WO2018108850A1

WO2018108850A1 - A method and apparatus for measuring a physiological characteristic of a subject

Info

Publication number: WO2018108850A1
Application number: PCT/EP2017/082300
Authority: WO
Inventors: Jacobus Josephus Leijssen; Siebe-Jan Van Der Hoef; Gerardus Johannes Nicolaas Doodeman; Rick BEZEMER
Original assignee: Koninklijke Philips N.V.
Priority date: 2016-12-15
Filing date: 2017-12-12
Publication date: 2018-06-21

Abstract

There is provided an apparatus for measuring a physiological characteristic of a subject, the apparatus comprising a signal generator for generating an alternating current; a magnetic field emitting device that is coupled to the signal generator, wherein the magnetic field emitting device comprises a first plurality of electrical elements that are each configured to generate an alternating magnetic field in response to an alternating current, wherein the first plurality of electrical elements are arranged with respect to each other to form a first Halbach array such that an alternating current from the signal generator causes the first Halbach array to generate an alternating magnetic field on a first side of the first Halbach array in a region of interest and substantially no magnetic field on a side of the first Halbach array that is opposite the first side; and a control unit that is configured to receive a current in response to the magnetic field emitting device emitting a magnetic field into the region of interest; process the received current to determine a phase shift and/or amplitude modulation of the received current relative to the generated alternating current; and determine a measurement of the physiological characteristic from the determined phase shift and/or amplitude modulation.

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 (such as the presence of fluid in the lungs) 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).

Magnetic fields penetrate more deeply into the human body and are less sensitive to measurement artefacts than electrical fields, and are therefore more suitable for unobtrusive vital sign and/or lung water measurements.

Commonly used antennas will create fields with two components, an electrical field and a magnetic field. To obtain a field with a primarily magnetic component, coils and/or loop antennas can be used, which ideally have a much smaller size than the emitted wavelength so that the current is equal throughout the loop resulting in a low or negligible electrical field. SUMMARY OF THE INVENTION

However, a problem with magnetic fields is that they are difficult to shape, especially when using small coils or antennas, and the magnetic field generated by a coil is equal on both sides of the coil, and this can cause artefacts in the measurement. For example, when placing a coil with one side to the body to perform measurements of a physiological characteristic, the other side of the coil is sensitive to the environment, which affects the measurements performed in the body.

Thus, there is a need for an improved method and apparatus for measuring a physiological characteristic of a subject using magnetic fields.

According to a first aspect, there is provided an apparatus for measuring a physiological characteristic of a subject, the apparatus comprising a signal generator for generating an alternating current; a magnetic field emitting device that is coupled to the signal generator, wherein the magnetic field emitting device comprises a first plurality of electrical elements that are each configured to generate an alternating magnetic field in response to an alternating current, wherein the first plurality of electrical elements are arranged with respect to each other to form a first array such that an alternating current from the signal generator causes the first array to generate an alternating magnetic field on a first side of the first array in a region of interest and substantially no magnetic field on a side of the first array that is opposite the first side; and a control unit that is configured to receive a signal (e.g. a current) in response to the magnetic field emitting device emitting a magnetic field into the region of interest; process the received current to determine a phase shift and/or amplitude modulation of the received current relative to the generated alternating current; and determine a measurement of the physiological characteristic from the determined phase shift and/or amplitude modulation.

In some embodiments, the electrical elements in the first array are arranged such that each electrical element is oriented such that the magnetic field generated by said electrical element is rotated with respect to the magnetic field generated by the previous electrical element in the first array. In some embodiments, the electrical elements in the first array are arranged such that each electrical element is oriented such that the magnetic field generated by said electrical element is rotated by 90° with respect to the magnetic field generated by the previous electrical element in the first array.

Preferably, the electrical elements in the first array are such that the electrical elements generate magnetic fields of the same strength. In some embodiments, each electrical element comprises a coil, and wherein one or more of the electrical elements in the first array have a configuration that is different to one or more other electrical elements in the first array, and wherein the electrical elements in the first array are such that the product of the alternating current provided to the electrical element, the number of windings of the electrical element and the magnetic flux density for each electrical element is the same value.

In some embodiments, the first array of electrical elements is a Halbach array, and in particular an electromagnetic Halbach array.

In some embodiments, the first array comprises three or more electrical elements.

In some embodiments, the control unit is configured to receive the signal/current from the magnetic field emitting device. In these embodiments, the received signal/current can correspond to a current induced in the magnetic field emitting device by a magnetic field emitted by the region of interest in response to the magnetic field emitted by the magnetic field emitting device.

