Patent Publication Number: US-2022218226-A1

Title: System and method for acquiring electrical impedance tomography data

Description:
This application claims priority from provisional patent application 201921019798 titled “System And Method For Acquiring Electrical Impedance Tomography Data” filed in Mumbai, India on 20 May 2019. 
     TECHNICAL FIELD 
     The present invention is in the field of electrical impedance tomography. More specifically, the invention relates to electrical and electronic methods for acquiring electrical impedance tomography data from a plurality of electrodes. 
     BACKGROUND ART 
     Electrical impedance tomography is a technique of acquiring information about the interior of an electrically conductive body by sensing the body&#39;s electrical behavior on its surface. Usually, electric current is passed through the body at specified points, and voltage is measured across other points of the body. Many such readings are taken, each current changing the points at which current is introduced and/or the points at which voltages are measured. Many procedures or algorithms are used to gain information about the interior of the body from the above information. 
     The same basic technique with field specific modifications is known by various names, such as electrical impedance tomography, impedance tomography, bio-impedance tomography, electrical sounding, etc. Electrical impedance tomography may be used for medical or biological imaging (where the conductive body is the human/biological body, and information to be gained about the body includes information about internal organs, etc.), geophysical/ground surveys (where the conductive body is the Earth, and information to be gained about the body includes information about presence of water, oil, minerals, etc.) and non-destructive testing of machines, engineered bodies, civil constructions or materials (where the conductive body is the machine, civil structure/building, material sample etc., and information to be gained includes information about interior material presence, cracks, interior structure, etc.). It may also be used in applications such as archeology, forensic science, security, etc. where knowledge of the interior of a body without accessing the interior of the body is important. 
     SUMMARY OF INVENTION 
     In the present invention, an AC current of a particular frequency is setup in the body to be sensed, and amplitudes and phases of AC voltages at many (or all) electrodes attached to the body are detected simultaneously. This measurement is performed without requiring a tremendous number of A-to-D converters. Two modulating signals of the same frequency as the introduced AC current, one in phase with the AC input and one 90 degrees out of phase with it, are provided at all electrode measurement circuits. At the electrode measurement circuit, each of the mixing signals is mixed separately with the electrode signal. The mixing may be done using non-linear mechanisms such as transistors or diodes. The output of this mixed signal is stored as a charge, possibly in a capacitor or a semiconductor device (e.g. a CMOS transistor) that can store charge when in a certain state. Upon assertion of a readout signal, the charge is converted back into a voltage and sent out on an analog voltage output line. The charge is reset upon the assertion of a reset signal. Voltages are read out from the analog voltage lines using ADCs. The circuit asserts a few of the electrodes circuits at a time, so as to achieve maximum speed of readout while minimizing the number of ADCs required. Using sufficient integration time, noise is minimized. Furthermore, since the readouts are amplitudes and not AC waveforms, data transfer requirements are also reduced. 
     In an embodiment, the electrodes are arranged in a rectangular or hexagonal matrix. An entire row is integrated together, and asserted for readout together. 
     Advantageous Effects of Invention 
     A benefit of the present invention is the speed of acquiring tomographic information. Hundreds or thousands of data points are gathered simultaneously instead of one at a time. Furthermore, a relatively few number of ADC (analog-to-digital converter) circuits are required. This improvement in cost and data acquisition time allows acquisition of data across thousands of electrodes instead of the tens of electrodes that are usually employed in electrical impedance tomography. This allows much higher resolution and more accurate reconstruction of the interior of the body being sensed. 
     By using higher integration time, noise in measurements can be significantly reduced. Furthermore, the ADC directly converts data equivalent to amplitudes and phases of the sensed AC signals, rather than the AC signals themselves. This reduces the data rate required and the amount of post processing required. This also allows data from a higher number of electrodes to be gathered at more time instants. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following figures more particularly illustrate the invention. 
         FIG. 1  illustrates a mixing sensor block, according to an embodiment. 
         FIG. 2  illustrates a system sensing signals from multiple electrodes according to an embodiment. 
         FIG. 3  illustrates a system sensing signals from multiple electrodes using multiple modulating signals according to an embodiment. 
         FIG. 4  illustrates a system sensing signals from multiple electrodes arranged in a matrix according to an embodiment. 
         FIG. 5  illustrates a schematic of a mixing sensor block according to an embodiment. 
         FIG. 6  illustrates a schematic of a mixing sensor block having a filter, according to an embodiment. 
         FIG. 7  illustrates a schematic of a mixing sensor block having multiple filters, according to an embodiment. 
         FIG. 8  illustrates a schematic of a mixing sensor block having two mixers, according to an embodiment. 
         FIG. 9  illustrates a schematic of a mixing sensor block that uses transistors as switches, according to an embodiment. 
         FIG. 10  illustrates a schematic of a mixing sensor block that uses a transistor as buffer, according to an embodiment. 
         FIG. 11  illustrates a schematic of a mixing sensor block without a transfer gate, according to an embodiment. 
         FIG. 12  illustrates a schematic of a mixing sensor block with an implicit integrating element, according to an embodiment. 
         FIG. 13  illustrates a schematic of a mixing sensor block with an implicit integrating element and no transfer gate, according to an embodiment. 
         FIG. 14  illustrates a schematic of a mixer according to an embodiment. 
