EP0923337A1

EP0923337A1 - Method for measuring a conductive volume and device for implementing this method

Info

Publication number: EP0923337A1
Application number: EP97938960A
Authority: EP
Inventors: Denis Duret
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1996-08-30
Filing date: 1997-08-29
Publication date: 1999-06-23
Also published as: FR2752935A1; FR2752935B1; CA2265040A1; US6434411B1; WO1998008435A1

Abstract

The invention concerns a method for measuring at least one dimension of a conductive volume, in which a measuring sensor (10) is placed inside a cavity present in a conductive or non-conductive homogeneous body (11), this cavity having large dimensions and high resistivity compared to the respective dimensions and resistivity of the volume to be measured, the sensor having features which vary according to the immediate volume in which it is found, this medium being a medium with electromagnetic losses. The invention also concerns a device for implementing this method.

Description

METHOD FOR MEASURING A CONDUCTIVE VOLUME AND DEVICE FOR CARRYING OUT SAID METHOD

DESCRIPTION

Technical area

The present invention relates to a method for measuring a conductive volume and to a device for implementing this method, this volume being placed in a homogeneous conductive body, which is itself placed in an environment of large dimensions and resistivity by compared to those of the volume to be measured.

State of the art

In the following description, we will consider, by way of example, a device for measuring the heart rate by implanted sensors.

Such a device is used for the detection of rhythm anomalies and the control of pacemakers in a certain number of pathologies, or to take account of the state of activity of a patient. The measured parameters related to cardiac activity are multiple; include partial pressure of blood oxygen, mechanical measurement of contraction, measurement of blood flow, direct measurement of electrical activity (ECG) by electrodes, measurement of blood pressure, measurement by Doppler effect, measurement of acceleration, etc.

The publications below, the references of which are given at the end of the description, describe various embodiments of the prior art. Reference [1] describes a system for monitoring the momentum or the speed of cardiac masses by an implanted sensor, this sensor being fixed on the internal muscular wall of the heart. The sensors that can be used are accelerometers, for example piezoelectric.

Reference [2] presents a stimulation device including an accelerometric sensor and an electrode.

Reference [3] uses mechanical deformation sensors based on propagation lines whose impedance varies according to their geometry.

Reference [4] describes a velocity measurement system using a catheter comprising at least two stepped electrodes. These electrodes form a polarized galvanic cell sensitive to the speed of blood flow.

Reference [5] presents a system based on the measurement of partial pressure of oxygen by electrodes.

The methods of measuring variations in the activity of the heart rhythm of the prior art do not use the overall volume information relating to a heart cavity. This quantity constitutes a marker of the highest interest since, with each beat, the ejection volume can vary from 20 to 60 I. In addition, arrhythmia disorders and certain other pathologies appear directly linked to the monitoring of this quantity.

The present invention relates to a method for measuring such a conductive volume. Statement of the invention

The present invention relates to a method for measuring at least one dimension of a conductive volume, characterized in that a measurement sensor is placed inside a cavity existing inside a homogeneous body. conductive or not, this cavity having large dimensions and resistivity with respect respectively to the dimensions and to the resistivity of the volume to be measured, the sensor having characteristics which vary according to the immediate environment which surrounds it. The measurement sensor comprises, for example, a self-inductance coupled by mutual inductance to the medium, which can be an electromagnetic loss medium.

The volume to be measured can vary over time.

In a first exemplary embodiment, the method of the invention is used for measuring the volume of a cardiac cavity, which is subject to variations with the cardiac revolution. Advantageously, the sensor can be placed at the end of the catheter, which can consist of a coaxial transmission line.

In this exemplary embodiment, the method of the invention makes the comparison to the normal cycle and the deviation from the standard makes it possible to detect, in real time and in situ, more or less serious dysfunctions.

In a second exemplary embodiment, the method of the invention is used to measure the αiameter of a pipe with cylindrical geometry, which can be, for example a blood vessel. The method of the invention can then be coupled to a speed measurement, to make it possible to measure the blood flow inside this vessel, this flow being an essential quantity for the evaluation of cardiovascular pathologies.

More generally, the invention can be applied to any measurement of volume or of pipe diameter, provided that the volume to be measured or the pipe has a resistivity contrast with the surrounding medium.

The invention also relates to a device for implementing this method in which the measurement sensor comprises a self-inductance wound with contiguous turns or not, and a reading electronics comprising successively:

- a 3dB coupler receiving the sensor input;

- an amplifier;

- a detection circuit, an amplifier being connected both to this circuit and to the coupler;

- a low-pass filter;

- a shaping circuit connected to a signal output.

Brief description of the drawings

- Figure 1 illustrates the electrical equivalent of a device for implementing the method of the invention; - Figure 2 illustrates the equivalent effect of the medium;

- Figure 3 illustrates an example of rudimentary embodiment for implementing the method of the invention; - Figures 4A and 4B show a simulation result;

- Figure 5 illustrates a measurement of impedance variation;

- Figure 6 illustrates electronics associated with the volume change measurement sensor.

