Abstract:
The invention provides a novel method of transmit beamforming, which allows compact analog implementation of complex digital algorithms without compromising their features. It is aimed to support envelope shaping, apodization, and phase rotation per channel and per firing. Each of three embodiments represents a complete transmit channel driven by pulse-width modulated (PWM) waveforms stored in a conventional sequence memory. PWM signals controls the transmit pulse envelope (shape) by changing the duty cycle of the carrier. Beamformation data are loaded prior to a firing via serial interface. Under the direction of a controller, the circuitry allows high precision (beyond sampling rate) phase rotation of the carrier. It also provides transmit apodization (aperture weighting), which maintains an optimal trade-off among low sidelobe level and widening of the mainlobe. Implementing such an IC, the manufacturing cost of a high-end ultrasound system can be reduced. Equally, the proposed solution makes the benefits of digital transmit beamformers available to midrange and entry-level machines since it merely requires a modified programming of the sequence memory.

Description:
RELATED APPLICATION  
       [0001]     This patent application is claiming the benefit of U.S. Provisional Patent Application having a Ser. No. 60/492,588, filed Aug. 4, 2003 in the name of Lazar A. Shifrin, and entitled “Ultrasound Transmit Beamformer Integrated Circuit And Method Therefor”. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to coherent ultrasound imaging systems and, in particular, phased array ultrasound imaging systems operating in different scan formats and imaging modes such as, B-, F-, M- and D- modes. Specifically, the invention relates to transmit beamforming and generating ultrasound transmit waveforms.  
         [0004]     2. Background of the Invention  
         [0005]     An ultrasound scanner with an array transducer comprises a number of identical transmit and receive channels. Acting together, these channels produce transmit and receive beams, respectively. This process is referred as beamforming. It is accomplished by combining transmit/receive signals having variable delay and weight.  
         [0006]     There are two mainstream approaches in transmit beamforming: analog and digital. Although prior analog beamformers support different apodization and delay profiles, their precision is inherently limited. Another problem with analog transmit beam formation is insufficient hardware resources to optimize the characteristics of transmit waveforms. Because of this factor, a typical excitation signal looks like a burst of a desired carrier frequency. The waveform shape or envelope is essentially fixed and, therefore, not optimal. The only adjustable parameter is the length of the gate in terms of an integer number of carrier cycles.  
         [0007]     There are significant advantages in enhancing the flexibility of a transmit beamformer using digital processing (by way of example, see U.S. Pat. Nos. 4,809,184, 4,896,287, 5,142,649, 5,549,111, 5,970,025, 6,104,673). For that reason, modern high-end ultrasound systems commonly employ digital techniques. The digital transmit beamformer architecture utilizes a plurality of programmable transmit processors each with a source of the desired waveform to be applied to one or more corresponding transducer elements. Since the initially produced waveform is digital, it is converted to analog in order to activate the transducer. The circuits provided this conversion comprises a digital-to-analog converter (DAC) and a transmit amplifier (see U.S. Pat. No. 6,537,216).  
         [0008]     Each channel of a digital transmit beamformer is a rather sophisticated signal processing system. Since there is a plurality of transmit channels, the manufacturing cost, power dissipation and space constraints per channel are quite tough. The major problems associated with this factor are as follows: 
        a) Operating at sampling frequencies within the range of 50-100 MHz, transmit DACs are fairly priced;     b) Conventionally used push-pull class B transmit amplifiers are characterized by low power efficiency.        
 