         FIG. 15  illustrates a schematic of a mixer comprising a single transistor according to an embodiment. 
         FIG. 16  illustrates a schematic of a mixing sensor block using transistors, according to an embodiment. 
         FIG. 17  illustrates a schematic of a mixing sensor block using transistors and no transfer gate, according to an embodiment. 
         FIG. 18  illustrates a schematic of a mixing sensor block using transistors and capacitors, according to an embodiment. 
         FIG. 19  illustrates a schematic of a mixing sensor block using two transistors, according to an embodiment. 
         FIG. 20  illustrates a schematic of a mixing sensor block having multiple signal mixing transistors, according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following embodiments more particularly describe the invention. 
       FIG. 1  illustrates a mixing sensor block  100 , according to an embodiment. 
     The mixing sensor block  100  is an electronic apparatus made up of electronic components. The mixing sensor block  100  has a sensing input line  101  that it senses the signal from. The mixing sensor block  100  has a modulating input line  102  that is used to input a modulating signal. The mixing sensor block  100  has an optional selection input line  103 . The mixing sensor block  100  has an output line  104 . In an embodiment, when the selection input line  103  is asserted, the mixing sensor block  100  produces a voltage corresponding to the sensed information on output line  104 . If the selection input line  103  is not asserted, a high-impedance output is produced on output line  104 . In this way, multiple output lines from multiple mixing sensor blocks may be ganged together and the signal may be passed to a single voltage sensor, which may be an analog to digital converter (ADC). In another embodiment, the selection input line  103  is not present, and signal is always presented on the output line  104 . One out of many such signals produced by multiple mixing sensor blocks may be selected to pass to an ADC using a signal multiplexer. 
     We will now describe how the sensed information to be presented as voltage on the output line  104  is created. The signal on the sensing input line  101  and the signal on the modulating input line  102  are mixed with each other. This mixing may be any non-linear function of both the signals. In an embodiment, this non-linear function is or approximates a multiplication of the two signals. Such mixing of two signals may be achieved using non-linear circuit components such as diodes and transistors. Specific circuits such as imbalanced, balanced and doubly-balanced signal mixers may be used. In an embodiment, both the sensing input line  101  and the modulating input line  102  are voltage signals. This mixed signal is then filtered to select a particular frequency component of the signal, such as the DC or base-band component. The amplitude of this component is the information to be produced at the output, a voltage corresponding to which will be created on the output line  104  (when the selection input line is asserted, or always). 
     In an embodiment, this mixed signal is integrated for a certain amount of time using an integrating circuit component such as a capacitor (which may be a specifically created component, or may be an implicit capacitance of another semiconductor component such as a transistor). This integration achieves filtering to select the DC frequency, as well as reduction in noise. In an embodiment, extra signals are provided to the mixing sensor block  100 . One such signal may expose or stop exposing the mixed signal to the integrating circuit component. Another such signal may reset the integrated signal to a standard value, to remove effects of previous use of the circuit. 
     In an embodiment, the modulating signal  102  is a sinusoidal signal. Such a sinusoidal signal may be produced using an oscillator circuit or ADC and provided to more than one mixing sensor blocks. In an embodiment, two modulating signals are available across the system of the present invention. Both these signals are at the same oscillating frequency, but they are out-of-phase with respect to each other by a certain amount (in an embodiment by 90 degrees). These signals may be present at the same time on two different circuits (or tracks), or may be present at different moments of operation of the present system. 
     Let the signal on the sensing input line  101  have multiple frequencies. This may be represented mathematically as 
     
       
         
           
             
               
                 
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     This allows us to detect the imaginary part of the amplitude x i . Thus, by using two modulating signals out of phase (either on different circuits or at different times), both the real and imaginary parts of the amplitude at a particular frequency can be extracted by the mixing sensor block  100 . In other words, the absolute amplitude and the phase angle of a particular frequency component of the sensed signal on the sensing input line  101  can be extracted. 
       FIG. 2  illustrates a system  200  sensing signals from multiple electrodes according to an embodiment. 
     Multiple electrodes  215 ,  225 ,  235  are electrically connected to the sensing input line of multiple mixing sensor blocks  210 ,  220 ,  230  respectively. A modulating signal source  205  provides the modulating signal to the mixing sensor blocks  210 ,  220 ,  230 . Separate selection signals  216 ,  226 ,  236  are provided as signals to the selection input line of the mixing sensor blocks  210 ,  220 ,  230  respectively. The output lines of multiple mixing sensor blocks  210 ,  220 ,  230  are tied together to a single output signal line  206 . 
     Even though the system is depicted as having three electrodes and three mixing sensor blocks, it may have a plurality of electrodes and corresponding mixing sensor blocks. 
     The same modulating signal (from the modulating signal source  205 ) is mixed with the various electrode signals in the mixing sensor blocks  210 ,  220 ,  230 . A single selection signal is asserted at a particular time, thus creating a voltage output from one of the mixing sensor blocks  210 ,  220 ,  230  as selected. This voltage output is present on the output signal line  206 . This output may be sensed using an ADC. 
     Over time, the modulating signal from modulating signal source  205  may be changed to various signal frequencies and/or phases. In an embodiment, the shape of the modulating signal is a sinusoidal. In another embodiment, the shape of the modulating signal is a square wave. 
