Ultrasound imaging transducer array for synthetic aperture

[0001] The present embodiments relate to synthetic aperture beam forming. Broad transmit beams with synthetic aperture processing assist three-dimensional imaging.

[0002] Three-dimensional real-time imaging uses complex ultrasonic transducer arrays, transmit circuitry and/or receive circuitry. Simple array construction, sufficient power/acoustic transmission and density of electrical connection may be difficult to achieve.

[0003] In one approach for three-dimensional imaging, a one-dimensional array electronically steers along one dimension and is mechanically steered along another dimension (i.e., wobbler array). However, the mechanical scan may limit the speed for scanning an entire volume. The mechanism for moving the array may be large and/or complex, resulting in loss of comfort for the user or reduction in life of use.

[0004] In another approach, a one-dimensional array is translated or rotated by the user. This free hand scanning may degrade resolution. The position of the array is estimated or measured for rendering. The estimation may be inaccurate. The measurements may require additional hardware for use.

[0005] In another approach, elements of a two-dimensional array of discrete elements operate for both transmit and receive functions. Many limitations and complexities arise from such an approach. Each element is connected with transmit circuitry that is capable of supporting high drive voltages and power levels required to adequately insonify the area of interest. For a relatively small array of 32 by 32 elements, 1024 connections are needed. Direct connection of the elements with the system electronics via a cable bundle is impractical, so drive electronics may be positioned in the transducer housing. Positioning drive electronics in the transducer housing complicates packaging, and thermal dissipation issues limit the complexity of the drive electronics.

[0006] For receive operation, each element drives connecting cables or electronics. Pre-amplification near the elements may provide sufficient drive capability. Elements of an N by M array may be smaller than a typical one-dimensional array, making impedance matching of a single layer piezo-ceramic element to the transmission line very poor. Multilayered two-dimensional arrays may better match the impedance of the transmission line, but are difficult to construct. Additionally, the receive electronics must be isolated from the high voltage drive circuitry, requiring some type of transmit/receive switching at the array, diode isolation circuits or connections to both sides of the piezoceramic element, doubling the number of necessary connections.

[0007] Making a connection to each of several thousand transducer elements and placing beam-forming electronics in the transducer to reduce the number of cables may be achieved. Where partial beamforming within the transducer limits the number of cables, less channel information is provided. The available data for the imaging system is the partial beams. However, availability of the raw channel data may be important in a variety of clinical scenarios.

[0008] The speed of acquisition may be limited by requirements of spatial beam sampling and the sound speed in tissue. For a square transducer, if M beams fill a plane, at least M² beams fill a volume. A typical beam is 2 wavelengths wide, and a typical transducer may be 200 wavelengths long, giving M=100. A typical beam requires 0.2 ms to acquire. If Z focal zones are needed to improve coherence, a total of M²Z² firings make up a volumetric image. For typical imaging depths, this results in a maximum imaging speed of 0.5/Z² volumes per second. This speed may be insufficient for real-time three-dimensional imaging.

[0009] One technique to increase acquisition speed is used for two-dimensional imaging. Synthetic transmit aperture imaging or co-array imaging uses broad transmit beams to insonify an entire area of interest. Receive beams within the area of interest are formed in response to the single transmission.

[0010] By way of introduction, the preferred embodiments described below include methods and transducer systems for synthetic transmit aperture in ultrasound imaging. A transducer may have separate transmit and receive elements. Broad beams are transmitted, allowing fewer transmit elements and/or more rapid scanning. A multidimensional receive array generates data in response to sequential transmissions, such as transmissions from different angles. The data is combined to increase resolution. A transducer array with offset transmit elements for forming a transmit line source may be used.

[0011] In a first aspect, a method provides synthetic aperture with an ultrasound imaging transducer. First and second transmit elements sequentially transmit broad wavefronts. A receive array receives echoes responsive to the sequential transmissions. Elements of the receive array are separate from the transmit elements. Signals responsive to the echoes of the sequential transmissions are combined.

[0012] In a second aspect, a method provides synthetic aperture with an ultrasound imaging transducer. First and second transmit elements transmit broadly at different angles relative to a volume of interest. A multidimensional receive array receives echoes responsive to the sequential transmissions. Elements of the receive array are separate from the transmit elements. Signals responsive to the echoes of the sequential transmissions are combined.

