Abstract:
An apparatus for performing electrical impedance detection and ultrasound scanning of body tissue, the apparatus including: an electrode array for performing electrical impedance detection by applying a first electrical signal to the body tissue, receiving an electrical response signal characteristic of the body tissue, and providing a first output signal representative of the electrical response signal; and an ultrasound transducer for performing ultrasound scanning by applying a first ultrasound signal to the body tissue, receiving an ultrasound response signal characteristic of the body tissue, and providing a second output signal representative of the ultrasound response signal, wherein the ultrasound transducer is mounted so as to be moveable during performance of the ultrasound scanning.

Description:
TECHNOLOGICAL FIELD 
       [0001]    The invention relates to an apparatus for, and a method of, performing electrical impedance detection and ultrasound scanning of body tissue. The apparatus and method may be used in applications such as medical diagnostics. 
       BACKGROUND 
       [0002]    Electrical impedance detection, as used in Electrical Impedance Mammography (EIM) and Electrical Impedance Imaging (EII), also referred to as Electrical Impedance Tomography (EIT), Electrical Impedance Scanning (EIS) and Applied Potential Tomography (APT), can provide an image of the spatial distribution of electrical impedance inside body tissue. This is attractive as a medical diagnostic tool because it is non-invasive and does not use ionizing radiation as in X-ray tomography or strong, highly uniform magnetic fields as in Magnetic Resonance Imaging (MRI). 
         [0003]    Typically a two dimensional or three dimensional array of evenly spaced electrodes is attached to the body tissue about the region of interest. Voltages are applied across pairs of input electrodes, and output electric currents are measured at output electrodes. Alternatively, input electric currents are applied between pairs of input electrodes, and output voltages are measured at output electrodes or between pairs of output electrodes. For example, a very small alternating electric current is applied between one pair of electrodes, and the voltage between all other pairs of electrodes is measured. The process is then repeated with the current applied between a different pair of the electrodes. 
         [0004]    The measured values of the voltage depend on the electrical impedance of the body tissue, and from these values an image is constructed of the electrical impedance of the body tissue. By performing a plurality of such measurements, both two dimensional and three dimensional images can be constructed. Spatial variations revealed in electrical impedance images may result from variations in impedance between healthy and non-healthy tissues, variations in impedance between different tissues and organs, or variations in apparent impedance due to anisotropic effects resulting for example from muscle alignment. 
         [0005]    Tissue or cellular changes associated with cancer cause significant localized variations in electrical impedance, and electrical impedance images can be used to detect breast carcinomas or other carcinomas. 
         [0006]    The electric current or voltage applied to the electrodes may have a broad range of different frequencies. Different morphologies that have insignificant impedance at one frequency may have a more significant variation in impedance at a different frequency. Signals with different frequencies may penetrate the object in different ways. For example, at one frequency a signal may penetrate most significantly through the inside of cells of body tissue (e.g. intro-cellularly) and at another frequency a signal may penetrate most significantly though spaces between cells of body tissue (e.g. extra-cellularly). 
         [0007]    Ultrasound scanning typically involves using a hand-held ultrasound probe that includes an array of ultrasound transducers which both transmit ultrasound energy into body tissue to be examined and receive ultrasound energy reflected from the body tissue. To generate ultrasound energy, a driver circuit of a processing unit sends precisely timed electrical signals to the transducers. Part of the ultrasound pulses is reflected in the body tissue under examination and returns to the transducers. The transducers then convert the received ultrasound energy into electrical signals which are amplified and processed to generate an image of the examined region. 
         [0008]    Electrical impedance detection can provide diagnostic information about body tissue, whereas ultrasound scanning can provide high resolution imaging of body tissue. 