         [0011]     For the theoretical class B amplifier, an absolute maximum of efficiency is 78%. A known alternative to conventional class B designs is those based on pulse-with modulation (PWM) or class D amplifiers. Such designs use analog signal processing to form the PWM signal by comparing the modulating signal with a high frequency sawtooth or triangle waveform that acts as a carrier. The resulting binary signal of the comparator feeds a suitable set of power switches connected to the power supply. Ideally, the switches dissipate no heat energy, i.e., the theoretical efficiency of class D amplifiers approaches 100%. Having low power dissipation, PWM amplifiers can be effectively integrated. Due to the same reason, class D amplifiers are more preferable in terms of heatsinking. These factors are the forces behind the motivation to develop a class-D transmit power amplifier suitable for ultrasound imaging. For instance, U.S. Pat. No. 6,135,963 entitled “Imaging System with Transmit Apodization Using Pulse-Width Variation” describes a method and apparatus for transmit apodization by controlling the duty cycle of the pulse. However, in view of foregoing consideration, building such a universally usable circuit seems to be problematic, despite today&#39;s advanced technology.  
         [0012]     To maintain low diffraction sidelobs from a sampled aperture, the error Δφ associated with a finite resolution of the delay profile must be kept small. Generally, the average error should be less than the signal provided by one channel. It leads to Δφ&lt;N/24, where N is the number of channels. For N=256, Δφ&lt;32 /2π. Thus, implementing a digital beamformer, one needs a 5- 6-Bit resolution DAC simply in order to provide an adequate precision of delay profile. However, in addition to fine focusing, DAC supports apodization and shaping. Evidently, that the resulting amplitude resolution of a channel is supposed to be sufficient to maintain the above tasks altogether. In sum, to support digital transmit beamforming, each transmitter should operate with dynamic range of at least 9 Bit.  
         [0013]     As well known (Black, H. S.  Modulation Theory.  New York: Van Nostrand Comp., 1953), in PWM waveform, the duty cycle is directly proportional to the modulating signal. Let F S  denote a sampling rate of the double-sided uniform pulse-width modulation (UPWM). Then, to maintain, for example, a 7-bit amplitude resolution, the carrier frequency, F C , would have to be sampled at F S =2·2 7 ·F C . Thus, for a 10 MHz transducer, the power switches would operate at a sub-nanosecond range. Given a 100-200 Volt pulse amplitude, building such a digital amplifier is quite challenging. Besides, running at higher frequencies, the efficiency of a Class D amplifier would be rather defined by dynamic power dissipation than the on/off switch resistance.  
       SUMMARY OF THE INVENTION  
       [0014]     By way of introduction, the present invention includes a method, a system, and device for ultrasound transmit beamforming.  
         [0015]     The general purpose of the invention, which will be described below in greater detail, is to provide a novel method of transmit beamforming that allows for compact analog implementation of complex digital algorithms without compromising their features.  
         [0016]     Another purpose of this invention is to provide a new transmit beamformer system that outperforms the prior art by simplicity, versatility, lower cost, and higher power efficiency, while maintaining programmability for carrier frequency, transmit waveform shape, delay and apodization profiles.  
       BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     In a first aspect, the waveform parameters and beamformation data are encoded differently. In particular, transmit waveform having gradually increased and gradually decreased envelope is approximated by a PWM signal. With reference to the resolution needed for focusing, the required number of pulse-width quantization levels for shaping is less. Consequently, the sampling rate can be reduced. At the same time, being greatly suppressed by the bandpass properties of the transducer and the frequency-dependent attenuation of the propagation medium, course quantization does not effect beamformation.  
         [0018]     In the second aspect, beamformation data, i.e., apodization and phase rotation values are binary-coded. These data to be converted to analog.  
         [0019]     In the third aspect, pulse-width amplitude modulation (PWAM) is provided. The resulting PWAM signals are originated by said PWM waveforms, which is then amplitude-modulated by the above mentioned analog signals. The two kinds of modulation, PWM followed by AM, constitute an analog multiplier circuit and yield a signal comprising both transmit and beamformation components.  
         [0020]     Further aspect and advantages of the invention are discussed below in conjunction with the preferred embodiments.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The invention will be described with respect to particular embodiments therefrom referring to the following drawings:  
         [0022]      FIG. 1  is a signal flow graph for the proposed method of phase rotation.  
         [0023]      FIG. 2  is a block diagram of the pulse height and width multiplier known in the art.  
         [0024]      FIG. 3  is a block diagram of a modified height and width multiplier.  
         [0025]      FIG. 4  is a block diagram of an embodiment of an ultrasound transmitter.  
         [0026]      FIG. 5  is a block diagram of another embodiment of an ultrasound transmitter.  
         [0027]      FIG. 6  is a schematic diagram of yet another embodiment of an ultrasound transmitter.  
         [0028]      FIG. 7  is a circuit diagram of a programmable current driver implemented within the embodiments. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]     An individual transmit waveform can be represented as an amplitude modulated cosine
 
 x ( t )= E ( t )·cos(2 πF   C   t +Ψ)
 
 where E(t) and Ψ are transmit envelope and carrier phase angle (in radians), respectively. For the carrier frequency F C , Ψ corresponds to a focusing time delay, τ, as
 
