Abstract:
An arbitrary waveform generator includes an arithmetic element that can access samples from a waveform sample memory and adjust values accessed from the waveform sample memory to modify waveform power or amplitude. In an illustrative embodiment of the arbitrary waveform generator, the arithmetic element is a multiplying digital-to-analog converter (DAC) that has a first input connection for receiving digitized waveform samples and has a second input connection for receiving a reference signal. An output signal from the multiplying DAC is a mathematical product of the digitized waveform samples and the reference signal. In one example, a reference digital-to-analog converter (DAC) generates the reference signal. In some examples, the digitized waveform samples are digitized samples of an analog waveform signal. In some examples, the arithmetic element is incorporated into the arbitrary waveform generator in a manner to maintain transmit signal resolution over a full range of transmit power settings.

Description:
BACKGROUND OF THE INVENTION 
     Ultrasound imaging systems, such as medical ultrasound systems, typically use a transducer comprised of a phased array of individually driven elements to generate interrogation signals. The most popular method, until recently, for driving ultrasonic transducer elements has been to apply timed electrical pulses to each element of the transducer. By properly adjusting the start time of the pulse for each transducer element, acoustic beams, that can be focused and steered, are formed. Images are created from the returned echoes of the acoustic beams as they are progressively swept across a target area. 
     Known pulse driving circuits for phased arrays transducer are described by Hans W. Persson in “Electric Excitation of Ultrasound Transducers for Short Pulse Generation,” Ultrasound in Med. &amp; Bio., Vol. 7, 1981. Such drive circuits are typically limited to generating rectangular waveforms that often exhibit exponentially decaying segments. The amplitude of drive signals created by pulse generator circuits is determined by the magnitude of a programmable d.c. voltage source. Programmable d.c. voltage source typically do not have the agility to change rapidly between transmit bursts, limiting the ability to create transmission signals with different amplitudes on alternate transmit bursts, a capability that is highly desirable for mixed mode operation. 
     Pulse generator circuits are often configured to provide signals of different magnitudes (amplitudes) to the individual elements of a transducer array. The arrangement of signal amplitudes applied to the elements of transducer arrays are often referred to as amplitude apodization profiles. Proper application of amplitude apodization reduces the magnitude of side lobes in transmitted acoustic beams. Apodization profiles are typically created using banks of pulse generating circuits powered by independent programmable d.c. voltage sources. The number of apodization levels is limited to the number of programmable d.c. voltage sources. For practical reasons, only a small number of programmable d.c. voltage sources can be provided, resulting in a piecewise approximation of the intended apodization profile. 
     The deficiencies of pulse generator circuits are most apparent in imaging modalities where a signal is transmitted at a fundamental frequency and images are constructed from received harmonic signals generated by non-linear acoustic propagation, so-called “harmonic imaging”. Similarly, pulse generator circuits exhibit less than satisfactory results when used to image harmonic signals generated by contrast agents. The basic reason for these unsatisfactory results is that the harmonic content of signals produced by pulse generator circuits typically exceeds levels required for optimal harmonic imaging modalities. In such imaging modalities, the harmonic content of the transmitted signal effectively increases the noise floor of the received harmonic signal. For optimal performance transmitted harmonics must be suppressed. 
     So-called arbitrary waveform generators have been advanced as a solution to the above noted problems with pulse driven phased array transducers. Arbitrary waveform generators use stored digital representations of shaped waveforms to generate, using a digital-to-analog converter, an analog drive signal for the transducer. The drive signals produced by an arbitrary waveform generator are typically gaussian or hamming modulated cosines individually formed for each transducer element. Arbitrary waveform generators can provide instantaneous change in transmit energy between transmit pulses, apodization profiles with greater resolution, and acoustic beams with lower harmonic content. 
     A basic arbitrary waveform generator, used to drive ultrasonic transducer elements, is described by R. Y. Liu in “The Design of Electric Excitations for the Formation of Desired Temporal Responses of Highly Efficient Transducers,” Acoustical Imaging, Vol., 12, 1982. The system described by Liu uses memory to store digitized waveform samples, a functional module to retrieve samples from memory, a digital to analog converter, and a broadband driver to excite a transducer element. In Liu&#39;s system, digitized waveform samples are pre-calculated and stored in memory. 