       BRIEF SUMMARY 
       [0009]    According to various, but not necessarily all, embodiments of the invention there is provided an apparatus for performing electrical impedance detection and ultrasound scanning of body tissue, the apparatus comprising: 
         [0010]    an electrode array for performing electrical impedance detection by applying a first electrical signal to the body tissue, receiving an electrical response signal characteristic of the body tissue, and providing a first output signal representative of the electrical response signal; and 
         [0011]    an ultrasound transducer for performing ultrasound scanning by applying a first ultrasound signal to the body tissue, receiving an ultrasound response signal characteristic of the body tissue, and providing a second output signal representative of the ultrasound response signal, 
         [0012]    wherein the ultrasound transducer is mounted so as to be moveable during performance of the ultrasound scanning. 
         [0013]    According to various, but not necessarily all, embodiments of the invention there is provided a method of performing electrical impedance detection and ultrasound scanning of body tissue, the method comprising: 
         [0014]    performing electrical impedance detection with an electrode array by applying a first electrical signal to the body tissue, receiving an electrical response signal characteristic of the body tissue, and providing a first output signal representative of the electrical response signal; and 
         [0015]    performing ultrasound scanning with an ultrasound transducer by applying a first ultrasound signal to the body tissue, receiving ultrasound response signals characteristic of the body tissue, and providing a second output signal representative of the ultrasound response signals, 
         [0016]    wherein the ultrasound transducer is moved during the performing of the ultrasound scanning. 
         [0017]    At least some embodiments of the invention therefore provide an apparatus and method for detecting both an electrical response signal and an ultrasound signal. The apparatus and method may therefore provide combined electrical impedance detection and ultrasound scanning. By moving the ultrasound transducer with respect to the electrode array, the ultrasound scanning can provide an image of the body tissue on which the electrical impedance detection is performed. 
         [0018]    In one embodiment, the ultrasound transducer may be moveable with respect to the apparatus along a path that is fixed relative to a location of the electrode array at which the electrode array is arranged to perform the electrical impedance detection. Likewise, according to the method, during the performing of the ultrasound scanning the ultrasound transducer may be moved along a path that is fixed relative to a location of the electrode array at which the electrode array is arranged to perform the electrical impedance detection. Therefore, a location of the electrode array and a path of the ultrasound transducer have a fixed relationship. By means of such a fixed relationship, the ultrasound response signals and the electrical response signals can have a high degree of spatial correlation, which can assist detection and characterization of features of the body tissue. 
         [0019]    The path of the ultrasound transducer may be a loop. This feature enables the apparatus to be compact. 
         [0020]    In one embodiment, the electrode array may be mounted on the apparatus so as to be moveable with respect to the apparatus. By moving the electrode array the first electrical signal may be applied, and electrical response signals may be detected, over a region of the body tissue larger than the area of the electrode array. Conversely, for a region of the body tissue of a given size, fewer electrodes can be deployed, which can reduce the complexity of electrical interconnections. Movement of the electrode array also enables electrical measurements with a fine resolution, using incremental positions of the electrodes more closely spaced than the physical spacing of the electrodes. 
         [0021]    In another embodiment the electrode array and the ultrasound transducer may be mounted on a common element that is moveable with respect to the apparatus. This enables the complexity of electrical connections to the ultrasound transducer and the electrode array to be reduced, for example by using common routing for the connections. 
         [0022]    The common element may be rotatable with respect to the apparatus. This feature enables the apparatus to be compact. 
         [0023]    The electrode array may be substantially flat. This enables a simple design and manufacture. However, the electrode array need not be flat. For example the electrode array may be profiled to complement the contours of the body tissue. 
         [0024]    The apparatus may comprise a container for receiving the body tissue, wherein the electrode array is provided at an inside bottom surface of the container. This enables the body tissue to be held in the container during the electrical impedance detection and ultrasound scanning, which can help to ensure that the body tissue is in an optimum position relative to the electrode array and the ultrasound transducer, and help to ensure that the body tissue remains stationary, resulting in improved resolution. It also enables the body tissue to be placed in a fluid, in particular an electrically conductive fluid, which can improve the electrical contact between the body tissue and the electrode array, enabling more reliable characterization. 
