Ultrasound apparatus

Abstract

A device and method using ultrasound for communication and for obtaining spatial information about an object probed by the ultrasound. The ultrasound communication system comprising an ultrasound transmitter 202; an ultrasound receiver 204; wherein the ultrasound transmitter is configured to be located at a different position to the ultrasound receiver and at least one of the ultrasound transmitter and ultrasound receiver is configured to be implanted in the body tissue structure. A controller 162 is configured to receive signalling representative of scattered ultrasound received by the ultrasound receiver, the scattered ultrasound resulting from data communication ultrasound transmitted by the ultrasound transmitter and interacting with the body tissue structure; the scattered ultrasound comprising at least one scatter component corresponding to a propagation channel of the data communication ultrasound; determine, in dependence on the signalling, a channel impulse response indicative of the propagation channel and obtain, in dependence on the channel impulse response, an indication of a property of the body tissue structure; and output the indication of the property of the body tissue structure.

Description

TECHNICAL FIELD The present disclosure relates to devices and methods for obtaining spatial information about an object through which ultrasound communications is being performed. Examples relate to ultrasound communication systems for performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure, methods for performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure using an ultrasound communication system, and computer program code for performing such methods. BACKGROUND Cardiac resynchronisation pacemakers are a mature technology, having originally being devised in the early 1950’s. In recent years, developments have been made to evolve the technology from large implantable pulse generators with leads going to the heart, to smaller devices that can be implanted in each chamber. However, there is still a challenge to control multiple chambers of the heart in synchrony. Furthermore, it can be challenging to obtain clinical information about a patient using pacemaker technology. Techniques such as Electrocardiograms (ECGs) or Magnetic Resonance Imaging (MRI) can be used to obtain scans of the heart, but these require a patient to attend a clinical setting, can take a significant amount of time to obtain the relevant data, and tend to be used once there is an indication of a need for clinical intervention. That is, such techniques are used for the investigation of a patient’s heart issues after a symptom has developed. They are not used as ongoing monitoring techniques due to the time, costs, expense, and limited availability of the equipment and clinicians required. Also, the information which can be extracted from an ECG can be limited, and while MRI scans can provide more detailed information, they are much more expensive, require more specialist equipment, and are not suitable for ongoing or constant monitoring. Developments are in progress to look at the use of techniques which can communicate with pacemaker systems and control the synchronisation of the electrodes. It would be advantageous to be able to control pacemaker technology in this way and understand how the patient’s heart function is. It would further be advantageous to be able to monitor other body structures such as other organs in a similar way. It is an aim of the present disclosure to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION In an aspect there is provided an ultrasound communication system for performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure; the ultrasound communication system comprising: an ultrasound transmitter; an ultrasound receiver; wherein the ultrasound transmitter is configured to be located at a different position to the ultrasound receiver and at least one of the ultrasound transmitter and ultrasound receiver is configured to be implanted in the body tissue structure; and a controller; wherein the controller is configured to: receive signalling representative of scattered ultrasound received by the ultrasound receiver, the scattered ultrasound resulting from data communication ultrasound transmitted by the ultrasound transmitter and interacting with the body tissue structure; the scattered ultrasound comprising at least one scatter component corresponding to a propagation channel of the data communication ultrasound; determine, in dependence on the signalling, a channel impulse response indicative of the propagation channel; obtain, in dependence on the channel impulse response, an indication of a property of the body tissue structure; and output the indication of the property of the body tissue structure. The scattered ultrasound may comprise a direct forward scatter component indicative of a direct propagation channel from the ultrasound transmitter to the ultrasound receiver. The scattered ultrasound may comprise at least one indirect scatter component indicative of a propagation channel comprising at least one reflection boundary of the body tissue structure from which the transmitted data communication ultrasound is reflected. The at least one indirect scatter component may comprise at least one multiply-reflected indirect scatter component that has reflected of plural surfaces of the body tissue structure. The controller may be configured to: compare the indication of the property of the body tissue structure to a prior stored indication of the property of the body tissue structure body tissue; determine whether a variation between the indication and the stored indication exceeds a predetermined variation threshold; and output an alert indication in dependence on the variation exceeding the predetermined variation threshold. The prior stored indication may be indicative of historical indications recorded over a plurality of prior obtained indications, such as an average, a moving average, a variation in rate of change of indication, etc. Obtaining the indication of the property of the body tissue structure may comprises: obtaining, in dependence on the channel impulse response, one or more of a signal amplitude and a propagation delay of the received scattered ultrasound; and determining the indication of the property of the body tissue structure in dependence on the one or more of the amplitude and the delay. Obtaining the indication of the property of the body tissue structure may comprise: obtaining, in dependence on the channel impulse response and a predefined model of the body tissue structure, a distance travelled by the received scattered ultrasound, based on a reference. The predefined model may comprise an indication of expected dimensions and expected ultrasound attenuation properties of the body tissue structure. The signalling may be representative of scattered ultrasound resulting from pulsed data communication ultrasound transmitted by the ultrasound transmitter. The controller may be configured to perform the intrabody data communication by: transmission of the pulsed data communication ultrasound by the ultrasound transmitter in a transmission signal portion of a plurality of periodic transmission periods; and reception of the pulsed data communication scattered ultrasound by the ultrasound receiver in a quiet signal portion between transmission signal portions. A distance between the ultrasound transmitter and the ultrasound receiver at the different position may be a known separation distance, or a distance determined based on a determination of a propagation delay in data communication ultrasound transmitted between the ultrasound transmitter and the ultrasound receiver. The body tissue structure may undergo periodic movement, and the controller may be configured to: determine, in dependence on the signalling, a periodicity of variation of the received scattered ultrasound; and determine a corresponding periodicity of movement of the body tissue structure in dependence on the periodicity of variation of the received scattered ultrasound. For example, the body tissue structure may be a heart, and the periodicity of movement may correspond to a heartbeat. The body tissue structure may undergo quasi periodic or chaotic movement, and the controller may be configured to determine, in dependence on the signalling, time and amplitude domain vectors of variation of the received scattered ultrasound; and determine corresponding time and amplitude domain vectors of movement of the body tissue structure in dependence on the of variation of the received scattered ultrasound. In this way information about the movement of the tissue structure can be obtained. The controller may be configured to output a plot of time versus a determined distance travelled by the at least one scattered ultrasound, wherein the indication of the property of the body tissue structure corresponds to a corresponding feature in the plot. The controller may be configured to: receive stimulation signalling representative of electrical stimulation provided to the body tissue structure; correlate the received signalling representative of scattered ultrasound with the received stimulation signalling; and output the indication of the property of the body tissue structure in dependence on the correlation. The body tissue structure may be a heart, for example, and the indication of the property of the body tissue structure may comprises an indication of geometry of at least a portion of the heart. The controller may be configured to: determine, in dependence on the indication of the property of the body tissue structure, a data communication feedback parameter indicative of a property of data communication ultrasound to be transmitted according to the property of the body tissue structure; and control the ultrasound transmitter to output data communication ultrasound according to the data communication feedback parameter. The property of the data communication ultrasound to be transmitted via the body tissue may be an energy loss, and the data communication