Detecting pressure ulcers using electromagnetic waves

Abstract

A system (20) includes multiple antennas (22) configured to transmit electromagnetic waves into tissue (52) of a subject and to convert, to analog signals, reflections of the waves received from the tissue, an analog-to-digital converter (37) configured to convert the analog signals to digital signals, and processing circuitry (26, 84) configured to detect, by processing the digital signals, a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue, and in response to detecting the difference, to output an indication that the first portion of the tissue includes a pressure ulcer. Other embodiments are also described.

Description

[0001] DETECTING PRESSURE ULCERS USING ELECTROMAGNETIC WAVES

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] The present application claims priority to US Provisional Application 63 / 655,616, entitled “Advanced contactless tissue status detection system using electromagnetic waves and sensor arrays,” filed June 4, 2024, whose disclosure is incorporated herein by reference.

[0004] FIELD OF EMBODIMENTS OF THE INVENTION

[0005] Embodiments of the present invention are related generally to the healthcare field, and specifically to the detection of pressure ulcers.

[0006] BACKGROUND

[0007] Pressure ulcers are localized injuries to the skin and underlying tissue, primarily caused by prolonged pressure on the skin. They commonly develop on bony areas of the body, such as the heels, ankles, hips, and tailbone, where the skin and tissue are compressed between a bone and a hard surface, like a bed or wheelchair. Individuals who are bedridden, use a wheelchair, or are unable to change positions frequently are at a higher risk of developing pressure ulcers. These ulcers can range from mild reddening of the skin to severe open wounds that expose muscle or bone. Prevention is crucial and involves regular repositioning, using supportive surfaces, maintaining good skin hygiene, and ensuring adequate nutrition and hydration. Early detection and treatment are essential to prevent complications, such as infections, which can lead to more serious health issues.

[0008] SUMMARY

[0009] There is provided, in accordance with some embodiments of the present invention, a system including multiple antennas configured to transmit electromagnetic waves into tissue of a subject and to convert, to analog signals, reflections of the waves received from the tissue, an analog-to-digital converter configured to convert the analog signals to digital signals. The system further includes processing circuitry configured to detect, by processing the digital signals, a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue, and in response to detecting the difference, to output an indication that the first portion of the tissue includes a pressure ulcer. In some embodiments, the electromagnetic waves include radio frequency waves.

[0010] In some embodiments, the processing circuitry is configured to detect the difference by detecting a greater change in the property at the first portion of the tissue over time, relative to the second portions of the tissue.

[0011] In some embodiments, the antennas are arranged in at least one two-dimensional array.

[0012] In some embodiments, the two-dimensional array includes multiple intercrossed one-dimensional arrays of the antennas, the antennas in each of the one -dimensional arrays being interconnected such that the antennas in each of the one-dimensional arrays transmit simultaneously and receive simultaneously, the one-dimensional arrays are configured to transmit the electromagnetic waves and to convert the reflections, and the processing circuitry is configured to locate the first portion of the tissue by correlating the digital signals between non-parallel ones of the one -dimensional arrays.

[0013] In some embodiments, at least one subset of the antennas are connected, via respective bandpass filters having different respective pass bands, to a common line, and the processing circuitry is configured to vary a frequency of an input signal delivered to the line such that the input signal is altematingly passed through each of the bandpass filters, and hence, the electromagnetic waves are altematingly transmitted from each antenna of the subset in response to the input signal.

[0014] In some embodiments, at least one other subset of the antennas are connected, via respective other bandpass filters, to another common line, and each antenna of the other subset is configured to convert the reflections in turn as the frequency of the input signal is varied, by virtue of the analog signals altematingly passing through each of the other bandpass filters.

[0015] In some embodiments, each one of the antennas is configured to transmit the electromagnetic waves at multiple frequencies.

[0016] In some embodiments, the property includes a frequency at which a power of the reflections is a minimum. In some embodiments, the property includes a frequency at which a dissipation ratio of the electromagnetic waves is a maximum.

[0017] In some embodiments, each one of the antennas is configured to transmit the electromagnetic waves at the multiple frequencies simultaneously.

[0018] In some embodiments, the system further includes a signal generator configured to generate a modulated input signal to the antennas using orthogonal frequency division multiplexing, such that each one of the antennas, in response to the input signal, transmits the electromagnetic waves at the multiple frequencies simultaneously using orthogonal frequency division multiplexing signaling.

[0019] In some embodiments, the reflections are received from multiple different overlapping pathways through the tissue, and the processing circuitry is configured to back-project the digital signals onto the tissue, thereby computing the property at multiple depths.

[0020] In some embodiments, the antennas are configured to transmit the electromagnetic waves at multiple different angles, such that the reflections are received from the multiple different overlapping pathways through the tissue.

[0021] In some embodiments, the processing circuitry is configured to control a phase of an input signal to the antennas such that the antennas transmit the electromagnetic waves at the multiple different angles by functioning as phased arrays.

[0022] In some embodiments, the antennas are configured to transmit the electromagnetic waves at the multiple different angles by virtue of at least some of the antennas being different from each other with respect to at least one antenna property.

[0023] In some embodiments, the antenna property includes a tilt angle.

[0024] In some embodiments, the antennas are configured to transmit the electromagnetic waves with multiple different penetration depths, such that the reflections are received from the multiple different overlapping pathways through the tissue.