         [0025]    The apparatus may comprise means for varying the depth of the container. This can assist placement of the body tissue in an optimum position relative to the electrode array and the ultrasound probe, and help to ensure that the body tissue is stationary during characterization, resulting in improved resolution. Furthermore, the signal propagation distance may be reduced, resulting in improved sensitivity of the apparatus in detecting signals. 
         [0026]    The ultrasound probe may be moveable around a side wall of the container. In particular, the path of the ultrasound probe may be around a side wall of the container. This enables the first ultrasound signal to be applied to a different side or surface of the body tissue than the first electrical signal, which is advantageous in providing the electrical response signal and the ultrasound response signal relating to different planes or surfaces of the body tissue, which can assist detection and characterization of features of the body tissue. The ultrasound transducer may be mounted behind the electrode array. This enables the first electrical signal and the first ultrasound signal to be applied to the same side of the body tissue. Such an arrangement is advantageous in providing the electrical response signals and the ultrasound response signals relating to a common plane or surface of the body tissue. This also contributes to a high spatial correlation of the electrical response signals and the ultrasound response signals, which can assist detection and characterization of features of the body tissue. 
         [0027]    The body tissue may be breast tissue. In this embodiment, the container may be dimensioned so as to receive a human or animal breast. 
         [0028]    The apparatus may comprise a display for displaying an image representative of the electrical impedance detection based on the first output signal and an image representative of the ultrasound scanning based on the second output signal. Likewise, the method may comprise displaying an image representative of the electrical impedance detection based on the first output signal and an image representative of the ultrasound scanning based on the second output signal. In particular, the display may be arranged to display the image representative of the electrical impedance detection and the image representative of the ultrasound scanning simultaneously. Likewise, the method may comprise displaying the image representative of the electrical impedance detection and the image representative of the ultrasound scanning simultaneously. Furthermore, the apparatus may comprise a display for displaying an image representative of a combination of the electrical impedance detection and the ultrasound scanning. Likewise, the method may comprise displaying an image representative of a combination of the electrical impedance detection and the ultrasound scanning. Therefore, images representing the electrical response signal and ultrasound response signal may be generated having a high degree of spatial correlation, which can assist detection and characterization of features of the body tissue. 
     
    
     
       BRIEF DESCRIPTION 
         [0029]    For a better understanding of various examples of embodiments of the present invention reference will now be made by way of example only to the accompanying drawings in which: 
           [0030]      FIG. 1  is a schematic diagram of an apparatus for detecting signals characteristic of a body tissue; 
           [0031]      FIG. 2  is a schematic diagram of an apparatus for detecting signals characteristic of a body tissue; 
           [0032]      FIG. 3  is a schematic three dimensional schematic view of an apparatus for detecting signals characteristic of a body tissue; and 
           [0033]      FIG. 4  is a schematic three dimensional schematic view of an apparatus for detecting signals characteristic of a body tissue. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Referring to  FIG. 1 , an apparatus  100  for detecting signals characteristic of body tissue comprises an electrode array  101  and an ultrasound probe  102 . The electrode array  101  comprises a plurality of electrodes  103  disposed on a face  104  of an electrode plate  105 . In use, the body tissue (not illustrated) is placed over the electrode plate  105 , adjacent to a face  104  of the electrode plate  105 , either in contact with, or space apart from, the face  104 . The electrodes  103  are able to apply a first electrical signal to the body tissue during electrical impedance measurements on the body tissue. The electrodes  103  are electrically coupled to a first controller  111  for transmitting the first electrical signal to the electrodes  103  for applying to the body tissue and for receiving a first output signal from electrodes  103 , which first output signal depends on electrical response signals, characteristic of the body tissue, received at the electrodes  103 . 
         [0035]    The ultrasound probe  102  comprises a plurality of ultrasound transducers  107  disposed on a face  106  of the ultrasound probe  102 . The ultrasound transducers  107  are able to apply a first ultrasound signal to the body tissue during ultrasound examination on the body tissue. The ultrasound transducers  107  are electrically coupled to a second controller  112  for providing a second input signal, generally in the form of electrical pulses, to the ultrasound transducers  107  that cause the ultrasound transducers  107  to apply the first ultrasound signal to the body tissue, and for receiving a second output signal from the ultrasound transducers  107 , which second output signal depends on ultrasound response signals, characteristic of the body tissue, received at the ultrasound transducers  107 . 