     John A. Hossack, et al., discloses the use of an arbitrary function generator to generate an excitation signal for an ultrasound transducer in “Improving the Characteristics of a Transducer Using Multiple Piezoelectric Layers,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 40, No. 2, March 1993. The Hossack et al. ultrasonic system includes a computer that stores a digital waveform in a memory integrated circuit chip. A digital counter sequentially addresses the memory integrated circuit and data is read from the memory when addressed. Data read from the memory integrated circuit is converted by a digital-to-analog converter (DAC) and amplified to drive an ultrasound transducer. 
     U.S. Pat. No. 5,675,554 (Christopher R. Cole et al.) describes an arbitrary waveform generator used to drive individual elements of an ultrasonic transducer array in medical imaging applications. This implementation utilizes a digital memory to store a pre-calculated time domain transmit waveform envelope for each transmit channel. These transmit waveform envelopes are base band (near zero Hz) and may be of arbitrary shapes including variations of Gaussian and Hamming. Upon the start of a transmit event and after an appropriate focusing delay, each channel&#39;s transmit waveform envelope is retrieved from memory and sent through a dedicated signal processing path where apodization weighting and fine focus delay adjustments are applied by digital multipliers. An amplitude modulator in each digital signal processing path modulates a high frequency carrier with the channel&#39;s base band waveform envelope. The resulting digital version of the amplitude modulated transmit signal is converted by a digital to analog converter and amplified prior to driving an element of an ultrasonic transducer array. 
     U.S. Pat. No. 5,608,690 (Hossack et al.) and U.S. Pat. No. 5,740,128 (Hossack et al.) disclose an arbitrary waveform generator for driving individual elements of a transducer array. The described arbitrary waveform generator operates similar to that described in the earlier mentioned academic paper published by Hossack et al., “Improving the Characteristics of a Transducer Using Piezoelectric Layers.” This architecture utilizes digital memory to store the actual time domain waveform samples that are sent to analog to digital converters and amplifiers so as to drive individual elements of transducer arrays. The waveforms stored in memory are digital versions of the envelope modulated drive signals sent to each ultrasound transducer element. 
     The arbitrary waveform generators described above have limitations. The amplitude of transmit signals generated by these implementations are controlled by adjusting the magnitude of the stored digital representation of the waveform. This requires the re-calculation and the re-storing of the waveform for each change in amplitude. Not only is this approach resource intensive, but it also results in the undesirable reduction of transmit signal resolution. To put it another way, as the amplitude of the signal being described decreases, the ability to accurately describe that signal also decreases since fewer digital bits are used to represent the signal. 
     Arbitrary waveform generators that adjust acoustic power by scaling the digital representation of transmit signals also increase harmonic content as acoustic power is reduced. This is because, as transmit signal resolution is reduced, the harmonic content of the transmit signal increases. While, the harmonic content can be removed from the transmit signal with high order low pass filters, such a solution is unwieldy due to the large bank of filters required for each transmit channel. 
     SUMMARY OF THE INVENTION 
     An arbitrary waveform generator that stores an optimized digital representation of a waveform in a memory and, after retrieval thereof, adjusts an amplitude of the waveform to generate an analog signal for exciting an element of an ultrasonic transducer. The arbitrary waveform generator includes an arithmetic element that accesses digitized waveform samples (or “digital waveform samples”) from a waveform sample memory and adjusts the amplitude thereof. Preferably, the arithmetic element is a multiplying digital-to-analog converter (MDAC) that has a first input connection for receiving digitized waveform samples and has a second input connection for receiving a reference signal. The output signal from the MDAC is a mathematical product of the digitized waveform samples and the reference signal. A controller provides the reference signal based on a requested power level and/or an imaging modality being utilized. The optimized digital representation of the waveform may be updated as needed, for example by the controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a block diagram of a multi-channel phased array ultrasound imaging system in accordance with the preferred embodiments of the present invention. 
     FIG. 2 is a block diagram of a multi-channel phased array ultrasound imaging system, showing one channel in detail, in accordance with a first preferred embodiment of the present invention. 
     FIG. 3 is a block diagram of a multi-channel phased array ultrasound imaging system, showing one channel in detail, in accordance with a second preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 1 is a block diagram of a multi-channel phased array ultrasound imaging system  100  in accordance with the preferred embodiments of the present invention. It will be appreciated by those of ordinary skill in the relevant arts that the ultrasound imaging system  100 , as illustrated in FIG. 1, and the operation thereof as described hereinafter is intended to be generally representative of such systems and that any particular system may differ significantly from that shown in FIG. 1, particularly in the details of construction and operation of such system. As such, the ultrasound imaging system  100  is to be regarded as illustrative and exemplary and not limiting as regards the invention described herein or the claims attached hereto. 
