding to the change ΔZ in the impedance Z. Then the voltages VZ and VΔZ (FIG. 10) are demodulated into the form DZ and DΔZ and measured in the measuring device 3, which provides an output corresponding to the impedance Z and its informative variation ΔZ, which are the results of the impedance measurement of the biological object 100 (FIGS. 1 to 4).

To check the correctness of measurements, the voltage V3 of the excitation current electrode 6 located at the output of the operational amplifier 9 is transferred to the measuring device 3 for evaluation, where the measurement result is compared with a predetermined voltage, exceeding of which indicates overloading of the device. Consequently, the measurement results are not correct and must cancelled. The final purpose is to measure the voltage V1 of the voltage pickup electrode 7 in the measuring device 3 (FIGS. 1 to 3) to verify the correctness of the contact between the excitation current electrode 5 and the object 100. The voltage V1 must be smaller than the set reference voltage. If any irregularities occur, the measuring device 3 issues an alarm signal. For the purposes of technical diagnostics, the values of the contact resistances r1 and r2 can be calculated based on the measurement results (FIGS. 2 to 4), and the stability and quality of the contacts between the electrodes and the body can be assessed, as well as conclusions can be drawn about the reliability of the measurements.

The advantages of the invention summed up in terms of the method.

1. The use of a strip form for designing the optimal configurations of both the excitation current electrodes 5, 6 and the voltage pickup electrodes 7, 8 according to the measurement task and the shape of the biological object 100 ensures the recording of signals containing maximum amount of information acquired from the voltage pickup electrodes 7, 8.

2. Marking the biological object 100 on the body in accordance with the configuration and position of the voltage pickup electrodes 7, 8 allows us to precisely define the area of the body impedance Z we want to measure. This ensures the true value of measurement results.

3. The use of warning signals developed based on the measurement of the electrode voltages V1 and V3 allows us to remove the measurement results corrupted by possible technical irregularities and disturbances. We achieve an increase in the reliability of medical diagnostics performed based on the bioimpedance measurement results.

4. Determination of the strength and frequency dependence of the excitation current i by selecting the circuit solution of the current-impeding circuit 10 (FIGS. 5 to 7) and setting the values of its components 21-24, the excitation current level maintained at the maximally allowed at all the excitation frequencies f. As a result, we achieve the best signal-to-noise ratio and measurement accuracy without exceeding the permitted electric current limits in biological subjects.

