Abstract:
Appliances and methods are disclosed for testing operation of an acoustic device that generates beams of acoustic energy. An imaging array generates electrical signals in response to impinging receipt of acoustic energy. An acoustic-energy direction system is disposed to focus acoustic energy onto the imaging array. A controller is electrically coupled with the acoustic device and with the imaging array. The controller has instructions to generate an image on a display from electrical signals received by the controller from the imaging array. The electrical signals are received by the controller in response to generation of a beam of acoustic energy by the acoustic device. The beam of acoustic energy is directed towards the acoustic-energy direction system. The image provides a representation of the generated beam of acoustic energy.

Description:
BACKGROUND OF THE INVENTION 
     This application relates generally to acoustic systems and probes. More specifically, this application relates to methods and appliances for testing acoustic system and/or probes. 
     Acoustic imaging is an important technique that may be used at different acoustic frequencies for varied applications that range from medical imaging to nondestructive testing of structures. The techniques generally rely on the fact that different structures have different acoustic impedances, allowing characterization of structures and their interfaces from information embodied by the different scattering patterns that result. While most applications use radiation reflected from structures, some techniques make use of information in transmitted patterns also. 
     Transmission of acoustic radiation towards a target and receipt of the scattered radiation may be managed by a modern acoustic-imaging system, which may take a variety of forms. For example, many modern systems are based on multiple-element array transducers that may have linear, curved-linear, phased-array or similar characteristics, and which may be embodied in an acoustic probe. Summing the contributions of the multiple transducer elements comprised by a transducer array allows images to be formed. The failure of a small number of elements in a given array, or a few defective receive channels in the acoustic system itself, may not be readily perceptible to users because of the averaging effect of summing many elements to form an acoustic beam. But the failure of even a small number of elements or receive channels can significantly degrade the performance of acoustic imaging systems, notably in certain modes of operation like those known as “Doppler” or “near-field” imaging modes. 
     While appliances have previously been developed to test acoustic systems and probes, they have been relatively complex, based on the direct electrical connection to probe elements or system channels. The development of two-dimensional “matrix array” probe technology has made the necessary reverse engineering of even a small number of probe and system models to support direct electrical-connection-based testing complex and expensive. Other solutions use an acoustic sensor to detect transmit energy emanating from a probe and to inject signals back into the ultrasound system for detection and display. Such solutions use a single sensor designed to sense elements in linear arrays, providing only a “signal” or “no signal” status relative to transmitted energy and require operator skill to scan a linear array probe for missing or nonfunctional elements. 
     There is thus a need in the art for convenient, inexpensive, and easy-to-use methods and appliances for evaluating ultrasonic probes and systems, both for acoustic power output as well as for other issues such as failed elements or channels, particularly to evaluate two-dimensional array probes and systems. 
     SUMMARY 
     Embodiments of the invention provide appliances for testing operation of an acoustic device that generates beams of acoustic energy. Such appliances may comprise an imaging array, an acoustic-energy direction system, and a controller. The imaging array generates electrical signals in response to impinging receipt of acoustic energy. The acoustic-energy direction system is disposed to focus acoustic energy onto the imaging array. The controller is electrically coupled with the acoustic device and with the imaging array. The controller has instructions to generate an image on a display from electrical signals received by the controller from the imaging array. The electrical signals are received by the controller in response to generation of a beam of acoustic energy by the acoustic device. The beam of acoustic energy is directed towards the acoustic-energy direction system. The image provides a representation of the generated beam of acoustic energy. 
     The acoustic-energy direction system may comprise an acoustic lens. 
     In some embodiments, the acoustic device generates beams of acoustic energy in a scanning pattern. The controller comprises instructions to capture a subset of the beams of energy defined by the scanning pattern, with the image providing a representation of the captured subset of the beams of energy defined by the scanning pattern. In one embodiment, the subset of the beams comprises a peak pulse of the scanning pattern. 
