WO2020020867A1

WO2020020867A1 - Ultrasound imaging system with selectable transmit aperture waveforms

Info

Publication number: WO2020020867A1
Application number: PCT/EP2019/069762
Authority: WO
Inventors: Changhong HU; Steven Russell FREEMAN
Original assignee: Koninklijke Philips N.V.
Priority date: 2018-07-23
Filing date: 2019-07-23
Publication date: 2020-01-30

Abstract

A digital transmit beamformer for an ultrasound system has a waveform sample memory which stores sequences of samples of different transmit waveforms. The memory is shared by a plurality of transmit channels, each of which can access its own selected sample sequence, independent of the selections by other channels. Waveform sample readout by all of the channels sharing the memory occurs substantially simultaneously during a transmit event. Packaging efficiency is provided by fabricating the digital memory and the channels which access the memory on the same integrated circuit.

Description

Ultrasound imaging system with selectable transmit aperture waveforms

This invention relates to ultrasound imaging systems and, in particular, to a digital transmit beamformer for an ultrasound system which is efficiently integrated and capable of applying different transmit waveforms to a transmit aperture.

Ultrasound systems which use multi-element array transducers employ beamformers to steer and focus the beams transmitted by and echoes received by the transducer arrays. Transmission is done by applying transmit signals to the elements of the array which are respectively delayed so that the resultant beam is steered in a desired direction and is focused at a desired focal point along the beam. The received echo signals are respectively delayed and then summed to produce a sequence of coherent echo signals from the beam direction from shallow to deeper depths. Originally beamformers were entirely analog and used analog components to delay, transmit, receive and process the transmitted beams and the received echo signals. The evolution of the beamformer since then has been marked by increasing digitization. The concepts of a digital receive beamformer appeared in the patent literature in the l970s as exemplified by US Pat. 4,301,501 (Caruso) and US Pat. 4,173,007 (McKeighen et al.) Commercial ultrasound systems with digital receive beamformers began to appear on the market in the l980s. The concepts of a digital transmit beamformer appeared in the patent literature in the early l990s as illustrated by US Pat. 4,893,629 (Lewis). Commercial ultrasound systems with digital transmit beamformers appeared later in the l990s.

Early ultrasound systems relied upon a simple pulser to excite the transducer elements into acoustic transmission. An electrical pulse would be applied to an element at the appropriate time for its contribution to the desired beam formation, causing the transducer element to vibrate and generate an acoustic pulse at its resonant frequency. Pulsed transmission produces broadband energy, which is desirable for imaging modes such as B mode, but other modes such as Doppler modes are more effective with narrower band transmission. Narrow band transmission requires longer, oscillatory waveforms for optimal frequency selectivity. Such linear waveforms are not possible with a simple pulser as the ultrasound transmitter. As the Lewis patent illustrates, the essence of a digital transmit beamformer is a digital memory which stores a sequence of digital samples of a transmit waveform. The value of each digital word defines the amplitude of the waveform at a particular point in time, and the rate at which the words are read from the memory defines the shape and frequency content of the transmitted waveform. Thus, the sequence of digital waveform samples can define a complex waveform such as an oscillating waveform. Since transducer array elements are by nature analog devices, the digital words delineating the transmit waveform are not directly applied to the transducer elements, but are converted to an analog waveform by a digital-to-analog converter. The analog waveform is then amplified and applied to an element of the transducer array. By applying the analog waveform to different transducer elements at different times, which effects the delays of a beamformer, the array produces a beam which is steered in a given direction and focused at a desired depth, with the desired frequency content.

Ultrasound systems are generally designed to operate with a wide variety of ultrasound probes. The transducer arrays of those probes are designed to transmit various forms of transmit waveforms, both pulsed and linear. A transmit beamformer designed to meet these requirements must be able to drive a transducer array with any of many possible waveforms, which implies a significant amount of waveform storage. Accordingly, it is desirable to configure a transmit waveform with these capabilities in an efficient and economical design architecture. It is a further object of this present invention to further provide complete flexibility in not only the possible waveforms for a transmit beam, but even the possible transmit waveforms for the transmit aperture for a single transmit event.

