Abstract:
The effective aperture of an ultrasound imaging probe can be increased by including more than one transducer array and using the transducer elements of all of the arrays to render an image can greatly improve the lateral resolution of the generated image. In order to render an image, the relative positions of all of the elements must be known precisely. Systems and methods for accurately calibrating and adjusting a multi-aperture ultrasound system are disclosed. The relative positions of the transducer elements can be computed and aligned prior to and during probe assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/210,015, filed Mar. 13, 2014, now U.S. Pat. No. 9,510,806, which application claims the benefit of U.S. Provisional Patent Application No. 61/780,366, filed Mar. 13, 2013, titled “Alignment of Ultrasound Transducer Arrays and Multiple Aperture Probe Assembly”, the contents of each application incorporated by reference herein. 
     
    
     INCORPORATION BY REFERENCE 
       [0002]    All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
       FIELD 
       [0003]    The present invention relates generally to imaging techniques, and more particularly to ultrasound imaging, and still more particularly to systems and methods for calibration and quality assurance measurement of ultrasound probes, particularly probes having multiple apertures. 
       BACKGROUND 
       [0004]    In conventional (scanline-based) ultrasonic imaging, a focused beam of ultrasound energy (a scanline) is transmitted into body tissues to be examined and echoes returning along the same scanline are detected and plotted. A complete image may be formed by combining multiple scanlines. While ultrasound has been used extensively for diagnostic purposes, conventional scanline-based ultrasound has been greatly limited by depth of scanning, speckle noise, poor lateral resolution, obscured tissues and other problems. 
         [0005]    Significant improvements have been made in the field of ultrasound imaging with the creation of multiple aperture imaging, some examples of which are shown and described in U.S. Pat. No. 8,007,439 titled “Method and Apparatus to Produce Ultrasonic images Using Multiple Apertures,” U.S. patent application Ser. No. 13/029,907, filed Feb. 18, 2010, now U.S. Pat. No. 9,146,313, titled “Point Source Transmission and Speed-Of-Sound Correction Using Multiple-Aperture Ultrasound Imaging, U.S. patent application Ser. No. 12/760,375, filed Apr. 4, 2010, titled “Universal Multiple Aperture Medical Ultrasound Probe,” and U.S. patent application Ser. No. 12/760,327, now U.S. Pat. No. 8,473,239, titled “Multiple Aperture Ultrasound Array Alignment Fixture,” all of which are incorporated herein by reference. Multiple aperture imaging methods and systems allow for ultrasound signals to be both transmitted and received from separate apertures. 
         [0006]    Ultrasound probes constructed to perform multiple aperture ultrasound imaging typically contain multiple separate transducer arrays. During construction of such a probe, the multiple arrays need to be aligned in a common imaging plane and in a desired orientation relative to one another. Some methods of performing such alignment and construction are shown and described in U.S. patent application Ser. No. 12/760,327, now U.S. Pat. No. 8,473,239. Room for further improvement remains. 
       SUMMARY 
       [0007]    In one embodiment, a method of building a multiple aperture ultrasound probe is provided, the method comprising the steps of forming a gasket with a first flowable solidifying material on a lower surface of a precision alignment element, securing the precision alignment element to a back surface of a transducer array with the gasket, evaluating and adjusting alignment of the transducer array relative to the precision alignment element, and injecting a second flowable solidifying material through at least one hole in the precision alignment element to secure the transducer array to the precision alignment element. 
         [0008]    In some embodiments, the injecting step comprises filling a volume defined by the back surface of the transducer array, the lower surface of the precision alignment element, and an inner surface of the gasket with the second flowable solidifying material. 
         [0009]    In some embodiments, the method further comprises allowing the second flowable solidifying material to solidify, and mounting the precision alignment element to a probe alignment bracket. 
         [0010]    In alternative embodiments, the method further comprises placing the probe alignment bracket into a probe housing, and injecting a third flowable solidifying material into a space between the transducer array and the probe housing. 
         [0011]    In other embodiments, the injected third flowable solidifying material surrounds at least a portion of the precision alignment element or the probe alignment bracket. 
         [0012]    In one embodiment, evaluating alignment of the transducer array relative to the precision alignment element comprises imaging a target with the transducer array and comparing a resulting image of the target with known information defining a geometry of the target. 
         [0013]    In some embodiments, the target comprises a plurality of pins oriented in a known configuration relative to the precision alignment element. 
         [0014]    In other embodiments, each of the pins has a flat surface substantially perpendicular to a longitudinal axis of the pins, the longitudinal axis being substantially perpendicular to an ultrasound wavefront transmitted from a single element of the transducer array and arriving at the pins. 
         [0015]    In some embodiments, adjusting alignment of the transducer array relative to the precision alignment element comprises adjusting at least one set screw to mechanically move the transducer array relative to the precision alignment element. 
         [0016]    In some embodiments, the method further comprises allowing the first flowable solidifying material to solidify prior to evaluating and adjusting alignment of the transducer array relative to the precision alignment element. 
         [0017]    In additional embodiments, the first flowable solidifying material and the second flowable solidifying material are the same material. 
         [0018]    In one embodiment, the pins are oriented with longitudinal axes that intersect at a single point. 
         [0019]    A method of evaluating an alignment of an ultrasound transducer array relative to a precision alignment element is also provided, the method comprising the steps of flexibly securing the ultrasound transducer array to the precision alignment element, mounting the precision alignment element in a fixed, known position and orientation relative to a target, the target having a plurality of reflectors in known reflector positions, imaging the reflectors of the target with the array, comparing imaged reflector positions with known reflector positions, and identifying a corrective adjustment based on the comparing step. 
         [0020]    In some embodiments, the method further comprises comparing a brightness of the reflectors with expected brightness values. 
         [0021]    In other embodiments, the method further comprises visually comparing imaged reflector positions with known reflector positions using a graphical user interface in which a first image comprising the imaged reflector positions is displayed simultaneously with a second image comprising the known reflector positions. 