         [0036]    The face  106  of the ultrasound probe  102  on which the ultrasound transducers  107  are disposed is adjacent to the electrode plate  105  and on the opposite side of the electrode plate  105  to the face  104  of the electrode plate  105  on which the electrodes  103  are disposed. Therefore, if the electrode plate  105  is placed horizontally with the face  104  of the electrode plate  105  on which the electrodes  103  are disposed upwards, then the ultrasound probe  102  is beneath the electrode plate  105  with the face  106  of the ultrasound probe  102  on which the ultrasound transducers  107  are disposed also upwards. Therefore, the ultrasound transducers  107  are arranged in a plane substantially parallel to the electrode plate  105 . This enables the electrical signals and the ultrasound signals to be applied to the body tissue in directions that are substantially parallel to each other. 
         [0037]    The ultrasound probe  102  and the electrode plate  105  are mechanically coupled, whereby the ultrasound probe  102  is rotatable about an axis  108  substantially perpendicular to the electrode plate  105 . In particular, the axis  108  is coupled to, central to, and substantially perpendicular to the electrode plate  105 . As the ultrasound probe  102  rotates, the path of each ultrasound transducer  107  traces an arc of a circle, and eventually a circular loop if the rotation continues for 360 degrees. 
         [0038]    In one embodiment, the ultrasound probe  102  is rotatable relative to the electrode plate  105 , whereas in another embodiment the electrode plate  105  also rotates, with the ultrasound probe  102 , about the same axis  108 . The mechanical arrangement for driving the rotation of ultrasound probe  102 , and optionally the electrode plate  105 , is omitted from  FIG. 1  for clarity; a conventional drive mechanism may be used. 
         [0039]    In the case that the ultrasound probe  102  is rotatable relative to the electrode plate  105 , the ultrasound probe  102  passes across one face of the electrode plate  105  for sounding body tissue located adjacent the opposite side, face  104 , of the electrode plate  105 . As only one of the electrode plate  105  and the ultrasound transducer  107  need move, the mechanical drive arrangement may be simple. The electrode plate  105  is at least partially transparent to ultrasound signals. The greater the transparency of the electrode plate  105  to ultrasound signals, the greater the sensitivity of the apparatus  100  in detecting the ultrasound response signals. 
         [0040]    Rotation of the ultrasound probe  102  enables an area to be sounded which is larger than the area of the ultrasound probe  102 , whilst maintaining a high degree of temporal and spatial correlation of the ultrasound response signals and the electrical response signals. Therefore the ultrasound probe  102  may be compact and employ relatively few ultrasound transducers  107 , which reduces the complexity of electrical interconnections and reduces the power required to drive the ultrasound transducers  107 . Rotation also enables sounding with a fine resolution, using incremental positions of the ultrasound transducers  107  more closely spaced than the physical spacing of the ultrasound transducers  107 . 
         [0041]    Likewise, rotation of the electrode plate  105  enables the first electrical signals to be applied, and the electrical response signals to be detected, over a region of the body tissue larger than the area of the electrode plate  105  over which the electrodes  103  are deployed. Conversely, for a region of the body tissue of a given size, fewer electrodes  103  may be deployed, which can reduce the complexity of electrical connections. 
         [0042]    Rotation of the electrode plate  105  also enables electrical measurements with a fine resolution, using incremental positions of the electrodes  103  more closely spaced than the physical spacing on the electrodes  103 . 
