TECHNICAL FIELD 
       [0001]    The following generally relates to ultrasound imaging and more particularly to an ultrasound imaging transducer array with individual elements and/or sub-groups of elements on a substrate rather than a fabricated entire array of physically connected elements mounted on the substrate. 
       BACKGROUND 
       [0002]    Micromachined ultrasonic transducers (MUT), such as capacitive (CMUT) or piezoelectric (PMUT) types, have demonstrated performance benefits relative to traditional piezoelectric-based transducers. These benefits include increased bandwidth, which enables a wider range of imaging options such as more harmonic frequencies and improved contrast agent imaging. However, very few MUT based probes have been marketed commercially. One challenge in the production of commercial MUT probes is a performance implication of the silicon backbone. The literature has explored the inter-element acoustic crosstalk that can be supported by the rigid, continuous backbone. Some have used diced or etched grooves on the backbone or between elements to reduce crosstalk, which has been fairly effective. In another approach, a compliant substrate was fabricated in the wafer processing to decouple adjacent CMUT cells. The ideal crosstalk configuration, though, would provide complete backbone separation between elements. 
         [0003]    Another challenge in the production of commercial MUT probes is the finite number of arrays that can be produced on each wafer. For instance, while it may be possible to fit twenty-three high-frequency linear arrays on a standard  6 ″ wafer ( FIG. 1 ) or eight abdominal arrays in the same circular area ( FIG. 2 ), the fabrication cost for any one wafer is roughly the same, regardless of the number of arrays produced on the wafer. Consequently, the cost per array scales with the area of the array. Some area of the wafer remains unused because of the limit to the number of rectangles/arrays that can be fit into the circular wafer (i.e. the empty spaces in  FIGS. 1 and 2 ). Furthermore, the yield of any individual element has consequences on the yield of the whole array. For instance, if one element has a poor etching characteristic, or if some contamination inhibits the sputtering adhesion on that element, then the entire array must be scrapped. In  FIGS. 1 and 2 , each rectangle is a fabricated monolithic array and include a plurality of physically connected elements, and the individual elements of an array are not physically separate and distinct components. 
         [0004]    Another challenge in commercial production is the fabrication of curved arrays. MUT designs use a solid backbone of silicon, which prohibits curving the array in azimuth. The silicon backbone could be thinned with chemical-mechanical polishing after the top-side processing is complete, but this puts the individual elements at risk, introduces additional variability, and does not eliminate the risk that the thinned silicon backbone could still break during bending. Another approach involves etching grooves into the backbone and optionally filling them with a flexible polymer, but this approach requires complex etching and alignment between the top and bottom surfaces of the wafer, complicates handling in the pre-polymer state, and requires a non-standard polymer groove-filling application to the silicon. A reverse fabrication process has been implemented in which the silicon backbone is completely removed late in the process, which could possibly facilitate curvature after the backbone removal. However, this technique makes it impossible to test elements at the wafer-level, thereby increasing the cost of any wafer-level failures that could not be discovered until the probe assembly is nearly complete. Unfortunately, all of these approaches add wafer-level complexity and the potential for increased cost. 
       SUMMARY 
       [0005]    Aspects of the application address the above matters, and others. 
         [0006]    In one aspect, a transducer array for an ultrasound imaging system includes a substrate and a single array comprising individual sub-sets of transducer elements disposed on the substrate, wherein the individual sub-sets are physically separate from each other and spatially arranged contiguous to each other. 
         [0007]    In another aspect, an apparatus includes a transducer array with a substrate and a single array comprising individual sub-sets of transducer elements disposed on the substrate, wherein the individual sub-sets are not in physical contact with each other and are serially arranged with respect to each other. The apparatus further includes transmit circuitry that conveys an excitation pulse to the transducer array, receive circuitry that receives a signal indicative of an ultrasound echo from the transducer array, and a beamformer that processes the received signal, generating ultrasound image data. 
         [0008]    In another aspect, a method includes transmitting an ultrasound signal with a transducer array having a plurality of individual and physically separate elements and/or sub-groups of elements, receiving an echo signal with the transducer array, beamforming the echo signal to create an image, and displaying the image. 
