WO2018138171A1

WO2018138171A1 - A method and apparatus for measuring a physiological characteristic of a subject

Info

Publication number: WO2018138171A1
Application number: PCT/EP2018/051762
Authority: WO
Inventors: Jacobus Josephus Leijssen; Rick BEZEMER; Josephus Arnoldus Henricus Maria Kahlman; Gerardus Johannes Nicolaas Doodeman; Bart Kroon
Original assignee: Koninklijke Philips N.V.
Priority date: 2017-01-25
Filing date: 2018-01-25
Publication date: 2018-08-02

Abstract

According to an aspect, there is provided a method of measuring a physiological characteristic of a subject, the method comprising controlling a first array of transducers to transmit radio frequency, RF, signals into a part of a body of the subject such that the energy of the RF signals is focused at a measurement position in the part of the body; receiving modulated RF signals from the part of the body of the subject using a second array of transducers, wherein the modulated RF signals are modulated by fluid movements in the part of the body; combining the received modulated RF signals into a first combined RF signal that originates from the measurement position in the part of the body; and analysing the first combined RF signal to determine a measurement of the physiological characteristic of the subject.

Description

A method and apparatus for measuring a physiological characteristic of a subject

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method and apparatus for measuring a physiological characteristic of a subject. BACKGROUND TO THE INVENTION

Unobtrusive continuous vital sign (physiological characteristic) monitoring is highly desired for ambulatory patients at hospital or for people at home. One way to measure vital signs such as heart rate and breathing rate in an unobtrusive way is to measure magnetic induction amplitude and/or phase modulations in the subject's chest. This can be done using an excitation magnetic field that covers the volume of the lung and/or heart of the subject. The magnetic induction (i.e. the generation of eddy currents in the tissue due to the application of an external alternating magnetic field) will be modulated by intra-thoracic fluid movements due to heart beats and breathing. These modulations (referred to herein as 'amplitude and/or phase modulations') can be measured (which are referred to herein as amplitude and/or phase measurements') and used to determine breathing rate, breathing depth, heart rate and/or other physiological characteristics that can be measured from changes in the electrical and/or magnetic properties of the body (e.g. due to fluid movements in the body).

Lower frequency signals (e.g. signals in the megahertz (MHz) range) penetrate more easily (i.e. further) into the body of the subject than higher frequency signals (e.g. signals in the gigahertz (GHz) range) but are less affected by tissue and fluid variations than higher frequency signals. Therefore there is a trade-off in measurement depth and

measurement quality to be made when selecting the frequency of the excitation magnetic field.

A common problem with these measurements for vital sign monitoring, applicable to nearly all, or all suitable frequencies, is the challenging signal-to-noise ratio due to the relatively small contribution of the fluid changes in the body to the measurement signals. As a consequence of a low signal-to-noise ratio, it difficult to measure such physiological characteristic in a robust manner. There is therefore a need for an improved method and apparatus for measuring a physiological characteristic of a subject.

SUMMARY OF THE INVENTION

According to a first aspect, there is provided a method of measuring a physiological characteristic of a subject, the method comprising controlling a first array of transducers to transmit radio frequency, RF, signals into a part of a body of the subject such that the energy of the RF signals is focussed at a measurement position in the part of the body; receiving modulated RF signals from the part of the body of the subject using a second array of transducers,; combining the received modulated RF signals into a first combined RF signal that originates from the measurement position in the part of the body; and analysing the first combined RF signal to determine a measurement of the physiological characteristic of the subject. Thus, the invention improves the signal-to-noise ratio of the contribution of fluid movements in the body in the signal by focussing the signal energy on both the transmit and receive sides. The modulated RF signals were modulated by fluid movements in the part of the body.

The step of controlling the first array of transducers can comprise generating an excitation signal that is for causing a transducer to transmit an RF signal; applying a respective adjustment to the phase and/or amplitude of the excitation signal for each transducer in the first array of transducers; and providing the excitation signal with the respective adjustment to each transducer in the first array of transducers.

The step of combining the received RF signals into a first combined RF signal can comprise applying a respective adjustment to the phase and/or amplitude of the RF signal received by each transducer in the second array of transducers; and combining the adjusted received RF signals to form the first combined RF signal.

In some embodiments, the step of analysing the first combined RF signal comprises measuring changes or modulations in the phase and/or signal strength of the first combined RF signal; and determining the measurement of the physiological characteristic from the measured changes or modulations in the phase and/or signal strength. This analysis enables physiological characteristics that cause changes in the fluid distribution of the part of the body to be measured.

In some embodiments, the step of measuring changes or modulations in the phase and/or signal strength of the first combined RF signal comprises measuring changes or modulations in the phase and/or signal strength of the first combined RF signal relative to an excitation signal used to cause the transducers in the first array of transducers to transmit the RF signals.

In some embodiments, the physiological characteristic is heart rate, breathing rate or breathing depth.

In some embodiments, the method further comprises the step of identifying the measurement position at which the energy of the transmitted RF signals is focussed in the part of the body by combining the received modulated RF signals into a plurality of second combined RF signals that each originate from a respective position in the part of the body; determining a signal characteristic for each of the second combined RF signals; and identifying the measurement position based on the determined signal characteristics. This embodiment enables the receiver side to be adjusted so that it is focussed at (i.e. the combined RF signal appears to originate from) the correct position (the measurement position) in the body, thereby maximising the signal-to-noise ratio of the desired signal.

In some embodiments, the signal characteristic comprises the amplitude of modulations of the second combined RF signal, the frequency of modulations of the second combined RF signal, the maximum modulation of the second combined RF signal, a signal- to-noise ratio of the second combined RF signal, or a peak amplitude of the second combined RF signal.