In accordance with the principles of the present invention, a digital transmit beamformer for an ultrasound system has the capability of selectively transmitting different pulsed or linear waveforms on a line-by-line basis. In a preferred implementation the choice of the transmit pulse or waveform can even be made on a channel-by-channel basis, enabling different elements within the active aperture to transmit different signals for a common transmit beam. In a preferred implementation, a plurality of channel address counters operate independently of each other and share a common digital waveform memory. The plurality of channel address counters and their shared memory are fabricated on the same integrated circuit for efficient and economical beamformer construction.

In the drawings: FIGURE 1 illustrates in block diagram form an ultrasound system configured in accordance with the principles of the present invention.

FIGURE 2 illustrates in block diagram form a digital transmit beamformer of the present invention suitable for use in the ultrasound system of FIGURE 1.

FIGURE 3 shows examples of some of the linear and pulse waveform types which may be transmitted by the digital transmit beamformer of FIGURE 2.

FIGURE 4 illustrates the channel address counter of the digital transmit beamformer of FIGURE 2 in further detail.

FIGURE 5 illustrates a transducer array with a transmit aperture of transducer elements driven by different transmit signals.

FIGURES 6a and 6b illustrate two other examples of transducer array transmit apertures of elements driven by different transmit signals.

Referring now to FIGURE 1, an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention is shown in block diagram form. A transducer array 12 is provided in an ultrasound probe 10 for transmitting ultrasonic waves and receiving echo information. The transducer array 12 may be a one- or two-dimensional array of transducer elements capable of scanning in two or three dimensions, for instance, in both elevation (in 3D) and azimuth. The transducer array 12 is coupled to an optional microbeamformer 14 in the probe which controls transmission and reception of signals by the array elements. Absent the microbeamformer, transmission and reception is controlled by the main system beamformer and beamformer controller. Microbeamformers are capable of at least partial beamforming of the signals received by groups or "patches" of transducer elements as described in US Pats. 5,997,479 (Savord et ah), 6,013,032 (Savord), and 6,623,432 (Powers et al.) The microbeamformer is coupled by the probe cable to a transmit/receive (T/R) switch 16 which switches between transmission and reception and protects the main beamformer 20 from high energy transmit signals. The transmission of ultrasonic beams from the transducer array 12 under control of the microbeamformer 14 is directed by a beamformer controller 18 coupled to the T/R switch and the main beamformer 20, which receives input from the user's operation of the user interface or control panel 38. Among the transmit characteristics controlled by the transmit controller are the number, spacing, amplitude, phase, frequency, polarity, and diversity of transmit waveforms. Beams formed in the direction of pulse transmission may be steered straight ahead from the transducer array, or at different angles on either side of an unsteered beam for a wider sector field of view. For some applications, unfocused plane waves may be used for transmission. Most 1D array probes of relatively small array length, e.g., a 128-element array, do not use a microbeamformer but are driven and respond directly to the main beamformer.

The echoes received by a contiguous group of transducer elements are beamformed by appropriately delaying them and then combining them. The partially beamformed signals produced by the microbeamformer 14 from each patch are coupled to the main beamformer 20 where partially beamformed signals from individual patches of transducer elements are combined into a fully beamformed coherent echo signal, or echo signals from elements of a one- dimensional array without a microbeamformer are combined. For example, the main beamformer 20 may have 128 channels, each of which receives a partially beamformed signal from a patch of 12 transducer elements, or from an individual element. In this way the signals received by over 1500 transducer elements of a two-dimensional array transducer can contribute efficiently to a single beamformed signal, and signals received from an image plane are combined.

