Apparatus and method for measuring the composition or constitution of human or animal tissue

Abstract

A tissue density assessment apparatus 200 comprises first and second transducer panels 210, 212 (30, 32 fig. 1) spaced apart facing each other. The transducer panels are connected to support arms 206, 208. One of the transducer plates 210, 212 is adjustable relative to the other e.g. by movement of arm 206 relative to fixed arm 208. At least one ultrasound emitter is disposed on the second panel, and at least one ultrasound receiver is disposed on the first panel. A processor determines from the received ultrasound signals a tissue composition / constitution measure of the assessed tissue, and may determine an overall tissue composition / constitution measurement across the entire area of the tissue. An adjustment element 220 may be provided between a panel and the body to reduce air gaps and increase comfort. The element may have a fluid filled bladder 220 and contact plate 222. Preferably a patient is assessed in an upright position (e.g. standing or seated). The apparatus may obtain a measure of an average breast tissue composition / constitution, which can be used to indicate a medical condition, or a risk of a medical condition, e.g cancer.

Description

Field of the Invention The present invention relates to apparatus and a method for obtaining a measure of the constitution or composition, such as tissue density, of a human or animal subject, for example for the assessment of a medical condition, and to a method of identifying a medical condition in a subject by use of apparatus or a method as taught herein. Background of the Invention Breast density assessment is an important part of personalised breast cancer risk assessment for future risk-adaptive screening services. Existing, approved measures of breast density assessment, in particular X-Ray Mammography (XRM) and Magnetic Resonance Imaging (MRI) do not meet the criteria set by the UK National Screening Committee (NSC) for population screening (or pre-screening) because these imaging methods are not simple, safe, precise, or validated tests. Specifically, they are susceptible to characterising women differently between readers or screening occasions. Additionally, existing breast density assessment techniques are generally not suitable for younger women who are under the screening age (40 in the US, and around 50 in Europe). More specifically, the UK National Screening Committee (NSC) considers changes to screening programmes when there is strong evidence that they could improve outcomes. In 2019, the committee considered the use of supplemental screening for women with dense breasts but found that the identification of this cohort (by mammographic breast density assessment) could not be done consistently, reliably or at scale (https: / / view-health-screeninq-recommendations.service.qov.uk / document / 465 / download). The NSC Report was accompanied by a consultation (https: / / view-health-screeninq-recommendations.service.qov.uk / review / breast-cancer-2019 / download-documents / cover sheet / ) which concluded: "It is essential to establish a safe, consistent and reliable method of measuring breast density before its routine use in clinical procedure could be considered. The current lack of a gold standard method must be addressed." Similarly, in the United States in 2023, the FDA published an update to the mammography regulations to require, among other things, mammography facilities to notify patients about the density of their breasts, in order to strengthen the FDA’s oversight and enforcement of facilities and to help interpreting physicians better categorize and assess mammograms. The update requires all mammographic facilities to comply by 10th September 2024. This will be by way of a mammographically derived breast density assessment. The intention is that women will be sent one of two Federal density notification statements ("not dense" or "dense") and that the mammogram report sent to referring providers will have to include an assessment (more detailed) of the patient's breast density. This follows a grassroots movement, spearheaded Dr Nancy Cappello and her “Are You Dense?” advocacy group, to push for breast density notification in the United States, as women are needlessly undergoing mastectomies, invasive treatment and dying after late detection because they were uninformed of breast density issues. Consistent with this, the American College of Radiology and the Society of Breast Imaging have recommended that breast cancer risk assessment is first carried out by the age 25, so that those at higher risk of developing breast cancer can be identified early and be offered risk management advice and with the prospect of early treatment of any cancer that subsequently develops. While the intention of such early assessment is laudable, breast density assessment cannot currently be included in early breast cancer risk assessment because mammograms are not recommended for young women in light of the health risks they entail. Similarly, in the “Report of The Independent Review of Adult Screening Programmes in England” of October 2019, Sir Michael Richards highlighted targeted risk-based screening as the main opportunity to further improve the uptake, coverage and functioning of breast screening programme. In light of all these recommendations and regulations seeking to establish screening of breast density particularly in younger women in order to identify early women at risk, in March 2023, the UK National Screening Committee gathered experts to discuss models for personalised breast screening based on risk (https: / / nationalscreeninq.bloq.qov.uk / 2023 / 03 / 28 / experts-discuss-models-for-personalised-breast-screeninq-based-on-risk / ) There is therefore a healthcare need and push to provide for quantitative breast density assessment for early detection of people at risk of developing breast cancer, even though no reliable system that is safe to be used particularly on younger women exists. In order to avoid X-ray and magnetic resonance imaging, the use of ultrasound imaging has been proposed, which is safe and can be applied repeatedly even in younger people. While ultrasound is a suitable candidate technology, traditional handheld scanners are heavily operator dependant and are not capable of consistent and reliable breast density assessment. Ultrasound computed tomography scanners in development are less dependant on the skill of the operator and are capable of breast density assessment. However, they are complex devices designed for breast screening and require thousands of ultrasound emitters and receivers to reconstruct images and map breast composition. Breast density assessment would be an adjunct to their primary use of breast screening. Furthermore, quantitative acoustic attenuation measurement of a heterogeneous tissue structure, such as breast tissue, is difficult in vivo due to complex ultrasound-tissue interactions, including refraction and phase cancellation effects, which cause significant measurement errors. To date there has not been proposed any ultrasound based system that can provide a reliable indication of breast tissue density. As a consequence, mammography remains the only currently accepted method for breast density assessment. However, as mammography uses ionising radiation, this limits the frequency of assessment, as well as requiring specialist facilities and operating personnel. Mammography is also an uncomfortable procedure which necessitates painful compression of the breast. Ultrasound tomography systems that have been proposed in the literature require water coupling to facilitate ultrasound propagation between a hemisphere / ring of ultrasound elements and the breast, which again is not an ideal solution. While there has been much focus by the health authorities in seeking to detect early the risk of a woman developing breast cancer, especially in younger women, there are more general clinical needs to early assessment of other medical conditions. These can include, for example, other cancers or the risk of developing cancer in other parts of the body and other benign growths. There is also a need for identifying early the risk or actual early onset of other changes to the condition of a person’s organs, including for example bone tissue and possible onset of osteoporosis, for instance. While X-ray and magnetic resonance imaging are capable to detecting such conditions, they are not routinely practiced out on a person until actual symptoms develop, which typically occurs only once the condition is advanced. This frustrates attempts for early detection and consequential management and early treatment of such conditions. Summary of the Present Invention The present invention seeks to apparatus and a method for obtaining a measure of tissue composition or constitution of a human or animal, particularly for the assessment of risk of developing or early onset of medical disorders. According to an aspect of the present invention, there is provided an organ tissue composition or constitution assessment apparatus comprising: first and second panel elements disposable in spaced apart and facing relationship, at least one of the first and second panel elements being adjustable relative to the other in spacing; at least one ultrasound emitter disposed at or on one of the first and second panel elements; at least one ultrasound receiver disposed at or on the other of the first and second panel elements, whereby the at least one ultrasound emitter and the at least one ultrasound receiver face one another; a support structure to which the first and second panel elements are attached; and a processing unit coupled to the at least one ultrasound transmitter and receiver and configured to determine from received ultrasound signals a tissue composition or constitution measure associated with the organ tissue assessed. Preferably, the processing unit is configured to determine an overall tissue composition or constitution measurement across a determined area of the organ tissue assessed. The measure may be of tissue density, for instance of a body part or organ. Advantageously, the support structure and panel elements permit a subject (for instance person) to undergo tissue composition or constitution assessment of a body part of that subject (person) while in a generally upright position, advantageously standing or seated. At least one of the first and second panel elements preferably comprises an adjustment element with a tissue facing plate orientable relative to the associated first or second panel element and relative to a part of a subject’s body to be assessed. The adjustment element is preferably configured to reduce or minimise air gaps between the transmitter and receiver elements and the part of a patient’s body to be assessed. Layers of acoustic coupling gel between the subject's body and the apparatus can provide for complete coupling. Another feature of the adjustment layer is that it can provide for improved patient comfort. The layer or layers can be designed to conform better to the patient's (human’s or animal’s) body part than more fixed transmitter and receiver plates. The adjustment element may be coupled to at a respective one of the first and second panel elements by a hinge or pivot and configured to rotate thereabout. In a practical embodiment, the adjustment element comprises a compliant mechanism disposed at a tissue remote side of the tissue facing panel and configured to allow adjustment of an angle of tilt of the tissue facing panel. The compliant mechanism may comprise a bladder selectively fillable with a fluid. For this purpose, the apparatus can comprise a fluid source, a pump coupled to the source and a pressure or volume sensor, the fluid source being coupled to the