WO2015028949A2 - A cmut-based ultrasound imaging system for wide frequency range imaging - Google Patents

A cmut-based ultrasound imaging system for wide frequency range imaging Download PDF

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Publication number
WO2015028949A2
WO2015028949A2 PCT/IB2014/064085 IB2014064085W WO2015028949A2 WO 2015028949 A2 WO2015028949 A2 WO 2015028949A2 IB 2014064085 W IB2014064085 W IB 2014064085W WO 2015028949 A2 WO2015028949 A2 WO 2015028949A2
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Prior art keywords
cmut cells
cmut
bias voltage
imaging
dc bias
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PCT/IB2014/064085
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French (fr)
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WO2015028949A3 (en
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Richard Edward DAVIDSEN
Junho Song
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Koninklijke Philips N.V.
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Priority to US61/870,273 priority
Priority to US201361887475P priority
Priority to US61/887,475 priority
Application filed by Koninklijke Philips N.V. filed Critical Koninklijke Philips N.V.
Publication of WO2015028949A2 publication Critical patent/WO2015028949A2/en
Publication of WO2015028949A3 publication Critical patent/WO2015028949A3/en

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B06GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06BMETHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/0292Electrostatic transducers, e.g. electret-type
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B06GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06BMETHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/0207Driving circuits
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B06GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06BMETHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B2201/00Indexing scheme associated with B06B1/0207 for details covered by B06B1/0207 but not provided for in any of its subgroups
    • B06B2201/70Specific application
    • B06B2201/76Medical, dental
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S15/00Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
    • G01S15/88Sonar systems specially adapted for specific applications
    • G01S15/89Sonar systems specially adapted for specific applications for mapping or imaging
    • G01S15/8906Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
    • G01S15/8909Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using a static transducer configuration
    • G01S15/8915Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using a static transducer configuration using a transducer array
    • G01S15/8925Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using a static transducer configuration using a transducer array the array being a two-dimensional transducer configuration, i.e. matrix or orthogonal linear arrays
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/52Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
    • G01S7/52017Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
    • G01S7/52046Techniques for image enhancement involving transmitter or receiver
    • G01S7/52047Techniques for image enhancement involving transmitter or receiver for elimination of side lobes or of grating lobes; for increasing resolving power
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/52Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
    • G01S7/52017Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
    • G01S7/52079Constructional features
    • G01S7/5208Constructional features with integration of processing functions inside probe or scanhead

Abstract

An ultrasonic diagnostic imaging system includes a CMUT transducer probe with an array of CMUT cells that can be configured to operate in different frequency ranges and with different acoustic element pitches for different clinical applications. The frequency response of the CMUT cells can be tailored for different clinical applications by increasing the DC bias voltage for higher frequency clinical applications, or decreasing the DC bias voltage for lower frequency applications. Individual acoustic elements including groups of CMUT cells can be configured to have a desired pitch that is optimal for the frequency being used for the imaging application.

Description

A Cmut-Based Ultrasound Imaging System For Wide Frequency Range Imaging

This invention relates to medical diagnostic ultrasound imaging systems and, in particular, to ultrasound systems including transducer probes with capacitive micromachined ultrasonic transducers (CMUTS) that can be selectively tailored for ultrasound imaging at different frequency ranges and for several different clinical applications.

When ultrasonic waves pass through tissue on both transmit and receive, the waves are affected by what is known as depth-dependent attenuation.

Ultrasound is progressively attenuated the further it travels through the body and the signal to noise ratio of echoes from extended depths in the body deteriorates. This attenuation is also frequency dependent, with higher frequencies being more greatly attenuated than lower frequencies. Accordingly, the frequency used for imaging can depend on the particular application being performed. The frequency will also be modulated during the collection of ultrasound images. Generally, higher frequency ultrasound is used for shallow, more superficial imaging while lower frequencies are used when imaging at greater depths.

The ultrasonic transducers used for medical imaging have numerous characteristics which lead to the production of high quality diagnostic images, including high sensitivity to low level acoustic signals at ultrasonic frequencies. Conventionally, the piezoelectric materials which possess these characteristics have been made of PZT and PVDF materials, with PZT being the most preferred.