The coherent echo signals undergo signal processing by a signal processor 26, which includes filtering by a digital filter and noise reduction as by spatial or frequency compounding. The filtered echo signals are coupled to a quadrature bandpass filter (QBP) 28. The QBP performs three functions: band limiting the RF echo signal data, producing in-phase and quadrature pairs (I and Q) of echo signal data, and decimating the digital sample rate. The QBP comprises two separate filters, one producing in-phase samples and the other producing quadrature samples, with each filter being formed by a plurality of multiplier- accumulators (MACs) implementing an FIR filter. The quadrature signal samples undergo signal processing by a signal processor 16, which includes filtering by a digital filter and speckle reduction as by spatial or frequency compounding. The signal processor can also shift the frequency band to a lower or baseband frequency range, as can the QBP. The digital filter of the signal processor 26 can be a filter of the type disclosed in U.S. Patent No. 5,833,613 (Averkiou et ah), for example. The beamformed and processed coherent echo signals are coupled to a B mode processor 30 which produces a B mode image of structure in the body such as tissue. The B mode processor performs amplitude (envelope) detection of quadrature demodulated I and Q signal components by calculating the echo signal amplitude in the form of (I²+Q²)^½. The quadrature echo signal components are also coupled to a Doppler processor 34. The Doppler processor 34 stores ensembles of echo signals from discrete points in an image field which are then used to estimate the Doppler shift at points in the image with a fast Fourier transform (FFT) processor. The rate at which the ensembles are acquired determines the velocity range of motion that the system can accurately measure and depict in an image. The Doppler shift is proportional to motion at points in the image field, e.g. , blood flow and tissue motion. For a color Doppler image, the estimated Doppler flow values at each point in a blood vessel are wall filtered and converted to color values using a look-up table. The wall filter has an adjustable cutoff frequency above or below which motion will be rejected such as the low frequency motion of the wall of a blood vessel when imaging flowing blood. The B mode image signals and the Doppler flow values are coupled to a scan converter 32 which converts the B mode and Doppler samples from their acquired R-q coordinates to Cartesian (x,y) coordinates for display in a desired display format, e.g. , a rectilinear display format or a sector display format. Either the B mode image or the Doppler image may be displayed alone, or the two shown together in anatomical registration in which the color Doppler overlay shows the blood flow in tissue and vessels in the image as shown in FIGURES 3a-3c. Another display possibility is to display side-by-side images of the same anatomy which have been processed differently. This display format is useful when comparing images.

The image data produced by the B mode processor 30 and the Doppler processor 34 are coupled to an image data memory 36, where it is stored in memory locations addressable in accordance with the spatial locations from which the image values were acquired. Image data from 3D scanning can be accessed by a volume renderer 42, which converts the echo signals of a 3D data set into a projected 3D image as viewed from a given reference point as described in US Pat. 6,530,885 (Entrekin et al.) The 3D images produced by the volume renderer 42 and 2D images produced by the scan converter 32 are coupled to a display processor 48 for further enhancement, buffering and temporary storage for display on an image display 40.

In accordance with the principles of the present invention, the beamformer 20 includes a digital transmit beamformer which is capable of actuating elements of a transducer array in probe 10 with either linear waveforms or transmit pulses. FIGURE 2 illustrates a channel of a digital transmit beamformer of the present invention in block diagram form. Digital samples of linear and pulse waveforms for transmission in both transmit modes are stored in a waveform sample memory 52. The digital samples may be acquired for the memory 52 by digitally sampling a desired analog transmit waveform with an analog to digital converter. The sample rate of the analog to digital converter should satisfy the Nyquist criterion for sampling by sampling at a frequency which is at least twice that of the highest frequency component of the waveform. Higher sampling rates are also desirable. A waveform may be produced by a waveform generator and sampled, for instance, or a waveform may be constructed in the frequency domain and then converted to the time domain for sampling by an inverse Fourier transform. Transmit pulse sequences may be constructed directly in digital form without the need for conversion by producing digital words of one, two, or three bits in accordance with the desired number of pulse levels and in consideration of a clock rate which will cause level transitions in a sequence of samples to occur at the desired times. Other techniques for generating transmit waveforms are well known to those skilled in the art.

A sequence of digital samples, which defines either a linear ( e.g ., shaped sinusoid) waveform or a pulse sequence, is stored in the memory in sequential memory locations. This enables the sequence to be read out in sequential order by addressing the memory with a read address counter 50, as shown in the drawing. The clock rate at which the samples are read out is chosen in consideration of the frequency of the desired waveform. For example, a pulse sequence can be read out at a low clock rate to produce a sequence of long, low frequency pulses, or the same sequence can be read out at a higher clock rate to produce a sequence of more narrow, higher frequency pulses. The same is true when reading out a linear sinusoidal waveform.

