CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. Pat. No. 14/636,537, filed on Mar. 3, 2015, and hereby incorporated by reference in its entirety. 
         [0002]    Application Ser. No. 14/636,537 claims the benefit of U.S. provisional patent application 61/947,241, filed on Mar. 3, 2014, and hereby incorporated by reference in its entirety. 
         [0003]    Application Ser. No. 14/636,537 claims the benefit of U.S. provisional patent application 62/127,101, filed on Mar. 2, 2015, and hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0004]    This invention relates to thermoacoustic imaging. 
         [0005]    BACKGROUND 
         [0006]    Thermoacoustic imaging involves illuminating a target with microwaves and performing acoustic imaging of sound generated in the target by the thermoacoustic effect. Photoacoustic imaging is a similar physical effect using an optical excitation as opposed to a microwave excitation. 
         [0007]    Although these effects have been used in imaging for some time now, it remains desirable to improve their performance to enable new and challenging applications. Accordingly, it would be an advance in the art to provide improved thermoacoustic imaging. 
       SUMMARY 
       [0008]    One potential application for thermacoustic imaging is biometric authentication based on the pattern of blood vessels in a user&#39;s finger. This is a challenging application for thermoacoustic imaging, since the relevant size scale is much less than in conventional applications such as whole-body or organ-scale imaging. 
         [0009]    In addition to various challenges related to this change of scale, we have unexpectedly found that the polarization of the microwave excitation is critical for getting good thermoacoustic images of vessel-like structures.  FIG. 5B  is a thermoacoustic image of the phantom of  FIG. 5A , and illustrates this problem. 
         [0010]    Structures in the phantom that are unfavorably aligned with the linear microwave polarization used for this image are not seen in the image, which is a catastrophic lack of image quality.  FIG. 5C  shows a corresponding thermoacoustic image of the phantom of  FIG. 5A  according to principles of the invention. Linear polarization of the microwaves is avoided, and the resulting image is much more accurate than the image of  FIG. 5B . Polarization can be quantified by referring to the eccentricity e of the polarization ellipse. Embodiments of the invention have e≦0.87. Preferred embodiments have e≦0.71. Particularly preferred embodiments have e 0.32. 
         [0011]    In some embodiments, optical excitation is combined with microwave excitation. It is convenient to regard these embodiments as also relating to thermoacoustic imaging, so we define thermoacoustic imaging as acoustic imaging based on microwave or microwave+optical excitation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a first embodiment of the invention. 
           [0013]      FIG. 2  shows a second embodiment of the invention. 
           [0014]      FIG. 3  shows a preferred microwave excitation array for use in embodiments of the invention. 
           [0015]      FIG. 4A  shows an exemplary unit cell of the microwave excitation array that includes several near-field excitation elements. 
           [0016]      FIG. 4B  shows notation for describing elliptical polarization. 
           [0017]      FIG. 5A  is an image of a phantom that simulates blood vessels in tissue. 
           [0018]      FIG. 5B  is a thermoacoustic image of the phantom of 
           [0019]      FIG. 5A  making use of linearly polarized microwaves from 2-element emitters. 
           [0020]      FIG. 5C  is a thermoacoustic image of the phantom of  FIG. 5A  making use of circularly polarized microwaves from 4-element emitters, in accordance with principles of the present invention. 
           [0021]      FIG. 6  shows an exemplary excitation and sensor array featuring integration of optical sources, microwave excitation and an acoustic sensor array. 
           [0022]      FIG. 7  schematically shows how improved depth uniformity can be provided by using optical excitation and microwave excitation at several different microwave frequencies. 
           [0023]      FIG. 8  shows a first exemplary alternating arrangement of optical sources and microwave emitter array unit cells. 
           [0024]      FIG. 9  shows a second exemplary alternating arrangement of optical sources and microwave emitter array unit cells. 
           [0025]      FIG. 10  shows an embodiment of the invention that is flexible in order to conform to the shape of the target being imaged. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows a first embodiment of the invention. In this example, a microwave source  102  is configured to provide a microwave output. A microwave emitter array  104  includes two or more unit cells  110 , which can be identical or dissimilar. As described in greater detail below, the unit cells each include three or more near-field excitation elements. Microwave emitter array  104  is configured to receive the microwave output from microwave source  102 , and to provide microwave excitation  112  to a target  114  via the near-field excitation elements of the unit cells  110 . The spacing between the microwave emitter and a typical position within the target is referenced as h. We define “near-field” to be within 10 wavelengths of the emitter within the target. For example, at 9 GHz the near-field region is within about 3 mm of the emitter in tissue. 