In some embodiments, the method further comprises the step of identifying the measurement position in the part of the body by (i) controlling the first array of transducers to transmit RF signals into the part of the body such that the energy of the RF signals is focussed at a first position in the part of the body; (ii) receiving modulated RF signals from the part of the body using the second array of transducers; (iii) combining the received modulated RF signals into a third combined RF signal that originates from the first position in the part of the body; (iv) measuring a signal characteristic of the third combined RF signal; (v) determining if the measured signal characteristic meets a criterion; (vi) if the measured signal characteristic meets the criterion, using the first position as the measurement position; (vii) otherwise repeating steps (i)-(v) for one or more further positions in the part of the body until the signal characteristic of the third combined RF signal meets the criterion, whereby said further position is used as the measurement position. This embodiment has the advantage that the part of the body can be 'scanned' to identify a suitable measurement position, e.g. tissue of interest, such as the heart or lungs.

In alternative embodiments, the method further comprises the step of identifying the measurement position in the part of the body by (i) controlling the first array of transducers to transmit RF signals into the part of the body such that the energy of the RF signals is focussed at a first position in the part of the body; (ii) receiving modulated RF signals from the part of the body using the second array of transducers; (iii) combining the received modulated RF signals into a third combined RF signal that originates from the first position in the part of the body; (iv) measuring a signal characteristic of the third combined RF signal; (v) repeating steps (i)-(iv) for one or more further positions in the part of the body; (vi) evaluating the measured signal characteristics to identify the position with the best measured signal characteristic; (vii) using the position with the best measured signal characteristic as the measurement position. This embodiment also has the advantage that the part of the body can be 'scanned' to identify a suitable measurement position, e.g. tissue of interest, such as the heart or lungs.

In the above two embodiments, the signal characteristic can comprise the amplitude of modulations of the third combined RF signal, the frequency of modulations of the third combined RF signal, the maximum modulation of the third combined RF signal, a signal-to-noise ratio of the third combined RF signal, or a peak amplitude of the third combined RF signal.

In the above alternative embodiment, the signal characteristic can comprise the amplitude of modulations of the third combined RF signal, and the measured signal characteristic meets the criterion if the amplitude of modulations in the third combined RF signal exceed a threshold.

In the above alternative embodiment, the signal characteristic can comprise the frequency of modulations of the third combined RF signal, and the measured signal characteristic meets the criterion if the frequency of modulations in the third combined RF signal is within a predetermined frequency range.

In some embodiments, the transducers in the first array of transducers and/or the second array of transducers are magnetic field transducers. In alternative embodiments, the transducers in the first array of transducers and/or the second array of transducers are electric field transducers.

According to a second aspect, there is provided a computer program product comprising a computer readable medium having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform any of the methods described above. In particular, the computer readable code can be configured to cause the computer or processor to control a first array of transducers in an apparatus to transmit RF signals into a part of a body of the subject such that the energy of the RF signals is focussed at a measurement position in the part of the body; receive modulated RF signals using a second array of transducers in the apparatus from the part of the body of the subject; combine the modulated received RF signals into a first combined RF signal that originates from the measurement position in the part of the body; and analyse the first combined RF signal to determine a measurement of the physiological characteristic of the subject. The modulated RF signals were modulated by fluid movements in the part of the body.

According to a third aspect, there is provided an apparatus for measuring a physiological characteristic of a subject, the apparatus comprising a first array of transducers for transmitting radio frequency, RF, signals; a second array of transducers for receiving RF signals; and a control unit configured to control the first array of transducers to transmit RF signals into a part of a body of the subject such that the energy of the RF signals is focussed at a measurement position in the part of the body; receive modulated RF signals using the second array of transducers from the part of the body of the subject; combine the received modulated RF signals into a first combined RF signal that originates from the measurement position in the part of the body; and analyse the first combined RF signal to determine a measurement of the physiological characteristic of the subject. Thus, the invention improves the signal-to-noise ratio of the contribution of fluid movements in the body in the signal by focussing the signal energy on both the transmit and receive sides. The modulated RF signals were modulated by fluid movements in the part of the body.

The control unit can further comprise a signal generator configured to generate an excitation signal that is for causing a transducer to transmit an RF signal; and one or more phase/amplitude control blocks that are configured to apply a respective adjustment to the phase and/or amplitude of the excitation signal for each transducer in the first array of transducers.

The control unit can be configured to combine the received RF signals into a first combined RF signal by applying a respective adjustment to the phase and/or amplitude of the RF signal received by each transducer in the second array of transducers; and combining the adjusted received RF signals to form the first combined RF signal.

In some embodiments, the control unit can be configured to analyse the first combined RF signal by measuring changes or modulations in the phase and/or signal strength of the first combined RF signal; and determine the measurement of the physiological characteristic from the measured changes or modulations in the phase and/or signal strength. This analysis enables physiological characteristics that cause changes in the fluid distribution of the part of the body to be measured.

In some embodiments, the control unit can be configured to measure changes or modulations in the phase and/or signal strength of the first combined RF signal by measuring changes in the phase and/or signal strength of the first combined RF signal relative to an excitation signal used to cause the transducers in the first array of transducers to transmit the RF signals.

In some embodiments, the control unit can be further configured to identify the measurement position at which the energy of the transmitted RF signals is focussed in the part of the body by combining the received modulated RF signals into a plurality of second combined RF signals that each originate from a respective position in the part of the body; determining a signal characteristic for each of the second combined RF signals; and identifying the measurement position based on the determined signal characteristics. This embodiment enables the receiver side to be adjusted so that it is focussed at the correct position (the measurement position) in the body, thereby maximising the signal-to-noise ratio of the desired signal.