         [0043]    Because the electrode plate  105  and the ultrasound transducers  107  are mechanically linked by the axis  108  of rotation, the ultrasound transducers  107  and the electrodes  103  have a defined spatial relationship. In the case that the electrode plate  105  and the ultrasound probe  102  rotate together, the defined spatial relationship is a fixed relationship. In the case that the ultrasound probe  102  is rotatable relative to the electrode plate  105 , the defined spatial relationship is a fixed path or trajectory. In either case, the path of the ultrasound probe  102  is fixed relative to a location of the electrode plate  105  at which the electrode plate  105  is used to perform the electrical impedance detection. Therefore, the electrical response signal and the ultrasound response signal can be ensured to have a defined relationship. 
         [0044]    In the case that the ultrasound probe  102  is rotatable relative to the electrode plate  105 , in use, the electrode plate  105  may be maintained in a constant position relative to the body tissue during rotation of the ultrasound probe  102 , thereby providing a fixed reference position, which can contribute to high resolution characterization of the body tissue. 
         [0045]    In the case that the electrode plate  105  and the ultrasound probe  102  rotate together, the complexity of electrical connections to the electrodes  104  and the ultrasound transducers  105  may be reduced, for example by using common routing for the connections. 
         [0046]    The electrodes  103  are coupled to a first port  109 , and the ultrasound transducers  107  are coupled to a second port  110 . The first port  109  is bidirectional, for conveying signals to and from the electrodes  103 . The second port  110  is also bidirectional, for conveying signals to and from the ultrasound transducers  107 . For clarity, connections between the first port  109  and the electrodes  103 , and between the second port  110  and the ultrasound transducers  107 , are not illustrated in  FIG. 1 . These connections may, for example, be located on the face of the electrode plate  105  opposite to the face  104 , or may be internal to the electrode plate  105 . 
         [0047]    There is a first controller  111  coupled to the first port  109 . The first controller  111  generates the first input signal which is delivered via the first port  109  to one or more of the electrodes  103  where, in response to the first input signal, the first electrical signal is transmitted to the body tissue. The first electrical signal passes through the body tissue and is received at other of the electrodes  103 . These received signals are termed electrical response signals in this specification and the accompanying claims. The first output signal, dependent on the electrical response signals is delivered to the first controller  111  via the first port  109 . 
         [0048]    There is a second controller  112  coupled to the second port  110 . The second controller  112  generates the second input signal which is delivered via the second port  110  to the ultrasound transducers  107 . The second input signal may be, for example an electrical signal or optical signal. The ultrasound transducers  107  convert the second input signal to the first ultrasound signal which is transmitted to the body tissue. The first ultrasound signal is reflected in the body tissue. These reflections are termed ultrasound response signals in this specification and the accompanying claims. The ultrasound response signals are detected by the ultrasound transducers  107 , which convert the ultrasound response signals to the second output signal which is delivered to the second controller  112  via the second port  110 . 
         [0049]    The first and second controllers  111 ,  112  are coupled to a data generator  113 . The data generator  113  generates electrical impedance data based on the first output signal, and ultrasound data based on the second output signal. The ultrasound data and the electrical impedance data are characteristic of the body tissue. 
         [0050]    The data generator  113  is coupled to a display  114  for displaying simultaneously an image representative of the electrical impedance data and an image representative of the ultrasound data. Because of the known spatial relationship of the images, a person interpreting the images is able to make direct comparison of portions of the images that are known to relate to the same region of the body tissue. 
         [0051]    Alternatively, the data generator  113  may combine the ultrasound data and the electrical impedance data, for example by correlation, and the display  114  may display an image representative of the combined ultrasound data and electrical impedance data. By this means, the electrical impedance data and the ultrasound data may be combined to provide an enhanced image, which can assist detection and characterization of features of the body tissue. Features of the body tissue that may not be apparent from solely the electrical impedance data or the ultrasound data may become apparent after the correlation of the electrical impedance data and the ultrasound data. The images may be two or three dimensional. 
         [0052]    Referring to  FIG. 2 , an apparatus  200  comprises an electrode plate  205  having a reduced number of electrodes  103  compared with the electrode plate  105  illustrated in  FIG. 1 . The electrodes  103  are deployed across a segment of a face  204  of the electrode plate  205 . Such an arrangement may be used in conjunction with an electrode plate  205  that rotates as described above, in which case the path of the electrodes  103  will trace an arc or full circle. All other elements of  FIG. 2  are identical to elements of  FIG. 1  and have identical reference numerals to those respective elements. 