       FIG. 3  illustrates a system  300  sensing signals from multiple electrodes using multiple modulating signals according to an embodiment. 
     A multiple of modulating signals from signal sources  305 ,  315 ,  325  are provided to the entire apparatus. Signal from each electrode from a plurality of electrodes  315 ,  325 ,  335  is connected to a plurality of mixing sensor blocks, one mixing sensor block for each pair of electrode and signal source. Selection signals  316 ,  326 ,  336  enable various mixing sensor blocks to produce output. For example, the selection signal  316  enables the mixing sensor blocks  310 ,  311 ,  312 , all corresponding to the electrode  315 , so that the mixing sensor blocks produce output corresponding to the signal of electrode  315  mixed with signals from signal sources  305 ,  315 ,  325  respectively to produce outputs on the output lines  306 ,  316  and  326  at the same time. Each of the output lines may be connected to a dedicated ADC, or the output lines may further be multiplexed onto the ADCs. Other combinations of enabling the mixing sensor blocks can be imagined as well. 
     Similarly, there are mixing sensor blocks  320 ,  321 ,  322  for electrode  325  and mixing sensor blocks  330 ,  331 ,  332  for electrode  335 . Even though 3 electrodes and three modulating signals are depicted, any plurality of electrodes and any plurality of modulating signals may be provided in the system of the present invention. In an embodiment, some of the modulating signals are 90 degrees out of phase (but of the same frequency) as other modulating signals. This allows detection of real and imaginary phase components as described about with reference to  FIG. 1 . 
       FIG. 4  illustrates a system  400  sensing signals from multiple electrodes arranged in a matrix according to an embodiment. 
     Electrodes  415 ,  416 ,  417 ,  425 ,  426 ,  427 ,  435 ,  436 ,  437  are arranged in a matrix comprising rows and columns. Depending on the topology required of the physical electrodes, some of the electrodes in such a matrix may be missing. In an embodiment, the physical electrodes are arranged in a geometric matrix as well. In another embodiment, physical electrodes are arranged in a geometric structure which is regular, such as a hexagonal (honeycomb) pattern, and the electrical arrangement is correspondingly that of a matrix of rows and columns. In yet another embodiment, the physical electrodes are not arranged in a matrix, but the circuit is electrically arranged in a matrix as shows. 
     The electrodes  415 ,  416 ,  417 ,  425 ,  426 ,  427 ,  435 ,  436 ,  437  are connected as sensing inputs to corresponding mixing sensor blocks  410 ,  411 ,  412 ,  420 ,  421 ,  422 ,  430 ,  431 ,  432 . All these mixing sensor blocks are given the same modulating signal  405  as modulating input. In an embodiment, more than one modulating signals may be present as described with reference to  FIG. 3 , and there may be correspondingly more than one mixing sensor blocks corresponding to each modulating signal. In an embodiment, two modulating signals are provided, of the same frequency but 90 degrees out of phase. Such signals may be sinusoidal signals or square waves. 
     A row selection signal  413  is provided as the selection input line to mixing sensor blocks  410 ,  411 ,  412  corresponding to the row of electrodes  415 ,  416 ,  417 . If the row selection signal  413  is asserted, the mixing sensor blocks  410 ,  411 ,  412  will produce voltage output corresponding to the sensed mixed (modulated) information from the row of electrodes  415 ,  416 ,  417  onto the output lines  440 ,  441 ,  442 . The output lines may have ADCs on each of them, or may further be multiplexed onto the ADCs. The output lines correspond to columns of electrodes. 
     If more than one modulating signals are provided, a single row selection signal may select all mixing sensor blocks of corresponding to all electrodes in a row, and all modulating signals. There will be output lines corresponding to each combination of electrode column and modulating signal. 
     If many ADCs are provided at the output lines they may be advantageously clubbed together in various ways to reduce the circuitry required. For example, a single ramp signal and clock signal may be provided across all the ADCs, and the ADCs individually compare the ramp signal to the signal on the corresponding output line (the output line is the input to the ADC), and when the comparison changes value, they will lock the count being counted up in a corresponding counter. 
     In another embodiment, the each ADC has a comparator and a capacitor that stores a voltage to be compared. A running voltage is provided on a global line, which takes values K/2, K/4, K/8, and so forth up to K/(2{circumflex over ( )}k)) where K is a reference voltage and k is the bit depth. This global line provides this voltage to all ADCs. Each ADC compares the voltage at its input (the voltage on the corresponding output line) to the sum of the voltages on the global line and the voltage stored in the capacitor. If the input voltage is higher, a 1 bit is output, and the summed voltage is fed back to the capacitor. If the input voltage is lower, a 0 bit is output and the capacitor maintains its original voltage. In this way, the ADC may complete the conversion in k steps. In another embodiment, the capacitor is initially set to the voltage to be converted; and if a incoming global line voltage is smaller than the capacitor voltage, a ‘1’ is output and the global line voltage is subtracted from the capacitor voltage and fed back to the capacitor voltage, else a ‘0’ is output and the capacitor voltage is maintained. 
       FIG. 5  illustrates a schematic of a mixing sensor block  500 , according to an embodiment. 
