Abstract:
An apparatus for electrical impedance imaging has electrodes arranged on an electrode carrier in an arrangement including a unit of repetition. The unit of repetition repeats over the electrode carrier and has an angle of rotational symmetry less than 90°. Specifically, the unit of repetition is an equilateral triangle or a hexagon.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates to an apparatus for electrical impedance imaging and a method of electrical impedance detection. Such an apparatus and method has application in medical diagnostics. 
     BACKGROUND TO THE DISCLOSURE 
     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). 
     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 electric current applied between a different pair of the electrodes. 
     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, for example resulting from muscle alignment. 
     Tissue or cellular changes associated with cancer cause significant localised variations in electrical impedance, and electrical impedance images can be used to detect breast carcinomas or other carcinomas. 
     The electric current or voltage applied to the electrodes may have a broad range of different frequencies. Different morphologies that have an insignificant impedance at one frequency may have a more significant variation in impedance at a different frequency. Also, different frequencies may penetrate the object to different depths, and reflections which can cause noise in images may vary with the frequency of the applied electric currents or voltages. 
     SUMMARY OF THE DISCLOSURE 
     According to a first aspect of the disclosure, there is provided an apparatus for electrical impedance imaging, the apparatus comprising electrodes arranged on an electrode carrier in an arrangement comprising a unit of repetition that repeats over the electrode carrier and that has an angle of rotational symmetry less than 90°. 
     According to a second aspect of the disclosure, there is provided a method of electrical impedance detection, comprising employing an electrode carrier having electrodes arranged on the electrode carrier in an arrangement comprising a unit of repetition that repeats over the electrode carrier and that has an angle of rotational symmetry less than 90°. 
     Electrodes deployed in such a manner enable measurement of electrical impedance to be made using a pattern of electrodes rotated through successive positions by a rotational displacement which is less than 90°, without rotating the electrode carrier and the electrodes physically. Therefore, the apparatus enables measurements of electrical impedance to be made with a finer resolution than the use of electrodes arranged in a rectangular grid, which enables rotation of a pattern of electrodes by multiples of 90° without rotating an electrode carrier. In other words, the apparatus enables measurements of electrical impedance to be made with rotational resolution less than 90° by selection of electrodes without rotating the electrode carrier and electrodes physically, in contrast to the use of a rectangular grid of electrodes which would require rotation of an electrode carrier to achieve the same rotational resolution. 
     In addition, electrodes deployed in such a manner enable measurement of electrical impedance to be made using a pattern of electrodes in different locations on the electrode carrier, where the pattern can be the same in each location. This facilitates comparison of measurements made in different locations of the electrode carrier. 
     Optionally, the unit of repetition can overlap from one repetition to another. This feature enables electrical impedance imaging with a fine resolution. 
     Optionally, the arrangement can comprise one or more different units of repetition that each repeat over the electrode carrier and have angles of rotational symmetry less than 90°. This feature enables electrical impedance imaging to be performed using initially a first unit of repetition providing, for example, a relatively coarse resolution for quickly identifying regions of interest in an object, followed by the use of a second unit of repetition providing a finer resolution for more detailed examination of an identified region. 
     Optionally, the angles of rotational symmetry can be equal. That is, the angle of rotational symmetry and another angle of rotational symmetry can be equal. This feature can reduce the number of electrodes required. 
     Optionally, the different unit(s) of repetition can have a different number of the electrodes. This feature can enable the speed of electrical impedance imaging to be increased. For example, initially a first unit of repetition using relatively few electrodes may be used for quickly identifying regions of interest in an object, followed by the use of a second unit of repetition with more electrodes providing a finer resolution for more detailed examination of an identified region. 
     Optionally, the different unit(s)s of repetition can extend across areas of different sizes. This feature can enable the speed of electrical impedance imaging to be increased. For example, initially a first unit of repetition extending across a relatively large area may be used for quickly identifying regions of interest in an object, followed by the use of a second unit of repetition extending over a smaller area for providing a finer resolution for more detailed examination of an identified region. 
     Optionally, the electrodes can be arranged at one or more corners of each triangle of a tessellation of triangles. In particular, the triangles can be equilateral triangles. 
     Furthermore, the triangles can be of equal size. Such an arrangement enables a rotational displacement which is a multiple of 60°. 
     Optionally, the apparatus can comprise: 
     means for applying a first input electrical signal at a first input electrode pair of the electrodes; 
     means for detecting first output electrical signals at a first output electrode group of the electrodes; 
     means for applying a second input electrical signal at a second input electrode pair of the electrodes; and 
     means for detecting second output electrical signals at a second output electrode group of the electrodes; 
     wherein the second input electrode pair is aligned relative to the first input electrode pair at an angle of rotational displacement equal to a multiple of (one of) the angle(s) of rotational symmetry. 
     Correspondingly, the method of electrical impedance detection can comprise: applying a first input electrical signal at a first input electrode pair of the electrodes; 
     detecting first output electrical signals at a first output electrode group of the electrodes; 
     applying a second input electrical signal at a second input electrode pair of the electrodes; and 
     detecting second output electrical signals at a second output electrode group of the electrodes; 
     wherein the second input electrode pair is aligned relative to the first input electrode pair at an angle of rotational displacement equal to a multiple of (one of) the angles of rotational symmetry. 
     In particular, the angle of rotational displacement can be equal to (one of) the angles of rotational symmetry. 
     An angle of rotational displacement which is a multiple of one of the angles of rotational symmetry for applying the input electrical signals enables electrical impedance measurements to be made having improved resolution. Where the electrodes are arranged at one or more corners of each triangle of a tessellation of equal size equilateral triangles, the rotational resolution can be 60°. 
     Optionally, the apparatus can comprise means for selecting the angle of rotational displacement from at least two of the set 60°, 120° and 180°. Correspondingly, the method can comprise selecting the angle of rotational displacement from at least two of the set 60°, 120° and 180°. 
