the embodiments of the invention are directed to an eit system and method adapted to detect changes within a body . they are particularly suited to detecting internal bleeding within the peritoneum . as previously mentioned , eit systems apply a current to a body and measure voltage between electrodes placed on the surface of the body . from these measurements it has been possible to calculate the electric field which is created in a two - dimensional plane or three dimensional volume as a result of the electric current flow . a variation in electric field results from changes in resistivity of the various tissues within the region . from these resistivity changes , it is possible to create an image of changes in organs and tissue present in the region . while this has been done in the past , the quality of the images that have resulted have been quite poor and thus the process has had limited practical imaging application . the inventors have recognised that while intrinsic image quality may be poor , the process may be used to provide a parameter calculated from the output of an eit imaging system which is subject to change and can be monitored in real - time . internal bleeding is a particular phenomenon which causes the specified parameter to vary due to the significantly different resistivity of free blood in comparison with that of other tissue in the abdomen . pooling of blood ( or other conductive fluids ) create a localised anomalous electrical conductivity which perturbs current flow within the body , and therefore perturb impedance measurements made on the body surface . these perturbations can be measured via the imaging process , to enable calculation of the rate of change , volume and location of the anomalous fluid . in this way the inventors have identified a manner in which eit can be used to identify and monitor internal bleeding by non - invasive means . as the most common cause of intraperitoneal bleeding is blunt trauma received in motor vehicle accidents the inventors have been concerned to develop apparatus that may be used for use with such patients who may well be suffering other injuries , including spinal injuries and / or be unconscious . eit equipment of the prior art is often not suitable for use in such circumstances . the first embodiment is described with reference to fig1 to 6 . as shown in fig1 , the first embodiment is an eit system 1 adapted to detect internal bleeding in the peritoneum and comprises a flexible electrode belt 3 , an on - patient module 2 and a processing means 4 . as better shown in fig2 a to 2g , the electrode belt 3 is of elongate form having a plurality of electrodes 7 equally spaced along its length . unlike the belts of the prior art which are adapted to be placed around the perimeter of the abdomen , the belt of the embodiment is arranged to be placed against the anterior surface of a patient &# 39 ; s abdomen , so that it runs substantially from one side of the patient &# 39 ; s abdomen to the other side . it is thus considerably shorter than conventional electrode belts which have been used for eit . in typical use , the belt 3 is placed proximate the umbilicus . the belt has some flexibility to enable it to adapt to the contour of the abdomen and hold the contact faces of all electrodes in firm contact with the skin of the abdomen to thereby ensure satisfactory electrical conduction . in use , the contact faces are pre - gelled with an appropriate conductive paste to assist contact with the skin , and contact is further assured by means of adhesive surrounding each electrode face . it can be seen that the belt thus define a substantially linear array of electrodes across the abdomen , as required for eit analysis . the electrode belt 3 is used to apply a current between a selected pair of the electrodes and to make measurements of voltages between remaining pairs of electrodes . the contact faces of the electrodes are elongate and preferably of substantially rectangular form . the elongate axes of the contact faces are oriented transversely to the elongate direction of the belt , so that when the belt is applied to the abdomen the contact faces lie substantially parallel to a central axis of the abdomen . elongate electrodes have been found to produce a more uniform parameter sensitivity in images . internal bleeding can be present anywhere within a large region of the abdomen , and , in use , the belt 3 is usually applied centrally in the region of the umbilicus . the elongate shape of the contact faces of the electrodes ensures that the electric field applied to the abdomen is relatively uniform over a wide region of the abdomen and that , consequently , the rates estimated and gathered from the abdomen are relatively insensitive to their axial location relative to the electrodes . the length of the contact faces is selected as a compromise to provided extended length while ensuring good contact with the body . typically , a length in the range of 75 mm to 100 mm has been found to be optimum , although lengths outside these limits will still function . the width is selected to ensure good contact area , while providing adequate spacing between electrodes . typically , a width in the range of 5 mm to 25 mm has been found to be suitable . the contact faces are composed of silver / silver chloride or any other suitable electrode material . each electrode 7 has a conductive track 8 connecting the electrode 7 to an electrical termination 9 . while the eit system of the invention can be adapted to function with electrode belts constructed in the manner of the prior art , it is very