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.

A common problem with these measurements for vital sign monitoring is the challenging signal-to-noise ratio due to the relatively small contribution of the fluid changes in the body to the measurements, and also the requirement for the transmitters and/or receivers to be precisely positioned and/or controlled in order to obtain the measurements. 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 obtaining a set of radio frequency, RF, signal measurements for RF signals transmitted into a part of a body of the subject, wherein a RF signal is transmitted sequentially from each of a plurality of transmitting positions with respect to the body of the subject, and wherein each transmitted RF signal is received at a plurality of receiving positions with respect to the body of the subject, wherein each RF signal measurement is based on a comparison of a phase and/or amplitude of each received RF signal to a phase and/or amplitude of the corresponding transmitted RF signal, wherein the set of RF signal measurements comprises a respective subset of RF signal measurements for each transmitted RF signal, wherein the RF signal measurements in a subset correspond to the RF signal measurements for the RF signals received at each of the plurality of receiving positions; applying, by a control unit, a set of time delays, a set of receiver beam steering parameters and a set of transmitter beam steering parameters to the set of RF signal measurements to form a virtual steered beam, wherein the set of time delays comprises a time delay for each subset, wherein each time delay is based on a timing of the transmission of the respective transmitted RF signal; wherein the set of receiver beam steering parameters comprises respective phase and/or amplitude adjustments to be applied to the RF signal measurements in a subset to effect the focusing of a beam formed from the received RF signals for the subset at a measurement position in the body of the subject, and wherein the set of transmitter beam steering parameters comprises respective phase and/or amplitude adjustments to be applied to the subsets to correspond to the focusing of a beam formed from the transmitted RF signals at a measurement position in the body of the subject; and determining, by a control unit, a measurement of the physiological characteristic of the subject from the virtual steered beam.

In some embodiments, the step of applying he set of time delays, the set of receiver beam steering parameters and the set of transmitter beam steering parameters to the set of RF signal measurements to form the virtual steered beam comprises applying the set of time delays, the set of receiver beam steering parameters and the set of transmitter beam steering parameters to the set of RF signal measurements and summing the RF signal measurements to form the virtual steered beam.

In some embodiments, the step of determining a measurement of the physiological characteristic from the virtual steered beam comprises determining changes in phase and/or signal strength of the virtual steered beam; and determining the measurement of the physiological characteristic from the determined changes in the phase and/or signal strength.

In some embodiments, the method further comprises the step of determining the set of time delays, set of receiver beam steering parameters and set of transmitter beam steering parameters such that the virtual steered beam is focussed at the measurement position.

In these embodiments, the step of determining the set of time delays, the set of receiver beam steering parameters and the set of transmitter beam steering parameters can comprise: (i) determining a first set of time delays, receiver beam steering parameters and transmitter beam steering parameters for a first measurement position; (ii) applying the first set of time delays, receiver beam steering parameters and transmitter beam steering parameters to the set of RF signal measurements to form a first virtual steered beam; (iii) determining a signal characteristic for the first virtual steered beam; and (iv) using the first set of time delays, a first set of receiver beam steering parameters and a first set of transmitter beam steering parameters to determine the measurement of the physiological characteristic if the determined signal characteristic meets a criterion.

In these embodiments, the method can further comprise (v) determining a second set of time delays, receiver beam steering parameters and transmitter beam steering parameters for a second measurement position if the determined signal characteristic does not meet the criterion; and repeating steps (ii)-(v) for the second set of time delays, receiver beam steering parameters and transmitter beam steering parameters.

In these embodiments, the signal characteristic comprises the amplitude of modulations of the virtual steered beam, the frequency of modulations of the virtual steered beam, the maximum modulation of the virtual steered beam, a signal-to-noise ratio of the virtual steered beam, or a peak amplitude of the virtual steered beam.

In some embodiments, the step of obtaining comprises using a first transducer array comprising one or more transducers to transmit RF signals into the part of the body of the subject; and using a second transducer array comprising one or more transducers to receive RF signals at respective receiving positions. In some embodiments, the control unit can be configured to obtain the set of RF signal measurements by receiving the received RF signals from the second transducer array; receiving the transmitted RF signals from a signal generator for the first transducer array; and comparing a phase and/or amplitude of each received RF signal to a phase and/or amplitude of the corresponding transmitted RF signal to determine each RF signal measurement.

In alternative embodiments, the step of obtaining comprises obtaining the RF signal measurements from a memory unit.

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 above.