[0025] In some embodiments, the processing circuitry is further configured to vary a frequency of the electromagnetic waves such that the antennas transmit the electromagnetic waves with the multiple different penetration depths.

[0026] In some embodiments, the processing circuitry is further configured to vary a spacing between a transmitting one of the antennas and a receiving one of the antennas, such that the reflections are received from the multiple different overlapping pathways through the tissue. There is further provided, in accordance with some embodiments of the present invention, a method for use with multiple antennas and an analog-to-digital converter. The method includes transmitting electromagnetic waves, from the antennas, into tissue of a subject, such that the antennas convert, to analog signals, reflections of the waves received from the tissue, and the analog-to-digital converter converts the analog signals to digital signals. The method further includes, by processing the digital signals, detecting a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue, and in response to detecting the difference, outputting an indication that the first portion of the tissue includes a pressure ulcer.

[0027] The present invention will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which:

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic illustration of a system for detecting pressure ulcers, in accordance with some embodiments of the present invention;

[0030] Figure 2 plots experimentally-derived Si l return losses as a function of frequency, which demonstrate principles of some embodiments of the present invention;

[0031] Figure 3 is a schematic illustration of a two-dimensional array of antennas, in accordance with some embodiments of the present invention;

[0032] Figure 4 shows a scheme for reducing connections and simplifying a multiplexer, in accordance with some embodiments of the present invention;

[0033] Figures 5A, 5B, and 5C are schematic illustrations of reflections of electromagnetic waves received from multiple different overlapping pathways through tissue, in accordance with some embodiments of the present invention; and

[0034] Figure 6 is a schematic illustration of orthogonal frequency division multiplexing (OFDM) circuitry, in accordance with some embodiments of the present invention.

[0035] DETAILED DESCRIPTION

[0036] OVERVIEW

[0037] As noted in the Background, early detection of pressure ulcers is essential to prevent complications. However, it is generally difficult to detect a pressure ulcer.

[0038] To address this challenge, embodiments of the present invention capitalize on the observation that as a pressure ulcer develops in an area of tissue, the response of this area to electromagnetic waves, such as radiofrequency waves, changes over time and becomes different from that of normal tissue. In particular, embodiments of the present invention provide a system configured to detect pressure ulcers by using electromagnetic waves to probe a subject's tissue. The waves are transmitted, and the reflections thereof are received, by multiple antennas positioned near the tissue but typically not in contact therewith, such as beneath a mattress or on a covering sheet. Typically, the antennas are arranged in at least one two-dimensional array, such as in multiple 2D arrays positioned to target different respective areas of the body most susceptible to pressure ulcers.

[0039] In some embodiments, to enhance the accuracy and precision with which pressure ulcers are identified and located, each antenna is capable of transmitting and receiving electromagnetic waves at multiple frequencies, often utilizing advanced modulation schemes such as orthogonal frequency division multiplexing (OFDM) to enable simultaneous multi-frequency operation. Alternatively or additionally, various techniques are used to differentiate between the response of the tissue at different depths, such that the three-dimensional coordinates of a pressure ulcer can be ascertained.

[0040] Advantageously, some embodiments provide schemes for simplified switching between the antennas.

[0041] SYSTEM DESCRIPTION

[0042] Reference is initially made to Fig. 1, which is a schematic illustration of a system 20 for detecting pressure ulcers, in accordance with some embodiments of the present invention.

[0043] System 20 comprises multiple antennas 22. Typically, antennas 22 are arranged in at least one two-dimensional array 23, such a rectangular or hexagonal array. For example, in some embodiments, array 23 comprises a rectangular arrangement of 8-20 x 4-10 antennas. In some embodiments, adjacent antennas in array 23 are spaced apart from each other by 20-40 mm.

[0044] In some embodiments, as shown in Fig. 1, antennas 22 are arranged in multiple separate two-dimensional arrays 23, which are placed opposite (e.g., beneath) respective areas of the subject’s body that are more susceptible to pressure ulcers such as the subject's shoulders, upper buttocks, lower buttocks, and / or ankles.

[0045] In some embodiments, each array 23 is printed on a flexible printed circuit board 34, which is optionally contained in a container, such as a plastic container. Antennas 22 are configured to transmit electromagnetic waves, such as radiofrequency waves, into tissue of a subject. Typically, the waves are transmitted with a power between -10 and 0 dBm. Antennas 22 are further configured to receive reflections of the waves from the tissue, and to convert the reflections to analog signals.

[0046] In some embodiments, each antenna 22 is configured to transmit and receive at multiple frequencies. In some such embodiments, each antenna is configured to transmit at at least some of the multiple frequencies simultaneously, e.g., using orthogonal frequency division multiplexing (OFDM) signaling, as further described below. In other such embodiments, the antenna is configured to transmit at the multiple frequencies sequentially.

[0047] Typically, antennas 22 do not contact the subject. For example, in some embodiments, antennas 22 are placed beneath a surface, such as a mattress 32 (drawn in transparent in Fig. 1) of a bed 30, on which the subject sits or lies. Alternatively, for example, antennas 22 are placed on a sheet that covers the subject.

[0048] System 20 further comprises a signaling unit 42. In some embodiments, unit 42 comprises a vector network analyzer (VNA) 36 comprising a signal generator 39, which is configured to generate input signals 33 (Fig. 4) that drive the transmission of antennas 22. VNA 36 further comprises a receiver (RCVR) 35, comprising a bidirectional coupler or Wheatstone bridge for example, configured to receive the analog signals from the antennas. VNA 36 further comprises an analog-to-digital (A / D) converter 37 configured to convert the analog signals to digital signals, typically after down-conversion to a suitable frequency range. In some embodiments, VNA 36 further comprises a digital signal processing (DSP) module 84 configured to process the digital signals in hardware.