     The imaging array and the acoustic-energy direction system may be disposed within an acoustically transmissive medium confined within a housing body, with the testing appliance further comprising an acoustic couplant coupled with the housing body. In such embodiments, the testing appliance may further comprise an acoustic standoff coupled with the acoustic couplant external to the housing body. In one such embodiment, the acoustic standoff comprises a second housing body filled with a second acoustically transmissive medium. In other embodiments. the testing appliance further comprises a volume of tissue-equivalent material coupled with the acoustic standoff. 
     An acoustic source transducer may be provided to generate an acoustic signal to be injected into the acoustic device. For example, an acoustic beamsplitter may be disposed to direct the generated acoustic signal into the acoustic device. In other instances, the acoustic source transducer may be disposed on a moveable stage. 
     The information collected by the imaging devices enables a number of diagnostic determinations. In one embodiment, for example, the controller has instructions to determine a total power of the generated beam of acoustic energy from the electrical signals. In other embodiments, the controller has instructions to determine an acoustic dosimetry provided by the generated beam, to calculate a thermal index for the generated beam, and/or to calculate a mechanical index for the generated beam. 
     In methods of the invention, operation of an acoustic device that generates beams of acoustic energy is tested. A beam of acoustic energy generated by the acoustic device is directed to an acoustic-energy direction system disposed to focus the acoustic energy onto an imaging array. Electrical signals generated by the imaging array in response to impinging receipt of acoustic energy are received. An image is generated on a display from the electrical signals. The image provides a representation of the generated beam of acoustic energy. 
     In some embodiments, the acoustic device is moved relative to the imaging array. 
     The acoustic-energy direction system may comprise an acoustic lens. 
     In some instances, the acoustic energy generated by the acoustic device comprises beams of acoustic energy in a scanning pattern, with electrical signals corresponding to a subset of the beams of acoustic energy defined by the scattering pattern being received. The image then provides a representation of the captured subset of the beams of acoustic energy defined by the scattering pattern. In one embodiment, the subset of the beams comprises a peak pulse of the scanning pattern. 
     The generated beam of energy may be directed to the acoustic-energy direction system through an acoustically transmissive medium confined within a housing body that contains the imaging array and the acoustic-energy direction system. In some instances, the generated beam of energy is directed through an acoustic standoff coupled with the housing body. The acoustic standoff may comprise a second housing body filled with a second acoustically transmissive medium. In other instances, the generated beam may be directed through a volume of tissue-equivalent material coupled with the acoustic standoff. 
     Embodiments also enable the injection of an acoustic signal into the acoustic device, as well as determining various characters that include an acoustic dosimetry provided by the generated beam, a thermal index for the generated beam, and/or a mechanical index for the generated beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, wherein like reference labels are used through the several drawings to refer to similar components. In some instances, reference labels are followed with a hyphenated sublabel; reference to only the primary portion of the label is intended to refer collectively to all reference labels that have the same primary label but different sublabels. 
         FIG. 1  is a schematic drawing that provides an overview of embodiment appliances of the invention for testing acoustic systems or probes; 
         FIG. 2  is a flow diagram that summarizes embodiment methods of the invention for testing acoustic systems or probes; 
         FIGS. 3A and 3B  provide illustrations of different implementations of the general embodiments of  FIG. 1 ; 
         FIG. 4  illustrates a structure of an array interface for coupling acoustic energy onto an imaging array that may be used in some embodiments; and 
         FIGS. 5A-5F  provide schematic illustrations of different arrangements for transducer arrays that may be tested using embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the invention provide methods and appliances for testing acoustic systems and/or probes. Such acoustic systems and probes are sometimes referred to collectively herein as “acoustic devices.” While much of the description below makes use of specific examples in discussing various aspects of the invention, such examples are intended merely for illustrative purposes; the invention is not intended to be limited by any operational characteristics used by the tested probe or system, such as the operational frequency of the tested acoustic device. As illustrated in further detail below, each of the acoustic probes and systems that may be tested with embodiments of the invention includes a plurality of “transducer elements,” which refers to elements adapted to transmit acoustic radiation and/or to receive acoustic radiation. While such elements are referred to generically herein as “transducer elements,” they may be distinguished at times according to their functions by describing them as “receiver elements” or “transmitter elements.” 