         [0022]    In alternative embodiments, the graphical user interface further comprises a graphical representation of a brightness of imaged reflectors within a predetermined radius of the known reflector positions. 
         [0023]    An ultrasound probe alignment system is provided, comprising a tank assembly comprising an ultrasound conducting material, an array affixing and adjusting assembly at least partially within the tank assembly, the array affixing and adjusting assembly supporting a precision alignment element in a known position and orientation relative to a target assembly, the target assembly being disposed in the tank assembly and comprising at least one reflector configured to reflect an ultrasound signal. 
         [0024]    Some embodiments further comprise a height adjustment assembly configured to adjust a distance between the array affixing and adjusting assembly and the target assembly. 
         [0025]    In other embodiments, the target assembly comprises a plurality of pins arranged so as to be coincident with a precisely aligned imaging plane of the ultrasound probe alignment system. 
         [0026]    In some embodiments, the pins are arranged so as to be displaced from one another in two dimensions in the imaging plane of the ultrasound probe alignment system. 
         [0027]    In one embodiment, the pins vary in length so as to lie on multiple different points of the imaging plane of the ultrasound probe alignment system. 
         [0028]    In alternative embodiments, the plurality of pins comprises a center pin and at least one pair of pins equidistant from the center pin. 
         [0029]    In other embodiments, the array affixing and adjusting assembly comprises structures for adjusting an orientation of an ultrasound transducer array relative to the precision alignment element. 
         [0030]    A multiple aperture ultrasound probe is provided, comprising a probe housing, a first transducer array secured to a first precision alignment element by a layer of a solidified polymer material, the first precision alignment element comprising a first plate secured to a back surface of the first transducer array, the first precision alignment element being secured to a probe bracket of the probe housing, a second transducer array secured to a second precision alignment element by a layer of a solidified polymer material, the second precision alignment element comprising a second plate secured to a back surface of the second transducer array, the second precision alignment element being secured to the probe bracket of the probe housing, and a filler solidified polymer material disposed in a space between the first and second transducer arrays and the probe housing. 
         [0031]    In some embodiments, the first and second arrays are precisely aligned relative to the first and second precision alignment elements, respectively. 
         [0032]    In other embodiments, the first precision alignment element comprises a plate having at least one hole through which a quantity of solidified polymer material extends. 
         [0033]    In one embodiment, the plate comprises two holes, at least one of which has a quantity of solidified polymer material extending therethrough. 
         [0034]    In some embodiments, the first precision alignment element is secured to a single surface of the first transducer array. 
         [0035]    In additional embodiments, the first precision alignment element is secured to the probe bracket by a plurality of mechanical fasteners. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
           [0037]      FIG. 1  is a perspective view of an embodiment of a fully assembled multiple aperture ultrasound imaging probe. 
           [0038]      FIG. 2  is a flow chart illustrating an embodiment of a high-level process for aligning transducer arrays during assembly of a multiple aperture ultrasound probe. 
           [0039]      FIG. 3  is a perspective view of an embodiment of a fixture assembly and a target for aligning a transducer array. 
           [0040]      FIG. 4  is an exploded view of an embodiment of the adjustment assembly section of the fixture assembly of  FIG. 3 . 
           [0041]      FIG. 5  is a perspective view of an embodiment of an alignment target made up of a plurality of pins. 
           [0042]      FIG. 6  is an exploded view of an embodiment of a transducer array, a gasket element, and a precision alignment element. 
           [0043]      FIG. 7  is a perspective view of an embodiment of the lower side of the precision alignment element of  FIG. 6 . 
           [0044]      FIG. 8  is a perspective view of an embodiment of jig for establishing the thickness of the gasket of  FIG. 6 . 
           [0045]      FIG. 9  is a perspective view of an embodiment of a transducer array mounted in an adjustment assembly. 
           [0046]      FIG. 10  is a block diagram illustrating an embodiment of an imaging controller for use with some embodiments of the alignment systems and methods herein. 
           [0047]      FIG. 11A  is an illustration of an embodiment of an array alignment display screen showing an image of an array that is out of alignment. 
           [0048]      FIG. 11B  is an illustration of an embodiment of an array alignment display screen showing an image of an array that is well-aligned. 
           [0049]      FIG. 12  is a perspective view of an embodiment of a probe alignment bracket for use in supporting transducer arrays in a designed orientation relative to a probe housing and relative to one another. 
           [0050]      FIG. 13  is a cross-sectional view of an embodiment of a completed multiple aperture ultrasound probe assembled using the systems and methods described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    The following disclosure provides embodiments of systems and methods for constructing accurately aligned multiple aperture ultrasound probes. Some embodiments provide systems and methods for checking, adjusting, and securing the alignment of an individual array relative to a precision alignment element (PAE). Some embodiments provide systems and methods for mechanically aligning and affixing multiple transducer arrays in a desired alignment relative to one another and relative to a probe housing. 
         [0052]    It is important that ultrasound probes to be used in high resolution multiple aperture ultrasound imaging be precisely constructed such that each of a plurality of transducer arrays be precisely aligned along a common imaging plane. It is further important that such arrays be mounted within a probe housing at a precise angle, orientation and position relative to each other and relative to the probe housing itself. 
         [0053]    As used herein, references to the “exact” or “precise” position of transducer elements (and similar terms) may imply a relatively tight tolerance. For example, in some embodiments ultrasound probe calibration systems and methods may provide information describing the acoustic position of each transducer element in an array to within a distance of a fraction of a wavelength of ultrasound being used. In some embodiments, the acoustic position of transducer elements may be determined to within 1/10 of a wavelength. In other embodiments, the acoustic position of transducer elements may be determined to within a tolerance of less than 1/10 of a wavelength. In some embodiments, such as for calibrating a standard (i.e., single aperture) ultrasound probe, much looser tolerances may also be used, provided that such tolerances meet the needs of a particular system. 
