FIELD OF THE INVENTION

This invention relates to the sensing of hemodynamic parameters, in particular flow and pressure. It relates in particular to a sensor arrangement for mounting at a distal portion of a catheter or guidewire.

BACKGROUND OF THE INVENTION

Minimally invasive surgery methods require the implementation of sensors for imaging or for physiological parameter monitoring at the tip of guidewires and catheters. For example, to assess the conditions of arterial veins, sensing wires (or catheters) are used to measure various physiological parameters, such as blood pressure and blood flow velocity. Guidewire-based blood flow sensors and blood pressure sensors are commercially available and widely used.

It would be desirable to combine these modalities into one wire, thereby to give a physician more information from a single procedure. However, the sensors are of different types and hence require different drive signals. As a result, the number of wires required along the catheter or guidewire increases, thereby complicating the implementation of a multi-functional sensor arrangement.

WO 2019/096812 discloses a sensor arrangement at the end of a catheter or guidewire, with a first sensor such as a Doppler ultrasound flow sensor and a second sensor such as a capacitive pressure sensor. The two sensors share a pair of electrical conductors, and different voltage and frequency signals are used for actuating the two sensors.

It would be desirable to have a simpler implementation to allow the sharing of control and readout wires between a flow velocity sensor and pressure sensor.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a sensor system, comprising: an elongate support; a sensor arrangement at a distal portion of the elongate support; a sensor readout circuit at a proximal end of the elongate support, wherein the sensor arrangement comprises: a piezoelectric sensor for measuring flow, and connected between first and second contact terminals; first and second piezoresistive pressure sensors in series between the first and second contact terminals, and defining a third contact terminal between them, wherein the first piezoresistive pressure sensor is exposed to a pressure to be measured and the second piezoresistive pressure sensor is insensitive to the pressure to be measured, wherein the elongate support comprises first, second and third conductors which connect to the first, second and third terminals, and wherein the sensor readout circuit comprises: an transmitter and an receiver coupled to the first and second conductors; a reference potential coupled to the third conductor; and a differential voltage measurement circuit coupled between the first and second conductors for pressure sensing measurement.

This sensor system enables the measurement of both blood flow and blood pressure using a single elongate shaft, e.g. guidewire or catheter. However, the configuration of piezoresistive pressure sensors and a piezoelectric sensor enables the multifunctional sensor to be implemented with only three conductors along the elongate shaft. This provides more useful information to a physician from a single interventional procedure. The use of two pressure sensors enables compensation to be implemented for thermal sensitivity.

The two sensors provide two separate information streams. To reduce cost and simplify wiring and interconnect design, the two streams are combined over the shared conductors and are separated at the sensor readout circuit, such as by a patient interface module. The pressure and flow velocity measurements can in particular be performed in a time-sequential way, and the during each measurement, the presence of the other sensor type in the sensor arrangement does not prevent the correct function of the sensor which is in use.

Combining the leads to the sensors reduces the cost of the guidewire or catheter and the overall system compared to a system in which sensors are connected separately.

The ultrasound transmitter for example comprises a first buffer between a transmission input and the first conductor, and a second inverting buffer between the transmission input and the second conductor. Thus, the ultrasound sensor is driven by opposite polarity signals, and hence a midpoint of the two drive signals is a virtual earth.

This virtual earth can then be used as a terminal of the pressure sensors.

The ultrasound receiver for example comprises a voltage measurement circuit between the first conductor and the second conductor. It measures a voltage across the ultrasound sensor.

The sensor readout circuit may comprise a first capacitor between the ultrasound transmitter and receiver and the first conductor, and a second capacitor between the ultrasound transmitter and receiver and the second conductor. These provide isolation between the ultrasound readout circuitry and the pressure sensing readout circuitry, to prevent the impedance of the ultrasound transmitter and receiver from influencing the pressure sensing measurements.

The sensor readout circuit may comprise: a first current source between the third conductor and the first conductor; and a second current source between the third conductor and the second conductor.

These drive a current through the piezoresistive pressure sensors so that a voltage measurement may be used to determine a resistance. The first and second current sources for example direct current away from the third conductor.

The sensor system may further comprise a controller for controlling the measurement of pressure and flow velocity in time-interleaved manner.

Although both modalities can make use of the same leads, they cannot use these at the same time. A time-interleaved mode is thus used. The flow velocity measurement is extremely sensitive to precise timing. One acquisition, that consists typically of 256 to 1024 ultrasound transmit and receive pulses, is executed as one uninterrupted sequence. The pressure measurement on the other hand is much less sensitive to timing. Complete flow velocity acquisitions are thus preferable interleaved with a pressure measurement.