     By selecting the angle of rotational displacement for applying input electrical signals, flexibility in making electrical impedance measurements is provided. For example, a single angle of rotational displacement may be selected, or the angle of rotational displacement may be varied, such as by using a coarse rotational resolution initially for quickly identifying regions of interest in an object, and then by using a finer rotational resolution for a more detailed examination of an identified region. Using a rotational resolution of 60° for applying input electrical signals, a total of six rotational positions are available, namely 0°, 60°, 120°, 180°, 240° and 300°. However, the rotational positions of 240° and 300° may be considered to be a rotational position of 120° and 60° respectively. 
     Optionally, the apparatus can comprise means for selecting a spacing of the electrodes of the first input electrode pair and a spacing of the electrodes of the second input electrode pair. Correspondingly, the method can optionally comprise selecting the spacing of the electrodes of the first input electrode pair and the spacing of the electrodes of the second input electrode pair. This feature provides flexibility in making electrical impedance measurements. For example, a relatively large spacing enables electrical impedance measurements to be made at a larger depth in an object, whereas a relatively small spacing enables electrical impedance measurements to be made having a high resolution. A larger spacing may be used initially for quickly identifying regions of interest in an object, and then a smaller spacing may be used for more detailed examination of an identified region. 
     Optionally, the electrodes of the first input electrode pair and of the first output electrode group can constitute a first electrode set having a first pattern, and the electrodes of the second input electrode pair and of the second output electrode group can constitute a second electrode set having a second pattern, and the second pattern can be identical to the first pattern rotated by the angle of rotational displacement. Use of such an identical pattern facilitates comparison of electrical impedance for different rotational displacements and detection of electrical impedance with a high resolution, thereby facilitating detection of characteristics of an object under evaluation. 
     Optionally, the apparatus can comprise means for selecting a number of electrodes in the first and second electrode sets. Correspondingly, the method can optionally comprise selecting a number of electrodes in the first and second electrode sets. This feature provides flexibility in making electrical impedance measurements. For example, a high number of electrodes may be selected for a large object under evaluation and a low number of electrodes may be selected for a small objection under evaluation. This can facilitate analysis of electrical impedance data by eliminating or reducing redundant data. As another example, a high number of electrodes may be used initially for quickly identifying regions of interest in an object, and then a small number of electrodes may be used for more detailed examination of an identified region. 
     The first pattern, and the identical second pattern, may be fixed or variable. Therefore, as a further feature, the apparatus can optionally comprise means for selecting the first pattern. Correspondingly, the method can optionally comprise selecting the first pattern. This feature provides flexibility in making electrical impedance measurements. For example, the first pattern may be selected to correspond to the shape of the object under evaluation. This can facilitate analysis of electrical impedance data by eliminating or reducing redundant data. As another example, a large first pattern may be used initially for quickly identifying regions of interest in an object, and then a first pattern matched to the shape of the region of interest may be used for more detailed examination of an identified region. 
     The first pattern may be devised by an operator or may be fixed. Therefore, as a further feature, the apparatus can optionally comprise means for selecting the first pattern from a plurality of fixed patterns. Correspondingly, the method can optionally comprise selecting the first pattern from a plurality of fixed patterns. This feature facilitates making electrical impedance measurements by removing or reducing the need for a first pattern to be devised for each object under evaluation. Suitable candidate first patterns can be stored in readiness for selection. The candidate first patterns may be, for example, suitable for typical objects to be evaluated. 
     Optionally, the boundary of the first pattern can be a hexagon, the hexagon having sides of length corresponding to a multiple of the length of a side of the equilateral triangles. A hexagon is a compact pattern that enables the same region of an object to be evaluated using the first electrode set and the second electrode set with the same electrodes. 
     Optionally, the first input electrode pair can comprise electrodes at opposite corners of the hexagon and the first output electrode group can comprise all other electrodes of the first electrode set. This feature can enable all electrodes of the first output electrode group to be used for detecting the first output electrical signals. 
     Optionally, the electrodes of the first electrode set can be a subset of the electrodes of the electrode carrier which form the hexagon. This feature can facilitate collection and analysis of electrical impedance data by eliminating or reducing redundant data. 
     Optionally, the means for detecting the first output electrical signals can be operable to detect the first output electrical signals at a plurality of combinations of electrodes of the first output electrode group, and the means for detecting the second output electrical signals is operable to detect the second output electrical signals at a plurality of combinations of electrodes of the second output electrode group. This feature enables electrical impedance to be detected with high resolution. 
     Optionally, the apparatus can comprise means for selecting a spacing of the electrodes of each of the combinations of electrodes. Correspondingly, the method can comprise selecting a spacing of the electrodes of each of the combinations of electrodes. This feature provides flexibility in making electrical impedance measurements. For example, a relatively large spacing enables electrical impedance measurements to be made at a larger depth in an object, whereas a relatively small spacing enables electrical impedance measurements to be made having a high resolution. A larger spacing may be used initially for quickly identifying regions of interest in an object, and then a smaller spacing may be used for more detailed examination of an identified region. 
     Optionally, the plurality of combinations of electrodes of the first output electrode group can be parallel to a line joining the electrodes of the first input electrode pair, and the plurality of combinations of electrodes of the second output electrode group can be parallel to a line joining the electrodes of the second input electrode pair. The use of such combinations can enable the first and second output electrical signals to be detected along lines of greatest voltage gradient, thereby contributing to a high accuracy of electrical impedance detection. 
     Optionally, the parallel combinations of electrodes can comprise parallel combinations of two adjacent electrodes. This feature can enable the number of parallel combinations to be maximised, thereby contributing to a high accuracy of electrical impedance detection. 
     Optionally, the apparatus can comprise means for applying a third input electrical signal at a third input electrode pair of the electrodes and means for detecting third output electrical signals at a third output electrode group of the electrodes, wherein the third input electrode pair is displaced from the first input electrode pair and the third output electrode group is displaced from the first output electrode group by a common distance and a common direction. Correspondingly, the method can comprise applying a third input electrical signal at a third input electrode pair of the electrodes and detecting third output electrical signals at a third output electrode group of the electrodes, wherein the third input electrode pair is displaced from the first input electrode pair and the third output electrode group of electrodes is displaced from the first output electrode group by a common distance and a common direction. This feature provides flexibility in making electrical impedance measurements. For example, displacement of electrodes by a common distance and a common direction may be used initially for quickly identifying regions of interest in an object, and then rotational displacement of electrodes may be used for more detailed examination of an identified region. 