In some embodiments, the control unit can be further configured to identify the measurement position in the part of the body by (i) controlling the first array of transducers to transmit RF signals into the part of the body such that the energy of the RF signals is focussed at a first position in the part of the body; (ii) receiving modulated RF signals from the part of the body using the second array of transducers; (iii) combining the received modulated RF signals into a third combined RF signal that originates from the first position in the part of the body; (iv) measuring a signal characteristic of the third combined RF signal; (v) determining if the measured signal characteristic meets a criterion; (vi) if the measured signal characteristic meets the criterion, using the first position as the measurement position; (vii) otherwise repeating steps (i)-(v) for one or more further positions in the part of the body until the signal characteristic of the third combined RF signal meets the criterion, whereby said further position is used as the measurement position. This embodiment has the advantage that the part of the body can be 'scanned' to identify a suitable measurement position, e.g. tissue of interest, such as the heart or lungs.

In alternative embodiments, the control unit can be further configured to identify the measurement position in the part of the body by (i) controlling the first array of transducers to transmit RF signals into the part of the body such that the energy of the RF signals is focussed at a first position in the part of the body; (ii) receiving modulated RF signals from the part of the body using the second array of transducers; (iii) combining the received modulated RF signals into a third combined RF signal that originates from the first position in the part of the body; (iv) measuring a signal characteristic of the third combined RF signal; (v) repeating steps (i)-(iv) for one or more further positions in the part of the body; (vi) evaluating the measured signal characteristics to identify the position with the best measured signal characteristic; (vii) using the position with the best measured signal characteristic as the measurement position. This embodiment also has the advantage that the part of the body can be 'scanned' to identify a suitable measurement position, e.g. tissue of interest, such as the heart or lungs.

In some embodiments, the transducers in the first array of transducers and/or the second array of transducers are magnetic field transducers. In alternative embodiments, the transducers in the first array of transducers and/or the second array of transducers are electric field transducers. 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 is a block diagram of an apparatus according to an embodiment of the invention;

Figure 2 is an illustration of the apparatus of Figure 1 in use;

Figure 3 is a block diagram illustrating the apparatus of Figure 1 in more detail;

Figure 4 is a flow chart illustrating a method of measuring a physiological characteristic of a subject according to the invention;

Figure 5 is a flow chart illustrating a method of determining a measurement position.

Figure 6 is a block diagram illustrating the operations of a control unit or controller according to an embodiment;

Figure 7 shows an exemplary amplitude signal;

Figure 8 shows an exemplary phase signal;

Figure 9 shows an exemplary frequency domain plot; and

Figure 10 is a block diagram illustrating the operations of a control unit or controller according to another embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Briefly, the invention proposes to improve the signal-to-noise ratio of measurements of RF modulations due to fluid movements in the body of a subject using beam- forming techniques, and in particular the invention proposes to use beam- forming techniques on both the transmitting and the receiving sides to improve the measurements of RF modulations due to fluid movements in the body.

An apparatus 2 for measuring a physiological characteristic of a subject according to an embodiment of the invention is shown in Figure 1. The physiological characteristic can be heart rate, breathing rate, breathing depth or any other physiological characteristic that can be measured from changes in the electrical and/or magnetic properties of the body (e.g. changes that are due to fluid movements in the body).

The apparatus 2 is mainly intended for use in the radio frequency (RF) near field, i.e. close to the body, and is not necessarily intended for image forming, but for measurement the changes in electrical and magnetic properties (permittivity, conductivity, etc.) of a part of the body of a subject.

The apparatus 2 comprises a first transducer array 4 that comprises a first plurality of transducers. The transducers in the first transducer array 4 are used to transmit respective radio frequency (RF) signals into part of the body of the subject, and thus each transducer in the first transducer array 4 is suitable for or capable of transmitting an RF signal in response to an applied excitation signal. Each transducer in the first transducer array 4 can be a magnetic field transducer, e.g. an antenna, for example a loop antenna or coil, or an electrical field transducer, e.g. an antenna).

The apparatus 2 also comprises a second transducer array 6 that comprises a second plurality of transducers. The transducers in the second transducer array 6 are for receiving RF signals from the part of the body of the subject, for example RF signals that have passed through the body of the subject, and thus each transducer is suitable for or capable of receiving RF signals. These RF signals will be modulated by fluid movements in the part of the body (e.g. caused by heart beats or breaths), and thus the transducers in the second transducer array 6 received these 'modulated' RF signals. Each transducer in the second transducer array 6 can be a magnetic field transducer, e.g. an antenna, for example a loop antenna or coil, or an electrical field transducer, e.g. an antenna).

The apparatus 2 also comprises a control unit 8 that is coupled to the first transducer array 4 and the second transducer array 6. The control unit 8 controls the operation of the apparatus 2, and specifically controls the transducers in the first transducer array 4 to transmit RF signals (for example by providing or supplying an excitation signal or respective excitation signals), and analyses or processes the RF signals received by the transducers in the second transducer array 6 (the modulated RF signals) to determine a measurement of the desired physiological characteristic.

The control unit 8 can be implemented in numerous ways, with software and/or hardware, to perform the various functions described below. The control unit 8 may comprise one or more microprocessors or digital signal processor (DSPs) that may be programmed using software to perform the required functions and/or to control components of the control unit 8 to effect the required functions. The control unit 8 may be implemented as a combination of dedicated hardware to perform some functions (e.g. amplifiers, preamplifiers, analog-to-digital converters (ADCs) and/or digital-to-analog converters (DACs)) and a processor (e.g., one or more programmed microprocessors, controllers, DSPs and associated circuitry) to perform other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, DSPs, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

In various implementations, the control unit 8 may be associated with or comprise one or more memory units (not shown in Figure 1) such as volatile and non- volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The control unit 8 or associated memory unit can also be used for storing program code that can be executed by a processor in the control unit 8 to perform the method described herein. The memory unit can also be used to store signals received by the second transducer array 6, the results of processing or analysis of the received signals and/or measurements of physiological characteristics determined by the control unit 8.

