METHOD AND DEVICE FOR MEASURING ELECTRICAL IMPEDANCE OF BIOLOGICAL TISSUES

Abstract

A device for measuring an electrical impedance of biologic tissue may include electrodes configured to contact the biologic tissue and generate a differential voltage thereon. The device may include a first circuit coupled to the electrodes and configured to force an oscillating input signal therethrough, and a differential amplitude modulation (AM) demodulator coupled to the plurality of electrodes. The differential AM demodulator may be configured to demodulate the differential voltage, and generate a base-band signal representative of the demodulated differential voltage. The device may further include an output circuit downstream from the differential AM demodulator and may be configured to generate an output signal representative of the electrical impedance as a function of the base-band signal.

Claims

1. A device, comprising: a plurality of electrodes configured to contact biologic tissue and generate a differential voltage thereon; a first circuit coupled to the plurality of electrodes and configured to force an oscillating input signal through the plurality of electrodes; a first differential amplitude modulation (AM) demodulator coupled to the plurality of electrodes without amplification circuitry therebetween, the first differential AM demodulator comprising a first pair of switches and a second pair of switches controlled in phase opposition to each other, the first differential AM demodulator being configured to demodulate a differential voltage and to generate a first base-band signal; a second differential AM demodulator coupled to the plurality of electrodes without amplification circuitry therebetween, the second differential AM demodulator comprising a third pair of switches and a fourth pair of switches controlled in phase opposition to each other, the second differential AM demodulator being configured to demodulate the differential voltage in quadrature with respect to the first differential AM demodulator and to generate a second base-band signal; an amplifier configured to amplify the first base-band signal and the second base-band signal; and an output circuit downstream from the amplifier and configured to generate an output signal representative of an electrical impedance of the biological tissue as a function of the first base-band signal and the second base-band signal, the electrical impedance having a real part and an imaginary part in a complex domain.

2. The device of claim 1, wherein the oscillating input signal comprises a square wave input signal.

3. The device of claim 2, wherein the first differential AM demodulator is configured to demodulate the differential voltage with a square wave demodulating signal in phase with the square wave input signal, the first base-band signal being represented as V.sub.ReM, wherein the second differential AM demodulator is configured to demodulate the differential voltage with a square wave demodulating signal in phase opposition with the square wave input signal, the second base-band signal being represented as V.sub.ImM, and wherein the output circuit is configured to receive the first base-band signal and the second base-band signal, the output circuit being further configured to generate output signals ReC and ImC representative of the real part and the imaginary part, respectively, according to the following equations:
ReC=b.sub.R*V.sub.ReM+c.sub.R*V.sub.ImM
ImC=b.sub.1*V.sub.ImM+c.sub.1*V.sub.ImM.sup.2 wherein b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are fixed parameters.

4. The device of claim 3, wherein the fixed parameters b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are equal to about 0.0606, −0.02, 0.093, and 0.0000056, respectively.

5. The device of claim 3, wherein the electrical impedance comprises a capacitive-resistive impedance.

6. A device for generating an output signal representative of an electrical impedance of biologic tissue, the device comprising: a plurality of electrodes configured to contact the biologic tissue and generate a differential voltage thereon; a differential amplitude modulation (AM) demodulator directly coupled to the plurality of electrodes without any intervening amplification circuitry therebetween and configured to demodulate the differential voltage, and generate a base-band signal representative of the demodulated differential voltage, the differential AM demodulator comprising a first pair of switches and a second pair of switches controlled in phase opposition to each other; and an output circuit coupled to the differential AM demodulator and configured to generate an output signal representative of the electrical impedance as a function of the base-band signal.

7. The device of claim 6, wherein the differential AM demodulator is configured to perform AM demodulation based upon a square-wave demodulation signal.

8. The device of claim 6, further comprising: a second differential AM demodulator coupled to the plurality of electrodes and configured to demodulate the differential voltage in quadrature with respect to the differential AM demodulator, and generate a second base-band signal representative of the quadrature demodulated voltage, wherein the output circuit is configured to generate the output signal based upon a real part and an imaginary part of the electrical impedance in a complex domain as function of the base-band signal and the second base-band signal.

