referring to fig1 , 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 fig1 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 fig1 , 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 fig1 . 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 fig1 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 fig2 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 fig8 . at step 610 , a first input electrode pair is selected by the controller 400 for the application of a first input signal . fig3 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 . fig3 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 : 50 and 60 ; 51 and 61 ; 52 and 62 ; 53 and 63 ; 59 and 68 ; 60 and 69 ; 61 and 70 ; 62 and 71 ; in fig3 , 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 . fig4 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 . fig4 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 : 50 and 59 ; 51 and 60 ; 52 and 61 ; 53 and 62 ; 60 and 68 ; 61 and 69 ; 62 and 70 ; 63 and 71 ; in fig4 , 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 fig3 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 . fig5 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 . fig5 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 : in fig5 , 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 fig3 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 fig2 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 . fig6 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 : 53 and 63 ; 54 and 64 ; 55 and 65 ; 56 and 66 ; 62 and 71 ; 63 and 72 ; 64 and 73 ; 65 and 74 ; in fig6 , 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 fig3 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 fig7 , 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 fig7 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 fig7 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 fig7 , 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 fig8 , 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 fig8 are identical to the elements illustrated in fig7 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 . although the embodiments of fig7 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 .
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 .