τ=Ψ/2πF C 
 
 From trigonometry identity,
 
 x ( t )= E ( t )·cos(2 πF   C   t )·cos Ψ− E ( t )·sin(2 πF   C   t )·sin Ψ
 
 In essence, the above equation depicts an arbitrary vector decomposed into in-phase and quadrature components. These components are representative of the primary I/Q vectors defined as
 
 I ( t )= E ( t )·cos(2 πF   C   t )
 
 Q ( t )= E ( t )·sin(2 πF   C   t )
 
 Finally,
 
 x ( t )= I ( t )·cos Ψ− Q ( t )·sin Ψ
 
         [0034]     As can be seen, there is the step of multiplying applied to the primary I/Q vectors. The multiplication factors are the sine and cosine functions of said phase angle Ψ. Therefore, to transmit an arbitrary waveform with a programmable phase of the carrier, the beamformer system should incorporate means for providing the primary vectors, computing sine/cosine values, multiplication, and summation.  
         [0035]     As used herein, the term “primary I/Q vectors” refers to a pair of quadrature waveforms aimed to originate a transmit signal. Also as used herein, the term “multiplication factors” stands for a pair of programmable constants having their ratio equal to the tangent function of said phase angle, Ψ. Autonomously, those constants are proportional to sinΨ and cosΨ. Their magnitude may characterize amplitude weighting (apodization) of a channel. In operation, the multiplication factors are provided prior to firing.  
         [0036]     It has been shown that there are two multiplications by a constant involved in phase rotating. Although high-performance analog multipliers are available, implementation on two wide-band four-quadrant devices per channel is quite expensive. However, encoding a primary vector into a digital PWM signal, the resulting pulse train can be used for amplitude sampling of a analog signal associated with a given multiplication factor. This procedure composes two (I/Q) pulse-width amplitude-modulated signals. Then, the obtained two PWAM signals are summed to produce transmit waveform tuned for fine focusing.  
         [0037]     Different techniques can be used to provide PWM representation of an analog waveform. Conventionally, PWM sequences are obtained by computing pulse widths in accordance with a modulated analog signal and programming a memory, which is then read out. FIG. 1  shows a signal flow graph for the proposed method of phase rotation regardless of the conversion details.  
         [0038]     An apparatus implementing PWAM for multiplication is often referred as “pulse height and width multiplier”.  FIG. 2  depicts a block diagram of such a circuit as known in the art (see “Electronics Engineers&#39; Handbook”, Fink, Donald G. &amp; Christiansen, Donald, 3d edition, McGraw-Hill, 1989, FIG. 13-125). The circuit behavior is illustrated by the accompanying truth table. It will be obvious to those skilled in the art that the switches shown in  FIG. 2  are to be bilateral. However, commercially available electronic switches of this kind are characterized by signal-dependent on-resistance that would degrade the accuracy of multiplication.  
         [0039]     The multiplier of  FIG. 2  can be modified to eliminate the bilateral switches needed for four-quadrant operation. An improved multiplier topology is shown in  FIG. 3 . Two important features characterize this topology: analog data are represented into the magnitude/sign format, and a single-ended architecture is replaced by a differential.  
         [0040]     The pulse height and width multiplier depicted in  FIG. 3  has two input ports: an analog  306  and a digital  305 . Said analog  306  and digital  305  ports are single-ended and differential, respectively. Using magnitude/sign format, the analog port  306  operates with unipolar signals, |V y |. Thus, in practice, switches  301 ,  302  may comprise conventional open-drain (open-collector) devices. Resistance of the pull-up resistors  308 ,  309  defines the amount of current provided by said switches  301 ,  302 . The PWM pulse train, V X , applied to the port  305  commands the state of said switches  301 ,  302 . To support four-quadrant computation, the phase of said pulse train, V X , is alternated in conjunction with the actual polarity of the analog input signal, V Y . In doing so, 2:1 multiplexers ( 303  and  304 ) select between the positive and inverted negative portions of the bipolar PWM waveform in response to the sign bit. Having the multiplexers&#39; inputs of the same name driven in reverse, this operation swaps the relative phases of the signals at nodes labeled as A and B. Finally, taking the difference between said nodes, a subtractor  307  provides bipolar PWAM-waveforms.  