desirable to minimise electrical noise . because the measurement of voltages between a pair of electrodes is intrinsically a high impedance measurement , when the voltage signals are processed , they are susceptible to noise . thus , noise suppression or insulation must be used . in the present embodiment , active shielding is provided in which a coaxial configuration is used by means of the novel construction of the belt . in this way , signals are transmitted from the belt to the on - patient module 2 , with the signal being applied both to the core and to the shield of the coaxial configuration . by doing this , there is no capacitive coupling between the shield and the core , because there is no differential voltage between the core and the shield . a low impedance shield voltage is generated using follower amplifiers in the on - patient module 2 . the electrode belt of the embodiment is constructed in a manner similar to a flexible printed circuit . in the present embodiment , the coaxial configuration is created using a seven - layer arrangement — as is illustrated in fig2 a to 2g . fig2 a to 2g each show one of the seven layers . the two outer layers 10 a , 10 b comprise an insulating material and one 10 a includes apertures 12 to allow the electrodes 7 to contact the patient &# 39 ; s skin . there is also provided an aperture 11 for a termination 9 . as mentioned above , each electrode 7 comprises a core and shield arrangement . fig2 c illustrates the core layer 10 c . the core layer comprises the electrodes 7 , each electrode having a printed conductive strip 8 , which will be used to connecting the electrode 7 to the coaxial cable 5 . each conductive strip 8 leads to a termination 9 , which in turn connects to the coaxial cable 5 . the conductive strips 8 are also made from a suitable conductive material such as silver . on either side of the core layer 10 c are insulating layers 10 d and 10 e , which provide a layer of insulation between the core and the shielding . it can be seen from fig2 a to 2g that one insulating layer 10 e provides insulation over the electrodes 7 , while the other layer 10 d does not , thereby allowing the electrodes 7 to protrude through the apertures 12 on the outer layer 10 b . on either outer side of the insulating layers 10 d and 10 e are the shielding layers 10 f and 10 g . these layers 10 f , 10 g include shielding for the electrodes 7 and conducting strips 8 of the core layer 10 c , and are also made of a conductive material such as silver . the two outer layers 10 a , 10 b are then placed on the outside . the belt 3 is printed using conventional printing techniques , with alternate layers of silver and insulating material . the traces of the different layers have different widths . the shielding layers 10 f , 10 g have the same , and thickest , width and are printed in silver . the insulating layers 10 e , 10 d have a smaller width and are printed in insulating material . the core layer 10 c has the smallest width and is also printed in silver . if the shielding layer 10 f is printed first , then subsequent layers laid down will take on the shape indicated in fig4 , thus forming a complete shielding layer around the core . fig4 shows , as mentioned above , the detail of a single electrode 7 , and illustrates this . as mentioned above , the core and shielding conductive strips are terminated with a suitable termination 9 including a core portion and a shield portion that can then be coupled to a coaxial cable 5 . the termination 9 comprises sixteen separate terminations — a core and shielding component for each electrode 7 . in this way , signals can be sent to and from the electrodes 7 from the on - patient module 2 , to carry out the appropriate measurements as will be described in more detail below . the on - patient module 2 comprises processing circuitry and a telemetry transceiver 21 that will allow data to be transferred to and from the processor 4 . in the embodiment , the data is transmitted by wireless communication to remove the need for a physical connecting cable between the body under test and the processor . nevertheless , it should be appreciated that a wired connection could be used as an alternative . the components of the on - patient module 2 are illustrated schematically in fig4 . these components are standard components and are mounted , in a conventional , known manner on a printed circuit board ( pcb ) ( not shown ). the on - patient module 2 applies current to a selected pair of adjacent electrodes 7 , and reads voltages from other pairs of adjacent electrodes 7 . current is supplied via a current multiplexer 13 and a constant current source in response to signals from a direct digital synthesiser ( dds ) 14 , and digital signal processor ( dsp ) 16 . as an example , the dsp 16 can be an analog devices adsp - 2181 and the dds 14 can be an analog devices ad9850 . the actual current is provided by a current source 23 provided between the dds 14 and the current multiplexer 13 . a current monitor 15 measures the actual current applied to the electrodes 7 — which may be slightly different to the constant current selected to be applied to the electrodes 7 , due to the source resistance of the body — and transmits the measured value of the current to the dsp 16 , to ensure that the correct current value of the current is communicated to the processor 4 . the dds 14 controls the frequency of the current signal to be applied to the current multiplexer 13 . in the present invention , the current is usually selected at around 3 ma and 62 khz , although this can be varied . when current