Ultrasound transmit waveforms having low harmonic content

Methods and apparatus for transmitting ultrasound energy having low harmonic content and for detecting harmonic reflections of the ultrasound energy are disclosed. According to the invention, the capacitance of an ultrasound transducer element, as well as any interconnection capacitance associated with one or more signal conductors attached to the transducer element, is utilized as part of a filter which conditions one or more electronic drive signals to provide an ultrasound transmit waveform having low harmonic content. Such a low harmonic content ultrasound transmit waveform may be used to expose a region of interest in a patient which reflects harmonics of the transmit waveform. Receive circuitry detects the harmonic reflections and generates an ultrasound image having improved contrast and reduced distortion.

FIELD OF THE INVENTION
 This invention relates to harmonic ultrasound imaging for medical
 applications and, more particularly, to methods and apparatus for
 transmitting ultrasound energy having waveforms with low harmonic content
 and for detecting harmonic reflections of the ultrasound energy.
 BACKGROUND OF THE INVENTION
 Ultrasound imaging systems for medical applications typically employ an
 array of individual ultrasound transducer elements which transmit and
 receive ultrasound energy. The transducer array transmits ultrasound
 energy into a region of interest in a patient, and receives reflected
 ultrasound energy, or echos, from various structures and organs within the
 patient's body. The imaging system then processes electronic signals
 generated by the elements of the transducer array, based on the received
 ultrasound energy, to form an image of the region of interest.
 Some ultrasound imaging applications make use of harmonic reflections from
 a region of interest for which an image may be desired. Harmonic
 reflections may result from a non-linear medium that is exposed to
 ultrasound energy at some fundamental transmit frequency. One example of
 such a non-linear medium includes water, which is present throughout body
 tissues and has different expansion and compression properties upon
 exposure to ultrasound energy. In this manner, non-linear body tissues and
 fluids may release acoustic energy at one or more harmonic frequencies of
 the fundamental transmit frequency of the ultrasound energy to which they
 are exposed.
 Contrast agents provide another example of a non-linear medium used in some
 ultrasound imaging applications. Non-linear contrast agents may be
 introduced into regions of interest in a patient, for example, into the
 blood stream or body tissues, to highlight these regions from surrounding
 tissue in the ultrasound image. These agents generally have stronger
 non-linear properties than the surrounding tissues. Typically, contrast
 agents have a fundamental resonant frequency and radiate ultrasound energy
 at a particular harmonic frequency when exposed to high intensity
 ultrasound energy at the fundamental resonant frequency. An ultrasound
 imaging system may therefore identify and isolate regions containing a
 contrast agent by differentiating the ultrasound energy at the particular
 harmonic frequency associated with the contrast agent from harmonic
 ultrasound energy associated with the surrounding tissue.
 Ultrasound imaging applications utilizing harmonic reflections from either
 body tissues and/or contrast agents may be limited, however, by the
 harmonic content of ultrasound energy transmitted to a region of interest.
 In particular, ultrasound transmit waveforms having a significant harmonic
 content may result in ultrasound images having undesired artifacts, as
 well as ultrasound images having reduced contrast between a contrast agent
 and surrounding tissues. Such artifacts or reduced contrast may be due to
 undesired high frequency components of the transmit waveform that
 interfere with desired harmonic reflections from the region of interest.
 For example, to reach a region of interest for which an image may be
 desired, ultrasound energy must often pass through body structures and
 tissues, such as a chest wall, which include inhomogeneous materials that
 may significantly distort the waveform profile of ultrasound energy. This
 distortion from inhomogenous materials often results in unwanted artifacts
 in the resulting image. Since ultrasound waveform distortion from
 inhomogeneous materials is typically less at lower frequencies, it is
 advantageous to transmit ultrasound energy at a low fundamental frequency.
 However, lower frequency ultrasound energy generally results in lower
 resolution images.
 To avoid a loss of resolution associated with lower frequencies, an
 ultrasound imaging system may detect harmonics reflected from any
 non-linear tissues being imaged, as discussed above. However, if the
 ultrasound transmit waveform itself includes a significant harmonic
 content, higher frequency components of the transmit waveform may not be
 discernible from the desired harmonic reflections. Moreover, distortion of
 the higher frequency components of the transmit waveform from
 inhomogeneous materials, notwithstanding the lower fundamental frequency,
 may contribute to unwanted artifacts in the resulting image.
 In view of the foregoing, it is desirable to transmit ultrasound energy as
 a waveform having a fundamental frequency, wherein the waveform is
 substantially free of higher frequency harmonic components of the
 fundamental frequency, or has low harmonic content. Since higher
 frequencies experience greater distortion in inhomogeneous materials, as
 discussed above, reducing higher frequency components in an ultrasound
 transmit waveform in turn reduces distortion of the ultrasound energy as
 it passes through body structures and tissues that include inhomogeneous
 materials. By utilizing ultrasound transmit waveforms having low harmonic
 content, it is possible for ultrasound imaging systems to obtain clearer
 images having fewer artifacts from distortion. Additionally, ultrasound
 imaging applications utilizing low harmonic content ultrasound transmit
 waveforms and contrast agents benefit from a higher contrast ratio between
 the contrast agent and surrounding tissue.
 In one known technique for reducing the harmonic content of an ultrasound
 transmit waveform, an electronic pulse generator supplies a burst of
 pulses having a fundamental frequency to a low pass filter. The duration
 of the pulses and the number of pulses in the burst are controllable and
 determine the resulting frequency spectrum of the burst. Such a burst of
 pulses has a primarily rectangular shape and has a frequency spectrum that
 includes a number of sidelobes having significant harmonic content. The
 low pass filter is typically designed as a Gaussian, Bessel or Chebyshev
 filter to substantially eliminate energy at a particular harmonic
 frequency from the burst of pulses. The filtered burst of pulses is then
 applied to ultrasound transducer elements which transmit ultrasound energy
 having a waveform similar to that of the filtered burst of pulses.
 Other proposed techniques for reducing the harmonic content of ultrasound
 transmit waveforms include using digital signal processing methods and
 apparatus, such as digital programmable waveform generators, to produce
 particularly shaped electronic waveforms having low harmonic content,
 which are then applied to ultrasound transducer elements. Such techniques
 typically involve synthesizing an electronic signal having a particularly
 shaped amplitude envelope which includes several cycles of a fundamental
 or "carrier" frequency. A variety of such electronic signals having
 frequency spectra that exhibit "sidelobe suppression," or low harmonic
 content, may be custom synthesized using digital signal processing methods
 and apparatus.
 In one such technique described in Hossack, et al., U.S. Pat. No.
 5,740,128, a desired frequency spectrum for an ultrasound transmit
 waveform is designed on a computer, and an inverse fast Fourier transform
 is performed to synthesize a corresponding time domain waveform. The
 synthesized waveform may be specifically designed to suppress ultrasound
 energy in a wide band around a particular harmonic frequency. A digital
 representation of the synthesized waveform is stored in memory, and is
 applied to a digital-to-analog converter which provides an analog
 electronic signal of the stored waveform. This analog signal may
 optionally be passed through a low pass analog filter, such as a Gaussian,
 Bessel filter or a Chebyshev filter, to further reduce any undesirable
 high frequency components of the analog signal. This low harmonic content
 electronic signal is then applied to ultrasound transducer elements to
 provide ultrasound energy having low harmonic content.