         [0054]    Conventional ultrasound (or “scanline based” ultrasound as used herein) utilizes a phased array controller to produce and steer a substantially linear transmit waveform groups. In order to produce a B-mode image, a sequence of such linear waveform groups (or “scanlines”) may be produced and steered so as to scan across a region of interest. Echoes are received along each respective scanline in a process known as receive beamforming. The echoes received along the individual scanlines may then be combined to form a complete image. 
         [0055]    In a ping-based imaging process, an unfocused circular wavefront is transmitted from a point source transmitter, and the echoes are received by a plurality of receive transducers. The received echoes may then be beamformed using a ping-based beamforming process in order to determine a display location for each reflector that returns an echo. Beamforming is generally understood to be a process by which imaging signals received at multiple discrete receptors are combined to form a complete coherent image. The process of ping-based beamforming is consistent with this understanding. 
         [0056]    Embodiments of ping-based beamforming generally involve determining the position of reflectors corresponding to portions of received echo data based on the path along which an ultrasound signal may have traveled, an assumed-constant speed of sound and the elapsed time between a transmit ping and the time at which an echo is received. In other words, ping-based imaging involves a calculation of distance based on an assumed speed and a measured time. Once such a distance has been calculated, it is possible to triangulate the possible positions of any given reflector. This distance calculation is made possible with accurate information about the relative positions of transmit and receive transducer elements. Further details of ping-based imaging are described in U.S. patent application Ser. No. 13/029,907, now U.S. Pat. No. 9,146,313, referenced above. 
       Embodiments of Alignment Array Fixtures and Assemblies 
       [0057]      FIG. 1  illustrates an assembled multiple aperture ultrasound probe  10 . The probe  10  of  FIG. 1  includes three separate transducer arrays  12 A,  12 B,  12 C, each of which may be secured (or “potted”) in a precise desired position and orientation within a probe housing  14 . In some embodiments, the arrays may be potted in the probe housing  14  with a flowable solidifying material such as a room temperature vulcanizing (RTV) silicone rubber or any other similarly suitable epoxy or polymerizing material. RTV silicone is particularly suitable due to its thermal and mechanical properties, but other materials with similar properties may also be used. Generally any reference herein to a “flowable solidifying material,” a “solidifying polymer material,” a “flowable hardening material” or an “acoustic damping material” may refer to any suitable material that transitions from a liquid to a solid by a curing, drying or polymerizing process. Such materials may include RTV silicone, two-part epoxy resins, or others. 
         [0058]    In general, it may be desirable for a flowable solidifying material to have properties in its solid state that are similar to properties of a medium to be imaged and similar to a lens material attached to a manufactured transducer array (which may also be specified for particular applications). RTV silicone is well-suited to medical applications while more rigid materials, such as hard-curing epoxies or a metal-impregnated epoxies may be well-suited to non-destructive testing applications. In still further embodiments, a flowable solidifying material may be a phase-changing material. For example, a molten plastic may be flowed as needed, and then allowed to solidify by cooling to a temperature below a melting point. 
         [0059]    In various alternative embodiments, multiple aperture probes may be constructed with 2, 3, 4, 5, 6, 7, 8, 9, 10 or more individual transducer arrays in a common housing. In some embodiments, all transducer arrays in a probe may be oriented in a common imaging plane. In other embodiments, some arrays may be aligned in multiple imaging planes so as to facilitate 3D or 4D imaging. Generally, multiple aperture probes are designed with relatively tight tolerances for the position and orientation of arrays within the probe housing. In order to meet these tolerances while assembling a probe, an alignment and affixing process may be needed. 
         [0060]      FIG. 2  illustrates an example of a process  20  for aligning one or more arrays relative to a precision alignment element (PAE) and affixing the one or more arrays to the PAE. The process  20  of  FIG. 2  may begin with the step of mounting an array to a PAE  22  in such a way as to allow for the position and/or orientation of the array to be adjusted relative to the PAE. The PAE may then be mounted  24  to an alignment/adjustment assembly. Using the alignment/adjustment assembly, the alignment of the array relative to the PAE may be tested  26 . The result of the test may be evaluated  28  to determine whether the array is sufficiently aligned with the PAE. If the testing  26  reveals that the alignment of the array relative to the PAE is outside of a desired tolerance  30 , then the array&#39;s alignment may be adjusted  32 , and the alignment may be re-tested  26 . Once the array is determined to be aligned with the PAE to within a desired degree of precision  34 , the array may be more permanently affixed to the precision alignment assembly  36 . The aligned array &amp; PAE assembly may then be mounted to a probe alignment bracket  38 , and when all such PAE/array assemblies are mounted to the probe bracket, the entire assembly may be placed into a probe housing  40 , and the entire assembly may be permanently potted into the probe housing  42 . The embodiments of various structures that may be used for such a process will now be described before describing further detailed embodiments of an alignment process  20 . 
         [0061]      FIG. 3  illustrates one embodiment of an alignment and adjustment assembly  50 . In the illustrated embodiment, the assembly  50  may include a tank-affixing section  52 , an array affixing and adjusting assembly  54 , and a target assembly  56 . The assembly  50  of  FIG. 3  may be generally configured such that a PAE  60  may be supported in a known precisely aligned position and orientation relative to a target to be imaged, such as pins  66 ,  66 T. The orientation of an array  62  attached to the PAE  60  may be tested by imaging the target  56  (or more particularly pins  66 ,  66 T in some embodiments) using the array  62  and evaluating the resulting image (as described in further detail below). If the array  62  is found to be out of alignment, the orientation of the array  62  relative to the PAE  60  may be adjusted using the adjustment assembly  54  (as described in further detail below with reference to  FIG. 9 ). 
         [0062]    In the illustrated embodiment, the PAE  60  may be secured to the assembly  54  by arms  57 . In alternative embodiments, any number of other structures may also be used depending on the shape and configuration of the PAE and other portions of the alignment assembly  50 . In other embodiments, the PAE  60  may also include further structures and features designed to enable precise alignment of the PAE  60  with components of the assembly  54 . An Adjustment cover  61  may also be provided to surround the array  62 , and to provide structure for a plurality of adjustment screws  63 . 