     Embodiments of the invention provide an appliance that senses and displays the energy distribution over a contact surface of an acoustic probe. It further provides a calibrated measurement appliance capable of integrating the total acoustic output of the acoustic system and thereby determining the acoustic power. In some implementations, the appliance is further capable of injecting various acoustic signals back into the acoustic system via the probe aperture for detection and display. A two-dimensional image of the acoustically active aperture of the probe with the corresponding energy level provides a convenient operator interface that may be used in establishing an acoustic signature for a given probe type. In some instances, such an image may be color coded and may be presented with an integrated total power measurement. It will be appreciated by those of skill in the art that the resultant information may find utility in verifying various output parameters such as the mechanical index (“MI”), the thermal index (“TI”), and the soft-tissue thermal index (“TIS”). 
     The testing appliance is generally configured to have adequate resolution to detect missing elements from a variety of different probe configurations, including one-dimensional and two-dimensional matrix-array probes. Because some current two-dimensional matrix-array probes have nearly 10,000 elements and future probes may have in excess of that number of elements, some implementations of the invention use a smaller sensor that has adequate resolution but not large enough to cover the complete array. The combination of such a sensor with position-sensing using any of a variety of techniques allows an operator to move the sensor over the surface of the probe being tested and thereby accumulate an imaging showing the acoustic energy distribution emanating from the probe. Examples of suitable sensor options include a mechanical linear-motion sensor attached to the probe or an optical position sensor. 
     In one example, the acoustic sensor is configured as a line array as long as the shortest dimension of the array to be tested. With such a configuration, the probe being tested may be swiped across the linear sensor with a resultant two-dimensional image assembled using the linear position sensor to track the motion of the probe to spatially register the detected acoustic information. In another example, the line array is expanded from element wide to two, three or more elements wide. This may improve signal-to-noise values and provide for additional image registration. In a further example, a small two-dimensional sensor array is scanned over the surface of the probe with motion patterns such as a raster pattern to produce a complete two-dimensional image of the probe&#39;s acoustic output energy. This approach may further allow the use of the detected acoustic information to collect and to spatially register images to form an image covering the full aperture of the probe&#39;s surface. Merely by way of illustration, a 12×12 mm 2  sensor allows scanning of almost all current ultrasound probes with a single swipe, although differently sized sensors may be used with other existing or future probes. 
     An overview of the functionality of testing appliances according to embodiments of the invention is provide with  FIG. 1 . In this drawing, a probe head  116  provides acoustic radiation denoted generally by rays  128 . The radiation is detected by an imaging array  104  after being directed using an acoustic-energy direction system  108  that may comprise such elements as acoustic lenses, acoustic mirrors, and the like. Generally, the acoustic-energy direction system  108  may comprise elements configured for gross or precise direction and/or shaping of the acoustic radiation  128  as appropriate for the application. For instance, the probe head  116  may emit unfocused acoustic radiation that is directed to the imaging array  104  by the acoustic-energy direction system  108 . The acoustic radiation  128  is propagated in a medium suitable for the propagation of acoustic radiation, such as water, with a couplant  112  being provided to couple the radiation emitted by the probe head  116  to the medium. 
     Any number of acoustic elements may be comprised by the acoustic-energy direction system  108 , particularly including multiple-acoustic-lens arrangements. Such systems may be configured according to known acoustic principles to provide dual-lens, triple-lens, and other arrangements. For example, the acoustic magnification provided by a lens is determined by the focal length f of the lens as related to the object distance d o  and image distance d i  by the lens equation: 
                 1     d   o       +     1     d   i         =       1   f     .           
Since the magnification M provided by a lens is the ratio of object to image distance, i.e. M≡d o /d i , variations in focal length result in variations in the magnification. A zoom lens arrangement provides certain advantageous characteristics and may be implemented in a variety of different ways. For example, a three-lens system having a first positive lens, a second negative lens, and a third positive lens may be used to implement a zoom system. In a particular embodiment, the first and second lenses have equal (but opposite) power so that with their focal lengths being denoted respectively as f 1  and f 2 , the range of effective focal length f (eff)  is given by
 
                   f   1     ⁢     f   2           f   1     +     f   2         ≤     f     (   eff   )       ≤         2   ⁢     f   1     ⁢     f   2           f   1     +     2   ⁢     f   2           .           