 One consequence of the techniques described above is that a significant
 amount of space is required for the electronic circuitry, for example, the
 analog filters or any digital signal processing electronics, such as a
 computer or programmable waveform generator, memory, and digital-to-analog
 converters, which provide the low harmonic content electronic signal
 applied to the ultrasound transducer elements. Accordingly, a method and
 apparatus for transmitting ultrasound energy having low harmonic content
 is desirable that uses fewer electronic components and requires less space
 than other proposed techniques.
 SUMMARY OF THE INVENTION
 The present invention is directed to methods and apparatus for transmitting
 ultrasound energy having low harmonic content and for receiving harmonic
 reflections. The methods and apparatus of the invention are simplified
 with respect to previous solutions for reducing harmonic content in
 ultrasound transmit waveforms, as discussed above. According to one aspect
 of the invention, the capacitance of an ultrasound transducer element, as
 well as any interconnection capacitance associated with one or more signal
 conductors attached to the transducer element, is utilized as part of a
 filter that conditions one or more electronic drive signals to provide an
 ultrasound transmit waveform having low harmonic content. An electronic
 drive signal input to the filter to drive the ultrasound transducer
 element may itself have an appreciable harmonic content. This is in
 contrast to prior art techniques for reducing ultrasound transmit waveform
 harmonic content, which use pre-filtered and/or synthesized low harmonic
 content electronic signals to drive an ultrasound transducer element.
 Additionally, in one aspect, an apparatus according to the invention may
 be fabricated as a monolithic integrated circuit. In this manner, the
 methods and apparatus of the invention facilitate the generation of
 ultrasound energy having low harmonic content and detection of harmonic
 reflections by using fewer components and circuitry and requiring less
 space than previous solutions.
 In one embodiment, an apparatus according to the invention includes an
 ultrasound transducer element to output ultrasound energy having an
 ultrasound transmit waveform with a low harmonic content, wherein the
 ultrasound transducer element has a capacitance. The apparatus further
 includes a transducer driver to charge and discharge the ultrasound
 transducer element based on at least one drive signal input to the
 transducer driver, wherein the at least one drive signal has a fundamental
 frequency. The transducer driver charges and discharges the ultrasound
 transducer element such that the ultrasound transmit waveform has a rise
 time based on the capacitance and one of an output impedance of the
 transducer driver and a drive current output by the transducer driver, the
 one of the output impedance and the drive current being selected such that
 the rise time is greater than one-fifth of a period given by a reciprocal
 of the fundamental frequency.
 In one aspect, the fundamental frequency of the drive signal is less than
 3.5 MHZ.
 In another aspect, the apparatus further includes receive circuitry to
 detect a reflected ultrasound waveform at a harmonic frequency of the
 fundamental frequency.
 In another aspect, the drive signal has a drive signal harmonic content,
 and the low harmonic content is substantially less than the drive signal
 harmonic content.
 In another aspect, the apparatus further includes at least one signal
 conductor electrically connected to the ultrasound transducer element and
 the transducer driver, wherein the at least one signal conductor has an
 interconnection capacitance. The interconnection capacitance and the
 capacitance of the ultrasound transducer element form a combined
 capacitance, and the rise time is based on the combined capacitance and
 the one of the output impedance and the drive current.
 In another aspect, the transducer driver comprises a controllable voltage
 source having the output impedance, the controllable voltage source and
 the ultrasound transducer element form a filter having a cutoff frequency
 based on the combined capacitance and the output impedance, and the output
 impedance is selected such that the cutoff frequency is less than a
 predetermined harmonic frequency of the fundamental frequency.
 In another aspect, the predetermined harmonic frequency is the second
 harmonic frequency.
 In another aspect, the transducer driver comprises a controllable switched
 current source including at least one of a charge circuit and a discharge
 circuit to output the drive current to the ultrasound transducer element
 based on the drive signal, and the drive signal includes digital pulse
 control signals to control at least one of the charge circuit and the
 discharge circuit.
 In another aspect, the digital pulse control signals control the
 controllable switched current source such that at least one of the charge
 circuit and the discharge circuit outputs at least two different current
 values for the drive current.
 In another aspect, the charge circuit comprises a plurality of first
 current sources having a first common terminal controlled by a charge
 control input, at least one of the first current sources having a second
 terminal controlled by a first auxiliary control input. The discharge
 circuit comprises a plurality of second current sources having a first
 common terminal controlled by a discharge control input, at least one of
 the second current sources having a second terminal controlled by a second
 auxiliary control input. The digital pulse control signals are applied to
 the charge control input, the discharge control input, the first auxiliary
 control input, and the second auxiliary control input in a predetermined
 manner to control the plurality of first controllable current sources and
 the plurality of second controllable current sources to output the drive
 current.
 In another aspect, the ultrasound transmit waveform is a substantially
 triangular waveform.
 In another aspect, the ultrasound transmit waveform comprises at least a
 first slope and a second slope, the first and second slopes being based on
 the combined capacitance and the drive current, the first slope having a
 different magnitude than the second slope.
 In another aspect, the ultrasound transmit waveform comprises at least two
 unique maxima.
 In another aspect, the apparatus is fabricated as a monolithic integrated
 circuit.
 In another embodiment, an apparatus according to the invention comprises a
 plurality of ultrasound transducer elements, each transducer element
 having a transducer impedance based on a capacitance of the transducer
 element, and a corresponding plurality of programmable-delay switched
 current sources to charge and discharge the plurality of ultrasound
 transducer elements in a selectable manner, each programmable-delay
 current source outputting a drive current to charge and discharge a
 respective ultrasound transducer element so as to transmit ultrasound
 energy having an ultrasound transmit waveform based on the drive current
 and the transducer impedance, wherein the ultrasound transmit waveform has
 a low harmonic content. The apparatus further comprises a transducer
 controller to output common control signals to the plurality of
 programmable-delay current sources, wherein the ultrasound transmit
 waveform transmitted by each transducer element is based on selectably
 delayed versions of the common control signals.
 In one aspect, each programmable-delay current source comprises a charge
 circuit to charge the respective transducer element, a discharge circuit
 to discharge the respective transducer element, and at least one
 programmable delay circuit to receive the common control signals and delay
 instruction signals, and to output the selectably delayed versions of the
 common control signals to the charge circuit and the discharge circuit,
 based on the delay instruction signals.
 In another aspect, the plurality of programmable-delay current sources and
 the transducer controller are fabricated as a monolithic integrated
 circuit.
 In another embodiment, a method according to the invention for transmitting
 ultrasound energy having a low harmonic content ultrasound transmit
 waveform comprises a step of charging and discharging an ultrasound
 transducer element using one of a drive voltage applied to the ultrasound
 transducer element through a drive impedance and a drive current, the
 drive voltage and the drive current each having a fundamental frequency,
 the ultrasound transducer element having a capacitance. The method further
 comprises a step of selecting one of the drive impedance and the drive
 current such that the ultrasound transmit waveform has a rise time greater
 than one-fifth of a period given by a reciprocal of the fundamental
 frequency, the rise time being based on the capacitance and the one of the
 drive impedance and the drive current.