A sequence of samples which is read out of the memory 52 is applied to three types of waveform and pulse transmitters in this example, a digital to analog converter 60, a two-level pulser 64, and a higher multi-level pulser 62 which can produce a pulse sequence of three, four or five levels. The transmitter comprises an individual set of these output devices for each channel of the transmit beamformer, with the outputs of the devices coupled to a transducer element 70. An enable signal, Em, En₂, or Em, from the channel address counter 50 enables the appropriate output device for that channel. The converter 60 will convert a sequence of digital words into a linear waveform such as the one shown in FIGURE 3 a. In this example each digital word is an eight-bit word and the word value defines the instantaneous amplitude as one of 256 possible levels. Each dot on the linear waveform 100 represents an eight-bit waveform sample, and the conversion of the digital word sequence to analog levels produces a linear waveform such as the one shown in the drawing. In this example, the analog waveform 100 is produced by digital to analog conversion of a sequence of N digital samples.

The two-level pulser 64 operates in the manner of a D-type flip-flop, with the clock Clk producing an output at the level of the digital word applied to the data input of the pulser. While different size digital samples can be employed, a two-level device only requires the digital sample to be a single bit, which will have either a one or zero value. When the sequence of bits changes from a zero to a one, the clock will transition the output to the high state of a pulse, which will remain high until the input bit sequences changes back to a zero. Transitions of the output pulse sequence will occur in phase with the clock signal. A typical two-level output pulse train 106 produced by the two-level pulser 64 is shown in FIGURE 3d. Since the pulse train only has two levels and phasing is controlled by the pulser clock, fewer samples are needed to define the pulse train as the more widely spaced sample dots in pulse train 106 indicates. In most instances, lower frequency read-out addressing by the channel address counter is also possible. With fewer samples needed to define the pulse train, and those samples being as small as one bit, fewer memory location of memory 52 are needed to store the digital samples for a two-level pulse. A large number of pulse train sequences can be stored in the same memory space required for one linear waveform of eight-bit words.

The multi-level pulser 62 produces a multi-level pulse train such as pulse trains 102 and 104 shown in FIGURES 3b and 3c. Pulser 62 can be constructed using a digital decoder driving stacked, oppositely poled switching transistors which are coupled to voltage supplies for the pulse levels. A three-level pulse train 104 as shown in FIGURE 3 c can require only two voltage levels (+HV and -HV) as shown in FIGURE 2, which will produce a pulse train such as 104 with a high (+HV), low (-HV) and intermediate (reference or ground) pulse level. A three-level pulse can be defined by two bits for the three levels, and the digital decoder can be as simple as a two-bit register into which the two-bit samples are sequentially loaded to drive the switching transistors.

For pulse trains of five, six or seven levels, additional supply voltage levels are required. The five-level pulse train 102 in FIGURE 3b, for instance, can be produced using two positive voltage supplies (+HV and ++HV) and two negative supplies (-HV and— HV), for instance. Three-bit words are necessary to define the five possible pulse levels, which can be decoded into the four signals necessary to drive the switching transistors, for example. Tike the two-level pulse train of FIGURE 3d, the digital sample sequences for the three- and five-level pulse trains 104 and 102 need fewer sample words than are needed for the linear waveform 100, and those words are of a lesser number of bits, meaning that they, too, require fewer memory storage locations than does the sample sequence for the linear waveform 100. The K, F, and M sample sequence lengths will generally be less than the N sample sequence needed for the linear waveform.

The beamformer transmitter shown in FIGURE 2 can drive a transducer element 70 with either a linear or a pulse transmit waveform, depending upon which stored pulse or waveform sequence is read from the memory 52 by the channel address counter 50, and which output device 60, 62 or 64 is enabled by an enable signal. FIGURE 2 shows the components for one channel of a beamformer transmitter, with each element or patch of an array transducer driven by its own transmit channel. Thus, each transmit beamformer channel is characterized by its own set of output devices and its own channel address counter.

In accordance with a preferred implementation of the present invention, a plurality of channels share the same waveform sample memory. That is, the linear and pulse samples for multiple channels and multiple transducer elements are stored in and read from a common waveform memory. Furthermore, the selection of a transmit waveform by one channel address counter for one transducer element is independent of the choices of transmit waveforms by the other channel for the other transducer elements. Thus, the active transmit aperture used to produce a transmit beam can, if desired, transmit different waveforms or pulses from different elements during the same transmit event. Moreover, many, and possibly all, of the channel address counters of the channels of the active transmit aperture are reading out their transmit waveform sequences from the waveform sample memory at the same time.

