Patent Publication Number: US-8526341-B2

Title: Systems and methods for microwave tomography

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/930,823 filed May 17, 2007, which is incorporated herein and made a part hereof in its entirety. 
    
    
     STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with a grant from the Government of the United States of America (Grant No. DK64659 from The National Institutes of Health). The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Tomography has revolutionized the way medical imaging has been done over the last few decades. The development of efficient and flexible wireless sensor networks creates wide variety of potentially new imaging modalities. One example technology is the IEEE 802.15.4 specification which provides the framework for an efficient wireless network. IEEE 802.15.4 emphasizes energy efficiency, flexibility and low cost of personal area networks (PAN) in sending data from one location to another. This and other wireless networks are designed and used for communication and analysis of data located in the payload of packets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of inventive subject matter may be best understood by referring to the following description and accompanying drawings, which illustrate such embodiments. In the drawings: 
         FIG. 1  is a block diagram of a wireless device for microwave tomography according to various embodiments; 
         FIG. 2  is a block diagram of a system for microwave tomography according to various embodiments; 
         FIG. 3  is a flow diagram illustrating a method for performing microwave tomography according to various embodiments; 
         FIGS. 4A and 4B  are block diagrams of wireless networks for microwave tomography according to example embodiments; 
         FIG. 5  is a diagram of an example single projection system according to an embodiment; 
         FIG. 6  is a flow diagram of a process for controller operation according to example embodiments; 
         FIG. 7  is a flow diagram of a process for remote device operation according to example embodiments; 
         FIG. 8  is a diagram of coordinator data output according to example embodiments; 
         FIG. 9  is a block diagram of a Fresnel zone according to various embodiments; 
         FIG. 10  is a diagram of example of a multiple projection system according to an embodiment; 
         FIG. 11  is a diagram of a system using a phase data gathering configuration according to various embodiments; and 
         FIG. 12  illustrates a computer system that executes programming for microwave tomography according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
       FIG. 1  is a block diagram of a wireless device  100  for microwave tomography according to various embodiments. The wireless device  100  includes a receiver  102 , and a packet analysis module  104 . The receiver  102  receives a packet over a wireless path  108 . The packet may include non-payload data which may be analyzed by the packet analysis module  104 . The term packet refers to any block of data that may be over a network which may include identities of sending and receiving nodes, and may be capable of carrying a payload. The non-payload data may include packet identifying data, source identification data, signal strength data and signal quality data, among other characteristic data. The non-payload data may be analyzed to determine the presence of a physical object  106  along the wireless path  108 . The wireless path may represent the straight-line path from a packet source to the receiver  102 . 
     The determination of a physical object  106  along the wireless path  108  may be done within the wireless device  100 , the packet analysis module  104 , or on an external computer or processor. 
       FIG. 2  is a block diagram of a system  200  for microwave tomography according to various embodiments. The system  200  includes the receiver  102 , the packet analysis module  104 , the physical object  106 , wireless transmitters  202 A- 202 C and associated wireless paths  204 A- 204 C. Similar to as described above with respect to  FIG. 1 , the receiver  102  may receive packets from transmitters  202 A-C. Non-payload information from the packets may be analyzed by the packet analysis module  104 . Using the non-payload information included with the packets from each transmitter  202 A-C, a tomographic analysis may be completed to take into account relative locations of the transmitters  202 A-C, as well as signal strength and quality information. This analysis may be used to determine of the presence of a physical object  106  along a wireless path  204 A-C. Additionally, the analysis may provide a spatial representation of the location and size of the physical object  106 . A tomographic analysis application may be used to compile the non-payload information from a number of packets (and transmitters  202 A-C) into data representing the position, size, shape, or other characteristics of the physical object  106 . This data may be in textual form or in image form as a tomogram. 
       FIG. 3  is a flow diagram illustrating a method  300  for performing microwave tomography according to various embodiments. The method  300  begins by receiving a packet (block  302 ). The packet may contain data, or it may have zero payload. The information not included in the payload of the packet may be analyzed (block  304 ). This information may include the source of the packet, signal quality and/or signal strength information. Data may be available to determine a relative location of the source of the packet. Using source location data and signal quality/strength data, a determination may be made regarding characteristics of interfering objects. An interfering object may be an object substantially along a straight-line path from the packet source to the packet receiver. The interfering object characteristics may include location, size, movement, reflectivity, and other characteristics. 