 In one aspect of the method according to the invention, the step of
 charging and discharging comprises a step of charging and discharging the
 ultrasound transducer element such that the ultrasound transmit waveform
 is a complex waveform comprising a sequence of three triangular pulses,
 wherein a first and last triangular pulse of the sequence each has a first
 slope magnitude and a first maximum, and a middle triangular pulse of the
 sequence, occurring between the first and last triangular pulses, has a
 second slope magnitude greater than the first slope magnitude and a second
 maximum greater than the first maximum.
 In another aspect of the method according to the invention, the step of
 charging and discharging comprises a step of applying digital pulse
 control signals having a pulse width to a switched current source to
 output the drive current, such that the ultrasound transmit waveform has a
 slope based on the capacitance and the drive current, and the rise time is
 comparable to the pulse width.
 In another aspect, a first magnitude of the low harmonic content of the
 ultrasound transmit waveform is at least 20 decibels below a second
 magnitude of a drive current harmonic content of the drive current in a
 frequency band around a second harmonic frequency of the fundamental
 frequency.
 Other advantages, novel features, and objects of the invention will be
 become apparent from the following detailed description of the invention
 when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION
 FIG. 1 shows a portion of an ultrasound imaging system for medical
 applications which includes an apparatus according to one embodiment of
 the invention. In the system of FIG. 1, one or more arrays 511 of
 individual ultrasound transducer elements 10 transmit ultrasound energy
 517 into a region of interest in a patient 501, and receive reflected
 ultrasound energy, or echos, from various structures and organs within the
 patient's body. Transmit circuitry 505 provides electronic signals to the
 transducer elements 10 of the array 511 to produce the ultrasound energy
 517, and receive circuitry 507 processes electronic signals generated by
 the elements 10 based on the ultrasound energy reflected from the region
 of interest. The ultrasound energy transmitted by the transducer elements
 10 may have a fundamental frequency, and the receive circuitry 507 may
 detect ultrasound energy reflected from the region of interest at one or
 more harmonic frequencies of the fundamental frequency.
 As illustrated in FIG. 1, the ultrasound imaging system may include a
 transmit/receive switch 508 to alternately couple, via one or more signal
 conductors 520, the transmit circuitry 505 and the receive circuitry 507
 to the transducer array 511 for respective transmit and receive
 operations. Alternatively, one group of transducer elements 10 or a
 transducer array similar to that of array 511, dedicated to transmitting
 ultrasound energy, may be connected directly to transmit circuitry 505,
 while another group of transducer elements 10 or another array similar to
 that of array 511, dedicated to receiving ultrasound energy, may be
 connected directly to receive circuitry 507. Additionally, transmit
 circuitry 505, receive circuitry 507, transducer array 511, and
 transmit/receive switch 509 may be located together in a single package
 502, for example a hand-held transducer head which is connected by a
 flexible cable 516 to an electronics unit (not shown), and may be
 fabricated as a monolithic integrated circuit. The electronics unit
 controls the circuitry of the transducer head to form an image of the
 region of interest in the patient, based on the transmitted and reflected
 ultrasound energy.
 The present invention utilizes the "clamp" capacitance of an ultrasound
 transducer element 10, as well as any interconnection capacitance between
 the transducer element and associated circuitry, as part of a "filter"
 that reduces the harmonic content of a waveform of ultrasound energy 517
 transmitted by the transducer element. For example, such a filter may
 include a portion or all of the transmit circuitry 505, the transducer
 element 10, and one or more signal conductors 520, and may receive one or
 more electronic drive signals having a fundamental frequency and a
 particular frequency spectrum, or harmonic content. An exemplary range of
 fundamental frequencies of the drive signals suitable for purposes of the
 invention may given as, but is not limited to, 1 MHz to 10 MHz.
 In the filter described above in connection with FIG. 1, the ultrasound
 transducer element 10 is capacitively charged and discharged through one
 or more signal conductors 520 by the transmit circuitry 505, based on the
 drive signals, and converts electrical energy from the drive signals into
 ultrasound energy 517 such that the ultrasound transmit waveform has low
 harmonic content. In particular, if the drive signals have an appreciable
 harmonic content, the ultrasound transmit waveform may have a harmonic
 content that is substantially less than the harmonic content of the drive
 signals. Such a low harmonic content ultrasound transmit waveform may be
 used to expose a region of interest in patient 501 that includes media,
 such as non-linear tissues and/or contrast agents, which reflect harmonics
 of the transmit waveform. Receive circuitry 507 may be used to detect such
 harmonic reflections and generate an ultrasound image of the region of
 interest having improved contrast and reduced distortion.
 A simplified block diagram of an example of an apparatus for transmitting
 ultrasound energy having low harmonic content, according to one embodiment
 of the invention, is shown in FIG. 2A. The apparatus of FIG. 2A includes
 an ultrasound transducer element 10, a transducer driver 20, and a signal
 conductor 11 connecting the transducer element 10 and the transducer
 driver 20. The transducer driver 20 may be included as part of the
 transmit circuitry 504 of the ultrasound imaging system shown in FIG. 1,
 and the signal conductor 11 may be, for example, one of the signal
 conductors 514 shown in FIG. 1. FIG. 2A also shows a capacitance 32
 associated with the transducer element 10 and the signal conductor 11. The
 capacitance 32 represents a combined capacitance based on a "clamp"
 capacitance of the ultrasound transducer element 10 and any
 interconnection capacitance associated with the signal conductor 11. For
 embodiments in which the interconnection capacitance may be negligible,
 the capacitance 32 represents primarily the capacitance of the ultrasound
 transducer element 10.
 FIG. 2A shows that, in one embodiment of the invention, the transducer
 driver 20 may include a controllable voltage source 34 having an output
 impedance 31, shown symbolically in FIG. 2A as resistor R.sub.0. In the
 apparatus of FIG. 2A, the controllable voltage source 34 charges and
 discharges the ultrasound transducer element 10 through the output
 impedance 31 and the signal conductor 11, based on one or more drive
 signals 40 input to the controllable voltage source 34. Accordingly, in
 the embodiment of FIG. 2A, the transducer driver 20 functions essentially
 as a "voltage mode" driver. As discussed above in connection with FIG. 1,
 the transducer element 10, the signal conductor 11, and the voltage mode
 transducer driver 20 including the output impedance 31 may be viewed as
 forming a filter 35 having a cutoff frequency based on the combined
 capacitance 32 and the output impedance 31.
 The cutoff frequency of a filter generally refers to that frequency at
 which the magnitude of the output of the filter is attenuated by
 approximately 3 dB (approximately 30%) from some maximum value. For
 first-order low-pass linear filters such as filter 35, the cutoff
 frequency in Hz may be expressed in terms of a time constant .tau. as
 1/(2.pi..tau.), where for the filter 35 .tau. is given by the product of
 the output impedance 31 and the combined capacitance 32. An exemplary
 range of values for the capacitance 32 suitable for purposes of the
 invention includes, but is not limited to, 10 to 200 picofarads.
 Similarly, an exemplary range of output impedance values for the voltage
 mode transducer driver 20 suitable for purposes of the invention includes,
 but is not limited to, 500 ohms or greater.
 In the embodiment of FIG. 2A, the value of the output impedance 31 may be
 selected and implemented during a fabrication process for the voltage mode
 transducer driver 20. In particular, the output impedance 31 may be
 selected such that the cutoff frequency of the filter 35 is less than a
 predetermined harmonic frequency of a fundamental frequency of one or more
 drive signals 40. For example, the output impedance 31 may be selected
 such that the cutoff frequency of filter 35 is less than a second harmonic
 frequency of the fundamental frequency of the drive signals 40.