     A sensing input line  510  connects to an electrode (not shown). A modulating input line  530  brings in a modulating signal. The signal mixer  520  mixes the signal on the sensing input line  510  with the signal on modulating input line  530  by combining them non-linearly. The output of the signal mixer  520  is connected to an integrating circuit element  550  (such as a capacitor) through an optional transfer gate  540  which may be controlled by a transfer input signal. Closing the transfer gate  540  starts integrating the output of the signal mixer  520 . The charge (or signal) accumulated in the integrating circuit element  550  may be reset to a reset voltage by asserting a reset input signal which connects the integrating circuit element  550  to the reset voltage line  570  through the reset gate  560 . The output voltage of the integrating circuit element  550  is fed through a buffer  580  and an output gate  590  to an output line  595 . When the output gate  590  is closed, it outputs the voltage at the integrating circuit element  550  onto the output line  595 . When the output gate  590  is open, it presents a high impedance to the output line  595 . In this way, the output line  595  may be shared between many mixing sensor blocks. The buffer  580  acts as a voltage follower and protects the signal stored in the integrating circuit element  550  from degradation, as the signal is output on the output line  595 . It may also be used to boost or amplify the signal. 
       FIG. 6  illustrates a schematic of a mixing sensor block  600  having a filter, according to an embodiment. 
     A signal mixer  620  mixes the signal on the sensing input line  610  with the signal on modulating input line  630  by combining them non-linearly. The output of the signal mixer  620  is connected to an integrating circuit element  650  through a transfer gate  640  and a frequency selective filter  625 . Closing the transfer gate  640  starts integrating the filtered output of the signal mixer  620 . In series with the transfer gate  650  (or in place of it), a filter  625  is used to select a narrow band of frequencies. The narrow band of frequencies may be a base band (DC), or another narrow band of frequencies which may be then rectified to convert them to a DC signal. In an embodiment, the integrating circuit element  650  (which may be a capacitor) is itself the filter  625 , which keeps the DC component and rejects the AC component. 
     The charge (or signal) accumulated in the integrating circuit element  650  may be reset to a reset voltage by asserting by connecting the integrating circuit element  650  to the reset voltage line  670  through the reset gate  660 . The output voltage of the integrating circuit element  650  is fed through a buffer  680  and an output gate  690  to an output line  695 . 
       FIG. 7  illustrates a schematic of a mixing sensor block  700  having multiple filters, according to an embodiment. 
     A signal mixer  720  mixes the signal on the sensing input line  710  with the signal on modulating input line  730  by combining them non-linearly. The output of the signal mixer  720  is connected to various integrating circuit element  750 ,  751  and  752  through a transfer gate  740  and frequency selective filters  725 ,  726  and  727 . Closing the transfer gate  740  starts integrating the filtered output of the signal mixer  720 , as filtered through various filters. Each frequency selective filter  725 ,  726  and  727  selects a narrow band of frequencies to pass, and further may rectify these narrow band of frequencies to the base band, to be integrated by the corresponding integrating circuit elements  750 ,  751   752 . 
     The charge (or signal) accumulated in the integrating circuit elements  750 ,  751 ,  752  may be reset to a reset voltage by closing the reset gates  760 ,  761 ,  762 . The output voltage of the integrating circuit elements  750 ,  751 ,  752  are fed through buffers  780 ,  781 ,  782  and output gates  790 ,  791 ,  792  to output lines  795 ,  796 ,  797 . The various reset gates may be controlled together, as may be the output gates. 
       FIG. 8  illustrates a schematic of a mixing sensor block  800  having two mixers, according to an embodiment. 
     A sensing input line  810  connects to an electrode (not shown). A modulating input line  830  brings in a modulating signal and another modulating input line  831  brings in another modulating signal. In an embodiment, the modulating signal on the modulating signal line  830  is of a high frequency (close to the frequency to be detected), whereas that on the modulating signal line  831  is of an intermediate frequency (between the frequency to be detected and DC). The signal mixer  820  mixes the signal on the sensing input line  810  with the signal on modulating input line  830  by combining them non-linearly. This produces a signal, one of whose components is an intermediate frequency signal which may be selectively passed to the next stage. The signal mixer  821  further mixes this intermediate frequency detected signal with the intermediate frequency modulating signal to produce a base band signal. The output of the signal mixer  821  is connected to an integrating circuit element  850  (such as a capacitor) through an optional transfer gate  840 . Closing the transfer gate  840  starts integrating the output of the signal mixer  821 . The charge (or signal) accumulated in the integrating circuit element  850  may be reset to a reset voltage by asserting by connecting the integrating circuit element  850  to the reset voltage line  870  through the reset gate  860 . The output voltage of the integrating circuit element  850  is fed through a buffer  880  and an output gate  890  to an output line  895 . 
     In an embodiment, multiple lines carry high frequency modulating signals (of different frequencies) and intermediate frequency modulating signals (of different frequencies). Each pair of high frequency and intermediate frequency modulating signals produces a different mixer output. Each electrode is connected to many mixing sensor blocks, one for each pair of modulating signals. Thus, many frequencies may be simultaneously sensed. The electric current entering the body is also created to have these same frequencies. 
       FIG. 9  illustrates a schematic of a mixing sensor block  900  that uses transistors as switches, according to an embodiment. 