feedback parameter may be indicative of a second energy loss of the data communication ultrasound, the second energy loss being lower than a first energy loss of the data communication ultrasound transmitted without dependence on the indication of the property of the body tissue structure. The ultrasound communication system may further comprise at least one pacemaker electrode, wherein: the body tissue structure is a heart; the ultrasound transmitter and ultrasound receiver are configured to be implanted in the heart; and the ultrasound transmitter and ultrasound receiver are configured to respectively transmit and receive ultrasound to communicate control signalling to the at least one pacemaker electrode to control operation of the at least one pacemaker electrode. The at least one pacemaker electrode may comprise a pair of electrodes configured to be implanted as part of a distributed pacemaker. The control signalling may be configured to synchronise the operation of the pair of electrodes. The ultrasound communication system may comprise a plurality of ultrasound receivers. The controller may be further configured to receive the signalling representative of scattered ultrasound received by the plurality of ultrasound receivers. The ultrasound transmitter(s) may be an omnidirectional transmitter(s). The ultrasound transmitter may be a tube-shaped, spherical, cuboid, or planar transmitter, depending on the desired directionality of the transmission or receipt thereof. The transducer may operate as an omnidirectional transmitter. The transducer may comprise lead zirconate titanate (PXT), polyvinylidene fluoride (PVDF), aluminium nitride (AIN), or zinc oxide (ZnO). The transducer(s) may be piezoelectric crystals or capacitive structures. In an aspect there is provided a method for performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure using an ultrasound communication system comprising an ultrasound transmitter and an ultrasound receiver, wherein the ultrasound transmitter is located at a different position to the ultrasound receiver; and at least one of the ultrasound transmitter and ultrasound receiver is implanted in the body tissue structure; the method comprising: receiving signalling representative of scattered ultrasound received by the ultrasound receiver, the scattered ultrasound resulting from data communication ultrasound transmitted by the ultrasound transmitter and interacting with the body tissue structure; the scattered ultrasound comprising at least one scattered component corresponding to a propagation channel of the data communication ultrasound; determining, in dependence on the signalling, a channel impulse response indicative of the propagation channel; obtaining, in dependence on the channel impulse response, an indication of a property of the body tissue structure; and outputting the indication of the property of the body tissue structure. The method may comprise: receiving data signalling representative of the data communication ultrasound transmitted by the ultrasound transmitter and received by the ultrasound receiver; and obtaining, in dependence on receipt of the data signalling transmitted by the ultrasound transmitter, a data packet. The method may comprise: receiving stimulation signalling representative of electrical stimulation provided to the body tissue structure; correlating the received signalling representative of scattered ultrasound with the received stimulation signalling; and outputting the indication of the property of the body tissue structure in dependence on the correlation. In an aspect there is provided computer program code which, when execute d on a processor, is configured to perform any method disclosed herein. Implementation thereof could be with a microprocessor, microcontroller, reconfigurable logic or an application specific integrated circuit. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more examples will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1A shows schematically an ultrasound communication system according to examples disclosed herein; Figure 1B shows schematically another ultrasound communication system according to examples disclosed herein; Figures 2A-2C illustrate forward scattering, tissue backscattering, and receiver backscattering scattering modes; Figures 3A and 3B illustrate pulsed ultrasound transmission and reception of the transmitted signal; Figures 4A-4C show example arrangements of a transmitter and a receiver of an ultrasound communication system with respect to a body tissue structure (a heart) according to examples disclosed herein; Figure 5 shows data of a body tissue structure (a heart) obtained from an ultrasound communication system according to examples disclosed herein; Figure 6 shows data obtained from an x-ray video of the heart of Figure 5 undergoing periodic motion; Figure 7 shows data obtained from an ultrasound communication system used with the body tissue structure of Figure 5 undergoing periodic motion, according to examples disclosed herein; Figure 8 illustrates determining dimensions of the body tissue structure of Figure 5 from a direct ultrasound path according to examples disclosed herein; Figure 9 illustrates determining dimensions of the body tissue structure of Figure 5 from one or more indirect ultrasound paths according to examples disclosed herein; Figure 10 illustrates determining dimensions of the body tissue structure of Figure 5 matched to a periodic movement of the structure according to examples disclosed herein; and Figure 11 shows an example method according to examples disclosed herein. DETAILED DESCRIPTION Synchronisation between electrodes requires communication. Biomedical communication may take a number of forms. A common practice is to use radiofrequency communication between external and implantable units. This can be effective, but also challenging to realise between implantable units, because signal attenuation scales with frequency, and antenna size scales with the reciprocal of frequency. As such, for implant to implant communication, ultrasound or pulsed electrical communication can be a more feasible method of communication. Ultrasound in particular readily transmits through the body and can thus provide an effective inter-implant communication method. Examples disclosed herein provide a method of utilising ultrasound communication and to also provide diagnostic information simultaneously without requiring further devices or electronics. Examples disclosed herein relate to ultrasonic communication systems that allows for both information transfer and determination of some property of the tissue through which the ultrasound passes. Examples described in detail below relate to ultrasound communication obtaining spatial information about a heart, for example communication between cardiac chambers for pacemaker control and 3D mechanical evaluation of the heart motion. However, the disclosed apparatus and method may be used with other body tissue structures, for example lungs, kidneys, liver, pulmonary structures, and others. The ultrasound communication system acts simultaneously to monitor the spatial environment of the tissue being investigated. The ultrasound from the transmitting ultrasound node scatters, echoes and reverberates around its environment to provide information on the spatial properties of the environment. In a heart environment the transmitted ultrasound can form a pattern with the heartbeat which can be detected as the heart walls move with the heartbeat. Thus the examples disclosed herein may provide a diagnostic tool for body tissue and organ behaviour. A motivation for the invention is the realisation that communication between devices can be used for an additional diagnostic functionality. Examples disclosed herein may be able to provide a comparable level of detail as an echocardiogram, but on a constant or continuous basis. They may be able to provide a wealth of helpful information, including (in the example of a heart) the size and shape of the heart (e.g. internal chamber size quantification), pumping capacity, location and extent of any tissue damage, and assessment of valves. It will be appreciated that discussions relating to determining dimensions and spatial properties of the body tissue structure also apply to determining changes relating to such dimensions and spatial properties. That is in some examples, an absolute dimension may not be obtained but detection of a variation in a dimension may be. In some examples an indication of absolute dimension may be obtained, for example with reference to a model. The capacity of the system to determine the properties of the surrounding tissue depends on the number of ultrasound nodes in use. More nodes allow for more information which can be reconstructed to obtain a functional model. The term “ultrasound node” or simply “node” may be understood to mean a transceiver (which may act as a transmitter and a receiver) and a controller. Such a node may in some examples be called a “bioelectronics node”. Each node may be a transceiver, which is configured to transmit and receive data. Indeed in some examples each transceiver is configured to both transmit and receive data, for example to acknowledge that data has been successfully received. Examples disclosed herein allow for the recordal of data that is piggy-backing on signals transmitted / received between two transceivers / nodes. A simple example which can perform this functionality is a transceiver which provides transmitting and receiving functionality using a shared ultrasound crystal, e.g. a piezoelectric (e.g. lead zirconate titanate, PZT) crystal, and has separate transmitter and receiver electronics. Thus where an element is described as a transmitter, the element may also have receiver functionality, and likewise, where an element is described as a receiver, the element may also have transmitter functionality. In some examples there may be a transmitter (without receiver capability) located separately from a receiver (without transmitter capability) thought this provides a more complex design. Figure 1 shows schematically