         [0009]    Those skilled in the art will recognize still other aspects of the present application upon reading and understanding the attached description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The application is illustrated by way of example and not limited by the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0011]      FIG. 1  schematically illustrates a prior art example of a finite number of arrays fabricated on a wafer; 
           [0012]      FIG. 2  schematically illustrates another prior art example of a finite number of arrays fabricated on a wafer; 
           [0013]      FIG. 3  schematically illustrates an example ultrasound imaging system with a transducer array including individual transducer elements and/or sub-groups of transducer elements; 
           [0014]      FIG. 4  schematically illustrates an example linear array with individual transducer elements and/or sub-groups of transducer elements thereon; 
           [0015]      FIG. 5  schematically illustrates another example linear array with individual transducer elements and/or sub-groups of transducer elements thereon; 
           [0016]      FIG. 6  schematically illustrates an example curved array with individual transducer elements and/or sub-groups of transducer elements on a flexible substrate disposed on a curved backbone; 
           [0017]      FIG. 7  schematically illustrates an example curved array with individual transducer elements and/or sub-groups of transducer elements disposed directly on the curved backbone; 
           [0018]      FIG. 8  schematically illustrates a wafer with a plurality of rows of individual elements; and 
           [0019]      FIG. 9  illustrates an example method in accordance with an embodiment herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 3  schematically illustrates an example ultrasound imaging system  300 . 
         [0021]    The ultrasound imaging system  300  includes a one-dimensional (1-D) or two-dimensional (2-D) transducer array  302  of transducer elements  304 . The transducer elements  304  are configured to transmit ultrasound signals, receive echo signals (which are generated in response to the transmitted ultrasound signal interacting with stationary and/or flowing structure), and generate electrical signals indicative of the received signals. Example arrays include sector, linear, curved, square, rectangular, circular, irregular and/or other shaped arrays. Example elements include CMUT, PMUT, TFUT (thick-film ultrasound transducer), PZT (lead zirconate titanate), PMN-PT (lead magnesium niobium-lead titanate), PIN-PMN-PT (lead indium niobium-PMN-PT), and/or other transducer elements. 
         [0022]    In the illustrated embodiment, the transducer elements  304  are only individual elements, only sub-groups of elements (each with two or more, but not all), or a combination of at least one individual element and at least one sub-group of elements, and are not all part of a fabricated monolithic array, such as those fabricated on wafers such as those shown in  FIGS. 1 and 2 . In one non-limiting instance, radiating surfaces of the elements  304  are planar on the order of ±0.015 millimeters (mm), lateral spacing of the elements  304  with respect to each other and along an array direction (azimuth) is on the order of ±0.015 mm, and/or consistency along element direction (elevation) is on the order of ±0.05 mm. Other surfaces, lateral spacing and/or consistency are contemplated herein. 
         [0023]    Non-limiting examples of the transducer array  302  are described in connection with  FIGS. 4 and 5  for linear arrays and  FIGS. 6 and 7  for curved arrays. Other array configurations are contemplated herein. 
         [0024]      FIG. 4  depicts an example in which element  402   1 ,  402   2 ,  402   3 , . . . ,  402   N  (where N is a positive integer), collectively referred to herein as elements  402 , are disposed on a (rigid or flexible) substrate  404 , which includes pairs of electrical traces  406   1 ,  406   2 ,  406   3 , . . . ,  406   N , one pair for each element. Each element  402  is electrically coupled to a corresponding pair of traces via a wire, etc. The substrate  404  is configured to include predetermined acoustic properties, e.g., to mitigate any reflections and/or suppress any vibrational modes (e.g., Lamb waves) that could couple into it from the element vibration. Alternately, the substrate  404  consists of integrated circuits (e.g., for signal conditioning and/or processing) that could have exposed attachment pads. 
         [0025]    An electro-mechanical machine  408  (or alternatively, tweezers) is shown placing the element  402   N  on the substrate  404 . For example, after the elements  406  are cut out of a wafer, each element  402  is individually and independently (with respect to the other element(s)) picked up and placed on the substrate  404  (i.e. into the array  302 ) using known technology. In this instance, each of the elements  402  is not physically connected to another element(s)  402  or fabricated on a same wafer with another element(s)  402 . For example, each of the elements  402  is only physically connected to the substrate  404 , with the exception of wires, active and/or passive components, etc., if any. The elements  402  are attached to the substrate  404  using an adhesive, solder, and/or other bonding mechanism. 