[0013] In a third aspect, a method provides synthetic aperture with an ultrasound imaging transducer. First and second confocal data sets are acquired in response to sequential transmissions. The first and second confocal data sets each represent a full volume. The acquisition uses a fully sampled two-dimensional receive array. The first and second confocal data sets are synthesized.

[0014] In a fourth aspect, a transducer system for synthetic aperture in ultrasound imaging is provided. A transmit circuit causes at least two transmit elements to transmit acoustic energy in at least two plane or diverging wavefronts, respectively, in sequential transmissions. A multidimensional receive array of N receive elements has receive elements which are separate from the transmit elements. A receive circuit electrically connects with the receive elements. The receive circuit combines data representing a same volume and responsive to the sequential transmissions.

[0015] In a fifth aspect, an ultrasound transducer for synthetic aperture is provided. A second array of elements is offset from a first array. The second array of elements is operable to form a diverging wavefront with a focal region closer to the first array than the second array.

[0016] The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments and may be later claimed independently or in combination.

[0017] The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

[0018] FIG. 1 is a block diagram of one embodiment of a transducer system for synthetic transmit aperture processing with a multidimensional receive array;

[0019] FIG. 2 shows top and side views of a transducer with separate transmit and receive elements in one embodiment;

[0020] FIGS. 3-7 are top views of transducers with separate transmit and receive elements in other embodiments;

[0021] FIG. 8 is a side view of the transducer of FIG. 7; and

[0022] FIG. 9 is a flow chart diagram of one embodiment of a method for synthetic aperture processing with an ultrasound imaging transducer.

[0023] A two-dimensional transducer array includes one or more (e.g. 4) radiating ultrasound transmit elements and a distinct or separate highly sampled two-dimensional receive array. The receive array includes hundreds or thousands of receive elements. Sound waves are launched into the area of interest by sequentially firing each or groups of the transmit elements. The transmit elements produce a generally unfocused or weakly focused sound field, broadly illuminating a region of interest. By locating the transmit elements at different locations, sequential broad wavefronts propagate through the region of interest at different angles. The returning echoes are received by the receive array. Receive beams are formed from the echoes. Each acoustic sample in the region of interest is synthesized through summation of multiple beams from the multiple transmit operations. The two-way response represents a focused or conventional receive beam and an unfocused transmit beam. The transmit beam is (1) completely isotropic, (2) focused in azimuth but unfocused in elevation, or vice versa, or (3) partially focused in both azimuth and elevation. The summation is incoherent (compounding) or coherent (synthesis), or a combination of the two.

[0024] Synthetic aperture imaging combines this transmit aperture with a fully sampled receive aperture for three-dimensional imaging. Acquisition speed is increased as compared to focused transmit scanning. The resulting image is in-focus throughout the region of interest. The acquired data may include samples for the entire region.

[0025] FIG. 1 shows a transducer system for synthetic aperture in ultrasound imaging. The transducer system includes transmit elements 12, a transmit circuit 14, a receive array 16, a receive circuit 18 and a transducer housing 22. Additional, different or fewer components may be provided. For example, the transducer housing 22 is not provided, or the transmit circuit 14 and/or all or a portion of the receive circuit 18 are outside of the transducer housing 22.

[0026] The transmit elements are piezoelectric elements, such as PZT slabs or composites. Alternatively, micromachined, capacitive, polyvinylidene di-fluoride (PVDF) or other materials operable to transduce from electrical to acoustic energy are used. In one embodiment shown in the side view of FIG. 2, the transmit elements 12 are multi-layer piezoceramic elements. Multiple layers may increase the electric field across each layer and output higher acoustic pressure for a given drive voltage compared to a single layer piezoceramic. In alternative embodiments, the transmit elements 12 are each a single layer of piezoceramic. Different or the same structures and/or materials are provided for each of the transmit elements 12.

[0027] Each transmit element 12 is relatively large as compared to individual receive elements, but same or smaller sized transmit elements 12 may be used. FIG. 2 shows rectangular transmit elements 12 positioned at the perimeter of the receive array 14. Alternatively, square, hexagonal or some other shape transmit elements are positioned at the corners, center and/or elsewhere relative to the receive array 16. FIG. 5 shows a segmented ring defining the transmit elements 12 with the receive array 16 within the ring. Each transmit element 12 may be unfocused or propagate a diverging beam, illuminating a pyramid shaped volume 20 (see FIG. 2).