an ultrasound communication system 100 according to examples disclosed herein. The ultrasound communication system 100 is for performing intrabody ultrasound communication (for example, to communicate with pacemaker elements in a heart) and for obtaining spatial information of a body tissue structure 150. The intrabody ultrasound communication may be bi-directional (between transceivers) or uni-directional (between a transmitter and a receiver). The ultrasound communication system 100 comprises an ultrasound transmitter 102 and an ultrasound receiver 104. The ultrasonic transmitter 102 may be located together with an ultrasonic receiver 104a in a first transceiver 101, and the ultrasonic receiver 104 may be located together with an ultrasonic transmitter 102a in a further transceiver 101a. These transceivers 101, 101a comprise electronic circuitry to drive ultrasonic oscillations from a vibrational unit and act as a transmitter 102, 102a, and / or record ultrasonic signals received from a vibrational unit and act as a receiver 104, 104a. The transmitter circuit may comprise a pulse modulation circuit which modulates a carrier wave according to amplitude, frequency or phase. It may also perform more complex modulations. The receiver circuitry may have amplifiers to increase the signal amplitude and filters to isolate the signal from noise. The transceiver 101 acting as the ultrasound transmitter 102 is configured to be located at a different position to transceiver 101a acting as the ultrasound receiver 104 in this example. At least one of the ultrasound transceivers 101, 101a is configured to be implanted in the body tissue structure 150. In some examples there may be a plurality of ultrasound receivers 104 (which may be in the form of transceivers), located at different places relative to the tissue 150 being investigated. The body tissue structure 150 is not part of the ultrasound communication system 100. The ultrasound communication system 100 comprises a controller 120. In some examples the controller 120 may comprise a transmission subsystem 122 which is configured to control the transmission of ultrasound from the transmitter(s) 102, 102a. The controller 120 is configured to receive signalling 106 representative of scattered ultrasound received by the ultrasound receiver 104 in step 108. The scattered ultrasound results from data communication ultrasound transmitted by the ultrasound transmitter 102 and interacting with the body tissue structure 150. The scattered ultrasound comprises at least one scatter component corresponding to a propagation channel of the data communication ultrasound. The illustrated controller 120 is an entity separate from the transceivers 101, 101a and in communication with at least the receiving transceiver 101a as shown. In some examples, each transceiver 101,101a may be a node and may comprise its own controller. There may in some such examples also be a separate controller as well, such as an overall implanted command and control controller in communication with the nodes (transceiver and controller units).The controller 120 is configured to determine, in dependence on the signalling 106, a channel impulse response indicative of the propagation channel in step 110. That is, the controller 120 is configured to estimate, or extract, a channel impulse response from the received signalling 106. A channel impulse response is indicative of the nature of the path taken by ultrasound on its path from the transmitter 102 to the receiver 104. By passing through, and / or reflecting from boundaries in the body tissue sample 150, the properties of the transmitted ultrasound are altered. These changes can be analysed to determine properties of the tissue which the ultrasound has interacted with. The controller 120 is configured to obtain, in dependence on the channel impulse response, an indication of a property of the body tissue structure 150 in step 112. That is, from the channel impulse response, some clinical information can be extracted. The channel impulse response may include a propagation delay, arising due to factors delaying the transmission of the ultrasound to the receiver - this may be due to the ultrasound taking an indirect path and reflecting from a boundary, and / or may be due to the ultrasound being slowed down because of passing through tissue having a density and elasticity which can affect propagation speed, for example. The channel impulse response may include a delay or delay spread, arising again due to factors changing the transmission of the ultrasound to cause the ultrasound to take multiple paths through the tissue sample before being detected at the receiver causing the received signal to be spread over a time of receipt of the different paths taken with associated amplitude changes due to propagating over those different multiple paths. In other words, the ultrasound communication system 100 may be configured to obtain an indication of the property of the body tissue structure by obtaining, in dependence on the channel impulse response, one or more of a signal amplitude and a propagation delay of the received scattered ultrasound; and determining the indication of the property of the body tissue structure 150 in dependence on the one or more of the amplitude and the delay. The controller 120 is configured to output the indication 116 of the property of the body tissue structure in step 114. The indication 116 of the property of the body tissue structure may indicate, for example, simply that something has changed compared to how the tissue structure was previously determined to be, such as a change in heartbeat cycle. The indication 116 of the property of the body tissue structure 150 may indicate, for example, some absolute information about the tissue structure 150, such as distances. Obtaining more absolute information in this way may use the obtained channel impulse response as well as additional information, such as some knowledge of the tissue types present, the nature and geometry of the structure being probed, and / or other parameters. As shown, the ultrasound system has the ultrasound transmitter 102 and the ultrasound receive 104 arranged in different locations so the system is operating in a forward scatter mode, and as such, the signalling 106 is representative of forward scatter ultrasound. A distance between the ultrasound transmitter 102 (or associated transceiver) and the ultrasound receiver 104 (or associated transceiver) at the different position may be a known separation distance in some examples (e.g. the transmitter 102 and receiver 104 have been placed at known positions with a known separation distance between then. A distance between the ultrasound transmitter 102 (or associated transceiver) and the ultrasound receiver 104 (or associated transceiver) at the different position may be a distance determined based on a determination of a propagation delay in data communication ultrasound transmitted between the ultrasound transmitter and the ultrasound receiver in some examples. That is, based on the received ultrasound signalling 106, a determination may be made as to the separation of the ultrasound transmitter 102 and the ultrasound receiver 104, for example by analysing the delay in signal reception of a directly propagated signal. In some examples, determining information about the direct path alone (i.e. without consideration of indirect propagation paths from ultrasound reflecting from a boundary in the tissue sample) of transmitted ultrasound from the transmitter 102 to the receiver 104 may be sufficient to obtain some clinical information, such as information on the movement of heart wall tissue during a heartbeat obtained from determining a propagation delay of ultrasound travelled along the direct path. As another example, in measuring ultrasound signal transmission in a heart in relation to a heart valve, the delay in ultrasound signal may not change significantly whether the valve is open or closed, but the magnitude of the signal can change significantly depending on whether the valve is open or closed. These different examples of peaks arising from particular structures in the body may be used, for example, to give an indication of the properties of a heart and may indicate an impending heart problem. In some examples, the controller 100 may be configured to determine, in dependence on the indication 116 of the property of the body tissue structure, a data communication feedback parameter indicative of a property of data communication ultrasound to be transmitted according to the property of the body tissue structure; and control the ultrasound transmitter 102 to output data communication ultrasound according to the data communication feedback parameter. The property of the data communication ultrasound to be transmitted via the body tissue may be an energy loss, and the data communication feedback parameter may be indicative of a second energy loss of the data communication ultrasound, the second energy loss being lower than a first energy loss of the data communication ultrasound transmitted without dependence on the indication of the property of the body tissue structure. That is, the communication system 100 may be able to provide feedback for the communication system in operating as a data communication system to indicate the best time to send a communication signal, e.g. with reduced signal attenuation. For example, in a body tissue system which has some periodic movement, there may be an optimum time during the period in which it is possible to send a communication signal with the least, or a low, energy loss and therefore obtain more efficient signal transmission, and the communication system 100 can help determine when that is for subsequent communication signal transmission by this feedback method. In some examples, the “control” (i.e. the steps 108,110,112,114 as shown) may be performed externally from the transceivers 101, 101a, for example by uploading data from the receiver 104 / transceiver 101a to a computer which can monitor what is being transmitted and recorded, and may periodically