         [0027]    An acoustic sensor array  106  includes one or more sensor elements  118  that are configured to receive acoustic signals  116  from target  114  responsive to microwave excitation  112 . A processor  108  is configured to receive signals from acoustic sensor array  106  and to provide an acoustic image of target  114 . The microwave emitter array can be periodic or aperiodic. As described in greater detail below, the microwave emitter array can be integrated with other sensors and/or excitation sources, such as optical excitation sources and/or acoustic sensors. 
         [0028]    In this example, the acoustic sensor array and the microwave emitter array are disposed on opposite sides of the target. It is also possible for the acoustic sensor array and the microwave emitter array to be disposed on the same side of the target, e.g. as in the example of  FIG. 6  below. 
         [0029]    Various possibilities exist for the microwave source. Preferred pulsed embodiments provide microwave pulses having a duration of 1 μs or less, a peak power of 0.1 W or more, and a repetition rate of 1 Hz or more. Alternatively, the microwave source can be configured to provide a continuous amplitude-modulated microwave output, such as Frequency-Modulated Continuous Wave (FMCW) or Stepped-Frequency Continuous Wave (SFCW). The frequency (or frequencies for multi-frequency excitation) is preferably between 0.1 GHz and 20 GHz. 
         [0030]    In preferred embodiments, crosstalk is mitigated using various strategies. For example non-adjacent unit cells of the microwave emitter array can be activated in sequence in order to reduce cross-talk between different unit cells. Another mitigation strategy is to activate non-adjacent sensor elements of the acoustic sensor array in sequence in order to reduce cross-talk between different sensor elements. This pattern of activation can be a simple interleaved pattern or a pseudo-random activation of elements. 
         [0031]    The sensor array elements can be individually tuned to different acoustic frequencies. These acoustic frequencies are preferably in a range from 1 MHz to 30 MHz to provide high sensitivity and high resolution simultaneously. 
         [0032]      FIG. 2  shows a second embodiment of the invention. This embodiment is similar to the embodiment of  FIG. 1 , except that one or more auxiliary sensors  202  are added. Such sensors can be microwave sensors, optical sensors or acoustic sensors. Various functions can be provided by such auxiliary sensors, including but not limited to: 1) monitoring and calibrating the microwave excitation, 2) monitoring and calibrating the optical radiation, 3) obtaining course-grain information about the target geometry and its motion, 4) providing signals for cancellation of microwave to acoustic leakage directly with analog electronics, 5) providing signals for cancellation of microwave to acoustic leakage computationally in post-processing, 6) providing signals for cancellation of optical to acoustic leakage directly with analog electronics, and 7) providing signals for cancellation of optical to acoustic leakage computationally in post-processing. 
         [0033]    Functions 2, 6 and 7 in this list relate to below-described embodiments that include optical excitation of the target in addition to microwave excitation of the target. 
         [0034]    As indicated above, it is important to avoid the use of a linearly polarized microwave excitation. Circular polarization is most preferred, although some deviation from circular polarization is permissible.  FIG. 3  schematically shows a preferred microwave excitation array for use in embodiments of the invention. Here each element emits with circular polarization (as shown), and there is also an alternating pattern of clockwise and counterclockwise rotation in the polarizations. The alternating pattern of clockwise and counterclockwise rotation in the polarization is a consequence of the sharing of excitation elements between adjacent excitation cells, to create a more economical design. However, this is not a strict requirement for proper device operation and thus it is possible to employ a single clockwise or counterclockwise polarization in all excitation cells by not sharing any excitation elements between cells. In general, such an emitter array can have N rows and M columns, although either M or N can be 1 to provide a 1-D array. 
         [0035]      FIG. 4A  shows an exemplary unit cell of the microwave excitation array that includes several near-field excitation elements. In this example, four T-shaped near field excitation elements ( 402 ,  404 ,  406 , and  408 ) are disposed in an aperture  410 . We can create a directionally uniform excitation by generating a rotating electric field using three or more near field excitation elements fed with RF phases that are equally spaced in the 0 to 360 degree range. In this example, 4 near field excitation elements are shown that are fed with 0, 90, 180, and 270 degree phases. The geometrical parameters a and s on  FIG. 4A  are preferably related to other design parameters as follows: a≈√2h, a≈λ/2, s≈a. Microwave frequency f and wavelength λ are related by fλ=c/√ε, where ε is the effective dielectric constant of the target material at frequency f, and c is the speed of light. 
         [0036]    Here it is helpful to consider the polarization of the microwaves in more detail.  FIG. 4B  shows notation for describing elliptical polarization. In general, the time evolution of the electric field of an electromagnetic excitation in a reference plane describes an ellipse, which we refer to as the polarization ellipse. Let E u  be the electric field magnitude of the major axis of the polarization ellipse (as shown). Let E v  be the electric field magnitude of the minor axis of the polarization ellipse (as shown). The eccentricity e of the polarization ellipse is defined by 
         [0000]    
       