         [0053]    Referring to  FIG. 3 , there is illustrated a three dimensional schematic view of an apparatus  300  comprising the electrode plate  105  and the ultrasound probe  102  of  FIG. 1 , in which additionally there is a container  301  for receiving the body tissue. For example, the container  301  may be dimensioned for receiving a breast. The container  301  has a side wall  302 . The electrode plate  105  forms the base of the container  301 , with the face  104  being inside the container  301 . The position of the electrode plate  105  within the container  301  may be varied, in order to vary the volume of the container  301  available for receiving the body tissue. The ultrasound probe  102  is beneath the container  301 , with its face  106  adjacent to the lower face of the electrode plate  105 . The ultrasound probe  102 , and optionally the container  301  including the electrode plate  105 , rotates about the axis  108 . The line of the axis  108  is denoted by the dashed line X. In use, the container  301  may contain fluid for enhancing the transmission of ultrasound and/or electrical signals. For clarity the first and second ports  109 ,  110 , the first and second controllers  111 ,  112 , the data generator  113  and the display  114  are omitted from  FIGS. 3 and 4 , but are identical to the corresponding elements of  FIG. 1 . 
         [0054]    Referring to  FIG. 4 , there is illustrated a three dimensional schematic view of an apparatus  400  comprising the ultrasound probe  102 , the electrode plate  105  and the container  301 , in which the electrode plate  105  forms the base of the container  301 , with the face  104  being inside the container  301 . The ultrasound probe  102  is beside the container  301 , with the face  106  of the ultrasound probe  102  adjacent the side wall  302  of the container  301 . The side wall  302  is vertical and the face  106  of the ultrasound probe  102  is also vertical. Therefore, the ultrasound transducers  107  are arranged vertically. In this embodiment, the ultrasound transducers  107  are arranged in a plane substantially perpendicular to the plane of the electrode plate  105 . In use, the ultrasound probe  102  passes externally across the side wall  302  of the container  301 , and the side wall  302  comprises a material which is at least partially transparent to ultrasound signals. 
         [0055]    There is a mechanical linkage  401  coupling the ultrasound probe  102  to the axis  108  and therefore to the electrode plate  105 . The ultrasound probe  102 , and optionally the container  301  including the electrode plate  105 , rotates about the axis  108 . The line of the axis is denoted by the dashed line X. In this way, the first ultrasound signal may be applied to the body tissue in the container  301  through the side wall  302 , and the first electrical signal may be applied to the body tissue by means of the electrodes  103  in the electrode plate  105  forming the base of the container  301 . Thus, the first ultrasound signal and the first electrical signal may be applied to the body tissue in planes that are substantially perpendicular to each other. 
         [0056]    Because the electrode plate  105  and the ultrasound probe  102  in the embodiment of  FIG. 4  are mechanically linked by the mechanical linkage  401  and the axis  108 , the ultrasound transducers  107  and the electrodes  103  have a defined spatial relationship, which is either a fixed relationship, if the ultrasound probe  102  and the electrode plate  105  rotate together, or a fixed path or trajectory if the ultrasound probe  102  rotates relative to the electrode plate  105 . Therefore, the electrical response signals and the ultrasound response signals can be ensured to have a defined relationship. 
         [0057]    In the embodiments described with reference to  FIGS. 3 and 4 , the container  301  is cylindrical. Containers of other shapes may be used. For example, a container may be used which has sides that taper outwards away from the electrode plate  105 . In this case the face  106  of the ultrasound probe  102  need not be vertical but may be substantially parallel to the tapered sides, such that the ultrasound transducers  107  are arranged in a plane at an angle greater than zero degrees and less than ninety degree to the plane of the electrode plate  105 . 