Unfortunately, conventional piezoceramic transducers exhibit limited bandwidths for imaging over wide frequency ranges. Accordingly, ultrasound systems will often be accompanied with a suite of transducers that can be used to cover the frequency ranges needed for performing different clinical applications. The need to have multiple transducers for various applications is a significant cost burden and causes additional inconvenience each time a sonographer has to change transducers and reoptimize system settings for imaging with a different transducer.

While transducer technology continues to advance, there is still a need for improved imaging systems can operate a transducer probe over different frequency ranges for different clinical applications selected by a user.

In accordance with the principles of the present invention, an ultrasonic diagnostic imaging system can be adapted to operate a single CMUT transducer probe over different ranges of frequencies for different clinical applications of ultrasound imaging. The ultrasonic diagnostic imaging system includes an ultrasound imaging control system comprising a user control for user selection of an imaging procedure. Upon selection of an imaging procedure for a particular clinical application, the imaging system is adapted to selectively vary a DC bias voltage to control a frequency of the groups of CMUT cells in the transducer probe and vary the configuration of the groups of CMUT cells in the probe to tailor the pitch of the transducer elements according to the selected imaging procedure.

In the drawings:

FIGURE 1 illustrates in block diagram form an ultrasonic diagnostic imaging system arranged to be operated in accordance with the principles of the present invention.

FIGURE 2 illustrates a CMUT cell controlled by a DC bias voltage and driven by an r.f. drive signal.

FIGURES 3a-3d illustrate the principles of collapsed mode CMUT operation applied in an implementation of the present invention.

FIGURE 4 illustrates the frequency response of a collapsed mode CMUT transducer with a fixed DC bias voltage.

FIGURE 5 illustrates the frequency response of a collapsed mode CMUT transducer with a DC bias voltage varied in accordance with the present invention.

FIGURE 6 illustrates an array of CMUT cells that is configurable to vary the dimensions of individual acoustic elements for imaging in different clinical applications.

FIGURE 7 illustrates an example of varying the number of rows which define an acoustic element in a CMUT array to tailor the pitch of the elements in a transducer probe.

FIGURE 8 illustrates a block diagram of an example embodiment of partial beamforming of signals from individual acoustic elements in the CMUT array.

FIGURE 9 illustrates a workflow in accordance with the present invention for varying DC bias voltage and configurations of groups of CMUT cells according to a selected imaging procedure.

Referring first to FIGURE 1, an ultrasonic diagnostic imaging system with a frequency-controlled CMUT probe is shown in block diagram form. In FIGURE 1, a CMUT transducer array 10' is provided in an ultrasound probe 10 for transmitting ultrasonic waves and receiving echo information. The transducer array 10' is a two-dimensional array of CMUT cells that can be grouped into acoustic elements capable of scanning in a 2D plane or in three dimensions for 3D imaging. The transducer array is coupled to a multiplexer 11 that controls which CMUT cells in the array are activated for transmit and/or receive according to the system design, which selectively activates groups of CMUT cells as individual acoustic elements. The configuration and number of CMUT cells in each acoustic element can be tailored for a selected imaging procedure for a desired clinical application. For partial beamforming of signals associated with acoustic elements, the multiplexer (MUX) 11 is further coupled to a microbeamformer 12 in the probe, which controls transmission and reception of signals by the CMUT cells which make up the acoustic elements. Microbeamformers are capable of at least partial beamforming of the signals received from individual acoustic elements of groups of CMUT cells.

Microbeamforming is described in US Pats. 5,997,479 (Savord et al.),

6,013,032 (Savord), and 6,623,432 (Powers et al.) The microbeamformer is coupled by the probe cable to a transmit/receive (T/R) switch 16 which switches between transmission and reception and protects the main beamformer 20 from high energy transmit signals.

The transmission of ultrasonic beams from the transducer array 10' under control of the microbeamformer 12 is directed by a transducer controller 18 coupled to the T/R switch and the main system beamformer 20. The transducer controller 18 receives input from the user's operation of the user interface or control panel 38. Transducer controller 18 and microbeamformer 12 are coupled to the CMUT transducer array 10' via a DC bias control 45. The DC bias control 45 sets DC bias voltage(s) that are applied to the CMUT cells. In combination with the user's operation of the control panel 38, the system is adapted to use the multiplexer 11 to set the configuration of groups of CMUT cells that are selectively operated together as individual acoustic elements. The system is also adapted to use the DC bias control 45 to set the voltage bias for the CMUT cells in order to have a desired frequency response.