         
           
             e 
             = 
             
               
                 
                   1 
                   - 
                   
                     
                       ( 
                       
                         
                           E 
                           v 
                         
                         
                           E 
                           u 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               . 
             
           
         
       
     
         [0037]    We define directional uniformity of the thermoacoustic signal as follows. Let TA +  be the thermoacoustic signal obtained from a linear target (such as a blood vessel) that is aligned with E u . Let TA −  be the thermoacoustic signal obtained from a linear target that is aligned with E v . Then the directional uniformity U is given by 
         [0000]    
       
         
           
             U 
             = 
             
               
                 
                   
                     TA 
                     + 
                   
                   - 
                   
                     TA 
                     - 
                   
                 
                 
                   TA 
                   + 
                 
               
               . 
             
           
         
       
     
         [0038]    We have U≦0.75, more preferably U≦0.5 and most preferably U≦0.1. Since 
         [0000]    
       
         
           
             
               
                 TA 
                 - 
               
               
                 TA 
                 + 
               
             
             = 
             
               
                 ( 
                 
                   
                     E 
                     v 
                   
                   
                     E 
                     u 
                   
                 
                 ) 
               
               2 
             
           
         
       
     
         [0000]    we have so we see that e≦0.87, more preferably e≦0.71, and most preferably e≦0.32. Relative phases of excitation elements of the unit cells of the emitter array are preferably selected to provide sufficiently low eccentricity in a reference plane within the target and parallel to the emitter array. It is convenient to refer to this arrangement of the reference plane as being “within the target”. 
         [0039]    The significance of providing directional uniformity as described above is clearly shown on  FIGS. 5A-C . Here  FIG. 5A  is an image of a phantom that simulates blood vessels in tissue. It includes long thin structures at various directions as can be expected of blood vessels in tissue.  FIG. 5B  is a thermoacoustic image of the phantom of  FIG. 5A  making use of linearly polarized microwaves from 2-element emitters. Here it is apparent that the image is seriously distorted. Investigation has confirmed that these poor imaging results are a result of unfavorable alignment between features being imaged and the linear polarization of the microwave excitation. 
         [0040]      FIG. 5C  is a thermoacoustic image of the phantom of  FIG. 5A  making use of circularly polarized microwaves from 4-element emitters, in accordance with principles of the present invention. Here it is seen that much better imaging results are obtained. 
         [0041]    Some embodiments of the invention further include providing optical radiation to the target to improve thermoacoustic imaging. The main idea is to make the overall energy deposition provided by the system more uniform by appropriately combining microwave excitation and optical radiation. In such embodiments, it is preferred to integrate optical sources, microwave emitters and the acoustic sensor array on the same surface.  FIG. 6  shows an example of this approach. 
         [0042]    Here  600  is the integrated device, shown in a view looking down on the device from where the target would be in operation. Here  602   a  and  602   b  are optical sources configured to provide optical radiation to the target, and in this design are intended to provide 0 to 2 mm penetration depth in tissue. Preferred optical wavelength(s) for this excitation are between 750 nm and 1500 nm. Microwave emitters  604   a ,  604   b ,  604   c , and  604   d  provide an outer microwave excitation  604   e , intended to provide excitation from 5 mm to 15 mm depth in tissue. Inputs  606   a ,  606   b ,  606   c ,  606   d  to emitters  604   a ,  604   b ,  604   c ,  604   d  have equally spaced relative phases as described above to provide sufficient directional uniformity. 
         [0043]    The 2 mm to 5 mm depth range in tissue is covered by an array of three unit cells. Inner microwave excitation  610   e  is provided by emitters  610   a ,  610   b ,  610   c ,  610   d . Inner microwave excitation  612   e  is provided by emitters  612   a ,  612   b ,  612   c ,  612   d . Inner microwave excitation  614   e  is provided by emitters  614   a ,  614   b ,  614   c ,  614   d . These emitters are driven by inputs  608   a ,  608   b ,  608   c ,  608   d  having equally spaced phases as described above to provide sufficient directional uniformity. The pattern of connections between inputs  608   a ,  608   b ,  608   c ,  608   d  and the emitters for the array lead to opposite polarization rotation for excitation  612   e  compared to excitations  610   e  and  614   e.    