According to a third aspect, there is provided an apparatus for measuring a physiological characteristic of a subject, the apparatus comprising a control unit configured to obtain a set of radio frequency, RF, signal measurements for RF signals transmitted into a part of a body of the subject, wherein a RF signal is transmitted sequentially from each of a plurality of transmitting positions with respect to the body of the subject, and wherein each transmitted RF signal is received at a plurality of receiving positions with respect to the body of the subject, wherein each RF signal measurement is based on a comparison of a phase and/or amplitude of each received RF signal to a phase and/or amplitude of the corresponding transmitted RF signal, wherein the set of RF signal measurements comprises a respective subset of RF signal measurements for each transmitted RF signal, wherein the RF signal measurements in a subset correspond to the RF signal measurements for the RF signals received at each of the plurality of receiving positions; apply a set of time delays, a set of receiver beam steering parameters and a set of transmitter beam steering parameters to the set of RF signal measurements to form a virtual steered beam, wherein the set of time delays comprises a time delay for each subset, wherein each time delay is based on a timing of the transmission of the respective transmitted RF signal, wherein the set of receiver beam steering parameters comprises respective phase and/or amplitude adjustments to be applied to the RF signal measurements in a subset to effect the focusing of a beam formed from the received RF signals for the subset at a measurement position in the body of the subject, and wherein the set of transmitter beam steering parameters comprises respective phase and/or amplitude adjustments to be applied to the subsets to correspond to the focusing of a beam formed from the transmitted RF signals at a measurement position in the body of the subject; and determine a measurement of the physiological characteristic of the subject from the virtual steered beam.

In some embodiments, the control unit is configured to apply the set of time delays, the set of receiver beam steering parameters and the set of transmitter beam steering parameters to the set of RF signal measurements and sum the RF signal measurements to form the virtual steered beam.

In some embodiments, the control unit is configured to determine the measurement of the physiological characteristic from the virtual steered beam by determining changes in phase and/or signal strength of the virtual steered beam; and determining the measurement of the physiological characteristic from the determined changes in the phase and/or signal strength.

In some embodiments, the control unit is further configured to determine the set of time delays, set of receiver beam steering parameters and set of transmitter beam steering parameters such that the virtual steered beam is focussed at the measurement position.

In these embodiments, the control unit can be configured to determine the set of time delays, the set of receiver beam steering parameters and the set of transmitter beam steering parameters by (i) determining a first set of time delays, receiver beam steering parameters and transmitter beam steering parameters for a first measurement position; (ii) applying the first set of time delays, receiver beam steering parameters and transmitter beam steering parameters to the set of RF signal measurements to form a first virtual steered beam; (iii) determining a signal characteristic for the first virtual steered beam; and (iv) using the first set of time delays, a first set of receiver beam steering parameters and a first set of transmitter beam steering parameters to determine the measurement of the physiological characteristic if the determined signal characteristic meets a criterion.

In these embodiments, the control unit can be further configured to determine the set of time delays, the set of receiver beam steering parameters and the set of transmitter beam steering parameters by (v) determining a second set of time delays, receiver beam steering parameters and transmitter beam steering parameters for a second measurement position if the determined signal characteristic does not meet the criterion; and repeating (ii)- (v) for the second set of time delays, receiver beam steering parameters and transmitter beam steering parameters.

In these embodiments, the signal characteristic can comprise the amplitude of modulations of the virtual steered beam, the frequency of modulations of the virtual steered beam, the maximum modulation of the virtual steered beam, a signal-to-noise ratio of the virtual steered beam, or a peak amplitude of the virtual steered beam.

In some embodiments, the apparatus further comprises a first transducer array comprising one or more transducers for transmitting RF signals into the part of the body of the subject; and a second transducer array comprising one or more transducers for receiving RF signals at respective receiving positions.

In these embodiments, the control unit is configured to obtain the set of RF signal measurements by receiving the received RF signals from the second transducer array; receiving the transmitted RF signals from a signal generator for the first transducer array; and comparing a phase and/or amplitude of each received RF signal to a phase and/or amplitude of the corresponding transmitted RF signal to determine each RF signal measurement.

In alternative embodiments, the control unit is configured to obtain the set of RF signal measurements from a memory unit.

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 block diagram illustrating the operations of a receiver module according to an embodiment;

Figure 5 is a flow chart illustrating a method of measuring a physiological characteristic of a subject according to the invention;

Figure 6 illustrates the set of RF signal measurements obtained following step 101 of Figure 5 for four transmitting transducers and four receiving transducers;

Figure 7 illustrates the formation of the virtual steering beam according to an embodiment of step 103 of Figure 5;

Figure 8 illustrates the formation of the virtual steering beam according to another embodiment of step 103 of Figure 5;

Figure 9 shows an exemplary amplitude signal;

Figure 10 shows an exemplary phase signal;

Figure 11 shows an exemplary frequency domain plot; and

Figure 12 is a flow chart illustrating a method of determining a measurement position.

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, and also improve the flexibility of the positioning of the receiving and transmitting transducers on or near the body of the subject, by using virtual beam-forming techniques, which allows multiple signals that pass through the body to be measured and combined in order to create or reconstruct 'virtual' beams focussed from or at a particular position in the body. In particular the invention proposes to construct a virtual steered beam that has been steered 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 one or more transducers. The transducer or 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 the transducer or 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. The transducer or 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 also comprises one or more transducers. The transducer or transducers in the second transducer array 6 are for receiving RF signals that have passed through the body of the subject, and thus the transducer or each transducer is suitable for or capable of receiving RF signals. The transducer or 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 transducer or transducers in the second transducer array 6 are different to the transducer or transducers in the first transducer array 4, i.e. no transducer is shared between the first transducer array 4 and the second transducer array 6. The first transducer array 4 can comprise the same or a different number of transducers to the second transducer array 4. In some embodiments, the first transducer array 4 comprises a single transducer, and the second transducer array 6 comprises a plurality of transducers. In other embodiments, the first transducer array 4 comprises a plurality of transducers, and the second transducer array 6 comprises a single transducer. In other embodiments, the first transducer array 4 and the second transducer array 6 both comprise a plurality of transducers.