In accordance with the invention, the control unit 8 controls the transducers in the first transducer array 4 to focus the energy of the transmitted RF signals at a measurement position in a part of the body of the subject (e.g. the chest, or more specifically the heart or lungs), and the control unit 8 controls the second transducer array 6 to focus the sensing of the second transducer array 6 at the measurement position (it will be appreciated that the focussing of the sensing of the second transducer array 6 at the measurement position has the effect of the modulated RF signal as received at the transducers in the second transducer array 6 appearing to originate from the measurement position). As described in more detail below, the control unit 8 adjusts the phase and/or amplitude of the RF signals transmitted by the transducers in the first transducer array 4 to focus the energy of the transmitted RF signals at the measurement position, and the control unit 8 adjusts the phase and/or amplitude of the signals received by the transducers in the second transducer array 6 to focus the second transducer array 6 at the measurement position.

As the RF signals pass through the part of the body of the subject, they are affected by the (time- varying) properties of the tissue or fluid, e.g. heart beats, breathing and fluid or blood movement, and in particular the strength and/or phase of the RF signals are modulated by the fluid movements in the part of the body. The control unit 8 analyses the received modulated RF signals to determine these strength and/or phase

modulations/changes, and determines a measurement of the physiological characteristic from the determined modulations.

Focussing the energy of the transmitted RF signals allows the energy to be targeted at a measurement position where the fluid movements are likely to be strongest (e.g. within the heart or major arteries). This increases the effect of the fluid modulations on the RF signals, and thus increases the signal-to-noise ratio of the strength and/or phase modulations that are analysed to determine the measurement of the physiological

characteristic. Focussing the second transducer array 6 at the measurement position (such that the received modulated RF signal appears to originate from the measurement position) reduces the influence of RF signals that have passed through or scattered from other parts of the body where the fluid modulations may be much weaker (e.g. the area surrounding the heart, or on the edges of the chest/torso) on the received RF signal that is analysed to determine the measurement of the physiological characteristic. This also increases the signal-to-noise ratio of the strength and/or phase modulations that are analysed to determine the measurement of the physiological characteristic.

In some embodiments, the first transducer array 4 is a fixed array in the sense that the transducers in the first transducer array 4 are in a fixed relationship with each other (i.e. the transducers cannot be moved relative to each other). In this embodiment the transducers in the first transducer array 4 can be arranged in a two-dimensional (2D) array. In alternative embodiments, one or more of the transducers in the first transducer array 4 can be freely positioned on or near the body of the subject as desired by a user of the apparatus 2. In this embodiment, the transducers in the first transducer array 4 can be held together by or be part of a flexible material, such as a plaster or fabric.

Likewise, in some embodiments, the second transducer array 6 is a fixed array in the sense that the transducers in the second transducer array 6 are in a fixed relationship with each other (i.e. the transducers cannot be moved relative to each other). In this embodiment the transducers in the second transducer array 6 can be arranged in a two- dimensional (2D) array. In alternative embodiments, one or more of the transducers in the second transducer array 6 can be freely positioned on or near the body of the subject as desired by a user of the apparatus 2. In this embodiment, the transducers in the second transducer array 6 can be held together by or be part of a flexible material, such as a plaster or fabric.

The first transducer array 4 and/or the second transducer array 6 are for use on or near the body of the subject, for example placed on the skin or on or in the clothing of the subject. In these embodiments the first transducer array 4 and/or the second transducer array 6 can be in the form of or part of an on-body sensor, an electronic plaster, or any other type of wearable article (e.g. a chest band, shirt, etc.). In alternative embodiments, the first transducer array 4 and/or the second transducer array 6 can be for use inside the body of the subject, and thus one or both of the first transducer array 4 and the second transducer array 6 can be configured to be implanted into the body, e.g. subcutaneously, or as part of the tip of a catheter) or as an e-pill that can be swallowed by the subject. In other embodiments, one or both of the first transducer array 4 and the second transducer array 6 can be in the form of a hand held unit that can be held close to the body of the subject when a measurement of the physiological characteristic is required.

It will be appreciated that Figure 1 only shows the components required to illustrate this aspect of the invention, and in a practical implementation the apparatus 2 may comprise additional components to those shown (for example a power source, a display for indicating a measurement of a physiological characteristic, and/or a transmitter for communicating a measurement of a physiological characteristic to another device, such as a smart phone, tablet computer, laptop, or desktop computer).

Figure 2 shows an example of how the first transducer array 4 and the second transducer array 6 can be arranged with respect to part of the body 10 of a subject. In the example of Figure 2, the first transducer array 4 and the second transducer array 6 are arranged on opposite sides of the part of the body 10 of the subject, for example on the skin or clothing of the subject, so that RF signals are emitted by the first transducer array 4 into the part of the body 10 and the second transducer array 6 receives the RF signals after they have passed through the part of the body 10.

An exemplary measurement position 12 in the body 10 is shown, and in this example, the measurement position 12 corresponds generally to the position of the heart 14 in the part of the body 10 (and thus the part of the body 10 is the chest). As noted above, the control unit 8 controls the first transducer array 4 to focus the energy of the transmitted RF signals at the measurement position 12 and controls the second transducer array 6 to focus the sensing of the second transducer array 6 at the measurement position 12 (such that the received modulated RF signals appear to originate from the measurement position 12).

It will be appreciated that other arrangements of the first transducer array 4 and the second transducer array 6 are possible. For example both arrays 4, 6 can be located on the same (or generally the same) side of the part of the body 10. In these cases a reflector can be positioned on the opposite side of the part of the body 10 that is configured to reflect RF signals that have passed through a part of the body 10 back towards the second transducer array 6.

It will also be appreciated that the measurement position 12 can be in parts of the body 10 other than the chest, for example the abdomen, the torso or an arm or leg. Figure 3 is a block diagram of an apparatus 2 according to an exemplary embodiment. In Figure 3 the functions of the control unit 8 are represented by various blocks. It will be appreciated that the blocks shown in Figure 3 can be implemented using dedicated hardware within the control unit 8, software modules or firmware, or any combination thereof.