Maximal effective focal length is achieved with a substantially zero separation between the two lenses, decreasing with separation until the separation approaches f 1 . The third lens in the arrangement uses the virtual image formed by the first two lenses as an object, collimating the diverging rays to form an image on the imaging array  104 . Axial movement of the central negative lens thus varies the power of the system, providing a zoom arrangement. Methods known in the art may be used to vary the position of the central negative lens, and it will be understood that a variety of other lens configurations may be used in providing zoom functionality.
 
     A controller system  120  may be used to coordinate the operation of the probe being tested with the imaging array  104 , particularly to provide triggering functions described in greater detail below. Images collected by the imaging array  104  may accordingly be displayed on a display  124  for evaluation by an operator. 
       FIG. 2  provides a flow diagram that summarizes methods for testing acoustic probes and/or systems in accordance with embodiments of the invention and that may use appliances like those illustrated with  FIG. 1 . At block  204 , the acoustic probe or system  116  is activated, coupling the radiation generated by a set of transducers in the acoustic propagation medium with the couplant  112 . The acoustic-energy direction system  108  directs the radiation  128  onto the imaging array  104  at block  208 . 
     At block  212 , a triggering function is applied to capture a specific energy pattern. Acoustic imaging systems generally use complex scanning patterns so that evaluation of its operation can be improved by consistent selection of a portion of the scanning pattern. In one embodiment, the largest pulse for a given probe is selected by capturing the peak pulse from successive image frames. This prevents mixing transducer pulses that may be pointed in various directions, and improves the calibration of such quantities as total power output, mechanical index, thermal index, soft-tissue thermal index, and the like. Triggering is implemented with the controller system  120  by causing the imaging array  104  to capture an image at appropriate times in accordance with the selection. 
     Accordingly, at block  216 , the image is generated for display on the display  124 . As explained above, defects in individual transducer elements may be readily detected by gaps in the resulting image. 
     At block  220 , the total power may be measured and included as part of the display. The total power measurement provides a ready and convenient basis for the calibration of individual probes and systems. Such calibration is difficult with prior-art methodologies, particularly since probes are prone to changing their overall power output over the typical lifetime of a probe. In medical environments, excessive output acoustic levels may increase tissue heating and cause possible injuries that are not otherwise easily detected. The ability of the invention to observe exactly what is emitted from the probe (and therefore into a patient when the probe is used medically) provides a particularly valuable measure. 
     In addition to determining the total power from the acoustic radiation received at the imaging array  104 , certain statistical measures may be derived at block  224 . For example, the mechanical index is defined as the quotient of the peak negative pressure P −  and the square root of frequency f 0 : 
             MI   =         P   _         f   0         .           
The mechanical index provides an indicator of possible nonthermal bioeffects from acoustic insonification of biological tissue, such as may result from cavitation and/or streaming. While existing acoustic systems generally display values for the mechanical index, it is known that the accuracy is highly variant; the value displayed by the system may differ from the actual value by 50% or more. The ability for accurate determination of the mechanical index thus provides a valuable source of information. Even in nonmedical contexts, the value of the mechanical index may be relevant in defining human-safety protocols in use of acoustic probes.
 
     The thermal index is defined as the ratio of the power used to the power required to produce a temperature rise of 1° C. It thus provides a measure of thermal effects on biological tissue resulting from insonification. Three sub indices provide specific measures in soft tissue (“TIS”), in bone (“TIB”), and in the cranium (“TIC”). Again, such measures are known to be determined with highly variable accuracy by the acoustic systems themselves so that an ability for direct determination of their values allows more reliable decisions to be made about the use of specific, individual probes in both medical and nonmedical contexts. 