         [0026]    In the illustrated example, the elements  402   1 ,  402   2 , and  402   3  are individual elements disposed on the substrate  404  (e.g., similar to the element  402   N ) and with a separation there between on the order of ±0.015 mm. In a variation, the sub-group of elements  402   1  and  402   2  or the sub-group of elements  402   1 ,  402   2 , and  402   3  are fabricated on and as part of a single unity sub-array and the sub-array (and not individual elements) is disposed on the substrate  404 . For example, the sub-group of elements  402   1  and  402   2  or the sub-group of elements  402   1 ,  402   2 , and  402   3  could be individual sub-arrays formed on a wafer. With this configuration, the sub-group of elements  402   1  and  402   2  or the sub-group of elements  402   1 ,  402   2 , and  402   3  are physically connected to each other, and neither the elements  402   1  and  402   2  nor the elements  402   1 ,  402   2 , and  402   3  are physically connected to the element  402   N . 
         [0027]      FIG. 5  depicts an example in which elements  502   1 ,  502   2 ,  502   3 , . . . ,  502   N  (collectively referred to herein as elements  502 ) are disposed on a substrate  504 . The substrate  504  includes a plurality of electrical interconnects  506  embedded in the substrate  504 . Each of the elements  502   1 ,  502   2 ,  502   3 , . . . ,  502   N  includes one or more electrical connections  508  through a thickness dimension of the element instead of from a top surface of the element (e.g., a through-silicon vias (TSV&#39;s) where the material is silicon, etc.) through which electrical signals are transferred from a respective one of the elements  502   1 ,  502   2 ,  502   3 , . . . ,  502   N  to the substrate  504 . Similar to the configuration described in  FIG. 4 , a machine, etc. can place the individual elements  502  on the substrate  504 , and the elements  502   1 ,  502   2 , and  502   3  are individually and independently disposed on the substrate  504 , or the elements  502   1  and  502   2  or the elements  502   1 ,  502   2 , and  502   3  are part of a single unity piece disposed on the substrate  504 . 
         [0028]      FIG. 6  depicts an example in which a flexible substrate  602  with elements  604  is disposed on a backing  606  having a rigid curved surface  608 . In one instance, the elements  604  are disposed individually on the flexible substrate  602  while the flexible substrate  602  is flat, e.g., similar to the element  402   N  of  FIG. 4 . In another instance, the elements  604  are disposed on the flexible substrate  602  while the flexible substrate  602  is flexed. For example, the flexible substrate  602  can first be flexed around and/or disposed on the rigid curved surface  608 , and then the elements  604  can be disposed on the flexible substrate  602 . Likewise, a machine, etc. can place the elements  604 , and the elements  604  are individually and/or group-wise placed on the flexible substrate  602 . 
         [0029]      FIG. 7  depicts the array in  FIG. 6  except that the flexible substrate  602  is omitted, and the elements  604  are individually disposed directly on the curved backing  606 . In this instance, the curved backing  606  serves as a substrate. Likewise, a machine, etc. can place the elements  604 , and the elements  604  are individually and/or group-wise placed on the backing  606 . 
         [0030]    For the approached described herein, the individual and/or sub-groups of elements (and not the entire array  302 ) are first fabricated in a wafer, then individually separated (e.g., etched, diced, etc.) from the wafer, and subsequently assembled individually into the transducer array  302 . Relative to a configuration (e.g.,  FIGS. 1 and 2 ) in which entire arrays are fabricated on a wafer, the approach described herein enables increased wafer area utilization optimizing cost per element, increased array geometry flexibility (e.g., elements can be assembled into curved transducers without additional wafer-level complexity), and/or a reduction of acoustic cross-talk, e.g., by the elimination of a common backbone.  FIG. 8  depicts an example wafer  800  with a plurality of rows  802  of individual and/or groups of fabricated transducer elements  804 , and not complete arrays. 
         [0031]    Returning to  FIG. 3 , transmit circuitry  306  generates pulses that excite a predetermined set of the elements to transmit the ultrasound signals. Receive circuitry  308  receives the electrical signals. In one instance, the receive circuitry  308  is configured to pre-process the received signals, e.g., by amplifying, digitizing, etc. the signals. A switch  310  switches between the transmit and receive circuitry  306  and  308 , depending on whether the transducer array  302  is in transmit or receive mode. A beamformer  312  processes the received signals, for example, by applying time delays and weights, summing, and/or otherwise processing the received signal. Other processing is also contemplated herein. 
         [0032]    A scan converter  314  scan converts the beamformed data, converting the beamformed data into the coordinate system of a display  316 , which visually displays the images. The data can be visually displayed in a graphical user interface (GUI), which allows the user to selectively rotate, scale, and/or manipulate the displayed data through a mouse, a keyboard, touch-screen controls, etc. A controller  318  controls one or more of the components of the system  300 , e.g., based on a mode of operation (e.g., B-mode, etc.). A user interface  320  includes an input device (e.g., a physical control, a touch-sensitive surface, etc.) and/or an output device (e.g., a display screen, etc.). A mode, scanning, and/or other function can be activated by a signal indicative of input from the user interface  320 . 