     By using non-payload data such as the received signal strength indication (RSSI) value of packets being received in the network with a known physical configuration, deflections in the RSSI value may indicate a physical obstruction in the network. In this way, the wireless network itself may be used as a dynamic sensor to collect data. A tomography application of a much larger scale than medical applications can be deployed on the scale of rooms, or larger depending on the wireless air interface used. The 802.15.4 direct sequence spread spectrum (DSSS) radios provide a means of filtering out noise. These radios are less susceptible to interference as they spread the signal over a band and also allow sharing of a single channel among many users. 
     In much the same way that a CT scan is done in a hospital to get a cross-sectional image of a human, a wireless data network can be used to obtain a cross-sectional image of a coverage area. Phantom objects may be discovered and identified by analyzing signal strength or quality values of transmitted packets. The ZigBee network protocol, for example, could be exploited for its ad hoc ability to autonomously create networks. In an embodiment, every remote node can be programmed with the exact same code and the network will configure itself. The number of nodes that can connect to the network is really practically limited by the amount of memory in a coordinator node. Possible applications include monitoring of fields, determining the contents of enclosed buildings/structures, security systems, and entertainment at venues. 
     The IEEE 802.15.4 standard is used as an example wireless system, but the present subject matter should not be taken to be limited to any particular wireless protocol or air interface. The previous and following examples which may be focused on 802.15.4 technology may include details which are specific to 802.15.4; however, it should be clear to one skilled in the art that the ideas may be equivalently applied to other wireless technologies as well. 
     The IEEE 802.15.4 standard was first approved in May 2003 to define a standard that would “provide a standard for ultra-low complexity, ultra-low cost, ultra-low power consumption, and low data rate wireless connectivity among inexpensive devices.” (IEEE std. 802.15.4-2003: Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications for Low Rate Wireless Personal Area Networks (LR_WPANs) 
     The 802.15.4 standard primarily outlines the PHY and MAC layers, but offers much in terms of guidelines for network layers and possible software architectures. The data rate for 802.15.4 varies depending on what range of frequencies a device is operating on. For the 2.4 GHz band, the raw data rate is 250 kb/s, 915 MHz data rate is 40 kb/s, and 868 MHz is 20 kb/s. 
     The 802.15.4 standard defines two types of devices that can participate in a network: a full-function device (FFD) and reduced-function devices (RFD). FFDs can serve as personal area network (PAN) coordinators. A reduced-function device (RFDs) is a device with a minimal implementation of the IEEE 802.15.4 protocol. The standard only allows RFDs to communicate with FFDs. The 802.15.4 specification defines two types of topologies of networks that can be formed using the standard: star and peer-to-peer. Both types require at least one FFD to serve as the PAN coordinator. 
       FIGS. 4A and 4B  are block diagrams of wireless networks  400  and  410  for microwave tomography according to example embodiments. Wireless network  400  represents a star network example, while wireless network  410  represents a peer to peer network. The wireless networks  400  and  410  include a coordinator  402 , FFDs  404  and RFDs  406 . 
     In the star network, a single, central controller talks with all devices  404  and  406  of the wireless network  400  directly. All of the other devices  404  and  406  are only allowed to communicate with other nodes via the coordinator  402 . All messages passed in the star network may be required to go through the coordinator  402 . 
     The peer to peer network also may have a single coordinator  402 ; however, FFDs  404  are allowed to communicate amongst themselves without routing messages through the coordinator  402 . This allows for more complex network setups where multiple hops can be used to deliver messages among nodes more efficiently. 
     By collecting RSSI information from several RFDs  406  lined up opposite of a coordinator  402 , several tomographic projections may be taken. Gathering information from the projections permit piecing together what and where physical objects may be present within the boundaries of the network. 
       FIG. 5  is a diagram of an example single projection system  500  according to an embodiment. The single projection system  500  includes a coordinator  502 , wireless devices  504  and a physical object  506 . 
     Multiple projections are collected by either by having multiple wireless devices  504  present around the coordinator  502 , or by moving the wireless devices  504  around the coordinator in spaced increments. This implementation allows for the coordinator  502  to receive varying RSSI values across multiple wireless paths. With the RSSI values and the path data, location information regarding the physical object  506  may be calculated. According to various embodiments, the coordinator  502  may collect and format the RSSI values along with other source identifying data to be communicated to an external computer or processor for tomographic processing. 