 Accordingly, for drive signals 40 having an appreciable harmonic content,
 the harmonic content of the ultrasound transmit waveform 44 may be
 substantially less than the harmonic content of the drive signal.
 Alternatively, the drive signal conditioning provided by the apparatus of
 FIG. 2A may be described in terms of a rise time of the ultrasound
 transmit waveform 44 output from the filter 35. The rise time of a
 waveform is commonly defined as the time in which the waveform changes
 from 10% to 90% of a span between a minimum and maximum value. In the
 apparatus of FIG. 2A, the ultrasound transmit waveform 44 has a rise time
 based on the combined transducer/signal conductor capacitance 32 and the
 output impedance 31 of the transducer driver 20. In a preferred
 embodiment, the rise time is greater than one-fifth of a period given by
 the reciprocal of the fundamental frequency of the drive signals 40.
 A simplified block diagram of an example of an apparatus for transmitting
 ultrasound energy having low harmonic content, according to another
 embodiment of the invention, is shown in FIG. 2B. In the apparatus of FIG.
 2B, unlike the apparatus of FIG. 2A, the transducer driver 20 functions
 essentially as a "current mode" driver. For example, the transducer driver
 20 of FIG. 2B may be a controllable switched current source having a high
 output impedance which approximates that of an ideal current source having
 a theoretically infinite output impedance. Similarly to the voltage mode
 transducer driver of FIG. 2A, the controllable switched current source
 transducer driver 20 of FIG. 2B may be included as part of the transmit
 circuitry 504 of the ultrasound imaging system shown in FIG. 1.
 In FIG. 2B, the controllable switched current source transducer driver 20
 is shown coupled between a supply voltage V.sub.H, typically in a range of
 10 to 200 volts, and ground. FIG. 2B also shows that the controllable
 switched current source 20 may include a charge circuit 12 to provide a
 charge current 26 (I.sub.c) to the ultrasound transducer element 10 and a
 discharge circuit 16 to conduct a discharge current 28 (I.sub.D) from the
 ultrasound transducer element 10. The charge circuit 12 may have a charge
 control input 14 and the discharge circuit 16 may have a discharge control
 input 18.
 The charge circuit 12 and the discharge circuit 16 of FIG. 2B are
 electrically coupled to provide a drive current 30 (I.sub.T) to the
 ultrasound transducer element 10. The drive current 30 is the sum of the
 charge current 26 and the discharge current 28 as a function of time. The
 charge control input 14 and the discharge control input 18 may receive one
 or more electronic drive signals 40 that determine the magnitudes of the
 charge current 26 and the discharge current 28 as a function of time and
 hence, determine the drive current 30.
 FIG. 2B shows that the electronic drive signals received on control inputs
 14 and 18 may be digital pulse control signals 40 having a pulse width 42.
 An exemplary range of digital control signal pulse widths suitable for
 purposes of the invention includes, but is not limited to, 50-200
 nanoseconds. According to the embodiment of FIG. 2B, the digital pulse
 control signals 40 may control at least one of the charge circuit 12 and
 the discharge circuit 16 to output the drive current 30 such that the
 ultrasound transmit waveform 44 has a slope 45 based on the combined
 transducer/signal conductor capacitance 32 and the drive current 30. In
 this embodiment, the ultrasound transmit waveform 44 may have a rise time
 46 comparable to the pulse width 42; namely, the rise time 46 and the
 pulse width 42 may have approximately the same order of magnitude.
 In the apparatus of FIG. 2B, typically the amplitude of the digital pulse
 control signals 40 determines the magnitudes of the charge current 26 and
 the discharge current 28, and the pulse width 42 determines the duration
 of the charge current 26 and the discharge current 28. Additionally, the
 digital pulse control signals 40 are generally applied to control inputs
 14 and 18 at different times in a predetermined manner and hence, only one
 of charge circuit 12 and discharge circuit 16 are operated at any given
 time. In this manner, the switched current source transducer driver 20 and
 the ultrasound transducer element 10 of FIG. 2B function together as a
 time-dependent filter 36.
 As discussed above in connection with FIG. 2A, in a preferred embodiment
 the rise time of the ultrasound transmit waveform 44 is greater than
 one-fifth of a period given by the reciprocal of the fundamental frequency
 of the drive signals 40. Specifically, in the apparatus of FIG. 2B, the
 amplitude of the digital pulse control signals 40 may be selected and the
 control signals 40 may be applied to control inputs 14 and 18 in a
 predetermined manner such that the rise time 46 of the ultrasound transmit
 waveform 44 is greater than one-fifth of the pulse width 42.
 In summary, each of the apparatus according to different embodiments of the
 invention shown in FIGS. 2A and 2B receives one or more electronic drive
 signals and conditions the drive signals to provide an ultrasound transmit
 waveform having low harmonic content by utilizing the capacitance of both
 an ultrasound transducer element and one or more signal conductors
 attached to the ultrasound transducer element as part of a filter. The
 apparatus according to the embodiment shown in FIG. 2A includes a "voltage
 mode" transducer driver, and the ultimate shape of the ultrasound transmit
 waveform is based on the combined transducer/signal conductor capacitance
 and an output impedance of the voltage mode transducer driver. The
 apparatus according to the embodiment shown in FIG. 2B includes a "current
 mode" transducer driver, and the ultimate shape of the ultrasound transmit
 waveform is based on the combined transducer/signal conductor capacitance
 and a drive current that charges and discharges the ultrasound transducer
 element.
 It is noteworthy that, while prior art transducer drivers are typically
 designed to have low impedance outputs so as to avoid any deleterious
 effects on the ultrasound transmit waveform due to the inherent
 capacitance of the transducer element, the present invention
 advantageously utilizes the inherent transducer element capacitance and
 signal conductor capacitance, in combination with either a selected driver
 output impedance or a drive current provided by a current source having a
 high output impedance which approximates a theoretically infinite
 impedance, to provide low harmonic content ultrasound energy.
 In both of the apparatus shown in FIGS. 2A and 2B, an electrical waveform
 is produced across the ultrasound transducer element 10 as a result of the
 charging and discharging of the transducer element due to either an
 applied voltage or current. The electrical energy of the electrical
 waveform is converted by the transducer element to ultrasound energy
 having an ultrasound transmit waveform 44 similar to that of the
 electrical waveform. In practice, the waveform 44 of ultrasound energy
 actually transmitted by the transducer element may not be exactly
 identical to the electrical waveform produced across the ultrasound
 transducer element and may have a slightly more "softened" or sinusoidal
 shape close to a waveform peak than that of the electrical waveform. This
 effect is primarily due to physical limitations of the transducer element,
 which mechanically vibrates in response to the electrical waveform to
 produced the ultrasound energy. Accordingly, such a waveform-softening
 effect may be more pronounced for electrical waveforms having sharp peaks,
 such as triangular waveforms. For purposes of the present discussion,
 however, the waveform-softening effect due to physical limitations of the
 transducer element is assumed to be negligible, and the harmonic content
 of the electrical waveform produced across the ultrasound transducer
 element 10 and the ultrasound transmit waveform 44 itself are assumed to
 be substantially similar.