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 transducer or 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 captures and analyses or processes the RF signals received by the transducer or transducers in the second transducer array 6 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.

The control unit 8 controls the transducer or transducers in the first transducer array 4 to transmitted RF signals through a part of the body of the subject, and specifically through at least a measurement position that corresponds to a part of the body of interest, for example the chest, or more specifically the heart or lungs, and the control unit 8 analyses or processes the received RF signals to determine a virtual beam that is focussed 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. Generally, the control unit 8 analyses the received RF signals to determine these strength and/or phase modulations, and determines a measurement of the physiological characteristic from the determined modulations. In some embodiments, if the relative positions of the first transducer array 4 and the second transducer array 6 are known (and more specifically the position of each transducer in the first transducer array 4 relative to the position of each transducer in the second transducer array 6 is known), then it can also be possible to determine a measurement of a physiological characteristic that includes a spatial element, for example breathing depth.

Virtual focussing of the transmitted RF signals as a beam allows the energy of the RF signals to be virtual 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. Virtual focussing of the received RF signals at 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 embodiments where the first transducer array 4 comprises a plurality of transducers, the first transducer array 4 can be a fixed array in the sense that the transducers 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. Alternatively, the transducers in the first transducer array 4 can be freely positioned with respect to each other 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 may be held together by or be part of a flexible material, such as a plaster or fabric, or they may be completely independent of each other (e.g. individually attached to the controller 8 via wires).

Likewise, in embodiments where the second transducer array 6 comprises a plurality of transducers, the second transducer array 6 can be 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. Alternatively, the transducers in the second transducer array 6 can be freely positioned with respect to each other 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, or they can be completely independent of each other (e.g. individually attached to the controller 8 via wires).

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). The transducer or transducers in the first transducer array 4 generally transmit RF signals omni-directionally, but as noted above, the control unit 8 processes the received RF signals to apply transmitter beam steering that would have focussed the transmitted signals at the measurement position 12. The transducer or transducers in the second transducer array 6 likewise receives RF signals from all directions, but the control unit 8 processes the received signals to determine a virtual receiver beam focussed at 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 the 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, but as noted above in some embodiments the first transducer array 4 can comprise a single transducer 16. Also 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, but as noted above in some embodiments the second transducer array 6 can comprise a single transducer 18. Again, each transducer 18 can be an antenna, for example a loop antenna or coil. Also as noted above, 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 the transducer 16 or all 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, or the signal generator 20 that generates a signal that is frequency, phase and/or amplitude modulated. 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.

The RF signal generator 20 operates under the control of a controller 26, for example the controller 26 can control when the RF signal generator 20 operates and/or control the frequency of the signal generated by the RF signal generator 20. The controller 26 also controls which of the transducers 16 in the first transducer array 4 are transmitting at any given time. In particular, the controller 26 controls the transducers 16 so that only one transducer 16 transmits RF signals at a time. In this example, the controller 26 can control the transducers 16 to transmit RF signals in sequence. As noted below, the frequency with which the controller 26 switches between the transducers 16 is preferably greater than the frequency of movements of the part of the body that the physiological characteristic is being measured from, or frequency of movements of the transducers 16/18 relative to the body.

Thus, for example, the frequency of switching is preferably greater than a possible heart rate or breathing rate (e.g. greater than 5 Hz). The control unit 8 can comprise one or more switches that can be controlled by the controller 26 to connect one or more transducers 16 to the RF signal generator 20 as required.

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 are provided to (optional) respective pre-amplifiers 28 that amplify the received RF signals prior to subsequent processing of the RF signals by respective receiver modules 30 that compare the phase and amplitude of the received RF signals to the phase and amplitude of the transmitted RF signal. The outputs of the receiver modules 30 are complex valued vector streams that are provided to the controller 26 for further processing and analysis. These complex valued vector streams are referred to a "RF signal measurements" herein that represent the result of the comparison between the phase and amplitude of the received RF signals and the phase and amplitude of the transmitted RF signals (e.g. as represented by the signal from the signal generator 20).

Figure 4 is a block diagram illustrating the functions of a receiver module 30 in determining the RF signal measurement from each individual receiving transducer 18 according to an embodiment. The receiver module 30 comprises two processing branches that handle the real and imaginary parts of the signal respectively. The received RF signal is multiplied with the real part of the orthogonal (complex) reference signal (which is a phase reference for the transmitted RF signals) from the RF signal generator 20 in multiplier 50 to produce a real output signal and multiplied with the imaginary part of the complex reference signal (which is also a phase reference for the transmitted RF signals) 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 part of a complex low frequency vector. This vector has a phase which is a measure of the phase of the received signal with respect to the phase of the reference signal from the RF signal generator 20. The vector has an amplitude which is proportional to the signal strength of the received signal. The analogue vector voltages are digitised by respective analog to digital converters (ADCs) 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.