         [0041]      FIG. 7  depicts one embodiment for the pulse height and width multiplier (PHWM) shown in  FIG. 3 . In essence, the embodiment represents a programmable current driver comprising a conventional 2:1 multiplexer  714 , a digitally programmable resistor (DPR)  740  with enable, a MOS transistor  720 , a reference input  702 , two clock inputs  704  and  706 , a polarity control input  708 , a data port  710 , and an output  712 . The transistor  720  is arranged to have its gate coupled to the reference input  702 , drain connected to the output  712 , and source connected to a common node  744  of said programmable resistor  740 . Multiplexer  714  comprises two selectable inputs, labeled A and B, an output  728  operative to enable the programmable resistor  740 , and a control input determining whether the A or B input get routed to the output. The programmable resistor  740  comprises a required plurality of 2-input open-drain NAND gates  716  and resistors  738 , said resistors  738  are binary weighted, each resistor  738  is connected between the open-drain output of a gate and the common node  744 . A first input of each gate receives a respective bit of a binary word applied to the data port  710 ; this word represents a desired resistance of the programmable resistor  740 .  
         [0042]     Referring to  FIG. 7 , programmable resistance is varied from 8R/15 to 8R. Thus, for a fixed voltage at the gate of the amplifier, the amplitude of the produced current pulses will approach the range of 24 dB. Other dynamic ranges, including lesser or larger ranges may be used. By any means, this range defines attainable amplitude resolution of a PHWM.  
         [0043]     All second inputs of each gate are coupled together exhibiting a node  742  and, thus, a logical “1” applied to said node enables conducting of the gates having a logical “1” on their first inputs. Therefore, applying a reference signal to the input  702 , the transistor  720  will generate a current pulse whose duration is determined by the width of a signal that drives one of the data inputs. In an alternative embodiment, thermometer coding scheme or its combination with binary techniques may be used. Apparently, the amplitude of this pulse will be directly proportional to said reference signal and inversely proportional to said resistance.  
         [0044]     There are several embodiments aimed to implement the proposed method of transmit beamforming. Operatively, these circuits generate transmit waveforms with programmable carrier frequency, envelop shape, fine delay, and apodization. Duplicating identical macrocells, the proposed solutions are suitable for integration. As used herein, the term “transmitter” refers to any circuit or device converting the primary I/Q PWM-signals into transmit waveforms applied to a transducer.  
         [0045]     In a first embodiment shown in  FIG. 6 , there are two identical pulse height and width multipliers,  601  and  602 , driving a transformer  613 . In operation, said multipliers  601  and  602  receive the primary I/Q PWM-sequences labeled SEQ_IP/N and SEQ_QP/N, respectively. Each of said PHWMs comprises two identical branches operating as an on/off switchable current driver. The switchable current driver includes a resistor  603 , an open-drain switch  604 , a 2:1 multiplexer  605  and a transistor amplifier  606  connected in a common gate configuration. The gate of the amplifier  606  is fed by a DAC  612 . In alternative embodiment, a bipolar transistor in a common base configuration may be used. In other alternative embodiments, different current drivers, such as conventional current mirrors, may be provided. In another alternative embodiment, an open collector switch may be employed. Regardless on the implementation, the current flowing trough the resistor  603  is proportional to the DAC output voltage. This current is turned on/off in response to the multiplexer output aligned with by the PWM pulse train. In consequence, the drain output of the amplifier  606  provides PWM current pulses with programmable amplitude. There is no difference in operation for the other current drivers.  
         [0046]     As well known, the common-gate configuration of a transistor amplifier provides high output impedance. Thus, coupling two drain outputs, the fractional currents are summed. In view of that, I/Q drain outputs are connected to the primary winding  613  of the transformer in pairs. The center tap of the primary winding is connected with a high voltage power supply. The secondary winding provides the transmitter output. The transformer may have a step-down or a step-up winding ratio.  
         [0047]     There is a transmit controller aimed to support beamformation. The controller comprises a sine/cosine look-up-table (LUT)  609 , two multiplying DACs  611 ,  612 , and a conventional DAC  610 . In operation, the LUT converts the input value of the phase angle into two digital words representing the sine and cosine function of said phase angle and deliver it to said multiplying DACs. The multiplying DACs provide the multiplication factors for phase rotation in the analog form. The apodization value is directly applied to the DAC  610 . Its output provides the reference voltage for both multiplying DACs,  611  and  612 . Thus, the amplitude of current pulses generated by said modulators  601 ,  602  is directly proportional to the product of said phase-rotating and apodization factors.  
         [0048]     Since the gate voltage in  FIG. 6  is formed as a product of phase rotating and apodization factors, it may have a wide dynamic range. Consequently, the threshold voltage tolerance of the transistor amplifiers may degrade the accuracy of beamformation. In view of that, the second embodiment shown in  FIG. 5  introduces a different architecture.  