 The concept of harmonic content of waveforms in general will now be
 discussed briefly with reference to FIGS. 3A-6C. FIGS. 3A and 3B through
 6A and 6B are graphs which illustrate four examples of waveforms and their
 corresponding frequency spectra. For each of FIGS. 3A, 4A, 5A, and 6A, the
 horizontal axis of the graphs represents time in microseconds (.mu.S), and
 the vertical axis represents waveform amplitude in arbitrary units. For
 each of FIGS. 3B, 4B, 5B, and 6B, the horizontal axis of the graph
 represents frequency in megahertz (MHz), and the vertical axis represents
 the relative strength of a particular frequency component of the waveform,
 or spectrum, in units of decibels (dB). While, for purposes of comparison,
 FIGS. 3A and 4A illustrate waveforms that are employed in known ultrasound
 imaging systems, FIGS. 5A and 6A show examples of ultrasound transmit
 waveforms according to the present invention, having an appreciably lower
 harmonic content than the waveforms of FIGS. 3A and 4A.
 FIG. 3A shows a single square pulse waveform 50 having a pulse width 49 of
 approximately 0.5 microseconds. The pulse width 49 is chosen for purposes
 of illustration only, and is not intended to be limiting. According to
 fundamental principles of Fourier transformation of waveforms between the
 time domain and the frequency domain, the fundamental frequency of such a
 square pulse waveform 50 is approximately equal to the reciprocal of the
 pulse width 49. Accordingly, the square pulse waveform 50 has a
 fundamental frequency of approximately 1/0.5 microseconds, or 2 MHz.
 FIG. 3B shows a spectrum 51 in the frequency domain corresponding to the
 single square pulse waveform 50. The spectrum 51 illustrates the relative
 strengths of the various frequency components present in the single square
 pulse waveform 50. A portion or "band" of the spectrum 51 that includes
 the fundamental frequency of 2 MHz is indicated along the horizontal
 frequency axis of FIG. 3B by reference character 600. Similarly, a band of
 the spectrum 51 including the second harmonic of the fundamental
 frequency, namely, a frequency component at 4 MHz, is indicated along the
 horizontal frequency axis of FIG. 3B by reference character 602. In each
 of FIGS. 3B, 4B, 5B, and 6B, the widths of the bands 600 and 602 are
 chosen for purposes of illustration to be equal to the fundamental
 frequency of the example waveforms, namely 2 MHz.
 The spectrum 51 of FIG. 3B comprises a plurality of sidelobes 58. In
 general, for Fourier transformation of waveforms from the time domain to
 the frequency domain, the relative strength of the sidelobes 58 decreases
 as the frequency difference from the fundamental frequency increases.
 Accordingly, the harmonic content, contained within the sidelobes 58 of a
 waveform, generally decreases with increasing frequency from the
 fundamental. More specifically, each waveform profile in time corresponds
 to a frequency spectrum having a particular distribution of sidelobes 58
 with varying relative strengths. Hence, the harmonic content of a waveform
 is directly related to the shape or profile of the waveform as a function
 of time. As a result, a waveform profile may be chosen to achieve an
 appreciable reduction of harmonic content, or "sidelobe suppression,"
 across the entire frequency spectrum, in one or more frequency bands of
 interest, or at or near a particular frequency of interest.
 Using 0.0 dB as a reference for the spectra of FIGS. 3B, 4B, 5B, and 6B at
 the fundamental frequency of 2 MHz, FIG. 3B shows that the square pulse
 waveform 50 has a spectrum 51 with significant frequency components in the
 band 600 around the fundamental frequency of 2 MHz. However, spectrum 51
 indicates that the harmonic content of square pulse waveform 50 is
 attenuated very near the second harmonic frequency of 4 MHz, and shows an
 absence of appreciable second harmonic content at 4 MHz. However, spectrum
 51 nonetheless shows a significant harmonic content in the band 602 around
 the second harmonic frequency, and in particular, a harmonic content of
 approximately -10 dB near the frequencies 3.3 and 5 MHz.
 FIG. 4A shows a plot of waveform 52 which includes two square pulses, each
 pulse having a pulse width 49 of approximately 0.5 microseconds. FIG. 4B
 shows the corresponding spectrum 53 for the waveform 52. As in FIG. 3B, a
 band of the spectrum 53 which includes the fundamental frequency 2 MHz is
 indicated by reference character 600, and a band of the spectrum 53
 including the second harmonic frequency 4 MHz is indicated by reference
 character 602. As compared to the spectrum 51 of FIG. 3B, the spectrum 53
 of FIG. 4B includes an increased number of sidelobes 58. Furthermore, the
 relative harmonic content of spectrum 53 in band 602 is notably decreased
 compared with that of the band 602 of spectrum 51. In particular, while
 the harmonic content of spectrum 51 in band 602 is as high as -10 dB or
 greater, the band 602 of spectrum 53 in FIG. 4B has a harmonic content of
 only up to approximately -15 dB or less, or a sidelobe suppression of
 approximately 15 dB or greater. Hence, the two-square-pulse waveform 52
 has a low harmonic content around the second harmonic frequency compared
 with that of the single-square-pulse waveform 50.
 FIGS. 5 and 6 show time domain and frequency domain illustrations, similar
 to the plots of FIGS. 3 and 4, for two exemplary ultrasound transmit
 waveforms according to the invention having triangular shapes. FIG. 5A
 shows a waveform 54 including two triangle pulses, each pulse having a
 rise time 59 of approximately 0.5 microseconds. FIG. 5B shows the spectrum
 55 corresponding to the waveform 54. The harmonic content of the spectrum
 55 in the band 602 around the second harmonic frequency has a maximum
 relative strength of between -25 and -30 dB, or a sidelobe suppression of
 between 25 and 30 dB.
 FIG. 6A shows a complex triangular waveform 56 having at least two unique
 maxima 80 and 82, including three triangle pulses, each pulse having a
 rise time 59 of approximately 0.5 microseconds. FIG. 6B shows a spectrum
 57 corresponding to the waveform 56. The harmonic content of spectrum 57
 in the band 602 around the second harmonic frequency has a relative
 strength of less than -30 dB, or a sidelobe suppression of greater than 30
 dB. Hence, of the four example waveforms illustrated in FIGS. 3 through 6,
 waveform 56 shown in FIG. 6A has the most significantly low harmonic
 content, or the greatest sidelobe suppression, particularly around the
 second harmonic frequency.
 It should be appreciated that while a particular time scale for the
 waveforms and corresponding frequency range for the spectra are given in
 the plots of FIGS. 3 to 6, a variety of waveform profiles according to the
 invention may be chosen to achieve sidelobe suppression, or low harmonic
 content in the spectra, over a variety of frequencies and ranges. For
 example, while predominantly triangular waveforms are illustrated in FIGS.
 5A and 6A as low harmonic content waveforms according to the invention, a
 variety of trapezoidal shaped waveforms may also provide a suitably low
 harmonic content for purposes of the invention. The time scale, frequency
 range, and waveform shapes of FIGS. 5-6 are chosen for purposes of
 illustrating the general concepts of sidelobe suppression and reducing
 harmonic content of waveforms according to the invention, and are not
 intended to be limiting.