Claims

1. A device for measuring electrical bioimpedance comprising a measuring device (3), a generator (4), excitation current electrodes (5, 6) and voltage pickup electrodes (7, 8), electrical contact resistances (r1, r2) between the excitation current electrodes (5, 6) and the biological object (100), characterized in that the device further comprises: an active electrode system (1), comprising an operational amplifier (9);a current-impeding circuit (10);an analog interface (2), comprising:a summator (11), having three summing units: a first summing unit (13), a second summing unit (14), and an amplifier (15);a demodulator (12), comprising two multipliers (16, 17), a formator (18), a low-pass filter (19), a band-pass filter (20);the voltage outputs leading to the inputs of the measuring device (3), wherein one of the outputs is connected to the excitation current electrode (6) of the operational amplifiers (9) output in the active electrode system (1) for measuring the voltage V3 and the other voltage input is connected to the voltage pickup electrode (7) for measuring the voltage V1, the two subsequent inputs of the measuring device (3) are connected with two other outputs of the analog interface (2) for measuring the voltages DZ and DΔZ detected in the demodulator (12).
2. The device according to claim 1, characterized in that the output of the generator (4) with voltage VG is connected to the input of the active electrode system (1) and the input of the analog interface (2), the informative voltage outputs with voltages V1 and V2 from the active electrode system (1) are connected to the inputs of the analog interface (2), and the output voltages V3 and V1 of the active electrode system (1) are directed for evaluation to the inputs of the measuring device (3).
3. The device according to claim 1, characterized in that the inverting input (−) of the operational amplifier (9) is connected to the one excitation current electrode (5) and the non-inverting input (+) is connected to a common ground holding the 0-potential (V=0), whereas the output of the operational amplifier (9) is connected to the second excitation current electrode (6).
4. The device according to claim 1, characterized in that the current-impeding circuit (10) with a resistance R is connected between the zero-potential (V≈0) inverting input (−) of the operational amplifier (9) and the output of the generator (4) voltage VG, and the current-impeding circuit (10) comprises a frequency-dependent resistance consisting of a fixed resistor (r3) and one or several parallel resistors (rp1, rp2) connected in parallel to it through one (C1) or several (C1, C2) capacitances.
5. The device according to claim 1, characterized in that the non-inverting input (+) and the inverting input (−) of the summator (11) are connected to the voltage pickup electrodes (7, 8) with voltages (V1, V2) and to the subtracting input (−−) of the summator (11), which is connected to the output of the generator (4) with the voltage VG.
6. The device according to claim 1, characterized in that the two signal inputs of the demodulator (12) are connected with the outputs of the summator (11) with voltages VZ and VΔZ, and the reference input is connected to the output of the generator (4) with voltage VG, the outputs of the demodulated voltages DZ and DΔZ are connected to inputs of the measuring device (3) that elaborates the numerical values for the impedance Z and its variation ΔZ.
7. The device according to claim 1 and 5, characterized in that the summator (11) comprises a first summing unit S1 (13) and a second summing unit S2 (14), whereas from the input voltage V2 at the non-inverting input (+) of the first summing unit (13) is subtracted the input voltage V1 at the inverting input (−), as a result of the subtracting, the output of the first summing unit (13) has the corresponding to the impedance Z modulated voltage VZ=V2−V1, which is directed to the non-inverting input (+) of the second summing unit (14), from which, in turn, the voltage VG entering the inverting input (−−) of the second summing unit (14) from the generator (4) is subtracted, wherein the amplifier (15) connected to the output of the second summing unit (14) gives out an amplified voltage VΔZ carrying information on the impedance variation ΔZ.
8. The device according to claims 1 and 3, characterized in that the demodulator (12) comprises one multiplier X1 (16) and another multiplier X2 (17), a formator (18), a low-pass filter LPF (19) and a bandpass filter BPF (20), wherein the signal input of the multiplier (16) is connected to the output of the summator (11) with the voltage signal VZ and the signal input of the multiplier (17) is connected to the output of the summator (11) with the voltage signal VΔZ, the output of the formator F (18) is connected to the reference inputs of both the multipliers (16 and 17), the input of the formator F (18) is connected to the output of the generator (4) with the voltage VG, wherein the output of the one multiplier (16) is connected to the input of the low-pass filter LPF (19), the output of which has a demodulated signal DZ corresponding to the impedance Z, and the output of the other multiplier (17) is connected to the input of the bandpass filter BPF (20), the demodulated output signal DΔZ of which corresponds to the impedance variation ΔZ.
9. The device according to claim 1, characterized in that the generator (4) is a voltage generator.
10. The device according to claim 1, characterized in that the generator (4) is a current generator.
11. The device according to claim 1, characterized in that the excitation current electrodes and the voltage pickup electrodes are long strip electrodes placed on the biological object to follow the shape of the region, so that on one side of the region there is one excitation current electrode and one voltage pickup electrode forming a pair of electrodes, and on the other side of the region there is another voltage pickup electrode and another excitation current electrode forming another pair of electrodes.
12. The device according to claim 11, characterized in that at least one voltage pickup electrode is longer than one of the excitation current electrodes.
13. The device according to claims 11 and 12, characterized in that the strip electrodes are designed to follow linearly the region to be measured of biological object.
14. The device according to claims 11 and 12, characterized in that the strip electrodes are formed to follow the region of biological object in an arched shape.
15. The device according to claims 1 and 14, characterized in that at least one of the strip electrodes connected at the ends of the strip to form a closed contour to surround the circular round-shape regions.
16. The device according to claims 1 and 15, characterized in that the innermost excitation current electrode is designed to a surface electrode.
17. The device according to claim 1 and 15, characterized in that the innermost excitation current electrode is designed to a point electrode.
18. The device according to claims 1 and 15, characterized in that the innermost excitation current electrode is designed to a line electrode.
19. The device according to claims 1 and 15, characterized in that one excitation current electrode and one voltage pickup electrode are joined together into a single common electrode resulting a three-electrode electrode system.