       FIG. 6  is a flow diagram of a process  600  for controller operation according to example embodiments. The process  600  starts with a coordinator powering up and setting up a network (block  602 ). The network may be set up on a clear channel using a unique ID. In an example embodiment, the unique ID may be a PAN ID where the coordinator is operating on a PAN. The coordinator may wait for remote devices to request to join the network (block  604 ). The coordinator may receive data packets from remote devices. As the data packets arrive, the coordinator may determine if the packet includes a join request (block  606 ). If the packet from a remote device includes a join request, the coordinator may assign the remote device a network address, and may also add that address to a neighbor table (block  612 ). 
     If the packet received by the coordinator does not include a join request, the coordinator may determine whether the packet includes a message request (block  608 ). If the packet includes a message request, the coordinator may then send any pending messages for the remote device (block  614 ). If the received packet does not include a message request, the coordinator may determine if the packet includes a message indication (block  610 ). If the packet does include a message indication, the coordinator may then receive the indicator from the message (block  616 ). The indicator may be RSSI, LQI, another similar indicator or a combination thereof. Upon receiving indicator, the coordinator may output a message summary (block  618 ). The output message summary may be analyzed and included in the production of a tomogram. 
       FIG. 7  is a flow diagram of a process  700  for remote device operation according to example embodiments. The process  700  may start with a remote device powering up (block  702 ) and searching for coordinators with which to join and network (block  704 ). If no coordinator is found (block  706 ), the remote device may continue searching for coordinators (block  704 ). Once a coordinator is found, the remote device may issue a join request (block  708 ). Upon being allowed to join with a coordinator, the remote device may receive a network address from the coordinator (block  710 ). The remote device may enter a sleep or idle mode once networked (block  712 ). The remote device may enter this idle state where it goes to sleep and occasionally checks for messages from the coordinator. Once awake, the remote device may start transmitting packets to the coordinator. In an example embodiment, the remote device may continue to transmit packets at a predetermined rate, possibly on the order of once every 0.5 seconds until it is powered down or reset. 
     Upon waking (block  714 ), the remote device may transmit a request for messages to the coordinator (block  716 ). If there is a message to process (block  720 ), the remote device may process the message (block  722 ), and then send a message for indicator processing (block  718 ). The indicator may be RSSI or LQI for example. If there is no message to process from the coordinator (block  720 ), the remote device may also send a message for indicator processing (block  718 ). 
       FIG. 8  is a diagram of coordinator data output  800  according to example embodiments. The data output  800  includes a source node address  802 , a packet identification (“ID”)  804 , a RSSI value  806  and a LQI value  808 . 
     A coordinator may receive packets from remote devices as source nodes. The payload of the packets transmitted is not a concern as the source node address  802 , packet ID  804 , RSSI value  806 , and LQI value  808  are of interest. These variables may be found in the PHY and MAC layers. Each remote device may fill the payload with data of some sort, whether it is a regular transmission or dummy data. The remote device may address the packets to an endpoint on the coordinator. The endpoint on the coordinator may make function calls on the stack to retrieve the variables of interest from the lower levels of the stack regarding the message received. 
     The coordinator may then report that it received a message in the format of the data output  800 . As an example configuration, “NNNN” is a short address  802  of the associated source node in hex (two bytes), PPP is the packet ID  804  number from the source node, RRR is the RSSI value  806  for that particular packet, and LLL is the LQI value  808  of the packet received. The LQI data may be merely collected at this point and may not necessarily be used in the creation of tomographic images. The example in  FIG. 8  shows that node  796 F sent packet ID  129  with an RSSI value of −53 dBm and LQI value of  204 . 
       FIG. 9  is a block diagram of Fresnel zones  900  according to various embodiments.  FIG. 9  includes a transmitter  902 , a receiver  904 , a first Fresnel zone F 1    906 , a second Fresnel zone F 2    908  and a surface  910 . Determining the placement of nodes with respect to known obstacles or surfaces  910  such as the ground may be useful in accurately measuring signal strength. Appropriate clearance may assure that the RSSI value reported by a coordinator is not stronger or weaker that the straight-line path signal strength. By calculating the Fresnel zone distances  906  and  908 , sufficient clearance of obstacles and surfaces for any two nodes participating in the network may be determined. The Fresnel zone expression for this geometry is: 
     
       
         
           
             
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     From this expression, example Fresnel Zone calculations may produce F 1 =0.3904 m, and F 2 =0.5521 m. Since the strongest signals lie within the first two Fresnel zones  906  and  908 , we need only be concerned with interference from multi-path along these. To stay clear of the second Fresnel zone, a distance of 0.5521 meters minimum must be clear. Using these numbers, placing the nodes at a height of above 0.5521 meters above the surface  910  they will be clear of any noticeable multipath due to ground reflection. 