     A signal mixer  920  mixes the signal on the sensing input line  910  with the signal on modulating input line  930  by combining them non-linearly. The output of the signal mixer  920  is connected to an integrating circuit element  950  (such as a capacitor) through an optional transfer gate transistor  940  which is controlled by a transfer input signal  945 . In an embodiment, one or more of the transistors referred to in this disclosure are CMOS transistors. Asserting the transfer input signal  945  starts integrating the output of the signal mixer  920 . The charge (or signal) accumulated in the integrating circuit element  950  may be reset to a reset voltage by asserting a reset input signal  961  which connects the integrating circuit element  950  to the reset voltage line  970  through the reset gate transistor  960 . The output voltage of the integrating circuit element  950  is fed through a buffer  980  and an output gate transistor  990  (that is controlled by a selection input line  999 ) to an output line  995 . When the selection input line  999  is asserted, the output gate transistor  990  outputs the voltage at the integrator onto the output line  995 . The buffer  980  acts as a voltage follower and protects the signal stored in the integrating circuit element  950  from degradation. 
       FIG. 10  illustrates a schematic of a mixing sensor block  1000  that uses a transistor as buffer, according to an embodiment. 
     A signal mixer  1020  mixes the signal on a sensing input line  1010  with the signal on a modulating input line  1030  by combining them non-linearly. The output of the signal mixer  1020  is connected to an integrating circuit element  1050  (such as a capacitor) through an optional transfer gate transistor  1040  which may be controlled by a transfer input signal  1045 . Asserting the transfer input signal  1045  starts integrating the output of the signal mixer  1020 . The charge (or signal) accumulated in the integrating circuit element  1050  may be reset to a reset voltage by asserting a reset input signal  1061  which connects the integrating circuit element  1050  to the reset voltage line  1070  through the reset gate transistor  1060 . The output voltage of the integrating circuit element  1050  is fed through a buffer transistor  1080  and an output gate  1090  (controlled by a selection input line  1099 ) to an output line  1095 . When the selection input line  1099  is asserted, the output gate transistor  1090  outputs the voltage at the integrator onto the output line  1095 . The buffer transistor  1080  acts as a voltage follower and protects the signal stored in the integrating circuit element  1050  from degradation. 
       FIG. 11  illustrates a schematic of a mixing sensor block  1100  without a transfer gate, according to an embodiment. 
     A signal mixer  1120  mixes the signal on a sensing input line  1110  with the signal on a modulating input line  1130  by combining them non-linearly. The output of the signal mixer  1120  is connected to an integrating circuit element  1150  (such as a capacitor). Since there is no transfer gate, this connection is permanent and integration is always being performed. The integration may only be reset (as described below), but may not be stopped. The charge (or signal) accumulated in the integrating circuit element  1150  may be reset to a reset voltage by asserting a reset input signal  1161  which connects the integrating circuit element  1150  to the reset voltage line  1170  through the reset gate transistor  1160 . The output voltage of the integrating circuit element  1150  is fed through a buffer transistor  1180  and an output gate transistor  1190  (controlled by an selection input line  1199 ) to an output line  1195 . When the selection input line  1199  is asserted, the output gate transistor  1190  outputs the voltage at the integrator onto the output line  1195 . The buffer transistor  1180  acts as a voltage follower and protects the signal stored in the integrating circuit element  1150  from degradation. 
       FIG. 12  illustrates a schematic of a mixing sensor block  1200  with an implicit integrating element, according to an embodiment. 
     A signal mixer  1220  mixes the signal on a sensing input line  1210  with the signal on a modulating input line  1230  by combining them non-linearly. The output of the signal mixer  1220  is connected to an integrating circuit element through an optional transfer gate transistor  1240  which may be controlled by a transfer input signal  1245 . The integrating circuit element is not an explicit, separate circuit element in this embodiment. The integrating circuit element is the gate capacitance of the buffer transistor  1280 , the junction-to-junction (source-to-drain) capacitance of the reset gate transistor  1260  (while it is off) and the source-to-gate capacitance of the transfer gate transistor  1240 . 
     Asserting the transfer input signal  1245  starts integrating the output of the signal mixer  1220 . The charge (or signal) accumulated in the integrating circuit element may be reset to a reset voltage by asserting a reset input signal  1261  which connects the charge-accumulating node of the integrating circuit element to the reset voltage line  1270  through the reset gate  1260 . The output voltage of the integrating circuit element is fed through a buffer transistor  1280  and an output gate  1290  (controlled by an selection input line  1299 ) to an output line  1295 . 
       FIG. 13  illustrates a schematic of a mixing sensor block  1300  with an implicit integrating element and no transfer gate, according to an embodiment. 
     A signal mixer  1320  mixes the signal on a sensing input line  1310  with the signal on a modulating input line  1330  by combining them non-linearly. The output of the signal mixer  1320  is connected to an integrating circuit element. The integrating circuit element is not an explicit, separate circuit element in this embodiment. The integrating circuit element is the gate capacitance of the buffer transistor  1380  and the junction-to-junction (source-to-drain) capacitance of the reset gate transistor  1360  (while it is off). The charge (or signal) accumulated in the integrating circuit element may be reset to a reset voltage by asserting a reset input signal  1361  which connects the charge-accumulating node of the integrating circuit element to the reset voltage line  1370  through the reset gate  1360 . The output voltage of the integrating circuit element is fed through a buffer transistor  1380  and an output gate  1390  (controlled by a selection input line  1399 ) to an output line  1295 . 