[0049] Alternatively or additionally, signaling unit 42 further comprises any other element comprising signal generator 39, receiver 35, (A / D) converter 37, and, optionally, DSP module 84. For example, in some embodiments, system 20 comprises an OFDM subunit 43 in which these modules are configured for OFDM signaling, e.g., as described below with reference to Fig. 6.

[0050] Typically, the raw digital signals and / or the output from DSP module 84 are passed, over a wired or wireless connection, to a processing unit 24 comprising a processor 26. Processor 26 is configured to process this input or to communicate the input, via a network interface 28, to a remote processor (such as a processor on a cloud-computing platform) configured to process the input. Alternatively, the processing is performed cooperatively by processor 26 and a remote processor.

[0051] Signaling unit 42 further comprises additional circuitry 38, typically comprising a multiplexer (MUX) 41 configured to switch input signal 33 (Fig. 4) between the antennas, e.g., as further described below with reference to Figs. 3-4. Typically, each switch in multiplexer 41 is connected to a respective antenna 22, or to a respective group of antennas, and is configured to be in one of three states: a high state, in which the connected antenna(s) can transmit electromagnetic waves in response to the input signals, a ground state, in which the connected antenna(s) can receive reflections of the electromagnetic waves, and a high-impedance state, in which the connected antenna(s) can neither transmit nor receive. Alternatively or additionally, circuitry 38 further comprises calibration electronics 45 configured, for example, to periodically execute a selfcalibration of system 20 by connecting signal generator 39 to a calibration load and / or based on historical data.

[0052] Typically, signaling unit 42 further comprises a microcontroller 40, which is configured to control VNA 36, OFDM subunit 43, and / or circuitry 38, e.g., in response to instructions received, from processor 26, over a wired or wireless connection.

[0053] Typically, for embodiments with multiple arrays 23 of antennas as shown in Fig. 1, system 20 comprises a separate signaling unit 42 for each array. In some embodiments, the components of each signaling unit 42 are printed on printed circuit board 34, printed on a separate printed circuit board near printed circuit board 34, or located within processing unit 24.

[0054] As described in detail below, the processing circuitry of system 20, which includes any one or more of processor 26, a remote processor, components of signaling unit 42 such as DSP 84 and microcontroller 40, phase controller 49 (Fig. 5B), and any other circuitry, is configured to perform various functions such as transmitting the electromagnetic waves from the antennas (i.e., driving the antennas to transmit the electromagnetic waves) by setting the properties (e.g., frequency and phase) of input signal 33 (Fig. 4), switching between the antennas, and processing the digital signals received in return. Typically, the processing of the digital signals includes the removal of noise and any interference, e.g., based on baseline signals received from one or more antennas that do not receive any reflections from the tissue. The processing of the signals further includes detecting a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue, e.g., as further described below with reference to Fig. 2. In response to detecting the difference, the processing circuitry outputs an indication that the first portion of the tissue includes a pressure ulcer. For example, in some embodiments, processor 26 outputs the indication on a display 44 and / or communicates the indication to a remote device, such as a smartphone, and / or a patient monitoring system. In some embodiments, as further described below with reference to Figs. 5A-C, the antennas receive the reflections of the electromagnetic waves from multiple different overlapping pathways through the tissue. In such embodiments, typically, the processing circuitry back- projects the digital signals onto the tissue, thereby computing the property at multiple depths. Advantageously, the mapping of the property in three spatial dimensions - i.e., the two (typically horizontal) spatial dimensions spanned by the two-dimensional arrangement of antennas and by the surface of the subject's body, along with the tissue depth - facilitates a more accurate location of any pressure ulcers.

[0055] In some embodiments, the processing circuitry detects the pressure ulcer at the first portion of tissue by comparing the value of the property at the first portion of the tissue with the value of the property at the second portions of the tissue at a single point in time. Thus, for example, the processing circuitry may detect a pressure ulcer immediately upon the admission of the subject, before the ulcer worsens.

[0056] Alternatively, the processing circuitry detects the pressure ulcer based on a change in the property at the first portion of the tissue over time, even without comparing this change to the change at other portions of tissue.

[0057] Typically, however, the processing circuitry detects the pressure ulcer by detecting a greater change in the property at the first portion of the tissue over time, relative to the second portions of the tissue. Advantageously, considering the time dimension, in addition to two or three spatial dimensions, facilitates better detection of any pressure ulcers.

[0058] In general, each of the processors described herein may be embodied as a single processor, or as a cooperatively networked or clustered set of processors. The functionality of the processor may be implemented solely in hardware, e.g., using one or more fixed-function or general -purpose integrated circuits, Application-Specific Integrated Circuits (ASICs), and / or Field-Programmable Gate Arrays (FPGAs). Alternatively, this functionality may be implemented at least partly in software. For example, the processor may be embodied as a programmed processor comprising, for example, a central processing unit (CPU) and / or a Graphics Processing Unit (GPU). Program instructions, including software programs, and / or data may be loaded for execution and processing by the CPU and / or GPU. The program instructions and / or data may be downloaded to the processor in electronic form, over a network, for example. Alternatively or additionally, the program instructions and / or data may be provided and / or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. Such program instructions and / or data, when provided to the processor, produce a machine or special-purpose computer, configured to perform the tasks described herein.

