WO2024047076A1

WO2024047076A1 - System and method for measuring common-mode signal

Info

Publication number: WO2024047076A1
Application number: PCT/EP2023/073738
Authority: WO
Inventors: Boyle ALISTAIR; Aggrawal HARI OM
Original assignee: Kite Medical Limited
Priority date: 2022-08-30
Filing date: 2023-08-29
Publication date: 2024-03-07

Abstract

The present invention relates to a device, a system, and a method for measuring the human body's direct current (DC) potential and raising the level of the instrument ground to the human body's DC potential to minimize the common-mode signal in an electrical impedance tomography instrument. The method, system, and device rely on a resistor-capacitor (RC) element and a discharging resistor that connects to the electrodes on the human body through switches and multiplexers.

Description

System and method for measuring common-mode signal

Field of the invention:

The present invention relates to a device, a system, and a method for minimizing the commonmode signal in an electrical impedance tomography (EIT) instrument.

Background information:

Electrical impedance tomography (EIT) is a well-known non-invasive medical imaging technique. It is generally used in clinics for lung and cardiac activity monitoring. In academic research, EIT has been evaluated for monitoring cerebral ischemia and kidney reflux among other applications. EIT uses a plurality of electrodes placed on the surface of a human body. Typically, a pair of these electrodes connects to the current source, passing a few milliampere of alternating current in the range of 10 kHz to 1 MHz into the human body; another pair of electrodes measures the change in voltage in response to the current source and any changes in conductivity or distribution of the internal tissues and fluids. New pairs of electrodes are selected for the current source and voltage measurements until a complete set of measurements is collected. The measured voltages are used to reconstruct an EIT image representing the spatio-temporal variation of the conductivity inside the human body.

EIT is generally very hard to implement properly. Numerous factors can degrade the quality of an EIT image; the common-mode signal is one of the critical factors for overall signal quality and is the main focus of the present invention. Other factors may include contact impedance at the skin-electrode interface, movement of electrodes during data acquisition, and the electrical design of the current source and measurement circuits, which can cause issues such as an unbalanced current source.

A difference amplifier gives a voltage output proportional to the voltage difference at its input terminals. An instrumentation amplifier is a common type of difference amplifier using input buffers to improve performance. The common-mode signal is either a DC or an AC voltage common to both terminals of an instrumentation amplifier; for example, used in the EIT system for amplifying the AC signal. Though ideally an instrumentation amplifier should reject common-mode signals completely, in practice, the common-mode rejection ratio (CMRR) is finite, typically lOOdB at DC. Hence, for a DC common voltage, the amplifier output generally has a DC offset (due to the comm on -mode signal) in the AC output signal. There exist many methods to remove DC offset from an AC signal. These methods mainly rely on the assumption that the DC offset remains the same for the entire time. But the present invention focuses on a more realistic setting where DC offset is assumed to be a slowly time-varying signal, removal of which requires special attention at a circuit level and is discussed in the present invention.

The common-mode signal impacts the EIT image quality, so the common-mode signal needs to be minimized in an EIT system. One of the primary sources of the common-mode signal is the potential difference between the patient's body and the instrument ground. To avoid any unacceptable risk of electrical shock under normal and single fault conditions, the patient's body and the medical instrument are electrically isolated from the earth ground (“earthed”). Both grounds are generally floating, but are indirectly coupled to the earth's ground through stray capacitances formed due to electromagnetic sources in the surrounding of the patient. This leads to a DC potential difference between the patient's body and the instrument ground.

In addition to ground potential difference, the other main source of DC comm on -mode voltage is the accumulated charge at the skin-electrode interface. An unbalanced current source can cause a systematic DC common-mode voltage. Switching between electrodes when not at a zero crossing can also cause a net current flow between electrode pairs and an accumulation of charge at the electrodes.

