Abstract:
A method of imaging biological tissue enables imaging structure in the biological tissue with an antenna tuned to emit radio energy through at least a covering layer of the biological tissue without significant attenuation of the radio energy. The method includes contacting skin covering tissue with a radio frequency emitting antenna, and adjusting at least one of a radiator and a feed in the antenna with reference to at least one measured electrical or physical property of the skin. The adjustment of the radiator and feed enable a combination of the antenna and skin to emit radio energy at a predetermined magnitude and frequency into a portion of the tissue covered by the skin.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from U.S. Provisional Application No. 61/319,715, which is entitled “WEARABLE MICROSTRIP ANTENNAS FOR SKIN PLACEMENT FOR BIOMEDICAL APPLICATIONS” and was filed on Mar. 31, 2010. 
     
    
     TECHNICAL FIELD 
       [0002]    The system and method described below relate to the detection and imaging of subdermal structures and growths in a living organism, in particular a human being. 
       BACKGROUND 
       [0003]    The field of medical imaging includes many well-known techniques for subdermal imaging of tissues and structures beneath the skin of a patient in a non-invasive manner. These imaging techniques commonly emit a form of electromagnetic energy that penetrates one or more tissue layers of the patient. Common examples of existing medical imaging techniques include X-Rays, positron emission tomography (PET) scans, and ultrasounds. A more recently developed medical imaging technique transmits microwave energy into a patient&#39;s body. This technique detects structures within the patient&#39;s body by detecting the reflections or backscatter occurring when the microwaves encounter a structure that reflects the microwave energy. 
         [0004]    Subdermal imaging has many possible medical applications, and one application of immediate importance is the detection of growths and tumors in breast tissue. Breast cancer is a serious disease that kills tens of thousands of women every year in the United States alone. The use of X-Ray mammography has helped to detect breast tumors earlier, allowing early treatment that greatly improves the survival rate. However, X-Rays are high-energy ionizing radiation known to promote the development of cancers in human tissue. Thus, there is an undesirable tradeoff between not testing breast tissue for cancer and risking an undetected cancerous growth, or testing more often and increasing the risk that the testing technique itself will cause cancer to develop. Other imaging techniques that avoid the use of ionizing radiation such as an ultrasound may be much less effective at detecting tumors in the breast tissue. 
         [0005]    Microwave imaging techniques present many advantages over other imaging techniques for detecting cancerous tumors, especially tumors in breast tissue. Unlike the energy used in an X-Ray, microwave energy is non-ionizing when penetrating human tissue. The non-ionizing radiation does not pose the cancer risk of the ionizing X-Ray radiation. Further, tumors in breast tissue are known to have a significant and consistent contrast to the microwaves in comparison to surrounding healthy breast tissue. The significant contrast characteristic means that a microwave that passes through healthy breast tissue also reflects off the surface of a tumor. By detecting the backscatter pattern of the reflected microwave energy, the tumor can be detected and imaged non-invasively. 
         [0006]    While microwave imaging techniques have many advantages, the structure of the human body presents difficulties in using microwave imaging devices to effectively identify and image tumors. One such problem is that the human body has many layers of heterogeneous tissues, with the skin being the outermost layer. As stated above, microwave energy passes through the healthy breast tissue and reflects off the surface of cancerous growths. However, the microwaves also tend to reflect off the surface of human skin, which is often referred to as the “skin reflection.” The skin reflection causes noise in the reflected microwave signals, potentially obscuring reflections that may come from a tumor inside the patient&#39;s body from reflections are merely from the patient&#39;s skin. A system for microwave imaging that reduces noise introduced by the skin reflection benefits the fields of biomedical imaging and oncology. 
       SUMMARY 
       [0007]    A method of imaging biological tissue enables imaging structure in the biological tissue with an antenna tuned to emit radio energy through at least a covering layer of the biological tissue without significant attenuation of the radio energy. The method includes contacting skin covering tissue with a radio frequency emitting antenna, and adjusting at least one of a radiator and a feed in the antenna with reference to at least one measured electrical or physical property of the skin. The adjustment of the radiator and feed enabling a combination of the antenna and skin to emit radio energy at a predetermined magnitude and frequency into a portion of the tissue covered by the skin. 
         [0008]    A system for imaging biological tissue has been developed. The system includes a radio signal generator, a radio signal receiver, at least one antenna electrically connected to the radio signal generator and radio signal receiver, and an imaging device communicatively coupled to the radio signal receiver. The at least one antenna is configured to couple with skin tissue to enable an antenna/skin tissue combination to radiate energy at a predetermined magnitude and frequency into at least one other layer of biological tissue covered by the skin tissue. The imaging device is configured to generate a two-dimensional or three-dimensional display of the biological tissue based on backscatter energy received from the biological tissue by the radio signal receiver. 