 Returning now to the apparatus shown in FIG. 2B having a current mode
 transducer driver, the charge circuit 12 and the discharge circuit 16 of
 the controllable switched current source transducer driver 20 may each
 include bipolar junction transistors (BJTS) or field effect transistors
 (FETs) as high impedance current sources to provide the charge current 26
 and conduct the discharge current 28. FIG. 7 shows an example of the
 charge circuit 12 including a BJT 22 as a current source according to one
 embodiment of the invention, providing charge current 26 from the
 collector of the BJT, while FIG. 8 shows an example of the charge circuit
 12 including an FET 24 as a current source according to one embodiment of
 the invention, providing charge current 26 from the drain of the FET.
 The BJT 22 or the FET 24 of FIGS. 7 and 8 may be high voltage devices,
 capable of withstanding voltages of between 25-100 volts. Additionally,
 while FIG. 7 shows only one BJT and FIG. 8 shows only one FET for the
 charge circuit 12, any number of BJTs or FETs, arranged in a variety of
 serial and/or parallel configurations, may be used for either the charge
 circuit 12 or the discharge circuit 16 to provide high impedance current
 terminations which approximate those of an ideal current source having a
 theoretically infinite impedance.
 FIG. 9 is a diagram of an example of the controllable switched current
 source transducer driver 20 of the apparatus of FIG. 2B according to one
 embodiment of the invention. Unlike the linear filter 35 of FIG. 2A which
 employs a voltage mode transducer driver, the switched current source 20
 of FIG. 9 essentially functions to "piece together" an electrical waveform
 across ultrasound transducer element 10, based on two or more different
 charge/discharge rates, or "slopes." In this manner, the switched current
 source 20 of FIG. 9 is particularly useful for generating an electrical
 waveform, and hence an ultrasound transmit waveform 44, similar to the
 triangular waveform 56 shown in FIG. 6A, which has at least two unique
 maxima and at least two different slope magnitudes.
 In the switched current source 20 of FIG. 9, the charge circuit 12 includes
 current sources 200 and 201 having a common terminal 205, serving as the
 output of the current sources 200 and 201. This output is controlled by
 charge control input 14, shown symbolically in FIG. 9 as operating
 (opening and closing) a switch 206. Current source 200 additionally has a
 terminal controlled by a first auxiliary control input 70, shown
 symbolically in FIG. 9 as operating switch 204.
 Similarly, the discharge circuit 16 of FIG. 9 includes current sources 202
 and 203 having a common terminal 209 which is controlled by discharge
 control input 18, shown symbolically in FIG. 9 as operating switch 210.
 Current source 202 additionally has a terminal controlled by a second
 auxiliary control input 72, shown symbolically in FIG. 9 as operating
 switch 208. While FIG. 9 shows the first and second auxiliary control
 inputs 70 and 72 connected together to receive the same control signal,
 auxiliary control inputs 70 and 72 may be separate and may operate
 independently, receiving two or more distinct control signals.
 The charge circuit 12 and discharge circuit 16 of FIG. 9 are electrically
 coupled to provide the drive current 30, and may be operated to output
 drive current 30 to the ultrasound transducer element 10 such that
 ultrasound energy having a wide variety of ultrasound transmit waveforms
 44 is transmitted from the transducer element 10. For example, as
 discussed above in connection with FIG. 2B, the switched current source 20
 of FIG. 9 may be operated by applying digital pulse control signals 40
 such that at least one of the charge circuit 12 and the discharge circuit
 16 outputs at least two different current values for the drive current 30.
 In particular, the digital pulse control signals 40 may be applied to the
 charge control input 14, the discharge control input 18, and the first and
 second auxiliary control inputs 70 and 72 in a predetermined manner, such
 that the drive current 30 produces an ultrasound transmit waveform 44
 having at least two distinct slopes and at least two unique maxima,
 similar to the waveform shown in FIG. 6A.
 FIG. 10 is a timing diagram of the digital pulse control signals applied to
 the switched current source 20 of FIG. 9 according to one embodiment of
 the invention, and the resulting electrical waveform generated across the
 ultrasound transducer element 10 and, hence, essentially the ultrasound
 transmit waveform 44 transmitted by the transducer element. The horizontal
 axis 100 of FIG. 10 indicates time t. The lower three plots of FIG. 10
 illustrate the digital pulse control signals PULL_UP, PULL_DOWN and BOOST.
 As shown in FIGS. 9 and 10, in this embodiment the PULL_UP control signal
 is applied to the charge control input 14, the PULL_DOWN signal is applied
 to the discharge control input 18, and the BOOST signal is applied to the
 first and second auxiliary control inputs 70 and 72.
 With reference to FIGS. 9 and 10, a first pulse 340 of the PULL_UP control
 signal closes switch 206 so that current source 201 charges ultrasound
 transducer element 10 during a "charge phase." As shown in FIG. 10, during
 pulse 340 the PULL_DOWN and BOOST control signals remain in a logic low
 state so that switches 204, 208, and 210 remain open. The ultrasound
 transducer element 10 charges for a time indicated by the pulse width 42
 of pulse 340. The resulting ultrasound transmit waveform 44 has a rise
 time 46 comparable to the pulse width 42, and a slope 45 based on the
 capacitance 32 and the drive current 30, which at this point is provided
 only by current source 201. The waveform 44 reaches a first maximum value
 80 after the rise time 46, at the end of pulse 340.
 After pulse 340, the PULL_UP control signal changes to a logic low state,
 at which time pulse 342 of the PULL_DOWN signal is applied to discharge
 control input 18. As a result, switch 206 is opened and switch 210 is
 closed, so that current source 203 conducts current from the ultrasound
 transducer element 10 during a "discharge phase." In the example of FIG.
 10, pulse 342 has a pulse width 42 similar to that of pulse 340, so that
 the slope 45' of ultrasound transmit waveform 44 during this discharge
 phase is substantially equal in magnitude but opposite in sign to the
 slope 45 during the charge phase of the ultrasound transducer element 10.
 While FIG. 10 shows similar pulse widths 42 for each of the pulses 340,
 342, 344, 348, 350, and 352 of the PULL_UP and PULL_DOWN control signal,
 it should be appreciated that each of the control signal pulses may have a
 unique pulse width, which in turn determines a respective rise or fall
 time of a portion of the ultrasound transmit waveform 44.
 Returning to the graph of FIG. 10, after pulse 342 the PULL_DOWN signal
 changes to a logic low state as pulse 344 of the PULL-UP signal is applied
 to charge control input 14. At this time, pulse 346 of the BOOST signal is
 applied to first and second auxiliary control input 70 and 72. While
 switch 210 is opened, switches 204 and 206 are both closed so that both of
 current sources 200 and 201 provide drive current 30 to the ultrasound
 transducer element 10. While pulse 344 has a pulse width 42 similar to
 that of pulses 340 and 342 in FIG. 10, the drive current 30 at this point
 is nonetheless greater than the drive current provided to, or drawn from,
 the ultrasound transducer element during the pulses 340 and 342,
 respectively, due to the two current sources 200 and 201 providing the
 drive current simultaneously. Accordingly, the magnitude of slope 47 of
 ultrasound transmit waveform 44 is greater than that of slope 45 as a
 result of the increased drive current, and hence, the magnitude of the
 ultrasound transmit waveform 44 reaches a higher maximum value 82 after
 substantially the same rise time 46.