How this is done is shown in further detail in the illustration of the bank of channel address counters 50 shown in FIGURE 4. This drawing shows a group of sixteen channel address counters 50 which are fabricated with a shared waveform sample memory 52 on one ASIC integrated circuit. An address counter 80 of each channel address counter is coupled to address lines of the shared waveform sample memory 52. A sequence of samples for a desired waveform is addressed by the counter, starting with a starting address and counting through the consecutively addressed samples until all of the samples for the waveform are read out. The starting address for the sequence of samples is initially loaded into the address counter 80 from a waveform select register 82. In order to steer and focus the transmit beam from an active transmit aperture of transducer elements, the times of transmission by each channel can be phased, or delayed, by a delay profile appropriate for the beam steering and focusing. This requires the start of transmission by each element to be uniquely delayed in accordance with the delay profile. In the special case of an unsteered beam, that is, one which is transmitted normal to the face of the array, the beam profile is symmetrical and elements equidistant on either side of the beam center will exhibit the same delay. In other instances, each channel will have its own delay for combined steering and focusing. The delay for a channel is implemented by a delay counter 84, which is initially loaded with a delay count appropriate for the delay required in that channel. A transmit synchronization signal, issued by a common controller for all of the channels, actuates the delay counters 84, each of which begins to count down the delay for its channel. When a delay counter reaches zero, the appropriate delay for the channel has expired and the delay counter 84 enables its address counter 80 to begin to count and issue consecutive addresses for the digital sample words of the waveform. As previously mentioned, the counting begins with the proper starting address for the sequence of samples to be transmitted. The counting of samples is controlled by a sample counter 86, which has been previously loaded with the number of samples comprising the sample sequence. When counting is enabled by the delay counter 84, the sample counter 86 begins to count down, with each count incrementing the address counter. Thus, the address counter produces a sequence of consecutive sample memory addresses, beginning with the starting address of the desired sample sequence for that particular channel, and continues to issue consecutive addressed to the waveform memory until the sample counter has counted the required number of samples of the sequence. When the sample counter reaches zero, counting and incrementing of the address counter stops, as the entire required sample sequence has been read from the memory.

An optional repeat counter 88 is coupled to the sample counter. The purpose of the repeat counter is to cause the sample counter and address counter to read out the same sequence of samples again. This enables production of a longer waveform by successively reading out a number of shorter waveforms. For example, suppose that a particular sample sequence defines one cycle of a transmit waveform. Further suppose that it is desired to transmit a three-cycle waveform. The repeat counter will cause the address counter and its associated components to address the same one-cycle sample sequence three times in succession, thereby producing a three-cycle waveform for transmission. In this example the repeat counter is initialized with a count of three, and when it reaches zero all three cycles will have been read from the waveform memory and memory addressing by the channel ends.

Each channel address counter 50 also includes a waveform enable register or decoder 90. This device is loaded with a digital word prior to transmission which enables the desired output device 60, 62, or 64 for transmission by that channel. The waveform enable word may be a three-bit word of a binary value of either 001, 010, or 100 which, when loaded into a register, will produce a high signal on one of three output lines for enable control. Alternately, the waveform enable device may be a two-bit decoder for the three enabling output lines.

A simple example will illustrate the sequencing and interaction of the components of a channel address counter and the sample waveform memory. Suppose that there are 256 samples in memory for a waveform which is to be transmitted by a particular transducer element, and that this sequence of samples is stored in sequence beginning at memory address 1024. Further suppose that transmission by the transducer element is to be delayed by the time of 100 clock cycles of the clock which increments the delay counter 84. This channel address counter is initialized by loading the starting memory address of 1024 into the address counter from the waveform select register 82, loading the delay counter 84 with the delay time of 100, loading the sample counter 86 with the sample sequence length of 256, and loading the repeat counter with a repeat count of one. When the transmit synchronization signal is received from a central or common controller, the delay counter 84 begins counting down from 100. When the delay counter reaches zero, the sample counter 86 is enabled and begins to count down from 256, incrementing the address counter which issues consecutive memory addresses to the waveform sample memory, beginning with the starting address of 1024. When the sample counter reaches zero, at which time the full waveform of 256 samples has been read from memory, the sample counter decrements the repeat counter 88 from one to zero. The zero value from the repeat counter halts the sample counter, and waveform sample readout is complete. Thus, following a delay time of 100 delay clock cycles, the desired waveform of 256 samples has been read from memory and transmitted by the transducer element. The other channels for the active transmit aperture are functioning in the same way with the same or different starting addresses for their waveform sample sequences so that pulse or linear waveforms for a plurality of transducer elements are provided concurrently by the waveform sample memory for the same transmit event.