     Optionally, the electrode carrier can be at least part of a base of a receptacle for electrical impedance imaging of an object located within the receptacle, and the receptacle can comprise further electrodes arranged on a wall of the receptacle for electrical impedance imaging of an object located within the receptacle. This feature facilitates two-dimensional electrical impedance imaging with a high sensitivity at greater distances from the electrode carrier by enabling current density to be increased at distances from the electrode carrier. 
     Optionally, the electrode carrier can be rotatable relative to the wall. Correspondingly, the method can comprise rotating the electrode carrier relative to the wall. This feature enables increased rotational resolution. 
     Optionally, the apparatus can comprise a spacing member for spacing the object from the electrode carrier. Correspondingly, the method can comprise spacing the object from the electrode carrier. By means of this feature, during rotation of the electrode carrier, the transfer of a rotational force from the rotating carrier to the object under evaluation can be reduced or eliminated, thereby decreasing patient discomfort. 
     Optionally, the spacing member can be electrically conductive. Optionally, the spacing member can have a conductivity less than the conductivity of the object. These features can reduce electrical influence of the spacing member on electrical impedance detection. 
     Optionally, the apparatus can comprise an electrically conductive liquid in the receptacle, wherein the spacing member has a conductivity at least 90% of the conductivity of the liquid. Correspondingly, the method can comprise providing an electrically conductive liquid in the receptacle, wherein the spacing member has a conductivity at least 90% of the conductivity of the liquid. This feature can provide improved electrical coupling between the electrode carrier and the object under evaluation. In one embodiment, the conductivity of the spacing member can be substantially equal to the conductivity of the liquid. 
     Optionally, the liquid can have a conductivity in the range 10 to 12 mS/cm. Such a range of conductivity is suited to typical objects to be evaluated by electrical impedance detection, such as human breast tissue. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is schematic diagram of an apparatus for electrical impedance imaging; 
         FIG. 2  is a flow chart of a method of electrical impedance detection; 
         FIG. 3  is a schematic diagram of electrodes mounted on an electrode carrier illustrating electrodes in use; 
         FIG. 4  is a schematic diagram of electrodes mounted on an electrode carrier illustrating other electrodes in use; 
         FIG. 5  is a schematic diagram of electrodes mounted on an electrode carrier illustrating further electrodes in use; 
         FIG. 6  is a schematic diagram of electrodes mounted on an electrode carrier illustrating yet further electrodes in use; 
         FIG. 7  is a schematic diagram of an apparatus for electrical impedance imaging comprising a receptacle having electrodes; and 
         FIG. 8  is a schematic diagram of an apparatus for electrical impedance imaging comprising a spacing member. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , there is a circular carrier  100  which is electrically non-conductive and which may be made, for example, of a plastic material. Electrodes  1 - 85  are deployed across a flat surface  105  of the electrode carrier  100  and are preferably recessed in the electrode carrier  100  so that they do not make physical contact with an object placed on the electrode carrier  100 . There are eighty five electrodes, each denoted in  FIG. 1  by a dot, and for ease of reference indicated by reference numerals  1  to  85  respectively. The electrodes  1 - 85  are arranged equidistant in a triangular matrix, such that the electrodes  1 - 85  are located at corners of equilateral triangles arranged in a continuum. In such an arrangement, each electrode  1 - 85 , except those adjacent the boundary of the arrangement, has six nearest neighbour electrodes  1 - 85  which are arranged in a hexagon. For example, the nearest neighbours to electrode  7  are electrodes  1 ,  2 ,  6 ,  8 ,  14 ,  15 . In  FIG. 1 , the electrodes  1 - 85  are illustrated positioned on a triangular grid  110 . This is purely for the purpose of illustrating the arrangement of the electrodes  1 - 85  and the grid is not necessarily present in the physical implementation of the electrode carrier  100 . A more dense triangular matrix could alternatively be provided by subdividing each equilateral triangle into four smaller equilateral triangles by means of additional lines parallel to the grid lines depicted in  FIG. 1 . 
     The electrodes  1 - 85  are numbered in the following manner in eleven horizontal rows of the triangular matrix. The numbers increase from left to right of each row and increase from the bottom row to the top row. The electrodes of each row are positioned symmetrically with respect to the electrodes of the adjacent row or rows. The first to eleventh rows contain, respectively, the electrodes having the following numbers:  1 - 4 ;  5 - 11 ;  12 - 19 ;  20 - 28 ;  29 - 38 ;  39 - 47 ;  48 - 57 ;  58 - 66 ;  67 - 74 ;  75 - 81 ;  82 - 85 . 
     Each electrode  1 - 85  is coupled to a switching device  200 , although in  FIG. 1  for clarity the individual couplings of each electrode  1 - 85  are not illustrated. The switching device  200  is coupled to a transceiver  300 . The transceiver  300  generates an input signal which is routed by the switching device  200  to a selected input electrode pair of the electrodes  1 - 85 . The transceiver  300  also receives output signals routed by the switch device  200  from a selected output electrode group of the electrodes  1 - 85 . The input signal may be an alternating voltage applied across the selected input electrode pair, in which case the output signals may be electric currents measured at the selected output electrode group. Alternatively, the input signal may be an alternating electric current applied at the selected input electrode pair, in which case the output signals may be voltages measured at the selected output electrode group. The transceiver  300  may generate the input signal at different frequencies in the range, for example 100 Hz to 10 MHz. 
     The switching device  200  is coupled to a controller  400  for controlling the selection of the input electrode pair at which the input signal is applied and the selection of the output electrode group from which the output signals are to be received. The transceiver  300  is coupled to the controller  400  which co-ordinates the generation of the input signal by the transceiver  300  and the selection of the input electrode pair and the output electrode group by the switching device  200 . 
     The switching device  200 , the transceiver  300  and the controller  400  may be integral with the electrode carrier  100 . This enables short couplings between the electrodes  1 - 85  and the switching device  200 , between the switching device  200  and the transceiver  300 , and between the transceiver  300  and the controller  400 , which facilitates the use of a high frequency for the input signals. 