 Continuing with FIG. 10, while pulse 346 of the BOOST signal is still
 applied to first and second auxiliary control inputs 70 and 72, after
 pulse 344 the PULL_UP control signal subsequently changes to a logic low
 state and pulse 348 of the PULL_DOWN signal is applied to discharge
 control input 18. As a result, switch 206 is opened and switches 208 and
 210 are closed, allowing both current sources 202 and 203 to draw current
 from ultrasound transducer element 10. During the assertion of pulse 348,
 the slope 47' of ultrasound transmit waveform 44 is substantially equal in
 magnitude but opposite in sign to the slope 47.
 Following pulse 348, both of the PULL_DOWN and BOOST signals change to a
 logic low state, thereby opening switches 210, 204, and 208, while pulse
 350 of the PULL_UP signal is applied to the charge control input 14, again
 closing switch 206, and allowing current source 201 to charge the
 ultrasound transducer element 10. Since only one current source 201 is
 charging ultrasound transducer element 10 as this point, the slope 45 of
 ultrasound transmit waveform 44 during pulse 350 is less than the slope 47
 and is similar to the slope of waveform 44 during pulse 340. Following
 pulse 350, the PULL_UP signal changes to a logic low state, and pulse 352
 of the PULL_DOWN signal is applied to discharge control input 18, thereby
 closing switch 210 and allowing current source 203 to discharge the
 ultrasound transducer element 10.
 According to the digital pulse control sequence described above in the
 example of FIG. 10, a complex triangular ultrasound transmit waveform 44
 is generated, having two distinct slope magnitudes 45 and 47 and at least
 two unique maxima 80 and 82, similar to the waveform shown in FIG. 6A. It
 should be appreciated that, while FIG. 9 shows two switched current
 sources for each of the charge and discharge circuits of controllable
 current source 20, any number of current sources and switches, controlled
 by any number of control inputs, may be employed in the charge circuit 12
 and the discharge circuit 16, in a number of configurations, to "piece
 together" a wide variety of low harmonic content waveforms having several
 distinct slope magnitudes and unique maxima. Additionally, as discussed
 above, while FIG. 10 shows similar pulse widths for the control signals, a
 variety of pulse widths and sequences may be used for the control signals,
 in conjunction with the apparatus of FIG. 9 or similar apparatus employing
 a different number of current sources and/or differently configured
 control inputs, to generate a variety of low harmonic content waveforms.
 FIGS. 11 and 12 each show a detailed circuit diagram of an example of the
 apparatus of FIG. 9 employing current mode transducer drivers according to
 one embodiment of the invention. The example of FIG. 11 uses BJTs and
 outputs the drive current 30 via collectors of BJTs, while the example of
 FIG. 12 uses FETs and outputs the drive current 30 via drains of FETs. As
 discussed above in connection with FIGS. 7 and 8, the collectors of BJTs
 and the drains of FETs provide high impedance current terminations which
 approximate those of an ideal current source having a theoretically
 infinite impedance.
 In the circuit of FIG. 11, the BOOST control signal and the PULL_UP control
 signal, as shown in FIG. 10, are applied to logic AND gate 604 to provide
 a gated BOOST signal 75. Similarly, the BOOST control signal and the
 PULL_DOWN control signal, as shown in FIG. 10, are applied to logic AND
 gate 606 to provide gated BOOST signal 77. The charge circuit 12 of FIG.
 11 includes two level shifters 78 and 79, two BJT switches 404 and 406, a
 first current source 400 comprising resistor R1 and BJT 414, and a second
 current source 401 comprising resistor R2 and BJT 414. The BJT switches
 404 and 406 function similarly to the switches 204 and 206 of FIG. 9,
 while the current sources 400 and 401 function similarly to the current
 sources 200 and 201 of FIG. 9.
 In the circuit of FIG. 11, level shifter 78 is coupled between charge
 control input 14 and BJT switch 406, and serves to shift the TTL logic
 level of the PULL_UP control signal to provide a switching current to the
 base of BJT switch 406 that is referenced to the supply voltage V.sub.H
 applied to the emitters of BJT switches 404 and 406. Similarly, level
 shifter 79 is coupled to receive the TTL logic level of the gated BOOST
 signal 75 and to provide a switching current to the base of BJT switch 404
 that is referenced to the supply voltage V.sub.H.
 When the PULL_UP control signal applied to charge control input 14 is such
 that BJT switch 406 conducts current, the current source 401 provides
 charge current 26 from the collector of BJT 414. The charge current 26
 provided by current source 401 is determined by the resistance value of
 resistor R2, the supply voltage V.sub.H, and a bias voltage BIAS1 applied
 to the base 74 of BJT 414. It should be appreciated that a variety of
 resistor values may be selected for resistor R2 and the supply voltage
 V.sub.H and the BIAS1 signal may be varied to provide a variety of charge
 currents 26 depending on a particular application.
 In a manner similar to that of current source 401, current source 400,
 which includes resistor R1 and BJT 414, may be controlled to provide
 current, in conjunction with current source 401, to output charge current
 26. When both the PULL_UP control signal and the BOOST control signal are
 at a logic high level, the gated BOOST signal 75, via level shifter 79,
 causes BJT switch 404 to conduct current, hence activating current source
 400. The charge current 26 is then the combined current contributions from
 current sources 400 and 401. As with current source 401, the current
 provided by current source 400 is determined by the resistance value of
 resistor R1, the supply voltage VH, and the BIAS1 signal applied to the
 base 74 of BJT 414. For many applications, the values of resistors R1 and
 R2 may be chosen to be equal.
 The discharge circuit 16 of FIG. 11 functions similarly to charge circuit
 12, as described above. In FIG. 11, discharge circuit 16 includes two BJT
 switches 408 and 410, which function similarly to switches 208 and 210,
 respectively, of FIG. 9. The base of BJT switch 408 is coupled to receive
 the gated BOOST signal 77, and the base of BJT switch 410 serves as the
 discharge control input 18 which receives the PULL_DOWN control signal.
 Current source 402 comprises resistor R3 and BJT 412, while current source
 403 comprises resistor R4 and BJT 412. Current sources 402 and 403
 function similarly to current sources 202 and 203, respectively, of FIG.
 9. A BIAS2 signal is applied to the base 76 of BJT 412, and together with
 the supply voltage V.sub.H and the resistance values of resistors R3 and
 R4, determines the current drawn by each of the current sources 402 and
 403, respectively. In the circuit of FIG. 11, the collector of BJT 414 and
 the collector of BJT 412 of the charge and discharge circuits,
 respectively, are coupled to provide the drive current 30. The switched
 current source 20 of FIG. 11 may be fabricated as a monolithic integrated
 circuit.
 The circuit of FIG. 12 functions similarly to that of FIG. 11, but employs
 FETs rather than BJTs. In the charge circuit 12 of FIG. 12, FET current
 mirror 500 and FET 513 essentially function as current source 201 of FIG.
 9, while current mirror 500 and FET 512 essentially function as current
 source 200 of FIG. 9. Reference current source 514, which is applied to
 the gates of FETs 512 and 513 in the circuit of FIG. 12, determines the
 magnitude of charge current 26. FET 506 corresponds to switch 206 of FIG.
 9, while FET 504 essentially functions as switch 204 of FIG. 9, receiving
 the PULL_UP control signal and the BOOST control signal on their
 respective gates. The output 515 of current mirror 500 provides the charge
 current 26.