In alternative embodiments, the apparatus further comprises a receiver arrangement that is coupled to the control unit, and the control unit receives the signal/current from the receiver arrangement. In these embodiments, the signal/current can correspond to a current induced in the receiver arrangement by a magnetic field emitted by the region of interest in response to the magnetic field emitted by the magnetic field emitting device. In some embodiments, the receiver arrangement is oriented with respect to the magnetic field emitting device such that the receiver arrangement is substantially insensitive to the magnetic field emitted by the magnetic field emitting device. In some embodiments, this means that the receiver arrangement is oriented orthogonally with respect to the magnetic field emitting device.

In some embodiments, each electrical element comprises a coil, such as a flat coil. In some embodiments, each electrical element can have a core formed of a magnetic material. In alternative embodiments, each electrical element can comprise an

electromagnetic horseshoe magnet.

In some embodiments, the apparatus can also comprise a plate that is positioned on the side of the first array that is opposite the first side, wherein the plate is formed from a ferrite material, a ferromagnetic material or compound or an alloy.

In some embodiments, the magnetic field emitting device can further comprise a second plurality of electrical elements that are each configured to generate an alternating magnetic field in response to an alternating current, wherein the second plurality of electrical elements are arranged with respect to each other to form a second array such that an alternating current from the signal generator causes the second array to generate an alternating magnetic field on a first side of the second array in a region of interest and substantially no magnetic field on a side of the second array that is opposite the first side. In some embodiments, the first array of electrical elements and the second array of electrical elements can be connected together to enable movement of the first array relative to the second array.

According to a second aspect, there is provided a method of measuring a physiological characteristic of a subject, the method comprising generating an alternating current; providing the alternating current to a magnetic field emitting device that comprises a first plurality of electrical elements that are each configured to generate an alternating magnetic field in response to the alternating current, wherein the first plurality of electrical elements are arranged with respect to each other to form a first array such that the alternating current causes the first array to generate an alternating magnetic field on a first side of the first array in a region of interest and substantially no magnetic field on a side of the first array that is opposite the first side; receiving a signal (e.g. a current) in response to the magnetic field emitting device emitting a magnetic field into the region of interest; processing the received current to determine a phase shift and/or amplitude modulation of the received current relative to the generated alternating current; and determining a measurement of the physiological characteristic from the determined phase shift and/or amplitude modulation.

Preferably, the electrical elements in the first array are such that the electrical elements generate magnetic fields of the same strength. In some embodiments, each electrical element comprises a coil, and wherein one or more of the electrical elements in the first array have a configuration that is different to one or more other electrical elements in the first array, and wherein the electrical elements in the first array are such that the product of the alternating current provided to the electrical element, the number of windings of the electrical element and the magnetic flux density for each electrical element is the same value. In some embodiments, the first array of electrical elements is a Halbach array, and in particular an electromagnetic Halbach array.

In some embodiments, the step of receiving comprises receiving the signal/current from the magnetic field emitting device. In these embodiments, the received signal/current can correspond to a current induced in the magnetic field emitting device by a magnetic field emitted by the region of interest in response to the magnetic field emitted by the magnetic field emitting device.

In alternative embodiments, the step of receiving comprises receiving the signal/current from a receiver arrangement. In these embodiments, the signal/current can correspond to a current induced in the receiver arrangement by a magnetic field emitted by the region of interest in response to the magnetic field emitted by the magnetic field emitting device. In some embodiments, the receiver arrangement is oriented with respect to the magnetic field emitting device such that the receiver arrangement is substantially insensitive to the magnetic field emitted by the magnetic field emitting device. In some embodiments, this means that the receiver arrangement is oriented orthogonally with respect to the magnetic field emitting device.

electromagnetic horseshoe magnet.