[0059] PRESSURE ULCER DETECTION

[0060] Reference is now made to Fig. 2, which plots experimentally-derived Si l return losses as a function of frequency, which demonstrate principles of some embodiments of the present invention.

[0061] Typically, to facilitate detecting differences between pressure ulcers and normal tissue, each antenna 22 (Fig. 1) transmits, into the tissue, electromagnetic waves at multiple frequencies (referred to below as "subcarriers"), such as between 100 and 1000 frequencies. In some such embodiments, the spacing between the frequencies is between 100 and 1000 kHz, such that, for example, the total bandwidth of the transmitted waves is at least 100 MHz, such as between 100 and 1000 MHz. The multiple frequencies may be centered, for example, at between 2 and 10 GHz. By virtue of the multiple transmission frequencies, the reflections are received from the tissue at multiple frequencies, as shown in Fig. 2. It is noted that the frequencies of the reflections may be shifted relative to the transmission frequencies.

[0062] In some embodiments, as further described below with reference to Fig. 6, at least some of the subcarriers are transmitted simultaneously using OFDM signaling. For example, in some embodiments, a set of simultaneous subcarriers having a bandwidth of between 10 and 100 MHz is transmitted multiple times, the subcarriers being frequency-shifted each time by a different amount such that the total bandwidth of the subcarriers is between 100 and 1000 MHz, for example. Alternatively, at least some of the subcarriers are transmitted simultaneously using other techniques, such as by using a chirp signal, a direct sequence spread spectrum, or multiple single carriers. Alternatively, all the subcarriers are transmitted sequentially, rather than simultaneously.

[0063] In some embodiments, following each transmission, the reflections from the tissue are received by the transmitting antenna.

[0064] In some such embodiments, the processing circuitry computes the Si l return loss, as a function of frequency, based on the digital signals. Based on the Si l return loss, the processing circuitry detects a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue.

[0065] For example, the top plot in Fig. 2 shows the S 11 return loss when transmitting into normal tissue at multiple frequencies between 4.6 GHz and 5.1 GHz with a central frequency of 4.85 GHz. In contrast, the bottom plot in Fig. 2 shows the SI 1 return loss when transmitting into a pressure ulcer at the same frequencies. As can be observed, there is a difference AF between the respective frequencies at which the power of the reflections is a minimum; in particular, this frequency is greater for the pressure ulcer than for the normal tissue. There is also a difference between these minimums: the minimum return loss is around -40 dB at the top of Fig. 2 but is slightly greater than -40 dB at the bottom of Fig. 2. (It is noted that in some cases, the frequency at which the power of the reflections is a minimum, and / or the minimum return loss, is greater for normal tissue, relative to a pressure ulcer.) Hence, in some embodiments, the processing circuitry detects a difference in at least one of the above properties - that is, AF and / or the difference between the minimum reflection powers - between the first portion of the tissue and the second portions of the tissue. Alternatively or additionally, for example, the processing circuitry detects a difference in the bandwidth of the reflections, and / or in a shift of the reflection frequencies relative to the transmission frequencies.

[0066] In some embodiments, following each transmission, the reflections are received by an antenna different from the transmitting antenna, alternatively or additionally to being received by the transmitting antenna, e.g., as described below with reference to Fig. 3, Fig. 4, and Figs. 5A-C. In such embodiments, differences between pressure ulcers and normal tissue can be identified based on the S12 and / or S21 return loss, alternatively or additionally to the SI 1 return loss. For example, in some embodiments, based on the S12 and / or S21 return loss, the processing circuitry detects a difference in any of the properties described above, a difference in the dissipation ratio at any one or more frequencies, or a difference in the frequency at which the dissipation ratio is a maximum.

[0067] In response to detecting the difference, the processing circuitry outputs an indication that the first portion of the tissue includes a pressure ulcer, as described above with reference to Fig. 1.

[0068] As described above with reference to Fig. 1, the difference can be detected by mapping the property over the tissue in two spatial dimensions without the tissue depth and without the time dimension. In other words, by transmitting into, and receiving from, the tissue at multiple locations (using multiple antennas), the processing circuitry can construct a two-dimensional map of the property. Alternatively, as described above with reference to Fig. 1, the processing circuitry can map the property with the added tissue-depth dimension. In other words, by transmitting into, and receiving from, the tissue at multiple locations and at multiple depths, the processing circuitry can construct a three-dimensional map of the property. Alternatively or additionally, as described above with reference to Fig. 1, the processing circuitry can perform the 2D or 3D spatial mapping repeatedly over a period of time, thereby adding a time dimension to the mapping. In some embodiments, alternatively or additionally to detecting the difference by analyzing the amplitudes of the reflections, the processing circuitry detects the difference by analyzing the phases of the reflections. For example, in some embodiments, the processing circuitry detects a difference in the phase shift of each subcarrier.

[0069] In some embodiments, the processing circuitry detects the difference between the portions of tissue using machine learning. For example, in some embodiments, the processing circuitry initially uses a model that requires relatively little training data, such as a support vector machine or a decision forest. Subsequently, as more data is acquired from multiple subjects, the processing circuitry trains a more complex model such as a convolutional neural network, and then uses this model, for subsequent subjects, for differentiating between the portions of tissue.