If the signals transmitted by the transducers 16 are modulated in frequency, phase and/or amplitude, the vector signal analysis can be expanded by using the benefits of the modulation of the transmitted signal.

The details of the operation of the vector signal analysis block 56 and subsequent processing by the controller 26 are set out in the mathematical description of the invention provided below.

As described in more detail below with reference to the flow chart in Figure 5, the controller 26 combines the outputs of the receiver modules 30 to form a virtual steered beam that is focussed at a sensing position in the part of the body, and the controller 26 analyses the virtual steered beam to measure changes in the signal strength and/or the phase of the virtual steered beam over time and determines a measurement of a physiological characteristic from the measured changes in the signal strength and/or the phase. In some embodiments, the controller 26 can use quadrature demodulation to detect amplitude and phase shifts in the virtual steered beam. The controller 26 can optionally apply filtering to the virtual steered beam 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 in terms of the volume of inhaled air), although it is necessary to know the positions of the transducers 16 in the first transducer array 4 relative to the positions of the transducers 18 in the second transducer array 6 to determine this depth measurement.

The flow chart in Figure 5 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. The method in Figure 4 is applicable to a first transducer array 4 that comprises one or more transducers 16 and a second transducer array 6 that comprises one or more transducers 18.

The method in Figure 5 is also described with reference to the specific embodiment shown in Figures 6-8 in which the second transducer array 6 comprises four transducers 18 located at respective receiving positions with respect to the part of the body 10, with the transducers 18 being labelled RX1, RX2, RX3 and RX4 respectively. Each receiving position can be a respective position on or near the skin or clothing of the subject. The first transducer array 4 can comprise a single transducer 16 that is moved between different transmitting positions (labelled TXl, TX2, TX3 and TX4 respectively) with respect to the part of the body 10 or a plurality of transducers 16 that are located in respective transmitting positions (which can also be labelled TXl, TX2, TX3 and TX4 respectively) with respect to the part of the body 10 and that are controlled to transmit individually.

In a first step, step 101, the control unit 8 obtains a set of RF signal measurements for RF signals transmitted into a part of the body 10 of the subject. Figure 6 shows the set of RF signal measurements 70 for RF signals transmitted from the four different transmitting positions TXl, TX2, TX3 and TX4 at respectively different times Tl, T2, T3 and T4 and received at four different receiving positions RXl, RX2, RX3 and RX4.

Each RF signal measurement in the set 70 corresponds to the output of a receiver module 30 for an RF signal transmitted from a transmitting position (TXl, TX2, TX3 or TX4) and received at a receiving position (RXl, RX2, RX3 or RX4). Thus, in the example of Figure 6, the set of RF signal measurements 70 comprises 16 RF signal measurements (i.e. there are four RF signal measurements per transmitting position and four transmitting positions).

The set of RF signal measurements that are obtained in step 101 can be formed as follows. An RF signal can be transmitted into the part of the body 10 from a first transmitting position (e.g. TXl) with respect to the part of the body 10 of the subject. This transmission will occur at a particular time, which is labelled Tl in Figure 6. Thus, a transducer 16 (or the transducer 16 in the case that the first transducer array 4 comprises a single transducer 16) that is in a first position (TXl) with respect to the part of the body 10 (e.g. on or near the skin or clothing) receives an excitation signal from the RF signal generator 20 and transmits an RF signal.

The transducers 18 in the second transducer array 6 receive the RF signal transmitted from the first transmitting position TXl after it has passed through the part of the body 10. As noted above, as the RF signal passes through the part of the body 10, the phase and/or signal strength of the RF signal will be modulated by the properties of the medium (e.g. by the fluid content and fluid movement). The RF signal is then received by the transducers 18 at receiving positions RXl, RX2, RX3 and RX4.

The received RF signals are provided to the respective receiver modules 30 (labelled Receiver 1, Receiver2, Receiver3, Receiver4 in Figure 6), which compare the phase and amplitude of the respective received RF signal to the amplitude and phase of the transmitted RF signal (or more specifically the signal produced by the signal generator 20). This comparison (optionally following pre-amplifying of the received RF signal by preamplifier 28) results in an RF signal measurement for each receiving position. The RF signal measurement can be in the form of a complex vector stream that has a lower frequency than the received RF signal. Each vector in the vector stream has the absolute amplitude of the received RF signal and a phase compared to the phase of the signal from the signal generator 20. The transmitted RF signals have a known phase relationship to the phase of the signal generator 20. The amplitude of the signal output by the signal generator 20 is constant, and so in practice this can be normalised.

The RF signal measurements for time period Tl in which an RF signal is transmitted from position TXl and received at receiving positions RXl, RX2, RX3 and RX4 are shown in Figure 6 and are considered as a subset 72 of the set of RF signal measurements 70 for position TXl .