9. The device of claim 8, wherein the differential AM demodulator is configured to demodulate the differential voltage with a square wave demodulating signal in phase with a square wave input signal, and is configured to generate a first demodulated signal V.sub.ReM; the second differential AM demodulator is configured to demodulate the differential voltage with a square wave demodulating signal in phase opposition with the square wave input signal, and is configured to generate a second demodulated signal V.sub.ImM; and the output circuit is configured to generate output signals ReC and ImC representative of the real part and of the imaginary part, respectively, of the electrical impedance of the biologic tissue, according to the following parametric equations:
ReC=b.sub.R*V.sub.ReM+c.sub.R*V.sub.ImM
IMC=b.sub.1*V.sub.ImM+c.sub.1*V.sub.ImM.sup.2 wherein b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are fixed parameters.

10. The device of claim 9, wherein the biologic tissue comprise human body tissue.

11. The device of claim 9, wherein the electrical impedance comprises a capacitive-resistive impedance, and wherein the fixed parameters b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are equal to about 0.0606, −0.02, 0.093, and 0.0000056, respectively.

12. A method of generating an output signal representative of an electrical impedance of biologic tissue, the method comprising: forcing an oscillating input current through a plurality of electrodes in contact with the biologic tissue to generate a differential voltage thereon; performing an amplitude modulation (AM) demodulation, in a differential fashion using a differential AM demodulator directly coupled to the plurality of electrodes without any intervening amplification circuitry therebetween, on the differential voltage on the plurality of electrodes to generate a base-band signal representative of the demodulated differential voltage, the differential AM demodulator comprising a first pair of switches and a second pair of switches controlled in phase opposition to each other; and generating an output signal representative of the electrical impedance as a function of the base-band signal.

13. The method of claim 12, wherein the oscillating input current is a square wave input current, and wherein the method comprises: performing the AM demodulation based upon a square wave demodulating signal in phase with the square wave input current to generate a first demodulated signal V.sub.ReM; performing a second AM demodulation in a differential fashion using a square wave demodulating signal in quadrature with the square wave input signal to generate a second demodulated signal V.sub.ImM; and generating output signals ReC and ImC representative of a real part and an imaginary part, respectively, of the electrical impedance by combining the first and second demodulated signals V.sub.ReM and V.sub.ImM according to the following equations:
ReC=b.sub.R*V.sub.ReM+c.sub.R*V.sub.ImM
ImC=b.sub.1*V.sub.ImM+c.sub.1*V.sub.ImM.sup.2 wherein b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are fixed parameters.

14. The method of claim 13, wherein the parameters c.sub.R and c.sub.1 are null, the method further comprising adjusting values of the parameters b.sub.R and b.sub.1 by: storing, in a look-up table, heuristically-determined values of the parameters b.sub.R and b.sub.1 as a function of ratios between the first and second demodulated signals V.sub.ReM, V.sub.ImM; calculating a ratio between first and second demodulated signals V.sub.ReM, V.sub.ImM; and updating the values of the parameters b.sub.R and b.sub.1 with the values stored in the look-up table corresponding to the calculated ratio.

15. The method of claim 12, wherein the oscillating input current is one of a sinusoidal input current and a square wave input current, and wherein the method comprises: sensing a phase difference between the sinusoidal input current and the corresponding differential voltage; performing the AM demodulation in a differential fashion using a square wave demodulating signal in phase with the corresponding differential voltage to generate a base-band demodulated signal Zo; and generating an output signal representative of a modulus of the electrical impedance by amplifying the base-band demodulated signal Zo with a gain k.

16. The method of claim 15, further comprising adjusting the gain k by: storing, in a look-up table, heuristically determined values of the gain k as a function of phase differences between the sinusoidal input current and the differential voltage; and updating the value of the gain k with the values stored in the look-up table corresponding to the sensed phase difference.

17. The method of claim 12, wherein the first pair of switches comprises a first switch and a second switch, the first switch of the first pair of switches being coupled between a first input terminal of the differential AM demodulator and a first output terminal of the differential AM demodulator, the second switch of the first pair of switches being coupled between a second input terminal of the differential AM demodulator and a second output terminal of the differential AM demodulator.

18. The method of claim 12, wherein the second pair of switches comprises a first switch and a second switch, the first switch of the second pair of switches being coupled between a second input terminal of the differential AM demodulator and a first output terminal of the differential AM demodulator, the second switch of the second pair of switches being coupled between a first input terminal of the differential AM demodulator and a second output terminal of the differential AM demodulator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 illustrates two architectures for measuring the impedance of a biologic tissue in accordance with the prior art.

[0023] FIG. 2 is a schematic diagram of an architecture for measuring the impedance of a biologic tissue in accordance with the prior art.