[0070] SIMPLIFIED SWITCHING

[0071] In some embodiments, each antenna is connected to a different respective switch. Typically, however, embodiments of the present invention utilize various techniques to simplify the switching system for the antennas. In this regard, reference is now made to Fig. 3, which is a schematic illustration of a two-dimensional array 23 of antennas 22, in accordance with some embodiments of the present invention.

[0072] In some embodiments, array 23 includes multiple intercrossed one-dimensional arrays of the antennas, the antennas in each of the one-dimensional arrays being interconnected such that the antennas in each of the one-dimensional arrays transmit simultaneously and receive simultaneously. For example, in some embodiments, as shown in Fig. 3, array 23 comprises multiple rows 25 (i.e., horizontal one-dimensional arrays) and columns 27 (i.e., vertical onedimensional arrays) of antennas, the antennas in each row 25 and in each column 27 being interconnected. Alternatively or additionally, array 23 includes one or more diagonal onedimensional arrays of interconnected antennas.

[0073] In such embodiments, the electromagnetic waves are transmitted from the one-dimensional arrays, i.e., in each one-dimensional array, the antennas transmit simultaneously, as if they were a single antenna, by virtue of being interconnected. Likewise, the reflections of the waves are converted by the one-dimensional arrays, i.e., in each one-dimensional array, the antennas convert the reflections simultaneously, as if they were a single antenna, by virtue of being interconnected. To compensate for the spatial resolution lost by the interconnectedness of each one -dimensional array, the processing circuitry correlates the digital signals between non-parallel one-dimensional arrays, thereby locating the portion of the tissue at which a pressure ulcer is possibly located. Thus, advantageously, the interconnectedness of the antennas reduces the connections to signaling unit 42 and simplifies multiplexer 41 (Fig. 1) without significantly compromising spatial resolution.

[0074] For example, Fig. 3 shows a first signal 80 obtained by activating, in sequence, each column 27, such that the column transmits and receives, and a second signal 82 obtained by activating, in sequence, each row 25, such that the row transmits and receives. To perform the correlation, the processing circuitry identifies the horizontal position of the greatest deviation in first signal 80 and the vertical position of the greatest deviation in second signal 82, and then locates the potential pressure ulcer at the intersection 47 of these positions.

[0075] It is noted that any suitable scheme for sweeping through the one-dimensional arrays is within the scope of the present disclosure, including, for example, a scheme per which the receiving one-dimensional array is different from, and optionally not parallel to, the transmitting one -dimensional array. For example, in some embodiments, a row receives reflections of electromagnetic waves transmitted by a column, or vice versa.

[0076] Another scheme for reducing the connections and simplifying the multiplexer, in accordance with some embodiments ofthe present invention, is shown in Fig. 4, to which reference is now made.

[0077] In some embodiments, at least one subset of antennas 22 are connected, via respective bandpass filters 29 having different respective pass bands, to a common line 31. In such embodiments, electromagnetic waves are altematingly transmitted from each antenna of the subset by varying the frequency of the input signal 33 delivered to the line, such that input signal 33 is altematingly passed through each of bandpass filters 29 connected to the line. In other words, the antennas in the subset are altematingly activated by switching the input frequency, rather than via physical switches.

[0078] As a purely illustrative example, Table 1 below shows one way in which eight antennas 22, which are numbered 1-8 in Table 1, can be altematingly activated using this scheme.

[0079] TABLE 1

[0080] In some such embodiments, there are at least two such subsets of the antennas. For example, in some embodiments, as shown in Fig. 4, antennas 22 are arranged in multiple rows or columns each having multiple antennas connected to a respective common line 31 via fdters 29 as described above. (Typically, the number of antennas in each row, and / or the number of antennas in each column, is the same.) As the frequency of input signal 33 is varied, thereby alternating the transmitting antennas in one of the subsets, the reflections are altematingly converted by each antenna of another subset (alternatively or additionally to being converted by the transmitting antennas), by virtue of the analog signals altematingly passing through each of the bandpass fdters connected to the other subset.

[0081] For example, in Fig. 4, some of the antennas are labeled with reference number 22 followed by an appended letter, which indicates the subset to which the antenna belongs, and an appended number, which indicates the ordering of the antenna within the subset. Likewise, some of the common lines are labeled with reference number 31 followed by an appended letter indicating the subset. With reference to this labeling, and supposing that antenna 22al is activated by an input frequency of 5 GHz, antennas 22b 1, 22c 1, ... may also be activated by an input frequency of 5 GHz . For example, antennas 22a 1 , 22b 1 , 22c 1 , . . . may each be connected to a bandpass fdter with a pass band of 4.92 - 5.08 GHz. Similarly, supposing that antenna 22a2 is activated by an input frequency of 5.23 GHz, antennas 22b2, 22c2, . . . may also be activated by an input frequency of 5.23 GHz. For example, antennas 22a2, 22b2, 22c2, . . . may each be connected to a bandpass filter with a pass band of 5.15 - 5.3 GHz. Thus, for example, given an input frequency of 5 GHz delivered to line 31a, antenna 22al may transmit electromagnetic waves, and antenna 22b 1 may convert the reflections of these waves to an analog signal that is passed over line 31b, and / or antenna 22c 1 may convert the reflections to an analog signal that is passed over line 31c.