       FIG. 14  illustrates a schematic of a mixer  1400  according to an embodiment. 
     The mixer  1400  mixes signals  1410  and  1430  by combining them non-linearly. In an embodiment, the signal  1410  may be a signal from an electrode, and signal  1430  may be a modulating signal. Any circuit that combines them non-linearly may be used, such as various known signal multiplier circuits. In this embodiment, the circuit consists of resistors  1421  and  1422  feeding the two signals to the inverting terminal of a differential amplifier  1424 , whose non-inverting terminal is grounded. The inverting terminal is also connected to the output of the differential amplifier  1424  through a diode  1423 . This has the effect of passing the sum of two signals through a non-linear function, in this case approximated as the logarithm. The output produced by the differential amplifier  1424  non-linearly combines the signals  1410  and  1430 . 
       FIG. 15  illustrates a schematic of a mixer  1500  comprising a single transistor according to an embodiment. 
     Signal  1510  from the electrode is given to one terminal (source or drain) of a transistor  1520  (that may be a CMOS transistor), and the modulating signal  1530  is given to the gate of the transistor  1520 . The second terminal of the transistor  1520  is used as the output signal that non-linearly combines the two signals. If the input from the electrode is very weak, it may be beneficial to connect the input from the electrode to the gate, and the modulating signal  1530  to a terminal (source or drain) of the transistor. 
       FIG. 16  illustrates a schematic of a mixing sensor block  1600  using transistors, according to an embodiment. 
     A sensing input line  1610  connects to an electrode (not shown). A modulating input line  1630  brings in a modulating signal. The signal mixing transistor  1620  mixes the signal on the sensing input line  1610  with the signal on modulating input line  1630  by combining them non-linearly. The output of the signal mixing transistor  1620  is connected to an integrating circuit element (such as a capacitor or implicit transistor capacitances) through an optional transfer gate transistor  1640  which may be controlled by a transfer input signal  1645 . Asserting the transfer input signal  1645  starts integrating the output of the signal mixing transistor  1620 . The charge (or signal) accumulated in the integrating circuit element may be reset to a reset voltage by asserting a reset input signal  1661  which connects the integrating circuit element to the reset voltage line  1670  through the reset gate  1660 . The output voltage of the integrating circuit element is fed through a buffer transistor  1680  and an output gate transistor  1690  (controlled by a selection input line)  1699  to an output line  1695 . When the selection input line  1699  is asserted, the output gate transistor  1690  outputs the voltage at the integrator onto the output line  1695 . The buffer transistor  1680  acts as a voltage follower and protects the signal stored in the integrating circuit element  1650  from degradation. 
       FIG. 17  illustrates a schematic of a mixing sensor block  1700  using transistors and no transfer gate, according to an embodiment. 
     A sensing input line  1710  connects to an electrode (not shown). A modulating input line  1730  brings in a modulating signal. The signal mixing transistor  1720  mixes the signal on the sensing input line  1710  with the signal on modulating input line  1730  by combining them non-linearly. The output of the signal mixing transistor  1720  is connected to an integrating circuit element (such as a capacitor or implicit transistor capacitances). The charge (or signal) accumulated in the integrating circuit element may be reset to a reset voltage by asserting a reset input signal  1761  which connects the integrating circuit element to the reset voltage line  1770  through the reset gate  1760 . The output voltage of the integrating circuit element is fed through a buffer transistor  1780  and an output gate transistor  1790  (controlled by a selection input line  1799 ) to an output line  1795 . When the selection input line  1799  is asserted, the output gate transistor  1790  outputs the voltage at the integrator onto the output line  1795 . The buffer transistor  1780  acts as a voltage follower and protects the signal stored in the integrating circuit element  1750  from degradation. 
       FIG. 18  illustrates a schematic of a mixing sensor block  1800  using transistors and capacitors, according to an embodiment. 
     A sensing input line  1810  connects to an electrode (not shown). A modulating input line  1830  brings in a modulating signal. The signal mixing transistor  1820  mixes the signal on the sensing input line  1810  with the signal on modulating input line  1830  by combining them non-linearly. In an embodiment, the sensing input line  1810  is connected to the gate of the signal mixing transistor and the modulating input line  1830  is connected to the terminal; in another embodiment (shown), the sensing input line  1810  is connected to the terminal of the signal mixing transistor and the modulating input line  1830  is connected to the gate. The output of the signal mixing transistor  1820  is connected to a capacitor  1850  that acts as an integrating circuit element. The charge (or signal) accumulated in the integrating circuit element  1850  may be reset to a reset voltage by asserting a reset input signal  1861  which connects the capacitor  1850  to the reset voltage line  1870  through the reset gate  1860 . The output voltage of the capacitor  1850  is fed through a buffer transistor  1880  and an output gate  1890  controlled by an selection input line  1899  to an output line  1895 . When the selection input line  1899  is asserted, the output gate transistor  1890  outputs the voltage at the integrator onto the output line  1895 . 
       FIG. 19  illustrates a schematic of a mixing sensor block  1900  using two transistors, according to an embodiment. 