         [0063]    In some such embodiments, the alignment assembly  50  of  FIG. 3  may be configured to allow the distance between the array  62  and the target  56  to be increased or decreased by known amounts. For example, the array affixing and adjusting assembly  54  may be mountable at a plurality of discrete locations  59  relative to the target assembly  56 . In alternative embodiments, a continuously variable height adjustment mechanism, such as a rack and pinion (or any other suitable mechanism) may be used to vary the height of the probe affixing assembly  54  relative to the target  56 . 
         [0064]    In some embodiments, all or part of the assembly  50  may be mounted relative to a tank containing a liquid bath such that at least the target assembly  56  and the emitting surface of the transducer array  62  may be submerged in a liquid medium with a known consistent speed of sound (e.g., water, gel, oil or other material as described in further detail below). In various embodiments, the tank-affixing section  52  may include any structure for affixing the assembly  50  relative to a water tank such that at least the array transducers  60  and the target  56  are submerged. 
         [0065]    In other embodiments, the water tank may be omitted. For example, a target assembly  56  may be encased within a solid material with a known consistent speed-of-sound (e.g., RTV silicone, ballistic gelatin, or any other solid elastic material suitable for use in ultrasound phantoms), and the transducer array  60  to be aligned may be acoustically coupled to a surface of the target assembly by an acoustic coupling gel or a conformable bladder containing a liquid or gel. In such embodiments, the material in which the target is encased may be selected based on the frequency and style of array under test. For example, whereas medical transducers are designed in the 1 to 18 MHz area, ideal target-encasing materials may have similar characteristics to human tissue. On the other hand, a transducer to be used in non-destructive testing (NDT) of industrial materials may be designed to operate at substantially higher frequencies in order to evaluate metals and composites. Some NDT arrays may also be air-coupled, doing its job without ever touching the work surface. Such devices typically work at much lower frequencies. A coupling medium is a bridge to allow energy of an appropriate frequency to travel back and forth from the testing array to the object under test (e.g., a phantom containing an alignment target). In the case of medical arrays and some low frequency NDT arrays, a coupling medium may include compatible gels, lotions, oils or water depending on the materials to be imaged. Higher frequency NDT arrays might use water, oil or a combination of liquids as a coupling medium. In some embodiments, a coupling medium may comprise a flexible bladder or pad of a material with suitable properties. 
         [0066]      FIG. 4  provides an exploded view further illustrating components of the array affixing and adjusting assembly  54  of  FIG. 3 . The array-holding assembly  54  may be secured in a consistent and known position and orientation relative to the target assembly  56 . Thus, depending on the shape and nature of the target assembly, various structures may be used to maintain the array-holder  54  in a known position relative to the target assembly. In the embodiment of  FIGS. 3 and 4 , the target assembly  56  may be secured to the array holder assembly  54  with rigid arms  58 . Any other alternative structures may also be used. 
         [0067]      FIG. 5  illustrates an embodiment of a target assembly  56  that may be used in an alignment process. In various embodiments, the target assembly  56  may include any structure with a known configuration of acoustic reflectors. A target  56  for use with an alignment process may generally have a pattern of reflectors that will allow for a clear indication of the array&#39;s alignment relative to the target  56 . Targets ideal for an alignment process are those that include a plurality of reflectors (or holes) that lie in a precise known pattern in a single plane that may be precisely aligned with the intended imaging plane of the array. For example, the target assembly  56  shown in  FIGS. 3-5  comprises a plurality of pins  66  (labeled as pins  66 A-D and T in  FIG. 5 ) arranged such that all of the pins  66  lie in a common plane that is coincident with the precisely aligned imaging plane. 
         [0068]    As best seen in  FIG. 5 , the pins  66 A-D and T may vary in length such that the tips of the pins may lie on multiple different points in the intended image plane (i.e., at different heights relative to a plane perpendicular to the imaging plane). In some embodiments, the pins may be oriented at angles such each pin&#39;s flat surface end may be oriented perpendicular to an arriving waveform transmitted from the array. In some embodiments, such angles may be selected assuming pings are transmitted from an origin at the center point of the array, even if pings are to be transmitted from multiple transmit elements at different locations on the array. 
         [0069]    In some embodiments, target pins may be arranged in a common imaging plane and oriented at angles such that longitudinal axes of the pins intersect at a single point near the transducer array (e.g., above the transducer array&#39;s transmitting surface in some embodiments). For example, in some embodiments, pins furthest from a center pin (the end pins) may be oriented such that, with the target positioned at a minimum distance from the array, the end pins lie at a desired angle relative to the precision alignment element. For example, the end pins may be oriented at an angle of about 30 degrees relative to a line perpendicular to the precision alignment element in the imaging plane. Larger or smaller angles may be desirable depending on an angle of sensitivity of transducer elements in transducer arrays to be aligned. 
         [0070]    When imaged by a transducer array to be aligned, each pin may appear as a dot. In this target configuration, each pin may appear as a dot in a known location on a display when the target is imaged by an aligned array supported in the array holder assembly. 
         [0071]    In some embodiments, a target  56  may include a plurality of reflectors positioned so as to evaluate the array&#39;s alignment at various discrete distances from the target. For example, a target may include a center pin  66 T and a plurality of pairs  66 A- 66 D of pins laterally spaced equal distances from the center pin  66 T. In various embodiments, a target may include any number of pairs of laterally-spaced pins. In some embodiments, the pairs of pins may be provided in a range of different lengths, meaning that some pairs of pins are closer to the transducer array than others. In various embodiments, alignment of an array may be evaluated at various distances from the target. In some embodiments, pin lengths may be calculated so as to place the faces of each pair of pins and the center pin  66 T coincident with an arc of a transmitted wavefront at a selected depth. 