     The controller  400  can have a user interface for controlling the selection of the input electrode pair and the output electrode group by the switching device  200 . Alternatively, the controller  400  can select a fixed input electrode pair for applying the input signal and select a fixed output electrode group for receiving the output signals. In addition, the controller  400  receives from the transceiver  300  indications of the output signals received by the transceiver  300 . From the indications of the output signals, and from data characterising the input signal, the controller  400  evaluates the electrical impedance of an object under evaluation. The controller  400  is coupled to a display device  500  for displaying an image representative of the electrical impedance. In addition, the controller  400  may perform diagnosis based on the electrical impedance. 
     Referring to  FIG. 2  there is illustrated a method of electrical impedance detection. At step  600 , an object to be evaluated, for example body tissue such as a human breast, is placed adjacent the electrode carrier  100 , either in contact with the electrode carrier  100 , or spaced apart from the electrode carrier  100  by an electrically conductive liquid  770  or spacing member  760 , described below with reference to  FIG. 8 . 
     At step  610 , a first input electrode pair is selected by the controller  400  for the application of a first input signal.  FIG. 3  shows an example of the first input electrode pair comprising electrodes  41  and  78 , highlighted by circles. For convenience, the first input electrode pair will be referred to as P 1 , where P 1 ={41, 78}. The electrodes of the first input electrode pair P 1  are spaced apart by a distance equal to four times the length of a side of the equilateral triangles. 
     At step  620 , a first output electrode group is selected by the controller  400  for receiving first output signals.  FIG. 3  shows an example of a first output electrode group comprising electrodes  42 ,  43 ,  51 - 53 ,  59 - 63 ,  68 - 71 ,  76  and  77 . For convenience the first output group of electrodes will be referred to as G 1 , where G 1 ={ 42 ,  43 ,  51 - 53 ,  59 - 63 ,  68 - 71 ,  76 ,  77 }. The electrodes of the first output electrode group G 1 , together with the electrodes of the first input electrode pair P 1 , will be referred to as a first electrode set S 1 . The nineteen electrodes of the first electrode set S 1  are arranged in a first pattern, and the boundary of the first pattern is a first hexagon comprising electrodes  41 - 43 ,  50 ,  53 ,  59 ,  63 ,  68 ,  71 ,  76 - 78 . Each side of the first hexagon has a length equal to twice the length of a side of the equilateral triangles. The electrodes of the first input electrode pair P 1  are located at opposite corners of the first hexagon. 
     At step  630  the first input signal is applied by the transceiver  300  to the electrodes  41  and  78  of the first input electrode pair P 1 . 
     At step  640  the first output signals are received by the transceiver  300  from the electrodes  42 ,  43 ,  51 - 53 ,  59 - 63 ,  68 - 71 ,  76  and  77  of the first output electrode group G 1 . In particular, within this first output electrode group G 1 , the first output signals are measured between the following combinations of electrodes: 
       42  and  52 ;  43  and  53 ; 
       50  and  60 ;  51  and  61 ;  52  and  62 ;  53  and  63 ; 
       59  and  68 ;  60  and  69 ;  61  and  70 ;  62  and  71 ; 
       68  and  76 ;  69  and  77 . 
     In  FIG. 3 , these combinations of electrodes are highlighted by being joined by bold lines. 
     Each of these combinations comprises two adjacent electrodes. The combination  51  and  61  and the combination  61  and  70  lie on a first line coincident with a line joining the electrodes  41  and  78  of the first input electrode pair P 1 . The combination  42  and  52 , the combination  52  and  62 , and the combination  62  and  71  lie on a second line parallel to the first line, and joining corners of the first hexagon adjacent to those corners of the first hexagon occupied by the electrodes  41  an  78  of the first input electrode pair P 1 . The combination  50  and  60 , the combination  60  and  69 , and the combination  69  and  77  lie on a third line parallel to the first line and on the opposite side of the first line to the second line, and joining further corners of the first hexagon adjacent to those corners of the first hexagon occupied by the electrodes  41  and  78  of the first input electrode pair P 1 . The combination  43  and  53  and the combination  53  and  63  lie on a fourth line parallel to the first line and on the same side of the first line as the second line, and joining corners of the first hexagon furthest from the electrodes  41  and  78  of the first input electrode pair P 1 . The combination  59  and  68  and the combination  68  and  76  lie on a fifth line parallel to the first line and on the same side of the first line as the third line, and joining further corners of the first hexagon furthest from the electrodes  41  and  78  of the first input electrode pair P 1 . 
     At step  650  the controller  400  receives from the transceiver  300  indications of the first output signals received by the transceiver  300 . The controller  400  stores these indications. 
     At step  660  the controller  400  determines whether the steps  610  to  650  are required to be repeated for another input electrode pair and another output electrode group. If the steps  610  to  650  are required to be repeated, flow returns to step  610 . 
     For example, if the steps  610  to  650  are required to be repeated for a second input electrode pair and a second output electrode group, at step  610  on the second pass, a second input electrode pair is selected by the controller  400  for the application of a second input signal.  FIG. 4  shows an example of a second input electrode pair P 2  comprising electrodes  43  and  76 , highlighted by circles, i.e. P 2 ={43, 76}. The second input electrode pair P 2  comprising electrodes  43  and  76  is aligned relative to the first input electrode pair P 1  comprising electrodes  41  and  78  at an anticlockwise rotational displacement of 60°, or equivalently a clockwise rotational displacement of 120°. Also, the spacing of the electrodes { 41 ,  78 } of the first input electrode pair P 1  is the same as the spacing of the electrodes { 43 ,  76 } of the second input electrode pair P 2 , namely a distance equal to four times the length of a side of the equilateral triangles. Thus the electrodes of the second input electrode pair P 2  occupy corners of the first hexagon adjacent to the corners occupied by the electrodes of the first input electrode pair. 