[0028] In alternative embodiments shown in FIGS. 4 and 6, the transmit elements 12 are small arrays. FIG. 4 shows a rectangular array of transmit elements 12. FIG. 6 shows annular arrays of transmit elements 12, such as annular arrays of 20 elements each. Transmit elements 12 formed as arrays may allow coarse or fine steering to concentrate on a particular region and reduce clutter. The transmit arrays may be used to transmit as a point source.

[0029] Two or more transmit elements 12 are used. Only one transmit element 12 may provided. The total number of transmit elements 12 is less than the total number of receive elements, such as being less than half the number of receive elements. FIGS. 2 and 5 show four transmit elements 12. FIG. 4 shows thirty six transmit elements 12. FIG. 3 represents a single transmit element 12, but a greater number of transmit elements 12 may be used.

[0030] The number of transmit connections with the transmit circuit 14 is reduced by several orders of magnitude from that of a two-dimensional transducer array with all of the elements used as both transmit and receive elements. Using a few transmit elements fired sequentially may reduce the heat generation in the transmit circuit 14 and allow higher transmit output. Additionally, the multi-layer architecture for the transmit elements 12 is simple to realize for the larger transmit elements 12. Due to the proportional increase in electric field with the number of layers, multilayer transmit elements 12 may output sufficient acoustic power with low voltage driven electronics or battery operation for a stand-alone device.

[0031] The transmit elements 12 are positioned adjacent to and/or within the receive array 16. Since the synthetic aperture is the convolution of the transmit aperture and the receive aperture, the transmit elements 12 are situated on the edges or adjacent to the receive array 16. FIGS. 2 and 5 show the transmit elements 12 surrounding a periphery of the receive array 16. FIGS. 4 and 6 show the transmit elements 12 at corners or opposite edges of the receive array 16. The transmit elements 12 are immediately adjacent receive elements, such as being separated by an air, gas, liquid, or solid filled kerf for acoustic isolation. Other acoustically isolating structures may be used, such as cured bonding agent for holding the transmit elements 12 adjacent to the receive array 16. The transmit elements 12 are formed from a same or different substrate as the receive array 16. Alternatively, a greater separation, such as a half or full wavelength gap is provided. The separation shown in FIGS. 7 and 8 may be even larger, such as tens of millimeters.

[0032] The transmit elements 12 do not overlap in azimuth or elevation with the receive array 16. In an alternative embodiment shown in FIG. 3, the transmit element 12 overlaps with the receive array 16 in elevation and azimuth, but is layered with the receive array 16 or does not overlap in a range dimension.

[0033] The transmit circuit 14 is a switch network, pulser, waveform generator, transmit beamformer, memory, digital-to-analog converter, beamformer or other now known or later developed circuit for generating ultrasound transmit waveforms. For example, the transmit circuit 14 is a switching bipolar or unipolar pulser with an amplifier for each transmit element 12. Alternatively, a fewer number of pulsers and amplifiers than transmit elements 12 is provided, such as multiplexing a single pulser and amplifier to different transmit elements 12 sequentially. The amplifier provides high drive voltages and power levels, such as for applying a transmit voltage of +/−200 volts.

[0034] The transmit circuit 14 is positioned in an imaging system and connects to the transmit elements 12 through cables, such as a coaxial cables. Alternatively, a portion or the entire transmit circuit 14 is within the transducer housing 22. A rechargeable battery or low voltage feed from the imaging system supplies power to the transmit circuit 14.

[0035] The transmit circuit 14 generates ultrasound transmit waveforms, such as pulsed waves of 1-10 cycles. Each transmit waveform is transmitted sequentially without transmissions from other transmit elements 12 and without any relative apodization. Alternatively, the transmit waveforms of each transmit element 12 are relatively delayed and apodized in sequential transmissions. The transmit elements 12 transduce the electrical waveform into an acoustic wavefront 20. The acoustic wavefront 20 is a broad wavefront, such as a plane or diverging wavefront. The transmit elements 12 generally act as point sources in one embodiment. In other embodiments, a weak focus, no focus, or a focal point on or behind the transmit array are used to generate the broad wavefront. The broad wavefront extends outward from the transmit element 12 and/or transducer housing 22 along azimuth and elevation dimensions. In alternative embodiments, the broad wavefront extends in azimuth or elevation and is focused more narrowly, such as an at least part converging wavefront, in the other of elevation or azimuth.