     An ultrasound system  100  includes a plurality of channels  110   a ˜ 110   n.  In the example shown, 128 channels are provided, although those of ordinary skill in the art will recognize that the number of channels may vary depending on the type of ultrasound system (for example expensive vs. inexpensive; general purpose vs. special purpose) and the type of transducer. Each Channel  110   n  includes a transmitter circuit and a receiver circuit. Collectively, the channels  110   n  drive an ultrasound transducer  108 . Preferably, each channel  110   n  drives a single element (not shown) of the ultrasound transducer  108 . 
     A control circuit  102  stimulates the transmitter circuit (described hereinafter with respect to FIGS. 2 and 3) of each channel  110   n  so as to generate an ultrasound signal from the transducer array  108 . The echoes of the ultrasound signal received by the elements in the transducer array  108  are transmitted to and combined by a receive beamformer  140 , in a known manner, to construct signals representing focused lines of acoustic reflection. The signals are passed from the receive beamformer  140  to an image detector and processor  160  that converts, using known techniques, the signals into a variety of useful formats. For example the image detector and processor  160  can apply techniques for amplitude detection to generate gray scale tissue images, or frequency detection to generate images of blood flow. Image data is passed to a video processor  162  that applies scan conversion to create image data in an X-Y format and subsequently converts the image data into an industry standard video format for display on a video display  164 . 
     FIG. 2 is a block diagram of a multi-channel phased array ultrasound imaging system  200 , showing one channel  210   a  in detail, in accordance with a first preferred embodiment of the present invention. As with FIG. 1, it will be appreciated by those of ordinary skill in the relevant arts that the ultrasound imaging system  200  is to be regarded as illustrative and exemplary and not limiting as regards the invention described herein or the claims attached hereto. 
     As with the system shown in FIG. 1, a plurality of channels  210   a ˜ 210   n  are provided. The transmitter circuits of the channels  210   a ˜ 210   n  of the ultrasound imaging system  200  operate as arbitrary waveform generators. While only one channel  210   a  is depicted for purposes of simplicity of understanding, those of ordinary skill in the art will recognize that each of the remaining channels  210   b - 210   n  may have similar configurations. 
     A control circuit  202  drives the transmitter circuit of each channel  210   n  so as to drive elements (not shown) in a transducer array  208  to produce an ultrasound signal. The echoes of the ultrasound signal are received by the elements in the transducer array  208 , processed by a receiver  224  in each channel and combined by a receive beamformer  240 , in a known manner, to construct signal representing focused lines of acoustic reflection. The signals are subsequently processed as in FIG. 1, however certain elements are omitted here so as to concentrate on the inventive transmitter and operation thereof. 
     The transmitter of each channel  210   n  includes a waveform memory  212 , a focus delay and data retrieval logic unit  214 , a multiplying digital-to-analog converter  216  (hereinafter MDAC  216 ), a reconstruction filter  218  and an amplifier  220 . A T/R switch  222  switches the operation of the transducer between transmitting and receiving in a known manner. 
     Generally, the MDAC  216  converts a digital representation of a waveform, stored in the waveform memory  212  in the form of digitized waveform samples (also referred to as “digitial waveform samples”), into an analog signal that is subsequently passed through the reconstruction filter  218  and amplified by the amplifier  220 . The MDAC  216  receives a digital input signal and produces an analog output signal that relates to the mathematical product of the digital input signal and an analog signal applied to a reference input terminal thereof. Generally, the analog output signal from the MDAC  216  is equal, or approximately equal, to a linear function, or otherwise a function of the digital input signal and the applied reference signal. As opposed to the prior art which adjust the amplitude of the acoustic signal output by the transducer by modifying the stored digitized waveform samples, the present invention modifies the amplitude of the signal output by the transducer by multiplying the digitized waveform samples after they have been converted to an analog signal. As described below, the amplifier  220  is used to make additional modifications to the acoustic signal based on an imaging modality used. 