     At step  620  on the second pass, a second output electrode group is selected by the controller  400  for receiving second output signals.  FIG. 4  shows an example of a second output electrode group G 2  comprising electrodes  41 ,  42 ,  50 - 53 ,  59 - 63 ,  68 - 71 ,  77  and  78 , i.e. G 2 ={ 41 ,  42 ,  50 - 53 ,  59 - 63 ,  68 - 71 ,  77 ,  78 }. The electrodes of the second output electrode group G 2 , together with the electrodes of the second input electrode pair P 2 , will be referred to as a second electrode set S 2 . The nineteen electrodes of the second electrode set S 2  are arranged in a second pattern, and the boundary of the second pattern comprises electrodes  41 - 43 ,  50 ,  53 ,  59 ,  63 ,  68 ,  71 ,  76 - 78 , which is coincident with the first hexagon. 
     At step  630  on the second pass, a second input signal is applied by the transceiver  300  to the electrodes { 43 ,  76 } of the second input electrode pair P 2 . 
     At step  640  on the second pass, second output signals are received by the transceiver  300  from the electrodes of the second output electrode group G 2 . In particular, within the second output electrode group G 2 , the second output signals are measured between the following combinations of electrodes: 
       41  and  50 ;  42  and  51 ; 
       50  and  59 ;  51  and  60 ;  52  and  61 ;  53  and  62 ; 
       60  and  68 ;  61  and  69 ;  62  and  70 ;  63  and  71 ; 
       70  and  77 ;  71  and  78 . 
     In  FIG. 4 , these combinations of electrodes are highlighted by being joined by bold lines. 
     Each of these combinations comprises two adjacent electrodes. The combination  52  and  61  and the combination  61  and  69  lie on a sixth line coincident with a line joining the electrodes  43  and  76  of the second input electrode pair P 2 . The combination  42  and  51 , the combination  51  and  60 , and the combination  60  and  68  lie on a seventh line parallel to the sixth line, and joining corners of the first hexagon adjacent to those corners of the first hexagon occupied by the electrodes  43  and  78  of the second input electrode pair P 2 . The combination  53  and  62 , the combination  62  and  70 , and the combination  70  and  77  lie on an eighth line parallel to the sixth line and on the opposite side of the sixth line to the seventh line, and joining further corners of the first hexagon adjacent to those corners of the first hexagon occupied by the electrodes  43  an  76  of the second input electrode pair P 2 . The combination  41  and  50  and the combination  50  and  59  lie on a ninth line parallel to the sixth line and on the same side of the sixth line as the seventh line, and joining corners of the first hexagon furthest from the electrodes  43  and  76  of the second input electrode pair P 2 . The combination  63  and  71  and the combination  71  and  78  lie on a tenth line parallel to the sixth line and on the same side of the sixth line as the eighth line, and joining further corners of the first hexagon furthest from the electrodes  43  and  76  of the second input electrode pair P 2 . 
     It can be readily appreciated from  FIGS. 3 and 4  that the second input electrode pair P 2  and the combinations of electrodes from the second output electrode group G 2  form an identical pattern to the first input electrode pair P 1  and the combinations of electrodes from the first output electrode group G 1 , except for an anticlockwise rotation by 60°, or equivalently a clockwise rotation by 120°. 
     At step  650  on the second pass, the controller  400  receives from the transceiver  300  indications of the second output signals received by the transceiver  300 . The controller  400  stores these indications. 
     At step  660  on the second pass, the controller  400  determines whether the steps  610  to  650  are required to be repeated for another input electrode pair and another output group of electrodes. If the steps  610  to  650  are required to be repeated, flow returns to step  610 . 
     For example, if the steps  610  to  650  are required to be repeated for a third input electrode pair and a third output group of electrodes, at step  610  on the third pass, a third input electrode pair is selected by the controller  400  for the application of a third input signal.  FIG. 5  shows an example of a third input electrode pair P 3  comprising electrodes  59  and  63 , highlighted by circles, i.e. P 3 ={ 59 ,  63 }. The third input electrode pair P 3  comprising electrodes  59  and  63  is aligned relative to the first input electrode pair P 1  and relative to the second input electrode pair P 2  at a rotational displacement of 60°, or equivalently 120°. Also, the spacing of the electrodes { 59 ,  63 } of the third input electrode pair P 3  is the same as the spacing of the electrodes { 43 ,  76 } of the first input electrode pair P 1  and of the second input electrode pair P 2 . 
     At step  620  on the third pass, a third output group of electrodes is selected by the controller  400  for receiving third output signals.  FIG. 5  shows an example of a third output group of electrodes G 3  comprising electrodes  41 - 43 ,  50 - 53 ,  60 - 62 ,  68 - 71 , and  76 - 78 , i.e. G 2 ={ 41 - 43 ,  50 - 53 ,  60 - 62 ,  68 - 71 ,  76 - 78 }. The electrodes of the third output group of electrodes G 3 , together with the electrodes of the third input electrode pair P 2 , will be referred to as the third electrode set S 3 . The nineteen electrodes of the third electrode set S 3  are arranged in a third pattern, and the boundary of the third pattern comprises electrodes  41 - 43 ,  50 ,  53 ,  59 ,  63 ,  68 ,  71 ,  76 - 78 , which is coincident with the first hexagon. 
     At step  630  on the third pass, a third input signal is applied by the transceiver  300  to the electrodes { 59 ,  63 } of the third input electrode pair P 3 . 
     At step  640  on the third pass, third output signals are received by the transceiver  300  from the electrodes of the third output group of electrodes G 3 . In particular, within the third output group of electrodes G 3 , the third output signals are measured between the following combinations of electrodes: 
       41  and  42 ;  42  and  43 ; 
       50  and  51 ;  51  and  52 ;  52  and  53 ; 
       60  and  61 ;  61  and  62 ; 
       68  and  69 ;  69  and  70 ;  70  and  71 ; 
       76  and  77 ;  77  and  78 . 
     In  FIG. 5 , these combinations of electrodes are highlighted by being joined by bold lines. 