intervene by updating the clinical protocol on the transceivers, for example. Figure 1B shows schematically another ultrasound communication system in use in a patient’s heart which may operate in this way. In this example, the controller comprises an external controller 164 and an internal controller 162 (in this example the internal controller 162 is implanted in a body for use). The external controller 164 is part of a computing system 166 whereby the controller 164 can communicate with the cloud 168 (or in some examples, an external server). Therefore there is communication between the cloud 168 to the external controller 164, between the external controller 164 and internal control 162, and between the internal controller 164 and the nodes 202, 204 implanted in the heart. Examples disclosed herein includes controllers which are configured to perform decision making computation, either locally or through transmission of data to an external processor or computer, by e.g. linear or nonlinear algorithms, threshold functions, neural networks, look up tables, or other mathematical means, defining interventions based on information received. Decisions may also be based on other factors such as sensed information from electrodes. Communication between implanted and external controller units may be via ultrasound, radiofrequency or other modality such as pulsed electrical body channel communication. Figures 2A-2C illustrate forward scattering, tissue backscattering, and receiver backscattering scattering modes. Each example shows, as in Figure 1, a transceiver 101 comprising a transmitter 102 and receiver 104a, and another transmitter 101a comprising a transmitter 102a and receiver 104. The ultrasound is transmitted from the transmitter 102 of the transducer 101, and takes different paths through the body tissue sample 150 which comprises different acoustic media 152,154 (i.e. different portions of the body tissue sample which affect the path of ultrasonic signals). The ultrasound may change direction as it reaches a boundary between the different acoustic media 152, 154 and these changes may be detected as discussed below. In Figure 2A, ultrasound is transmitted, changes path as it reaches the boundary between the different acoustic media 152, 154, and is detected by the detector 104 of the other transducer 101a. In Figure 2B, ultrasound is transmitted, changes path as it reaches the boundary between the different acoustic media 152, 154, and is reflected from the boundary back to be detected by the detector 104a of the same transducer 101 which transmitted the outgoing ultrasound. In Figure 2C, ultrasound is directly transmitted to the other transducer 101a, reflected from the other transducer 101a, changes path as it reaches the boundary between the different acoustic media 152, 154, and is reflected back to be detected by the detector 104a of the same transducer 101 which transmitted the outgoing ultrasound. In the examples of Figure 2C, a communication transmitter 102 may communicate with a satellite node 104, which modifies the signal prior to reflection. In such a case the backscatter (which may be thought of as forward scatter in the reverse direction) would be received. It will be appreciated that in these backscattering modes, the transceiver 101 which transmits ultrasound also receives the backscattered echoes and the other transceiver 101a receives forward transmitted ultrasound signalling. In the backscattering modes of operation, after transmission, the transmitter transceiver 101 can also observe the backscattered ultrasound signal coming back via the receiver 104. In a bi-directional communication system, when using pulsed ultrasound as shown in Figures 3A and 3B discussed below, during the pulse interval (while the receiver side 104 is analysing the forward scatter ultrasound), the transmitter side 102 may also receive the backscattered ultrasound and perform analysis from its side. Such an example would be a combination of forward scatter ultrasound as in Figure 2A, and backscattered ultrasound as in Figures 2B and / or 2C. The examples disclosed herein, in relation to Figures 3A-3B, 4A-4C and 5-10, show operation of the ultrasound communication system 100, 200 which operates in a forward scatter mode as shown in Figure 2A. However, other ultrasound communication systems covered by this disclosure may operate in a tissue backscattering mode as in Figure 2B, or a receiver backscattering mode as in Figure 2C. In some examples as shown in relation to Figures 3A and 3B, the transmitted ultrasound from the transmitter 102 is pulsed, to perform pulsed data communication ultrasound. Figures 3A and 3B illustrate pulsed ultrasound in a forward scattering transmission mode. Figure 3A shows the forward scattering mode of Figure 2A, and indicates that the transmitted ultrasound from the transmitter 102 may comprise a carrier signal 160 of a series of pulses followed by a pause, and this pattern is repeated to provide plural pulses. The received forward scattered ultrasound received at the receiver 104 comprises a signal comprising a primary signal 162 (a high amplitude peak or trough) form the direct propagation path, and a multipath dispersion signal 164 from the scattered ultrasound. Figure 3B shows an example of measured data indicating a transmitted carrier signal 160 “Tx” on the top of the plot, and a received signal “Rx” on the bottom of the plot. The received signal “Rx” shows a propagation delay 166 between transmission of the pulse 160 and receipt of the primary signal 162, a primary signal 162, and a multipath signal 164, compared to the transmitted pulsed ultrasound of pulsed carrier signals 160. A delay ATpnm is a time delay of the primary signal 162 and a delay ATwuiti is a time delay of the multipath signal 164. In such examples using this ultrasound transmission scheme, the controller 100 may be configured to perform the intrabody data communication by transmission of the pulsed data communication ultrasound by the ultrasound transmitter 102 in a transmission signal portion of a plurality of periodic transmission periods; and reception of the pulsed data communication scattered ultrasound by the ultrasound receiver 104 in a quiet signal portion between transmission signal portions. It should be noted that the pulsed transmission scenario illustrated in Figures 3A and 3B is an example, though in other examples, similarly to some operating protocols in radar and sonar communications systems, the channel impulse response can be by transmission of a phase or frequency modulated signal that is longer in duration than the channel delay spread (i.e. operating in a pulse compression or matched filtering mode). It will be appreciated that the examples disclosed herein are not necessarily limited to the transmission of ultrasound in a specific pulsed or non-pulsed modulation. Figures 4A-4C show example arrangements of a transmitter 202 and a receiver 204 of an ultrasound communication system 200 with respect to a body tissue structure 250 (a heart). This example thus comprises an ultrasound communication system as in Figure 1, as well as at least one pacemaker electrode 206, wherein: the body tissue structure is a heart 250; the ultrasound transmitter 202 and ultrasound receiver 204 are configured to be implanted in the heart 250; and the ultrasound transmitter 202 and ultrasound receiver 204 are configured to respectively transmit and receive ultrasound to communicate control signalling to the at least one pacemaker electrode 206 to control operation of the at least one pacemaker electrode 206. The at least one pacemaker electrode 206 may in some examples comprise a pair of electrodes configured to be implanted as part of a distributed pacemaker. The at least one pacemaker electrode 206 is configured to stimulate the cardiac muscle and sense cardiac electrical activity for recordal. In such an example, the control signalling may be configured to synchronise the operation of the pair of electrodes. In some examples, there may be a plurality of ultrasound receivers 204, and the communication system 200 may be configured to receive signalling representative of scattered ultrasound received by the plurality of ultrasound receivers 204. In the examples described herein, the implanted transducers 202, 204 are suspended on wires in the heart, and as such they remain relatively static with respect to each other, such that movement of the transducers is not correlated with any movement of the heart walls. Thus the time of flight remains approximately constant as any movement of the suspended transducers is not correlated with any movement in the plane of signal propagation. However, there are variations in the channel impulse response as discussed as the signals are propagated through the body from transmitter to receiver. In other example systems in which at least one transducer is implanted in, or attached to, the body tissue structure, such as in or on the heart wall, it is expected that would be variation in the time of flight and also variation in the channel impulse response. Figure 4A shows an x-ray of a communication system in situ in a pig heart taken as part of an experimental investigation, and Figure 4B shows the same image schematically. The transmitter 202 is located in the right atrium, and the receiver 204 is located in the right ventricle, at a separation distance of between 55 to 61 mm dependent on the geometry of the heart 250 at the time of determining the distance because of the heart moving as it beats. The sense circuit in 202 is configured to detect the ECG signal and an algorithm can decide what information is transmitted to the receiver 204 and when, in order to control stimulation. Also shown in this example in Figures 4A and 4B is the stimulation electrode 206 forming part of a pacemaker system to control the heart beating. Also illustrated are two external chest ECG electrodes 208, 210 which were placed on the pig’s chest and used to monitor the heartbeat of the pig during the investigations. Figure 4C schematically illustrates the ultrasound communication system 200 used