In some embodiments, the magnetic field emitting device can further comprise a second plurality of electrical elements that are each configured to generate an alternating magnetic field in response to an alternating current, wherein the second plurality of electrical elements are arranged with respect to each other to form a second array such that the step of providing the alternating current comprises causing the second array to generate an alternating magnetic field on a first side of the second array in a region of interest and substantially no magnetic field on a side of the second array that is opposite the first side. In some embodiments, the first array of electrical elements and the second array of electrical elements can be connected together to enable movement of the first array relative to the second array.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a plurality of magnets arranged in parallel (Figure 1(a)) and in a Halbach configuration (Figure 1(b));

Figures 2(a) and 2(b) show magnetic field emitting devices according to two exemplary embodiments;

Figure 3 shows a magnetic field emitting device formed from a plurality of coils according to an embodiment;

Figure 4 shows the magnetic field produced by a magnetic field emitting device according to an embodiment (Figure 4(a)) and the magnetic field produced by a magnetic field emitting device according to another embodiment (Figure 4(b));

Figure 5 is a block diagram of an apparatus according to an embodiment of the invention;

Figure 6 is an illustration of the apparatus of Figure 5 in use on a subject; Figure 7 is a flow chart illustrating a method of measuring a physiological characteristic of a subject according to an embodiment;

Figure 8 is a diagram illustrating an input signal for a magnetic field emitting device and an output signal received from a body;

Figure 9 is an illustration of another embodiment of a magnetic field emitting device;

Figure 10 is a side view of another embodiment of a magnetic field emitting device; and

Figure 11 is an illustration of a magnetic field emitting device according to a further embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, there is a problem with conventional magnetic induction- based measurements of a physiological characteristic in that a coil placed with one side to the body to perform measurements will be sensitive to the environment on the other side of the coil, and this can affect quality of the measurements performed of the body. In addition the magnetic field produced by a coil or similar element can be difficult to shape.

Thus, the invention provides improvements to the device used to emit magnetic fields into the body of a subject for the purpose of measuring a physiological characteristic of a subject using magnetic fields.

It is known that permanent magnets can be arranged in a Halbach configuration, which is an array of permanent magnets that leads to the augmentation of the magnetic field on one side of the array while cancelling the field to near zero on the other side. Figure 1(a) illustrates an array 2 of five permanent magnets 4 arranged in a line with the magnetic field of each permanent magnet 4 (indicated by the arrows) aligned in the same direction, and the magnetic field generated by this array 2. Figure 1(b) shows an array 6 of five permanent magnets 8 arranged in a line, with the magnetic field of each magnet 8 in the line rotated by 90° with respect to the previous magnet 8 in the array 6. This is known as a Halbach configuration, and it can be seen from the magnetic field lines in Figure 1(b) that the magnetic field is enhanced on one side of the array 6, and reduced to substantially zero on the other side of the array 6. It will be appreciated that the strength of the magnetic field produced by each of the magnets 8 should be the same to produce the field pattern shown in Figure 1(b).

However, permanent magnets create a permanent magnetic field which is not suitable for performing measurements of a physiological characteristic. Instead, for obtaining measurements of physiological characteristics using magnetic induction, alternating magnetic fields are required.

Thus, in the invention, a magnetic field emitting device is provided and used for physiological characteristic measurements that comprises a number of electrical elements (such as coils) that are arranged with respect to each other to form an electromagnetic

Halbach array such that an alternating current from a signal generator causes the array to generate an alternating magnetic field on a first side of the array in a region of interest (e.g. a body, when the magnetic field emitting device is used near or on a body) and substantially no magnetic field on the opposite side of the array.

This magnetic field emitting device is driven with an alternating current to create an alternating magnetic field which is able to penetrate into the body of a subject and that has little to no sensitivity on the other side (i.e. the side responsible for artefacts, such as a hand moving towards the sensor). Figures 2(a) and 2(b) show magnetic field emitting devices that use a Halbach configuration according to two exemplary embodiments. Figure 2(a) shows a first magnetic field emitting device 10 that comprises five electrical elements 12 arranged in a row. Each electrical element 12 can be a coil, or an electromagnetic horseshoe magnet (i.e. a horseshoe or U-shaped core that has wire or other conductive material wrapped around it). The electrical elements 12 are arranged in the magnetic field emitting device 10 such that the magnetic field of each electrical element 12 is rotated by 90° with respect to the previous electrical element 12 in the device 10. In this embodiment, the magnetic fields rotate 90° anticlockwise when moving from left to right along the device 10. This arrangement of electrical elements 12 produces a magnetic field on a first side of the device 10, as indicated by arrows 13 in Figure 2(a), and substantially no magnetic field on the opposite side of the device 10.