       FIG. 10  is a diagram of example of a multiple projection system  1000  according to an embodiment. The system  1000  includes multiple projection points  1002 , an output image  1006  and a phantom  1004 . The projection points  1002  may include a number of points in a rotation around a physical object. In an example, 36 projection points  1002  may be used at 10 degrees of rotation each. Remote devices may be arranged upon one or more of the projection points  1002 , with coordinators generally arranged opposite each remote device. Wireless transmissions or projections between the remote devices and the coordinators may then be used to determine the presence of a physical object within the network boundary. The capture of RSSI data from any one projection may be averaged. Once all the data is gathered, the data may be inverted using computed tomographic algorithms to generate an output image  1006  or images of the physical space the network covers. Within an output image  1006 , a phantom  1004  may exist identifying a physical object within the network boundary. 
     In an example embodiment, using 802.15.4/ZigBee technology, Microchip PICDEM Z boards may be used. The PICDEM Z supports a 28- or 40-pin DIP microcontroller. A list of built-in peripherals include a R S -232 DB-9 connector, three momentary push buttons, LEDs, an in circuit serial programmer (ICSP), 9V battery clip, and an external power supply jack. 
     Ten PICDEM Z nodes may be utilized for a test conducted to take advantage of their pre-packaged 802.15.4 ready radio and software stack. Nine may serve as RFDs and one as the coordinator in a star topology network. There are several components to the PICDEM Z boards, including the wireless nodes themselves. The radio for each node is a Chipcon CC2420, ZigBee-certified transceiver which is integrated on a printed circuit board (PCB) with an inverted F-type PCB antenna. 
     The motherboards that the radios connect to are run with a Microchip PIC18LF4620 microcontroller that is clocked at 4 MHz. The controller effectively runs at 16 MHz after the 4× phase-locked loop (PLL) is engaged. This may be the highest speed possible with the oscillator included on each PICDEM Z board. The motherboard provides power to everything with a 9V DC to 3.3V DC voltage regulator rated at 100 mA. 
     The PICDEM Z boards also provide a RJ-11 connection for Microchip&#39;s in-circuit debugger (ICD2) so that projects created in MPLAB IDE can easily be programmed onto he microcontroller in a matter of seconds. This also frees the application developer from having to remove the microcontroller from the PICDEM Z every time a node&#39;s firmware needs to be updated. 
     Code for the microcontroller may be written using the MPLAB Integrated Development Environment (IDE). The IDE also provides a software interface with an ICD2 device which connects to a PICDEM Z board. 
     The ZigBee software stack uses the Microchip&#39;s C18 compiler to create a hex file that is programmed onto the microcontroller. Note that compiler optimizations should to be enabled so that the Coordinator code can fit onto the microcontroller. 
     Microchip provides a software stack of the ZigBee protocol which is designed to run on most of Microchip&#39;s 18F series microcontrollers and is written in the C programming language. 
     When the appropriate files from the stack are included, a programmer may focus on the application layer file which makes calls on the ZigBee protocol functions. The stack may handle everything except primitives that come back to the application-level in which code may be used to handle those primitives. 
     The PIC18LF4620 is a 40-pin DIP package microcontroller produced by Microchip. It is nearly identical to the PIC18F4620 other than this microcontroller has had its integrated electronics designed to operate over the range of 2.0V to 5.5V rather than 4.2V to 5.5V to reduce power consumption. Key characteristics include 64Kbytes of Flash program memory and 3986 bytes of data memory. There are four timers, one 8-bit timer (TMR 2 ) and 3 16-it timers (TMR 0 , TMR 1 , TMR 3 ). The stack uses TMR 0 , while the other timers are free. 