is applied to a pair of adjacent electrodes , then the resulting voltages between other pairs of electrodes on the electrode belt 3 are measured these voltages are input via a voltage multiplexer 17 to a differential amplifier 18 to provide a voltage difference , dv , which is then input to an analogue to digital converter ( adc ) 24 , which then provides a 14 - bit data signal to the dsp 16 corresponding to this voltage difference . controlling signals and data can be sent to and from the dsp 16 to the remote processor 4 via a serial communications port 20 . as mentioned above , this data is sent using radio telemetry , and a suitable radio telemetry transceiver 21 is provided on the on - patient module 2 . however , it will be understood that other communications means — either wireless of fixed line — could be used . the on - patient module 2 also includes a battery ( not shown ), which supplies power to the components , as well as to the electrode belt 3 . the software for the on - patient module 2 resides on an eprom 22 . upon reset of the dsp 16 , it boots from the eprom memory . the software consists of a main routine and approximately 20 subroutines carrying out various functions . the main routine carries out the initialisation of the various circuit elements on the pcb and then enters an infinite loop waiting for events , to which it responds . events are initiated by the receipt of characters on the serial port 20 of a uart board coupled to the dsp 16 and memory mapped into the dsp data area . the arrival of particular character strings causes selected activities to be executed within the software subroutines . several interrupts are enabled for the dsp 16 . a timer interrupt is used to start and stop activities that need to be done in a timely fashion . the transmission and reception of characters on the uart connected to the dsp is also done using interrupts . character strings sent to the uart 20 from the processor 4 are used to invoke the following activities : test whether the on - patient module 2 is on and communicating properly . select two electrodes on the electrode belt 3 , to which the current is to be supplied . select electrodes on the electrode belt 3 , from which voltages are to be measured — this is usually all other possible pairs of electrodes on the belt , that is apart from the electrodes to which the current is supplied . select the gain of the differential amplifier 18 used to amplify the voltage measured on the selected voltage electrodes . select the frequency for transmission of the signal to the electrodes i . e . that of the current applied to the belt 3 . four frequencies are available — 15625 hz , 31250 hz , 62500 hz and 125000 hz . the default is 62500 hz . carry out a single measurement of current ( using the current monitor 15 ), and the voltage using the presently selected current and voltage electrode pairs from the belt electrodes 7 . carry out a complete measurement of all possible voltage and current readings from all possible current electrode pairs on the electrode belt 3 . in one instantiation , eight current source positions and forty voltage measurements are made in total . electrode pairs include the two end electrodes between which measurements are taken / current is applied . stop all measurements , calculations and activities being undertaken . measure on - patient module 12 battery health . prior to the initiation of the above functions — by the receipt of a character string by the uart serial port 20 connected to the dsp 16 — the dsp 16 transmits a string back out the serial port 20 to the processor 4 , to verify that the command was received . the dsp 16 carries out all the logic to convert the bit stream arriving at its serial port into meaningful characters . characters sent through the uart serial port 20 are mapped into the memory of the dsp 16 . the clocking signal of serial port 1 is used to control the triggering of the adc 24 . the timer interrupt is set up to allow timer interrupts to be used to start and stop data gathering . interrupts for the reception and transmission of data on the serial port of the uart serial port 20 are enabled . all extraneous serial port interrupts are cleared and nesting of interrupts is disabled . programmable flag pins are set to be outputs rather than inputs . the current monitor 15 includes a programmable gain amplifier that amplifies the signal to the current monitor 15 . the differential amplifier 19 amplifies either one of the battery signal or the voltage from the selected voltage electrodes . both amplifiers have their gains set to one of 4 values . the gain of the current monitor pga may be set to 1 , 10 , 100 or 1000 . in practice it is set to 1000 because the current monitor signal is small . the gain of the other pga is set to 1 ( for battery signal ) or 10 , 100 or 1000 ( for voltage measurement ). two control lines are required for both amplifiers to program one of the four gains . these control lines are connected to programmable flag outputs on the dsp 16 and thus gains are set by the dsp 16 . a programmable switch is used to select what signals are sent to the second pga above — that is either the voltages from electrodes 7 or battery voltage and subsequently to the adc 24 . there are four 8 - channel multiplexers on the pcb . two are in the current multiplexer 13 and two are in the voltage multiplexer 17 . the multiplexers select one of the eight electrodes 7 to be the positive current electrode , and a second to be the negative current electrode i . e . each selects one of the pair of electrodes to which the current is applied ; and the positive voltage electrode and the negative voltage electrode i . e . the pair of electrodes between which a voltage measurement is taken . the multiplexers are set by logic levels supplied by two 8 - output programmable latches connected to the dsp 16 . one latch is programmed to output the four logic levels ( 3 inputs to set the channel and 1 to enable ) required for each of the 2 multiplexers in the electrode current circuit and the other is likewise programmed to output settings for the voltage multiplexers . the latches reside on the data bus of the dsp 16 and are programmed with a write to the dsp &# 39 ; s data bus . a programmable switch is used to select whether the adc 24 is supplied with a signal from the current monitor 15 , or via the differential amplifier 18 . this switch is set by a logic level output from a programmable flag on the dsp 16 . clocking of the adc 24 is carried out by the clock line from serial port 1 on the dsp 16 . no other use is made of serial port 1 . clocking of the adc 24 is undertaken at a rate of 32 times the frequency transmitted through the current electrodes of the electrode belt 3 . the clocking rate is set by writing a counter value into a register on the dsp 16 . the output of the adc 24 is wired to the idma port on the dsp 16 . prior to enabling a voltage or current measurement the idma port is set up to start writing into a particular memory location in the dsp &# 39 ; s random access memory ( ram ), thus storing the measured value at the selected memory location . the dsp &# 39 ; s idma port increments the pointer to the write location after each analogue to digital conversion . in one instantiation , 8000 samples of waveforms are recorded . this corresponds to 250 periods of the transmitted waveform with 32 measurements per period . the 14 bits from the adc 24 are written into the 14 least significant bits of the chosen 16 - bit location in the ram . the second most significant bit is zero . the most significant bit is an overflow test bit from the adc 24 . the 8000 samples are then used to provide a measured value for the current and voltage . thus , after 250 periods are recorded for the required measurement , that measurement is processed and the results transmitted to the processor 4 via the uart serial port 20 . a baseline level for each measurement is calculated . this is done to allow for voltage offsets on amplifiers , the adc 24 and the electrodes 7 . in one instantiation , each of the 32 voltages in the period of the current or voltage waveform is estimated by averaging them over the 250 periods . this results in 32 numbers . once appropriately normalised they are compared with the full averages over 250 periods to derive a standard deviation measure of the validity of signal measurement . after subtraction of the baseline level , the amplitude of the ac signal from the current monitor or voltage electrodes is calculated by summing the square of the 32 averaged samples . for calculation of the battery voltage ( a dc signal ), a simple normalised average of the 8000 samples is calculated . data and standard deviations are formatted as 32 - bit real floating - point numbers and transmitted out the uart serial port 20 to the processor 4 . the processing of the data by the processor 4 is represented diagrammatically by fig5 . the processor 4 may take any suitable form such as a handheld computer or personal digital assistant , for example a hewlett packard jornada or ipaq palm size computer or any serial capable device . in the embodiment , the processor is associated with a version of the windows ce operating system , although it will be recognized that other suitable operating systems may be used . the analysis of the data is directed to the calculation of a parameter which is representative of conductivity within the portion of the body being monitored . it is the change of this parameter which provides an indication in increase in volume of body tissue as in the case of internal bleeding . this parameter can be monitored over time to determine the rate of internal bleeding . this parameter has been termed the “ resistivity index ” ( ri ). in order to process the information , the abdomen may be modelled as a disk - shaped region . this is represented in fig6 . as shown in fig6 , the disk - shaped region 60 is represented by a 16 × 16 array 61 . a 16 × 16 array is used on the basis that 16 electrodes would be placed substantially equidistantly around the circumference i . e . abdomen , with the applied electric field patterns resulting in values for electrical conductivity in the surrounding tissue at each of 256 array locations . this is typical of eit processing that has been conducted in the prior art . however in principle there is no restriction on the number of pixels used in an image . the array may be represented as a planar surface in two dimensions as shown in fig6 , or may be mapped as a three dimensional cylindrical array of voxels . the resistivity index involves adding up the total conductivity change observed within an image since this total change should reflect the total volume of anomaly that has appeared . in the case of the pixellation shown in fig6 , calculating the resistivity index will just involve adding all pixel values since all pixels are the same area , but in general should allow for variations in pixel size and therefore in general the resistivity index should be calculated by summing the quantity ( duds ) where s is the region described in the model , e . g . a cylinder . the equation defining this relationship may thus be expressed as shown