bladder for filling the bladder with fluid to a determined pressure or volume. The compliant mechanism is advantageously made of an acoustically transmissible material, wherein the at least one ultrasonic emitter or the at least one ultrasonic receiver is disposed behind the compliant mechanism relative to the tissue facing surface such that in use ultrasonic beams pass through the material of the compliant mechanism. Preferably, the at least one ultrasound receiver is shaped and sized to be at least as large as an area expected to be covered by a person’s body part to be assessed when placed between the lower and upper panel elements. In practical terms, it is not essential for the receiver to be bigger than the body part under investigation, as it just needs to be of a size and shape to capture all of the acoustic energy transmitted through the body part under investigation. Advantageously, the apparatus comprises an array of ultrasound emitters, the array advantageously being shaped and sized to be at least as large as an area expected to be covered by a person’s body part to be assessed when placed between the lower and upper panel elements. In practical terms, as with the receiver, it is not essential for the emitter or emitters to have an area bigger than the body part under investigation, as it / they just need(s) to be of a size and shape to transmit acoustic energy transmitted through the entire body part under investigation. The at least one ultrasound emitter is preferably adjustable and / or movable relative to the at least one receiver so as to be able to emit an adjustable or movable ultrasonic beam towards the at least one emitter. In some embodiments, the at least one ultrasound emitter has an adjustable ultrasonic beam angle. In practice this can be by generating narrow and / or broad ultrasound beams, or by beam steering. Preferably, the at least one ultrasonic receiver's output is proportional to the time-averaged intensity of ultrasound incident on its surface. The at least one ultrasonic receiver may be a phase insensitive receiver. In the preferred embodiments, the processing unit is configured to determine on the basis of an ultrasonic signal sensed at the at least one ultrasonic receiver and of the detected distance between the at least one ultrasonic emitter and the at least one ultrasonic receiver an attenuation of the ultrasonic energy and to determine therefrom a measure of tissue composition or constitution of the body part. Advantageously, the processing unit is configured to determine on the basis of a thickness of compliant and / or other material between the at least one ultrasonic emitter and / or the at least one ultrasonic receiver and the or a tissue contact panel of the apparatus a lost ultrasonic attenuation from the compliant and other material and to adjust the measured attenuation on the basis of the determined lost ultrasonic attenuation. It in the preferred embodiments, the measure of attenuation determined is an attenuation coefficient, such as dB per cm. The processing unit may be configured to calculate thickness of compliant and / or other material on the basis of a measured angle of the tissue facing surface relative to the at least one ultrasonic emitter or receiver, or on the basis of volume of fluid in the or a fillable bladder. Advantageously, the processing unit is configured to determine acoustic attenuation across substantially the whole area of an organ and to determine an average ultrasound attenuation through the tissue of the organ. In the preferred embodiments, the processing unit is configured to determine acoustic attenuation across a plurality of select areas of an organ and to determine an ultrasound attenuation map across the areas of the organ. The processing unit is preferably configured to operate the at least one transmitter at different frequencies, for the analysis of different tissue types. Advantageously, the processing unit is configured to determine a measure of tissue composition or constitution of a selected tissue type within a body part being tested. In practical embodiments, one or both of the first and second panel elements comprises a pivot or hinge configured to tilt at least a part of the panel element. The apparatus may be configured to determine a two dimensional image of tissue composition or constitution. Preferably, the apparatus is configured to obtain a determination of tissue composition or constitution in a plurality of planes by adjustment of the orientation of the transmitter and receiver elements relative to a body part being analysed. Preferably, the transmitter and receiver elements are adjustable in orientation and / or disposition (location relative to the body part), for example by being translatable across a body part. The apparatus may comprise a gimbal mechanism coupled to the panel elements and operable to adjust the orientation of the first and second panel elements. In an embodiment, the apparatus is configured to assess breast tissue density, or composition or constitution. The apparatus is advantageously operated substantially free of water or other liquid between the ultrasonic emitter and receiver elements and contacting the organ of a patient. According to another aspect of the present invention, there is provided a method of assessing organ tissue composition or constitution comprising: disposing first and second panel elements in spaced apart and facing relationship, at least one ultrasound emitter being disposed at or on one of the first and second panel elements and at least one ultrasound receiver disposed at or on the other of the first and second panel elements, whereby the at least one ultrasound emitter and the at least one ultrasound receiver face one another; and determining from received ultrasound signals a tissue composition or constitution measure associated with the organ tissue assessed. Preferably, an overall tissue composition or constitution measurement is determined across an area of the organ tissue assessed. Advantageously, the method is practiced on a person while in a generally upright position, advantageously standing or seated. At least one of the first and second panel elements may comprise an orientation adjustable tissue facing plate, wherein the method comprises adjusting the orientation of the plate relative to the associated first or second panel element and relative to a part of a patient’s body to be assessed. The method preferably comprises adjusting the orientation of the body facing plate to reduce or minimise air gaps between the transmitter and receiver elements and the part of a patient’s body to be assessed. The method advantageously comprises receiving ultrasonic signals across an area at least as large as an area covered by a subject’s (person’s) body part to be assessed. It may alternatively or additionally comprise emitting ultrasonic signals across an area at least as large as an area covered by a subject (person’s) body part to be assessed. The method may include adjusting and / or moving the at least one ultrasound emitter relative to the at least one receiver so as to emit an adjustable or movable ultrasonic beam towards the at least one emitter. Preferably, the method includes determining on the basis of an ultrasonic signal sensed at the at least one ultrasonic receiver and of the detected distance between the at least one ultrasonic emitter and the at least one ultrasonic receiver an attenuation of the ultrasonic energy and to determine therefrom a measure of tissue composition or constitution of the organ. It may include determining on the basis of a thickness of compliant and / or other material between the at least one ultrasonic emitter and / or the at least one ultrasonic receiver and the or a tissue contact panel of the apparatus a lost ultrasonic attenuation from the compliant and other material and to adjust the measured attenuation on the basis of the determined lost ultrasonic attenuation. It may also include calculating a thickness of compliant and / or other material on the basis of a measured angle of the tissue facing surface relative to the at least one ultrasonic emitter or receiver, or on the basis of volume of fluid in the or a fillable bladder. Preferably, the method includes determining acoustic attenuation across substantially the whole area of an organ and determining an average ultrasound attenuation through the tissue being assessed. It may include determining acoustic attenuation across a plurality of select areas of an organ and an ultrasound attenuation map across the areas of the organ. Advantageously, the method includes operating the at least one transmitter at different frequencies, for the analysis of different tissue types and / or for different assessment specificity. It may determine a measure of tissue composition or constitution of a selected tissue type within a body part being tested. The method may include determining a two dimensional image of tissue composition or constitution. Preferably, the method includes obtaining a determination of tissue composition or constitution in a plurality of planes by adjustment of the orientation of the transmitter and receiver elements relative to a body part being analysed. According to another aspect of the present invention, there is provided apparatus or a method according to any preceding claim, for the assessment of at least one of: a) risk of developing cancer; b) assessment of an appropriate screening modality c) the existence of a potential cancerous growth; d) an acute or chronic muscle condition; e) the existence or risk of muscular dystrophy; f) an acute or chronic bone condition; g) the existence or risk of osteoporosis. According to another aspect of the present invention, there is provided a method of identifying a medical condition in a patient by use of apparatus or a method according to any preceding claim, wherein the method is preferably practiced in the assessment of at least one of: a) risk of developing cancer; b) assessment of an appropriate screening modality c) the existence of a potential cancerous growth; d) an acute or chronic muscle condition; e) the existence or risk of muscular dystrophy; f) an acute or chronic bone condition; g) the existence or risk of osteoporosis. Other aspects and advantages of the present invention will become apparent to the person skilled in the art having regard to the disclose of the preferred embodiments which follows. Brief Description of the Drawings Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side elevational view of a preferred embodiment of tissue composition or constitution assessment apparatus according to the present invention; Figure 2 is a front elevational view of the embodiment of apparatus of Figure 1; Figure 3 is a side elevational view of a part of the transducer plates of the embodiment of apparatus of Figure 1; Figure 4 is a front elevational view of the part of the transducer plates of Figure 3; Figure 5 is a side elevational view of the embodiment of apparatus of Figure 1 in use; Figure 6 is a front elevational view of a part of the apparatus of Figure 1 in use; Figure 7 is a front elevational view of a part of the embodiment of apparatus of Figure 1 showing the operation of a gimbal of the apparatus; Figure 8 shows front and side elevational views of a rendered casing for the apparatus of Figure 1; Figure 9 is a side perspective view from above of the rendered casing of Figure 8; Figure 10 is a side perspective view of the transducer plates of the rendered casing of Figure 8; Figure 11 is a side elevational view of the transducer plates of Figure 10; Figure 12 is a schematic representation of a sensor plate of the embodiment of apparatus of Figure 1; Figure 13 is a schematic representation of the sensor plate of Figure 12 in an example mode of operation; Figure 14 is a