[0082] In such embodiments, before each transmission, the processing circuitry controls multiplexer 41 (Fig. 1) so as to select a transmitting line 31 and a receiving line 31 (or multiple receiving lines). Subsequently, the processing circuitry selects the transmitting and receiving antennas by setting the input frequency. Thus, for example, multiple groups of antennas may be activated sequentially, whereby each group includes an antenna in one common line 31 and a corresponding antenna in at least one other common line 31. By way of illustration, one example sequence is 22al / [22al, 22bl], 22a2 / [22a2, 22b2], ... 22bl / [22al, 22bl, 22cl], 22b2 / [22a2, 22b2, 22c2], ... , where for each group of antennas, the reference number preceding the forward-slash refers to the transmitting antenna and the reference numbers following the forward-slash refer to the receiving antennas. Another example sequence is 22al / 22bl, 22bl / 22al, 22bl / 22cl, 22cl / 22bl, ...

[0083] In some embodiments, to further simplify the switching, the switches in the multiplexer are arranged in a tree structure such that, assuming N intercrossed one -dimensional arrays (Fig. 3) or N common lines 31 (Fig. 4), the number of switches is log2(N).

[0084] COMPUTING THE PROPERTY AT MULTIPLE DEPTHS

[0085] As noted above with reference to Fig. 1, in some embodiments, the reflections are received from multiple different overlapping pathways through the tissue, thereby facilitating computing the property (e.g., the frequency at which the power of the reflections is a minimum or the dissipation ratio is a maximum) at multiple depths. For further details in this regard, reference is now made to Figs. 5A-C, which are schematic illustrations of reflections of electromagnetic waves received from multiple different overlapping pathways 50 though tissue 52, in accordance with some embodiments of the present invention.

[0086] In some embodiments, the reflections are received from multiple different overlapping pathways through the tissue by varying the spacing between the transmitting antenna and the receiving antenna. For example, in Fig. 5A, a first antenna 22_1 transmits electromagnetic waves, as indicated by an arrow pointing from first antenna 22_1, and second and third antennas 22_2 and 22_3 receive reflections of these waves, as indicated by arrows pointing to these antennas. (Optionally, antennas 22_1, 22_2, and 22_3 belong to different respective one-dimensional arrays of interconnected antennas as shown in Fig. 3, or are connected to different respective common lines 31 as shown in Fig. 4.) Due to the spacing between third antenna 22_3 and first antenna 22_1 being different from the spacing between second antenna 22_2 and first antenna 22_1, the pathway 50 from which the reflections are received by third antenna 22_3 partially overlaps the pathway from which the reflections are received by second antenna 22_2, this overlapping region 51 being explicitly marked in an inset portion of Fig. 5A.

[0087] Alternatively or additionally, the reflections are received from multiple different overlapping pathways through the tissue by transmitting the electromagnetic waves at multiple different angles. For example, in Fig. 5B, first antenna 22_1 transmits electromagnetic waves at a first angle and receives reflections thereof, as indicated by a bidirectional arrow, second antenna 22_2 transmits electromagnetic waves at a second angle, as indicated by an arrow pointing from this antenna, and first antenna 22_1 receives reflections of these waves, as indicated by an arrow pointing to this antenna. Due to the second angle differing from the first angle, first antenna 22_1 receives reflections from two partially-overlapping pathways 50, this overlapping region 51 being explicitly marked in an inset portion of Fig. 5B.

[0088] In some such embodiments, the electromagnetic waves are transmitted at multiple different angles using phased arrays of the antennas. For example, in some embodiments, each antenna comprises multiple sub-antennas 22s and a phase controller 49 configured to vary the phase of the input signal across sub-antennas 22s, such that each antenna can function as a phased array. Alternatively or additionally, the phase is varied between a subset (e.g., a row or column) of the antennas such that, when transmitting simultaneously, the subset functions as a phased array.

[0089] Alternatively or additionally, the electromagnetic waves are transmitted at multiple different angles by virtue of at least some of the antennas being different from each other with respect to at least one antenna property, such as a tilt angle. For example, in Fig. 5B, the tilt angle of second antenna 22_2 differs from that of first antenna 22_1.

[0090] Alternatively or additionally, the reflections are received from multiple different overlapping pathways through the tissue by transmitting the electromagnetic waves with multiple different penetration depths, such as a set of penetration depths between the tissue surface and the underlying bone. For example, in Fig. 5C, first antenna 22_1 transmits electromagnetic waves at two different penetration depths, such that first antenna 22_1 (and / or another antenna) receives reflections of these waves from two different overlapping pathways 50, this overlapping region 51 being explicitly marked in an inset portion of Fig. 5C.

[0091] In some such embodiments, the electromagnetic waves are transmitted with multiple different penetration depths by varying the frequency of the electromagnetic waves; in particular, lower frequencies penetrate tissue more, relative to higher frequencies. For example, Fig. 5C assumes first antenna 22 1 to be a multi-band antenna configured to transmit at multiple different frequency bands. As another example, in some embodiments, at least some of the antennas have different sizes, and hence different resonant frequencies, from each other.

[0092] It is emphasized that any suitable combination of the techniques illustrated in Figs. 5A-C is within the scope of the present invention. OFDM TRANSMISSION

[0093] Reference is now made to Fig. 6. which is a schematic illustration of OFDM subunit 43, in accordance with some embodiments of the present invention.