         [0009]    An antenna system has been developed for the biological tissue imaging system. The antenna system includes a plurality of antennas, a substrate measurement device, and a selector. Each antenna in the plurality of antennas is configured to couple to a substrate layer primarily composed of biological tissue having at least one predetermined electrical property, and to enable an antenna/substrate combination to have an input impedance that enables radio energy at a predetermined range of frequencies to radiate from the antenna/substrate layer combination. One antenna in the plurality of antennas has a configuration that enables the one antenna and the substrate layer to emit radio energy through tissue covered by the substrate. The substrate measurement device is configured to identify the at least one predetermined electrical property of the substrate. The selector is configured to select the one antenna from the plurality of antennas for coupling with the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  is a perspective view of a patch antenna suitable for transmitting microwave energy when placed in contact with skin. 
           [0011]      FIG. 1B  is a partially exploded perspective view of another embodiment of a patch antenna suitable for transmitting microwave energy when placed in contact with skin. 
           [0012]      FIG. 1C  is a perspective view of still another patch antenna suitable for transmitting microwave energy when placed in contact with skin. 
           [0013]      FIG. 2A  is an illustration of a model of a human breast with the antenna of  FIG. 1  placed in contact with the breast. 
           [0014]      FIG. 2B  is an illustration of the model of the human breast from  FIG. 2A  with a measurement device electrically connected to skin on the model. 
           [0015]      FIG. 3A  is a perspective view of an antenna such as the antennas in  FIG. 1A-FIG .  1 C that is positioned to transmit microwave energy through layers of breast tissue. 
           [0016]      FIG. 3B  is a perspective view of an antenna such as the antennas in  FIG. 1A-FIG .  1 C that is covered by a signal enhancing and/or comforting material and is configured to transmit microwave energy through layers of breast tissue. 
           [0017]      FIG. 4A  is an illustration of a model of a human breast with the antenna of  FIG. 1  showing the current density pattern of microwave energy in the skin. 
           [0018]      FIG. 4B  is an illustration of the model and antenna of  FIG. 4A  showing the current density pattern of microwave energy in a tumor. 
           [0019]      FIG. 5  is a flow diagram of a method for adjusting the antenna of  FIG. 1  to operate at a desired frequency when placed in contact with human skin. 
           [0020]      FIG. 6  is a schematic view that depicts four antennas positioned about a breast and regions within the breast tissue. 
           [0021]      FIG. 7  is a perspective view of an arrangement of a patch antenna arranged in a wearable garment adapted to be worn on a breast. 
           [0022]      FIG. 8  is a cross-sectional view of structural elements in a wearable garment including antennas. 
           [0023]      FIG. 9  is an illustration of a model of a human breast with a prior art antenna showing the current density pattern of microwave energy in the skin. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Medical imaging devices generate images corresponding to various types of biological tissue. Such tissue includes, but is not limited to, skin, bones, muscles, adipose, cartilage, tendons, cysts, growths, and tumors. The tissue may belong to an animal including humans, or to plants and other organisms. The imaging may be performed on biological tissue of a living or deceased organism. The term “skin” as used herein may refer to human skin or the skin of an animal. Skin may also be more generic, referring to the outermost layer of biological tissue of an organism that undergoing medical imaging. 
         [0025]    As used herein, the term “radiator” refers to a structure in an antenna that emits electromagnetic energy when an electrical signal is applied to the radiator. The term “feed” refers to an electrically conductive structure in an antenna that couples a source of an electrical signal, such as a signal generator providing an electrical signal through an electrically conductive wire, to the radiator. As used herein, the term “via” refers to any electrical conductor that establishes an electrical connection between two different electrical conductors that are positioned approximately parallel to one another. 
         [0026]    Patch antennas configured to transmit microwave energy through human skin are depicted in  FIG. 1A ,  FIG. 1B , and  FIG. 1C . In  FIG. 1A  antenna  100  is a rectangular microstrip antenna including a radiator  108  of length  116  and width  120 , feed  110 , and adhesive backing  104 . In the example of  FIG. 1A , antenna  100  is primarily composed of a flexible, copper-clad material such as Pyralux. The copper surface of radiator  108  is exposed in order to be placed in contact with the surface of the skin of a test subject. Feed  110  provides an interface for connection of lead wires that electrically connect antenna  100  to a microwave signal generator (not shown). Adhesive backing  104  provides an adhesive area that adheres to the skin of a patient, maintaining contact between radiator  108  and the skin. In one embodiment, the radiator  108  bends into a curved shape when adhered to the skin to conform to the shape of the breast during operation. When adhered to the skin, antenna  100  is held in a stable position relative to the underlying tissue being imaged. 