     A sensing input line  1910  connects to an electrode (not shown). A modulating input line  1930  brings in a modulating signal. The signal mixing transistor  1920  mixes the signal on the sensing input line  1910  with the signal on modulating input line  1930  by combining them non-linearly. The output of the signal mixing transistor  1920  is connected to an integrating circuit element  1950  (such as a capacitor) through an optional buffer transistor  1940 . The charge (or signal) accumulated in the integrating circuit element  1950  may be reset to a reset voltage by asserting a reset input signal  1961  which connects the integrating circuit element  1950  to the reset voltage line  1970  through the reset gate  1960 . The output voltage of the integrating circuit element  1950  is fed through a buffer transistor  1980  and an output gate  1990  controlled by an selection input line  1999  to an output line  1995 . Both the buffer transistors  1940  and  1980  may be connected to the same voltage source  1985 . When the selection input line  1999  is asserted, the output gate transistor  1990  outputs the voltage at the integrator onto the output line  1995 . The buffer transistor  1980  acts as a voltage follower and protects the signal stored in the integrating circuit element  1950  from degradation. 
       FIG. 20  illustrates a schematic of a mixing sensor block  2000  having multiple signal mixing transistors, according to an embodiment. 
     A sensing input line  2010  connects to an electrode (not shown). A modulating input line  2030  brings in a modulating signal. The signal mixing transistor  2020  mixes the signal on the sensing input line  2010  with the signal on modulating input line  2030  by combining them non-linearly. The output of the signal mixing transistor  2020  is connected to another signal mixing transistor  2021  which combines the signal with the second modulating signal on the modulating input line  2031 . The output of the signal mixing transistor is connected to an integrating circuit element  2050  (such as a capacitor) through an optional transfer gate (not shown) and an optional buffer transistor  2040 . The charge (or signal) accumulated in the integrating circuit element  2050  may be reset to a reset voltage by asserting a reset input signal  2061  which connects the integrating circuit element  2050  to the reset voltage line  2070  through the reset gate  2060 . The output voltage of the integrating circuit element  2050  is fed through a buffer transistor  2080  and an output gate  2090  (controlled by a selection input line  2099 ) to an output line  2095 . When the selection input line  2099  is asserted, the output gate transistor  2090  outputs the voltage at the integrator onto the output line  2095 . The buffer transistor  2080  acts as a voltage follower and protects the signal stored in the integrating circuit element  2050  from degradation. Both the buffer transistors may be connected to the same voltage source  2085 , or to separate voltage sources. 
     Other Embodiments 
     An AC electric current of a particular frequency is introduced into the body to be sensed. In an embodiment, a superposition of multiple frequencies is introduced simultaneously into the body. This causes voltages to be produced at various points of the surface of the body. Electrodes are connected to various points of the surface of the body. These electrodes pick up the amplitudes and phases of the particular frequencies specifically introduced into the body, as described in various embodiments above. This sensing is done simultaneously across multiple electrodes. These electrodes may number in the hundreds, or may be a matrix of electrodes numbering in the thousands. Simultaneous sensing across a plurality of electrodes achieves speeds of acquisition which are impossible when sensing only across one pair of electrodes at a time (as is usually the norm). 
     The readout signal may be provided as more than one signal, for example a row and a column signal, assertion of both of which will cause the readout to take place. In this way, a 2D array of electrodes can be managed one electrode at a time. Alternatively, only a row signal may be provided, causing simultaneous readout of an entire row, requiring an A-to-D converter per column, but faster readout. In this way, we can reduce the number of A-to-D converters to just one (if only one electrode is read out at a time) or equal to the number of logical columns of electrodes (number of electrodes in a row). Other number of A-to-D converters are possible as well, by appropriate wiring. 
     Assertion of the transfer signal (enabling transfer gates) may be performed for the entire matrix of electrodes at a time, or for a row at a time, rolling from one end of the matrix to the next. The readout, may be synchronized in such a way that the most exposed row of electrodes is being read while transfer is disabled, immediately after which the row charges are reset and transfer into the integrators is started again. 
     In an embodiment, the charge recorded at each electrode, after the exposure time is complete, is transferred to a separate readout capacitor or semiconductor device that can store charge, so that the next exposure period can start sooner. Alternatively, data may be read one row at a time, and the exposures are also reset row-wise. A row-wise or device-wide reset signal is provided so that the stored charge may be reset so that a new measurement can start after the charge is reset or drained. Implicit or explicit integration time control is provided row-wise or device-wide which decides if the capacitor is charging or lying idle. 
     In an embodiment, there may be a linear or non-linear relation between the sensed levels and the actual AC amplitude and phase, which is carefully calibrated. 
     In an embodiment, instead of measuring various voltages, various currents are measured. This measurement may be performed by measuring voltages across known resistors. The known resistors may be variable resistors as described below. 
     In an embodiment, the same electrodes that are used for sensing may also be used for setting up the AC current that moves through the body being sensed. The AC current may be setup as being between two specific electrodes, or more than two electrodes may participate in the process. For example, spatial patterns may be created such as the 2D Fourier basis (real part and imaginary part produced separately at separate times). In this way, current is not concentrated at a single electrode, but still the entire vector space of possible current settings is traversed. 
     In an embodiment, these spatial patterns are changed gradually (compared to the AC frequency itself), not abruptly. The sensing may be done as before in multiple frames, or the gradual shift in the spatial pattern may itself be used as additional modulation for the electrode measurement circuit to record more than two values per electrode, and then finally read everything out. 