to probe a heart 250, wherein the transmitter 202 can transmit ultrasound in the heart which is received by the receiver 204. The stimulation electrode 206 can stimulate the heart and the communication signals received by the receiver 204 can be fed into the stimulation electrode 206 system to control its operation. The system 200 illustrated in Figures 4A-4C was developed by the inventors and used as a benchtop proof of concept electronic system 200 that incorporates both ultrasonic transmitters 202 with associated drivers, and ultrasound receivers 204 with associated circuitry. The transmitters 202 and receiver 204 in this example comprised custom tubular ultrasonic elements for omnidirectional transmission, via an encapsulated flexible lead for connection to the drivers I circuitry. Regarding other factors of the experiment, additional circuits were developed for neural recording inside the heart, which were connected to electrode leads. Custom glucose fuel cells were created and connected to power electronics for energy harvesting. The communication system 100 in this example operated using Differential Pulse Position Modulation (DPPM) and incorporated cyclic redundancy checks to ensure reliable communications. Using the arrangement shown in Figures 4A - 4C, the experimental operation was as follows: (i) stimulation of the right atrium; (ii) communication from the right atrium to the right ventricle; (iii) stimulation of the right ventricle; (iv) Time of flight analysis of the communication commands. Figure 5 is titled “Hypothesis for Time of Flight Data” shows data obtained from the ultrasound communication system set up as shown in Figures 4A-4C. This figure shows, on the left, a surface plot of the received signalling representative of forward scatter ultrasound received by the ultrasound receiver on the horizontal axis against the pulsed traversal distance (in millimetres) on the vertical axis. The data shows the received ultrasound over six heartbeat cycles. The plot may be understood to show Time of Flight (ToF) data. Various peaks can be seen in the data which each indicate ultrasound received which has travelled on a particular path through the body tissue structure (the pig heart in this example). In the centre of Figure 5 is a surface plot of the average received signalling representative of forward scatter ultrasound received by the ultrasound receiver from the six cycles shown in the left hand plot, on the horizontal axis, plotted again against the pulsed traversal distance (in millimetres) on the vertical axis. This is an intracardial measurement. The average is taken to improve the signal to noise ratio and is valid because the same features can clearly be seen repeated in each of the six cycles in the left hand plot. The cycle time, or time for one cycle to complete, is approximately 655 milliseconds. On the right side of this central plot, a series of 2D line plots show cuts taken along the vertical axis at different points through the cycle to illustrate the presence of the peaks shown between around 120 mm and 400 mm, which are discussed in more detail below. The data shows, at a traversal distance of between about 85 mm and 110 mm, a main (high intensity) peak as shown in the upper right of Figure 5. This peak is indicative of a direct travel path of the ultrasound from the transmitter to the receiver. It can be seen that there is an onset of the primary (direct) path signal at around 88mm traversal distance, with a central maximum of the pulse train at around 95 mm traversal distance which was determined to be stronger at the point in the heartbeat cycle when the atrioventricular valve is open (thereby providing less attenuation of the ultrasound signal than when the valve is closed). The pulse train is determined to be around 18mm long (vertical measurement). The primary path dispersion (shape of the primary peak) changes through the cycle as the atrial-ventricular activity changes. The data also shows, at a traversal distance of between about around 120 mm and 400 mm, particularly between arounds 200mm to 350 mm, secondary (lower intensity) peaks which may be called secondary path features. These peaks arise from ultrasound being transmitted and taking an indirect path through the body tissue structure to the ultrasound receiver. The peaks between around 140 mm and 250 mm arise from ultrasound interaction with ipsilateral intracardial features, and the peaks between around 240 mm and 380 mm arise from ultrasound interaction with contralateral intracardial features. This is discussed in more detail with reference to Figure 9. At pulse traversal distances larger than around 450 mm, peaks arise from deeper echoes and reverberations of the transmitted ultrasound with body tissue structures beyond the heart, i.e. outside the body tissue structure being investigated, such as the lungs. Figure 5 illustrates that the controller 120 may be configured to output a plot (or at least data which may be shown as a plot) of time versus a determined distance travelled by the at least one forward scatter ultrasound, as shown on the left of Figure 5. The indication of the property of the body tissue structure corresponds to a corresponding feature in the plot. This is explored more in relation to Figures 6 to 9. Figure 6 is titled “Obtain the separation cycle from the video” because the movement of the transmitter and receiver may be deduced from an x-ray video of the heart beating as controlled by a pacemaker, as shown in Figures 4A-4C. On the left of Figure 6, at the top, there is a schematic illustration of a heart with the atrioventricular valve closed. As the heart moves higher and wider the ultrasound transmitter and ultrasound receiver will move apart, increasing the distance between them. This can be seen in an x-ray video of the heart movement, a still of which is shown in Figure 6 showing the distance. At the bottom of Figure 6 on the left there is a schematic illustration of a heart with the atrioventricular valve open. As the heart moves it elongates and becomes thinner, so the ultrasound transmitter and ultrasound receiver will move together, decreasing the distance between them. This can also be seen in the x-ray video of the heart movement, another still of which is shown in Figure 6 showing the distance. The graph on the right of Figure 6 shows a plot of the separation of the transmitter and receiver on the vertical axis as a function of time on the horizontal axis. It can be deduced that the heart beats at 150 beats per minute (BPM) by monitoring the number of movements of the transmitter and receiver. The ToF data indicates a fixed distance between the ultrasound transmitter and receiver of around 88 mm as determined using the Pythagoras relationship shown in the bottom right of Figure 6 whereby the ToF separation is shown as the hypotenuse of the right angled triangle with the out of plane and in plane separations as the two shorter sides. The estimated ToF, R, can be obtained from the in-plane and out-of-plane separations from the relation R = Vx2 + T2 + Z2 .The in-plane separation can be determined from the x-ray video. The out of plane separation can be estimated based on the ToF distance and the separation identified from the x-ray video., Thus from the x-ray video, the distance between the transmitter and receiver can be determined. This is useful for interpreting the received ultrasound signalling. As noted above, in this example, the implanted transducers are suspended on wires in the heart so they remain relatively static with a fixed separation of around 88 mm. In other examples, if at least one transducer is implanted in, or attached to, the body tissue structure, there may be expected to be a variable transducer separation as the body tissue structure to which the transducer is attached moves. The movement of the transmitter and receiver in the out of plane (y) direction is shown in the upper plot with separation between approximately 63 mm and 70 mm, and the movement of the transmitter and receiver in the in-plane (x) direction is shown in the lower plot with separation between approximately 55 mm and 61 mm. The point in the movement curves at which the in-plane component is a minimum and the out of plane component is a corresponding maximum corresponds to the valve through which the ultrasound is travelling being closed. The point of the movement curves at which the in-plane component is a maximum and the out of plane component is a corresponding minimum corresponds to the valve through which the ultrasound is travelling being open. Figure 7 shows the data of Figure 5 obtained from an ultrasound communication system used with the heart undergoing periodic motion and focussing on the main, or direct, travel peak at between 85 mm and 110 mm. Figure 7 is titled “Connecting the ToF signal to the ECG signal” because the direct peak can be matched up, or correlated with, a corresponding feature in the ECG data to help understand the origin of the direct peak. On the left of Figure 7, at the top, there is a schematic illustration of the heart with the atrioventricular valve closed. The ultrasound plot shows that with the valve closed, the primary propagation path of the ultrasound is attenuated by the closed valve, causing scattering and reflection of the ultrasound which significantly reduces the intensity of the direct peak in the plot. At the bottom of Figure 7, there is a schematic illustration of the heart with the atrioventricular valve open. The ultrasound plot shows that with the valve open, the primary propagation path of the ultrasound is less attenuated because the valve is open, allowing a greater proportion of the ultrasound to get through along the direct path to the receiver, which causes a greater intensity of the direct peak in the plot. In the central portion of Figure 7, there is an illustration of matching up the ultrasound data with the corresponding ECG signal. The ECG signal is well understood and comprises a P wave, a PR segment (the P wave and PR segment forming the PR interval), a Q wave, an R wave, an S wave (the Q wave, R wave and S wave