         [0044]    Here  616  is the acoustic sensor array. In this example, the acoustic sensor array and the microwave emitter array are disposed on the same side of the target. 
         [0045]    The microwave excitation and the optical radiation can be provided to the target synchronously using a single trigger signal. Alternatively, the microwave excitation and the optical radiation can be provided sequentially in alternation. 
         [0046]    This is an example of combining one large, low-frequency microwave excitation cell with several smaller high-frequency microwave excitation cells and optical cells to create a more uniform excitation along the depth. Uniform field distribution in depth is improved by using a combination of optical excitation together with multiple-frequency microwave excitation. Optical excitation is optimized in the closest region, high frequency microwave is optimized in the mid region, and low frequency microwave is optimized in the far region. A combination of frequency selection together with selection of the excitation cell geometries and locations can provide a uniform excitation. 
         [0047]    The spacing and geometry of the excitation cells can be optimized for different frequencies. 
         [0048]      FIG. 7  schematically shows how improved depth uniformity can be provided by using optical excitation and microwave excitation at two or more different microwave frequencies. Here  702  is the optical excitation,  704  is microwave excitation at a first RF frequency f 1 ,  706  is microwave excitation at a second RF frequency f 2 , and  708  is microwave excitation at a third RF frequency f 3 . Microwaves at lower frequencies have greater penetration depth in tissue, so we have f 1 &gt;f 2 &gt;f 3  for biometric applications. The overall energy deposition is schematically shown as  710 , and is significantly more uniform than the deposition from any one of the sources. Thus the microwave signals at the two or more frequencies are configured to enhance depth uniformity of thermoacoustic imaging. 
         [0049]    Depth uniformity is defined with respect to energy deposition in a homogeneous target. A deposition pattern is considered to have depth uniformity if the energy deposition at a depth of minimum energy absorption within a predetermined depth range is 25% or more of the energy deposition at a depth of maximum energy absorption within the predetermined depth range. For example, this predetermined depth range can be 0-10 wavelengths of the microwave excitation being used. 
         [0050]    In the example of  FIG. 7 , the target has a first region closer to a surface of the target than a second region, and the optical radiation is mainly provided to the first region, while the microwave excitation is mainly provided to the second region. 
         [0051]    Although the example of  FIG. 7  shows use of only a single optical wavelength, two or more optical wavelengths can be employed to improve depth uniformity. In such cases, the optical source can be configured to provide optical radiation at two or more optical wavelengths, and the optical radiation at the two or more optical wavelengths can be configured to enhance depth uniformity of thermoacoustic imaging. 
         [0052]    Optical emitters and the unit cells of the microwave emitter array can alternate.  FIG. 8  shows a checkerboard pattern of alternating microwave unit cells  802  and optical emitters  804 .  FIG. 9  shows row by row alternation of microwave unit cells  802  and optical emitters  804 . 
         [0053]    Practice of the invention does not depend critically on the overall mechanical configuration. In some cases, the apparatus is rigid and planar. In other cases, the apparatus is flexible in order to conform to a shape of the target.  FIG. 10  shows an example along these lines. Here microwave emitter array  1002  is flexible and can conform to the shape of target  114  in operation. The apparatus can also be rigid and have a non-planar geometry, such as cylindrical and hemispherical geometries. These geometries are expected to be useful for the application of biometric authentication using finger blood vessels. For example, a cylindrical excitation/sensor array can surround the finger.
Improved thermoacoustic imaging is provided by ensuring directional uniformity of the microwave excitation provided to the target being imaged. This directional uniformity can be quantified in terms of the eccentricity e of the polarization ellipse of the microwave excitation. We have e<=0.87, preferably e<=0.71, and more preferably e<=0.32. Optical excitation can be provided in addition to the microwave excitation. Excitation can be performed at multiple optical wavelengths and/or microwave frequencies to improve depth uniformity. In addition, the employment of excitation cells with optimized spacing and geometry provides the uniformity in another two degrees of freedom. One potential application is to detect blood vessel in user's finger for biometric authentication.