[0094] In some embodiments, signal generator 39 comprises a binary data module 56, which supplies binary data for the modulation, and a Quadrature Phase Shift Keying (QPSK) module 58, which modulates the binary data using QPSK modulation symbols. The output from QPSK module 58 is passed through an M-point Inverse Fast Fourier Transform (IFFT) module 60, M being the number of frequencies that are to be simultaneously transmitted. The output from M-point IFFT module 60 is passed through cyclic-prefix-adding module 62, which adds a cyclic prefix for orthogonality (and hence, elimination of cross-talk). The output from cyclic -prefix-adding module 62 is passed through a programmable up-converter 64, which shifts the frequencies by varying amounts to allow a relatively large total bandwidth, such as a bandwidth of at least 100 MHz, e.g., between 100 and 1000 MHz. The output from programmable up-converter 64 is input signal 33, which is delivered to the antennas via MUX 41 (Fig. 1).

[0095] In such embodiments, receiver 35 comprises a programmable down-converter 68, which performs the inverse function of programmable up-converter 64. The output of programmable down-converter 68 is passed through A / D converter 37. Optionally, the digital signals then undergo preliminary processing by DSP module 84, which comprises, for example, a cyclic- prefix-removing module 72 and an M-point Fast Fourier Transform (FFT) module 74, which perform the inverse functions of cyclic-prefix-adding module 62 and M-point IFFT module 60, respectively, and a phase-and-amplitude estimator 76, which utilizes the QPSK modulation symbols, which are received from QPSK module 58, to estimate the amplitude and phase at each frequency, typically after removing any noise or interference. The output 78 from phase-and- amplitude estimator 76 is passed to processor 26 (Fig. 1), which analyzes output 78, e.g., as described above with reference to Fig. 2.

[0096] It is noted that the use of OFDM as described herein differs significantly from conventional uses of OFDM. In particular, conventionally, OFDM is used to transmit information to a remote site, whereby the receiver, at the remote site, attempts to infer the information from the received signal without knowing the QPSK modulation symbols. In contrast, in embodiments of the present invention, the receiver is collocated with the transmitter, and uses the known modulation symbols to estimate properties of the channel (in particular, the tissue) through which the signal is transmitted. Moreover, conventional OFDM typically does not repeat the same waveform across multiple frequencies, such that conventional OFDM typically uses a narrower bandwidth than do embodiments of the present invention, and typically does not employ programmable up-converter 64 or programmable down-converter 68 as do embodiments of the present invention.

[0097] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.

Claims

CLAIMS1. A system, comprising: multiple antennas configured to: transmit electromagnetic waves into tissue of a subject, and convert, to analog signals, reflections of the waves received from the tissue; an analog-to-digital converter, configured to convert the analog signals to digital signals; and processing circuitry, configured to: by processing the digital signals, detect a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue, and in response to detecting the difference, output an indication that the first portion of the tissue includes a pressure ulcer.

2. The system according to claim 1, wherein the electromagnetic waves include radiofrequency waves.

3. The system according to claim 1, wherein the processing circuitry is configured to detect the difference by detecting a greater change in the property at the first portion of the tissue over time, relative to the second portions of the tissue.

4. The system according to claim 1, wherein the antennas are arranged in at least one two- dimensional array.

5. The system according to claim 4, wherein the two-dimensional array includes multiple intercrossed one-dimensional arrays of the antennas, the antennas in each of the one-dimensional arrays being interconnected such that the antennas in each of the one-dimensional arrays transmit simultaneously and receive simultaneously, wherein the one-dimensional arrays are configured to transmit the electromagnetic waves and to convert the reflections, and wherein the processing circuitry is configured to locate the first portion of the tissue by correlating the digital signals between non-parallel ones of the one-dimensional arrays.

6. The system according to claim 1, wherein at least one subset of the antennas are connected, via respective bandpass filters having different respective pass bands, to a common line, andwherein the processing circuitry is configured to vary a frequency of an input signal delivered to the line such that the input signal is altematingly passed through each of the bandpass filters, and hence, the electromagnetic waves are altematingly transmitted from each antenna of the subset in response to the input signal.

7. The system according to claim 6, wherein at least one other subset of the antennas are connected, via respective other bandpass filters, to another common line, and wherein each antenna of the other subset is configured to convert the reflections in turn as the frequency of the input signal is varied, by virtue of the analog signals altematingly passing through each of the other bandpass filters.

8. The system according to any one of claims 1-7, wherein each one of the antennas is configured to transmit the electromagnetic waves at multiple frequencies.

9. The system according to claim 8, wherein the property includes a frequency at which a power of the reflections is a minimum.

10. The system according to claim 8, wherein the property includes a frequency at which a dissipation ratio of the electromagnetic waves is a maximum.

11. The system according to claim 8, wherein each one of the antennas is configured to transmit the electromagnetic waves at the multiple frequencies simultaneously.

12. The system according to claim 11, further comprising a signal generator configured to generate a modulated input signal to the antennas using orthogonal frequency division multiplexing, such that each one of the antennas, in response to the input signal, transmits the electromagnetic waves at the multiple frequencies simultaneously using orthogonal frequency division multiplexing signaling.

13. The system according to any one of claims 1-7, wherein the reflections are received from multiple different overlapping pathways through the tissue, and wherein the processing circuitry is configured to back-project the digital signals onto the tissue, thereby computing the property at multiple depths.

14. The system according to claim 13, wherein the antennas are configured to transmit the electromagnetic waves at multiple different angles, such that the reflections are received from the multiple different overlapping pathways through the tissue.