Previous efforts toward digital transmit beamformers have tried to use various shortcuts in attempts to reduce memory size requirements. All of these approaches have, in various ways, limited transmitter performance, increased hardware or software complexity, or suffered combinations of these effects. A conventional approach has been to provide each channel with its own waveform memory. If the same waveform is to be used by multiple channels, it must be stored in the memory of each channel with this approach. A common, shared memory makes more efficient use of memory storage space. Another proposal is to store waveform samples for two waveforms as halves of double words in memory, so that half of a readout word is used for one waveform and the other half is used for another waveform. This necessarily requires the two waveforms to be used for the same transmission, preventing each channel from selecting a waveform independently of the other. It also requires different delays for the two waveforms to be applied in subsequent processing. Another proposal has been to read out samples for two waveforms in interleaved fashion, ping-ponging back and forth between the two waveforms. This approach mandates redirecting the samples into two paths for the two waveforms, and also halves the high frequency resolution of the resultant transmit beams since the waveform sample rate is cut in half, a problem which can be overcome by doubling the read speed of the memory, which may not be feasible or desirable. Yet another approach has been to store only the envelope of the transmit waveform, and subsequently modulate the envelope with a high frequency carrier. This requires a modulator to be implemented and operate before a waveform can be transmitted. Still another approach is to store only a few waveforms in memory, then perform interpolations of them to generate additional waveforms. This requires a waveform interpolator to be implemented and operate before a waveform can be transmitted. Other approaches have called for storing one or only a few waveforms in memory, then weighting or phasing them or adding successive waveform increments to generate the additional required waveforms. Again, additional hardware or software is needed to perform these operations, and the time required to execute them before transmission must be figured into the waveform production time. And the delays must be implemented in a subsequent operation. A preferred implementation of a transmit beamformer of the present invention avoids all of these deficiencies and complexities by allowing each channel to address a desired waveform in memory independently of the selections by other channels. This independent operation of the transmit channels is made possible by having all channel address counters on an ASIC share a common waveform sample memory, which all of the channels can access simultaneously and independently. Since the channel address counters and their common memory are all on the same integrated circuit, all address bus routing is done on the IC so that no IC pins are needed for waveform sample addressing. The implementation of FIGURE 4 shows sixteen address counters which, together with their waveform sample memory, are fabricated on the same ASIC. Thus, eight of these ASICs will support a 128-element array requiring 128 transmit channels. Higher integration is also possible and desirable. When 32 or 64 or 128 channel address counters and their shared memory are fabricated on the same ASIC, only four, two or one such transmit ASIC is required to support a 128- element array.

The capability of each channel of a digital transmit beamformer to select one of a plurality of waveform or pulse train sample sequences in memory, independently of the other channels, enables the beamformer to transmit a beam from an active aperture in which different elements transmit different waveforms. Conventional beamformers transmit the same pulse train or waveform, with appropriate delays, from each element of the transmit aperture. The constructive and destructive interference of wavefronts in an image field scanned with a single transmit waveform are well understood and accepted. But the ability to transmit different signals from different elements of the aperture enables additional imaging features to be obtained. One such application of the use of different transmit waveforms to produce a beam with a line focus, whereby different frequencies are focused at different depths. A transducer which is operated to produce such frequency-dependent focusing is referred to as a Hanafy lens, after its eponymous developer. See US Pat. 5,415,175 (Hanafy et al.) FIGURE 5 illustrates ultrasound transmission by a transmit beamformer of the present invention to effect a Hanafy lens. In the drawing, a transmit aperture of a transducer array 112 comprises a row of transducer elements 112. As shown by the waveform illustrations above the aperture, different transducer elements are driven with transmit waveforms of different frequencies. The individual per- element transmit frequencies range from high frequencies 122 in the center of the aperture to progressively lower frequencies 120 at the outer ends of the aperture. Since the channel addressing and waveform sample readout by a transmit beamformer of the present is independent from channel to channel, each channel address counter 50 can read out the waveform samples of its selected waveform at the appropriately delayed time for transmission by the element it is driving. Conventional operation of the array of FIGURE 5 could cause the transmit beam to have a nominal focus at focal point F. For Hanafy lens focusing, the elements for higher frequencies have delays chosen to focus the highest frequencies in the near field as indicated by FH, and the elements producing lower frequencies have delays chosen to focus the lowest frequencies in the far field as indicated by FL. Frequencies intermediate the highest and lowest are delay to focus them along a line between FH and FL, thereby providing the line focus.