[0036] The receive array 16 is an array of PVDF or capacitive membrane elements. PVDF or capacitive membranes may not be as effective as piezoceramics for relatively high powered transmit operations. However, PVDF or capacitive membranes may operate more efficiently for receive only purposes. PVDF for example is several times more efficient on reception than PZT (e.g., Yr=1.35 vs Yr=0.2). The achievable bandwidth with these devices likewise may be significantly higher than that of piezoceramics. Alternatively, the receive array 16 includes piezoelectric elements. Singe or multiple layer elements may be used.

[0037] In one embodiment, the receiving array 16 is a sheet of PVDF with electrodes selectively applied to form an N by M array. N and M are greater than 1. Dicing operations for the receive array 16 may be avoided since the elements are defined by lithography and/or electrode deposition. Finer pitch and higher frequency receive arrays 16 may be provided.

[0038] The sheet entirely fills the region between the transmitting elements 12. Alternatively, the receiving array 16 entirely covers the transmitting elements 12 and the regions between the transmitting elements 12 as shown in FIG. 3. Through selection of materials and/or thickness, the receive array 16 of FIG. 3 is effectively or substantially transparent to the transmit pressure signal, and the receive array elements are effectively backed by intermediate layers between the transmit elements 12 and the PVDF layer of the receive array 16.

[0039] In another embodiment, the receive array 16 is a capacitive membrane ultrasonic transducer (cMUT) with N by M elements. The cMUT is formed by integrated circuit fabrication processes. Other microelectromechanical devices may be used, such as flexible beams. Each of the receive elements in the cMUT array has one or more, such as up to several hundred, cavities covered by a thin membrane. In one embodiment, the cavities within each receive element have varying dimensions, so are sensitive to different incoming pressure levels with the same bias voltage applied. Such an arrangement is disclosed in U.S. Pat. No. ______ (Publication No.______ (Application No. ______ (Attorney Docket No. 2005P05011 US))), the disclosure of which is incorporated herein by reference. A digitized version of the incoming radio frequency acoustic signal with up to 8 bit resolution is output from each of the N by M receive elements. In other embodiments, the cMUT receive elements operate as analog devices.

[0040] The elements of the receive array 16 are distributed in a rectangular, hexagonal, triangular or other grid. The receive array 16 is rectangular, square, cross-shaped, circular, oval or other shape. The elements are along both azimuth and elevation dimensions. The receive array 16 is flat or curved. Multidimensional arrays include two-dimensional, 1.25, 1.5, 1.75 or other arrays in any distribution of elements along two dimensions. The receive array 16 is fully sampled, but sparse or substantially fully sampled receive arrays may be used. Fully sampled receive arrays 16 may include internal locations for transmit elements 12, such as shown in FIG. 7.

[0041] The receive array 16 and receive elements are separate from the transmit elements 12. The receive elements are free of possible electrical connection to the transmit circuit, and the transmit elements are free of possible electrical connection to the receive circuit. For example, the transmit elements 12 are dedicated to transmit operation without a transmit/receive switch or other isolation circuits allowing connection of the transmit elements 12 to the receive circuit 18. Separating the transmit elements 12 from the receive array 16 may eliminate the need for signal isolation electronics or methods. The transducer is segregated into high-power transmitters and high-sensitivity receivers without compromise to use the same elements for both transmit and receive operation.

[0042] The receive circuit 18 includes one or more preamplifiers, multiplexers, mixers, delays, multipliers, phase rotators, summers, buffers, memories or combinations thereof. The receive circuit 18 electrically connects with the receive elements of the receive array 16 in a plurality of channels. PVDF and cMUT receive arrays 16 are directly grown on to silicon integrated circuits containing the receive pre-amplifiers. Alternatively, monolithic integration is used, such as growing the arrays 16 directly on top of electronics. By eliminating the necessity for transmit electronics from the same chips, the surface area of a single electronic receive channel may be on par with an acoustic receive element. Flexible circuit, z-axis backing blocks, wire jumpers, cables or other electrical connections between the elements of the receive array 16 and the receive circuit 18 may be used.