     Each of these combinations comprises two adjacent electrodes. The combination  60  and  61  and the combination  61  and  62  lie on an eleventh line coincident with a line joining the electrodes  59  and  63  of the third input electrode pair P 3 . The combination  68  and  69 , the combination  69  and  70 , and the combination  70  and  71  lie on a twelfth line parallel to the eleventh line, and joining corners of the first hexagon adjacent to those corners of the first hexagon occupied by the electrodes  59  and  63  of the third input electrode pair P 3 . The combination  50  and  51 , the combination  51  and  52 , and the combination  52  and  53  lie on a thirteenth line parallel to the eleventh line and on the opposite side of the eleventh line to the twelfth line, and joining further corners of the first hexagon adjacent to those corners of the first hexagon occupied by the electrodes  59  an  63  of the third input electrode pair P 3 . The combination  76  and  77  and the combination  77  and  78  lie on a fourteenth line parallel to the eleventh line and on the same side of the eleventh line as the twelfth line, and joining corners of the first hexagon furthest from the electrodes  59  and  78  of the third input electrode pair P 3 . The combination  41  and  42  and the combination  42  and  43  lie on a fifteenth line parallel to the eleventh line and on the same side of the eleventh line as the thirteenth line, and joining further corners of the first hexagon furthest from the electrodes  59  and  63  of the third input electrode pair P 3 . 
     It can be readily appreciated from  FIGS. 3 and 5  that the third input electrode pair P 3  and the combinations of electrodes from the third output electrode group G 3  form an identical pattern to the first input electrode pair P 1  and the combinations of electrodes from the first output electrode group G 1 , except for a clockwise rotation by 60°, or equivalently an anticlockwise rotation by 120°. 
     At step  650  on the third pass, the controller  400  receives from the transceiver  300  indications of the third output signals received by the transceiver  300 . The controller  400  stores these indications. 
     At step  660  on the third pass, the controller  400  determines whether the steps  610  to  650  are required to be repeated for another input electrode pair and another output group of electrodes. If the steps  610  to  650  are required to be repeated, flow returns to step  610 . 
     If the steps  610  to  650  are not required to be repeated, flow continues to step  670  where the controller  400  employs the stored indications of the first, second and third output signals to evaluate the electrical impedance of the object being evaluated. For this evaluation the controller  400  may also employ data characterising the first, second and third input signals. 
     At step  680 , the controller  400  may format for display data representative of the electrical impedance of the object being evaluated, and transmit the formatted data to the display device  500  which displays an image representative of the electrical impedance. 
     At step  690 , the controller  400  may perform diagnosis based on the electrical impedance of the object being evaluated. For example, the diagnosis may be performed by comparing the evaluated electrical impedance with stored reference data representative of the impedance of an object in different medical conditions. 
     The method step  610  to  650  illustrated by the flow chart of  FIG. 2  may also be performed in further passes for further pairs of electrodes which are displaced from the first, second and third input electrode pairs P 1 , P 2 , P 3 , and for further output electrode groups which are displaced from the first, second and third output electrode groups G 1 , G 2 , G 3  by a common distance and a common direction.  FIG. 6  shows an example of a fourth input electrode pair P 4  comprising electrodes  44  and  81 , highlighted by circles, i.e. P 4 ={ 44 ,  81 }, and a fourth output electrode group G 4  comprising electrodes  44 - 46 ,  53 - 56 ,  62 - 66 ,  71 - 74 ,  79 - 81 , i.e. G 4 ={ 44 - 46 ,  53 - 56 ,  62 - 66 ,  71 - 74 ,  79 - 81 }. The electrodes of the fourth output electrode group G 4 , together with the electrodes of the fourth input electrode pair P 4 , will be referred to as a fourth electrode set S 4 . The nineteen electrodes of the fourth electrode set S 4  are arranged in a fourth pattern, and the boundary of the fourth pattern comprises the electrodes  44 - 46 ,  53 ,  56 ,  62 ,  66 ,  71 ,  74 ,  79 - 81  and form a second hexagon. 
     With such a displacement, at step  630  on a fourth pass, a fourth input signal is applied by the transceiver  300  to the electrodes { 44 ,  81 } of the fourth input electrode pair P 4 , and at step  640  on the fourth pass, fourth output signals are received by the transceiver  300  from the electrodes of the fourth output group of electrodes G 4 . In particular, within this fourth output group of electrodes G 4 , the first output signals are measured between the following combinations of electrodes: 
       45  and  55 ;  46  and  56 ; 
       53  and  63 ;  54  and  64 ;  55  and  65 ;  56  and  66 ; 
       62  and  71 ;  63  and  72 ;  64  and  73 ;  65  and  74 ; 
       71  and  79 ;  72  and  80 . 
     In  FIG. 6 , these combinations of electrodes are highlighted by being joined by bold lines. 
     Each of these combinations comprises two adjacent electrodes. The combination  54  and  64  and the combination  64  and  73  lie on an sixteenth line coincident with a line joining the electrodes  44  and  81  of the fourth input electrode pair P 4 . The combination  53  and  63 , the combination  63  and  72 , and the combination  72  and  80  lie on a seventeenth line parallel to the sixteenth line, and joining corners of the second hexagon adjacent to those corners of the second hexagon occupied by the electrodes  44  and  81  of the fourth input electrode pair P 4 . The combination  45  and  55 , the combination  55  and  65 , and the combination  65  and  74  lie on an eighteenth line parallel to the sixteenth line and on the opposite side of the sixteenth line to the seventeenth line, and joining further corners of the second hexagon adjacent to those corners of the second hexagon occupied by the electrodes  44  an  81  of the fourth input electrode pair P 4 . The combination  62  and  71  and the combination  71  and  79  lie on a nineteenth line parallel to the sixteenth line and on the same side of the sixteenth line as the seventeenth line, and joining corners of the second hexagon furthest from the electrodes  44  and  81  of the fourth input electrode pair P 4 . The combination  46  and  56  and the combination  56  and  66  lie on a twentieth line parallel to the sixteenth line and on the same side of the sixteenth line as the eighteenth line, and joining further corners of the second hexagon furthest from the electrodes  44  and  81  of the fourth input electrode pair P 4 . 