Commonly, the common-mode voltage is minimized through a feedback circuit where a reference electrode is attached to the body whose primary purpose is to measure the body potential and adjust it so that the common-mode signal reduces to zero. This strategy is known for electrocardiogram (ECG) from Bruce B. Winter et al., Driven-Right-Leg Circuit Design, IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983; and for EIT from US 9968272 B2, Device for Detecting Electric Potentials. Though the feedback circuit is quite common in ECG applications, it is not a very reliable technique in EIT applications due to the higher AC current source and measurement frequencies used in EIT. In practice, most EIT systems now take measurements without any reference electrode. Reference electrodes are avoided, as an additional electrode that needs to be placed on the patient away from the existing electrode groupings leads to reduced usability of the system in practical applications.

Accordingly there is a need for a system and method that can minimize the effect of DC common-mode voltage on the useful AC signal in an EIT measurement. Summary

Accordingly there is provided a system and method as detailed in the claims that follow.

Brief description of the drawings

Figure l is a schematic view of a circuit according to the present invention for measuring the human body's potential.

Detailed description of the invention:

An objective of the present invention is to provide an electrical circuit that minimizes the effect of DC common-mode voltage on the useful AC signal in an EIT measurement. Per the present teaching a system (that incorporates an electrical circuit) and method are provided that raise the level of the instrument ground to the human body potential to minimize the common-mode signal. In this way, the present inventors have identified that it is possible to minimize timevarying DC common-mode signals without requiring any additional electrode or a feedback circuit.

Referring to the drawings, Fig 1 shows a schematic view of a device according to the present invention for measuring the human body's potential. In this case, the potentials are measured on a patient's skin through a plurality of electrodes 100.

These pluralities of electrodes 100 are connected either to a current source 101, 102 through a multiplexer SI or to a differential (instrumentation) amplifier through a multiplexer S2. SI is always in the open state for measuring the human body's DC potential and in the closed state for measuring the useful signal that reflects the change in impedance in the internal organs. While collecting the useful signal, switch S3 is always in the open state, and switch S4 is always in the closed state.

For measuring the human body's DC potential, switch S3 is in the closed state. Each electrode connects sequentially to the Rl-Cl element through multiplexer S2 and charges the capacitor Cl of the Rl-Cl element so that the voltage across Cl equals the human body's DC potential. Whenever S3 is in the closed state, S4 is in the open state. Once the switching cycle completes sampling through all electrodes, S4 is closed, and the Cl discharges through R2, raising the instrument ground level. As is exemplified by the circuit of Figure 1, per the present teaching there is provided a device having an RC element connected to a plurality of electrodes through switches or multiplexers and a discharging resistor whose one terminal is connected to the instrument ground (floating), and another terminal is connected to the RC element through a switch. The RC element forms a low-pass filter (passing low frequency signals, while blocking high frequency signals) where the voltage across the capacitor is time-varying as a function of the resistor and capacitor values such that 7(t) = FQ(1 ^—

with T = RC for resistance R and capacitance C giving an “RC time constant” reflecting the rate of change of the filter.

In the device, the RC element measures the DC potential of the human body and, through the discharging resistor, raises the level of instrument ground to the measured DC potential, thus minimizing the potential difference between grounds, which reduces the common-mode voltage in the EIT measurements.

A pair-drive EIT system is one where a single current source (or pair of current sources) is used to provide AC current to the patient. Switches and multiplexers are integral parts of pair-drive EIT systems, which are exploited in the present invention. The human body's DC potential is measured, relative to the instrument ground, only when current sources are disconnected (through switches) from the human body, and no AC current flows from the current source to the electrodes attached to the body. In this case, the human body's potential is primarily due to stray capacitances formed due to surrounding electric fields or charges accumulated at the skinelectrode interface. This potential has only a DC component. It is to note that the prior art adjusts the human body's potential while the current source is active and current flows through the body where the measured human body potential has both the AC and DC components that complicate handling common-mode signals.