below for a two - dimensional array : alternatively , these may be expressed in discrete format . for a two - dimensional array , the relationship below is given , where a p is the area of the pixel in question and tp is the total number of pixels : for a three - dimensional array , the relationship below is given , where v p is the volume of the voxel in question and tp is the total number of voxels : as shown in fig5 , the processor 4 receives data from the on - patient module 2 which is operated on at 42 with the stored reconstruction matrix 43 to provide a reconstructed image 44 . the image per se may be displayed on the display 53 of the processor . this image is then combined with stored spatial filter matrix 45 to provide a filtered image 47 . a resistivity index is then calculated at 49 . preferably , the resistivity index is processed with a temporal filter 50 at 51 to remove breathing effects . finally a scaling is applied to derive a blood volume estimation . this process is now described in more detail below . the processor 4 therefore receives information as to the voltages measured between respective electrode pairs for a given measured current between a predetermined pair of electrodes . measurements are received for all possible electrode pairs . in an instantiation using 8 adjacent current electrode pair positions and adjacent voltage measurements this comprises a total of 40 measurements — that is for each electrode pair to which current is applied , there are 5 voltage measurements to be taken . there are 8 different electrode pairs ( including the two end electrodes )— making the total number of measurements 40 i . e . 8 × 5 . in addition , the actual current value measured between each of the eight adjacent current pairs is measured and transmitted , making a total of 48 measurements sent to the processor 4 . these 40 voltage measurements , normalised by dividing by each relevant current measurement , are saved to file in memory in the processor 4 . processing these 40 measurements will give an indication of the conductivity of the tissue within the abdomen at the time that the measurement is taken . by repeating these measurements regularly at predetermined intervals , any changes in the measured conductivity indicates changes in the tissue composition e . g . through the presence of internal bleeding , as well as the approximate location of that change . once all the measurements have been saved to a file associated with the processor 4 , then the processor 4 is operable to carry out the reconstruct routine mentioned above , to determine the rate of change of blood volume in the abdominal cavity , and , if necessary , trigger any alarm . in the present embodiment , eight electrodes are provided over half the abdomen — numbered “ 0 ” to “ 7 ”— and this must be accounted for , and which will be discussed further below . by solving laplace &# 39 ; s equation for the region of interest ( for the abdomen the region shape will usually be assumed cylindrical ), the expected resultant electric field from current electrodes placed at positions around the circumference of the region 60 can be determined and can be used to derive expected conductivity values measured at any of the 256 locations within the array 61 , for current electrode pairs placed around the circumference . thus , a 256 × 256 reconstruction matrix can be derived from these calculations for all possible electrode pairs . this reconstruction matrix will be used by the processor 4 to provide indications of changes in tissue conductivity within the abdomen — as will be discussed in more detail below . 1 . process — that is calculate the difference between reference ( ref ) and current ( exp ) data sets to obtain vector , w 1 . 1 . reconstruct — this comprises multiplying the data vector w ( 256 × 1 matrix ) by a reconstruction matrix ( 256 × 256 ) to obtain a reconstructed image . 2 . if necessary , reduce spatial variation of image parameters by filtering , and apply temporal filtering to remove breathing artefacts 3 . integrate the pixels in ( 256 × 1 or 16 × 16 ) reconstructed image to obtain an ri estimate 4 . scale rate estimate using empirical sensitivity to obtain ri in terms of blood volume 5 . divide estimated blood volume by time interval between reference and current data sets 6 . depending on the rate that has been calculated , determine alarm category : thus , the process subroutine calculates the change between the present measurements and the last set of measurements — referred to herein as “ exp ” and “ ref ” respectively . thus the change =( exp − ref ). this is done by calculating a vector , w , of changes in data from the reference (“ ref ”) data , which can later be multiplied by the reconstruction matrix to produce an image and hence an estimation of blood volume — as will be discussed in more detail below . the resultant 256 × 1 matrix is an approximate reconstructed image , and , from there , values for the “ resistivity index ” ( ri ) can be obtained by integrating pixel values in this reconstructed image . this ri value can then be used to provide an estimate of blood volume . this is done by using empirically derived values of blood volume as a function of ri . the value of estimated blood volume can be used to determine the rate of change of blood value by dividing the estimated blood volume by the time interval between the reference ( ref ) and current ( exp ) data sets , and if this value falls greater than a predetermined value , then an alarm can be triggered . as mentioned above , variations to the measurements that can be attributed