     The CC2420 is an IEEE 802.15.4 compliant RF transceiver manufactured by Chipcon/TI and is the radio used with the PICDEM Z boards. The CC2420 can be found on a daughterboard manufactured by Microchip which plugs into the motherboard with the microcontroller. The communication between the CC2420 and PIC18LF4620 may be done using 4-wire serial peripheral interface (SPI) (SDO, SDI, SCLK, CSn). In this configuration, the CC2420 acts as a slave and the PIC18LF4620 is the master. The CC2420 is suitable for both RFD and FFD operation since the determination of what role each node plays is in the code on the microcontroller. The CC2420 RF output power is controllable with a register named TXCTRL.PA_LEVEL. There are eight levels of output ranging from 0 dBm to −25 dBm. 
     The CC2420 provides 8-bit resolution of the RSSI value. The digital value is in 2&#39;s complement (−128 to +127) and has a dynamic range of approximately 100 dB from 0 dBm to −100 dBm. The accuracy of the RSSI value is +/−6 dB. The raw RSSI value given by the CC2420 then needs to have an offset value added to the raw RSSI value to obtain actual RF input power. In an embodiment, −45 as the offset may be used to add to this value to obtain the actual RSSI. For example, if the value read from the CC2420 is −20, the RF input power is then calculated to be −65 dBm. 
     A frame check sequence (FCS) may be automatically generated and verified by the hardware onboard the CC2420. An option exists to disable the automatic detection and the cyclic redundancy check (CRC) would then need to be performed on the microcontroller firmware using the FCS polynomial stated by the IEEE 802.15.4 standard. For this example, the default option of having the FCS checked by the CC2420 is chosen to free up resources on the microcontroller. 
     The CC420 also has the capability to be used as not only a DSSS radio, but a Frequency Hopping Spread Spectrum (FHSS). The trade-off is a system that is not 802.15.4 compliant and a MAC layer that would need to be customized so that all the radios are synchronized to operating on the same channel at the same time. 
     Using MPLAB and the ZigBee stack, a straight forward application may be developed. With reference to  FIGS. 6 and 7 , a coordinator first establishes a network on a clear channel. Afterwards, each RFD can be activated and automatically search for and connect to the coordinator. At this point the coordinator may assign a 16-bit network (NWK) address to the joining node. In an example, in order to decrease network formation and discovery time, every node may be forced to work on channel  15  and transmit at full power (0 dBm). 
     The RFD may enter an idle state where it goes to sleep and occasionally checks for messages from the coordinator. Once a button on the RFD motherboard is pressed, an external interrupt is triggered and sets a flag to let the RFD know it is time to start transmitting packets to the coordinator. The RFD will continue to transmit packets at a pre-determined rate, currently set at once every 0.5 seconds until it is powered down or reset. This is done by using TMR 1  on the PIC18LF4620 and counting the number of times it interrupts which equates to 0.5 seconds at the operating clock frequency. 
     A computer connected to the coordinator may be equipped with a graphical user interface (GUI) that was written to interface with the described system. The GUI may provide tomographic imagery from stored data or in real time illustrating measured areas of the network. The GUI may also provide a means of quickly analyzing and troubleshooting the network for data anomalies and connectivity of the nodes. There may also be a means for recording data to a file while running tests. 
     The GUI may include several text fields and bar graphs. Each bar graph corresponds to an RFD in the network that is connected to the coordinator. If an RFD is transmitting packets, the RSSI value received from the RFD is reported to the GUI and displayed graphically. Timers are used to keep track of how recently a packet has been received from a node. If an extended period of time has passed (set at 2.0 seconds) since an RFD last sent a packet, the GUI may display “- - - -” for that particular RFD and clear it&#39;s RSSI bar graph. This allows quick determination of what nodes are connected and actively transmitting to the coordinator. 
     One button on the GUI may allow a user to take samples of the RSSI data from all of the transmitting nodes and average the RSSI value seen over the next 16 transmissions from each RFD. This feature may help calibrate each RFD and provide a way to graphically display the RSSI in an intuitive fashion. 
     Another button located on the GUI enables a user to collect data from the time the button is first clicked to when it is clicked again. All of the raw data sent to the GUI from the coordinator can then be saved as a .txt file on the computer that the GUI is running. In addition to the raw data that is saved, if samples had been taken for the configuration, the averages are saved at the top of the file. This feature provides a method of preserving the data for future analysis. 
     The methods and systems described herein may additionally include additional nodes, varying frequencies of wireless transmission, varying positioning of nodes with respect to each other or objects, and varying power levels. The use of different antennas may also alter the accuracy of the devices. Reduced node size would allow for practical use in embedding at regular intervals in the walls of buildings where the power could then come from flat conductors within the wall. 