Figure 2(b) shows a magnetic field emitting device 14 according to a second embodiment. In this embodiment, the magnetic field emitting device 14 comprises five electrical elements 16 arranged in a row, with the magnetic field of each electrical element 16 rotated by 90° clockwise with respect to the previous electrical element 16 when moving from left to right along the device 14. As in Figure 2(a), each electrical element 16 can be a coil, or an electromagnetic horseshoe magnet (i.e. a horseshoe or U-shaped core that has wire or other conductive material wrapped around it). This arrangement of electrical elements 16 produces a magnetic field on a first side of the device 14, as indicated by arrows 17 in Figure 2(b), and substantially no magnetic field on the opposite side of the device 14.

Although the embodiments in Figure 2 show a 90° rotation with each electrical element 12, 16, it will be appreciated that in other embodiments, other rotations can be used (e.g. more or less than 90°). However, it will also be appreciated that the magnetic field pattern produced by such an arrangement would not be as strong and coherent on the side that is to face the body, and the arrangement will be more sensitive to external magnetic fields on the other side of the body.

In the embodiments of Figure 2, each electrical element 12, 16 can be driven with a respective alternating current (with the current supplied to each element having the same phase), or each electrical element 12, 16 can be electrically interconnected so that an alternating current only needs to be supplied to one of the electrical elements 12, 16.

It will also be appreciated that in order to produce the required magnetic field pattern, e.g. as shown in Figure 1(b), the field strength produced by each electrical element in Figures 2(a) and 2(b) should be equal. Figure 3 shows another embodiment of a magnetic field emitting device that uses a Halbach configuration. This magnetic field emitting device 18 comprises five electrical elements 19 in the form of coils. In some embodiments, the coils 19 are flat coils. The coils are formed in a single piece of wire (to enable all of the coils 19 to be driven with an alternating current at one end of the device 18), and are formed such that the magnetic fields produced by each coil are rotated by 90° with respect to the previous coil. The magnetic field produced by the magnetic field emitting device 18 in Figure 3 is shown by arrows 20.

In the embodiment of Figure 3, each coil 19 comprises a core 21, for example a magnetic core formed from a magnet or a ferromagnetic material, to improve the strength of the magnetic field produced by each coil 19. However, it will be appreciated that the core 21 can be omitted in other embodiments, and the electrical elements 19 can just comprise a coil 19.

Each coil 19 can comprise any number of windings, and the number of windings shown in Figure 3 is merely exemplary. In some embodiments, each coil 19 has the same configuration, i.e. they comprise the same number of windings and the same magnetic flux density. Thus, provided each coil 19 is supplied with the same alternating current (which is ensured if the coils 19 are interconnected or formed from the same wire), each coil 19 with generate a magnetic field with the same strength. However, one or more of the coils 19 can have a different configuration and still produce a magnetic field pattern as shown in Figure 1(b), provided that the product of the alternating current provided to a particular coil 19, the number of windings of the particular coil 19 and the magnetic flux density for the particular coil 19 is the same as the product for all other coils 19 in the magnetic field emitting device 18. Thus, for example, a coil 19 that has 10% of the magnetic flux density of the other coils 19 can generate the same magnetic field strength if it is driven with a current that is 10 times bigger than that used to drive the other coils 19.

Two further embodiments of a magnetic field emitting device having a Halbach configuration according to the invention are shown in Figure 4. Figure 4(a) shows a magnetic field emitting device 22 that comprises three electrical elements 24 in the form of flat coils. The flat coils 24 are arranged such that the magnetic fields produced by each coil 24 are rotated by 90° with respect to the previous coil. The middle coil 24 is shown side on and the other two coils 24 are shown end on in Figure 4(a). Area 26 indicates the

approximate magnetic field generated by the magnetic field emitting device 22. Figure 4(b) shows a modification of the magnetic field emitting device 24 in Figure 4(a), in that the magnetic field emitting device 28 comprises three electrical elements 30 in the form of flat coils arranged in a similar way to coils 24 in Figure 4(a). However, in this embodiment the magnetic field emitting device 28 also comprises a plate 32 that is positioned on the side of the coils 30 opposite to the side on which the magnetic field strength is concentration. That is, the plate 32 is positioned on the side of the magnetic field emitting device 28 that would face the environment when the magnetic field emitting device 28 is in use on a subject. The plate 32 can be formed from a ferrite material, a ferromagnetic material or compound or an alloy, for example an alloy containing iron, nickel, cobalt or some rare earth metals. The plate 32 acts to further improve the magnetic field pattern on the emitting side of the magnetic field emitting device 28. In some embodiments, the plate 32 can be formed from a flexible material to enable the shape of the magnetic field emitting device 24 to be adapted more closely to the shape of the body on which the magnetic field emitting device 24 is being used.