         [0027]    Antenna  100  is tuned to resonate at a desired frequency and impedance when placed in contact with skin. In the example of  FIG. 1A , antenna  100  is tuned to a primary frequency of 2.45 GHz with a 50 Ωimpedance when it is placed in contact with human skin. While this frequency and impedance setting is an example of a configuration found to be effective in microwave imaging of human breast tissue, alternative frequencies and impedances are be employed in different imaging system embodiments. Because human skin has electrical properties including conductance and a dielectric coefficient, antenna  100  does not operate at the tuned frequency if it transmits microwave energy when separated from the surface of the skin. A method for adjusting antenna  100  to operate at the desired frequency and impedance when in contact with skin is described in more detail below. Once tuned, antenna  100  is coupled to a microwave signal generator by feed  110 , and antenna  100  is able to emit a continuous microwave signal, or modulated microwave signals generated by the microwave signal generator. 
         [0028]      FIG. 1B  depicts another embodiment of a microstrip antenna  124  including a radiator  140  having a length  148  and width  152 , feed  142 , an electrically non-conductive adhesive backing  128 , a slot  132  formed through the radiator  140 , and another slot  136  formed through an electrical ground layer  144 . Slots  132  and  136  each have a length and width that are adjusted to tune the antenna  124 . In the example of  FIG. 1B , antenna  124  is primarily composed of a flexible, copper-clad material, such as Pyralux. The copper surface of radiator  140  is exposed in order to be placed in contact with the surface of the skin of a test subject. Feed  142  provides an interface for connection of lead wires that electrically connect antenna  124  to a microwave signal generator (not shown). Adhesive backing  128  provides an adhesive area that adheres to the skin of a patient, maintaining contact between radiator  140  and the skin. The adhesive backing  128  is formed from an electrically non-conductive material that separates the radiator  140  from the ground layer  144 . 
         [0029]    As described in more detail below, antenna  124  is tuned to a selected primary frequency and impedance. In the example of  FIG. 1B , antenna  124  is tuned to a primary frequency of 2.45 GHz with a 50Ω impedance when it is placed in contact with human skin. In one embodiment, the radiator  140  bends into a curved shape when adhered to the skin to conform to the shape of the breast during operation. When adhered to the skin using adhesive backing  128 , antenna  124  is held in a stable position relative to the underlying tissue being imaged. 
         [0030]      FIG. 1C  depicts a third a microstrip antenna embodiment  160 . The microstrip antenna  160  includes a radiator  168  having a feed  170 , electrically non-conductive adhesive backing  164 , electrical ground layer  180 , an electrically conductive top layer  176 , dielectric layer  172 , and an electrical short  184 . In the example of  FIG. 1C , antenna  160  is primarily composed of a flexible, copper-clad material, such as Pyralux. The radiator  168  has a shape similar to radiators  108  and  140  as depicted in  FIG. 1A  and  FIG. 1B , respectively. The top conductor  176  has a length  188 , a width  192 , and includes a copper surface  178 . The copper surface  178  is exposed in order to be placed in contact with the surface of the skin of a test subject. Electrical short  184  is depicted as a via that establishes an electrical connection between the top conductive layer  176  and the ground  180 . Thus, the top conductive layer  176  is also an electrical ground. Feed  170  provides an interface for connection of lead wires that electrically connects the radiator  168  to a microwave signal generator (not shown). 
         [0031]    As described in more detail below, antenna  160  is tuned to a selected primary frequency and impedance. In the example of  FIG. 1C , antenna  160  is tuned to a primary frequency of 2.45 GHz with a 50Ω impedance when it is placed in contact with human skin. In one embodiment, the antenna  160  bends into a curved shape when adhered to the skin to conform to the shape of the breast during operation. When adhered to the skin using adhesive backing  164 , the antenna  160  is held in a stable position relative to the underlying tissue being imaged. 
         [0032]    While the embodiments of  FIG. 1A-FIG .  1 C depict antenna configurations having rectangular radiators, various different antenna configurations include radiators having different shapes including ellipsoidal, curved, and polygonal shapes, as well as ground layers and dielectric layers having different shapes. The sizes, shapes, positions, and selected materials used in the various structures in each of the antennas  100 ,  124 , and  160  are selected to tune the antennas to a selected resonant frequency, impedance, and radiation pattern for tissue imaging. 