[0043] The receive circuit 18 also includes a summer, multiplier, filter and/or memory (e.g., look-up table) operable to combine data representing a same volume and responsive to the sequential transmissions. Channel data or beamformed data prior to or after detection is combined. The data representing the same or substantially same spatial locations from different transmissions is combined to synthesize the aperture.

[0044] In one embodiment, the elements of the receive array 16 connect to electronics of the receive circuit 18 positioned near (e.g., flip chip bonding or within the transducer housing 22) the array for pre-amplification and/or impedance buffering. The receive circuit 18 may implement coarse or partial beamforming for reducing the number of channels to a quantity accommodated by the imaging system. For example, one of the channel reduction receive circuits disclosed in U.S. Pat. Nos.______ and ______ (Publication Nos. 20050148878 and 20050148873), the disclosures of which are incorporated herein by reference, are used. In another embodiment, one of the receive circuits 18, such as a sub-array mixing circuit, disclosed in U.S. Pat. No.______ (Publication No. 20050203392), the disclosure of which is incorporated herein by reference, is used. Other receive circuits 18 may be used, such as the circuits disclosed in U.S. Pat. Nos.______ and ______ (Publication Nos. 20050192499 and ______ (Ser. No. 10/834,779, filed Apr. 28, 2004)), the disclosures of which are incorporated herein by reference.

[0045] Using time or frequency domain multiplexing, partial beamforming, sub-array mixing or other now known or later developed techniques, the receive circuit 18 reduces the number of channels for communicating information from the transducer to the imaging system. The reduced number of receive signals are sent via coaxial cable to the system. Alternatively, full beamform processing and mixing to baseband is accomplished at the transducer array, and in-phase and quadrature data is sent to the system via cable, or digitized and transmitted wirelessly or over a wire. In the case of the digital cMUT receiving array, full or partial digital beamformation may be performed by the receive circuit 18 beneath the array or in the transducer housing 22, or the data is sent to the system via a high bandwidth digital interface like Firewire, optical fiber or WAN (802.11b).

[0046] In another embodiment, the receive circuit 18 is one of the receive circuits disclosed in U.S. Pat. No.______ (Publication No.______ (application Ser. No. 10/982,693, filed Nov. 5, 2004), the disclosure of which is incorporated herein by reference. The receive circuit 18 for each channel includes signal conditioning circuitry, such as a low noise amplifier, variable gain stage, anti-aliasing filter, sample and hold circuit and time-domain multiplexer. Data is output for each channel to other circuits in the transducer housing 22 or the imaging system. In other embodiments, the receive circuit 18 also includes a multiplexer, delays, phase rotators, and/or summers. For example, the receive circuit 18 is implemented on a chip or as an application-specific integrated circuit and provides partial beam formation or multiplexed data. Partial beamforming may allow the output of elevation channel data at a lower frequency than for multiplexing channel signals. The receive circuit 18 forms azimuth beams with receive delays in the under-chip electronics. Digital control of the receive circuit 18 schedules each azimuth element's delay prior to summation as a function of depth. This delay may be the same for all elements in a given azimuth column. The remaining beamformation is provided in the imaging system.

[0047] The receive circuit 18 in combination with detection and image processing provide digital reconstruction of an ultrasound volume in real-time. For example, data from many receive elements are multiplexed into high-speed analog signals for transmission to the imaging system through a practical number of interconnects. The signals are first conditioned by a matching amplifier which applies gain and bandwidth shaping defined by the input specification of the analog-to-digital converter and the characteristics of the interconnect and on-chip line output amplifier. The analog signal is then converted to a digital signal. Several analog-to-digital converters with outboard sample-and-hold circuitry may be used for a single analog line. The digital signals are demultiplexed into individual channels. The individual signals then pass into a signal processing block for demodulation to baseband, low pass filtering, decimation, and inversion of any pulse compression applied on transmit (i.e., a spiking filter). The baseband in-phase and quadrature data then enter a channel memory operating as a three-dimensional array with axes of azimuth channel, elevation channel and sample number. An array of digital signal processors beam form from the channel data. The processors may allow implementation of programmable aberration correction, correlation functions for motion correction, strain imaging and vector flow, beam-to-beam filtering for super-resolution, or adaptive artifact canceling. Alternatively, hardware based receive beamformers are used. The aperture is synthesized by circuitry in the transducer housing or in the imaging system.