The controller is thus preferably adapted to: implement a flow velocity measurement by performing a sequence of ultrasound transmit and receive pairs; and before and/or after the sequence, implement a pressure measurement.

The sequence of ultrasound transmit and receive pairs for example typically comprises 256 to 1024 ultrasound transmit and receive pairs as mentioned above.

The controller may be adapted to implement pressure measurements with a repetition frequency of at least 100 Hz. A minimum update rate of 100 Hz may be desired and is achievable. The sensor readout part of the system can also be used for either a separate pressure or a separate flow velocity measurement wire.

The elongate support for example is comprised by an interventional device such as a guidewire or catheter and the sensor readout circuit for example may be comprised either in the proximal portion of the interventional device, in a patient interface module or a console that is configured to be connected to the interventional device.

The invention also provides a method for sensing flow velocity and pressure using the sensor arrangement as defined above, the method comprising performing the measurement of pressure and flow velocity in time-interleaved manner.

The method for example comprises performing the flow velocity measurement by performing a sequence of ultrasound transmit and receive pairs and before and/or after the sequence, performing the pressure measurement. The sequence of ultrasound transmit and receive pairs may typically comprise 256 to 1024 ultrasound transmit and receive pairs and the method may comprise performing pressure measurements with a repetition frequency of at least 100 Hz.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 shows a known flow velocity sensor;

Figure 2 shows a known pressure sensor; and

Figure 3 shows a sensor system for measuring both flow velocity and pressure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures.

It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

The invention provides a sensor system which comprises a piezoelectric ultrasound sensor for measuring flow velocity and first and second piezoresistive pressure sensors for measuring pressure. The sensors are at a distal portion of an elongate support such as a catheter or guidewire, which comprises first, second and third conductors, shared between the two sensor types. A time-interleaved readout of the sensor signals is used.

Blood flow velocity measurement is typically performed using ultrasound Doppler measurement. For example, sensing wires exist comprising an ultrasound transducer at the tip of a guide wire (or catheter) that uses ultrasound frequencies of 12 MHz.

One ultrasound sensing event consists of two phases.

During a first phase, the ultrasound element at the tip of the wire first receives a burst of pulses (typically between 16 and 64 pulses) at ultrasound frequencies, such as 12 MHz. These pulses typically have amplitude in the order of 30 V (peak to peak). During this burst of pulses, the ultrasound element emits strong acoustic waves at the ultrasound frequencies.

After the burst of transmission pulses, the system enters a receive mode for the second phase. The acoustic signal received by the ultrasound element is converted to electrical signals, which are transferred to the measurement system for processing. The listening period is typically of the order of 10 ps.

A number of these ultrasound sensing events are repeated (typically between 256 and 1024 events) with a very well-defined spacing in time. The total measurement period is thus for example of the order of 10ms. From the resulting data, the blood flow velocity can be computed in known manner.

Figure 1 shows in simplified form a system for blood velocity measurement.

The system comprises an elongate support 10 such as a guidewire or catheter, along which a pair of electrical conductors 12 is provided. A piezoelectric ultrasound sensor 20 is provided at a distal end of the elongate support 10. A sensor readout circuit 30 is at a proximal end of the elongate support 10, and it may comprise part of a patient interface module. A transmission circuit comprises a first unity buffer 32 and a second, inverting, unity buffer 34 which each receive a transmission pulse signal Vtx. The outputs of the two buffers are provided across the sensor 20. For receiving a reflected ultrasound pulse, the two conductors connect to a (differential) voltage measurement circuit 36 which generates a receive signal Vrx. This mechanism requires a two-lead interconnect.

Figure 2 shows in simplified form a piezoresistive blood pressure measurement system.

Again, the system comprises an elongate support 10 such as a guidewire or catheter, along which three electrical conductors 12 are provided. A series connection of first and second piezoresistive pressure sensors 22, 24 is provided at the distal end of the elongate support 10. The sensor readout circuit 30 is again at a proximal end of the elongate support 10

The first and second piezoresistive pressure sensors 22, 24 are in series between the first and second conductors. The first piezoresistive pressure sensor 22 is exposed to a pressure to be measured whereas the second piezoresistive pressure sensor 24 is insensitive to the pressure to be measured. This can simply be achieved based on the locations of the pressure sensor, e.g. with respect to an opening in the body which defines the guidewire or catheter.

Pressure sensors in the form of piezoresistive elements have low sensitivity for the detected strain and a much higher sensitivity for temperature, which therefore needs compensation. The use of two elements provides a practical solution to this problem, whereby one is strained by blood pressure, and the other is not. From the difference in resistance, the blood pressure can be deduced, compensating for the common temperature of the two elements.