The setting of the voltage bias and the configuration of the CMUT cells for each acoustic element are set for a selected clinical application. Some example clinical applications include thyroid imaging, breast imaging, carotid imaging, deep abdominal imaging, vascular imaging, and obstetrical imaging. The location (e.g., deep or shallow) and tissue characteristics of the anatomy for each clinical application varies and involves different frequencies and transducer pitches for optimal image formation. In certain embodiments, the DC bias voltage and/or configuration of the acoustic elements can be manually set by a user. For example, the user may select a desired frequency for imaging, which will cause the system to configure the voltage applied to the CMUTs and the pitch of the probe accordingly. In other embodiments, the frequency and/or configuration of the acoustic elements can be preset in the system for different clinical applications. The display 40 includes icons that are associated with different clinical applications. For example, touchscreen icons (or icons activated with a mouse) can be present on the display 40 and selected individually by a user. After selection, the system configures the array transducer for that particular application. As part of the imaging procedure, the transmit and receive frequencies as well as the pitch of the array transducer are quickly configured by the system through control of the multiplexer 11 and the DC bias control 45. If, for example, deep abdominal imaging is selected, the DC bias voltage will be set to a level that causes the CMUT cells to transmit and receive at lower frequencies for better penetration. The pitch of the transducer will also be tailored by changing the number and/or configuration of CMUT cells that are operated together as individual acoustic elements, such that the element pitch is optimized for reduced grating lobes during imaging.

The partially beamformed signals produced by the microbeamformer 12 on receive are coupled to a main beamformer 20 where partially beamformed signals from individual acoustic elements are combined into a fully beamformed signal. For example, the main beamformer 20 may have 128 channels, each of which receives partially beamformed signals from individual acoustic elements that each include tens to hundreds of CMUT transducer cells. In this way, the signals received by the individual cells of a CMUT transducer array can contribute efficiently to a single beamformed signal.

The beamformed signals are coupled to a signal processor 22. The signal processor 22 processes the received echo signals in various ways, such as bandpass filtering, decimation, I and Q component separation, and harmonic signal separation which acts to separate linear and nonlinear signals so as to enable the identification of nonlinear (higher harmonics of the fundamental frequency) echo signals returned from tissue and microbubbles. The signal processor may also perform additional signal enhancement such as speckle reduction, signal compounding, and noise elimination. The bandpass filter in the signal processor can be a tracking filter, with its passband sliding from a higher frequency band to a lower frequency band as echo signals are received from increasing depths, thereby rejecting the noise at higher frequencies from greater depths where these frequencies are devoid of anatomical signal information.

The signals can also be processed for a wide variety of imaging techniques, such as B-mode imaging, M-mode imaging, Doppler imaging, and others. Here, the processed signals are coupled to a B mode processor 26 and a Doppler processor 28. The B mode processor 26 employs detection of an amplitude of the received ultrasound signal for the imaging of structures in the body such as the tissue of organs and vessels in the body. B mode images of structure of the body may be formed in either the harmonic image mode or the fundamental image mode or a combination of both as described in US Pat. 6,283,919 (Roundhill et al.) and US Pat. 6,458,083 (Jago et al.)

The Doppler processor 28 processes temporally distinct signals from tissue movement and blood flow for the detection of the motion of substances such as the flow of blood cells in the image field. The Doppler processor typically includes a wall filter with parameters which may be set to pass or reject echoes returned from selected types of materials in the body. For instance, the wall filter can be set to have a passband characteristic which passes signal of relatively low amplitude from higher velocity materials while rejecting relatively strong signals from lower or zero velocity material. This passband characteristic will pass signals from flowing blood while rejecting signals from nearby stationary or slowing moving objects such as the wall of the heart. An inverse characteristic would pass signals from moving tissue of the heart while rejecting blood flow signals for what is referred to as tissue Doppler imaging, detecting and depicting the motion of tissue. The Doppler processor receives and processes a sequence of temporally discrete echo signals from different points in an image field, the sequence of echoes from a particular point referred to as an ensemble. An ensemble of echoes received in rapid succession over a relatively short interval can be used to estimate the Doppler shift frequency of flowing blood, with the correspondence of the Doppler frequency to velocity indicating the blood flow velocity. An ensemble of echoes received over a longer period of time is used to estimate the velocity of slower flowing blood or slowly moving tissue.