schematic diagram of the electronic circuitry of the embodiment of apparatus of Figure 1; Figure 15 is a schematic diagram of a part of the electronic circuitry of Figure 14; Figure 16 is a schematic diagram showing and alternative implementation of a part of the apparatus circuit apparatus of Figure 14; Figure 17 is a schematic diagram of the preferred structure of ultrasonic receiver for the apparatus disclosed herein; Figure 18 is a graphical illustration of a simulation of the intensity of an ultrasound field produced by an ideal plane-piston transducer; Figure 19 is a graph showing the frequency dependence of ultrasound attenuation and backscatter in breast tissue; Figures 20 to 22 are schematic representations of a receiver array showing transducer coupling to the object of interest; Figures 23 to 25 are schematic representations of a receiver array showing the effect of selecting only transducers positioned over the region of interest; Figure 26 is a series of graphs depicting pyroelectric voltage measurement across transducers of an array; Figure 27 shows the series of graphs of Figure 26 when subjected to conditions of poor signal to noise; Figure 28 is a graph depicting the effect of measurements performed in quick succession when the sensor does not have time to return to the “rest” state; Figure 29 is a schematic side elevations view of transducer plates of the apparatus with no compliant coupling provided; Figure 30 is a schematic side elevations view of transducer plates of the apparatus with compliant coupling provided; and Figure 31 is a graph of simulated measurements of attenuation through the same 70 mm pathlength of tissue showing how attenuation is directly proportional to acoustic attenuation in the example shown. Description of the Preferred Embodiments The disclosures herein relate to the measurement of tissue composition or constitution of a part of a human or animal subject, for the assessment of the condition of the subject. While the disclosures focus on breast density assessment as one example implementation of the teachings herein, it is to be understood, and it is disclosed herein, that the method and apparatus taught can be used to the detection of conditions in other parts of the body and also other medical conditions. The method and apparatus can be used for the determination of risk of a patient to develop breast cancer, for example. They can equally be used to determine cancerous growths in other parts of the body, including parts made up of different tissue types such as muscle, fatty tissue, bone and so on. They can equally be used to detect other conditions or the risk of developing other conditions, such as osteoporosis and the like. Examples are described below. Breast density is a measure of the relative proportion of fat to fibro-glandular tissue in the breast and is a key indicator of breast cancer risk and mammographic sensitivity. Fat and fibro-glandular tissue differ in their ultrasonic propagation properties. In the preferred embodiments of the present invention, a series of through-transmission ultrasound measurements of the breast at several positions are taken and used to determine the bulk / volume-averaged acoustic attenuation of the breast, which can act as a surrogate measure of mammographic breast density. The quantitative through-transmission ultrasound measurement by phase-insensitive detection as disclosed in the embodiments of the invention described below can overcome these challenges and meet regulatory criteria by providing: 1) simple ultrasound-based through-transmission measurements in the preferred embodiments only requiring a single emitter and receiver pair which are directly-coupled to the patient (that is not through water), and in which imaging is avoided so as to avoid complex reconstruction computations; 2) safe non-ionising diagnostic ultrasound conditions, which can incur reduced and preferably no compression of the breast or other body part and with direct-coupling thereby avoiding water-based signal cross-contamination; 3) precise quantitative measurements which can be largely operator independent, and which can provide high contrast. Technical benefits of the disclosed apparatus and method in the field of breast density assessment and in other fields include the following: 1) in comparison to conventional mammography: avoidance of non-ionising radiation, quantitative measurement, and avoidance of compression of the patient’s breasts; 2) in comparison to ultrasound computed tomography: a simpler and cheaper technology which requires less computation and fewer ultra-sound elements; faster signal processing and determination (no complex reconstruction algorithm is required); and the avoidance of water coupling; 2) in comparison to hand held ultrasound: a quantitative determination and which is largely operator independent. Quantitative, traceable measurements attainable by the system and method taught herein can provide confidence and assurance in the assessment for patients and clinicians. Quantitative data can more readily allow for the intercom parison of density across patients and also (or in the alternative) over time, which will provide great value for national screening programmes and the knowledgebase of breast density. The configuration of the apparatus and the method for ultrasound breast density assessment are safe, affordable and can be automated, which can enable procedures to be performed outside a typical hospital setting, for instance, in pharmacies, general doctor surgeries and private breast cancer risk clinics. In one aspect, the teachings herein can provide ultrasound breast density assessment for comprehensive personalised breast cancer risk assessment (optionally combining with information on lifestyle, familial history, genetics, and so on) which can be used to inform risk stratified screening. These new screening requirement strategies can be used to notify individuals of optimum screening recruitment including, for example: within an age range (such as: 50-70, 40-70...), at a certain frequency (such as every 1, 2, or 3 years), recommended imaging modality (such as: mammography-only, mammography plus handheld ultrasound, MRI-only, as so on), refinement of existing risk stratification for high-risk individuals. It can also be used as a research tool for pharmaceutical companies developing risk-reducing medicines, such as Tamoxifen™ used to reduce breast density. Other example applications include providing through-transmission acoustic attenuation measurements of large muscle masses (such as quadricep, calf, bicep and so on) for example for use by sports scientists. The preferred embodiments disclosed herein make use of phase-insensitive pyroelectric sensors for obtaining quantitative ultrasound determinations by ultrasound computed tomography, although embodiments are envisaged that could use other sensor types. Examples of such sensors have been disclosed by the applicant in, for example, EP-3,588,023, WO-2010 / 084319. As these sensors are known in the art and described in detail in these patent publications, they are not described in further detail herein. Should it be deemed necessary, for the avoidance of doubt, the disclosures in these patent publications are incorporated herein by reference. While these patent publications relate to quantitative acoustic attenuation tomography (imaging) of breast phantoms and use cases of this quantitative imaging technology for neo-adjuvant therapy response monitoring and breast cancer diagnosis, the apparatus and method disclosed herein use such sensors in a different manner, as described in detail below. The preferred embodiments determine a bulk acoustic attenuation measurement, preferably using phase-insensitive pyroelectric sensors, by obtaining simple through-transmission ultrasound measurements in a modified non-imaging configuration. The measurement can be of an entire body part, or a section of a body part or targeted to a specific tissue type. The preferred embodiments provide apparatus and a process which: 1) enables a patient to remain upright during the procedure, 2) provides direct coupling of the sensors to the patient avoiding passage through water, 3) provides signal pathlengths which can be readily determined from plate separation distance and compliance mechanism angle, and 4) reduces the amount of body part compression required compared to traditional imaging systems. Figures 8 to 11 are different views depicting an example of tissue composition or constitution assessment device in what could be described as a final implemented form. The device 200 comprises the components of the embodiment shown in Figures 1 to 7. The device 200 includes a main casing 202 which in this example is intended to be floor standing, and a frame component 204 which includes first and second arms 206, 208 extending substantially horizontally from the frame 204 and which support the transducer plates 210, 212. The arm 206 is able to move upwardly and downwardly, that is to bring the transducer plates 210 further away or closer together. The lower arm 208 is typically fixed to the frame 204, although the frame 204 can itself move upwardly and downwardly and also rotate in a manner similar to that shown in Figure 7. The upper transducer element, in this example, comprises a conformance mechanism 220, which includes an inflatable bladder 220 and a contact plate 222, which are equivalent to the conformance mechanism 40 described above. The skilled person will appreciate that this is just one example of final implementation of the apparatus taught herein and that it can take different forms, for example to assess other body parts. The example shown in Figures 8 to 11 is designed specifically for breast tissue composition or constitution (especially density) assessment and it is considered that the form shown in this example is a material improvement over existing imaging system devices. Referring now to Figures 12 and 13, these show schematic representations of the transducer plate 32 of the embodiment of Figures 1 to 7 (and equally of the embodiment of Figures 8 to 11). It will be appreciated that the images of Figures 12 and 13 could relate to an embodiment comprising an array of transmitters or an array of receivers. While the mechanism will be different, the principle remains the same. Referring first to Figure 12, this shows a silhouette of a breast 100 located adjacent the transducer plate 32. Typically, and in order to ensure that the apparatus is able to assess the entirety of a breast, in this example, the transducer array 32 is generally larger than the area of the breast 100, such that there is a transducer aligned at every location of the breast 100 (or other body part). It will be appreciated that some of the transducers will fall outside the area of the breast 100 or other body part and that therefore any signal that is generated as a consequence of those outlying transducer elements will in practice need to be discounted. The preferred embodiment, however, goes further than that by identifying the signals that reliably pass through the body part (in this example breast 100) and to discount the signals associated with all of the transducers that do not satisfy this criteria. In the example depicted in Figure 13, a number of the transducers within the perimeter of the body part (breast 100 in this example) are discounted on the basis that they provide a poor quality signal. This is achieved by analysing the pulse-echo return of the signal emitted by the individual transceivers and it is determined from the pulse-echo whether the transducers are in a zone of good return, such that all transducers disposed that exhibit a poor return will have their signals discounted during the subsequent assessment of the data. In the case of an embodiment in which an