[0024] FIG. 3 is a schematic diagram of an architecture disclosed in Rafael Gonzàlez-Landaeta, Oscar Casas, and Ramon Pallàs-Areny, Heart rate detection from plantar bioimpedance measurements, IEEE Transactions on Biomedical Engineering, 55(3)1163-1167, 2008 for measuring impedance of biologic tissues in accordance with the prior art.

[0025] FIG. 4 is a schematic diagram of a device for measuring impedance of biologic tissues having a differential AM demodulator directly connected to the electrodes in accordance with the present invention.

[0026] FIG. 5 is a device generating an electrocardiogram (ECG), having a channel for sensing the impedance of biologic tissues using a square-wave AM demodulating carrier in accordance with the present invention.

[0027] FIG. 6 is a more detailed schematic diagram of another architecture of the device that uses a square-wave AM demodulating carrier for sensing the AC and the DC components of the real part of the impedance in the complex domain in accordance with the present invention.

[0028] FIG. 7 is a graph of the voltage on the electrodes vs. the phase of the impedance in the complex domain when a resynchronization technique is implemented and when it is not implemented.

[0029] FIG. 8 is a graph of a test case of the error in estimating the modulus of the impedance vs. the phase thereof with and without using a compensation technique.

[0030] FIG. 9 is a graph of the ratio between an impedance modulus Zo estimated by injecting a sinusoidal current with the electrodes and by using a demodulating square-wave and the effective modulus M of the impedance of a biologic tissue for various phase angles of the impedance in the complex domain, with and without applying a resynchronization technique.

[0031] FIG. 10 is a schematic diagram of an embodiment of a device for measuring impedance of biologic tissues having two differential AM demodulators directly connected to the electrodes in accordance with the present invention.

[0032] FIG. 11 is a schematic diagram of a two-channel architecture of a device that uses square-wave AM demodulating carriers in quadrature for generating signals representing the real part and the imaginary part of the impedance in the complex domain in accordance with the present invention.

[0033] FIGS. 12a and 12b are graphs of percentage error on the estimation of the real part and of the imaginary part vs. the phase of the impedance in the complex domain.

[0034] FIG. 13a is a graph of the corrected and the uncorrected ratio between the estimations of the real part and of the imaginary part vs. the phase of the impedance in the complex domain.

[0035] FIG. 13b is a graph of the error on the estimations of the real part and of the imaginary part vs. the phase of the impedance in the complex domain.

[0036] FIG. 14 is a graph of the error on the estimations of the real part and of the imaginary part and of the evaluated modulus of the impedance vs. the phase of the impedance in the complex domain when a compensation technique is implemented.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0037] An embodiment of a device for measuring the impedance of biologic tissues is illustrated in FIG. 4. The circuit blocks in common with the prior devices of FIGS. 2 and 3 are identified by the same labels.

[0038] The applicant found that it is not necessary to amplify the voltage on the electrodes that inject a current throughout a biologic tissue, and thus, that it is possible to connect directly the AM differential demodulator directly to the electrodes as shown in FIG. 4. The demodulated base-band signal is, in most practical cases, adapted to be amplified and processed for generating signals representing with a sufficient accuracy the modulus and the phase of the impedance in the complex domain. The so AM demodulated base-band differential signal may be supplied in input to an INA, that generates an amplified replica thereof.

[0039] Differently from the known device of FIG. 2, the INA amplifies always a base-band signal, thus it has a large gain and a relatively good CMRR in the base-band range of frequencies. Therefore it is possible to use a low cost and low power consumption INA.

[0040] According to an embodiment, the voltage generator VOLTAGE GENERATOR generates a sinusoidal voltage, to force a sinusoidal current throughout the electrodes. According to a more preferred embodiment, throughout the electrodes, a square-wave current is forced and the AM demodulated base-band signal is obtained by demodulation using a square-wave demodulating carrier. Therefore, in this particular case the block VOLTAGE GENERATOR generates a square-wave voltage that is converted into a square-wave current injected throughout the biologic tissue. In this embodiment the realization of the differential AM demodulator is simplified and the resulting AM demodulated base-band signal may be processed, as will be shown hereinafter, to generate signals that represent accurate estimations of the modulus and the phase (or of the real part and of the imaginary part) of the impedance.