together forming the QRS complex, an ST segment, and a T wave, the ST segment and T wave with the QRS complex forming the QT interval). When the valve is closed there is a smaller primary signal which correlates with the portion of the ECG waveform from the peak of the R wave to the end of the T wave. When the valve is open there is a larger primary signal which correlates with the portion of the ECG waveform from the end of the T wave to the peak of the R wave in the following waveform. The illustrated example shows a PR interval of 160 ms, a QRS complex of 100 ms, QT interval of 360 ms, and a R to R peak distance (a heart rate, HR) of 70 (beats) per minute. The plot on the right of Figure 7 shows that the video data discussed in Figure 6 can be matched up to the ECG data and therefore with the received ultrasound. When the valve is closed there is a smaller ultrasound signal and this matches with the QT interval of the ECG. When the valve is open there is a larger ultrasound signal and this matches with the portion of the ECG signal from the end of the T wave to the peak of the R wave of the next ECG waveform. The example shown in Figure 7 may be considered to illustrate that the communication system 100 may be used to obtain information about a change in property of the body tissue structure being investigated. A subject may have the system 100 in place and in use, for example to regulate a pacemaker. The data communications between ultrasound transducers to regulate the pacemaker operation may also advantageously be used to obtain spatial information about the subject’s heart and a change in behaviour may be detected through use of the ultrasound communication system for the different purpose of controlling the pacemaker. That is, the communication system 100 may be used to compare an indication of the property of the body tissue structure (such as a separation of ultrasound transducers during a heartbeat) to a prior stored indication of the property of the body tissue structure body tissue (i.e. a previously determined ultrasound transducer separation). The communication system 100 may determine whether a variation between the indication and the stored indication exceeds a predetermined variation threshold; and output an alert indication in dependence on the variation exceeding the predetermined variation threshold. Of course the “predetermined variation threshold” may involved one or more factors and may be variable, for example according to one or more parameters of, or determined by, the ultrasound communication system. Comparing the indication of the property of the body tissue structure to the prior stored indication of the property of the body tissue structure body tissue may be performed locally using nodes in some examples (in which the transceiver and controller are housed together), and used to determine decision making locally. In other examples, the received data may be communicated externally to a computer which processes the data and determines an intervention to make (for example, in a pacemaker example, the stimulation provided by the pacemaker may be adjusted according to the received and processed data). For example, if a variation in average or peak transducer separation of >5% over a 48 hour period is determined, this may be an indication of a health problem with the heart which warrants further investigation, e.g. by an ECG or MRI scan. Of course other thresholds and parameters may be used depending on what provides a useful indicator for the body tissue structure being monitored. The prior stored indication may be indicative of historical indications recorded over a plurality of prior obtained indications, such as an average, moving average, variation in rate of change of indication, or other measure. As well as determining the mechanical properties of the heart as described, the ultrasound which, as illustrated, indicates spatial properties of the heart, was also used to drive a distributed pacemaker which can be placed separately in both atrial and ventricle chambers of the heart. Using ultrasound, the communication system 100 was used to synchronise the two pacemaker nodes and modulate the heartrate to between 95-145 BPM. The example illustrated in Figures 6 and 7 may be understood to illustrate that the controller 120 of the ultrasound communication system 100, 200 may be configured to receive stimulation signalling representative of electrical stimulation provided to the body tissue structure (i.e. signalling indicating the operation of the stimulation electrode 206); correlate the received signalling representative of forward scatter ultrasound with the received stimulation signalling (as shown in the right of Figure 7 whereby the ToF primary peak from the ultrasound can be matched up with the ECG and transducer separation according to the heartbeat); and output the indication of the property of the body tissue structure in dependence on the correlation (for example, the property of the heartbeat correlating with transducer separation). As such the communication systems 100 disclosed herein may advantageously provide communication efficacy; the experiments described above allowed for real time ultrasonic communication between the right atrium and right ventricle to be performed in a live pig model. Further, cardiac synchrony can be monitored as illustrated in the live pig experiments, whereby heart activity between 95 and 145 beats per minute were measured using the ultrasound system disclosed herein. Further, a mechanical evaluation of the heart may be undertaken using the communication systems disclosed herein: the communication system simultaneously allows performance of diagnostic analysis of the mechanical environment around the heart as well as communication for pacemaker electrode control. In the example so far, the direct ultrasound propagation path has been considered and used to demonstrate that the received ultrasound includes information indicative of spatial properties of the body tissue under investigation. In the examples of Figures 8 and 9, further spatial information can be obtained to provide additional information on the body tissue under investigation by consideration of the indirect propagation paths of the ultrasound (e.g. those paths leading to the peaks having a secondary path length of greater than around 140 mm in the illustration in Figure 5, i.e. the peaks having a longer propagation path length than the path length of the direct propagation peak length giving rise to the primary peak). Figure 8 illustrates determining dimensions of the body tissue structure from a direct ultrasound path. The forward scatter ultrasound received by the ultrasound receiver comprises a direct forward scatter component which is indicative of a direct propagation channel from the ultrasound transmitter to the ultrasound receiver. This direct forward scatter component, which may be called a “primary path” of the ultrasound from transmitter to receiver, may be used to determine dimensions of the body tissue structure being investigated. As a note, in some examples, the delays of the multipath peaks (i.e. indirect scattering, discussed in relation to Figure 9) can be determined relative to the peak of first arrival (i.e. the primary peak arising from ultrasound travelling on a direct path from the transmitter to the receiver), which arises from the shortest travel path of the ultrasound from transmitter to receiver. Because of the two-way communication of systems disclosed herein in which the transducers may be configured to act as both transmitters and receivers (i.e. transceivers), the delay relative to T = 0, i.e. the time of initial transmission, can also be determined to work out information about the first arrival peak, such as distance travelled. Figure 8 shows an approximate model of a porcine heart adapted from a human heart model which provides a sufficiently accurate model to demonstrate the feasibility of the technique. The ultrasound transducers are located in the heart as before in the right atrium and the right ventricle, at a separation of around 88 mm which is the distance travelled by ultrasound on the direct / primary transmission path. The ultrasound signal was transmitted as a pulse train showing a spread in the vertical distance axis of around 18 mm. No signals were observed at distances less than around 80 mm. The primary path signal is seen to start at a distance of around 88 mm which is consistent with the heart model. The central maximum of the primary path signal shows variation in intensity through the heartbeat cycle, consistent with the opening and closing of the valve through which the ultrasound signal passes in the direct path between transducers. The primary path dispersion (spread) changes with atrial-ventricular activity. These factors suggest that the primary peak arises from direct ultrasound transmission along a path consistent with the direct distance between transducers. Thus plots such as those illustrated here indicate paths taken by ultrasound in the body tissue structure. Peaks appearing at larger distances indicate ultrasound which has travelled on a path longer than the direct path, i.e. ultrasound which has indirectly arrived following reflection or multipath transmission between transducers. This is illustrated with reference to Figure 9. Figure 9 illustrates determining dimensions of the body tissue structure from one or more indirect ultrasound paths. The ultrasound communication systems disclosed herein may make use of scattered ultrasound which comprises at least one indirect scatter component indicative of a propagation channel comprising at least one reflection boundary of the body tissue structure from which the transmitted data communication ultrasound is reflected. The at least one indirect scatter component detected by the ultrasound apparatus may comprise at least one multiply-reflected indirect scatter component that has reflected of plural surfaces of the body tissue structure, i.e. the path includes more than one reflection in some examples. Figure 9 shows that the longer secondary, or indirect, paths