The magnetic field emitting devices 22 and 28 in Figures 4(a) and 4(b) can be considered as flat magnetic field emitting devices, with the coils 24, 30 being formed on a printed circuit board (PCB), with Figures 4(a) and 4(b) showing a side view of the PCBs. The middle ('horizontal') coil 24, 30 can be flat, e.g. 2mm, and can be driven with 10 times the current of the outer two coils 24, 30 (which are termed 'vertical' field coils) due to the differences in construction of the horizontal coil 24, 30 and vertical coils 24, 30 in order for the strength of the magnetic field generated by the coils 24, 30 to be equal to each other. In this way, flat magnetic field emitting devices can be created for use in plasters or other lightweight and/or unobtrusive monitoring devices. The PCB can be rigid or flexible to enable to the magnetic field emitting device to adapt to the shape of the body.

Although the magnetic field emitting devices in Figures 2 and 3 comprise five electrical elements and the magnetic field emitting devices in Figure 4 comprise three electrical elements, it will be appreciated that the number of electrical elements is merely exemplary, and a magnetic field emitting device according to the invention can be formed using three or more electrical elements that form a Halbach configuration.

An apparatus 40 for measuring a physiological characteristic of a subject using a magnetic field emitting device as described above is shown in Figure 5. The physiological characteristic can be heart rate, breathing rate, breathing depth or any other physiological characteristic (e.g. lung fluid content) that can be measured from changes in the magnetic properties of the body (e.g. changes that are due to fluid movements in the body). The apparatus 40 is mainly intended for use in the near field, i.e. close to the body, and is not necessarily intended for image forming, but for measurement of the changes in magnetic properties (permittivity, conductivity, etc.) of a part of the body of a subject.

The apparatus 40 comprises a magnetic field emitting device 42 and a signal generator 44 that is configured to generate an alternating current. As described above the magnetic field emitting device 42 comprises a plurality of electrical elements that are each configured to generate an alternating magnetic field in response to the alternating current from the signal generator 44, and the elements are arranged with respect to each other to form an array such that the alternating current causes the array to generate an alternating magnetic field on a first side of the array and substantially no magnetic field on the opposite side of the array.

The magnetic field emitting device 42 is used to emit a magnetic field into part of the body of the subject close to the magnetic field emitting device 42. The magnetic field emitting device 42 can be constructed or configured according to any of the embodiments described above.

The apparatus 40 also comprises a control unit 46 that is coupled to the signal generator 44 to control the generation of the alternating current. The control unit 46 also controls the operation of the apparatus 40, and specifically receives a current in response to the magnetic field emitting device 42 emitting a magnetic field and processing the received current to determine a measurement of the physiological characteristic.

The control unit 46 can be implemented in numerous ways, with software and/or hardware, to perform the various functions described below. The control unit 46 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 46 may be

implemented as a combination of dedicated hardware to perform some functions (e.g.

amplifiers, pre-amplifiers, 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 46 may be associated with or comprise one or more memory units 48 such as volatile and non- volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The control unit 46 or associated memory unit 48 can also be used for storing program code that can be executed by a processor in the control unit 46 to perform the method described herein. The memory unit 48 can also be used to store a signal representing the received current, the results of processing or analysis of the received current and/or measurements of physiological characteristics determined by the control unit 46.

As the magnetic field (which can be referred to as the primary magnetic field) enters the part of the body of the subject, the alternating magnetic field causes magnetic induction, i.e. the generation of eddy currents in the tissue due to the application of an external magnetic field, and this eddy current is affected or modulated by the (time- varying) properties of the tissue or fluid, e.g. heart beats, breathing and fluid or blood movement. In particular the time- varying properties of the tissue or fluid will affect the amplitude and/or phase of the eddy current. The eddy current itself creates a magnetic field in the body (known as the secondary magnetic field) and this magnetic field induces a current in a receiver component of the apparatus 40. This current is processed by the control unit 46 to determine the measurement of the physiological characteristic.

In some embodiments, the control unit 46 obtains the received current from the magnetic field emitting device 42, i.e. the magnetic field emitting device 42 is used to both generate the magnetic field and sense the magnetic field generated by the eddy current. In this case the received current can be obtained as changes or modulations in the magnetic field emitting device's return loss.