     The control circuit  202  generally comprises a controller  203 , a DAC  204 , and a noise reduction filter  206 . The amplitude of the signal output by the MDAC  216  is controlled by the output of the DAC  204  (preferably at least a 12-bit DAC), which converts a digital signal from the controller  203  into an analog signal applied to the reference terminal of the MDAC  216 . The analog signal from the DAC  204  is preferably filtered by a noise reduction filter  206  prior to being applied to the reference terminal of the MDAC  216 . The noise reduction filter  206  is preferably a second-order 5 Hz, low-pass noise reduction filter that reduces low frequency spectral and 1/f noise in the range of detectable continuous-wave Doppler frequencies. 
     The waveform memory  212  functions as storage for a digital representation of a waveform in the form of a plurality of digitized waveform samples. Preferably, each waveform memory  212  provides at least 512 bytes of memory for storing the digitized waveform samples. The digitized waveform samples preferably have a format of seven magnitude data bits with one sign bit per sampling point (although other formats may be used). Additionally, the block of memory assigned to a channel may contain multiple digital representations of waveforms that may be independently retrieved for a particular imaging modality. 
     The resolution of a digital representation waveform is proportional to the number of digital codes representing the waveform&#39;s magnitude at the point of sampling. The greater the number of digital codes used to describe each digitized waveform sample, the higher the resolution of the digitized waveform sample and the overall digital representation of the waveform. The maximum number of digital codes is limited by the number of bits per sample within the waveform memory  212 . In accordance with the preferred embodiment, each digitized waveform sample within the waveform memory  212  preferably comprises eight bits (seven amplitude data bits plus one sign bit) for a maximum of 256 digital codes. 
     An optimized digital representation of a waveform has the maximum available resolution. To optimize a digital representation of a waveform, the digitized waveform samples describing the waveform, such as those stored in waveform memory  212 , are scaled by software so that their maximum peak to peak amplitude corresponds to the maximum number of available digital codes. Typically, digital representations of waveforms that have been optimized for maximum resolution are used for all modes of operation including gray scale imaging (2-D), color flow imaging, pulse Doppler imaging, and continuous wave (CW) Doppler imaging. A distinct advantage of the present invention is that optimized digital representations of waveforms are stored in waveform memory remain in an optimized state with maximum waveform resolution independent of acoustic power settings (which are controlled by setting the gain of the MDAC  216  and/or the amplifier  220 ). The optimized digital representations of waveforms may be further optimized to compensate for errors and distortions induced by the circuitry in the channels. The goal is to produce a digital representation of a waveform that results in an acoustic signal by the transducer with controllable harmonic content. 
     In operation, the controller  203  provides data and timing signals to the individual channels  210   n  to coordinate the generation and reception of ultrasound signals and echoes. The channel  210  is prepared for operation by the controller  203  calculating and loading digitized waveform samples describing an acoustic signal into the waveform memory  212 . Next, the focus delay and data retrieval logic unit  214  retrieves data from the waveform memory  212  according to timing set by start and focus delay signals sent by the controller  203 . The controller  203  may employ various known control techniques to specify timing. In any event, in accordance with a preferred mode of operation, the controller  203  conveys a trigger signal indicative of the start of a transmit event and sets a focusing delay load in the focus delay and data retrieval logic unit  214 . Alternatively, other resources inside or outside the ultrasound imaging device  200  may produce the trigger and delay signals. Following the trigger event and after the focusing delay is finished, focus delay and data retrieval logic unit  214  retrieves digitized waveform samples from waveform memory  212  and conveys the samples to the MDAC  216 . In accordance with the preferred embodiment, the MDAC  216  preferably receives the digitized waveform samples at a rate of 40 MHz. 
     The MDAC  216  outputs an analog signal based on the product of the digitized waveform samples and an analog signal from the control circuit  202  (via the DAC  204 ). The analog signal output by the MDAC  216  is preferably filtered by the reconstruction filter  218  to smooth or average the discrete steps of the sampled waveform. Preferably, the reconstruction filter  218  is a fourth-order, 20 MHz, low-pass reconstruction filter, although other alternatives are know to those of ordinary skill in the art. The filtered signal from the reconstruction filter  218  is sent to the amplifier  220  (preferably a linear amp) that drives an element of the ultrasonic transducer  208 . The gain of the amplifier  220  is controlled by the controller  203 . Alternatively, a gain control signal may be supplied from sources other than the controller  203 . 