[0041] In a four electrode configuration, as the architecture shown in FIG. .sub.4, it is optionally possible to connect an ECG front-end to measure also the electrocardiogram of a patient. An architecture of a device for measuring the impedance of biologic tissues and for generating an ECG of a patient is illustrated in FIG. 5. The blocks X include two pairs of switches controlled in phase opposition by the control phases φ.sub.1 and φ.sub.2, as shown in FIG. 5. The function of each block is resumed in the following table:

TABLE-US-00001 INPUT STAGE Instrumentation Amplifier stage OS Output stage with chopping spike filter PGA Programmable Gain Amplifier ≈ Analog Filter x1 Voltage buffer X Chopper switches

[0042] The frequency spectra on the right side of FIG. 5 highlights that the output of the channel at the top of the figure is the sum of a base-band signal representing the ECG of the patient and of a modulated signal at the switching frequency f of the square-waves φ.sub.1 and φ.sub.2, that represent the impedance of the biologic tissue between the electrodes. The channel at the bottom of the figure outputs the sum of a base-band signal representing the impedance of the biologic tissue between the electrodes and of a modulated signal at the switching frequency f of the square-waves φ.sub.1 and φ.sub.2, that represent the ECG of the patient. Low-pass filters (not shown in the figure) extract the base-band signals representing the ECG and the impedance from the output signals of the two channels.

[0043] Injecting a square-wave current throughout the biologic tissue greatly simplifies the architecture of the differential AM demodulator and also of the AM modulator. Moreover, using a differential AM demodulator immediately downstream from the electrodes without interposing any signal processing stage between the demodulator and the electrodes, allows to use an amplification stage INPUT STAGE with a reduced bandwidth, because it has to amplify a base-band signal.

[0044] A more detailed representation of an architecture of a device for measuring electrical impedance of biologic tissues is illustrated in FIG. 6. Given that the current injected throughout the biologic tissue is a square-wave, the AM demodulator immediately downstream the electrodes may be easily realized with two pairs of switches controlled in phase opposition by the control phases φ.sub.1 and φ.sub.2. In the embodiment of FIG. 6, the amplifier INA generates signals that represent the AC (ReM AC) and the DC (ReM DC) components of the real part ReM of the impedance in the complex domain. A digital circuit DIGITAL PART processes the signals ReM AC and ReM DC and generates signals representing the modulus and the phase of the impedance of the biologic tissue under test.

[0045] In some applications, the parameter of interest is the modulus of the bioimpedance, that may be roughly approximated with the real part of the impedance in the complex domain, that is greater than the imaginary part. According to a more accurate method of estimating the modulus of the bioimpedance, the demodulating carrier is resynchronized with the sensed voltage on the electrodes using the technique disclosed in Italian patent application No. VA2010A000078 in the name of the applicant of the present application. Both the above mentioned methods may be relatively easily implemented with the devices disclosed herein.

[0046] In particular, the method disclosed in the Italian patent application No. VA2010A000078 may be implemented in the device of FIG. 10, even if this prior method has been disclosed only for AC currents injected throughout the tissue, and allows accurate estimation of the modulus of the bioimpedance. For illustrative purposes, the graph of FIG. 7 compares, obtained in a test case, the voltages sensed on the electrodes and the mean values thereof (that estimate the modulus of the bioimpedance) when a square-wave current is injected in a biologic tissue with and without applying the resynchronization technique disclosed in the cited prior Italian patent application, assuming that the correct value of the modulus of the bioimpedance is 100. It is evident that the resynchronization technique relevantly improves the accuracy of the estimations given by the mean values of the sensed voltages. The graph in FIG. 8 illustrates the corresponding percentage error vs. the phase of the bioimpedance without implementing the cited prior resynchronization technique and implementing it with different resynchronization steps.

[0047] It is also possible to estimate the impedance of biologic tissues by forcing an AC sinusoidal current therethrough, and by demodulating the voltage on the electrodes with a square-wave carrier. In this case, only the main harmonic of the square-wave demodulating carrier contributes in generating the base-band AM demodulated signal. For this reason, the graph of the ratio between the amplitude Zo of the base-band signal and the modulus M of the bioimpedance is as shown in FIG. 9, as a function of the phase difference between the voltage on the electrodes and the square-wave demodulating carrier.

[0048] Therefore, the modulus M of the bioimpedance may be obtained by multiplying the amplitude Zo of the base-band signal by a correction factor k:

M=k*Zo.

[0049] The values of the correction factor k for various phase angles may be heuristically estimated with tests and stored in a look-up table. With this technique, when the phase angle of the bioimpedance is known, for example, by using a resynchronization technique as disclosed in Italian patent application No. VA2010A000078, it may be possible to determine the value of the correction factor k, and thus obtain the modulus of the bioimpedance.