         [0033]    In one instance, the transducer array  302  is part of a probe and the transmit circuitry  306 , the receive circuitry  308 , the switch  310 , the beamformer  312 , the scan converter  314 , the controller  318 , the user interface  320 , and the display  316  are part of a console. Communication there between can be through a wired (e.g., a cable and electro-mechanical interfaces) and/or wireless communication channel. In this instance, the console can be a portable computer such as a laptop, a notebook, etc., with additional hardware and/or software for ultrasound imaging. The console can be docked to a docketing station and used. 
         [0034]    Alternatively, the console can be part (fixed or removable) of a mobile or portable cart system with wheels, casters, rollers, or the like, which can be moved around. In this instance, the display  316  may be separate from the console and connected thereto through a wired and/or wireless communication channel. Where the cart includes a docking interface, the console can be interfaced with the cart and used. An example of such a system is described in US publication 2011/0118562 A1, entitled “Portable ultrasound scanner,” and filed on Nov. 17, 2009, which is incorporated herein in its entirety by reference. 
         [0035]    Alternatively, the transducer array  302 , the transmit circuitry  306 , the receive circuitry  308 , the switch  310 , the beamformer  312 , the scan converter  314 , the controller  318 , the user interface  320 , and the display  316  are all housed by and enclosed within a hand-held ultrasound apparatus, with a housing that mechanically supports and/or shields the components within. An example of a hand-held device is described in U.S. Pat. No. 7,699,776, entitled “Intuitive Ultrasonic Imaging System and Related Method Thereof,” and filed on Mar. 6, 2003, which is incorporated herein in its entirety by reference. 
         [0036]    The transducer array  302  described herein can have alternating high- and low-frequency elements (e.g., odd elements are low frequency, even elements are high frequency). Such a design could enhance tissue harmonic imaging (transmit primarily on low-frequency elements, receive primarily on high-frequency elements) and/or improve image quality through the entire depth (use smaller-elevation high-frequency elements for shallow imaging and large-elevation low-frequency elements for deep imaging). 
         [0037]    Furthermore, the surface profile of the elements can be varied across elevation by depositing multiple rows of varying thicknesses. This allows for elevation focusing, which is difficult to do on a basically planar piece of silicon. Furthermore, elements from varying sizes, shapes, and/or compositions can be assembled. For instance, the array  302  can have alternating CMUT and PZT elements, e.g., where the PZT elements are used for transmit and the CMUT elements for receive. This arrangement would be very difficult without the fine-scale assembly approach described herein. 
         [0038]    The element and/or group-wise construction described herein is compatible with a wide variety of cell designs (e.g., square, hexagonal, circular, extra membrane mass, non-flat surfaces, etc.), element designs (e.g., multi-frequency, apodization, patterned electrodes, etc.), wafer processing techniques (e.g., sacrificial release, wafer bonding, etc.), and/or interconnect configurations (e.g., top surface only for wire bonding or other attachment, through-silicon vias, trench connections, etc.). Generally, the element and/or group-wise construction described herein is independent of the specific design of the transducer element  304 . 
         [0039]      FIG. 9  illustrates an example method. At  902 , an ultrasound signal is transmitted via a transducer array that includes a plurality of individual elements (i.e., elements not part of a common substrate) and/or a plurality of sub-groups of elements (i.e., each sub-group is part of a common substrate, and the different sub-groups are part of different substrates). At  904 , an echo signal is received via the transducer array. At  906 , the echo signal is processed to produce an image. 
         [0040]    The application has been described with reference to various embodiments. Modifications and alterations will occur to others upon reading the application. It is intended that the invention be construed as including all such modifications and alterations, including insofar as they come within the scope of the appended claims and the equivalents thereof.
A transducer array for an ultrasound imaging system includes a substrate and a single array comprising individual sub-sets of transducer elements disposed on the substrate, wherein the individual sub-sets are physically separate from each other and spatially arranged contiguous to each other. An apparatus includes a transducer array with a substrate and a single array comprising individual sub-sets of transducer elements disposed on the substrate, wherein the individual sub-sets are not in physical contact with each other and are serially arranged with respect to each other. The apparatus further includes transmit circuitry that conveys an excitation pulse to the transducer array, receive circuitry that receives a signal indicative of an ultrasound echo from the transducer array, and a beamformer that processes the received signal, generating ultrasound image data.