Detailed description of embodiments

The method of the invention uses a sensor whose characteristics vary as a function of the immediate volume which surrounds it, which can be an electromagnetic loss medium. The sensor is for example a self-inductance coupled by mutual in the middle. If the volume is variable, both the coupling coefficient and the losses of the secondary circuit formed by the blood medium, in this case, are variable and directly linked to variations in volume. FIG. 1 represents the electrical equivalent of such a device with:

Ll: L measure L2: L medium R: losses

M: coupling coefficient. The losses of the medium can be ohmic or dielectric. In an application for measuring the volume of a cardiac cavity and at the frequencies which are considered by way of example (typically below 100 MHz), they are mainly ohmic. FIG. 2 represents the effect of the impedance of the medium, brought back to the primary, measurement circuit, with:

Ll: L measure R _s : R series (measure) Z: Z brought back. The equations of the coupled circuits make it possible to connect Z to L2 and to R by the following formula

R represents the intrinsic losses of the measurement inductance.

FIG. 3 represents a rudimentary embodiment making it possible to implement the method of the invention, the measurement inductance being a self-inductance with cylindrical geometry and the measurement volume being a conductive cylinder.

The measuring inductor 10 has a diameter d, a length 1, the loss medium 11 of conductivity σ occupies the entire volume limited by the cylinder of diameter D and length L.

The variations in volume of the cylinder limiting the conducting medium act on the inductance L2, on the term R and on the mutual fund M: they therefore have an effect on the impedance brought back in series with the measurement inductance. This effect becomes zero if the external medium has the same conductivity σ. The greater the contrast in resistivity between the cylinder of variable volume and the external environment.

The frequency at which the impedance is measured is linked to the field of influence of the measurement (the diameter D in this case): the higher the frequency, the smaller the influence diameter. FIGS. 4A and 4B represent a realistic simulation result for which the working frequency f (ω = 2πf) is 10 MHz, the conductivities of the variable cylinder of 0.7 S / m eι of external medium of 0.1 S / m. The plots represent the magnetic energy in relative units for two geometric cases: D = 6 and 10cm, L = 8 cm, d = 5 mm, 1 = 15 mm. An envelope medium of intermediate conductivity was considered. The dimensions and conductivities roughly represent a heart cavity filled with blood, the heart muscle and the lungs. These Figures 4A and 4B represent the variation of magnetic energy by simulation. The total magnetic energy varies by 2.8% between the two geometric cases, which roughly corresponds to the variations that can be expected on the reduced impedance. Figure 4A corresponds to a 3 cm core, with the following ranges: 15: -9 to -8

16: -10 to -9

17: -11 to -10

18: -12 to -11

19: -13 to -12 20: -14 to -13

21: -15 to 14

22: -16 to -15.

FIG. 4B corresponds to a 5 cm core, the ranges identical to those of FIG. 4A bearing the same references.

In an exemplary embodiment, the invention proposes a method for global measurement of the variations in volume of the cardiac cavity, using its electromagnetic influence on an inductance placed in the cavity. These variations modify the apparent impedance of the measurement inductance. They can be highlighted by conventional means of bridge mounting or reflectometry and collected at the end of a cable of constant characteristic impedance, after adaptation.

The invention has many advantages. The measured parameter is global. It is purely geometric and does not seem to have been the subject of particular use for applications in implanted cardiac monitoring. The sensor preferably uses at the end of the catheter, the catheter being for example constituted by a coaxial transmission line, is particularly simple and robust (in this case, the line is of any length). Its influence with the medium is only electromagnetic, therefore without contact.

On the other hand, any physiological coating reaction does not disturb the measurement very much since it only slightly modifies the geometrical parameters of the coil. The safety of the measurement is ensured: only a very low energy of coupling to the medium is used (of the order of 1 m at most), at a frequency of the order of a few MHz. In the case of a conductive surrounding medium, the measurement device thus produced is self-shielded: in fact the conductive surrounding medium strongly attenuates the propagation outside. For the same reason and by reciprocity, this device is very insensitive to the surrounding electromagnetic disturbances.

We will now consider an exemplary embodiment.

The measurement sensor comprises a cylindrical self-inductance 10 mm long and 2 mm in diameter. It is wound with contiguous turns with dj 0.1 mm copper wire. For these parameters, the value of the inductance is of the order of 1.3 μH. The The circuit in FIG. 5 gives a method of measuring the variations in impedance in which the resonant circuit is tuned for example to a frequency close to 10 MHz by a capacitance C1 of the order of 100 pF. This circuit is itself adapted to the characteristic impedance of a transmission line LT, usually 50 Ω, for example by a C2 series capacitor. The value of this capacity depends on the intrinsic and coupling losses, typically a few tens of pF.

As regards the reading electronics, a bridge method is advantageously used, such as that shown in FIG. 6. This electronics successively comprises: - a 3dB coupler 25 receiving the sensor input E;

- an amplifier 27;

a detection circuit 28, an amplifier 26 being connected both to this circuit 28 and to the coupler 25;

- a low-pass filter 29;

- a shaping circuit 30 connected to a signal output S.