array of receivers is used, receivers in a location of poor return can be identified by determining whether the signals they receive have been subjected to no or minimal attenuation, in particular an attenuation that is not consistent with passage of the ultrasonic beam through body tissue. In the example shown at Figure 13, the reason why some of the transducer elements falling within the perimeter of the body part (breast 100 in this example) are deemed to provide a poor return will generally result from poor direct contact of the body to the apparatus at those locations. This will typically be as a consequence of an air gap between the transducer element or contact plate and a part of the body. While the embodiments described above include transducer plates facing one another, in which could be described as a vertical orientation such as one transducer plates lies above the other, it is to be understood that this is just one example implementation and that in other implementations the transducer plates could be disposed in other orientations including facing one another in a horizontal direction. What is relevant is that the transducer plates 30, 32, 200-210 are preferably disposed so as to face one another, in particular to have the same spacing between the array of transducers and the receiver (or in the alternative a array of receivers and one transmitter or an array of transmitters). Having the transducer plates parallel to one another simplifies the subsequent signal processing because in normal conditions the attenuation will be the same across the array. It is not excluded, however, that in other embodiments the transducer plates may not be parallel, with the processing unit of the system being configured to take into account the different spacings across the transducer plates. Similarly, while in the preferred embodiment the transducer plates are preferably flat plates, in other embodiments they may be curved, for example to conform better to the shape of a body part. Again, in the case of curved transducer plates, the processor unit will be configured to take into account differences in the spacings between the transducer plates across the area of the transducer elements. As a consequence, the person skilled in the art will appreciate that a practical implementation of the teachings above can take a variety of different forms. Having described the principal elements of the apparatus, the disclosure proceeds to describe the signals obtained by the apparatus and how these are processed. In brief terms, the apparatus produces a series of ultrasound bursts which travel through tissue under investigation and which are then transduced into a voltage by an ultrasound sensor. The following describes how the apparatus produces the ultrasound bursts and how the received sensor voltages can be used for tissue characterisation. Referring to Figure 14, this shows in schematic form the transmission stage 300 of the system. The purpose of the transmission stage 300 is to generate timed drive signals to send to the one or more ultrasound transducers 320 (labelled "Tx" in Figure 14) to generate the desired ultrasound pulses used to "interrogate" the tissue 100 under investigation. The transmission stage 300 comprises a signal generator 302 which generates drive voltages. A computer or control unit 304 drives the signal generator 302, in particular the: (i) voltage amplitude; (ii) sine wave frequency; (iii) tone burst duration; (iv) tone burst spacing of the signals outputted by the signal generator 302. An amplifier and multiplexer module 306 is connected to the output of the signal generator 302 and includes a first amplifier stage 308 which amplifies the dive signals from the signal generator 302 to the voltages required by the planepiston ultrasound transducers to produce sufficient ultrasound power that will be measurable by the sensor after attenuation from the tissue under investigation. The transmission stage 300 amplifies the drive signals from the signal generator 302 and sends the drive signals to each of the ultrasound transducers such that measurable signal level is detected by the sensor. The speed of the switching and the electrical power the switch is required to handle determines the type of switching network and the amplifier configuration around it. The transmission stage 300 in this embodiment comprises a multistage amplification configuration to a plurality of transmitters. More specifically, the amplifier and multiplexer stage includes a switch network 310 connected to and controlled by the computer or processor unit 304, for switching the transmission signal to one of a series of second stage amplifiers 312, each associated with a respective ultrasonic transmitter 320. The switch network 310 typically sends the drive signal to the plurality of ultrasound transducers 320 in quick succession, preferably at a switching rate in the order of 10s of milliseconds. In the case of the use of a switch network 310 designed for fast switching and which is unable to handle high electrical power, and as shown in the embodiment of Figures 14 and 15, the circuit amplifies the drive signal to an "intermediate" drive level with a high-quality class A first stage amplifier 308, switches through the switching network 310 the "intermediately" amplified drive signals to each of the channels, and then amplifies these to the "full" drive signal power with cheaper, lower quality class A / B second stage amplifiers 312. An enlarged view of this section of the circuitry is shown in Figure 15. For a tomography implementation of the system and method described herein a typical scan would involve 10,000 individual measurements. For the tissue density assessments disclosed herein, fast switching is not necessary so it is possible to use a more simple and less expensive configuration of a type as shown in Figure 16, comprising for example a single class A amplifier 308 and a high power, slower switching network 310. The second stage amplifiers 312 would not be necessary. The purpose of the ultrasound transmission array 320 is to produce ultrasound beams from one or more positions in the array to interact with a volume of tissue 100 under investigation and produce a signal that can be registered by the sensor 352. The characteristics of the ultrasound tone bursts (frequency, amplitude, duration, timing and so on) are determined by transmission electronics and are chosen to register sufficient signal in the sensor 352 (after attenuation by the tissue 100 under investigation) but without exceeding diagnostic ultrasound output power limits set by regulation. This is the ALARA principle (as low as reasonably achievable), and the limits are tissue dependent. The type of ultrasound transducers 320 used for this purpose can be plane-piston transducers. This type of transducer produces highly directional, high intensity beams with a directionality that depends on its size compared to the wavelength. The detection stage 350 includes a sensor unit 352 which comprises a reference signal receiver 354 and a signal receiver 356 configured and disposed to receive the ultrasound beams from the transmitter array 320. The reference receiver 354 is designed to obtain a reference signal for use in normalising the signals received by the signal receiver 356, in a manner disclosed below. The reference receiver and signal receiver 354, 356 are coupled to a signal amplifier 358, which amplifies the receiver signals to the required level for an oscilloscope unit 360. The oscilloscope unit 360 is fed with the signal from the receiver 356, the signal from the reference receiver 354 and a difference signal, being the difference between the signal of the receiver 356 and that of the reference receiver 354. The output of the oscilloscope unit 356, is connected to the computer or control unit 354 for analysis of the receiver signals and determination of a measure of tissue density of the body part 100, in the preferred embodiments of an average tissue density across an area identified for analysis (such as the entire body part being analysed or a part thereof). The person skilled in the art will be familiar with specific circuit components can be used with the circuit, these being part of the skilled person’s common general knowledge. Figure 17 shows an example of the preferred structure for the sensor 352. The structure of the ultrasound receiver (transducer) is as disclosed in and covered by the applicant’s earlier EP-3,588,023, for example, and is able to provide phase insensitive ultrasound detection for use in a variety of ultrasound apparatus and fields, including the medical field. The receiver structure could be described as being a tri-laminar array formed of first and second pyroelectric layers separated by a spacer. The skilled person will appreciate that the structure includes a number of other layers that provide ancillary functions to the three layer core structure. Referring to Figure 17, this shows a preferred embodiment of receiver array or transducer 410, which includes first and second pyroelectric electrodes 414, 412 preferably formed of poled polyvinylidene difluoride (PVDF). The pyroelectric layers 414, 412 in this embodiment have a thickness of 28 micrometres, although may in other embodiments have a thickness anywhere in the range from 1 to 60 micrometres. The thickness of the pyroelectric layers 414, 412 is typically dependent upon the nature of the material used for these layers and upon the pyroelectric characteristics desired for the detector. Thinner layers 412 and 414 can provide greater gains in peak amplitude and reduced time to peak due to more rapid heat diffusion through the material of the layer. PVDF layers that are too thick can lead to higher reflections and reduced sensitivity. The first and second pyroelectric electrode layers 414, 412 are in the preferred embodiments identical in constitution (material) and size (thickness and volume). This ensures that the signals from the first and second pyroelectric elements are directly comparable. The skilled person will appreciate, however, that in other embodiments the first and second pyroelectric layers 412, 414 may be different, for example to be of different size (e.g. thickness) or material, in which case it is preferred that the detection apparatus is configured to calibrate the signals from the first and second pyroelectric elements so as to produce a reliable and usable differential signal therefrom. In the case where the two electrodes are precisely the same, it is envisaged that one could reverse the polarity of the two signals (by flipping one of the membranes around) and sum their outputs directly to derive a difference signal. In practice, it is advantageous to be able to monitor the signals separately (as well as the differential or summation) as this can provide diagnostic information about how good any particular measurement is during the scan to reduce artefacts. Disposed between the first and second pyroelectric electrode layers 414, 412 is a spacer layer 16 that acts: 1) to separate electrically the pyroelectric layers 412, 414; and 2) to keep the two pyroelectric layers 412, 414 close enough such that they see effectively the same vibration or background acoustic excitation; while 3) maintaining sufficient distance to ensure that the pyroelectric layers 412, 414 generate significant differences in their pyroelectric responses. The first or lower (as viewed in Figure 17) pyroelectric electrode layer 414 is disposed adjacent the absorbing highly backing layer 418, which acts as the heat source. This structure provides two distinct signals (at separate timestamps), the difference between which provides a more accurate signal. The spacer layer 416 of the embodiment of transducer 410 shown in Figure 17 has a thickness of 9 micrometres, which has