In Figure 3 the first transducer array 4 is shown as comprising a plurality of transducers 16. As noted above, each transducer 16 can be an antenna, for example a loop antenna or coil. The second transducer array 6 is also shown as comprising a plurality of transducers 18. Again, each transducer 18 can be an antenna, for example a loop antenna or coil. It will be appreciated that the first transducer array 4 and the second transducer array 6 can comprise the same or a different number of transducers 16/18.

The control unit 8 comprises a signal generator 20 that is for generating an excitation signal for all of the transducers 16 in the first transducer array 4. The signal generator 20 generates an excitation signal at a particular frequency, for example 400 MHz (although it will be appreciated that other frequencies can be used). The signal generator 20 can be, for example, an oscillator. The excitation signal is supplied or provided to each of the transducers 16 in the first transducer array 4 via respective amplifiers 22. The amplified excitation signal from each amplifier 22 drives or excites the respective transducer 16 to transmit an RF signal corresponding to the amplified excitation signal.

In order to allow the focus of the energy of the transmitted RF signals to be controlled (i.e. in order to form a beam of RF energy), prior to amplification by the amplifier 22, the excitation signal from the signal generator 20 is input to a respective phase/amplitude control block 24 that is provided for each transducer 16. Each phase/amplitude control block 24 selectively applies a phase delay and/or modulates the amplitude of the excitation signal under the control of a controller 26. The controller 26 determines a required amount of phase delay and/or amplitude modulation required for each transmitted RF signal in order to focus the RF signal energy at the required position in the part of the body 10, through constructive interference of the transmitted RF signals. Thus, based on the location of the desired measurement position, the controller 26 controls the phase/amplitude control blocks 24 to apply a respective required amount of phase delay to the excitation signal so as to create a focus point at the desired measurement position where the wave fronts from the transducers 16 all have the same phase. Thus, at the measurement position the amplitude of the RF signals will be at a maximum (i.e. the energy is high). The controller 26 can comprise one or more programmed microprocessors, DSPs or processors and associated circuitry, and may be an application specific integrated circuit (ASIC) or a field-programmable gate array (FPGA).

On the receiving side, the RF signals received by each transducer 18 (which have been modulated by fluid movements in the part of the body) are provided to respective pre-amplifiers 28 that amplify the received modulated RF signals prior to subsequent processing and analysis of the RF signals.

In order to allow the sensing focus of the second transducer array 6 to be directed at the measurement position 12 (i.e. in order to make the second transducer array 6 most sensitive to RF signal energy from the measurement position 12 such that the RF signal energy appears to originate from the measurement position 12)), each received and pre- amplified modulated RF signal is input to a respective phase/amplitude control block 30 that is provided for each transducer 18. Each phase/amplitude control block 30 selectively applies a phase delay and/or modulates the amplitude of the received modulated RF signal under the control of controller 26. The controller 26 determines a required amount of phase delay and/or amplitude modulation required for each received modulated RF signal in order to focus the sensing (i.e. maximise the sensitivity) of the second transducer array 6 at the measurement position 12 in the part of the body 10, through constructive interference of the received modulated RF signals. Thus, based on the location of the desired measurement position, the controller 26 controls the phase/amplitude control blocks 30 to apply a respective required amount of phase delay to each received modulated RF signal so as to create a focus point at the desired measurement position and maximise the RF signal from the measurement position.

The phase/amplitude controlled RF signal from each of the phase/amplitude control blocks 30 are then combined in adder 32 and provided to the controller 26 for analysis.

The controller 26 receives the excitation signal from the signal generator 20 and analyses the combined signal (which can also be referred to as a 'summed' signal) to measure changes or modulations in the signal strength and/or the phase of the combined signal over time (compared to the excitation signal) and determine a measurement of a physiological characteristic from the measured changes in the signal strength and/or the phase. For example the controller 26 can use quadrature demodulation to detect amplitude and phase shifts in the combined signal compared to the excitation signal. The controller 26 can optionally apply filtering to the combined signal before determining the measurement of the physiological characteristic. The physiological characteristic can be the heart rate, the breathing rate, or any other physiological characteristic that can be measured from changes or movements in the fluid in the part of the body of a subject. In some embodiments, the physiological characteristic can be breathing depth (i.e. how deep each breath by the subject is), although it is necessary to know the positions of the first transducer array 4 and the second transducer array 6 relative each other to determine this depth measurement.

The flow chart in Figure 4 illustrates a method of measuring a physiological characteristic of a subject according to an aspect. The method can be performed by the apparatus 2, and specifically the steps in the method can be performed by the control unit 8 in conjunction with the first transducer array 4 and the second transducer array 6.

Thus, in a first step, step 101, the transducers 16 in the first transducer array 4 are controlled to transmit respective RF signals into the part of the body 10 of the subject. The excitation signal provided to each transducer 16 is such that the energy of the RF signals is focussed at a measurement position 12 in the part of the body 10. In some embodiments, step 101 can comprise generating an excitation signal, for example using signal generator 20, applying a respective adjustment to the phase and/or amplitude of the excitation signal for each transducer 16, for example using phase/amplitude control blocks 24, and providing the adjusted excitation signals to each transducer 16.

In the second step, step 103, the transducers 18 in the second transducer array 6 receive modulated RF signals from the part of the body 10.

The modulated RF signals received by the second transducer array 6 are optionally amplified by pre-amplifiers 28, and then combined into a combined RF signal that is focussed at the measurement position 12 (i.e. appears to originate from the measurement position 12) in the part of the body 10 (step 105). Step 105 can comprise applying a respective adjustment to the phase and/or amplitude of the modulated RF signal received by each transducer 18 in the second transducer array 6, and combining (e.g. summing) the adjusted received modulated RF signals to form the combined RF signal, for example using adder 32.