     It can be readily appreciated from  FIGS. 3 and 6  that the fourth input electrode pair P 4  and the combinations of electrodes from the fourth output electrode group G 4  form an identical hexagonal pattern to the first input electrode pair P 1  and the combinations of electrodes from the first output electrode group G 1 , except for a displacement of each electrode of the fourth electrode set S 4  relative to the corresponding electrodes of the first electrode set S 1  by a common distance of three times the length of the side of the equilateral triangles, and by a common direction, namely horizontally to the right. 
     At step  650  on the third pass, the controller  400  receives from the transceiver  300  indications of the fourth output signals received by the transceiver  300 . The controller  400  stores these indications for subsequent evaluation of the electrical impedance of the object being evaluated. 
     Further passes of steps  610  to  650  may be made, if desired, with a fifth input electrode pair P 5  and a fifth output electrode group G 5  which are a horizontal displacement to the right by three electrode positions of, respectively, the second input electrode pair P 2  and the second output electrode group G 2 , and with a sixth input electrode pair P 6  and a sixth output electrode group G 6  which are a horizontal displacement to the right by three electrode positions of, respectively, the third input electrode pair P 3  and the third output electrode group G 3 . The displacement of the arrangement of the input electrode pairs and of the pattern of the output electrode groups by a common distance and a common direction, that is a lateral displacement, may be used for localisation of an area of interest in an object, whereas the rotational displacement of the arrangement of the pairs of electrodes and of the pattern of the groups of electrodes may be used for increasing the resolution of the electrical impedance evaluation. 
     Referring to  FIG. 7 , a receptacle  700  which is cylindrical has a base  710  which is circular comprising the electrode carrier  100 , with the electrodes  1 - 85  exposed to the interior of the receptacle  700  for electrical impedance imaging of an object within the receptacle  700 . The electrodes  1 - 85  are not illustrated in  FIG. 7  for clarity. The electrode carrier  100  may be the whole base  710 , or a part of the base  710 . The receptacle  700  has a wall  720 . Further electrodes  730  are deployed across the wall  720  and are exposed to the interior of the receptacle  700  for electrical impedance imaging of an object within the receptacle  700 . Preferably, the further electrodes  730  are recessed in the wall  720  so that they do not make physical contact with the object in the receptacle  700 , but can make electrical contact with the object by means of an electrically conductive liquid in the receptacle  700 . The further electrodes  730  are arranged in a first ring  740  and a second ring  750 . Each ring has thirty-six of the further electrodes  730  equally spaced. Alternatively, other numbers of the further electrodes  730  may be used. The switching device  200  can be coupled to each of the further electrodes  730 , although in  FIG. 7  for clarity the individual couplings to each of the further electrodes  730  are not illustrated. The input signal generated by the transceiver  300  can be routed by the switching device  200 , under the control of the controller  400 , to a selected input electrode pair, or more than one pair in different ones of the first and second rings  740 ,  750 , of the further electrodes  730 . The transceiver  300  receives output signals routed by the switch device  200  from selected ones of the further electrodes  730  under the control of the controller  400 . The controller  400  co-ordinates the generation of the input signal by the transceiver  300  and the selection of the further electrodes  730  by the switching device  200 . The selection of the further electrodes  730  may be made from the first ring  740  or from the second ring or from both rings  740 ,  750 . Furthermore, none of the further electrodes  730  may be selected if it is desired to make electrical impedance measurements using only the electrodes  1 - 85  of the electrode carrier  100 . Also, the selection of the further electrodes  730  may be made simultaneous to, or independently of, the selection of the input electrode pair P 1 , P 2 , P 3 , P 4 , P 5 , P 6  at which the input signal is applied and the selection of the output electrode group G 1 , G 2 , G 3 , G 4 , G 5 , G 6  from which the output signals are to be received. The controller  400  receives from the transceiver  300  indications of the output signals received by the transceiver  300  from the selected further electrodes  730  and evaluates the electrical impedance of the object under evaluation. 
     In a variation of the embodiment described with reference to  FIG. 7 , the electrode carrier  100 , which is the whole or a part of the base  710 , is rotatable relative to the wall  720  of the receptacle  700 . The rotation takes place in the plane of the flat surface  105 . This enables electrical impedance detection using a finer resolution by enabling the position of the electrodes  1 - 85  to be changed relative to the position of the further electrodes  730 . 
     Referring to  FIG. 8 , another embodiment of an apparatus for electrical impedance imaging comprises a spacing member  760  for spacing an object under evaluation from the electrode carrier  100 . All other elements illustrated in  FIG. 8  are identical to the elements illustrated in  FIG. 7  and have corresponding reference numerals, and will not be described again. The spacing member  760  is located within the receptacle  700  and in use an object to be evaluated is placed against the spacing member  760  on the opposite side of the spacing member  760  to the electrode carrier  100 . The spacing member  760  may be in contact with the base  710 , or spaced from the base  710 . When the electrode carrier  100  rotates relative to the wall  720 , the spacing member  760  and the object do not rotate relative to the wall  720 . In this way, the object is shielded from rotational forces from the rotating carrier  100 , and discomfort to a patient can be reduced or eliminated. 
     The spacing member  760  may be electrically conductive. Typically, the spacing member  760  has a conductivity less than the conductivity of the object under evaluation. 
     In use, the receptacle  700  may contain an electrically conductive liquid  770 . The liquid  770  may occupy the volume, if any, between the base  710  and the spacing member  760 , and may also be present on the opposite side of the spacing member  760  to the base  710 , where the object is placed for evaluation. The conductivity of the spacing member  760  and of the liquid  770  is typically substantially the same. Preferably, the conductivity of the spacing member  760  is at least 90% of the conductivity of the liquid  770 . A preferred conductivity of the liquid  770  will depend on the conductivity of the object to be evaluated by electrical impedance imaging, but a conductivity of the liquid  770  in the range 10 to 12 mS/cm is suited to typical objects, such as human breast tissue. Such a liquid may be, for example, a saline solution having a concentration of 0.9% at 20° C. 