In other embodiments, the apparatus 40 comprises a receiver arrangement 48 that is connected to the control unit 46 and that is positioned on or near the body so that it is sensitive to the eddy current induced in the body of the subject. The magnetic field generated by the eddy current in the body induces a current in the receiver arrangement 48, and this current is passed to the control unit 46. In use, the receiver arrangement 50 can be arranged or positioned so that it is insensitive, or minimally sensitive, to the magnetic field emitted by the magnetic field emitting device 42. For example, the receiver arrangement 50 can be arranged so that the primary sensing direction of the receiver arrangement 50 (i.e. the direction from which the receiver arrangement 50 is most sensitive) is orthogonal to the primary field emitting direction of the magnetic field emitting device 42.

The receiver arrangement 50 can correspond in structure to the magnetic field emitting device 42 in the apparatus 40, or an alternative configuration of the magnetic field emitting device 42 as described above. In that case, the receiver arrangement 50 can be positioned on the subject so that the side most sensitive to the magnetic field (i.e. the side that would emit the magnetic field if the magnetic field emitting device was driven with an alternating current) facing the body of the subject, so that the magnetic field emitting device is sensitive to the magnetic field generated by the eddy current.

In other embodiments, the receiver arrangement 50 can comprise a conventional antenna, such as a coil or loop antenna.

The magnetic field emitting device 42 and/or the receiver arrangement 50 (if present) 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 magnetic field emitting device 42 and/or the receiver arrangement 50 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 magnetic field emitting device 42 and/or the receiver arrangement 50 can be for use inside the body of the subject, and thus one or both of the magnetic field emitting device 42 and/or the receiver arrangement 50 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 magnetic field emitting device 42 and/or the receiver arrangement 50 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 5 only shows the components required to illustrate this aspect of the invention, and in a practical implementation the apparatus 40 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 6 shows the apparatus 40 of Figure 5 in use on a subject 60. In Figure 6, the magnetic field emitting device 42 is used to both emit the magnetic field into the body of the subject and to detect the eddy current generated by the emitted magnetic field. Thus, the magnetic field emitting device 42 is placed on the chest of the subject 60, so that the magnetic field (indicated by dashed line 62) is emitted into the body of the subject 60. The side of the magnetic field emitting device 42 that emits substantially no magnetic field is facing away from the chest of the subject 60, and is therefore largely insensitive to magnetic fields or currents external to the body of the subject 60. The flow chart in Figure 7 illustrates a method of measuring a physiological characteristic of a subject 60 according to an embodiment. This method can be performed using the apparatus 40 described above. The method is performed when a magnetic field emitting device 42 is placed on or near a body of a subject 60. If the apparatus 40 comprises a separate receiver arrangement 50 for detecting or measuring the magnetic field generated by the eddy currents in the body of the subject 60, the receiver arrangement 50 is also placed on or near the body of the subject 60, preferably such that the receiver arrangement 50 is insensitive (or minimally sensitive) to the magnetic field generated by the magnetic field emitting device 42.

Thus, in a first step, step 101, the method comprises generating an alternating current for a magnetic field emitting device 42. This step can comprise controlling signal generator 44 to generate an alternating current. As noted above, in some embodiments of the magnetic field emitting device 42, individual electrical elements in the magnetic field emitting device 42 may require different current levels due to the construction of the magnetic field emitting device 42, in which case step 101 can comprise generating an alternating current at the required level for multiple electrical elements.

Next, in step 103, the alternating current (or currents, if required) are provided to magnetic field emitting device 42. In response to the alternating current(s), the magnetic field emitting device 42 generates an alternating magnetic field on a first side of the magnetic field emitting device 42 in a region of interest (i.e. in the body of the subject 60), and substantially no magnetic field on the opposite side of the magnetic field emitting device 42. As noted above, this magnetic field pattern is produced due to the arrangement of the electrical elements in the magnetic field emitting device 42.

In step 105 the apparatus 40, and specifically the control unit 46, receives a current in response to the magnetic field emitting device 42 emitting the magnetic field into the region of interest (body). In some embodiments step 105 can comprise receiving a signal representing the current, and processing the signal to determine the current.

In steps 107 and 109 the control unit 46 processes the received current to determine a measurement of a physiological characteristic of the subject 60.