The combined RF signal is then analysed to determine a measurement of a physiological characteristic of the subject (step 107). Step 107 can comprise measuring changes or modulations in the phase and/or signal strength of the combined RF signal and determining the measurement of the physiological characteristic from the measured changes or modulations in the phase and/or signal strength. In particular, the changes or modulations in the phase and/or signal strength of the first combined RF signal can be measured with respect to the excitation signal.

In embodiments where the first transducer array 4 and/or the second transducer array 6 are positioned freely with respect to the part of the body 10 of the subject, it can be useful to know the positions of the first transducer array 4 with respect to the part of the body 10 and the positions of the second transducer array 6 with respect to the part of the body 10 in order to enable effective control/steering of the beam from the first transducer array 4 and effective control of the sensitivity of the second transducer array 6 to the modulated RF signals from the measurement position (i.e. to determine how much phase delay and/or amplitude modulation is to be applied to focus the RF signal energy at the required measurement position). These relative positions can be determined using measurements of RF signals that are transmitted between the transducers 16, 18. These measurements can be analysed using time-of- flight and triangulation techniques to determine the relative positions of the transducers 16, 18 or the transducer arrays 4, 6.

Alternatively, in embodiments where the first transducer array 4 and/or the second transducer array 6 are positioned freely with respect to the part of the body 10 of the subject, the control unit 8 can effect a 'scanning' technique to determine a desired or optimum measurement position 12 in the part of the body 10. This way, it is not necessary to determine the relative positions of the first transducer array 4 and the second transducer array 6 in advance. This scanning technique is described in more detail below. Of course, it will be appreciated that the scanning technique can also be applied when the relative positions of the first transducer array 4 and the second transducer array 6 are known as described above.

It will be appreciated that in order to achieve the dual signal-to-noise ratio benefits provided by the invention (i.e. focussing of the transmitted RF energy and focussing the sensitivity of the receiving array), the modulated RF signals received by the second transducer array 6 need to be combined into a combined RF signal that is focussed at the same position in the part of the body of the subject as the RF signals transmitted from the first transducer array 4. Therefore, in some embodiments, the control unit 8 can carry out a scanning operation with the second transducer array 6 to identify the correct measurement position (i.e. the correct phase and/or amplitude adjustments to focus the sensing at the measurement position such that the received modulated RF signals appear to originate from the measurement position).

The scanning operation can comprise performing step 101 as described above so that RF signals are transmitted into the part of the body 10 of the subject, with the energy of those signals focussed at a measurement position 12. The control unit 8 then determines the correct measurement position by combining the received modulated RF signals into a plurality of different combined RF signals, with each combined RF signal being focussed on (i.e. most sensitive to or originate from) a different position in the part of the body 10. Thus, different phase and/or amplitude adjustments are applied to the modulated RF signals to form each combined RF signal with a different focus position.

The control unit 8 then determines a signal characteristic for each of the plurality of combined RF signals. The signal characteristic can be any one or more of the amplitude of modulations of the combined RF signal, the frequency of modulations of the combined RF signal, the maximum modulation of the combined RF signal, a signal-to-noise ratio, and a peak amplitude of the combined RF signal.

The control unit 8 can then identify the measurement position (and thus the required phase and/or amplitude modulations required to determine a combined RF signal that originates from the measurement position) based on the determined signal

characteristics. In particular, the control unit 8 can identify the measurement position as the position associated with the combined RF signal with the best value for the signal

characteristic (e.g. the highest amplitude of modulations, the frequency modulations being within a defined frequency range (e.g. corresponding to typical heart rates or breathing rates), the highest maximum modulation, the highest signal-to-noise ratio, or the highest peak amplitude).

In further embodiments, in addition to the scanning operation described above for 'locking' the second transducer array 6 on to the measurement position 12, it may be desirable to perform a scanning operation with the first transducer array 4 in order to focus the RF signal energy at an appropriate position in the part of the body 10. For example, where the apparatus 2 is to measure the heart rate, the first transducer array 4 should ideally focus the RF signal energy at the heart 14 (i.e. the measurement position should coincide with the heart 14), since a measurement position outside the heart 14, for example in positions A or B in Figure 2, the effect of the fluid movements from the heart beating on the RF signals will be much less, reducing the signal-to-noise ratio of the measured heart rate.

Thus, prior to measuring the physiological characteristic, the method can further comprise a method of identifying the best measurement position. This method is shown in Figure 5. Thus, in a first step, step 111, the first transducer array 4 is controlled to transmit RF signals into the part of the body 10 such that the energy of the RF signals is focussed at a first position in the part of the body. This first position could be position A as shown on Figure 2. Focussing of the RF energy from the first transducer array 4 is as described above (i.e. through phase and/or amplitude adjustments of the RF signals to be transmitted by each transducer 16).

The second transducer array 6 receives the modulated RF signals from the part of the body 10 (step 113).

The received modulated RF signals are combined into a combined RF signal that is originates from the first position in the part of the body 10 (step 115). This corresponds to focussing of the sensing of the second transducer array 6 at the first position is as described above (i.e. through phase and/or amplitude adjustments of the modulated RF signals received by each transducer 18). It will be appreciated that it may be necessary to perform the receiver-side scanning operation described above to make sure that the combined RF signal is focussed at/originates from the first position.

Next, in step 117, a signal characteristic of the combined RF signal originating from the first position is determined. The signal characteristic can be any one or more of the amplitude of modulations of the combined RF signal, the frequency of modulations of the combined RF signal, a signal-to-noise ratio of the combined RF signal, the maximum modulation of the combined RF signal, or a peak amplitude of the combined RF signal.

The measured signal characteristic is evaluated to determine if it meets a criterion (step 119). The criterion is used to determine if the position is a suitable position to use as the measurement position, and thus the criterion can be a threshold value or acceptable range of values for the signal characteristic. For example the measured signal characteristic can meet the criterion if the amplitude of modulations in the combined RF signal exceeds a threshold. Alternatively, the measured signal characteristic can meet the criterion if the frequency of modulations in the combined RF signal is within a predetermined frequency range (e.g. corresponding to typical heart rates).