to the patient &# 39 ; s breathing can be accounted for within the processing — if required . the elongate shape of the electrodes 7 , on the electrode belt 3 , enable correlation between the reconstructed image and the amount of tissue that the image represents . elongate electrodes overcome the problem that the use of conventional - shaped ( small diameter , circular ) electrodes would present , in that , if an amount of tissue such as blood , were to move a small axial distance out of the plane of the electrodes then the resistivity index would be very different . providing elongate electrodes — as described above — overcomes this problem to some extent and allows the vector / reconstructed image to be used to derive the information required . the processor can be used to select to a variety of parameters for operation and function . for example , the following functions are accessible using the processor 4 : starts automatic collection at specified time intervals stop automatic measurements changes time interval between measurements checks rms noise appearing on adjacent electrode voltage measurements to determine contact quality change the bleeding rate displayed between / sec , / min or / hr executes a measurement restarts measurements ( change patient ) saves data from a session checks for communication between the processor 4 and on - patient module 2 setup communication ( serial port ) between the processor 4 and on - patient module 2 checks battery of module 2 changes measurement frequency ( at present measurements are made at 62 . 5 khz ). change phase of measurement e . g . to take quadrature ( reactive ) measurements rather than resistive measurements . it will be obvious to person skilled in the art , that variations are possible within the scope of the present invention . for example , the apparatus could be used to detect other fluids or other tissue — such as cancerous tissue — and in other areas of the human body , and could be adapted for use with animals . it will be appreciated that advances in technology may lead to other ways of implementing certain aspects of the embodiments . those skilled in the art will appreciate that the wireless communication may be implemented in other ways than that described . in an adaptation of the embodiment , the belt is provided with some stiffness to hold the belt in curved form , having greater curvature proximate the ends . such a belt is adapted to support electrodes very close to the sides of patient , maintaining those electrodes in good contact with the skin . in the present embodiment mechanical contact between the skin and electrodes is facilitated by adhesive electrode surrounds . it will be clear that the invention is not restricted to a belt having the number of electrodes described in the embodiment . with too few electrodes , there is insufficient resolution of voltage variations across the abdomen , so that is becomes impractical to generate a clear enough reconstruction using this method . for some uses , acceptable results may be obtained with a belt having only four electrodes , although for most uses , at least 8 electrodes would be preferred . the number of electrodes might also be increased above eight to improve resolution . clearly , such an array will require more processing power and data transmission bandwidth to be effective . but the effectiveness of taking this step will be limited in any event . as the number of electrodes is increased , the relative resolution improvement reduces so that the benefit becomes insignificant . it is also to be appreciated that the purpose of the belt is to provide a straightforward means of applying a group of electrodes to the skin in the desired area . it would also be possible provide a linear array of electrodes which are adapted to contact the skin in an arrangement which would not be considered as a belt in conventional terminology . for instance , the electrodes might be associated with a mattress such that contact with the electrodes might be maintained merely because the patient was to lie upon the mattress . such an array would still contact the abdomen on one side only and require eit analysis in the same manner as previously described to thereby provide the monitoring for internal bleeding . throughout the specification , unless the context requires otherwise , the word “ comprise ” or variations such as “ comprises ” or “ comprising ”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers .
an eit system 1 adapted to detect internal bleeding in a body portion , the eit system 1 comprising a plurality of electrodes 3 adapted in use to extend in a substantially linear orientation across one side only of the body portion and to be applied in electrical contact with the skin of the body portion , a current source adapted to cyclically apply an electric current between one pair of the electrodes 3 , a voltage measuring means to measure the voltage across each of the other pairs of the electrodes resulting from the current , a data collection system 2 and a data analysis system 4 to analyse data resulting from the voltages that are measured by the voltage measuring means , wherein the analysis system 4 is configured to obtain quantitative information related to amounts and rates of conductive tissue changes occurring in the body , based on an eit analysis equivalent to that obtained from data derived from electrodes spaced around the full perimeter of the body portion . also disclosed is an electrode belt suitable for bioelectrical use and in particular for detection of change of volume of tissue in a body portion .