         [0058]    As another example, a container may be used which has sides that are curved. In this way the shape of the container may be contoured in a similar shape to the body tissue. The face  106  of the ultrasound probe  102 , and the arrangement of ultrasound transducers  107  may be profiled to complement the shape of the sides of the container. 
         [0059]    Such shaping of the container  301  and arrangements of the ultrasound transducers  107  enables the first electrical signal and the first ultrasound signal to be applied to different sides or surfaces of the body tissue, and can be advantageous in providing the first and second output signals relating to different projections of a common region of the body tissue, enabling the characterization of the body tissue to be determined with increased resolution, and is particularly advantageous for three dimensional characterization of the body tissue. 
         [0060]    Similarly, although the electrode plate  105  illustrated in  FIGS. 1 to 4  is flat, this is not an essential feature, and the electrode plate  105 , or at least the face  104 , may be non-flat. For example, the face  104  may be profiled in a similar shape to the body tissue. This enables distortion of the shape of the body tissue to be reduced or avoided. The face  106  of the ultrasound probe  102 , and the arrangement of ultrasound transducers  107  may be profiled to complement the shape of the adjacent electrode plate  105 . 
         [0061]    By employing shapes which are complementary to the shape of the body tissue, the length of the signal path between the electrode plate  105  and the body tissue, and between the ultrasound transducers  107  and the body tissue, may be reduced, resulting in improved sensitivity of the apparatus in detecting the response signals 
         [0062]    In the embodiments illustrated in  FIGS. 1 to 4  the electrode plate  105  is circular. This is not an essential feature, and other shapes may be used. 
         [0063]    Furthermore, the axis  108  need not be located at the center of the electrode plate  105 . Also, the axis  108  may be located asymmetrically with respect to the ultrasound probe  102 , and in particular with respect to the arrangement of ultrasound transducers  107 . The greater the asymmetry, the greater the radius of the arc which the ultrasound transducers  107  may trace. 
         [0064]    The signals delivered via the first port  109  and the second port  110  may be electric currents or voltages, or may be optical signals. Also, they may be analogue or digital signals. Where optical signals are used, conversion between optical and electrical signals may be performed by the ultrasound transducers  107 , by the electrodes  103  and by the first and second controllers  111 ,  112 . Digital to analogue conversion, and analogue to digital conversion, may be performed by the ultrasound transducers  107 , by the electrodes  103  and by the first and second controllers  111 ,  112 . The ultrasound transducers  107 , the electrodes  103  and the first and second controllers  111 ,  112  may include signal processing, for example amplification and filtering. The first controller  111  may be integral with the electrode plate  105  and the second controller  112  may be integral with the ultrasound probe  102 , in which case either or both of the first and second ports  109 ,  110  may be internal to the electrode plate  105  or ultrasound probe  102  respectively. Alternatively the first controller  111  may be spaced apart from the electrode plate  105  by means of cables, and/or the second controller  112  may be spaced apart from the ultrasound probe  102  by means of cables. 
         [0065]    The first controller  111  and the second controller  112  may be coupled, and indeed may be a common controller. This enables the generation of the first and second signals to be synchronized. For example, the relative timing and/or the magnitude of the first and second signals may be controlled. 
         [0066]    The features of the embodiments of  FIGS. 3 and 4  may be combined, by providing an ultrasound probe  102  that has a part beneath the electrode plate  105  as in  FIG. 3  and a part beside the container  301  as in  FIG. 4 . This arrangement enables more detailed evaluation of body tissue characteristics. 
         [0067]    From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are known in the art of electrical impedance imaging and ultrasound techniques for medical diagnostics, and which may be used instead of, or in addition to, features described herein. 
         [0068]    Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of embodiments of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
         [0069]    Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. 
         [0070]    For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality. 
         [0071]    Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. 
         [0072]    Features described in the preceding description may be used in combinations other than the combinations explicitly described. 
         [0073]    Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. 
         [0074]    Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. 
         [0075]    Whilst endeavoring in the foregoing specification to draw attention to those features of the invention 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. 
         [0076]    I/we claim