     The general purpose of the TI is to translate the relative acoustic output power of an acoustic system or probe into a quantity that correlates with models for tissue heating due to acoustic exposure. As the output power of an acoustic system or probe increases, so does the TI. The temperature rise in vivo is also significantly influenced by how the acoustic energy propagates through the tissue. Using highly focused model of operation with stationary acoustic beams such as “spectral Doppler” (e.g., pulsed wave and continuous wave) and TM-mode focus the acoustic energy into a smaller area (i.e., high time average acoustic intensity) and in the case of continuous-wave Doppler, results in higher localized temperature increases. In non-scanning modes, the location of the maximum temperature rise varies as a function of the size of the active aperture of the probe. For example, with a smaller aperture, the tissue closer to the energy source is the area of highest temperature rise. 
     Conversely, scanning modes of operation such as B-mode imaging disperse the acoustic power over a wider area, known as the field of view, resulting is less-localized tissue heating at increased depths. In vivo acoustic intensity also decreases as a function of depth because of attenuation of acoustic energy (i.e., tissue attenuation coefficient). In addition to acoustic intensity, the resulting temperature rise also depends on the rate at which the energy is converted into heat in the tissue (i.e., tissue absorption coefficient), the diffusion of the heat in the tissue (i.e., thermal conductivity), and the rate at which the heat is removed by blood flow in the tissue (i.e., blood perfusion). The tissue location that receives the maximum acoustic exposure for the longest time, i.e. acoustic dose, is the point of probe contact at the tissue contact surface. 
     It is widely held that the TI gives a relative indication of the potential for temperature increase at a specific point along the acoustic beam and therefore that the TI provides a relative indication of exposure conditions that are more likely than others to produce thermal effects. But the duration of exposure is known to be a critical factor in determining the likelihood of inducing a thermal effect, with the risk of thermal damage increasing exponentially with exposure time. The values of TI displayed on current ultrasound system monitors do not include any dependence on exposure time. As a result, a longer exposure at a lower TI might be more of a risk to a patient than a shorter exposure at a higher TI. Since the system-displayed values do not inform the operator of risk based on exposure duration, the TI value is, at best, a weak indication of potential patient harm from excessive thermal load. 
     The dimensionless TI and MI values displayed on contemporary ultrasound system monitors are not measures of the actual energy present at the aperture of a specific probe on a specific system; they are rather a statistical composite of a population of in-kind probes on in-kind systems and may not provide an adequate measure of exposure risk in newer modalities. Embodiments of the invention are able to better respond to the growing complexity of systems, probes, modalities, clinical applications, and concerns about patient safety by directly visualizing and measuring the energy present at the probe aperture (tissue contact surface) and providing an acoustic dosimetry measurement as indicated at block  228 . 
     Acoustic sensors in the imaging array  104  may be made of a variety of different materials in different embodiments, including both polyvinylidene fluoride (“PVDF”) and ceramics. PVDF is both flexible and inexpensive, but is less efficient than the ceramic lead zirconate titanate (“PZT”). While PVDF is a relatively poor transmitter, this is not a significant barrier to its use in embodiments of the invention because output levels equivalent to returning tissue echoes are also very low and readily sensed by medical ultrasound systems. 
     Specific implementations of the testing appliances are illustrated in  FIGS. 3A and 3B . While the specific features shown in those drawings may be implemented separately or in combination, they are shown separately in the drawings for ease of illustration. In both cases, a portion of the structures are embodied in an acoustic camera. In  FIG. 3A , for example, the acoustic camera is denoted generically by reference number  302  and comprises a body  304  within which the imaging array  308  and acoustic-energy direction system  312  are disposed within an acoustically transmissive medium  306 . A couplant  316  allows acoustic energy to be coupled between the exterior of the housing  304  and the acoustically transmissive medium  306 , and may be made of a material such as silicone rubber, urethane, or latex. 
     In some embodiments, depending on the specific configuration of the elements, it is possible to couple acoustic radiation emanating from a probe directly to the couplant  316 . But in other instances, geometrical considerations indicate that electrical interference may result from such a direct coupling because of the proximity of the surface aperture of the acoustic probe  328 . Accordingly, an acoustic standoff  320  may be used to provide a physical path that isolates the acoustic signal from the electrical signal by the time delay between them. Such an acoustic standoff  320  may be implemented in a variety of ways. In the illustrated embodiment, the acoustic standoff comprises a body filled with an acoustically transmissive medium, taking advantage of the focal imaging approach used by the acoustic camera  302  and described above. The acoustic-energy direction system  312  focuses the energy at a specific distance from the sensor, and may be set to provide an image of the energy as it leaves the probe  328 . In other embodiments, the acoustic standoff is provided as a thin element to avoid excessive divergence of the acoustic signal from the array elements over the additional path distance. 