The sensor readout circuit in this case comprises a reference potential (ground) coupled to a third conductor which connects to the junction between the two pressure sensors 22, 24. A voltage measurement circuit 38 is again coupled between the first and second conductors for pressure sensing measurement, and it outputs pressure measurement Vpr.

A first current source 50 is provided between the third conductor and the first conductor and a second current source 52 is between the third conductor and the second conductor. The first and second current sources direct current away from the third conductor so that the voltage on the first and second conductors correspond to the voltage across the first and second sensors. The current sources enable a resistance measurement to be made of the piezoresistive pressure sensors, and the difference in resistance is effectively measured by the voltage measurement circuit. Blood pressure measurements are in this way implemented by using piezoresistive elements. The pressure being sensed, e.g. a blood pressure, introduces a strain in the piezoresistive element which is exposed to the pressure, which in turn causes a change in resistance. This change in resistance can be measured electrically and the blood pressure can be derived.

The arrangement shown in Figure 2 is a half bridge configuration for the two pressure sensors and hence it requires a three-lead interconnect. Typical measurement currents are 60 mA per piezoresistive element. Very low frequencies are involved in the measurement of the blood pressure.

The two sensing modalities each provide an information stream that has to be transferred to the data processing and visualization system.

However, combining the two modalities as described above while keeping the systems completely separate would require five leads, which results in a very high cost of the wire and interconnect system.

To minimize the cost of the wire and the interconnect system, it is beneficial to combine these two information streams and transfer these using a reduced amount of wires, as has been recognized in WO 2019/096812.

The invention provides a time-interleaved approach for combining the signals for transfer over the shared conductor arrangement. In this way, the shared conductors are used either to measure blood flow or blood pressure, but never at the same time. The leads can be then used for one modality at a time.

However, various considerations are needed to enable implementation of such an approach. In particular, in order to avoid elaborate multiplexing switching mechanisms in the guidewire or catheter, the following requirements need to be met:

(i) Both the velocity and flow sensors are connected to the same leads;

(ii) The measurement of flow can be performed without damaging the connected pressure sensor;

(iii) The measurement of pressure can be performed without damaging the connected flow sensor;

(iv) The measurement of flow can be performed without the connected pressure sensor affecting this measurement; and

(v) The measurement of pressure can be performed without the connected flow sensor affecting this measurement. These conditions are challenging particularly at the sensor (distal) side of the system. Once the sensor side requirements have been met, the electronics at the (proximal) side of the system, namely either in the proximal portion of the interventional device (e.g. handle of the catheter, at the patient interface module (PIM) connecting the proximal portion of the interventional device (e.g. guidewire), or a console that is configured to be in connection with the interventional device, can be implemented to allow the two modalities to coexist.

Figure 3 shows a system for velocity using ultrasound Doppler measurements as well as for measuring flow using pressure sensing.

The system combines the two modalities of Figures 1 and 2, and the same reference numbers are used.

The sensor system comprises an elongate support 10, which is comprised in an interventional device, with a sensor arrangement 14 at a distal portion of the elongate support and a sensor readout circuit 30 at a proximal end of the elongate support. The sensor arrangement has a piezoelectric ultrasound sensor 20 for measuring flow velocity between first and second contact terminals 26,27 as well as first and second piezoresistive pressure sensors 22, 24 in series between the first and second contact terminals 26, 27. A third contact terminal 28 is defined between the pressure sensors 22, 24. The first piezoresistive pressure sensor 22 is exposed to a pressure to be measured and the second piezoresistive pressure sensor 24 is isolated from the pressure to be measured. Of course, the system is symmetrical, so either one of the pressure sensors is exposed to the pressure to be measured and the other is not (the configuration chosen will affect the sign of the measurement signal). In an embodiment the flow sensor is located at the distal end of the elongate support and the first pressure sensor is at the distal portion of the elongate support exposed to the medium of which pressure is to be measured at a lateral side of the elongate support. In other embodiments both, the flow sensor 20 and the first pressure sensor 22 are positioned such that they are exposed to the medium to be measured along the side of the elongate support. In a further embodiment the flow sensor 20 and the first pressure sensor 22 are positioned to such that they are exposed to the medium to be measured along the longitudinal axis of the elongate support, e.g. at the distal end of the elongate support.

The elongate support comprises first, second and third conductors 12 which connect to the first, second and third terminals 26, 27, 28.

The sensor readout circuit comprises the ultrasound transmitter (i.e. the first and second buffers 32, 34) and the ultrasound receiver 36 (i.e. the voltage measurement circuit) coupled to the first and second conductors. A reference potential (ground) is coupled to the third conductor and hence to the third terminal 28. The sensor readout circuit also includes the voltage measurement circuit 38 coupled between the first and second conductors for pressure measurement as well as the first and second current sources 50, 52.