If the measured signal characteristic meets the criterion, the first position is used as the measurement position (step 121), and thus the phase and/or amplitude

modulations required to focus the sensing of the second transducer array 6 at the

measurement position are known.

However, if the measured signal characteristic does not meet the criterion (for example as might be the case for position A in Figure 2), then steps 111-119 are repeated for another position in the part of the body 10, until a position is found where the signal characteristic of the combined RF signal meets or exceeds the criterion (e.g. measurement position 12 in Figure 2). This position is then used as the measurement position in the method of Figure 4.

In some embodiments, rather than the criterion being based on the evaluation of a signal characteristic for a single position, signal characteristics can be determined for RF signals focussed at (originating from) a plurality of positions, and the criterion can be to select the measurement position as the position providing the best measured signal characteristic (e.g. the highest signal-to-noise ratio). In these embodiments, steps 111-117 can be repeated for a plurality of positions, and then their signal characteristics evaluated together in step 119 to determine which position provides the best signal characteristic. The position with the best signal characteristic can be, for example, the position with the highest amplitude of modulations of the combined RF signal, the position with the frequency of modulations of the combined RF signal in a predetermined range, the position with the highest signal-to-noise ratio of the combined RF signal, the position with the highest maximum modulation of the combined RF signal, or the position with the highest peak amplitude of the combined RF signal. That position is then used as the measurement position 12.

The method in Figure 5 can also be used if the desired measurement position moves relative to the transducer arrays 4, 6 to refocus the transmitted RF signal energy at the measurement position and to refocus the sensing of the second transducer array 6 at the measurement position. This can be useful where, for example one or both of the transducer arrays 4, 6 are part of an item of clothing and move relative to the part of the body 10, or the location of the measurement position changes (e.g. the organ of interest, e.g. heart, can move in the body of the subject).

Figure 6 is a block diagram illustrating the functions of the control unit 8 or controller 26 in determining the measurement of the physiological characteristic from a combined RF signal according to an embodiment. The combined RF signal is input to an amplitude demodulator 36 and a phase demodulator 38 that perform amplitude demodulation and phase demodulation of the combined RF signal respectively. The amplitude demodulator 36 demodulates the combined RF signal to produce an amplitude (e.g. signal strength) signal. An exemplary amplitude signal is shown in Figure 7. The phase demodulator 38

demodulates the combined RF signal to produce a signal proportional to the phase variation over time. An exemplary phase signal is shown in Figure 8.

The amplitude signal is provided to a signal analysis block 40. The signal analysis block 40 analyses the amplitude (signal strength, e.g. received signal strength indication, RSSI) signal to determine a measurement of the physiological characteristic (e.g. heart rate and/or breathing rate). Typically, the breathing rate is determined from the amplitude signal, and it can be seen in Figure 7 that the amplitude is generally periodic. The heart rate is typically determined from the phase signal.

The phase signal from the phase demodulator 38 is provided to a time integrator 42 and a subtraction block 44. The time integrator 42 integrates the phase signal over time to determine an average phase (e.g. mean phase) for the signal. The average (e.g. mean) phase is used as a reference signal and is provided to the subtraction block 44. The reference signal (i.e. average phase) is subtracted from the phase signal from the phase demodulator 38 to give a signal showing the deviation in phase of the combined RF signal from the average phase.

The phase deviation signal is provided to the signal analysis block 40. The signal analysis block 40 analyses the phase deviation signal to determine a measurement of the physiological characteristic (e.g. heart rate and/or breathing rate).

In some embodiments, the signal analysis block 40 can combine the amplitude and phase signals into a vector vs time signal. This time domain signal can be converted to a frequency domain plot with a Fourier transform. An exemplary frequency domain plot is shown in Figure 9. It can be seen that there are two peaks in the plot, one at 0.1 Hz and the other at about 1.2 Hz. Since the rate of breathing is typically much less than the heart rate, the breathing rate corresponds to the lower peak and the heart rate corresponds to the higher peak. In this example, the breathing rate has a frequency around 0.1 Hz and is therefore around 6 breaths per minute, and the heart rate is about 1.2 Hz (around 72 beats per minute).

It will be appreciated that in some embodiments, the physiological characteristic may be determined from just one of the amplitude changes of the combined RF signal and the phase changes of the combined RF signal. In this case the control unit 8 or controller 26 may only comprise one of the amplitude demodulator 36 and the phase demodulator 38, and the control unit 8 or controller 26 will only include the

components/functionality required for processing the appropriate one of the amplitude and phase.

Figure 10 is a block diagram illustrating the functions of the control unit 8 or controller 26 in determining the measurement of the physiological characteristic from a combined RF signal according to an alternative embodiment. The control unit 8 or controller 26 comprises two processing branches that handle the real and imaginary parts of the signal respectively. The combined RF signal is multiplied with the orthogonal (complex) excitation signal from RF signal generator 20 in multiplier 50 to produce a real output signal and in multiplier 60 to produce an imaginary output signal. The multiplied real signal is low pass filtered by filter 52 and the multiplied imaginary signal is low pass filtered by filter 62. The filter cut-off frequencies can be set to, for example, 100 Hz. The output of the filters 52, 62 are the real and imaginary parts of a complex low frequency vector. This vector has a phase which is a measure for the phase of the combined RF signal with respect to the excitation signal from the RF signal generator. The vector has an amplitude which is proportional to the received vector signal strength. The analogue vector voltages are digitised by respective analog to digital convertors (ADC) 54, 64 and processed by vector signal analysis block 56. The vector signal analysis block 56 extracts the vector from the digitised complex signals by a suitable mathematical analysis, e.g. a Fourier transform.

There is therefore provided an improved method and apparatus for measuring a physiological characteristic of a subject.