The above is repeated for at least a second transmitting position TX2 (that is a different transmitting position to the first transmitting position TXl) during a subsequent time period, T2. That is, an RF signal is transmitted from a transducer 16 at a second transmitting position TX2 with respect to the part of the body 10 during a time period T2. Depending on the configuration of the first transducer array 4, this can occur after a user has moved the single transducer 16 from TX1 to TX2, or activating/using a different one of the transducers 16 in the first transducer array 4 to transmit an RF signal). The receiving transducers 18 in the second transducer array 6 receive the RF signal from the second transmitting position after it has passed through the part of the body 10.

This is repeated for any number of transmitting positions in respective time periods, as required (e.g. for four different transmitting positions in the example of Figure 6). Each transmitting position (and thus each time period) produces a respective subset of RF signal measurements, labelled subset 74, subset 76 and subset 78 for transmitting positions TX2, TX3 and TX4 respectively.

It will be appreciated that in some embodiments step 101 can comprise obtaining the RF signal measurements by transmitting the RF signals from the first transducer array 4 and receiving the RF signals at the second transducer array 6 and processing the received RF signals in the receiver modules 30 associated with each receiving transducer 6. This embodiment can be used where the measurement of the physiological characteristic is required in real-time or near-real-time. In alternative embodiments, step 101 can comprise the control unit 8 obtaining a set of RF signal measurements from a memory unit. This set of RF signal measurements may have been obtained previously, and thus this embodiment can be used where real-time measurements of the physiological characteristic are not required.

After the set of RF signal measurements 70 are obtained in step 101, the control unit 8 or controller 26 processes the set 70 to form a virtual steered beam that has been steered on both the transmitter and receiver sides. In particular, the control unit 8 (or controller 26) applies beam steering parameters and time delays to the RF signal

measurements to form the virtual steered beam (step 103).

In particular, the control unit 8 applies two sets of beam steering parameters (referred to as the "receiver beam steering parameters" and the "transmitter beam steering parameters") and time delays, and combines the adjusted RF signal measurements into a virtual steered beam that has been virtual steered/focussed on the transmitter side and that has been focussed on the receiver side.

The sets of beam steering parameters comprise phase and/or amplitude adjustments that can be applied to a plurality of RF signal measurements in order to effect the steering or focussing of a beam formed from those RF signal measurements at a measurement position 12 in the body 10. Ways in which phase and amplitude adjustments can be applied to RF signals or RF signal measurements are known to those skilled in the art (e.g. by multiplication), and thus details are not provided herein.

Thus the set of receiver beam steering parameters comprise respective phase and/or amplitude adjustments for the RF signals received at each of the receiving positions that effect the steering or focussing of the received RF signals at a sensing position in the body 10 of the subject (e.g. measurement position 12).

The set of transmitter beam steering parameters comprise respective phase and/or amplitude adjustments for the RF signals transmitted from the different transmitting positions that, if they had been applied to the transmitted RF signals (i.e. before transmission by the first transducer array 4), would have effected the steering of the transmitted RF signals from the different transmitting positions into a beam that is focussed at the measurement position 12.

Since the subsets of RF signal measurements (e.g. subsets 72, 74, 76, 78) are obtained for different time periods (due to the time sequential transmission of the RF signals from the different transmitting positions), the subsets of RF signal measurements are not aligned in time, and thus the set of time delays are applied in order to shift the subsets of measurements in time so that they are aligned.

Figures 7 and 8 illustrate two embodiments of the application of the sets of beam steering parameters and time delays to the set of RF signal measurements 70 to form a virtual steered beam.

In both Figures 7 and 8, the set of receiver beam steering parameters 80 comprises phase and/or amplitude adjustments for each of the four receiving positions RX1, RX2, RX3 and RX4, which are shown as Amp/Phase RX1 , Amp/Phase RX2, Amp/Phase RX3, and Amp/Phase RX4 respectively. The set of transmitter beam steering parameters 82 parameters comprises phase and/or amplitude adjustments for each of the four transmitting positions TX1, TX2, TX3 and TX4, which are shown as Amp/Phase TX1, Amp/Phase TX2, Amp/Phase TX3, and Amp/Phase TX4 respectively. The set of time delays 84 comprises a respective time delay for each of the four transmitting periods Tl, T2, T3 and T4 (which correspond to the four transmitting positions TX1, TX2, TX3 and TX4), which are shown as Delay 1 , Delay 2, Delay 3 and Delay 4 respectively.

In Figure 7, the control unit 8 applies the set of receiver beam steering parameters 80 to each of the subsets of RF signal measurements. Thus, for example, in the subset of RF signal measurements 72 that result from the transmission from the first transmitting position TX1, the receiver beam steering parameters Amp/Phase RXl are applied to the RF signal measurement for the first receiving position RXl, the receiver beam steering parameters Amp/Phase RX2 are applied to the RF signal measurement for the second receiving position RX2, and so on. In the case of the second transmitting position TX2, the receiver beam steering parameters Amp/Phase RXl are applied to the RF signal measurement from TX2 for the first receiving position RXl, the receiver beam steering parameters Amp/Phase RX2 are applied to the RF signal measurement from TX2 for the second receiving position RX2, and so on.