In this case, the bridge is a 3 dB coupler whose output is zero when the load (the circuit extended by the line) is equal to 50 Ω, and takes an increasing value with the mismatch. The bridge is excited at the resonant frequency of the tuned and matched circuit. An amplifier accentuates the imbalance, and an envelope or coherent detection (case of the figure) makes it possible to extract the mismatch information which, after filtering and shaping, makes it possible to go back to the information of variation of the volume . As mentioned above, in a first industrial application the invention relates to the evaluation of the variations in volume of one of the cardiac cavities of a patient during a cycle. The volume of a heart cavity, or ventricle, is subject to variations with the cardiac revolution (systole and diastole); the ejection volume can vary from 20 to 60% with each beat, the resistivity contrasts between the cardiac cavity and the muscular tissue (2,5) and between the cardiac cavity and the lung (14) allow to use the variation overall conductive blood liquid (0.7 mhos / m) as a marker by measuring the losses induced by mutual insurance with the sensor. The comparison to the normal cycle and the deviation from the norm makes it possible to detect more or less serious malfunctions in real time and in situ. This detection allows the action on a stimulation organ and possibly the triggering of a defibrillation. A second application concerns the measurement of the diameter of a pipe with cylindrical geometry: if the length of the pipe considered is large compared to the influence distance of the sensor, the latter is sensitive to the average of the diameter of the pipe along its length. distance of influence. In the health field, this system is advantageously used for measuring the diameter of blood vessels. Coupled with a speed measurement using a different principle, the device of the invention makes it possible to measure blood flow, an essential quantity for the evaluation of cardiovascular pathologies. In such an application the typical dimensions are obviously different, likewise possibly the operating frequency can advantageously be higher. The geometry, which allows us to understand how this second application works, obtained by considering L as infinite in Figure 3.

It is obviously possible to envisage applications of the invention in fields other than that of health.

REFERENCES

[1] EP-A-0 582 162 (SORIN BIOMEDICA)

[2] EP-A-0 515 319 (SORIN BIOMEDICA) [3] WO 95/15784 (PACESETTER AB)

[4] WO 95/26677 (PACESETTER AB)

[5] US-A-5 431 172 (PACESETTER AB)

Claims

1. Method for measuring at least one dimension of a conductive volume, characterized in that a measurement sensor (10) is used which is placed inside a cavity existing inside a body homogeneous conductive or not (11), this cavity having large dimensions and resistivity relative to the dimensions and resistivity of the volume to be measured, respectively, and in that the sensor, which has characteristics varying according to the immediate environment which l environment, includes a self-inductance coupled by mutual inductance to this medium.

2. Method according to claim 1, characterized in that the volume to be measured is variable over time.

3. Method according to claim 1, characterized in that it is used to measure the diameter of a pipe with cylindrical geometry.

4. Device for implementing the method according to any one of the preceding claims, characterized in that the measurement sensor comprises a self-inductance wound with contiguous or non-contiguous turns, and in that the reading electronics successively comprises :

- a 3 dB coupler (25) receiving the sensor input (E);

- an amplifier (27);

- a detection circuit (28), an amplifier (26) being connected to both this circuit

(28) and to the coupler (25);

- a low-pass filter (29);

- a shaping circuit (30) connected to a signal output (S).

5. Device according to claim 4, characterized in that the self-inductance is coupled by mutual inductance to the medium, which is an electromagnetic loss medium.

6. Device according to claim 4, characterized in that the sensor is arranged at the end of the catheter.

7. Device according to claim 6, characterized in that the catheter consists of a coaxial transmission line.

8. Device for measuring the volume of a cardiac cavity, which is subject to variations with the cardiac revolution, characterized in that it comprises a measurement sensor (10) arranged inside this cavity, this cavity having large dimensions and resistivity respectively with respect to the dimensions and the resistivity of the volume to be measured, the sensor having characteristics which vary according to the immediate environment which surrounds it.

9. Device according to claim 8, characterized in that the measurement sensor comprises a self-inductance coupled by mutual inductance in the middle.

10. Device according to claim 8, characterized in that the sensor is arranged at the end of the catheter.

11. Device according to claim 10, characterized in that the catheter consists of a coaxial transmission line.

12. Device according to claim 11, characterized in that it comprises means for comparing to a normal cycle and means for measuring the deviation from the standard, which makes it possible to detect in real time and in situ malfunctions.

13. Device according to claim 8, characterized in that the measurement sensor comprises a self-inductance wound with contiguous or non-contiguous turns, and in that the reading electronics successively comprises:

- a 3 dB coupler (25) receiving the sensor input (E);

- an amplifier (27), -

- a detection circuit (128), an amplifier (26) being connected to both this circuit

(28) and to the coupler (25);

- a low-pass filter (29);

- a shaping circuit (30) connected to a signal output (S).

14. Use of the device according to any one of claims 4 to 7 for measuring the diameter of a blood vessel.

15. Use of the device according to any one of claims 4 to 7, coupled with a speed measurement, for measuring the blood flow inside a blood vessel.