In particular, as noted above, as the primary magnetic field from the magnetic field emitting device 42 enters the body of the subject 60, the alternating magnetic field causes magnetic induction and generates an eddy current in the tissue. The properties of this eddy current are affected or modulated by the time -varying properties of the tissue or fluid, e.g. heart beats, breathing and fluid or blood movement. The eddy current itself creates a secondary magnetic field and this magnetic field induces a current in a receiver component of the apparatus 40. This received current will exhibit changes in phase and/or changes in amplitude compared to the current used to drive the magnetic field emitting device 42, and these change in phase and/or changes in amplitude depend on the time- varying properties of the body (i.e. the physiological characteristics). Thus, step 107 comprises processing the received current to determine a phase shift and/or amplitude modulation of the received current relative to the generated alternating current. Thus, the control unit 46 can compare the received current to the generated current to determine the changes in phase (i.e. phase shift) and/or changes in amplitude (i.e. amplitude modulation).

Finally, in step 109, the measurement of the physiological characteristic can be determined from the determined phase shift and/or amplitude modulation. For example, for cyclical or periodic physiological characteristics (i.e. breathing or heart beats) the period or frequency of the phase shift and/or amplitude modulation can provide the physiological characteristic measurement. For example the period or frequency of the phase shift and/or amplitude modulation can indicate the breathing rate or heart rate. Those skilled in the art will be aware of various techniques for extracting a physiological characteristic measurement from a received current, and therefore further details are not provided herein.

As noted above, in some embodiments the magnetic field emitting device 42 can be used to both emit the primary magnetic field into the body of the subject 60 and detect the secondary magnetic field from the body. In this case, the received current (i.e. the current that the control unit 46 receives and processes to determine the measurement of the physiological characteristic), can be obtained as follows.

When providing an alternating current to a magnetic field emitting device 42 to create an alternating magnetic field, part of the alternating current is reflected by the magnetic field emitting device 42 due to a mismatch between impedance of the electrical elements (e.g. coils) in the magnetic field emitting device 42 and the impedance of the environment around the electrical elements. When the body of the subject 60 is close to the sensitive side of the magnetic field emitting device 42, the magnetic field is directly influenced (i.e. modulated) by changes in the magnetic properties within the magnetic field in the body. For example, breathing modulates the air and tissue content in the thorax and thus the magnetic properties in the thorax, and this will therefore modulate the extent of the impedance mismatch between the electrical elements and the environment (including the lungs), and hence modulate the reflected alternating current. As such, to measure

changes/modulations in the body's magnetic properties (e.g., due to fluid and air shifts as a result of heartbeats or breathing) without requiring a separate receiver arrangement, changes or modulations in the return loss of the magnetic field emitting device 40 could be monitored. This is illustrated in Figure 8 which shows a generated alternating current 70 that is provided to magnetic field emitting device 71, and the return loss signal 72. The return loss of the magnetic field emitting device 71 is the ratio (expressed in dB) of the power of the original alternating current 70 vs. the power of the reflected current 72.

Figures 9 and 10 illustrate two alternative embodiments of a magnetic field emitting device that use a Halbach configuration according to the invention in which a more compact form factor is achieved by shifting the electrical elements into each other. In particular, Figure 9 shows a magnetic field emitting device 76 that comprises three electrical elements 78 in the form of coils that are arranged more closely together than in the embodiment of Figure 4(a). In particular, the electrical elements 78 can overlap with each other (when looking at the magnetic field emitting device 76 in a direction parallel to the direction in which the magnetic field is emitted), and so the elements 78 can be understood as sharing the same volume space. The magnetic field pattern produced by this arrangement is shown by area 80, and it can be seen that there is more 'leakage' of the magnetic field on the opposite side of the magnetic field emitting device 76 than with the embodiment shown in Figure 4(a), which is the trade-off for the more compact form factor.

Figure 10 shows a side view of a magnetic field emitting device 82 that comprises seven electrical elements, four 'horizontal' oriented elements 84 and three

'vertical' oriented elements 86. These elements 84, 86 are overlapped to form the magnetic field emitting device 82. Furthermore, the structure of the 'vertical' electrical elements 86 are such that they require an alternating current that is ten times the alternating current required to drive the 'horizontal' elements 84.

Figure 11 shows a magnetic field emitting device according to a further embodiment. In this embodiment, the magnetic field emitting device 92 comprises multiple Halbach arrays 94 that can be used to emit magnetic fields into a larger region of interest than a single Halbach array. Thus, the magnetic field emitting device 92 comprises a number of electrical elements 96, that are each arranged into a respective Halbach array 94. Each of these Halbach arrays 94 can be according to the embodiments shown in any of Figures 2, 3, 4, 9 or 10. Figure 11(a) shows a top view of the magnetic field emitting device 92, and it can be seen that the multiple arrays 94 can be interconnected such that each array 94 can move with respect to the other arrays 94 to enable the magnetic field emitting device 92 to cover and adapt to the shape of a larger region of interest, such as the thorax. Figure 11(b) shows a side view of the magnetic field emitting device 92.