         [0033]      FIG. 2A  depicts the patch antenna of  FIG. 1A ,  FIG. 1B , or  FIG. 1C  in contact with the surface of model of a human breast.  FIG. 2A  shows a model  204  of a human breast in a conical form that is 10 cm in width and 5 cm in height with sides divided into eight facets. Antenna  208  is a patch antenna tuned to transmit microwave energy when placed in contact with the surface of model  204 . The radiator of antenna  208  is placed in contact with the surface of model  204 . The interior of model  204  includes an example of a tumor  212  that is modeled as a sphere having a diameter of 1 cm. The outer layer of model  204  includes a skin that is 2 mm in thickness. 
         [0034]    The antenna  208  is electrically connected to a radio frequency (RF) transceiver  220  through an electrically conductive cable or wire  218  that is connected to antenna feed  216  and the RF transceiver  220 . The RF transceiver  220  includes a transmitter that generates electrical signals for the antenna  208  to emit as electromagnetic signals, and a receiver that detects electrical signals that correspond to radio frequency signals received by the antenna  208 . The signal generator in the RF transceiver  220  has an output impedance that corresponds to the input impedance of a combination of antenna  208  and the skin on the model  204  when the antenna  208  is coupled to the model  204 . In the example of  FIG. 2A , the RF transceiver  220  generates and detects RF signals in the microwave band, although alternative RF transceivers send and receive in different frequency ranges. In another embodiment, a separate RF transmitter and RF receiver are electrically connected to the antenna  208 . While  FIG. 2A  depicts a single antenna connected to the RF transceiver  220 , two or more antennas are placed in contact with the breast  204  at different locations and are connected to the RF transceiver in alternative configurations. 
         [0035]    When antenna  208  is positioned in contact with the skin covering the breast tissue and tumor, microwave energy emitted through antenna  208  penetrates the skin and travels through surrounding breast tissue until reaching the surface of tumor  212 . Tumor  212  causes some of the microwave energy to scatter. A portion of this scattered microwave energy returns through the surface of model  204 . The antenna  208  and transceiver  220  detect the scattered microwave energy. A signal processor  224  is configured to generate a visual depiction of the tissue in the breast  204  and tumor  212 . In some configurations, the signal processor  224  also generates command signals to operate the RF transceiver  220 . Signal processor  224  includes a processor such as a central processing unit (CPU), graphical processing unit (GPU), digital signal process (DSP) or other suitable components and computer programs that enable the signal processor to generate two-dimensional and three-dimensional visualizations of the breast tissue  204  and tumor  212 . A display  228 , such as an LCD display or the like, is connected to the signal processor  224  and displays the two or three dimensional visualizations generated by the signal processor  224 . 
         [0036]    A cross-sectional view of breast tissue through which the microwave energy travels is seen in  FIG. 3A , with antenna  304  placed in contact with a first skin layer  308 . The first skin layer  308  acts as a substrate with the antenna  304 , and the coupled skin layer  308  and antenna  304  radiate radio energy into the breast tissue  312 . Healthy breast tissue  312  is composed of fatty tissue, which may contain one or more structures, such as muscles, glands, cysts or tumors, that separate layer  308  from a second skin layer  316 . 
         [0037]      FIG. 3B , depicts an alternate configuration for coupling antenna  304  to the breast skin  308  through a layer of signal enhancing or comforting material  318 . The signal enhancing or comfort material  318  provides comfort to the patient wearing the antenna  304 , while also having selected electrical properties that are similar to the breast skin  308  to enhance propagation of radio signals into the breast tissue  312 . Signal enhancing or comforting materials have electrical properties that are similar to the skin layer  308 , such as a dielectric permittivity of approximately 40. In some configurations, the signal enhancing materials also have an electrical conductivity that is similar to the skin layer  308 , such as a conductivity of approximately 1.1 S/m. Materials that are suitable to provide signal enhancement and comfort include polymers, silicone, flexible glass fibers, flexible ceramic fibers, cloths, and foams, such as Polyurethane foam. Various embodiments of signal enhancing or comforting material  318  are in the form of a tape or adhesive material to enhance contact with the skin layer  308 . The signal enhancing or comforting material  318  conforms to the shape of a breast or other portion of the body to improve contact with the skin  308  and be comfortable to wear. Some commercially available brassiere products include suitable signal enhancing or comforting materials, or may be fitted with signal enhancing or comforting materials for use with the antenna  304 . 