FIGURE 6a illustrates Hanafy lens focusing by a transmit beamformer of the present invention which drives the elements 112 of the transmit aperture (shaded elements) of an array transducer 110. In this example the transmit aperture is sixteen elements in width and is driven by eight different frequency signals fi to f₈, with the frequencies ranging from the highest in the center of the aperture to the lowest at the ends. In this example of an unsteered (straight ahead) beam, like frequency signals have the same delays; the delay profile is symmetrical. This causes the highest frequencies f₈ of the center elements to be focused at F_H and the lowest frequencies f_L of the end elements to be focused at F_L, with the intermediate frequencies focused in a line between these two depths. As the beam profile 114 indicates, the result is a line focus from F_H to F_L. The sixteen-element transmit aperture of FIGURE 6a could be driven by sixteen transmit channels with their channel address counters and shared sample waveform memory fabricated on one ASIC, as in the example of FIGURE 4.

FIGURE 6b is an example of Hanafy lens focusing by a transmit beamformer of the present invention which drives the elements 112 of a thirty- two element transmit aperture 116 of an array transducer. In this example the line focusing is performed with only five different frequency signals, which could require less storage space in a waveform sample memory than that required by the eight different signals of the previous example. While thirty-two channels provide the transmit signals for the thirty-two elements, so that each element can be operated with its own unique focus (or focus and steering) delay, the same frequency transmit signal is applied to four adjacent elements, except for the two end pairs of elements. In this example the highest f frequencies are focused around focus F_H and the lowest fi frequencies are focused around the deeper focus F_L, with the other frequencies focused in between these two points. This progression of frequencies is sufficient for the desired line focus precision in most applications, with the benefit of using less storage area of the waveform sample memory. The thirty-two element transmit aperture of FIGURE 6b could be driven by thirty-two transmit channels with their channel address counters and shared sample waveform memory fabricated on one ASIC.

Other implementations using a transmit beamformer of the present invention to independently control the transmit signals of the elements of a transmit aperture will readily occur to those skilled in the art. It may be desirable to reduce the occurrence of grating lobe artifacts in an image by transmitting higher frequency signals from one side of the transmit aperture and lower frequency signals from the other side when a beam is steered off-axis, for instance. The use of per-channel transmit signal variation to provide transmit apodization is described in a concurrently filed patent application by the same inventors.

It should be noted that an ultrasound system suitable for use in an implementation of the present invention, and in particular the component structure of the ultrasound system of FIGURES 1, 2, and 4, may be implemented in hardware, software or a combination thereof. The various embodiments and/or components of an ultrasound system, for example, the channel address counter and its controller, or components and controllers therein, also may be implemented as part of one or more computers or microprocessors. The computer or processor may include a computing device, an input device, a display unit and an interface, for example, for accessing the internet. The computer or processor may include a microprocessor. The microprocessor may be connected to a communication bus, for example, to access a PACS system or the data network for importing training images. The computer or processor may also include a memory. The memory devices such as the waveform sample memory 52 may include Random Access Memory (RAM) and Read Only Memory (ROM). The computer or processor further may include a storage device, which may be a hard disk drive or a removable storage drive such as a floppy disk drive, optical disk drive, solid-state thumb drive, and the like. The storage device may also be other similar means for loading computer programs or other instructions into the computer or processor.