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. A single processor or other unit may fulfil the functions of several items recited in the claims. 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. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method of measuring a physiological characteristic of a subject, the method comprising:

controlling a first array of transducers to transmit radio frequency, RF, signals into a part of a body of the subject such that the energy of the RF signals is focussed at a measurement position in the part of the body;

receiving modulated RF signals from the part of the body of the subject using a second array of transducers;

combining the received modulated RF signals into a first combined RF signal that originates from the measurement position in the part of the body; and

analysing the first combined RF signal to determine a measurement of the physiological characteristic of the subject.

2. A method as defined in claim 1, wherein the step of analysing the first combined RF signal comprises:

measuring modulations in the phase and/or signal strength of the first combined RF signal; and

determining the measurement of the physiological characteristic from the measured modulations in the phase and/or signal strength.

3. A method as defined in claim 2, wherein the step of measuring modulations in the phase and/or signal strength of the first combined RF signal comprises measuring modulations in the phase and/or signal strength of the first combined RF signal relative to an excitation signal used to cause the transducers in the first array of transducers to transmit the RF signals.

4. A method as defined in any of claims 1-3, wherein the method further comprises the step of identifying the measurement position at which the energy of the transmitted RF signals is focussed in the part of the body by:

combining the received modulated RF signals into a plurality of second combined RF signals that each originate from a respective position in the part of the body; determining a signal characteristic for each of the second combined RF signals; and

identifying the measurement position based on the determined signal characteristics.

5. A method as defined in claim 4, wherein the signal characteristic comprises the amplitude of modulations of the second combined RF signal, the frequency of modulations of the second combined RF signal, the maximum modulation of the second combined RF signal, a signal-to-noise ratio of the second combined RF signal, or a peak amplitude of the second combined RF signal.

6. A method as defined in any of claims 1-5, wherein the method further comprises the step of identifying the measurement position in the part of the body by:

(i) controlling the first array of transducers to transmit RF signals into the part of the body such that the energy of the RF signals is focussed at a first position in the part of the body;

(ii) receiving modulated RF signals from the part of the body using the second array of transducers;

(iii) combining the received modulated RF signals into a third combined RF signal that originates from the first position in the part of the body;

(iv) measuring a signal characteristic of the third combined RF signal;

(v) determining if the measured signal characteristic meets a criterion;

(vi) if the measured signal characteristic meets the criterion, using the first position as the measurement position;

(vii) otherwise repeating steps (i)-(v) for one or more further positions in the part of the body until the signal characteristic of the third combined RF signal meets the criterion, whereby said further position is used as the measurement position.

7. A method as defined in any of claims 1-5, wherein the method further comprises the step of identifying the measurement position in the part of the body by:

(i) controlling the first array of transducers to transmit RF signals into the part of the body such that the energy of the RF signals is focussed at a first position in the part of the body; (ii) receiving modulated RF signals from the part of the body using the second array of transducers;

(iv) measuring a signal characteristic of the third combined RF signal;

(v) repeating steps (i)-(iv) for one or more further positions in the part of the body;

(vi) evaluating the measured signal characteristics to identify the position with the best measured signal characteristic;

(vii) using the position with the best measured signal characteristic as the measurement position.

8. A computer program product comprising a computer readable medium having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform the method of any of claims 1-7.

9. An apparatus for measuring a physiological characteristic of a subject, the apparatus comprising:

a first array of transducers for transmitting radio frequency, RF, signals;

a second array of transducers for receiving RF signals; and

a control unit configured to:

control the first array of transducers to transmit RF signals into a part of a body of the subject such that the energy of the RF signals is focussed at a measurement position in the part of the body;

receive modulated RF signals using the second array of transducers from the part of the body of the subject;

combine the received modulated RF signals into a first combined RF signal that originates from the measurement position in the part of the body; and

analyse the first combined RF signal to determine a measurement of the physiological characteristic of the subject.

10. An apparatus as defined in claim 9, wherein the control unit is configured to analyse the first combined RF signal by: measuring modulations in the phase and/or signal strength of the first combined RF signal; and

11. An apparatus as defined in claim 10, wherein the control unit is configured to measure modulations in the phase and/or signal strength of the first combined RF signal by measuring modulations in the phase and/or signal strength of the first combined RF signal relative to an excitation signal used to cause the transducers in the first array of transducers to transmit the RF signals.

12. An apparatus as defined in any of claims 9-11, wherein the control unit is further configured to identify the measurement position at which the energy of the transmitted RF signals is focussed in the part of the body by:

combining the received modulated RF signals into a plurality of second combined RF signals that each originate from a respective position in the part of the body;

determining a signal characteristic for each of the second combined RF signals; and

13. An apparatus as defined in claim 12, wherein the signal characteristic comprises the amplitude of modulations of the second combined RF signal, the frequency of modulations of the second combined RF signal, the maximum modulation of the second combined RF signal, a signal-to-noise ratio of the second combined RF signal, or a peak amplitude of the second combined RF signal.

14. An apparatus as defined in any of claims 9-13, wherein the control unit is further configured to identify the measurement position in the part of the body by:

(ii) receiving the modulated RF signals from the part of the body using the second array of transducers; (iii) combining the received modulated RF signals into a third combined RF signal that originates from the first position in the part of the body;

(iv) measuring a signal characteristic of the third combined RF signal;

(v) determining if the measured signal characteristic meets a criterion;

(vii) otherwise repeating (i)-(v) for one or more further positions in the part of the body until the signal characteristic of the third combined RF signal meets the criterion, whereby said further position is used as the measurement position.

15. An apparatus as defined in any of claims 9-13, wherein the control unit is further configured to identify the measurement position in the part of the body by:

(ii) receiving the modulated RF signals from the part of the body using the second array of transducers;

(iv) measuring a signal characteristic of the third combined RF signal;

(v) repeating (i)-(iv) for one or more further positions in the part of the body;