15. The system according to claim 14, wherein the processing circuitry is configured to control a phase of an input signal to the antennas such that the antennas transmit the electromagnetic waves at the multiple different angles by functioning as phased arrays.

16. The system according to claim 14, wherein the antennas are configured to transmit the electromagnetic waves at the multiple different angles by virtue of at least some of the antennas being different from each other with respect to at least one antenna property.

17. The system according to claim 16, wherein the antenna property includes a tilt angle.

18. The system according to claim 13, wherein the antennas are configured to transmit the electromagnetic waves with multiple different penetration depths, such that the reflections are received from the multiple different overlapping pathways through the tissue.

19. The system according to claim 18, wherein the processing circuitry is further configured to vary a frequency of the electromagnetic waves such that the antennas transmit the electromagnetic waves with the multiple different penetration depths.

20. The system according to claim 13, wherein the processing circuitry is further configured to vary a spacing between a transmitting one of the antennas and a receiving one of the antennas, such that the reflections are received from the multiple different overlapping pathways through the tissue.

21. A method for use with multiple antennas and an analog -to-digital converter, the method comprising: transmitting electromagnetic waves, from the antennas, into tissue of a subject, such that the antennas convert, to analog signals, reflections of the waves received from the tissue, and the analog-to-digital converter converts the analog signals to digital signals; by processing the digital signals, detecting a difference, in at least one property, between the reflections from a first portion of the tissue and the reflections from one or more second portions of the tissue; and in response to detecting the difference, outputting an indication that the first portion of the tissue includes a pressure ulcer.

22. The method according to claim 21, wherein the electromagnetic waves include radiofrequency waves.

23. The method according to claim 21, wherein detecting the difference comprises detecting a greater change in the property at the first portion of the tissue over time, relative to the second portions of the tissue.

24. The method according to claim 21, wherein the antennas are arranged in at least one two- dimensional array.

25. The method according to claim 24, wherein the two-dimensional array includes multiple intercrossed one-dimensional arrays of the antennas, the antennas in each of the one-dimensional arrays being interconnected such that the antennas in each of the one-dimensional arrays transmit simultaneously and receive simultaneously, wherein transmitting the electromagnetic waves comprises transmitting the electromagnetic waves from the one-dimensional arrays such that the one-dimensional arrays convert the reflections, and wherein processing the digital signals comprises locating the first portion of the tissue by correlating the digital signals between non-parallel ones of the one-dimensional arrays.

26. The method according to claim 21, wherein at least one subset of the antennas are connected, via respective bandpass filters having different respective pass bands, to a common line, and wherein transmitting the electromagnetic waves comprises varying a frequency of an input signal delivered to the line such that the input signal is altematingly passed through each of the bandpass filters, and hence, the electromagnetic waves are altematingly transmitted from each antenna of the subset in response to the input signal.

27. The method according to claim 26, wherein at least one other subset of the antennas are connected, via respective other bandpass filters, to another common line, and wherein the reflections are altematingly converted by each antenna of the other subset as the frequency of the input signal is varied, by virtue of the analog signals altematingly passing through each of the other bandpass filters.

28. The method according to any one of claims 21-27, wherein transmitting the electromagnetic waves comprises transmitting the electromagnetic waves at multiple frequencies from each one of the antennas.

29. The method according to claim 28, wherein the property includes a frequency at which a power of the reflections is a minimum.

30. The method according to claim 28, wherein the property includes a frequency at which a dissipation ratio of the electromagnetic waves is a maximum.

31. The method according to claim 28, wherein transmitting the electromagnetic waves at the multiple frequencies comprises simultaneously transmitting the electromagnetic waves at the multiple frequencies.

32. The method according to claim 31, wherein simultaneously transmitting the electromagnetic waves at the multiple frequencies comprises simultaneously transmitting the electromagnetic waves at the multiple frequencies using orthogonal frequency division multiplexing signaling.

33. The method according to any one of claims 21-27, wherein the reflections are received from multiple different overlapping pathways through the tissue, and wherein processing the digital signals comprises back-projecting the digital signals onto the tissue, thereby computing the property at multiple depths.

34. The method according to claim 33, wherein transmitting the electromagnetic waves comprises transmitting the electromagnetic waves at multiple different angles, such that the reflections are received from the multiple different overlapping pathways through the tissue.

35. The method according to claim 34, wherein transmitting the electromagnetic waves at the multiple different angles comprises transmitting the electromagnetic waves at the multiple different angles using phased arrays of the antennas.

36. The method according to claim 34, wherein transmitting the electromagnetic waves at the multiple different angles comprises transmitting the electromagnetic waves at the multiple different angles by virtue of at least some of the antennas being different from each other with respect to at least one antenna property.

37. The method according to claim 36, wherein the antenna property includes a tilt angle.

38. The method according to claim 33, wherein transmitting the electromagnetic waves comprises transmitting the electromagnetic waves with multiple different penetration depths, such that the reflections are received from the multiple different overlapping pathways through the tissue.

39. The method according to claim 38, wherein transmitting the electromagnetic waves with the multiple different penetration depths comprises transmitting the electromagnetic waves with the multiple different penetration depths by varying a frequency of the electromagnetic waves.

40. The method according to claim 33, further comprising varying a spacing between a transmitting one of the antennas and a receiving one of the antennas, such that the reflections are received from the multiple different overlapping pathways through the tissue.

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