         [0049]     FIG. 5  depicts the second embodiment  500  comprises two identical pulse height and width multipliers  501 ,  520 , a transmit controller  570 , and an output multiplier  580 . Each of the PHWMs  501  and  520  comprises two identical programmable current drivers  530 ,  540  and  550 ,  560 . Interface and architecture of these drivers are shown in  FIG. 7 .  
         [0050]     Referring to the PHWM  501 , the first and second clock terminals  504 ,  506  of the first and the second drivers are connected in reverse. These terminals are fed by the PWM pulse train as discussed previously.  
         [0051]     Switching inputs  508  of the first and second drivers  530 ,  540  are controlled by the polarity node  514 . Reference inputs  502  of the first driver  530  and the second driver  540  are connected together. Voltage at their connection point is defined by one of the phase rotating factors provided by the transmitter controller  570  through the node  542 .  
         [0052]     Data ports  510  of the current drivers are supplied by the controller  570  altogether. As can be seen, in the second embodiment, the apodization settings are provided in a digital format.  
         [0053]     Outputs  512  of the first  530  and second drivers  540  from the first PHWM  501  are respectively connected to the outputs of the first  550  and the second  560  drivers from the second PHWM  520 . Then, these signals are applied to a transformer-coupled class B push-pull amplifier  580  having its first  582  and second transistors  584  connected in a common-gate configuration. Consequently, the power amplifier  580  is driven by a sum of the currents produced by both PHWMs.  
         [0054]     Referring to the transmit controller  570 , LUT  516  converts the input value of the phase angle into the sine and cosine function of said angle feeding afterward two conventional DACs,  518  and  519 . Thus, the amplitude of current pulses generated by the PHWMs  501 ,  520  becomes directly proportional to the sine/cosine rotating factors and inversely proportional to a resistance of the programmable resistor, i.e., to the apodization factor. This partition reduces the phase rotation errors due to the threshold voltage deviation.  
         [0055]     A third embodiment shown in  FIG. 4  comprises a transmit controller  470  having a LUT  416 , a DAC  418 , a power amplifier  480 , and two identical PHWMs,  401  and  420 . The transistor amplifier  480  operates in a common-gate mode providing essentially low input impedance. Each PHWM comprises a pair of programmable current drivers ( 430  and  440  vs.  450  and  460 ) aimed to feed the power amplifier  480 . The drivers&#39; interconnection within a PHWM duplicates the one discussed in respect of the second embodiment. However, the beamformation data are encoded differently.  
         [0056]     In operation, transmit controller  470  receives the apodization settings in a digital format and converts it into analog. Accordingly, the reference nodes  412  are connected altogether with the output of DAC  418 .  
         [0057]     The phase rotating data are distributed digitally. Referring  FIG. 4 , LUT  416  feeds the first  420  and the second rotation  422  ports. Agreeably, the phase rotation data are directly applied to the driver&#39;s data ports  410  grouped in pairs. Thus, as previously, the PWM pulse train will determine timing of the current pulses produced by a driver. In contrast, the amplitude of these pulses is directly proportional to the apodization settings and inversely proportional to resistance of the DPR, i.e., to the one of phase rotating factors.  
         [0058]     If the DPR resistors are binary-weighted as shown in  FIG. 7 , this programmable current driver can be interpreted as a linear current-output DAC. However, it will be appreciated that its conversion speed is defined by the firing rate rather than by sampling frequency.  
         [0059]     Alternatively, to avoid using a LUT, a nonlinear current driver can be implemented. In this architecture, the driver provides both digital-to-analog and trigonometric conversion at the same time. Technically, it can be done by selecting distinct resistors for each switch/resistor cell and increasing the number of cells or by making the cell topology more complicated. Both approaches, however, require extensive hardware resources.  
         [0060]     While the invention has been described above by reference to various embodiments, it would be understood that many changes and modifications could be made without departing from the scope of the invention. For example, different multipliers, current drivers, switches, or output amplifier configurations may be used. It is therefore intended that the foregoing detailed description be understood as an illustration of the presently preferred embodiments of the invention, and not as a definition of the invention. It is only the following claims or added claims, including all equivalents, are intended to define the scope of this invention.