     In an embodiment, many electrodes are provided in a line, 2D matrix, honeycomb arrangement etc. Each electrode is a rod with a hemispherical tip, with at least the tip being conductive. The tip and rod may have another shape such as a hexagonal prismatic rod with a filleted hexagonal cap, etc. These rods are individually spring loaded, meaning they can move in and out of a base on which they are mounted. When these rods are pressed against a body, they will conform to the contours of the body, and each will make electrical contact with the body. Measurement of how much each rod is pushed may be provided, e.g. using proximity sensors, measurements using rheostats (position dependent resistances), measurements using strain gauges, etc. The resistance measurements transducing how much each rod is pushed may be transmitted over the same lines as the electrode measurements are. Each rod tip may be broken into multiple electrodes, with 2, 3, 4, 6 side electrodes and zero or one center electrode. This will allow an accurate measurement of the local impedance seen by the electrode (contact impedance, skin impedance, etc.). 
     The 2D matrix or honeycomb of electrodes may be arranged in a flat arrangement which conforms to a certain extent to the body being measured. This flat arrangement may be placed horizontally as a bed for a patient or object to lie on, or as the backrest of a chair or a wall etc. More than one such panels may be used to surround the object being measured. The arrangement may not be flat but some other shape such as a cylinder or even a wearable vest or other piece of clothing. 
     In an embodiment, the contact between the electrodes and the body being measured is direct. In another embodiment, a conductive gel is placed on each electrode, or a single conducting gel is spread on the entire body. The tips of the electrodes may be disposable, or may be a single disposable sheet. 
     In an embodiment, dynamic resistance circuits (resistances that can be set to various values) are present at every electrode. A fixed AC voltage waveform is applied through this resistance to the body. Resistance values are known, and voltage amplitude and phase is measured. In another embodiment, dynamically set voltage patterns are applied to the body through fixed resistances. The voltages at the non-driven ends of the resistances (which are also the voltages seen by the electrodes) are sensed by the electrode circuits. From knowledge of the resistance and the voltages at its two ends (one being set and the other being measured by the system), the current through the electrode can be calculated. Thus both the current and voltage at every electrode is known. 
     In an embodiment, spatial patterns of electrode excitation may be set up by setting each electrode in one of a finitely many states. The states could be 
     (a) current drive +ve 
     (b) current drive −ve 
     (c) zero current (floating electrode) 
     (e) current drive +90 deg 
     (f) current drive −90 deg 
     (g) voltage drive +ve 
     (h) voltage drive −ve 
     (i) voltage drive ground 
     (j) voltage drive +90 deg 
     (k) voltage drive −90 deg 
     In various embodiments, various subsets of the above states may be employed. In particular, the following combinations to be provided in particular embodiments: 
     (g), (i) 
     (g), (i), (c) 
     (g), (h), (i) 
     (g), (h), (c) 
     (g), (h), (i), (c) 
     (g), (h), (i), (j), (k), (c) 
     Global AC voltage lines (+ve signal, −ve signal, +90 deg phase shifted, −90 deg phase shifted, ground) may be provided to each electrode circuit, and a multiplexer chooses one of these signals (or none, to float the electrode) to connect to the electrode that connects to the body to be measured. Furthermore, one or more than one of these signals may be passed through fixed or variable resistors to produce newer effective phases and amplitudes. The global lines may be voltage drives or current drives. In the case of current drives, the electrodes are still connected on a single current drive in parallel, thus producing the same voltage across all electrodes connected to a single current drive. The current drive helps to regulate the current flowing into the body, which may be important for regulating the amount of current entering the body being sensed, for reasons of safety. It is also possible that after creating a voltage signal using the above techniques at each electrode, a voltage-to-current converter (amplifier) is used so that each electrode is an independent current drive. This will ensure that safety regulations regarding current density are met. 
     In an embodiment the spatial excitation patterns which are set up are spatially periodic patterns. Some patterns have a smaller spatial period and others have a larger spatial period. The periodic pattern may comprise of a unit that is repeated, where the unit itself has some excitations surrounded by a region that is electrically floating. (The electrically floating region can still sense voltage). In such situations, each repeating unit may be approximately thought to produce a response that is independent of other units. Thus the sampling speed is improved. Smaller repeating units will provide information about shallower objects whereas larger repeating units will provide information of deeper objects. 
     In an embodiment, the modulating signal amplitude and phase changes due to unavoidable circuit reasons at various mixing sensor blocks. (Frequency does not change.) This variation in amplitude and phase across the system is carefully calibrated and then corrected for while the signal is being digitally processed. 
     INDUSTRIAL APPLICABILITY 
     Electrical impedance tomography may be used for medical or biological imaging (where the conductive body is the human/biological body, and information to be gained about the body includes information about internal organs, etc.), geophysical/ground surveys (where the conductive body is the Earth, and information to be gained about the body includes information about presence of water, oil, minerals, etc.) and non-destructive testing of machines, engineered bodies, civil constructions or materials (where the conductive body is the machine, civil structure/building, material sample etc., and information to be gained includes information about interior material presence, cracks, interior structure, etc.). It may also be used in applications such as archeology, forensic science, security, etc. where knowledge of the interior of a body without accessing the interior of the body is important.