taken by ultrasound are the result of reflection in the body tissue structure due to acoustic impedance differences. Figure 9 shows a schematic model of heart muscle tissue with a blood-filled cardiac chamber to the left and fluid space, such as interstitial fluid, on the right. Incoming ultrasound from the cardiac chamber can reflect from the blood-muscle boundary and the muscle-interstitial fluid boundary. Some example acoustic impedances are illustrated in units of 106 kg / m2s: lung, 0.18; fat, 1.34; interstitial fluid, 1.48; blood, 1.65; muscle, 1.71 and bone, 7.8. Suggested identified secondary paths giving rise to peaks in the plotted data on the right include ipsilateral intracardial secondary multipath transmission, and contralateral intracardial secondary multipath transmission. As illustrated and discussed above, the peaks between around 140 mm and 250 mm arise from ultrasound interaction with ipsilateral intracardial features, and the peaks between around 240 mm and 380 mm arise from ultrasound interaction with contralateral intracardial features. A minimum ipsilateral path may be taken to be around 122 nm in this model and a minimum contralateral path may be taken to be 228 mm in this model. These known dimensions can be used to interpret the original / paths taken by ultrasound signals giving rise to peaks in the plot. Thus, with reference to Figures 8 and 9, it may be understood that examples of the ultrasound communication system disclosed herein allow for obtaining the indication of the property of the body tissue structure by obtaining, in dependence on the channel impulse response (as illustrated in the 2D plots) and a predefined model of the body tissue structure (e.g. a human or porcine heart as illustrated), a distance travelled by the received scattered ultrasound, based on a reference. The predefined model may comprise an indication of expected dimensions and expected ultrasound attenuation properties of the body tissue structure. That is, by understanding the geometry of the body tissue structure and from monitoring the channel impulse response by way, for example, of a plot of distance versus time of received ultrasound, inferences can be made regarding the properties of the body tissue. This may find application in systems where the body tissue structure is continually or regularly and periodically analysed by way of the received ultrasound signals. Any change in particular received signals beyond a predetermined threshold may be understood to infer a possible change in the properties or structure of the body tissue structure which may warrant further investigation, for example by ECG or MRI. For example, if a secondary peak can be determined to arise from ultrasound reflecting from an atrial wall or ventricular wall, and a change in this peak is observed beyond an acceptable tolerance, this may be indicative of a weakening or change in that structure which should be medically investigated. Another example may be monitoring the behaviour of a peak determined to be related to an interface between a heart wall and proximal lung, which may indicate characteristics of the heart’s structure. Changes over time of such a peak may indicate a change in the properties of the heart wall - lung interface, and may indicate a medical problem for further investigation. Thus, ultrasound communication systems disclosed herein may allow for obtaining an indication of the property of the body tissue structure by obtaining, in dependence on the channel impulse response and a predefined model of the body tissue structure, a distance travelled by the received scattered ultrasound, based on a reference. The predefined model may comprise an indication of expected dimensions and expected ultrasound attenuation properties of the body tissue structure, as shown in Figure 9. Figure 10 (“Final overlay comparison”) illustrates determining dimensions of the body tissue structure matched to a periodic movement of the structure. As discussed above, data from an ECG can be aligned to the features in the ultrasound time of flight plot, for example by offsetting the ECG to start at an appropriate start point to match with the ultrasound time of flight plot. Plural heartbeat cycles may be measured and plotted as time of flight data (shown as ultrasound path distance in mm plotted against time in s) and matched with the ECG data (shown as voltage in mV against time in s in the central portion of Figure 10) and with transducer separation data obtained from x-ray video (shown as separation in mm against time in s). This clearly indicates that there is comparable periodic repetition in the ultrasound data. The ultrasound communication system disclosed herein may therefore be used to investigate body tissue structures which undergo periodic movement. The controller of the ultrasound communication system may be configured to determine, in dependence on the signalling, a periodicity of variation of the received scattered ultrasound; and determine a corresponding periodicity of movement of the body tissue structure in dependence on the periodicity of variation of the received scattered ultrasound. As shown for example, the body tissue structure may be a heart, and the periodicity of movement may correspond to a heartbeat. In other examples, the body tissue structure may undergo quasi periodic or even chaotic movement. In examples in which the body tissue structure undergoes some movement, the controller may be configured to determine time and amplitude domain vectors of variation of the received scattered ultrasound; and determine corresponding time and amplitude domain vectors of movement of the body tissue structure in dependence on the of variation of the received scattered ultrasound. By obtaining an indication of a change in behaviour of a body tissue structure in this way using example apparatuses as disclosed herein, an ongoing monitoring can be undertaken and provide an insight into whether a further medical investigation is warranted if a change in usual behaviour is detected or if a determined parameter such as a body part dimension (e.g. wall thickness) or body part property (e.g. opening and closing of a valve) is not within expected healthy limits. Rather than scheduling expensive, expert-led and time consuming MRI scans in hospitals for monitoring purposes, patients may be able to regularly monitor their own body e.g. heart, and potentially proactively prevent impending problems before they become more involved or too serious to treat. Figure 11 shows an example method 1100 according to examples disclosed herein of performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure using an ultrasound communication system as discussed herein, comprising an ultrasound transmitter and an ultrasound receiver, wherein the ultrasound transmitter is located at a different position to the ultrasound receiver; and at least one of the ultrasound transmitter and ultrasound receiver is implanted in the body tissue structure. The method 1100 comprises receiving signalling 1102 representative of scattered ultrasound received by the ultrasound receiver. The scattered ultrasound results from data communication ultrasound transmitted by the ultrasound transmitter and interacting with the body tissue structure; the scattered ultrasound comprising at least one scatter component corresponding to a propagation channel of the data communication ultrasound. The methods 1100 comprises determining, in dependence on the signalling, a channel impulse response indicative of the propagation channel 1104; obtaining, in dependence on the channel impulse response, an indication of a property of the body tissue structure 1106; and outputting the indication of the property of the body tissue structure 1108. The method 1100 may comprise receiving data signalling representative of the data communication ultrasound transmitted by the ultrasound transmitter and received by the ultrasound receiver; and obtaining, in dependence on receipt of the data signalling transmitted by the ultrasound transmitter, a data packet. The method 1100 may comprise receiving stimulation signalling representative of electrical stimulation provided to the body tissue structure; correlating the received signalling representative of scattered ultrasound with the received stimulation signalling; and outputting the indication of the property of the body tissue structure in dependence on the correlation. Also included in this disclosure is computer program code which, when executed on a processor, is configured to perform any method disclosed herein. It will be appreciated that various changes and modifications can be made to the present disclosed examples without departing from the scope of the present application as defined by the appended claims. Whilst endeavouring in the foregoing specification to draw attention to those features believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. An ultrasound communication system for performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure; the ultrasound communication system comprising:an ultrasound transmitter;an ultrasound receiver; wherein the ultrasound transmitter is configured to be located at a different position to the ultrasound receiver and at least one of the ultrasound transmitter and ultrasound receiver is configured to be implanted in the body tissue structure; anda controller; wherein the controller is configured to:receive signalling representative of scattered ultrasound received by the ultrasound receiver, the scattered ultrasound resulting from data communication ultrasound transmitted by the ultrasound transmitter and interacting with the body tissue structure; the scattered ultrasound comprising at least one scatter component corresponding to a propagation channel of the data communication ultrasound;determine, in dependence on the signalling, a channel impulse response indicative of the propagation channel;obtain, in dependence on the channel impulse response, an indication of a property of the body tissue structure; andoutput the indication of the property of the body tissue structure.