[0048] A memory stores the synthesized output of these processors or beamformer. This beam memory is, conceptually, a three-dimensional array with axes of azimuth beam, elevation beam and sample number. Once a volume of data is stored in the beam memory, a display processor converts the data to the appropriate format on the screen of the imager. Examples of display processing include slicing of the data at arbitrary angles, maximum intensity projection, segmentation, speckle reduction, surface rendering, alpha blending, multiplanar reconstruction or other now known or later developed representations derived from data representing a volume or distributed along three dimensions.

[0049] The transducer housing 22 is plastic, fiberglass, metal, wood or other now known or later developed material. The transducer housing 22 is sized and shaped for hand-held use. Alternatively, the transducer housing 22 is sized and shaped for use within a patient, such as a catheter, endocavity or intraoperative probe. The transmit elements 12, the receive array 16 and at least part of the receive circuit 18 are in a transducer housing 22. For example, pre-amplifiers and other signal reduction circuitry of the receive circuit 18 are within the transducer housing 22 and the synthesis circuitry is outside of the transducer housing 22. In alternative embodiments, the receive circuit 18 is outside of the transducer housing 22. Other distributions of the receive circuit 18 may be used. The transmit circuit 14 is within, outside of or partially within the transducer housing 22.

[0050] FIG. 9 shows a method for synthetic aperture with an ultrasound imaging transducer. A plurality of confocal data sets is acquired in response to sequential transmissions. Each confocal data set represents a same or full volume or three-dimensional region of interest. A few transmit beams impinging at different angles illuminate the region of interest. Synthetic or compound beam formation from the confocal data sets may approach the theoretical maximum spatial resolution and may provide high temporal resolution. Only a few transmit firings scan the entire volume, possibly allowing imaging speeds in excess of 1000 volumes per second.

[0051] The acts shown in FIG. 9 are performed in the order shown or a different order. The different ones of act 82 and the corresponding act 84 are performed sequentially, but may be performed simultaneously. Additional, different or fewer acts may be provided.

[0052] In act 82, acoustic energy is transmitted broadly from transmit elements. Spatially separate transmit elements generate the acoustic transmissions. Sequential use of spatially separate transmit elements results in scanning of the volume of interest from different angles. The acoustic energy wavefront passes through the volume at different angles.

[0053] The transmissions are from signal elements sequentially. For example, one transmit element 12 at one side or corner of a receive array 16 transmits broadly. Subsequently, another transmit element 12 on a different side or corner of the receive array 16 transmits broadly. In one embodiment with four transmit elements 12 or four transmit arrays, the data for synthesis is acquired in two, three or four transmit firings, such as one firing of each of the four transmit elements 12.

[0054] Alternatively, the sequential transmissions are from the same or different transmit arrays. The transmit arrays may be formed from adjacent or spaced transmit elements. For example, the transmit arrays at the corners of the receive array 16 in FIGS. 4 or 6 sequentially transmit broad wavefronts. As another example, the transmit arrays or transmit elements 12 from more than one edge or corner transmit at a same time, but with coding, for sequential transmissions.

[0055] The broad wavefront is a plane wave, a diverging (spherical or cylindrical) wave or weakly focused. Omni-directional transducers may be used. Transmit elements 12 which are small as compared to wavelength generate hemispherical waves. A weakly focused or unfocused phased array or larger transmit elements 12 may output higher output pressures. The broad wavefront may emulate a virtual point source for synthesis.

[0056] Since a fewer number of transmit elements 12 are used in the transmissions of act 82, less acoustic energy may be transmitted as compared to transmission with a full array. If a traditional imager has an aperture of area A_T, and the area of each defocused transmit element 12 is A_D, the emitted power decreases by a factor of A_T/A_D. Moreover, the focusing gain of the transmit aperture is lost or reduced. These effects, when combined, can provide a loss of signal or poor signal-to-noise ratio. The signal-to-noise ratio may be sufficient for imaging without additional techniques. The use of PVDF or a cMUT receive array 16 may provide sufficient signal-to-noise ratio. Alternatively, the acoustic energy output by the transmit elements 12 may be increased, but may be limited by regulations or performance capability.