         [0038]    An illustration depicting microwave energy reflected from the skin of a breast model using the antennas of  FIG. 1  is depicted in  FIG. 4A .  FIG. 4A  uses breast model  404 , which has the same configuration as the breast model of  FIG. 2A . Antenna  408  is similarly placed in contacted with the skin on the surface of model  404 . Contour lines  416  surrounding the center of antenna  408  indicate the current density of electrical currents induced in the skin of model  404 . Areas with a higher measured current density indicate that microwave energy emitted by antenna  408  is being reflected by the skin of model  404 . The reflected energy depicted by contour lines  416  is localized to the periphery of the antenna, with the greater portion of the emitted energy entering the breast tissue and traveling towards tumor  412 . This emission pattern occurs because the skin of breast model  404  acts as a substrate layer of antenna  408  when antenna  408  is placed in contact with the breast skin. The combination of antenna  408  and skin on the breast model  404  emit a greater portion of the radio energy into the breast tissue within model  404  than the prior art antenna. 
         [0039]    In  FIG. 4B , reflected microwave energy is minimized because the breast skin itself becomes an emitting element instead of a passive reflective element. The results of  FIG. 4A  are contrasted with a prior art antenna arrangement seen in  FIG. 9 . In  FIG. 9 , antenna  908  is placed at a predetermined distance  916  from the surface of breast model  904 . In the example of  FIG. 9 , distance  916  is 5 cm. Antenna  908  may emit the same microwave energy as antenna  408 , but the skin on model  904  reflects a substantial portion of the microwave energy. The contour lines  920  that cover each facet of the surface of model  904  show the induced current density over the entire surface of model  904 , indicating a large amount of reflected microwave energy. The reflected energy is unable to penetrate the model, and it acts as a noise source, obscuring reflected energy from tumor  912 . If prior art antenna  908  is placed in contact with the surface of model  904 , the electrical properties of the skin covering  904  interfere with operation of the prior art antenna, preventing it from operating at the desired frequency. These electrical properties include the electrical conductivity and dielectric permittivity of the skin. 
         [0040]      FIG. 4B  depicts the same model  404  and antenna  408  of  FIG. 4A . In this illustration, contour lines  420  on the surface of tumor  412  indicate the density of an electrical current induced in the surface of tumor  412 . As discussed above, the current density indicates that microwave energy is reaching and reflecting off of tumor  412  through the skin and surrounding breast tissue. An example of the maximum strength of an induced current density using the example embodiment of  FIG. 4B  is 69.65 A/m 2 . 
         [0041]    A process  500  for adjusting an antenna to operate at a desired resonant frequency when placed in contact with skin is shown in  FIG. 5 . First, the final desired resonant frequency and input impedance for the antenna when placed in contact with skin are determined (block  504 ). The desired resonant frequency may change depending upon the type of material being imaged. The desired input impedance may be selected to match the output impedance of a microwave signal generator to prevent signal reflection from the antenna to the generator. 
         [0042]    Next, one or more electrical properties of the skin being coupled to the antenna are measured. The dielectric constant of permittivity in the skin is measured using a known method such as measuring the capacitance of the skin using a multimeter and identifying the dielectric constant from the capacitance and thickness of the skin (block  508 ). A dielectric constant of approximately 40 is typical for human skin, although the measured result may vary based on various factors affecting the condition of the skin. The electrical conductivity of the skin is also measured (block  512 ) using a measurement device such as an ohmmeter. In human skin, a measured conductivity on the order of 1.1 S/m is typical, although this value may also vary based on the condition of the skin. As seen in  FIG. 2B , a meter  250 , such as an ohmmeter or multimeter, is electrically coupled to a breast model  204  using electrodes  252  and  254 . The meter  250  measures various electrical properties of the skin on the breast  204  model, including conductance and capacitance. The electrical properties of the skin are identified based on the measurements generated by the meter  250 . 
         [0043]    After measuring the electrical characteristics of the skin, the emitter element of the antenna is adjusted so that the combined antenna and skin emit microwave energy at the desired resonant frequency and input impedance (block  516 ). The resonant frequency of the antenna may be adjusted by increasing or decreasing length and/or width of the radiator, such as radiators  108 ,  140 , and  168  depicted in  FIG. 1A ,  FIG. 1B , and  FIG. 1C , respectively. For example, if the resonant frequency of the antenna is higher than the desired frequency, the length of the radiator is increased and vice versa. The input impedance of the antenna may be adjusted by changing the position of the feed and/or increasing or decreasing the width of the radiator. The input impedance of the antenna may be increased by moving the feed away from the center of the corresponding radiator towards one corner of the radiator and/or by decreasing the width of the radiator. A transformer, such a balun, may be electrically connected between the antenna and the microwave signal generator to provide a tunable input impedance for the antenna when placed in contact with skin and the output impedance of the microwave signal generator. 