     The spatial resolution may be limited by the wavelength of the electromagnetic radiation used. This is approximately 12.5 cm with 802.15.4 technology. The actual spatial resolution of the system may also be affected by the number and spacing of the RFD nodes. Actual resolution can be increased by increasing the number of RFD nodes. 
     The imaging rate may be limited by the packet transmission rate of the network. Transmitting packets containing zero bytes in the payload of the packet (zero payload is the minimum packet size permitted by the standard), the minimum packet length required would be 16 bytes. Assuming use of the 802.15.4 specification&#39;s maximal data rate of 250 kbps, and assuming a 9 RFD node configuration (as an example implementation), it would be possible to acquire one image every 4.6 mS. Further reduction in the acquisition rate may be expected due to dropped packets, inter-nodal clock jitter, and other system nonidealities. 
     A non-linear configuration of the RFDs would utilize more robust tomographic algorithms, but provides for more flexibility in application uses. It may be possible to accurately recreate tomographic images with irregular geometries of sensors. The capability of deploying RFDs in non-uniform orientations is useful in many applications. 
     Wherever there is a wireless network, the framework is already in place to piggyback a tomographic application to gather cross-sectional data about objects that may lie within its boundary. 
     A possible life-saving application example, according to an embodiment, includes the use of the present subject matter in difficult to reach places of buildings, particularly skyscrapers. By embedding a compliant radio in the walls of each floor of the building at known locations, cross-sectional images of each floor of a building could be generated. During the event of an emergency, such as a fire or earthquake, the data from the network could be used to help rescuers determine where to focus their efforts. A wireless sensor network has the benefit of not being deterred by smoke and flames that IR sensors and video cameras would encounter. 
     The simplicity and power-saving properties of 802.15.4 may be implemented to pass periodic, small packets among nodes that are dispersed over large and remote areas, such as farm fields or forests. These nodes may also then be used to measure air moisture and precipitation that falls within a region by utilizing the tomography aspect of the microwaves in the network. The results from the tomography analysis could then be compared to radar and satellite estimates. This will allow meteorologist to improve their tools and predictions. The ability to measure rain over a large area rather than at discrete locations where rain gauges are located is a highly sought-after tool in hydrology. 
       FIG. 11  is a diagram of a system  1100  using a phase data gathering configuration according to various embodiments. The system  1100  includes a coordinator  1102 , a first row or remote devices  1104 , a second row of remote devices, and a microwave diffracting object  1108 . Using diffraction tomography (DT) algorithms, phase, as well as signal intensity of transmitted packets among nodes (remote devices and coordinators) in the network may be needed. It is possible to obtain phase data in such a network using a second row of remote devices  1106  placed behind the first row of remote devices  1104  at a known distance, Δ. With two sets of intensity data and known distances for each projection calculating phase is possible. 
       FIG. 12  illustrates a computer system that executes programming for microwave tomography according to various embodiments. A general computing device  1210  may include a processing unit  1202 , memory  1204 , and storage  1212 . Computer-readable instructions stored on a computer-readable medium are executable by the processing unit  1202  of the computing device  1210 . A hard drive, CD-ROM, and RAM are some examples of articles including a computer-readable medium. Instructions for implementing any of the above described methods and processes may be stored on any of the computer readable media for execution by the processing unit  1202 . The memory  1204  may include volatile memory  1206  and/or non-volatile memory  1208 . Additionally, the memory  1204  may include program data  1214  which may be used in the execution of various processes. Storage for the computing device may include random access memory (RAM), read only memory (ROM), erasable programmable read-only memory (EPROM) &amp; electrically erasable programmable read-only memory (EEPROM), flash memory, one or more registers, or other memory technologies, compact disc read-only memory (CD ROM), Digital Versatile Disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium capable of storing computer-readable instructions. 
     The computing device  1210  may include or have access to a computing environment that may include an input  1216 , an output  1218 , and communication connections  1220 . The computing device  1210  may operate in a networked environment using a communication connection to connect to one or more other computing devices, processors or other circuits. In some embodiments, the computing device  1210  may determine signal strength or quality data from received wireless packets. The computing device  1210  may additionally determine tomographic information based on the signal strength or quality data from the received wireless packets. 
     The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.