been found particularly effective in tests, although could have a thickness from 1 to 100 micrometres. The spacer 416 may be made of polyethylene terephthalate, another polyester such as Mylar®, polymethyl methacrylate, or any other suitable polymer in the case of electrodes formed, for example, of PVDF. For electrode layers made of other materials, the spacer layer could likewise be made of a different material, as explained above. The spacer 416 provides electrical separation between the first and second pyroelectric layers 414, 412 but is preferably heat conductive (most preferably being substantially transparent to heat). In some embodiments the spacer layer 416 may be made of polyvinylidene fluoride (PVDF). This configuration provides the best acoustic impedance match as is same as the material either side. The spacer layer has been formed as an unpoled layer so as not to be piezo or pyroelectrically active. Poling is the process which essentially lines up the molecules so that the material responds to changes in pressure and temperature. There is an optimum thickness for the spacer layer 416. Specifically, the inventor has discovered that too thick a spacer layer has a deleterious effect on the directional response of the sensor, that is the way the output of the device responds to ultrasound striking the surface of the device at an angle (rather than perpendicularly). The preferred device should be omnidirectional, such that at whatever angle the ultrasound is incident, the output of the device should be the same. Spacers of up to 100 micrometres in thickness can greatly enhance the response to non-perpendicular incidence and as a consequence tissue imaging, while significant departures from this thickness will adversely affect tissue reconstructions. Too thin a spacer layer 416 can result in the pyroelectric response of the two layers 412, 414 being very similar to one another, such that differential operation will reduce the output voltage waveform and therefore the sensitivity of the device. Tests to date have shown that the optimum thickness of the spacer layer is in the region of 9 micrometres. The sensor structure 410 preferably also includes a high energy absorbency base or backing layer 418 for boosting sensor sensitivity. A suitable material for the backing absorber is based on a di-functional polytetramethylene glycol. In order to achieve significantly increased absorption above that of the base material, small micro-balloons of the material Expancel® may be used. Additionally, in order to modify the acoustic impedance of the backing material so that it is better matched to water, a high-density filler may be added to increase the material density to a value of 1,910 kg m'3. The absorption coefficient of the material at 3 MHz is preferably greater than 950 dB cm’1. The absorbing layer 418, with which the first pyroelectric layer 414 is preferably in direct, or intimate, contact, is very absorbing of ultrasound energy at the generated frequency. In practice, following transmission through the first pyroelectric layer 414, the majority of the acoustic power is absorbed within a millimetre or so of the pyroelectric layer, leading to heat being generated which dissipates across the various sensor layers. There is preferably also provided a protective layer 420 disposed so as to overlie the top of the array 410, which is transparent to ultrasound and made of electrically insulating and preferably water impermeable material. This may be of the same material as the spacer layer, although in the preferred embodiment the outside is metallised (for example with a thin spray coating) and grounded for electrical shielding from stray radiofrequency sources. The protective layer 420 may be of any suitable thickness, 9 micrometres for example. The protective layer is provided for physical protection and optimally should have no or minimal effect on the performance of the first and second pyroelectric elements 414, 412. In practice, the layers 412-420 may be bonded to one another by a suitable bonding or adhesive, preferably having non-matched characteristics. In the preferred embodiments, the glue has properties (acoustic etc.) that are substantially identical to the properties of the material layers on either side, as they are deposited from the material applied in solvent form. The glue layers can also be made conducting by doping with metallic flakes. The preferred glue layer only has an adhesive function, with the matching of the properties meaning that the sensitivity of the device can be maximised. In practical embodiments, the structure disclosed herein can be bonded together with a non-property matched glue. This is advantageously made as thin as possible to reduce losses, preferably just a few micrometres thick. In practice, an off-the shelf glue can be used that is adapted to bond together layers of PVDF and non-matched means in terms of acoustic impedance (Z) in relation to the (PVDF) layers. An example is a nitrile-rubber-based adhesive such as BOSTIK® 1755 (diluted to 5 wt% with BOSTIK 6322 thinner). In preferred embodiments, further thin layers may be disposed in intimate contact with the pyroelectric layers 412, 414 to dissipate the signal away more rapidly (essentially increasing thermal conductivity), thereby ensuring that the signal from the sensor 410 decays to background as quickly as possible (a factor which affects scanning speed as it dictates when the next firing of the transducer can be executed. In summary, the preferred structure of ultrasonic sensor includes first and second overlaying pyroelectric layers and an electrically insulating spacer layer disposed between the first and second pyroelectric layers so as to separate The first and second pyroelectric layers are respectively a measurement electrode and a reference electrode. Further details of this ultrasonic sensor can be found in the applicant’s patent publication EP-3,588,023. Figure 18 shows a simulation of the intensity of an ultrasound field produced by an ideal plane-piston transducer. The information derived from tissue 100 under investigation comes from only the tissue encountered by the whole beam. This fact can be used in the following ways: i) narrow beams can investigate narrow portions of tissue 100, and multiple measurements (either one transducer 356 scanned or an array of transducers) can build up a projection image to understand the spatial distribution of acoustic properties. Narrow beams can be made narrower (focused) using acoustic lenses; ii) broad beams (weakly focused) can investigate larger portions of tissue in a single measurement, therefore reducing the need for multiple narrow measurements. This effectively uses the acoustic field to achieve spatial averaging. Again, acoustic lenses can be used to achieve this effect. Piezo-ceramic transducers can be used to produce megahertz frequency ultrasound beams and have a resonant frequency at which they operate with maximum efficiency. To generate at multiple frequencies from a single device, the configuration of the circuit could: (i) have multiple transducers with different resonant frequencies, (ii) drive individual transducers at harmonics of the resonant frequency, (iii) drive the transducer off-resonance (for sufficiently broadband devices), for instance driving a 2 MHz transducer at both 1.5 and 2.5 MHz. Frequencies for optimal transmission through different human tissues have been found to be as follows: (ii) 0.2 -1 MHz -> bone (ii) 2 - 5 MHz -> soft tissue Operation of the circuit at multiple frequencies can be used to determine the frequency dependent acoustic attenuation properties of tissue, which can provide additional diagnostic information. Figure 18 is a graph from F. T. D’Astous and F. S. Foster, “Frequency dependence of ultrasound attenuation and backscatter in breast tissue” (Ultrasound in Medicine and Biology, vol. 12, no. 10, pp. 795-808, 1986.), which shows the differing frequency dependence of ultrasound attenuation (d I df) for various breast tissue components. Operating the apparatus at different frequencies can enable the detection of different tissue / organ types within a volume of interest, such as soft tissue, bone, and so on. Transmission frequency can therefore be used to filter or tune the system to a specific tissue type. The purpose of the detection stage 350 of the preferred embodiment is to transduce the attenuated ultrasound energy into a voltage signal and record the full sensor response for signal processing and to calculate an acoustic attenuation measurement. The purpose of the sensor 356 is to transduce the attenuated ultrasound energy into a voltage signal. In an embodiment, the system comprises a single trilaminar pyroelectric sensor 356 of the type disclosed in the applicant’s earlier patent publications identified above. The characteristics of this sensor type are advantageous for the following reasons: (i) they can have a large-area, able to collect the whole transmitted beam energy after refraction and diffraction from ultrasound interaction with a heterogeneous medium, (ii) they are phase-insensitive, which responds to power or time-averaged intensity and is immune to phase-related ultrasound measurement errors such as phase cancellation, (iii) they are omni-directional and are able to receive transmitted beam energy from a large range of incident angles (flat to ±40°), which means that issues of alignment (parallelism of the sensor relative to the transducer array) are much less important, (iv) they have a tri-laminar construction, able to measure low ultrasound powers (as a result of high attenuation by the medium or low initial transmission powers) in the presence of significant environmental noise that might be expected in clinical environments, (v) they are broadband, and able efficiently to transduce incident ultrasound from multifrequency sources using a single sensor. A trilaminar sensor of the type disclosed herein and in EP-3,588,023 and / or WO-2010 / 084319 generates two signals from a single ultrasound exposure, one from each membrane (labelled as "Sig Rx" and "Ref Rx" in the signal chain diagram of Figure 14). The character of these signals is determined by: (1) the characteristics of the ultrasound exposure, and (2) the signal amplifier electronics. A further advantage can be gained with the use of multiple receivers. Two advantages of multiple receivers include: (i) faster measurement sequences by measuring different parts of the tissue under investigation at the same time, (ii) more diagnostic information about the tissue from understanding how the ultrasound beam has travelled through the tissue, that is of the sound speed properties of the medium. The purpose of the signal amplifier 356 is: (i) to work in sympathy with the sensor electrical properties to produce a voltage whose amplitude (during ultrasound exposure) is proportional to the received acoustic power (if the sensor is sufficiently large to collect the whole beam area), (ii) to amplify and condition the sensor response such that the pyroelectric component is measurable above sources of noise. In addition to the amplified and conditioned "signal" and "reference" sensor responses from the receivers 354, 356, the detection unit 350 also produces a third output, which is an electronic difference between the two sensor responses. This can be described as the "difference" sensor response and is labelled "Diff" in Figure 14. The difference signal provides an important signal on which subsequent processing is carried out. The details of the signal amplifier operation are outlined in the applicant’s earlier EP-3,588,023 and WO-2010 / 084319. The purpose of the oscilloscope unit 360 is to record the timeseries voltage responses from the "signal", "reference" and "difference" output channels of the signal amplifier 358. The