         [0044]    The embodiments of  FIG. 1B  and  FIG. 1C  may be adjusted to a selected frequency, an input impedance, and a radiation pattern by altering the length and width of the radiator, position of the feed, and by adjusting other structures in the antenna. For example, in the embodiment of antenna  124  shown in  FIG. 1B , the length, width, surface area, and position of the slots  132  and  136  are adjusted to tune the antenna to a desired frequency, input impedance, and radiation pattern. In the embodiment of antenna  160  shown in  FIG. 1C , the length  188  and width  192  of the top conductor  176  and the size and location of the via  184  is adjusted to tune the antenna  160  to a desired frequency, input impedance, and radiation pattern. 
         [0045]    As seen in  FIG. 1A-FIG .  1 C, antennas  100 ,  124 , and  160  may be manufactured with various different sizes and configurations of structures in each antenna for a range of input impedance, resonant frequency, and radiation pattern parameters. In one configuration, a pre-fabricated antenna is selected from a plurality of differently configured pre-fabricated antenna types. The selected antenna has the desired input impedance, resonant frequency, and radiation pattern parameters when coupled to the skin for tissue imaging. The selection of the antenna is made after measuring the electrical and physical properties of the skin, such as the dielectric permittivity and electrical conductivity of the skin. 
         [0046]    The plurality of antennas each include variations to the structures in the antenna that enable different antenna characteristics including resonant frequency and input impedance when coupled with skin. For example, variations of antenna  100  include a range of widths  120  for the radiator  108 , and various different positions of the feed  110  with respect to the radiator  108 . Variations of antenna  124  further include different sizes and positions of the slots  132  and  136 . Variations of antenna  160  further include different sizes of the top conductor  176  and different connection positions of the via  184 . In some embodiments, multiple antennas are coupled to the skin. Each of the antennas that are coupled to the skin have the selected antenna structure. 
         [0047]    In one example, the selected characteristics of an antenna coupled to the skin include a 2.45 GHz resonant frequency and an input impedance of 50Ω. The skin is measured with a conductivity of 1.1 S/m, and a dielectric constant of 40. In some configurations, some or all of the electrical properties of the skin are estimated instead of being measured directly. An antenna is selected from a group of differently configured antennas that each have a configuration that produces the selected resonant frequency and input impedance when coupled to the skin. One antenna is selected that is configured to emit radio waves at the selected 2.45 GHz frequency and 50Ω input impedance when the antenna is coupled to skin having the measured conductivity of 1.1 S/m and dielectric constant of 40 in the skin. An operator may select an appropriate antenna using a reference table, computer program, or any device that correlates the measured skin parameters to a selected set of antenna parameters. 
         [0048]    Selection of a suitable antenna from a group of pre-fabricated antennas eliminates the need to adjust a single antenna by cutting or otherwise altering structures in the antenna. A pre-fabricated antenna is identified based on the relationship between measured properties of the skin, and selected antenna parameters such as the resonant frequency and input impedance. Each pre-fabricated antenna is configured to operate over a predetermined range of resonant frequencies, and input impedances for a predetermined range of skin properties. 
         [0049]    After the antenna is selected or configured to operate at the desired resonant frequency, it is placed in contact with the skin of a test subject (block  520 ). This may include cleaning the area under the antenna to promote a uniform electrical contact. The placement may additionally include application of a moisturizing lotion or a topical cream to the area of skin placed in contact with the antenna. The application of this lotion or cream may enhance the electrical contact between the antenna and the underlying skin layer. The topical cream may also include an electrically conductive gel that promotes electrical contact with the skin, such as commercially available gels used for affixing electrodes to human skin. An adhesive backing may also be used to ensure that the antenna remains in the same relative position on the patient during the microwave imaging process. Alternatively, a layer of additional material is used on the antenna that contacts the skin. The additional layer enhances the microwave signal penetration through the skin or the comfort to the patients or both. 
         [0050]    Once the antenna is configured and attached to the patient&#39;s skin, the microwave signal generator may operate at the desired frequency and impedance (block  524 ). The antenna emits microwaves that pass through the skin and into tissue layers such as breast tissue. Other structures in the breast tissue including tumors reflect the microwave energy. The backscatter of microwave energy may be detected in order to identify possible tumors disposed in the breast tissue. Detected microwave energy may be received by one or more receiver antennae, be processed by a signal processor such as signal processor  224  seen in  FIG. 2A , and be displayed on an imaging device such as display  228 . 
         [0051]      FIG. 6  depicts four microstrip antennas  604 A- 604 D positioned around a breast  608  in contact with skin  612 . The antennas  604 A- 604 D may be incorporated in a garment, such as the single breast cup  700  depicted below in  FIG. 7 , that is fitted to the breast  608 . The antennas  604 A- 604 D may have the same structure, tuning, and impedance as microstrip antennas  100 ,  124 , and  160  from  FIG. 1A-FIG .  1 C. 