         [0072]    In some embodiments, different reflectors of the target  56  may be configured and used for evaluating the array&#39;s alignment at different distances from the target  56  and/or for evaluating the array transmitting at different frequencies. For example, the target shown in  FIG. 5  may include several pairs of reflectors of different lengths to be imaged at different vertical distances between the array and the target. As shown, the first pair of reflectors  66 A may be configured for evaluating an array&#39;s alignment at a target-distance of about 1.5 cm, the second pair  66 B of reflectors may be configured for evaluating an array&#39;s alignment at a target-distance of about 3.0 cm, the third pair  66 C may be configured for evaluating an array&#39;s alignment at a target-distance of about 4.5 cm, and the fifth pair  66 D may be configured for evaluating an array&#39;s alignment at a target-distance of about 6.0 cm. In alternative embodiments, targets may be configured for testing an array at any distance as desired. 
         [0073]    In further alternative embodiments, the target  56  may include phantom structures with reflectors made of any suitable ultrasonically-reflecting material in a variety of alternative configurations. For example, the target may comprise a plurality of small sphere-shaped reflectors embedded in a solid material. Generally, a target may include any number of reflectors made of an appropriate echogenic material, as determined by the frequency and array style, that provides a small reflective surface relative to the wave length of the sound being used. Such reflective objects may be encased in precisely known locations in a sonolucent material. The sonolucent material to be used may be selected to be similar to conditions to be experienced by the array in an intended application. As such, in some cases, a sonolucent material may or may not offer attenuation. Reflectors in a target assembly need not be arranged in a symmetrical pattern, but preferably include multiple points at multiple different known locations such that alignment may be evaluated. The target may generally include any pattern of reflectors which may be supported within a solid, liquid or gaseous medium (depending on the intended use application). In some embodiments, a target may also include one or more “holes”—regions or objects that substantially absorb and do not reflect significant ultrasound signals. In some embodiments it may be desirable for reflectors or holes to be aligned in a single plane that is may be aligned with the ideal imaging plane. 
         [0074]    In some alternative embodiments, a target may include any substantially static object that may be imaged with an ultrasound probe. For example, any number of phantoms designed for sonographer training are widely commercially available from various suppliers of medical equipment. Some commercially available phantoms are made to mimic the imaging characteristics of objects to be imaged such as specific or generic human tissues. Such properties may or may not be used in combination with various embodiments described herein. An object need not be purpose-built as a phantom to be used as a phantom for the alignment processes described herein. 
         [0075]    As shown in  FIG. 6 , in some embodiments, the PAE  60  may include a plate  70  with precisely positioned mounting holes  72  precisely arranged for attachment to the holder  54 . For example, in some embodiments the PAE  60  may include two alignment mounting holes  72  configured to mount the PAE  60  to mounting arms  57  of the adjustment assembly (shown in  FIGS. 3 and 4 ). The PAE  60  may further include corner holes  74 A- 74 D for receiving set screws and for mounting the PAE  60  to a probe alignment bracket in a final probe assembly (e.g., as described below with reference to  FIG. 12 ). In some embodiments, temporary set screws in the corner holes  74 A- 74 D may also be used to adjust the position of an array relative to the PAE during an alignment procedure as described in further detail below. In some embodiments, the corner holes  74 A- 74 D may be tapped with fine pitch threads. 
         [0076]    In alternative embodiments, a precision alignment element may be provided in a variety of different structures, and may include any suitable features to ensure and/or to verify accurate and precise positioning of the PAE  60  relative to the target  56 . For example, the PAE may include holes, pins, recesses or other structures configured to engage (or to be engaged by) corresponding structures on a holder assembly. In general, a precision alignment element (or PAE) may be any structure that may be mounted in a known precise position relative to a target in an alignment test assembly. Similarly, alternative holder assembly structures may include clamps, screws, pins, holes, recesses and various other mechanical structures configured to engage corresponding portions of a PAE and to hold a PAE in a consistent known position and orientation relative to a target. 
         [0077]    In some embodiments, precision alignment features may be integrated into a probe alignment bracket (such as that described below with reference to  FIG. 12 . In such embodiments, a probe alignment bracket configured to support transducer arrays in a desired orientation relative to one another may include array-mounting sections, gasket-supporting sections and holes for injecting a flowable solidifying material once an array is aligned. In such some embodiments, a plurality of targets may be provided such that each array has a corresponding target arranged perpendicular to the aligned array plane. Alternatively, a target or a PAE holder may be adjustable so as to position the bracket PAE and the target(s) in a perpendicular orientation. 
         [0078]      FIG. 6  illustrates an embodiment of a PAE  60  and a gasket  76  for adjustably securing the PAE  60  to a transducer array  62 . In some embodiments, the PAE  60  may comprise a plate with a lower surface  78  sized and configured to be bonded to a back surface  80  of a transducer array. 
         [0079]    As shown in  FIG. 7 , in some embodiments the lower surface  78  of the PAE  60  may include a recessed section  81 . The recessed section  81  may be machined (or otherwise formed) to a precise depth and dimensions for creating a gasket  76 . In some embodiments, a gasket  76  may be formed by extruding a bead or injecting a flow of a liquid or flowable solidifying material (e.g., RTV silicone, epoxy or other flowable solidifying material) around the perimeter of the recessed section  81  on the lower surface  78  of the PAE  60 . In some embodiments, before the solidifying material cures, the PAE  60  and the gasket  76  may be pressed onto the back surface  81  of the transducer array  62 . 
         [0080]    In some embodiments, a jig  82 , such as that shown in  FIG. 8 , may be used to ensure that the gasket  76  is compressed to a consistent thickness. Using the jig  82 , a consistent desired gasket thickness may be achieved by placing the PAE  62  in the provided slot  84 , and then placing the transducer array  62  into the space above the PAE  62  until it abuts the shoulders  86  such that the height of the shoulders  86  above the PAE&#39;s lower surface  78  ensures a consistent spacing between the lower surface  78  of the PAE  60  and the back surface  80  of the transducer array  62 . 