     The gain settings of the amplifier  220  are preferably varied between at least two gain settings by a gain select signal issued by the controller  202 . During transmission of signals for tissue harmonic imaging, two-dimensional imaging, color flow imaging, and other applications that utilize relatively large-amplitude transmit waveforms, the amplifier  220  is preferably set to a high gain setting to drive the large amplitude signals to the transducer. In contrast, for low amplitude waveform signal applications such as continuous-wave Doppler data acquisition, a low gain setting is preferable so as to increase the signal-to-noise ratio of the electrical signal driving the transducer. Transmit signals for continuous-wave Doppler are significantly lower in amplitude than signals in other operating modes. With the amplifier  220  set for high gain, the spectral noise contributed by the MDAC  216  is unacceptable for continuous-wave Doppler mode operation. Accordingly, the gain of the linear amplifier  220  is preferably reduced so that the noise contributed by the MDAC  216  is significantly reduced, resulting in a transmit waveform with an improved signal-to-noise ratio and a lower continuous-wave Doppler noise floor. 
     Under typical operating conditions, the controller  203  scales the magnitude of the digitized waveform samples in the waveform memory  212  to maximize digital resolution. The controller  203  then adjusts acoustic power simply by changing the setting of the DAC  204 . During normal operation, the samples in the waveform memory  212  remain constant. Such arrangement reduces the formatting time for power changes since new digitized waveform samples do not need to be calculated and loaded into waveform memory  212 . In addition, the resolution of the digital representation of the waveform remains optimal for all power settings. The gain setting of the amplifier  220  also affects acoustic power, e.g. as the gain of the linear amplifier  220  changes, the scale factor relating the digital input signal to the acoustic power of the output signal also changes. Control logic, such as control functions implemented in system software, may track the gain of the amplifier  220  and adjusts the scale factor of the DAC  204  accordingly. 
     It will be appreciated by those skilled in the art that changes may be made in the first embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents. The illustrative ultrasound imaging system  200  has a controller  202 , DAC  204 , and noise reduction filter  206 . However, those of ordinary skill in the art will recognize that one or more central controllers may be used. The amplitude control DAC  204  is optional and may be excluded from a particular example of a ultrasound imaging device  200 . Alternatively, some ultrasound imaging devices may include one or more amplitude control DACs. Furthermore, the noise reduction filter  206  is optional and may be excluded from a particular example of a ultrasound imaging device  200 . An ultrasound device with multiple amplitude control DACs may have a noise reduction filter connected to the output line from each amplitude control DAC. Alternatively, a plurality of noise reduction filters may be cascaded at the output line of the amplitude control DAC to effectively operate as a single filter. Additionally, the acoustic power setting operation may be executed by logic other than the controller  203 . For example, the acoustic power may be set by dedicated logic within the ultrasound imaging device  200 , by logic external to the ultrasound imaging device  200 , or by other suitable logic. The MDAC  216  could also be replaced by a DAC and a separate, subsequent, analog multiplier. 
     FIG. 3 is a block diagram of a multi-channel phased array ultrasound imaging system  300 , showing one channel in detail, in accordance with a second preferred embodiment of the present invention. The ultrasound imaging system  300  shown in FIG. 3 is similar to the ultrasound imaging system  200  shown in FIG. 2, with the modification that a DAC  304  and associated noise reduction filter  306  are provided for each transmit circuit in each channel  31  On, as opposed to the use of a central DAC  204  and noise reduction filter  206 , as shown in FIG.  2 . In this configuration, a control circuit  302  comprises a controller  303 . As described above, a receive beamformer  340  is provided to combine received echoes. Thus, each channel  310   n  has a transmitter that drives an element (not shown) of a transducer array  308 . The transmitter is provided with a waveform memory  312 , a focus delay and data retrieval logic  314 , a MDAC  316 , a reconstruction filter  318 , and an amplifier  320 , along with the aforementioned DAC  304  and associated noise reduction filter  306 . Each channel  310   n  also has a transmit/receive switch  322  and a receiver  324 . 
     Those of ordinary skill in the art will recognize that modifications described above with respect to the ultrasound system  200  can be made to the ultrasound system  300 . 
     While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiments described are possible. For example, those skilled in the art will readily implement the structures necessary to provide the structures and methods disclosed herein, and will understand that the process parameters, materials, and dimensions are given by way of example only and can be varied to achieve the desired structure as well as modifications which are within the scope of the invention. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims. 
     In the claims, unless otherwise indicated the article “a” is to refer to “one or more than one”.