oscilloscope unit 360 is preferably of a type that is able to measure a wide range of voltage sizes (few volts to few milli volts) over the duration of a few milliseconds. Accurate and precise recordings of these pyroelectric signals is advantageous for the subsequent analysis and conversion to an acoustic attenuation measurement. The computer or processing unit 304 preferably coordinates the following components of the signal chain: (i) drive signal generation settings (including frequency) and signal triggering (turning the ultrasound exposure ON and OFF), (ii) switching the drive signal to the desired ultrasound transducers positioned over different parts of the tissue under investigation, (iii) signal amplifier settings, (iv) oscilloscope settings and signal capture triggering. The computer or processing unit 304 preferably allows for both "static" measurement sequences (settings are set once in the measurement setup and do not change) and "dynamic" (closed loop) measurement sequences, used to optimise data acquisition. Examples of "dynamic" algorithms include: (i) drive signal settings and sequencing can be adjusted on-the-fly to respond to the resultant sensor signals to adjust ultrasound power to safe diagnostic levels following the ALARA principle; (ii) the switching network can be programmed to select transducers which have the desired sensor response or perhaps the expected transducer impedance. The utility of this sequence type can be demonstrated with a couple of examples, explained below. With reference first to the example of Figures 20 to 22, this shows how the computer system can select only transducers with only good coupling to the object of interest, that is the body tissue to be analysed. The example refers to breast tissue, although the skilled person will appreciate from the teachings herein how they would be implemented with other body tissues and tissue types. In the first stage, ultrasound is fired from all of the transducers. Once fired, the control unit measures and grades the response signal, for example to set categories as shown in these Figures, for instance, a good response (green) category, a poor response (brown) category, and no response (black) category. The grading could be by means of a threshold and look-up table, which may be predetermined or could also be adjusted by means of feedback through machine learning, or artificial intelligence. In the third stage (Figure 22) on those transmitters that have been deemed to provide a good response for the measurement sequence are then fired, with the other transmitters being left idle. In the example of Figure 22, the transmitters in green are those selected to be used, whereas the transmitters in blue are not. With reference now to Figures 23 to 25 these show another example implementation, which enables a selection of transducers positioned over a region of interest, for example to select bone-only measurement positions in a scan of a patient’s limb, for example a leg. In the first stage (Figure 23), ultrasound is fired from all of the transducers. In the second stage (Figure 24), the system measures and grades the responses through the body part. As with the earlier embodiment, the grading could be based upon a threshold and look-up table associated with the expected response for that particular body part (bone for example, soft tissue and so on) with in this embodiment there being three grade categories, one representative of the expected response (green in Figure 24), one representative of poor response or a response that is not associated with bone tissue (brown in for example Figure 24) and regions in which there has been no response (those indicated in black in Figure 24). The system then only makes use of those transmitters from which it has been determined that there is a good response. In the example of Figure 25 those transmitters that are identified in green, whereas the transmitters identified in blue are not used. It will be apparent in particular from Figure 25 that while several other ultrasonic beams have passed through body tissue, the system has filtered the receiver signals to identify only body tissue of particular interest. As explained above, the filtering for particular body tissue types, based upon frequency of the ultrasonic signal, which exhibits different rates of attenuation through different body types, thereby enabling the apparatus and method to focus on particular body types for the assessment of that tissue type. Use of different frequencies can also be used for different assessment specificity In order to achieve these practical implementations, the signal amplifier settings can be adjusted by the computer or control unit 304, preferably dynamically, to minimise the effect of varying noise sources on the sensor signal during a measurement sequence. Furthermore, the oscilloscope unit 360 preferably comprises an auto-ranging algorithm to account for large variations and received sense of voltages with measurement through different types. The apparatus and method preferably use an algorithm which converts a pyroelectric sensor response into a measurement of volume-averaged acoustic attenuation of the tissue under investigation. The following parameters are preferably determined for the calculation of acoustic attenuation: (i) pyroelectric voltage experienced by the sensor 352 with the tissue 100 under investigation in the ultrasound path (this will be dependent on frequency, if the transducer is driven at various frequencies), (ii) pyroelectric voltage experienced by the sensor 352 with the calibration material(s) in the ultrasound path (system calibration is described below), (iii) temperature of the calibration material(s) during the reference measurement, (iv) ultrasound pathlength through the tissue under investigation. To calibrate the apparatus and method, measurements can be completed using a block(s) of material of known attenuation which could simulate the approximate attenuation of body tissue to be analysed. The loss expected at the frequency or frequencies of interest should be known. In principle, such calibration could be carried out daily, and the voltages and uncertainties (repeatability) loaded in the software prior to measurements on a person or animal. This can also be beneficial for quality assurance and understanding the uncertainty contributions to the measurement. Temperature of the device elements, calibration materials and tissue under investigation are all need measured, controlled and / or accounted for. One possible solution to avoiding large temperature variation across elements and over time can be to heat consistently all elements (device and calibration materials) approximately to body temperature. Figure 26 is a graph of pyroelectric voltage measurement, being an example of the three signals recorded by the oscilloscope (the graph Tear” = sig / signal and the graph “front” = ref / reference). Capturing all three signals can provide information on the quality of the measurement and potential further diagnostic information. The change in voltage (V, or the peak signal), from the "rest" state to the "excited" state of the sensor during ultrasound exposure, is the important measurement in the preferred embodiments. Measuring this voltage can be difficult, particularly in conditions of poor signal to noise of a type as depicted in Figure 27, and / or when measurements are performed in quick succession and the sensor has not had time to return to the "rest" state, as depicted in Figure 28. There are a few ways to measure the pyroelectric voltage which have advantages and disadvantages in these conditions. Two example ways include: (i) direct measurement, which is the simplest, most naive approach which requires least computation; (ii) maximum likelihood estimation, which has demonstrated the best performance for noise resistance although requires more computation and an extra measurement step. As detailed in the applicant’s earlier patent publications, the pyroelectric sensor response (V) is proportional to the received acoustic power (P), if a set of special conditions are met. The calculation of insertion loss (IL) is as follows: IL = 10 log10 = 10 log1Q where subscripts ref and t denote measurement made with the reference material and tissue paths, respectively. This is a form of correction, cancelling the effect of time-invariant differences in the sensitivities of the ultrasound transducers and variations in the sensitivity of the sensor across its face. The sensor is large enough to capture the whole ultrasound beam, and therefore, the insertion loss measured at a particular position is dependent on the acoustic properties of the material encountered by the whole beam. Next, the system and method convert from a "total loss encountered (relative to the reference) by the ultrasound beam" to a "loss per unit distance encountered (relative to reference) by the ultrasound beam". To do the system obtains a measure of the distance the loss was measured over and the conditions of the reference measurement, and therefrom calculates: IL(n + ILCAL(n = ------dt------ where dt is the ultrasound pathlength through the tissue under investigation (assuming a simple straight-line path between source and receiver), dt can be simple to determine if there is no compliant coupling mechanism, and a little more complicated (but relatively simple with some trigonometry) if there is. Parameters such as the separation of the transmitter and receiver plates (ds) and the coupling paddle angle (0) are recorded by the device and used in the calculation of dt. Figures 29 and 30 are two examples of dt considerations with and without a compliant mechanism. In Figure 29 dt = ds, because the tissue thickness is equal to the plate separation. In Figure 30, dt + ds, although the tissue thickness can be calculated with knowledge of the plate separation, coupling paddle angle, ultrasound transducer position, and other standoff distances. Any uncertainty in the measurements can be addressed in known way by rounding or thresholding, for instance with reference to the standard deviation in the signals. The derived acoustic attenuation coefficient measurements can be used in the following manner. In the case of breast density assessment, for example, the inventors have demonstrated (through acoustic simulation and in-vivo testing) that acoustic attenuation in breast tissue is related to the ratio of fat and fibro-glandular tissues, that is to breast tissue density. The graph of Figure 31 shows the results of acoustic simulations through simulated breasts of varying breast density (% fat on the x axis) showing the relation to the measured parameter (IL on the y axis). For this example, the simulated measurements where through the same 70mm pathlength of tissue so IL is directly proportional to acoustic attenuation in this case. In principle, acoustic attenuation measurements, with the apparatus and methodology described in this document, can be used as a biomarker for other useful parameters in other tissue of the human or animal body, including for example: Muscle - varying fibrous content, which can be objectively and longitudinally assessed with the disclosed apparatus and method, and can be a useful indication of acute and chronic muscle conditions, such as sports injury assessment and muscular dystrophy monitoring, respectively; Bone - varying bone density, which again can be objectively and longitudinally assessed with the disclosed apparatus and method, and can be a useful indication of acute and chronic bone conditions, such as fracture assessment and osteoporosis monitoring, respectively. The skilled person will appreciate that the method and apparatus disclosed herein can be used in the analysis of other bodily tissues and organs, all within the competence of the skilled person.