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

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

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

a signal generator for generating an alternating current;

a magnetic field emitting device that is coupled to the signal generator, wherein the magnetic field emitting device comprises a first plurality of electrical elements that are each configured to generate an alternating magnetic field in response to an alternating current, wherein the first plurality of electrical elements are arranged with respect to each other to form a first Halbach array such that an alternating current from the signal generator causes the first Halbach array to generate an alternating magnetic field on a first side of the first Halbach array in a region of interest and substantially no magnetic field on a side of the first Halbach array that is opposite the first side; and

a control unit that is configured to:

receive a current in response to the magnetic field emitting device emitting a magnetic field into the region of interest;

process the received current to determine a phase shift and/or amplitude modulation of the received current relative to the generated alternating current; and determine a measurement of the physiological characteristic from the determined phase shift and/or amplitude modulation.

2. An apparatus as claimed in claim 1, wherein the electrical elements in the first Halbach array are arranged such that each electrical element is oriented such that the magnetic field generated by said electrical element is rotated with respect to the magnetic field generated by the previous electrical element in the first Halbach array.

3. An apparatus as claimed in claim 1 or 2, wherein the electrical elements in the first Halbach array are such that the electrical elements generate magnetic fields of the same strength.

4. An apparatus as claimed in claim 1, 2 or 3, wherein the control unit is configured to receive the current from the magnetic field emitting device.

5. An apparatus as claimed in any of claims 1-4, wherein the received current corresponds to a current induced in the magnetic field emitting device by a magnetic field emitted by the region of interest in response to the magnetic field emitted by the magnetic field emitting device.

6. An apparatus as claimed in any of claims 1-3, wherein the apparatus further comprises:

a receiver arrangement that is coupled to the control unit, wherein the control unit receives the current from the receiver arrangement.

7. An apparatus as claimed in claim 6, wherein the received current corresponds to a current induced in the receiver arrangement by a magnetic field emitted by the region of interest in response to the magnetic field emitted by the magnetic field emitting device.

8. An apparatus as claimed in claim 6 or 7, wherein the receiver arrangement is oriented with respect to the magnetic field emitting device such that the receiver arrangement is substantially insensitive to the magnetic field emitted by the magnetic field emitting device.

9. An apparatus as claimed in any of claims 1-8, wherein each electrical element comprises a coil or an electromagnetic horseshoe magnet.

10. An apparatus as claimed in any of claims 1-9, wherein the apparatus further comprises:

a plate that is positioned on the side of the first Halbach array that is opposite the first side, wherein the plate is formed from a ferrite material, a ferromagnetic material or compound or an alloy.

11. An apparatus as claimed in any of claims 1-10, wherein the magnetic field emitting device further comprises:

a second plurality of electrical elements that are each configured to generate an alternating magnetic field in response to an alternating current, wherein the second plurality of electrical elements are arranged with respect to each other to form a second Halbach array such that an alternating current from the signal generator causes the second Halbach array to generate an alternating magnetic field on a first side of the second Halbach array in a region of interest and substantially no magnetic field on a side of the second Halbach array that is opposite the first side.

12. An apparatus as claimed in claim 11, wherein the first Halbach array of electrical elements and the second Halbach array of electrical elements are connected together to enable movement of the first Halbach array relative to the second Halbach array.

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

generating an alternating current;

providing the alternating current to a magnetic field emitting device that comprises a first plurality of electrical elements that are each configured to generate an alternating magnetic field in response to the alternating current, wherein the first plurality of electrical elements are arranged with respect to each other to form a first Halbach array such that the alternating current causes the first Halbach array to generate an alternating magnetic field on a first side of the first Halbach array in a region of interest and substantially no magnetic field on a side of the first Halbach array that is opposite the first side;

receiving a current in response to the magnetic field emitting device emitting a magnetic field into the region of interest;

processing the received current to determine a phase shift and/or amplitude modulation of the received current relative to the generated alternating current; and

determining a measurement of the physiological characteristic from the determined phase shift and/or amplitude modulation.

14. A method as claimed in claim 13, wherein the step of receiving comprises receiving the current from the magnetic field emitting device.

15. A method as claimed in claim 13, wherein the step of receiving comprises receiving the current from a receiver arrangement.