[0057] In one embodiment, the signal-to-noise ratio is increased by coding the transmissions as a function of transmit element. Spatial transmit coding increases transmit power for synthesized aperture. Without spatial coding, the image is formed by a summation of the data from each of the four transmit elements 12 or arrays on separate firings. Spatial coding allows transmission with two or more, such as all four, transmit elements or arrays at once. Multiple or sequential transmit firings are used, such as four transmit firings. The signal processing for data associated with each corner is separated. For example, different transmit elements 12 use inverse transmit waveforms for different transmit firings. Any now known or later developed spatial coding schemes may be used. The received data is multiplied by the inverse of the coding matrix. In one embodiment, a Hadamard matrix is used.

[0058] In another embodiment, bias variation in cMUT based transmit elements 12 performs spatial coding. Instead of receiving from a certain element selected by a multiplexer, a group of elements is selected using bias control. The polarity of transduction in a cMUT is controlled by the bias polarity. To illustrate, consider a 2×2 matrix of transmit elements 12. Over four firings, the bias polarities are switched in a pattern so that linear combinations of the received data recreate the signal from each transmit element 12. Larger processing gains can be obtained from choosing larger sub-apertures.

[0059] In another embodiment, temporal coding increases the signal-to-noise ratio. The transmit waveforms are coded, such as with chirp, Golay or Barker codes. The “spiking filter” or inverse code operation is performed on the channel data prior to beam formation. At this point in the signal processing chain, no focusing has occurred. Consequently, long time-bandwidth products may be achieved with a concomitant improvement in signal-to-noise ratio.

[0060] Additional or alternative signal-to-noise ratio improvement may be provided by motion compensation. The data from several transmitter firings is coherently combined, yielding 10 log₁₀ F dB of improvement if F is the number of firings added. As the number of firings for signal-to-noise ratio improvement increases, tissue motion is estimated to preserve coherence. Motion is estimated using correlations between the data sets, such as a cross-correlation or sum of absolute differences. The motion is used to align the data sets locally or globally. Coherence is maintained over the entire volume.

[0061] In act 82, each sequential transmission illuminates a volume of interest. The volume of interest is an entire field of view for imaging. Each transmission insonifies the entire field of view. The transmitted plane wave or diverging wavefront extends over azimuth and elevation dimensions. Different amounts of focus may be provided along different dimensions. For example, the wavefront may be broad for both the azimuth and elevation dimensions. Alternatively, each transmission insonifies less than the entire field, such as a quarter, more or less.

[0062] In one embodiment, sequential transmissions illuminate a same plane or volume, but different sets of sequential transmissions illuminate different planes or volumes. For example, the wavefront is more focused, such as being divergent or broad along one dimension and focused relatively narrower along another dimension. Traditional beamforming is provided along one dimension, such as in azimuth. The synthetic aperture is formed along the other dimension, such as in elevation. Each firing covers an elevation plane at a single azimuthal angle. MZ beams are needed to create a volume with focal zones, which makes the imaging speed about 50/Z volumes per second.

[0063] Focus along one dimension may increase the power density of the transmission, increasing the signal-to-noise ratio. A transducer with a one-dimensional focus in azimuth and broad transmission in elevation is shown in FIGS. 7 and 8. FIG. 7 shows three linear transmit arrays of transmit elements 12. Fewer or additional arrays may be used. For example, the central transmit array of elements 12 is not provided. The center transmit array may assist with imaging tissue close to the face of the receive array 16. The center transmit array is used with the outer transmit arrays and is focused in azimuth.

[0064] The transmit arrays are phased in azimuth for azimuth focus. The transmit arrays may or may not have multiple elements along the elevation dimension. For example, the center transmit array in FIG. 7 is a one-dimensional array for broad transmit in elevation. As another example, the edge transmit arrays in FIG. 7 have a curved shape (e.g., half pipe) or relative elevation phasing for transmitting broadly along the elevation dimension. FIG. 8 shows the edge transmit arrays as curved and offset behind the receive array 16. The focal line of the transmit arrays is along an edge of the receive array 16, effectively transmitting a broad wavefront in elevation emanating from the edge of the receive array 16. A focal line or other region behind, in front, further to the side or another location may be used. For example, the receive array 16 is offset from the acoustic window or lens by about 10 mm or other distance. The volume between the transmit and receive arrays and any lens or acoustic window is filled with fluid or other acoustic matching material. The transmit array creates a tight focus at a given point in azimuth while maintaining a large angular spread in elevation. In alternative embodiments, the transmit arrays have greater or less focus. Different relative sizes of the arrays may be provided.