         [0081]    It is generally desirable for the gasket to secure the PAE to the array while remaining somewhat flexible, allowing a small degree of movement between the PAE and the array during the alignment and adjustment process. Such flexibility may be achieved through selection of an appropriate flowable solidifying material and/or selecting a gasket thickness and width that allows sufficient flexibility. Alternatively, flexibility of the gasket may be achieved by performing the adjustment process before a hardening material completely cures. 
         [0082]    In some embodiments, the PAE  60  and array  62  may be held within the jig  82  for a sufficient time for the gasket material to cure. Once cured, the PAE  62  will be temporarily secured to the back surface  80  of the transducer array  62  by the gasket, while allowing a small range of movement between the PAE  60  and the array  62 . Although the use of a jig may provide a certain degree of precision to the assembly, the actual acoustic position of the transducer elements may not necessarily be precisely consistent with the physical back surface  80  of the transducer array  62  simply due to inevitable manufacturing variability. 
         [0083]    In some embodiments, the PAE  60  may also include features and structures configured to facilitate precise alignment of the PAE  60  and an attached transducer array  62  with elements of a final probe assembly. For example, as shown in  FIG. 6 , the plate  70  may also include a plurality of holes  74 A- 74 D precisely positioned for precisely mounting the alignment element  60  to a probe alignment bracket as will be described in further detail below with reference to  FIG. 12 . In some embodiments, the plate  70  may also include one or more channels  92  precisely sized oriented to engage corresponding structures in a probe alignment bracket. 
         [0084]    In some embodiments, the PAE  60  may include one or more injection holes  94  through which a flowable solidifying material may be injected once the transducer array is determined to be perfectly aligned with the PAE (as will be described in further detail below with reference to  FIG. 9 ). The PAE  60  may also include a relief  96  surrounding the injection holes  94  to prevent any overflowing affixing material from interfering with the fit of the PAE in a probe alignment bracket (as described in further detail below). 
         [0085]      FIG. 9  illustrates an array  62  to be aligned to a precision alignment element  60  and mounted in an adjustment assembly  54 . The adjustment assembly  54  may generally include one or more adjustment mechanisms configured to move the array  62  relative to the PAE  60 . In the illustrated embodiment, a plurality of set screws  63 A- 63 C may be provided as adjustment mechanisms. A spring  98  (or other resilient material or device) may also be provided to mechanically bias the array towards the adjustment mechanisms so as to maintain contact between the array  62  and the set screws  63 A- 63 C. In some embodiments, a point-contact device may be positioned between the spring  98  and the array  62 . A point-contact device may be any structure that creates a small point of contact with the array, such as a pin, a nail, a sphere, a cone, or otherwise shaped structures. Any number of set screws in any desired arrangement may be used to adjust the position of the array  62  relative to the PAE  60 . 
         [0086]      FIG. 9  illustrates several set screws  63 A- 63 C for adjusting the position of the array  62  relative to the PAE. The six adjustment set screws in the front surface of the adjustment cover  61  may be used for adjusting the position of the array by displacing a portion of the array in the Y direction. For example, tightening the bottom of the center screws  63 B will tend to cause the array to pivot about the longitudinal axis  102 , while tightening the right-side screws  63 C or the left-side screws  63 A will tend to cause the array  62  to pivot about the vertical axis  106 . Tightening all of the front screws (or at least the top left and right side screws) may cause the array  62  to translate along the elevation axis  104 . In some embodiments, adjustment set screws may also be used in one or more of the four corner holes  74 A- 74 C in the PAE  60 . Tightening a set screw in the screw right rear hole  74 B will tend to cause the array to pivot about the longitudinal axis and the elevation axis. Thus, depending on the degree and the direction of misalignment detected during a testing step, one or more set screws may be adjusted until a desired adjustment of the array&#39;s position relative to the PAE  60  is achieved. 
         [0087]    In various embodiments, ribbon connectors extending from the array may be electrically connected to a controller in order to transmit and/or receive ultrasonic signals using the transducer array. Any suitable connector may be used for achieving such electrical connections. 
       Alignment Imaging Controller Embodiments 
       [0088]      FIG. 10  illustrates a block diagram of a controller  200  that may be used for controlling, transmitting, and receiving of ultrasound signals using the transducer array  62  during an alignment process. The controller  200  may also be configured to generate and display images based on the received echo data. In some embodiments, the controller  200  may further be configured to store raw echo data for later retrieval and analysis. 
         [0089]    As shown in  FIG. 10 , a controller  200  may electronically and logically connected to a transducer array  202  to be aligned. In some embodiments, at least some of the transducer elements may be designated as transmit elements, while others may be designated as receive elements. In some embodiments, each transducer element may convert ultrasound vibrations into time-varying electrical signals and vice versa. In various embodiments, the array  62  to be aligned to a PAE may include any number of ultrasound transducer elements in any desired configuration. 
         [0090]    The controller  200  may contain software and hardware elements configured to control an imaging process. In various embodiments of the alignment methods described herein, any imaging method (e.g., ping-based imaging, scanline-based imaging or any other available ultrasound imaging method) may be used for imaging the target assembly. Due to the use of transducer element position in ping-based beamforming methods, such methods may be particularly suited for an alignment evaluation process. 
         [0091]    The transmission of ultrasound signals from elements of the array  62  may be controlled by a transmit controller  204 . Upon receiving echoes of transmit signals, the transducer elements may generate time-varying electric signals corresponding to the received ultrasound vibrations. Signals representing the received echoes may be output from the array  62  and sent to a receive subsystem  210 . In some embodiments, the receive subsystem may include multiple channels (e.g., one channel for each transducer element in some embodiments). Each channel may include an analog front-end device (“AFE”)  212  and an analog-to-digital conversion device (“ADC”)  214 . In some embodiments, each channel of the receive subsystem  210  may also include digital filters and data conditioners (not shown) after the ADC  214 . In some embodiments, analog filters prior to the ADC  214  may also be provided. In some embodiments, the output of each ADC  214  may be directed into a raw data memory device  220 . Notably, the controller  200  need not include a scan converter for systems configured to use a ping-based imaging method. 