         [0052]    In  FIG. 6 , the interior of the breast  608  is divided into regions R 1 -R 9 . The breast  608  contains a tumor  616  located in a region R 1 . The arrangement of antennas  604 A- 604 D may detect both the position and the size of the tumor  616 . In operation, one of the antennas, such as antenna  604 A, emits a signal that is directed into the tissue of breast  608 . In the example of  FIG. 6 , the signal is emitted in a microwave frequency range of 2.1 GHz-2.5 GHz, although alternative signals may use different frequency ranges. As the signal propagates through the breast  608 , various tissues in the breast, including the tumor  616 , scatter and attenuate the signal. The tumor  616  has a higher water content than the surrounding tissue in the breast  608 , and the water in the tumor  616  scatters the signal to a greater degree than the surrounding tissue. Each of the antennas  604 A- 604 D are configured to receive the scattered signal, including a backscatter signal reflected from the tumor  616  and detected by antenna  604 A. The breast tissue absorbs a portion of the energy in the scattered signal emitted by the antenna  604 A, which may be referred to as a “scatter loss.” The antennas  604 A- 604 D may be calibrated using simulated breast material, such as glycerine, to identify signal noise and scatter loss that occur in healthy breast tissue prior to testing breast tissue that may include tumors. 
         [0053]    To detect and identify the position of a tumor in the breast  608 , antenna  604  emits a signal through the skin  612  into the tissue in breast  608 . The signal scatters as the signal propagates through the breast tissue, and antennas  604 A- 604 D record the scattered signal. In the example of  FIG. 6 , the signal encounters the tumor  616  in region R 1 . The tumor  616  scatters the signal emitted from the antenna  604 A to a greater degree than normal breast tissue, resulting in a larger magnitude of backscatter radiation detected by the antenna  604 A. Each of the antennas  604 B- 604 D emit a signal in a similar manner with the remaining antennas detecting the scattered signal. Due to the position of the tumor  616  proximate to antenna  604 A, the greatest magnitude of backscatter radiation is detected at antenna  604 A. Antennas  604 B and  604 D are each positioned at approximately equal distances from the tumor  616  and each receive backscatter signals having roughly equal magnitudes. The antenna  604 C is farthest from the tumor  616  and receives a backscatter signal having a magnitude that is lower than the magnitude of backscatter signals received by the other antennas  604 A,  604 B, and  604 D. 
         [0054]    The position of the tumor may be identified using the relative magnitudes of backscatter signal detected at each of the antennas  604 A- 604 D. In the example of  FIG. 6 , the highest backscatter signal strength occurs near antenna  604 A, with equal strength backscatter signals detected by antennas  604 B and  604 D indicating that the tumor  616  is located in region R 1 . Further, in some embodiments, existing signal conditioning methods are applied to the received backscatter signal. In these embodiments, the signal is fed to existing image reconstruction computer programs, which use existing image reconstruction algorithms to generate two dimensional and three dimensional images of the breast. These images show different parts of the breast, some of which may include tumors. While  FIG. 6  depicts four antennas positioned on the breast  608 , alternative embodiments may include more antennas around the breast  608 . Additional antennas enable measurement of the position of tumors in the breast over a greater number of regions in the breast, increasing the precision of identification of the position of tumors in the breast tissue. 
         [0055]    The antenna configuration of  FIG. 6  may also estimate the size of a tumor in the breast using the measured scatter return signal. The amplitude of the measured scatter return signal increases as the size of a tumor, such as tumor  616 , increases. In the embodiment of  FIG. 6 , the magnitude of detected backscatter for a signal emitted through healthy breast tissue is approximately −56 decibels (dB). This value may be identified by calibrating the antennas  604 A- 604 D using healthy breast tissue or glycerine as described above. When antenna  604 A emits a signal proximate to the tumor  616 , the magnitude of the detected backscatter signal increases. If tumor  616  has a diameter of 5 mm, the measured scatter loss is −47 dB, while the measured scatter loss for tumor  616  is −38 dB when the diameter of the tumor is 7.5 mm. Note that the magnitude of backscatter signal increases as the decibel number approaches zero, so the −38 dB signal for the 7.5 mm tumor represents the largest magnitude of detected backscatter, while the −56 dB measured loss for healthy breast tissue is the smallest magnitude of detected backscatter. Using empirical calibrations for known tumor sizes, the antenna arrangement depicted in  FIG. 6  may identify tumors of various sizes in the breast  608 . Both the size and position of a detected tumor may be display visually using a display device such as an LCD display the display a model of the breast tissue and tumor. Using various visualization techniques, the display depicts a two dimensional or three dimensional representation of the breast tissue and tumor. 