[0065] Phasing one of the three transmit arrays generates focused azimuth beams. One or more focal zones are chosen to cover the depth range of interest with sufficient coherence and depth of field. In elevation, the transmit arrays generate a diverging, cylindrical wave. The elevation aperture is synthesized from two or more firings, such as one from the upper line source and one from the lower source.

[0066] The channel count for the transmit array of elements 12 may be minimized using bias line polarity for defocusing. A two-dimensional array of transmit elements 12 is provided. The bias is controlled to defocus along one or both dimensions of the array, such as disclosed in U.S. Pat. Nos.______ or ______ (Publication Nos.______ or ______ (application Ser. No. 10/819,094, filed Apr. 5, 2004, or Ser. No. 10/959,665, filed Oct. 4, 2004)), the disclosures of which are incorporated herein by reference. Multiple transmit firings may be used to improve the quality of the elevation line sources.

[0067] For harmonic imaging of contrast agents or free of contrast agents, increased transmit power may provide stronger second or other harmonic response. For stronger response, such as for harmonic tissue response, the transmit arrays may be more focused along the elevation dimension, such as through bias based focusing. The transducer system is used with or without synthetic aperture.

[0068] In another embodiment, broad transmit beams are more strongly focused to cover a portion of a volume rather than an entire volume or a plane. Transmit subarrays of elements 12 are distributed around and within the receive array 12. The size of each transmit subarray is small enough so as not to disrupt receive synthesis, but large enough that the lateral extent of the transmitted radiation is controlled. The wavefront is confined to the near vicinity of a block of receive lines. The receive circuitry 18 may be capable of forming receive lines simultaneously for a subset of all of the receive lines. A modicum of focal control is provided along both axes. The same group of receive lines is used for multiple consecutive transmit events with different transmit sub arrays to synthesize for the corresponding receive lines.

[0069] In acts 84, the receive array 16 receives echoes responsive to the sequential transmissions of act 82. The elements of the receive array 16 are separate from or independent of the transmit elements 12. The echoes are received with a fully sampled multidimensional receive array 16, but acquisition of data may be performed with a sparse or substantially fully sampled receive array 16. Analog waves are generated by the transducer receive elements, but digital sensing may be used.

[0070] In act 86, the number of data channels is reduced. In one embodiment, the channel data is multiplexed as a function of time, frequency or other coding. For example, data from subarrays, such as a block of elements or elements in rows, are multiplexed together onto one interconnect with the imaging system. Greater or lesser numbers of elements may be multiplexed together. Alternatively, partial beamforming or subarray mixing are used. In another alternative embodiment, the channel data is transmitted from the receive array 16 without channel reduction.

[0071] The data from the receive array 16 is processed to form samples representing a volume. For example, receive beamformation is provided by parallel beamformers, providing for simultaneous beamformation along a plurality of receive lines. Alternatively, the channel data is transformed, such as with a Fourier transform, to generate data representing the volume. Due to the sequential transmissions, two or more sets of data represent the substantially same volume or spatial locations.

[0072] In act 88, the signals responsive to the echoes of the sequential transmissions are combined. Signals representing the same or substantially same spatial location are combined. Averaging, weighted averaging, interpolation or other combination functions synthesize the data. A nearest neighbor or combination of values from adjacent spatial locations may be used. The data may be phase adjusted for coherence prior to combination. The combination occurs prior to beamformation, after beamformation and prior to detection or after detection. By synthesizing two or more confocal data sets, the volume is represented with higher resolution. To achieve exactly the same point-spread function as a focused transmit and receive apertures, the apodizing function in the synthesis is set equal to the product of the transmit and receive apodize functions of a traditional array.

[0073] In alternative embodiments, a combined transmit and receive array is used. The elements are both transmit and receive elements, so are not separate other than as a function of time. A transmit/receive switch connects each element with transmit and receive circuits sequentially. Such arrays may allow scanning of a smaller anatomical window. In another alternative, virtual lines sources for transmit are provided within the receive array or only a sub-set of the receive elements are also switchable to transmit operation. Bias line polarity defocusing may be used in the elevation aperture with delay or phase focusing in azimuth.

[0074] While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.