While the current source is disconnected, a multiplexer sequentially connects the RC element to each electrode. The voltage across the capacitor readjusts as per the human body's potential at the plurality of electrodes. The RC time constant is large enough such that each electrode is sampled at least once to measure the average human body potential across the capacitor. Therefore, the RC time constant depends on the multiplexer switching time. In the RC element, the resistor should limit the worst-case leakage current to electrically safe limits for humans, and the capacitor should be large enough that the capacitor’s stored charge can adjust the instrument ground. After one switching cycle through all electrodes, the charge across the capacitors is discharged through a resistor connected to the instrument ground to adjust the instrument ground. At each new switching cycle, the voltage across the capacitor re-adjusts as per the human body's potential during that cycle. In this way, the present invention takes care of the time-varying DC potential by adjusting the instrument ground at regular intervals.

It is worth to note that the present invention works for both single-ended and differential measurements.

Accordingly there is provided an arrangement that minimizes the effect of DC common-mode voltage on the useful AC signal in an EIT measurement.

Claims

We claim:

1. A system for minimizing common-mode signal in electrical impedance tomography, EIT, measurements, the system comprising: an EIT instrument comprising: at least one current source configured for passing an alternating current into the human body, a measurement circuit configured for measuring a change in voltage in response to passing the alternating current into the human body, and an instrument ground; a plurality of electrodes for operative placement on the surface of a human body, wherein the plurality of electrodes comprises at least two electrodes connected to the at least one current source through switches or multiplexers, and at least two measurement electrodes connected to the measurement circuit through switches or multiplexers; and a common-mode signal minimizing device, wherein the common-mode signal minimizing device comprises an RC element and a discharging resistor, wherein the RC element is connected to each of the plurality of electrodes through at least one switch or multiplexer and is connected to the instrument ground through a capacitor, and wherein the discharging resistor has a first terminal connected to the instrument ground and a second terminal connected to the RC element through a switch.

2. A system according to claim 1 wherein the plurality of measurement electrodes comprises at least four measurement electrodes.

3. A system according to claim 1 or 2 wherein the RC element forms a low-pass filter.

4. A system according to any preceding claim wherein the system is a pair-drive EIT system.

5. A system according to any preceding claim wherein the measurement circuit is configured for making single-ended measurements.

6. A system according to any one of claims 1 to 5 wherein the measurement circuit is configured for making differential measurements.

7. A system according to any preceding claim wherein the measurement circuit comprises a differential amplifier.

8. A system according to any preceding claim wherein the EIT instrument comprises a pair of current sources.

9. A method of minimizing common-mode signal in an electrical impedance tomography, EIT, measurement, the method comprising: providing an EIT instrument comprising a current source for passing an alternating current into the human body, a measurement circuit for measuring a change in voltage in response to passing the alternating current into the human body, and an instrument ground; providing a plurality of electrodes for placement on the surface of a human body, wherein the plurality of electrodes comprises at least two electrodes connected to the current source through switches or multiplexers and at least two measurement electrodes connected to the measurement circuit through switches or multiplexers; providing a device for minimizing common-mode signal, wherein the device for minimizing common-mode signal comprises an RC element and a discharging resistor, wherein the RC element is connected to each of the plurality of electrodes through at least one switch or multiplexer and is connected to the instrument ground through a capacitor; and wherein the discharging resistor has a first terminal connected to the instrument ground and a second terminal connected to the RC element through a switch; connecting the at least two electrodes to the current source and the at least two measurement electrodes to the measurement circuit; disconnecting the at least two electrodes from the current source and the at least two measurement electrodes from the measurement circuit; sequentially connecting the RC element to each electrode of the plurality of electrodes to charge the capacitor while the switch that connects the RC element to the discharging resistor is open; and connecting the capacitor to the discharging resistor to raise the instrument ground level while the RC element is disconnected from the plurality of electrodes.

10. A method according to claim 9 wherein connecting and disconnecting is achieved by closing and opening the switches or multiplexers.