     In addition to the acoustic standoff  320 ,  FIG. 3A  illustrates the incorporation of a tissue-equivalent material  324  disposed between the probe  328  and other elements of the appliance. Such tissue-equivalent material  324  may serve multiple functions, particularly for the testing of probes intended for medical applications. In such medical applications, there are two principal sources of tissue heating—the direct effect of acoustic energy propagated into tissue and also conductive heating that results from the conversion efficiency of piezoelectric transducers. The inclusion of tissue-equivalent material both absorbs acoustic energy and allows models to matched for combining instruments to measure power and surface-temperature rises. This enables the use even of acoustic cameras  302  designed for the detection of highly attenuative signals despite the fact that the acoustic signals generated by acoustic probes are only weakly attenuated. The reduction in the signal level allows optimization of the dynamic range of the sensor. There are a variety of materials that may be used for the tissue-equivalent material  324 , including agar, silicone, polyvinyl alcohol gel (“PVA”), and polyacrylamide gel (“PAA”), among others. 
       FIG. 3B  illustrates another embodiment in which a portion of the structures are embodied within an acoustic camera denoted generically in the drawing by reference number  352 , with a body  354  containing the imaging array  358  and acoustic-energy direction system  362  within an acoustically transmissive medium  356 . Similar to the embodiment of  FIG. 3A , this embodiment includes a couplant  366  and may include an acoustic standoff  370  implemented with any of the variations described in connection with  FIG. 3A . In addition, though, this embodiment includes an acoustic source transducer  382 . While shown implemented as part of the acoustic camera  352  by having it disposed within the camera body  354 , this is not a requirement of the invention and other possible positions may be used. 
     The acoustic source transducer  382  may be used to inject acoustic signals back into the probe  378 . When incorporated as part of the acoustic camera  352 , an acoustic beam splitter  360  may be included to manage direction of the source acoustic signals injected back into the probe  378  and the received acoustic signals generated by the probe  378 . The beam generated by the acoustic source transducer  382  impacts the acoustic beam splitter  360 , where the beam is split such that any desired fraction of the beam is reflected from the beam splitter  360  and out through the couplant  366 . 
     The acoustic beam splitter  360  may be of any of a variety of types, including those composed of a material that has an acoustic-impedance mismatch with the acoustically transmissive medium  356 . Because of the acoustic-impedance mismatch, the material of the acoustic beam splitter  360  reflects a portion of the acoustic beam transversely while transmitting a portion of the beam so that it propagates axially. The thickness of the material of the acoustic beam splitter  360  can advantageously be selected such that the thickness is greater than half a wavelength of the source acoustic beam to ensure a sufficient acoustic mismatch, but different material thicknesses may be used in different embodiments. In one exemplary embodiment, the thickness of the acoustic beam splitter  360  is selected such that a ratio of the transverse reflected beam to the axial transmitted beam is approximately 50%, although other ratios may be used in different embodiments. Examples of materials suitable for use for the acoustic beam splitter  360  include thin sheets of glass or metal such as aluminum or steel, but other materials may also be used. 
       FIG. 3B  also shows the inclusion of a second acoustic source transducer  386  that may aid in accurate determinations of the mechanical index for the probe  378 . The second acoustic source transducer  386  may advantageously be deployed on a moveable stage (not shown) adapted to be moved manually or with a motorized mechanism. Such a motorized mechanism may be controlled by the controller system  120  described in connection with  FIG. 1 . The ability to position the second acoustic source transducer  386  enables the determination of the peak negative pressure P_at a known depth, permitting accurate determination of the mechanical index. In one embodiment, the second acoustic source transducer  386  comprises a single-sheet PVDF transducer placed about 6 cm from the probe face, although different types of transducers and different geometrical arrangements may be used in different embodiments. 