The resulting adjusted RF signal measurements for each subset 72, 74, 76, 78 are summed (as shown by add blocks 86 in Figure 7) to form a respective receiver-steered beam for each subset/transmitting position.

Next, the set of time delays 84 are applied to the receiver-steered beams in order to align the receiver steered beams in time. Thus, for example, for the receiver-steered beam for the transmissions from the first transmitting position, TX1, the time delay Delay 1 is applied to the receiver-steered beam, for the receiver-steered beam for the transmissions from the second transmitting position, TX2, the time delay Delay 2 is applied to the receiver- steered beam, and so on.

The set of transmitter beam steering parameters 82 are then applied to the time-delayed receiver-steered beams to effect the virtual steering or focussing of the transmitted RF signals at the measurement position 20. Thus, for example, for the time- delayed receiver- steered beam for the first transmitting position TX1, the transmitter beam steering parameters Amp/Phase TX1 are applied to that time-delayed receiver- steered beam, for the time-delayed receiver-steered beam for the second transmitting position TX2, the transmitter beam steering parameters Amp/Phase TX2 are applied to that time-delayed receiver-steered beam, and so on.

Following the application of the set of transmitter beam steering parameters 82 the resulting signals are summed (as shown by add block 88) to form the virtual steered beam (which has been steered on both the transmitter and receiver sides).

As noted above, Figure 8 shows an alternative embodiment of the application of the sets of beam steering parameters 80, 82 and set of time delays 84. In this embodiment, the add blocks 86 are omitted, and the set of time delays 84 and set of transmitter beam steering parameters 82 are applied to each of the receiver-beam-steering-adjusted RF signal measurements. The final add block 90 sums all of the individual adjusted RF signal measurements to form the virtual steered beam.

It will be appreciated that the embodiments shown in Figures 7 and 8 are mathematically equivalent and thus the virtual steered beams produced by Figures 7 and 8 are identical. Those skilled in the art will also appreciated that as the application of the sets of beam steering parameters 80, 82 are mathematical operations, the order in which they are applied to the RF signal measurements can be altered from that shown in Figures 7 and 8 without affecting the outcome (i.e. without changing the virtual steered beam). For example the order in which the set of time delays 84 and the sets of beam steering parameters 80, 82 are applied can be altered from that shown in Figures 7 and 8, e.g. by applying the set of time delays 84 before any sets of beam steering parameters 80, 82 are applied, or applying the set of time delays 84 after both sets of beam steering parameters 80, 82 have been applied.

Likewise, the order in which the set of receiver beam steering parameters 80 and the transmitter beam steering parameters 82 are applied can be reversed.

Finally, in step 105, a measurement of a physiological characteristic of the subject is determined from the virtual steered beam. In some embodiments, this step can comprise analysing changes in the amplitude of the virtual steered beam over time and/or analysing changes in the phase of the virtual steered beam over time. The changes in amplitude and/or phase can be caused by movements of fluid in the body 10 of the subject, for example caused by heart beats or breathing, and thus analysis of these amplitude and/or phase changes can be used to determine a measurement of a physiological characteristic (e.g. heart rate, breathing rate, heart rate variability, etc.).

Figure 9 is a plot showing the changes in amplitude over time of an exemplary virtual steered beam according to the invention. It can be seen that the amplitude is generally periodic and corresponds to the breathing rate of the subject, and thus the breathing rate, or other breathing related parameters, can be determined from the change in amplitude of the virtual steered beam over time.

Figure 10 is a plot showing the changes in phase over time of an exemplary virtual steered beam according to the invention. The changes in phase over time can be analysed, for example in the frequency domain using Fourier analysis, as shown in Figure 11, and the dominant frequencies will correspond to one or more physiological characteristics. It can be seen that in Figure 11 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).

As noted above, the method in Figure 5 can be applied to a second transducer array 6 that comprises a single receiving transducer 18. In this case, the set of RF signal measurements 70 can be obtained by, for each transmitting position, the user moving the single receiving transducer 18 to a number of different receiving positions and receiving the RF signal from the transmitting transducer 16. This produces similar measurements for each transmitting position as those shown in Figure 6, except that the received RF signals (e.g. RX1, RX2, RX3 and RX4) are themselves separated in time. In this case respective delays can be added to the RF signal measurements such that each of the RF signal measurements from a particular transmitting position are aligned in time with each other. As noted in more detail below in the mathematical description of the method, the time shift of the RF signal measurements is required to fit the different RF signal measurements on to the same rhythm of the physiological characteristic being measured (e.g. a time shift is required to fit the RF signal measurements on to the same rhythm of a stationary heart beat in order to take measurements at the same phase of the heart contraction/expansion).

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 transducers 16 in the first transducer array 4 with respect to the part of the body 10 and the positions of the transducers 18 in the second transducer array 6 with respect to the part of the body 10 in order to enable effective control/steering of the RF signal measurements to form the virtual steered beam (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 analysis of RF signals that are transmitted between the transducers 16, 18. These signals can be analysed using time-of- flight and/or triangulation techniques to determine the relative positions of the transducers 16, 18 or the transducer arrays 4, 6.