2. The ultrasound communication system of claim 1, wherein the scattered ultrasound comprises:a direct forward scatter component indicative of a direct propagation channel from the ultrasound transmitter to the ultrasound receiver; andat least one indirect scatter component indicative of a propagation channel comprising at least one reflection boundary of the body tissue structure from which the transmitted data communication ultrasound is reflected.

3. The ultrasound communication system of any preceding claim, wherein the controller is configured to:compare the indication of the property of the body tissue structure to a prior stored indication of the property of the body tissue structure body tissue;determine whether a variation between the indication and the stored indication exceeds a predetermined variation threshold; andoutput an alert indication in dependence on the variation exceeding the predetermined variation threshold.

4. The ultrasound communication system of any preceding claim, wherein obtaining the indication of the property of the body tissue structure comprises:obtaining, in dependence on the channel impulse response, one or more of a signal amplitude and a propagation delay of the received scattered ultrasound; anddetermining the indication of the property of the body tissue structure in dependence on the one or more of the amplitude and the delay.

5. The ultrasound communication system of any preceding claim, wherein obtaining the indication of the property of the body tissue structure comprises:obtaining, in dependence on the channel impulse response and a predefined model of the body tissue structure, a distance travelled by the received scattered ultrasound, based on a reference.

6. The ultrasound communication system of any preceding claim, wherein the signalling is representative of scattered ultrasound resulting from pulsed data communication ultrasound transmitted by the ultrasound transmitter.

7. The ultrasound communication system of any preceding claim, wherein a distance between the ultrasound transmitter and the ultrasound receiver at the different position is:a known separation distance; ora distance determined based on a determination of a propagation delay in data communication ultrasound transmitted between the ultrasound transmitter and the ultrasound receiver.

8. The ultrasound communication system of any preceding claim, wherein the body tissue structure undergoes periodic movement, and wherein the controller is configured to:determine, in dependence on the signalling, a periodicity of variation of the received scattered ultrasound; anddetermine a corresponding periodicity of movement of the body tissue structure in dependence on the periodicity of variation of the received scattered ultrasound.

9. The ultrasound communication system of any preceding claim, wherein the controller is configured to output a plot of time versus a determined distance travelled by the at least onescattered ultrasound, wherein the indication of the property of the body tissue structure corresponds to a corresponding feature in the plot.

10. The ultrasound communication system of any preceding claim, wherein the controller is configured to:receive stimulation signalling representative of electrical stimulation provided to the body tissue structure;correlate the received signalling representative of scattered ultrasound with the received stimulation signalling; andoutput the indication of the property of the body tissue structure in dependence on the correlation.

11. The ultrasound communication system of any preceding claim, wherein the controller is configured to:determine, in dependence on the indication of the property of the body tissue structure, a data communication feedback parameter indicative of a property of data communication ultrasound to be transmitted via the body tissue structure; andcontrol the ultrasound transmitter to output data communication ultrasound according to the data communication feedback parameter.

12. The ultrasound communication system of claim 11, wherein:the property of the data communication ultrasound to be transmitted via the body tissue structure is an energy loss, andthe data communication feedback parameter is indicative of a second energy loss of the data communication ultrasound, the second energy loss being lower than a first energy loss of the data communication ultrasound transmitted without dependence on the indication of the property of the body tissue structure.

13. The ultrasound communication system of any preceding claim, further comprising at least one pacemaker electrode, wherein:the body tissue structure is a heart;the ultrasound transmitter and ultrasound receiver are configured to be implanted in the heart; andthe ultrasound transmitter and ultrasound receiver are configured to respectively transmit and receive ultrasound to communicate control signalling to the at least one pacemaker electrode to control operation of the at least one pacemaker electrode.

14. The ultrasound communication system of claim 13, wherein the at least one pacemaker electrode comprises a pair of electrodes configured to be implanted as part of a distributed pacemaker, and wherein the control signalling is configured to synchronise the operation of the pair of electrodes.

15. The ultrasound communication system of any preceding claim, comprising a plurality of ultrasound receivers, wherein the controller is further configured to receive the signalling representative of scattered ultrasound received by the plurality of ultrasound receivers.

16. The ultrasound communication system of any preceding claim, wherein the ultrasound transmitter is an omnidirectional transmitter.

17. A method for performing intrabody ultrasound communication and for obtaining spatial information of a body tissue structure using an ultrasound communication system comprising an ultrasound transmitter and an ultrasound receiver, wherein the ultrasound transmitter is located at a different position to the ultrasound receiver; and at least one of the ultrasound transmitter and ultrasound receiver is implanted in the body tissue structure;the method comprising:receiving signalling representative of scattered ultrasound received by the ultrasound receiver, the scattered ultrasound resulting from data communication ultrasound transmitted by the ultrasound transmitter and interacting with the body tissue structure; the scattered ultrasound comprising at least one scatter component corresponding to a propagation channel of the data communication ultrasound;determining, in dependence on the signalling, a channel impulse response indicative of the propagation channel;obtaining, in dependence on the channel impulse response, an indication of a property of the body tissue structure; andoutputting the indication of the property of the body tissue structure.

18. The method of claim 17, comprising:receiving data signalling representative of the data communication ultrasound transmitted by the ultrasound transmitter and received by the ultrasound receiver; andobtaining, in dependence on receipt of the data signalling transmitted by the ultrasound transmitter, a data packet.

19. The method of claim 17 or claim 18, comprising:receiving stimulation signalling representative of electrical stimulation provided to the body tissue structure;correlating the received signalling representative of scattered ultrasound with the received stimulation signalling; and5 outputting the indication of the property of the body tissue structure in dependenceon the correlation.

20. Computer program code which, when executed on a processor, is configured to perform the method of any of claims 17 to 19.1031

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