     Although embodiments have been described which have eighty five electrodes mounted on the electrode carrier  100 , a greater or smaller number of electrodes may be used. In a non-illustrated variant of the electrode carrier  100 , some of the electrodes  1 - 85  may be omitted, in which case the electrodes  1 - 85  are not equidistant. In such an arrangement, the electrodes  1 - 85  are nevertheless located at corners of equilateral triangles arranged in a continuum, although not at all available corners of the equilateral triangles. This variant may be used when the union of the different input electrode pairs and output electrode groups {P 1 , P 2 , P 3 , P 4 , P 5 , P 6 , G 1 , G 2 , G 3 , G 4 , G 5 , G 6 } for all selectable input electrode pairs and output electrode groups does not encompass all available corners of the equilateral triangles. This variant enables the number of connections to the electrodes  1 - 85  to be reduced. 
     Preferably, one or more of the electrodes  1 - 85  and further electrodes  730  have a surface that has been polished to make it smooth. A smooth surface can enhance the repeatability of the measurements of the output signals. 
     Although embodiments have been described in which the electrode carrier  100  is circular, this is not essential and an electrode carrier having a different shape can be used. Although embodiments have been described in which the electrodes  1 - 85  are deployed across a flat surface  105  of the electrode carrier  100 , it is not essential for the surface to be flat. For example, the surface may be curved, or contoured to match the shape of an object to be evaluated. Although embodiments have been described in which rotation of the electrode carrier  100  takes place in the plane of the flat surface  105  of the electrode carrier  100 , if the surface of the electrode carrier  100  is not flat, rotation may take place about an axis that is perpendicular to the surface of the electrode carrier  100  and that preferably passes through the centre of the electrode carrier  100 . 
     Although embodiments have been described which employ up to five input electrode pairs P 1 , P 2 , P 3 , P 4 , P 5 , P 6  and five output electrode groups G 1 , G 2 , G 3 , G 4 , G 5 , G 6 , other numbers may be employed. The number of different input electrode pairs P 1 , P 2 , P 3 , P 4 , P 5 , P 6  and output electrode groups G 1 , G 2 , G 3 , G 4 , G 5 , G 6  may be selectable by a user or by the controller  400 , or may be fixed. By using more input electrode pairs P 1 , P 2 , P 3 , P 4 , P 5 , P 6  and output electrode groups G 1 , G 2 , G 3 , G 4 , G 5 , G 6 , more data can be generated from the output signals, enabling the electrical impedance of the object to be determined with greater resolution. 
     The rotational displacement of the different input electrode pairs P 1 , P 2 , P 3 , P 4 , P 5 , P 6  may be selected by a user or by the controller  400 , or may be fixed. In general, this rotational displacement is a multiple of 60°, that is 60°, 120 or 180°. Correspondingly, the rotational displacement of the different output electrode groups G 1 , G 2 , G 3 , G 4 , G 5 , G 6  may be selectable by the user or by the controller  400 , or may be fixed. This rotational displacement is also a multiple of 60°, that is 60°, 120° or 180°. 
     Although embodiments have been described which employ a single spacing of the electrodes of the input electrode pairs P 1 , P 2 , P 3 , P 4 , P 5 , P 6 , more than one spacing may be employed. The spacing or spacings may be selectable by a user or by the controller  400 , or may be fixed. 
     Although embodiments have been described in which each output electrode group G 1 , G 2 , G 3 , G 4 , G 5 , G 6  employs seventeen electrodes, more or fewer electrodes may be employed. The number of electrodes in the different output electrode groups G 1 , G 2 , G 3 , G 4 , G 5 , G 6  may be selectable by a user or by the controller  400 , or may be fixed. 
     Although embodiments have been described which employ a hexagonal pattern of electrodes, other patterns may be used. 
     Although embodiments have been described which employ a hexagonal pattern of electrodes in which the sides of the hexagon have a length equal to twice the length of the side of the equilateral triangles, other multiples of the length of the side of the equilateral triangles may be used. 
     Although embodiments have been described which employ a single pattern for the output electrode groups G 1 , G 2 , G 3 , G 4 , G 5 , G 6 , the pattern may be selectable by the user or by the controller  400 . The selection may be made from a plurality of fixed patterns, or may be made by selecting a plurality of individual electrodes. The patterns may be of different shapes, or may have the same shape and be of different sizes. 
     Although embodiments have been described in which the electrodes of an electrode set comprise all electrodes of the electrode carrier within a boundary, and in particular a hexagonal bounday, it is not essential that the set comprises all of the electrodes within the boundary. In other words, one or more of the electrodes within the boundary may be excluded from the electrode set, and therefore remain unused for the electrode set. For example, in the case of a hexagonal pattern of electrodes having a hexagonal boundary, one or more electrodes in a central region of the hexagon may remain unused. Although embodiments have been described in which the first and second output electrical signals are detected at a plurality of combinations of electrodes of the first and second output electrode group respectively, each of the combinations comprising adjacent electrodes, it is not essential for the electrodes of each combination to be adjacent. Furthermore, the spacing of the electrodes of the combinations of electrodes may be selectable. The electrodes of each combination, and their spacing, may be selected by the controller  400 . 
     Although embodiments have been described in which lateral displacement of a pattern of electrodes is performed horizontally, other directions of lateral displacement may be used. 
     Preferably, the first, second and third input signals are identical. 
     Although the embodiments of  FIGS. 7 and 8  comprise a receptacle  700  which is cylindrical, the receptacle  700  need not be cylindrical. Similarly, the base  710  need not be circular. The further electrodes  730  may be arranged in any number of rings, and indeed need not be arranged in rings  740 ,  750  but may instead be arranged in other configurations. The electrode carrier  100  may be moveable relative to the wall  720  in order to varying the volume of the receptacle  700 , for example to adjust the volume to objects of different sizes under evaluation. 
     Although embodiments have been described in which the electrodes are arranged at corners of triangles, this is not essential and other arrangements of electrodes may be used, for example the electrodes may be arranged on five or more lines passing through a common point. 
     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 already known in the art of electrical impedance imaging for medical diagnostics, and which may be used instead of, or in addition to, features already described herein. 
     Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation 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. 
     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. 
     It should be noted that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single feature may fulfil the functions of several features recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims. It should also be noted that the Figures are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the present invention.