         [0092]    In some embodiments, raw echo data may be stored in a raw data memory device  220  prior to any beamforming or image formation. In some embodiments, echo data may be passed directly from a receive subsystem  210  to an image formation sub-system  230 . 
         [0093]    The image formation sub-system  230  may include a beamformer  232  and an image layer combiner (“ILC”)  234 . If needed, image data may be temporarily stored in an image buffer memory device  236 . In some embodiments, the image formation subsystem may retrieve stored echo data from the raw data memory device rather than receiving real-time echo data from the receive sub-system. The beamformer  232  may include or may have logical access to a memory device  238  containing transducer element position data. In the case of a new un-aligned and un-calibrated transducer array, such transducer element position data may be based on an idealized case for transducer arrays of a particular type. Alternatively, the transducer element position data may be based on calibration analysis of a plurality of previously-aligned arrays. 
         [0094]    An alignment overlay subsystem  235  may include stored data including information describing known positions of reflectors in the target  56 . In some embodiments, such an alignment overlay sub-system may include information for several targets which may be selectable by a user depending on which target is to be used. The alignment overlay subsystem may also include hardware and software for forming an image of expected reflector positions and additional information for assisting in assessing the alignment of a transducer array under examination. 
         [0095]    The controller  200  may be further configured to output image data to a display sub-system  240 . A display subsystem  240  may include a video processor  242  for performing various digital video image processing steps, a video memory  246  for storing “cine loop” data (e.g., processed video clips), and a display controller  244  configured to output image pixels to a display device. 
       Array Alignment Testing and Adjustment Method Embodiments 
       [0096]    Thus, returning to the process diagram of  FIG. 2 , an embodiment of a process for aligning a transducer array  62  relative to a PAE  60  using an alignment apparatus such as that shown in  FIGS. 3-6  will now be described. Once a PAE  60  has been temporarily mounted to an array  62  with a gasket  76 , the PAE  60  and array  62  may be mounted in the adjustment section  54  of an alignment assembly  50 . Ribbon connectors extending from the array  62  may then be electronically connected to an alignment imaging controller  200 . In some embodiments, the degree of alignment (or misalignment) may then be tested by imaging the target assembly  56  with the array  62 . 
         [0097]    Because the position of the pin tips  66  may be known with a high degree of precision, the expected image produced by a perfectly-aligned array may be predicted with a high degree of precision. Thus, the actual obtained image may be compared with the theoretically ideal image, and the alignment of the array may be quantitatively and/or qualitatively evaluated. In some embodiments, such a qualitative comparison may be performed visually by a user. In order to assist in visually comparing the actual image with the theoretical image, a software layer may be configured to overlay a schematic representation of the theoretical image with the actual image. In some embodiments, the two images may be displayed in contrasting colors to further aid in distinguishing the actual image from the theoretical image. 
         [0098]      FIGS. 11A and 11B  illustrate an embodiment of an actual image of a target with five reflectors in precisely known positions. The reflector images are indicated by the amorphous-shaped patterns  110 A- 110 E, and an overlaid theoretical image is indicated by the circles  112 A- 112 E.  FIG. 11A  illustrates an example of an actual image that is misaligned with the target (and therefore, the array  62  is misaligned with the PAE  60 , since the PAE  60  is known to be precisely aligned with the target). In some embodiments, a bar graph  114  (and/or a numerical value, a line graph or other quantitative visual information) may be displayed along with the theoretically correct image. Each bar of the bar graph  114  may indicate the intensity of reflectors lying within the ideal target region defined by one of the circles  112 . Thus, each bar  115 A- 115 E may correspond to each circle  110 A- 110 E, which correspond to known positions of the reflectors (e.g., pins  66 ). A higher bar level may indicate better alignment of the actual image with the theoretical image for a given reflector position  110 . 
         [0099]    In the example of  FIG. 11A , the center pin image  110 C appears to be well-aligned while the images of the pins on the left  110 A,  110 B appear too high, and the image of the pins on the right  110 D,  110 E appear too low. This pattern may indicate that the array is misaligned in rotation about the elevation axis  104  (i.e., the left side of the array is too close to the target, and the right side of the array is too far away from the target), In view of this misalignment, the array  62  may be adjusted by tightening the right-side set screws  74 B,  74 C. 
         [0100]    Misalignment due to rotation about the longitudinal axis  102  may be detected by recognizing that the images of all of the pins  110 A- 110 C (or at least the center pin image  110 C) is not as bright as expected. Such misalignment may be corrected by adjusting either the front screws  74 D,  74 C in the PAE or the rear PAE screws  74 A,  74 B depending on the suspected direction of misalignment about the longitudinal axis. In some cases, similar adjustments may be made my adjusting screws  63 A- 63 C in the adjustment cover  61 . 
         [0101]    Misalignment about the vertical axis  106  may result in the images of pins further from the center being progressively less bright than the center pin image  110 C. Such misalignment about the vertical axis may be corrected by tightening one or more of the adjustment screws  63 A or  63 C in the front plate  61  of the adjustment assembly  54 . 
         [0102]    In some embodiments, an assessment of the alignment of an array under test may be made based primarily on the imaged position of the center pin  110 C and a single pair of pins equidistant from the center pin  110 C. For example, the degree and direction of any misalignment of the array may be determined by evaluating the imaged position of only the center pin  110 C and the two next-closest pins  110 B and  110 D relative to the expected positions of those pins. 
         [0103]      FIG. 11B  illustrates an example of an image that may be produced by an array that is substantially perfectly aligned with the target  56  and the PAE  60 . In some embodiments, the degree of variation from the ideal image that may be allowable within a designed tolerance may be determined by experimentation. 