         [0056]    An arrangement of patch antennae suitable to be worn on a human breast is depicted in  FIG. 7 . A breast cup  700  is formed including multiple antennae shaped to fit the outer wall of the breast cup  700 , such as antennae  704 A- 704 D. The cup is formed with an inner cavity  708  that may be adapted to a variety of sizes and shapes to comfortably accommodate a breast. The radiators of each of the antennae are exposed to the inner cavity, as exemplified by the radiator of antenna  704 D. Each of the radiators may contact the skin covering breast tissue. Breast cup  700  may hold the antennae in a fabric sheath with pockets arranged to hold an antenna in each pocket. This allows for individual antennae to be removed, adjusted, and then reinserted into breast cup  700 . In the arrangement of  FIG. 7 , the multiple antennae of breast cup  700  are arranged to emit microwave energy into the breast from multiple sides of the breast and from multiple angles. Each antenna may receive backscatter microwave signals that reflect off structures in the breast tissue, including tumors. 
         [0057]    Multiple antennae in breast cup  700  may be selectively activated in groups, and different antenna groups may be activated sequentially to scan the breast in three dimensions. Antenna group  712 A includes four antennae placed in a two by two formation. Antenna group  712 A may emit microwave signals from all four antennae simultaneously, and the other antennae in breast cup  700  may receive backscatter from those signals. Antenna group  712 B is another group of four antennae, including two of the antennae from antenna group A. It may be activated at a different time from antenna group  712 A to image the breast from a different angle. Selecting groups of smaller antennae such as groups  712 A and  712 B results in a higher image resolution than using a single antenna of a size equivalent to antenna groups  712 A or  712 B. By activating on or more antennae in a sequential manner, microwave energy may be sent into the breast tissue from multiple angles for imaging. 
         [0058]    The single breast cup  700  of  FIG. 7  may be paired with a second breast cup adapted for a second breast, and the two may be further held together in a brassiere that is worn by the test subject. Such an arrangement allows the antennae to remain in a fixed position relative to the breast tissue being imaged, and increases the comfort of the test subject. While  FIG. 7  depicts a wearable breast cup, alternative garments adapted for use in imaging different portions of the body of a human or an animal are envisioned. For example, a circular waist-band garment could be used for imaging structures within the abdomen of a test subject. 
         [0059]      FIG. 8  depicts a cross-sectional view of an antenna array formed in a wearable garment that conforms to a breast, such as breast cup  700 . A breast  804  includes a layer of skin  812  that encloses breast tissue  808 . The antenna array includes antennas  816 A and  816 B, dielectric layer  820 , and ground layer  824 , and may include optional separator layer  828  and reflector layer  832 . 
         [0060]    The antenna array depicts two microstrip antenna elements  816 A and  816 B in cross section, although alternative configurations may include a different number and arrangement of antennas. The antenna elements  816 A and  816 B may be microstrip antennas as seen in  FIG. 1A ,  FIG. 1B , and  FIG. 1C . A dielectric layer  820  is positioned between the antenna elements  816 A and  816 B and a ground layer  824 . The dielectric layer  820  is formed from an electrical insulator that resists a flow of electricity between the ground layer  824  and either of the antenna elements  816 A and  816 B. The dielectric layer  820  polarizes in the presence of an electrical field that is applied to the antenna elements  816 A or  816 B when emitting a signal into the breast  804 . The ground layer  824  may be formed from a planar electrical conductor, such as copper, aluminum, or any other suitable electrically conductive material. 
         [0061]    The embodiment of  FIG. 8  includes an optional separator layer  828  formed over the ground layer  824  and a reflector layer  832  formed over the separator layer  828 . Separator layer  824  is an electrical insulator, such as a polymer, that electrically isolates the reflector layer  832  from the ground layer  820 . The reflector layer  832  is formed from a material that reflects radio energy frequencies emitted by the antenna elements  816 A and  816 B. In the example of  FIG. 8 , the reflector layer  832  is configured to reflect microwave energy in a frequency range of approximately 2 GHz-3 GHz emitted by the antenna elements  816 A and  816 B. The reflector layer  832  is formed from a material that reflects microwave energy, such as a sheet of copper or aluminum, that is shaped to conform to the separator layer  828 . The separator layer  828  and reflector layer  832  are optional layers that reflect signals emitted by the antenna elements  816 A and  816 B into the breast tissue  808 . The reflected signals improve the efficiency of operation of the antenna elements  816 A and  816 B. 
         [0062]    Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. For example, while the embodiments disclosed above are primarily directed towards the detection of tumors in breast tissue, the improved antenna and imaging techniques described herein are applicable to imaging many different structures in the bodies of humans, animals, and other organisms. Therefore, the following claims are not to be limited to the specific embodiments illustrated and described above. The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.