     Signals from the second acoustic source transducer  386  can be used to identify the central beam transmitted to form a two-dimensional image and can also be analyzed to determine the center frequency of the transmitted energy. The appliance of the invention may thus be used to calculate a TI value—or a TIS, TIB, or TIC value—that can be compared with the value displayed be the acoustic system, enabling a calibration of the display values. In addition, setting the acoustic system to focus at the distance of the second acoustic source transducer  386  provides a pressure signal that can be used to calculate an estimate of the current MI value that is also displayed by the acoustic system. 
     The acoustic beam focused by the acoustic-energy direction system in any of these embodiments may sometimes be coupled onto the imaging array through an array interface like that shown in  FIG. 4 . Such an array interface is not required in all embodiments of the invention but advantageously prevents the acoustically transmissive medium within the housing of the acoustic camera from contacting the imaging array. The imaging array is denoted generically by reference number  416  in  FIG. 4  and corresponds to imaging array  104 ,  308 , or  358  in  FIG. 1 ,  3 A, or  3 B. The array interface  400  comprises a diaphragm  412  having a truncated conical segment and a retainer ring  408 . The diaphragm  412  is sealed to a backplate  404  using the retainer ring  408 . The diaphragm  412  comprises an acoustically transmissive material such as polyurethane, allowing the coupling of acoustic energy from the acoustically transmissive medium onto the imaging array  416  while preventing the acoustically transmissive medium from contacting the imaging array  416 . 
     Advantageously, embodiments of the invention are not limited by the particular structure of the acoustic probe or system being tested, are not limited by the frequency characteristics of the acoustic probe or system being tested, and do not require the use of adapters to accommodate different transducer arrangements used by different probes. Examples of different probe structures that may readily be accommodated is provided with  FIGS. 5A-5F . 
     The array  504  shown in  FIG. 5A  is a conventional one-dimensional array in which individual acoustic elements  508  are distributed along a length to define the array  504 . While the length is shown to be linear in the drawing, the length may more generally be curvilinear, with some acoustic devices having curved distributions of acoustic elements  508 . 
     The array  516  shown in  FIG. 5B  comprises a plurality of one-dimensional arrays  524  spaced along an elevational height. Such an array  516  is sometimes referred to in the art as a “1.5-dimensional array.” Acoustic devices having such a 1.5-dimensional array are less successful at near-field imaging, particularly when the elevational height is relatively large. The success of such imaging depends also on the frequency used by the array  516 . Accordingly, such arrays  516  sometimes use all of the one-dimensional arrays  524  when imaging the far field, but use only a smaller subset of the one-dimensional arrays  524 —perhaps only a single one-dimensional array  512 —when imaging the near field. 
     The array  528  shown in  FIG. 5C  is a two-dimensional matrix array, with the individual transducers  532  distributed in a regular pattern. Such probes may have an active area divided in two dimensions in different elements, and allow the acoustic beam to be driven in three dimensions by combining electronic focusing and deflection. 
     The array  536  shown in  FIG. 5D  is an annular array made up of a set of concentric rings  540 . They allow the beam to be focused to different depths along an axis. While the structure illustrated is one in which the radial width of each ring  540  is constant, other structures use a different width for each ring  540  so that the surface area of the rings  540  is constant. 
     Like the array  528  shown in  FIG. 5C , the array  544  shown in  FIG. 5E  provides an example of a two-dimensional matrix array. The transducers  548  are also arranged in a regular pattern, but as elements arranged in circles. The elements can be directed towards the interior, towards the exterior, or along the axis of symmetry of the circle in different embodiments. When directed along the circle&#39;s axis of symmetry, an acoustic mirror may be used to provide the beam with the desired angle of incidence. 
     The array  552  shown in  FIG. 5F  is also a two-dimensional array, but one in which the individual acoustic elements  556  are distributed irregularly. Such an array is referred to in the art as a “sparse array.” 
     While any of the array structures illustrated in  FIGS. 5A-5F  may be tested in different embodiments of the invention, even by using the same testing appliance, their illustration is not intended to be exhaustive. Still other array structures not explicitly illustrated may be tested using the methods and appliances described herein. 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.