[0050] Another architecture of the device with two demodulation channels for determining in-phase and quadrature components of the bioimpedance in the complex domain is shown in FIG. 10. The blocks with the same name of those of FIG. 5 have the same function. Taking into consideration what has been stated for the architecture of FIG. 5, the functioning of the architecture of FIG. 10 will be immediately evident and for this reason this last architecture will not be further described.

[0051] An architecture similar to that of FIG. 6 for the device of FIG. 10 is illustrated in FIG. 11. Differently from the embodiment depicted in FIG. 6, the voltage on the electrodes is AM demodulated with a square-wave carrier in phase (φ=0°) and in quadrature (φ=90°) to generate signals that represent the AC and DC components of the real part V.sub.ReM and of the imaginary part V.sub.ImM of the bioimpedance in the complex domain.

[0052] Exemplary graphs of percentage error vs. the phase of the bioimpedance in the complex domain for the real part ReM and for the Imaginary part ImM thereof are illustrated in FIGS. 12a and 12b.

[0053] According to an innovative aspect, the signals V.sub.ReM and V.sub.ImM are processed by the block DIGITAL PART for generating signals representing in a very accurate fashion the effective modulus and phase of the bioimpedance. This is done substantially by calculating the effective real ReC and imaginary ImC parts of the bioimpedance using the following parametric equations:

ReC=b.sub.R*V.sub.ReM+c.sub.R*V.sub.ImM

ImC=b.sub.1*V.sub.ImM+c.sub.1*V.sub.ImM.sup.2

wherein b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are fixed parameters.

[0054] According to an embodiment, the impedance is assumed capacitive-resistive, and the parameters b.sub.R, c.sub.R, b.sub.1 and c.sub.1 are about equal to 0.0606, −0.02, 0.093 and 0.0000056, respectively. As a general rule, the values of the parameters b.sub.R, c.sub.R, b.sub.1 and c.sub.1 depend on the working frequency and may be heuristically determined with tests depending on the application and stored in a look-up table in function of the values of the signals V.sub.ReM and V.sub.ImM.

[0055] According to another embodiment, the parameters c.sub.R and c.sub.1 are null, and the values of the parameters b.sub.R and b.sub.1 are adjusted according to the following procedure:

[0056] preliminarily filling-in a look-up table of heuristically determined values of the parameters b.sub.R and b.sub.1 as a function of phase differences between the square wave input current and the corresponding differential voltage; sensing the phase difference between the square wave input current and the corresponding differential voltage; and updating the values of the parameters b.sub.R and b.sub.1 with the values stored in the look-up table corresponding to the sensed phase difference.

[0057] Substantially, according to this embodiment, the phase of the bioimpedance is used as an indicator that allows adjustment of the values of the parameters b.sub.R and b.sub.1. Exemplary percentage error characteristics vs. the phase of the bioimpedance on the estimated real part ReC and imaginary part ImC of the bioimpedance, and the estimated modulus M of the bioimpedance obtained using the above method are compared in the graph of FIG. 14.

[0058] The phase of the bioimpedance may not be used as the indicator that allows adjustment of the parameters b.sub.R and b.sub.1. Instead, any quantity tied to the phase with a bijective law may be used.

[0059] Referring to the graph in FIG. 13a, it is possible to notice that the ratio V.sub.ImM/V.sub.ReM is bijectively tied to the phase of the bioimpedance, i.e. by knowing the ratio V.sub.ImM/V.sub.ReM it is possible to determine the phase of the bioimpedance, and thus to adjust the values of the parameters b.sub.R and b.sub.1 to estimate the effective ratio Im/Re between the imaginary part and the real part of the bioimpedance. The percentage error in approximating the effective real part and imaginary part with the values V.sub.ImM and V.sub.ReM is shown in the graph of FIG. 13b.

[0060] Accordingly, the real part and the imaginary part of the bioimpedance are calculated with the following parametric equations:

ReC=b.sub.R*V.sub.ReM+c.sub.R*V.sub.ImM

ImC=b.sub.1*V.sub.ImM+c.sub.1*V.sub.ImM.sup.2

being c.sub.R and c.sub.1 null, wherein the values of the parameters b.sub.R and b.sub.1 are adjusted according to the following procedure: preliminarily filling-in a look-up table of heuristically determined values of the parameters b.sub.R and b.sub.1 in function of ratios between the demodulated signals first V.sub.ReM and second V.sub.ImM; calculating the ratio between the demodulated signals first V.sub.ReM and second V.sub.ImM; and updating the values of the parameters b.sub.R and b.sub.1 with the values stored in the look-up table in correspondence of the calculated ratio.