         [0104]    In some embodiments, the step  26  (in the process of  FIG. 2 ) of testing the alignment of an array  62  relative to a PAE  60  may be performed using a tank assembly such as that shown and described in U.S. patent application Ser. No. 12/760,327, now U.S. Pat. No. 8,473,239. In that system, the alignment of an array supported at an upper part of a tank may be tested by transmitting an ultrasound signal from the array and receiving echoes using a separate set of hydrophones located at the bottom of the tank. 
         [0105]    With reference to  FIGS. 6 and 9 , once the array is found to be sufficiently aligned, the array  62  may be fixed in the new position relative to the PAE  60  by injecting a low viscosity flowable solidifying material through the injection holes  94  in the PAE. The solidifying material used in this step may have a sufficiently low viscosity to allow easy injection and filling of the space between the PAE and the array without altering the array&#39;s alignment relative to the PAE. The solidifying material may then be allowed to cure. In some embodiments, a quantity of flowable solidifying material may be injected into one hole  94  until the liquid solidifying material is seen extruding from the second hole  94 . In other embodiments, a measured quantity of the flowable solidifying material approximately equal to the volume of the space between the PAE  60  and the back surface  80  of the array  62 . Excess solidifying material may be allowed to extrude from the second hole. Once the solidifying material has cured, the array  62  will be secured to the PAE  60  in the aligned orientation, thus forming an aligned array-PAE assembly. At this point, the set screws may be removed or backed out from the adjusted positions, and the aligned array-PAE assembly may be removed from the adjustment and alignment assembly  54 . If needed, the process may be restarted for a new array. 
         [0106]    In various embodiments, some or all of the process of testing and adjusting alignment of a transducer array may be automated. For example, software may be provided and configured for evaluating misalignment of an array and selecting a suitable corrective adjustment as described above. Furthermore, robotic elements may be provided and configured to adjust the various set screws in order to automatically apply a corrective adjustment selected by a software agent. A robotic element may also be provided for injecting a quantity of a flowable solidifying material into the space between the PAE and the array. 
       Probe Assembly Method Embodiments 
       [0107]    Once a sufficient number of arrays have been aligned to their respective PAEs, the aligned arrays may be mounted to a probe alignment bracket  120  such as that shown in  FIG. 12  before final assembly into a probe housing  14  ( FIG. 13 ). In some embodiments, a probe alignment bracket  120  may be provided with a plurality of array-receiving sections  122 A- 122 C. Each array-receiving section  122 A- 122 C may include structural features for receiving a PAE  60  attached to an aligned array  62 . In some embodiments, the receiving sections  122 A- 122 C may include ribs configured to engage channels  92  in the PAE  60  ( FIG. 6 ). The receiving sections  122 A- 122 C may also include a plurality of screw holes  124  through which mounting screws may pass for attaching PAEs  60  to the probe alignment bracket  120 . The alignment bracket  120  may also include flanges  126  and/or other features to assist in positioning the PAEs in the proper positions. In other embodiments, a probe alignment bracket may have a wide range of shapes and configurations beyond that illustrated here depending on the number and designed orientation of arrays to be included in a probe. 
         [0108]    In some embodiments, the probe alignment bracket  120  may also include attachment flanges  128  for securing an electronic connection board (not shown). An electronic connection board may be configured with a plurality of connectors configured for electrical connection to the flex connectors extending from each transducer array. In some embodiments, the connector board may further include traces connecting the transducer array connections to a common connector that may be configured for connection to a cable. Details of some embodiments of such connector boards and cabling assemblies may be seen in Applicants&#39; U.S. patent application Ser. No. 13/272,098 titled “Multiple Aperture Probe Internal Apparatus and Cable Assemblies,” which is incorporated herein by reference. 
         [0109]    The probe internal assembly including the probe alignment bracket  120 , connector board and aligned array-PAE assemblies may then be inserted into a probe housing  14  as shown in  FIG. 13 . In various embodiments, a probe housing  14  may include a one-piece construction, a clamshell construction, or any other suitable configuration. In some embodiments, portions of the internal assembly may be attached to portions of the probe housing by screws, bolts, clamps, clips, pins, or any other suitable attachment device. 
         [0110]    Once the internal assembly is fully inserted into a probe housing  14 , the aligned arrays  12 A- 12 C and the probe alignment bracket  120  to which they are mounted may be permanently potted by injecting a flowable solidifying material  130  such as RTV silicone into the shell housing, surrounding at least portions of the arrays  12 A- 12 C. In some embodiments, a flowable solidifying material  130  may also be injected further into the probe housing  14  so as to surround all or portions of the probe alignment bracket  12 . In some embodiments, the flowable solidifying material may be used to substantially fill the space between the arrays and the sides of the probe housing  14 . The solidifying material may also be smoothed out so as to provide a substantially consistent surface with the front surfaces of the arrays  12 A- 12 C. 
       Embodiments of Completed Probe Assemblies 
       [0111]    In various embodiments, a final probe assembled using the systems and methods described above may have some unique characteristics, some of which are illustrated in  FIG. 13 . As shown in the cross-sectional view of  FIG. 13 , a completed probe may include a plurality of transducer arrays  12 A- 12 C potted into the probe housing  14  by a quantity of a solidified potting material  130  (e.g., RTV silicone or any other solidified flowable solidifying material). Each transducer array  12 A- 12 C may be seen to be secured to a precision alignment element  60 A- 60 C by an additional layer of a solidified material  132  between the precision alignment element  60  and the transducer array  12  ( 62 ). The layer of solidified material  132  may include the gasket ( 76  in  FIGS. 6 and 7 ) and the affixing layer of solidifying material injected after aligning the array to the PAE. The precision alignment elements  60 A- 60 C are, in turn, mounted to a probe alignment bracket  120  in precise positions. 
         [0112]    Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Various modifications to the above embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow. 
         [0113]    In particular, materials and manufacturing techniques may be employed as within the level of those with skill in the relevant art. Furthermore, reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “and,” “said,” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, unless explicitly stated otherwise, the term “or” is inclusive of all presented alternatives, and means essentially the same as the commonly used phrase “and/or.” It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.