Claims

1. A tissue composition or constitution assessment apparatus comprising: first and second panel elements disposable in spaced apart and facing relationship, at least one of the first and second panel elements being adjustable relative to the other in separation;at least one ultrasound emitter disposed at or on one of the first and second panel elements;at least one ultrasound receiver disposed at or on the other of the first and second panel elements, whereby the at least one ultrasound emitter and the at least one ultrasound receiver face one another;a support structure to which the first and second panel elements are attached; anda processing unit coupled to the at least one ultrasound transmitter and receiver and configured to determine from received ultrasound signals a tissue composition or constitution measure associated with the organ tissue assessed.

2. Apparatus according to claim 1, wherein the processing unit is configured to determine an average tissue composition or constitution measurement across a determined area of the organ tissue assessed.3 Apparatus according to claim 1 or 2, wherein the support structure and panel elements permit a subject to undergo tissue density assessment of a body part of that subject while in a generally upright position, advantageously standing or seated.

4. Apparatus according to any preceding claim, wherein at least one of the first and second panel elements comprises an adjustment element with a tissue facing plate orientable relative to the associated first or second panel element and relative to a part of a patient’s body to be assessed.

5. Apparatus according to claim 4, wherein the adjustment element is configured to reduce or minimise air gaps between the transmitter and receiver elements and the part of a patient’s body to be assessed.

6. Apparatus according to claim 4 or 5, wherein the adjustment element is coupled to at a respective one of the first and second panel elements by a hinge or pivot and configured to rotate thereabout.

7. Apparatus according to any one of claims 4 to 6, wherein the adjustment element comprises a compliant mechanism disposed at a tissue remote side of the tissue facing panel and configured to allow adjustment of an angle of tilt of the tissue facing panel.

8. Apparatus according to claim 7, wherein the compliant mechanism comprises a bladder selectively fillable with a fluid.

9. Apparatus according to claim 8, comprising a fluid source, a pump coupled to the source and a pressure or volume sensor, the fluid source being coupled to the bladder for filling the bladder with fluid to a determined pressure or volume.