20. The device according to claims 11, 12 and 13, characterized in that one or more strip electrodes are formed from the point and line electrodes, wherein point electrodes are in electric interconnections using programmable connecting.
21. A method for continuous monitoring of electrical bioimpedance measurement according to claims 1 to 20 of the device, comprising the steps of: placing the electrodes of the excitation current and the electrodes of the voltage pickup on the biological object;generating an electrical excitation current;directing the excitation current through the excitation current electrodes of the biological object;modulating the voltage generated by the excitation current in the biological object by the electrical bioimpedance Z of the region of object;recording the voltage modulated by the electrical bioimpedance Z from the voltage pickup electrodes;measuring the value of the voltage from the voltage pickups and the value of the electrical bioimpedance Z is derived by dividing the voltage drop measured from the voltage pickup electrodes by the value of the excitation current directed through the biological object,
22. The method according to claim 21, characterized in that the steps of forming and installing the electrodes are: forming the electrodes from long and narrow electrically conductive materials with a strip-shaped design;placing the strip-shaped excitation current and voltage pickup electrodes on the body surface surrounding the selected regions of the biological object so that, following the surface shape of the object, one excitation current electrode (5) and one voltage pickup electrode (7) are placed on one side of the object as a pair, and on the other side of the object as the second pair of second voltage pickup electrode (8) and the second excitation current electrode (6);forming the electrodes as for cutting off the arc-shaped sections;forming the electrodes as for cutting off the sector of a circular object;forming the electrodes as the closed concentric circles;forming one of the electrodes as an internal surface electrode;forming one of the electrodes as a point electrode;forming the electrodes as a series of point electrodes connected with each other;designing the single strip-shaped voltage pickup electrode as common electrode comprising one excitation current electrode and one voltage pickup electrode as a pair, wherein the excitation current electrode involved in the pair is for the purpose for the collectable voltage to grip the whole voltage drop generated by the excitation current.
23. The method according to claim 21, characterized in that it comprises the steps of measuring and evaluating the voltages V1 and V3: the voltage V1 of the voltage pickup electrode (5) paired with the excitation current electrode (7) connected to the inverting input of the operational amplifier is measured to determine the resistance r1=V1/i between this excitation current electrode and the object, where i denotes the excitation current, to ensure that it does not exceed a set limit;the voltage V3 of the excitation current electrode (6) connected to the output of the operational amplifier measured to ensure that its value does not exceed a set limit;the voltage V3 of the excitation current electrode (6) connected to the output of the operational amplifier measured to determine the resistance r2=(V3−V2)/i between this excitation current electrode and the object to ensure that it exceeds the set value.
24. The method according to claim 21, characterized in that both the bioimpedance Z of the object and its variation ΔZ are measured simultaneously in a wide frequency band from 10 Hz to 10 MHz, where in order to achieve the maximally possible measurement accuracy, the measurements are carried out with a frequency-dependent excitation current strength, performing the following steps in accordance with the safety standards (Medical Devices IEC 60601-1-Electrical Safety EN 60601-1): the excitation current passing through the biological object is set to the maximum permissible level in the frequency band from 10 Hz to 1 kHz by determining the value of the low-frequency resistance r3 of the current-impeding circuit;the excitation current passing through the biological object is set to increase linearly in the frequency band from 1 kHz to 10 kHz to the constant permissible level by setting the capacitance of the capacitor C1 located in the current-impeding circuit;keeping the excitation current passing through the biological object constant in the frequency range from 10 kHz to 100 kHz by adjusting the parallel resistance rp1;setting the excitation current to an increased level at frequencies exceeding 100 kHz by supplementing the parallel resistance rp1 in the current-impeding circuit by adding the parallel resistance rp2, which is switched on via the capacitance of the capacitor C2.
25. The method according to claims 21 and 24, characterized in that the demodulation of the voltage difference (V2−V1)=VZ is performed by synchronous detection and averaging the detected voltage signal corresponding to the bioimpedance Z using a low-pass filter in the frequency range from 0 to 0.2 Hz, which comprises the following steps: the voltage difference VZ is directed to the signal input of the first synchronous detector located in the analog interface;the generator voltage VG is directed to the reference signal formator of the synchronous detector, in which a normalized reference signal is formed from the voltage VG, directed to the reference input of the first synchronous detector;the voltage difference VZ multiplied by the normalized reference signal coming from the formator and the result presented in the form of the output voltage of the first multiplier;the output voltage of the first multiplier filtered out from the by-products and noise of the multiplication by passing through a low-pass filter and presented in the form of the demodulated voltage DZ.
26. The method according to claims 21 and 24, characterized in that the demodulation of the voltage VΔZ corresponding to the variation ΔZ of the bio-impedance Z is performed by synchronous detection and band-pass filtering the detected voltage signal as its result—the detected voltage in the frequency range of 0.2 to 30 Hz, comprising the following steps: the voltage VΔZ corresponding to the variations in the bioimpedance is directed to the signal input of the second synchronous detector located in the analog interface;the electrical excitation voltage VG directed to the reference former of the synchronous detector, in which a normalized reference signal is formed, directed to the reference input of the second synchronous detector;the voltage VΔZ corresponding to the variation in bioimpedance is multiplied by the normalized reference signal coming from the former, and the measurement result is presented in the form of the output voltage of the second multiplier;the output voltage of the second multiplier is filtered out from the multiplication by-products and noise by passing it through the band-pass filter and presented in the form of the demodulated voltage DΔZ.

Priority Claims (1)

Number	Date	Country	Kind
P202300029	Nov 2023	EE	national

Device and Method for measuring electrical bioimpedance

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Priority Claims (1)