In some embodiments, the location of an optimum sensing position can be determined by the controller 26 through an evaluation of the received RF signals. Once the optimum sensing position is found, the received RF signals can be evaluated for that position using the method in Figure 5 to determine the measurement of the physiological

characteristic. Briefly, the controller 26 evaluates the individual received RF signals for each receiving transducer 18 received from each transmitting position in order to determine the properties of the signal path from each transmitting position to each receiving position. This evaluation will provide a raw indication of the properties of the part of the body between the first transducer array 4 and the second transducer array 6. Next, multiple virtual steered beams can be formed that are focussed at respectively different positions in the part of the body, and these beams evaluated to find the position at which the parameters for signal transfer are optimised. The beam steering parameters (e.g. phase adjustment and/or amplitude adjustment required to direct the beams at this position) for the 'best' virtual steered beam can be used in step 103 to enable a measurement of a physiological

characteristic to be obtained in step 105.

The flow chart in Figure 12 illustrates a method of determining the location of an optimum sensing position according to an embodiment. In particular, after obtaining the RF signal measurements in step 101, and prior to performing steps 103 and 105 of Figure 5, the method can comprise steps that identify the best (or at least a suitable) measurement position.

Thus, after step 101, the method can comprise steps 121-131 that aim to determine a set of time delays, a set of receiver beam steering parameters and a set of transmitter beam steering parameters for focussing a virtual steered beam at the best or a suitable measurement position. Thus, in step 121, a first set of time delays, a first set of receiver beam steering parameters and a first set of transmitter beam steering parameters for focussing a virtual steered beam at a first measurement position are determined. It will be appreciated that the position of the first position in the part of the body can be arbitrary, for example it can correspond to a measurement position obtained using arbitrary phase and/or amplitude adjustments (in other words the parameters can be determined without regard to where the measurement position might be).

Next, in step 123, the determined time delays and beam steering parameters are applied to the obtained set of RF signal measurements 70 to form a virtual steered beam that is focussed at the first measurement position. Step 123 corresponds in operation to step 103 in Figure 5.

Next, in step 125, a signal characteristic is measured from the virtual steered beam. The signal characteristic can be any one or more of the amplitude of modulations of the virtual steered beam, the frequency of modulations of the virtual steered beam, a signal- to-noise ratio of the virtual steered beam, the maximum modulation of the virtual steered beam, or a peak amplitude of the virtual steered beam. Where the transmitted signal is frequency, phase and/or amplitude modulated, the signal characteristic can be the difference in the modulation between the transmitted RF signals and the virtual steered beam. The measured signal characteristic is evaluated to determine if it meets a criterion (step 127). 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 virtual steered beam exceeds a threshold. Alternatively, the measured signal characteristic can meet the criterion if the frequency of modulations in the virtual steered beam 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 129), and thus the set of time delays and the sets of beam steering parameters determined in step 121 (i.e. the phase and/or amplitude

modulations required to focus the virtual steered beam at the first measurement position) are used in step 103 of Figure 5 to determine the measurement of the physiological characteristic.

However, if the measured signal characteristic does not meet the criterion (for example if the first position is not within the heart or lungs, then the set of time delays, set of receiver beam steering parameters and set of transmitter beam steering parameters are modified such that a virtual steered beam formed therefrom is focussed at a different (second) measurement position or the focus is otherwise altered or adjusted (step 131). Thus, step 131 can comprise adjusting any of the parameters (i.e. phase and/or amplitude adjustments) in either or both of the set of the receiver beam steering parameters or transmitter beam steering parameters. It will be appreciated that the adjustments in step 131 can change the

measurement position and/or improve the focus of the resulting virtual steered beam on the transmitter and/or receiver side. It will also be appreciated that the set of time delays may not need to be modified in this step, since the time delay between the transmission of each of the RF signals will be known.

The method then returns to step 123 and the modified set of time delays and beam steering parameters are applied to the set of RF signal measurements 70 to determine a virtual steered beam focussed at the second measurement position. The method then repeats from step 123 until a set of parameters (and thus a measurement position) are determined for which the signal characteristic of the virtual steered beam meets the criterion.

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 virtual steered beams focussed at a plurality of positions, and the criterion can be to select the measurement position (or associated parameters) as the position providing the best measured signal characteristic (e.g. the highest signal-to-noise ratio). In these embodiments, steps 123- 131 can be repeated for a plurality of positions, and then their signal characteristics evaluated together in step 127 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 virtual steered beam, the position with the frequency of modulations of the virtual steered beam in a predetermined range, the position with the highest signal-to-noise ratio of the virtual steered beam, the position with the highest maximum modulation of the virtual steered beam, or the position with the highest peak amplitude of the virtual steered beam. That position is then used as the measurement position 12.

The method in Figure 12 can also be used if the part of the body moves relative to the transducer arrays 4, 6 to determine the beam steering required to refocus the virtual steered beam at the required 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).