Figure 3 additionally shows a first capacitor 60 between the ultrasound transmitter and receiver and the first conductor, and a second capacitor 62 between the ultrasound transmitter and receiver and the second conductor. These capacitors isolate the high frequency blood flow related signals from the low frequency pressure signals.

Figure 3 also shows a controller 70 for controlling the measurement of pressure and flow velocity in a time-interleaved manner. The controller can be implemented either in the proximal portion of the interventional device (e.g. handle of the catheter, at the patient interface module (PIM) connecting the proximal portion of the interventional device (e.g. guidewire), or in a console that is configured to be in connection with the interventional device.

In principle, the flow and pressure measurements can be conveyed to a console, with is configured to provide a visualization of the measurement results, by a wireless transmission or by electrical wired connection. When the sensor readout circuit 30 is implemented in the handle of the catheter or in the patient interface module, it is more advantageous the wireless transmission of the measurement signals. The sensor readout circuit may be provided with autonomous power supply by providing a battery or by powering the sensor readout circuit and/or the controller by wireless power transmission, in which case the sensor readout circuit and/or controller is provided with suitable electronics to receive electromagnetic energy wirelessly.

The way this circuit configuration allows the conditions above to be met is now explained in more detail.

(i) Both the velocity and flow sensors are connected to the same leads

The piezoelectric ultrasound sensor 20 is connected between the two sides of the half bridge of the pressure sensor. Other configurations would be possible, but are asymmetrical and will therefore degrade the common mode rejection needed for EMI suppression during flow measurement.

(ii) The measurement of flow can be performed without damaging the connected pressure sensor In the configuration shown, the two piezoresistive elements in series are directly exposed to the transmit pulses required for flow measurement. Two possible negative effects could occur: too high dissipation, or electrical breakdown.

The pressure sensitive elements 22, 24 have a typical resistance of 3.5 kQ. Assuming a continuous flow measurement, the dissipation per element under typical flow pulses is therefore: Npulses elem f,

R event elem f, US

In which:

P _eiem The dissipated power per element

Vpuise The peak voltage of the pulse (assuming a bipolar square wave transmit pulse)

(typical pulse height 30 V)

R _g iem The resistance of an element (typically 3.5 kQ)

Npuises The number of pulses in a burst (typically 64) f _us The ultrasound frequency used (typically 12.5 MHz) fevent The ultrasound event frequency (typically 50 kHz)

An "ultrasound event" is the combination of a transmit pulse and a receive pulse.

With typical values, the dissipation in the complete sensor would be 17 mW, which is acceptable both for avoiding excessive heat dissipation and electrical or thermal breakdown.

(iii) The measurement of pressure can be performed without damaging the connected flow sensor

The measurement of pressure involves sending small DC currents through the piezoresistive elements, resulting in voltages well below a few Volts. Such a voltage will not have any effect on an ultrasound element. To prevent the impedance of the ultrasound transmitter and receiver influencing this measurement, the capacitors 60, 62 are placed between the ultrasound transmitter and receiver and the part that measures the pressure, as is shown in Figure 3. (iv) The measurement of flow can be performed without the connected pressure sensor affecting this measurement

The pressure sensor is an additional mostly resistive load to the ultrasound sensor element 20. This load will decrease the output signal of the ultrasound element by a small fraction. The impedance of the wiring to the patient interface module and of the patient interface module itself is typically of the order of 100 W. Thus, the load change due to the pressure sensor would typically be less than 1.5%, and is considered insignificant.

(v) The measurement of pressure can be performed without the connected flow sensor affecting this measurement

For low frequency signals, a piezoelectric ultrasound element behaves as a small capacitor of typically 35pF. This capacitor is connected in parallel with the pressure sensor which presents a combined resistance of 7 kQ using the 3.5 kQ example above. The RC time constant is therefore less than 0.3 ps. A very short stabilization time is therefore required before measuring the voltage of the half bridge of the pressure sensor.

Although both modalities can make use of the same leads, they cannot use these at the same time. A time-interleaved mode is used as explained above. The Doppler velocity flow measurement is extremely sensitive to precise timing. One acquisition consists of typically 256 to 1024 ultrasound events, and this acquisition is preferably executed as one action. Pressure measurement on the other hand is much less sensitive to timing. It therefore makes sense to interleave complete velocity acquisitions with a pressure measurement.

The desired minimum pressure update rate of 100 Hz is achievable.

The sensor readout system may be used drive a pressure-only guidewire or catheter or a flow velocity-only guidewire or catheter.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.