 In the discharge circuit 16 of FIG. 12, FET 510 functions similarly to
 switch 210 of FIG. 9, wherein the gate of FET 510 serves as the discharge
 control input 18 which receives the PULL_DOWN control signal. FET 508
 functions similarly to switch 208 of FIG. 9, wherein the gate of FET 508
 serves as the second auxiliary control input 72 which receives the BOOST
 control signal. FET 519 functions similarly to current source 203 of FIG.
 9, while FET and 518 functions similarly to current source 202 of FIG. 9.
 FETs 518 and 519 are biased at their gates by reference current source 514
 which, similarly to the charge current 26, determines the magnitude of the
 discharge current 28. As a result, the magnitude of the reference current
 514 determines an appropriate range of values for the charge current 26
 and the discharge current 28, and hence, the drive current 30. As in the
 circuit of FIG. 11, the switched current source 20 of FIG. 12 may be
 fabricated as a monolithic integrated circuit.
 While the foregoing discussion has been directed primarily to an ultrasound
 transmit waveform generated by a single ultrasound transducer element, the
 method and apparatus of the invention may be effectively implemented to
 provide an ultrasound transmit waveform having low harmonic content from a
 number of ultrasound transducer elements. FIG. 13 is a block diagram of an
 apparatus for controlling a number of ultrasound transducer elements to
 transmit ultrasound energy having low harmonic content, according to one
 embodiment of the invention.
 The apparatus of FIG. 13 includes a plurality of ultrasound transducer
 elements 10, each transducer element having a capacitance 32. While in
 practice the respective capacitances 32 of the transducer elements 10 may
 not be precisely equal, substantially similar capacitances are assumed,
 which is suitable for purposes of the invention. Additionally, as
 discussed above in connection with FIG. 2A, the capacitance 32 is assumed,
 for purposes of discussion, to include any interconnection capacitance due
 to the signal conductors 11. Programmable delay current sources 90 charge
 and discharge the ultrasound transducer elements 10 via the signal
 conductors 11 in a selectable manner. Each programmable delay current
 source 90 charges and discharges a respective ultrasound transducer
 element 10 so as to transmit ultrasound energy having an ultrasound
 transmit waveform, shown in FIG. 13 as waveforms 440, 442, 444 and 446 for
 respective transducer elements 10, wherein the ultrasound transmit
 waveform has a low harmonic content based on the capacitance 32.
 The apparatus of FIG. 13 may further include a transducer controller 60 to
 output common control signals 300 to the plurality of programmable delay
 current sources 90. The ultrasound transmit waveforms 440, 442, 444, and
 446 transmitted by each transducer element 10 are based on selectably
 delayed versions of the common control signals 300. The common control
 signals 300 may be, for example, digital pulse control signals similar to
 those described above in connection with FIGS. 9 and 10. One example of a
 transducer controller 60 suitable for purposes of the invention includes,
 but is not limited to, a synchronous state machine which receives a clock
 signal 64 and a trigger signal 62, and generates one or more digital
 pulses in a predetermined timing sequence as the common control signals
 300.
 FIG. 14 is a block diagram of an example of one programmable delay current
 source 90 of the apparatus of FIG. 13. Each programmable delay current
 source 90 may comprise a switched current source 20 that includes a charge
 circuit 12 to charge a respective transducer element 10, and a discharge
 circuit 16 to discharge the respective transducer element. Each current
 source 90 may additionally include at least one programmable delay circuit
 92 to receive the common control signals 300 and delay instruction signals
 302 and to output selectively delayed versions 310 and 312 of the common
 control signals 300 to the charge circuit 12 and the discharge circuit 16,
 based on the delay instruction signals 302.
 With reference again to FIG. 13, each programmable delay current source 90
 may receive unique delay instructions signals 302, 304, 306, and 308, so
 that the respective ultrasound transmit waveforns 440, 442, 444 and 446
 output by the ultrasound transducer elements 10 have similar waveform
 profiles but are delayed with respect to one another in time. By
 selectively delaying the common control signals 300, the apparatus of FIG.
 13 allows a waveform of ultrasound energy transmitted by a plurality of
 transducer elements to be steered and/or focused to a particular region of
 interest.
 In FIG. 14, the switched current source 20, including charge circuit 12 and
 discharge circuit 16, may be similar to any one of the circuits discussed
 in connection with FIGS. 2, 9, 11, or 12. While FIG. 14 shows two
 selectively delayed versions 310 and 312 of the common control signals 300
 coupled to the charge circuit 12 and the discharge circuit 16,
 respectively, any number of selectively delayed control signals may be
 provided by the programmable delay circuit 92 and applied to either one or
 both of the charge circuit 12 and the discharge circuit 16 to operate the
 controllable current source 20. Additionally, with reference again to FIG.
 13, the programmable delay current sources 90 and the transducer
 controller 60 may be fabricated as a monolithic integrated circuit.
 FIG. 15 is a more detailed block diagram of a portion of the circuit of
 FIG. 13, constructed and arranged to provide ultrasound transmit waveforms
 from a plurality of transducer elements in a manner similar to that
 described in connection with FIGS. 9 and 10 for a single ultrasound
 transducer element. More specifically, the apparatus of FIG. 15 may be
 utilized to provide a complex triangular ultrasound transmit waveform
 having at least two distinct slope magnitudes and at least two unique
 maxima from a plurality of transducer elements.
 In FIG. 15, common control signals 300 include signals 320, 322, and 324,
 output from transducer controller 60. Each signal 320, 322, and 324 is
 applied to a respective programmable delay 94 in the programmable delay
 circuit 92 of current source 90 (FIG. 13) for each ultrasound transducer
 element. Each programmable delay circuit 92 receives delay instruction
 signals as described above in connection with FIGS. 13 and 14, shown in
 FIG. 15 as signals 302 and 304, and outputs selectively delayed versions
 of the common control signals 320, 322, and 324 based on the delay
 instruction signals. While FIG. 15 shows only two programmable delay
 circuits 92 receiving delay instruction signals 302 and 304, respectively,
 any number of programmable delay circuits 92 may be utilized, as
 determined by the number of ultrasound transducer elements employed in the
 ultrasound imaging system. Furthermore, the delay instruction signals for
 each programmable delay circuit 92 may or may not be unique, and depends
 on the manner in which ultrasound energy is to be directed by the
 transducer elements.
 In FIG. 15, the selectively delayed versions of the common control signals
 300 output by the programmable delay circuits 92 correspond to the BOOST,
 PULL_UP and PULL_DOWN signals shown in FIGS. 9 and 10. In the apparatus of
 FIG. 15, the outputs of programmable delay circuits 92 are labeled, for
 purposes of illustration, with reference characters 14, 18, 70, and 72,
 and 14', 18', 70', and 72', respectively, to correspond to the charge
 control input, the discharge control input, and the first and second
 auxiliary control inputs of the circuit of FIG. 9 for two of the
 ultrasound transducer elements shown in FIG. 13. In practice, the
 apparatus of FIG. 15 may generate a selectively delayed version of the
 BOOST, PULL_UP and PULL_DOWN control signals for each ultrasound
 transducer element of an imaging system so that a waveform of ultrasound
 energy having low harmonic content may be directed by the ultrasound
 transducer elements into a region of interest.
 Having thus described at least one illustrative embodiment of the
 invention, various alterations, modifications and improvements will
 readily occur to those skilled in the art. Such alterations, modifications
 and improvements are intended to be within the spirit and scope of the
 invention. Accordingly, the foregoing description is by way of example
 only and is not intended as limiting.