Mathematical description of the method

The following section sets out a mathematical description of the methods shown in Figures 5 and 12. In particular, the mathematical description below covers the scanning process shown in Figure 12 and then the measurement process shown in Figure 5. The mathematical description is based on there being n transmitting transducers 16 (or rather n transmitting positions) and m receiving transducers 18 (or rather m receiving positions) and a scatter point in the body, representing the measurement position. As noted above, 'm' and 'n' can be any integer number greater than 0.

In order to scan a defined scatter point in the part of the body, the transfer from the transmitting transducers 16 to the scatter point of interest should be adjusted by virtual beam forming (i.e. by virtual adjusting the amplitude and phase of the individual transmitting transducer signals).

The transmitted electric-field (E-field) by a transmitting transducer n, with RF transmit generator voltage u _tx is:

E _T (n, t) = a _tx n). e^ ⁿ u _tx {t) = c _tx (n). u _tx (t) (1) where a _tx (n) is the amplitude conversion for transmitting transducer n, el*** is the phase adjustment for transmitting transducer n and c _tx (n) the control factor for beam steering. In other words, the required amplification factor 'a' is also the conversion from transducer voltage to E- fie Id strength.

The E-field at the scatter point depends on complex transfer parameters h _tx (which depend on the body medium) and is the summation of the received signals from all transmitting transducers 16:

^■• scatterpoint

The scatter point scatters the E-field omnidirectional with (time-dependent) efficiency s(t), where s is a scalar. The time dependency is, for example, due to the change in fluidal properties in the heart or chest.

The receiving transducers 18 receive E-fields through the body medium with transfer functions h _rx , where rx indicates the path from the scatter point to the receiving transducers 18: s(t) ^■ E _sca tf- _er p ₀ i _n f-

h _rx (iy

Beam steering at the receiver side 6 comprises adapting amplitude and phase with control parameter c _rx (l) to c _rx (m) at the signals received by each individual receiving transducer 18. The summation of the received signals leads to beam steering with the second transducer array 6:

' E _sca tt _er p ₀ i _n t (4)

Transmitter beam steering at the first transducer array 4 comprises adapting

c(m, n) = a _rx {m). a _tx {m) . e rxin)+j p _tx (n) (6) The beam factor (control parameter) c for each transducer 18 is found by the controller 26 by a scanning algorithm (e.g. as shown in the flow chart of Figure 12), and is adjusted to compensate for the path transfer function. Ideally c is equal to the inverse of h for the corresponding path. However, h is unknown by the controller 26 and can only be estimated by the controller 26 using all of the information for all of the direct paths from each transmitting transducer 16 to each receiving transducer 18.

The properties of s(t) are derived from the measured data u _rx (t)/u _tx (t) with an uncertainty about the path parameters h.

It is assumed that h _rx and h _tx are time independent, or at least contain a time independent part and a time dependent part with the same influence on the measurement as the time dependency of s. In other words, it can be assumed that any changes in h over time increase the effect of the changes of s over time.

Receiver beam steering - The signals received by each of the receiver transducers 18 are collected, and the receiver beam steering can be applied on the data set.

The collected received signals are represented by:

The receiver beam steering is applied by the controller 26 by multiplying each received signal by a phase and amplitude factor c _rx i) and taking the sum of all adjusted signals, i.e.:

Virtual transmitter beam steering - Virtual transmitter beam steering is only possible if the transmitted signals are time multiplexed (since only one transmitting transducer 16 is active at the same time, although the transmitting transducers are active in a fast time sequence).

Each received signal from an active transmitting transducer 16 is stored in memory.

The signals received from a particular transmitting transducer 16 are, with actual receiver beam steering:

c _rx ( . h _rx {t)} ^■ h _tx {n) ^■ s(tj (9)

The actual receiver beam steering is captured with The virtual transmitter beam steering (step 103) is applied by combining the individual active transmitting transducer signals:

= C _tx (j) ^■ h _tx (j) ^■ S (tj)} (11)

Due to the discrete sample moments the average properties of s are recovered. The multiplexing period time has to be short enough for s(t) to be approximately constant.

Recovering the signals from measured data without actual beam steering - With the recovered average property s(t), both virtual transmitter and receiver beam steering (i.e. the formation of the virtual steered beam) can be executed by the controller 26:

g| = c _tx (j ^■ h _tx (j } ^■ (i2)

Receiver Virtual transmitter

beam steering beam steering

The absolute properties of s(t) are not of interest, but the changes over time of s(t) are.

(13)

Receiver Virtual transmitter

beam steering beam steering

The accuracy of the beam steering acts as a constant with a gain which can be optimized by scanning the virtual steered beam over the part of the body (i.e. by adjusting the steering parameters) to find an optimum measurement position.

If the measurement technique of transmitting signals into the part of the body without active beam steering is used, data is collected about the transfer functions from every transmitting transducer 16 directly to every receiving transducer 18. This data contains enough information to start the scanning process in Figure 12 by a best guess or best estimate. The advantage is that no calibration procedure is needed as part of the

measurement, as all information can be extracted from the measured data.

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.