10. Apparatus according to any one of claims 7 to 9, wherein the compliant mechanism is made of an acoustically transmissible material, wherein the at least one ultrasonic emitter or the at least one ultrasonic receiver is disposed behind the compliant mechanism relative to the tissue facing surface such that in use ultrasonic beams pass through the material of the compliant mechanism.

11. Apparatus according to any preceding claim, wherein the at least one ultrasound receiver is shaped and sized to be at least as large as an area expected to be covered by a person’s body part to be assessed when placed between the lower and upper panel elements.

12. Apparatus according to any preceding claim, comprising an array of ultrasound emitters, the array advantageously being shaped and sized to be at least as large as an area expected to be covered by a person’s body part to be assessed when placed between the lower and upper panel elements.

13. Apparatus according to any preceding claim, wherein the at least one ultrasound emitter is adjustable and / or movable relative to the at least one receiver so as to be able to emit an adjustable or movable ultrasonic beam towards the at least one emitter.

14. Apparatus according to claim 13, wherein the at least one ultrasound emitter has an adjustable ultrasonic beam angle.

15. Apparatus according to any preceding claim, wherein the at least one ultrasonic receiver's output is proportional to the time-averaged intensity of ultrasound incident on its surface.

16. Apparatus according to any preceding claim, wherein the processing unit is configured to determine on the basis of an ultrasonic signal sensed at the at least one ultrasonic receiver and of the detected distance between the at least one ultrasonic emitter and the at least one ultrasonic receiver an attenuation of the ultrasonic energy and to determine therefrom a measure of tissue composition or constitution of the organ.

17. Apparatus according to claim 16, wherein the processing unit is configured to determine on the basis of a thickness of compliant and / or other material between the at least one ultrasonic emitter and / or the at least one ultrasonic receiver and the or a tissue contact panel of the apparatus a lost ultrasonic attenuation from the compliant and other material and to adjust the measured attenuation on the basis of the determined lost ultrasonic attenuation.

18. Apparatus according to claim 17, wherein the processing unit is configured to calculate thickness of compliant and / or other material on the basis of a measured angle of the tissue facing surface relative to the at least one ultrasonic emitter or receiver, or on the basis of volume of fluid in the or a fillable bladder.

19. Apparatus according to any preceding claim, wherein the processing unit is configured to determine attenuation across substantially the whole area of a body part and to determine an average attenuation through the body part.

20. Apparatus according to any preceding claim, wherein the processing unit is configured to determine acoustic attenuation across a plurality of select areas of an organ and to determine an ultrasound attenuation map across the areas of the organ.

21. Apparatus according to any preceding claim, wherein the processing unit is configured to operate the at least one transmitter at different frequencies, for the analysis of different tissue types and / or for different assessment specificity.

22. Apparatus according to claim 21, wherein the processing unit is configured to determine a measure of tissue composition or constitution of a selected tissue type within a body part being tested.

23. Apparatus according to any preceding claim, wherein one or both of the first and second panel elements comprises a pivot or hinge configured to tilt at least a part of the panel element.

24. Apparatus according to any preceding claim, wherein the apparatus is configured to obtain a determination of tissue composition or constitution in a plurality of planes by adjustment of the orientation and / or disposition of the transmitter and receiver elements relative to a body part being analysed.

25. Apparatus according to claim 24, comprising a gimbal mechanism coupled to the panel elements and operable to adjust the orientation of the first and second panel elements.

26. Apparatus according to any preceding claim, wherein the apparatus is configured to assess breast tissue composition or constitution.

27. A method of assessing organ tissue composition or constitution comprising: disposing first and second panel elements in spaced apart and facing relationship, at least one ultrasound emitter being disposed at or on one of the first and second panel elements and at least one ultrasound receiver disposed at or on the other of the first and second panel elements, whereby the at least one ultrasound emitter and the at least one ultrasound receiver face one another;and determining from received ultrasound signals a tissue composition or constitution measure associated with the organ tissue assessed.

28. A method according to claim 27, wherein an overall tissue composition or constitution measurement is determined across an area of the organ tissue assessed.

29. A method according to claim 27 or 28, wherein the method is practiced on an organ of a person while in a generally upright position, advantageously standing or seated.

30. A method according to any one of claim 27 to 29, wherein at least one of the first and second panel elements comprises an orientation adjustable tissue facing plate, wherein the method comprises adjusting the orientation of the plate relative to the associated first or second panel element and relative to a part of a patient’s body to be assessed.

31. A method according to claim 30, comprising adjusting the orientation of the body facing plate to reduce or minimise air gaps between the transmitter and receiver elements and the part of a patient’s body to be assessed.

32. A method according to claim 30 or 31, comprising operating a compliant mechanism disposed at a tissue remote side of the tissue facing panel to allow adjustment of an angle of tilt of the tissue facing panel.

33. A method according to claim 32, comprising selectively filling with a fluid a bladder of the compliant mechanism by operating a fluid source with a pump coupled to the source and a pressure or volume sensor, and selectively filling the bladder to a determined pressure or volume.

34. A method according to any one of claims 27 to 33, comprising receiving ultrasonic signals across an area at least as large as an area covered by a person’s body part to be assessed.

35. A method according to any one of claims 27 to 34, comprising emitting ultrasonic signals across an area at least as large as an area covered by a person’s body part to be assessed.

36. A method according to any one of claims 27 to 35, including adjusting and / or moving the at least one ultrasound emitter relative to the at least one receiver so as to emit an adjustable or movable ultrasonic beam towards the at least one emitter.

37. A method according to claim 36, including adjusting an ultrasonic beam angle of the at least one ultrasound emitter.

38. A method according to any one of claims 27 to 37, including determining on the basis of an ultrasonic signal sensed at the at least one ultrasonic receiver and of the detected distance between the at least one ultrasonic emitter and the atleast one ultrasonic receiver an attenuation of the ultrasonic energy and to determine therefrom a measure of tissue composition or constitution of the organ.

39. A method according to claim 38, including determining on the basis of a thickness of compliant and / or other material between the at least one ultrasonic emitter and / or the at least one ultrasonic receiver and the or a tissue contact panel of the apparatus a lost ultrasonic attenuation from the compliant and other material and to adjust the measured attenuation on the basis of the determined lost ultrasonic attenuation.

40. A method according to claim 39, including calculating a thickness of compliant and / or other material on the basis of a measured angle of the tissue facing surface relative to the at least one ultrasonic emitter or receiver, or on the basis of volume of fluid in the or a fillable bladder.

41. A method according to any one of claims 27 to 40, including determining an attenuation across substantially the whole area of an organ and determining an average attenuation through the tissue of the organ.

42. A method according to any one of claims 27 to 41, including determining an attenuation across a plurality of select areas of a body part and an attenuation map across the areas of the body part.

43. A method according to any one of claims 27 to 42, including operating the at least one transmitter at different frequencies, for the analysis of different tissue types.

44. A method according to claim 43, including determining a measure of tissue density of a selected tissue type within a body part being tested.

45. A method according to any one of claims 27 to 44, including obtaining a determination of tissue composition or constitution in a plurality of planes byadjustment of the orientation of the transmitter and receiver elements relative to a body part being analysed.

46. Apparatus or a method according to any preceding claim, for the assessment of at least one of:a) risk of developing cancer;b) assessment of an appropriate screening modalityc) the existence of a potential cancerous growth;d) an acute or chronic muscle condition;e) the existence or risk of muscular dystrophy;f) an acute or chronic bone condition;g) the existence or risk of osteoporosis.

47. A method of identifying a medical condition in a patient by use of apparatus or a method according to any preceding claim.

48. A method according to claim 47, in the assessment of at least one of:a) risk of developing cancer;b) assessment of an appropriate screening modalityc) the existence of a potential cancerous growth;d) an acute or chronic muscle condition;e) the existence or risk of muscular dystrophy;f) an acute or chronic bone condition;g) the existence or risk of osteoporosis.

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