As used herein, the term "computer" or "module" or“processor” or “workstation” may include any processor-based or microprocessor-based system including systems using microcontrollers, reduced instruction set computers (RISC), ASICs, logic circuits, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and are thus not intended to limit in any way the definition and/or meaning of these terms.

The computer or processor executes a set of instructions that are stored in one or more storage elements, in order to process input data. The storage elements may also store data or other information as desired or needed. The storage element may be in the form of an information source or a physical memory element within a processing machine.

The set of instructions of an ultrasound system including those controlling the acquisition, processing, and display of ultrasound images as described above may include various commands that instruct a computer or processor as a processing machine to perform specific operations such as the methods and processes of the various embodiments of the invention. The set of instructions may be in the form of a software program. The software may be in various forms such as system software or application software and which may be embodied as a tangible and non-transitory computer readable medium. Further, the software may be in the form of a collection of separate programs or modules such as a transmit control module, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to operator commands, or in response to results of previous processing, or in response to a request made by another processing machine.

Furthermore, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35

U.S.C. 112, sixth paragraph, unless and until such claim limitations expressly use the phrase "means for" followed by a statement of function devoid of further structure.

Claims

WHAT IS CLAIMED IS:

1. A beamformer system for an ultrasound system comprising: a digital memory adapted to store digital samples of a plurality of linear or pulse transmit waveforms;

a plurality of channel address counters, coupled to the digital memory, each adapted to address a sequence of digital samples of the linear or pulse transmit waveforms stored in the digital memory independently of the addressing of a digital sample sequence by the other channel address counters; a plurality of transmitter output devices, coupled to the digital memory and responsive to a sequence of digital waveform samples and adapted, when enabled, to produce the pulse or linear waveforms for an array of transducer elements; and

an array of transducer elements, each of which is coupled to an output of one or more transmitter output devices, and adapted to produce an ultrasound beam from a transmit aperture when actuated by a pulse or linear waveform, wherein a plurality of the channel address counters and the digital memory addressed by the plurality of channel address counters are on a common integrated circuit.

2. The beamformer system of Claim 1 , wherein the plurality of channel address counters comprise at least sixteen channel address counters.

3. The beamformer system of Claim 2, wherein the plurality of channel address counters comprise at least thirty-two channel address counters.

4. The beamformer system of Claim 1, wherein the digital memory is adapted to be addressed simultaneously by the plurality of channel address counters.

5. The beamformer system of Claim 1, wherein the channel address counters are adapted to address different sequences of digital samples of different transmit waveforms for a single transmit event.

6. The beamformer system of Claim 5, wherein the different transmit waveforms further comprise different frequency transmit waveforms.

7. The beamformer system of Claim 6, wherein the array of transducer elements further comprises a row of transducer elements exhibiting an active transmit aperture,

wherein lower frequency transmit waveforms are applied to end elements of the active transmit aperture and higher frequency transmit waveforms are applied to central elements of the active transmit aperture.

8. The beamformer system of Claim 1, wherein each channel address counter further comprises an address counter coupled to the digital memory; and

a waveform select register, coupled to the address counter, and adapted to load a starting address for a sequence of waveform samples into the address counter.

9. The beamformer system of Claim 8, wherein each channel address counter further comprises a delay counter, coupled to the address counter, which is adapted to count a delay time prior to production of a sequence of digital samples by the digital memory in response to the address counter.

10. The beamformer system of Claim 9, wherein each channel address counter comprises a sample counter that is coupled to the address counter and is adapted to count the number of samples of a sequence.

11. The beamformer system of Claim 10, wherein each channel address counter comprises a repeat counter that is coupled to the address counter and is adapted to enable addressing of a sequence of digital samples of the linear and pulse transmit waveforms an additional time.

12. The beamformer system of Claim 1 , wherein the plurality of transmitter output devices each comprise a digital to analog converter and a pulser for each channel of the transmit beamformer.

13. The beamformer system of Claim 1, wherein the digital samples of the linear waveforms stored in the digital memory comprise a sequence of eight-bit digital words.

14. The beamformer system of Claim 1, wherein the digital samples of the pulse waveforms stored in the digital memory comprise a sequence of one-bit digital words.

15. The beamformer system of Claim 1, wherein the digital samples of the pulse waveforms stored in the digital memory comprise a sequence of three-bit digital words.