The structural and motion signals produced by the B mode and Doppler processors are coupled to a scan converter 32 and a multiplanar reformatter 44. The scan converter arranges the echo signals in the spatial relationship from which they were received in a desired image format. For instance, the scan converter may arrange the echo signal into a two dimensional (2D) sector- shaped format, or a pyramidal three dimensional (3D) image. The scan converter can overlay a B mode structural image with colors corresponding to motion at points in the image field corresponding with their Doppler-estimated velocities to produce a color Doppler image which depicts the motion of tissue and blood flow in the image field. The multiplanar reformatter will convert echoes which are received from points in a common plane in a volumetric region of the body into an ultrasonic image of that plane, as described in US Pat. 6,443,896 (Detmer). A volume renderer 42 converts the echo signals of a 3D data set into a projected 3D image as viewed from a given reference point as described in US Pat. 6,530,885 (Entrekin et al.) The 2D or 3D images are coupled from the scan converter 32, multiplanar reformatter 44, and volume renderer 42 to an image processor 30 for further enhancement, buffering and temporary storage for display on an image display 40. In addition to being used for imaging, the blood flow velocity values produced by the Doppler processor 28 are coupled to a flow quantification processor 34. The flow quantification processor produces measures of different flow conditions such as the volume rate of blood flow. The flow quantification processor may receive input from the user control panel 38, such as the point in the anatomy of an image where a measurement is to be made. Output data from the flow quantification processor is coupled to a graphics processor 36 for the reproduction of measurement values with the image on the display 40.

The graphics processor 36 can generate graphic overlays for display with the ultrasound images. These graphic overlays can contain standard identifying information such as patient name, date and time of the image, imaging parameters, and the like. For these purposes the graphics processor receives input from the user interface 38, such as a typed patient name. As described above, the user interface is also coupled to the transmit controller 18 to control the generation of ultrasound signals from the transducer array 10' and hence the images produced by the transducer array and the ultrasound system. The user interface is also coupled to the multiplanar reformatter 44 for selection and control of a display of multiple multiplanar reformatted (MPR) images which may be used to perform quantified measures in the image field of the MPR images.

In an implementation of the present invention the transducer array 10' includes CMUT cells. FIGURE 2 shows a CMUT cell with a membrane or diaphragm 114 suspended above a silicon substrate 112 with a gap 118 therebetween. A top electrode 120 is located on the diaphragm 114 and moves with the diaphragm and a bottom electrode is located on the floor of the cell on the upper surface of the substrate 112 in this example. Other realizations of the electrode 120 design can be utilized, such as embedding electrode 120 in the membrane 114 or depositing the electrode on the membrane 114 as an additional layer. In this example, the bottom electrode 122 is circularly configured and embedded in the substrate layer 112. In addition, the membrane layer 114 is fixed relative to the top face of the substrate layer 112 and configured and dimensioned so as to define a spherical or cylindrical cavity 118 between the membrane layer 114 and the substrate layer 112. The cell and its cavity 118 may define alternative geometries. For example, cavity 118 could define a rectangular or square cross-section, a hexagonal cross-section, an elliptical cross-section, or an irregular cross-section.

In some embodiments, the CMUT cells in the array may have different diameters. The diameter of the CMUT cell is the biggest lateral dimension of the cell. The different diameters allow transmission of ultrasound waves at variable fundamental frequencies. For example, CMUT cells of bigger diameter have lower fundamental frequency compared to the cells of the smaller diameter. Accordingly, the diameter of the individual CMUT cells can be fabricated to inherently exhibit a desired center frequency that is independent of a bias voltage applied to the CMUT during operation.

The CMUT cells can be fabricated to include a variety of layered configurations. For example, the bottom electrode 122 is typically insulated on its cavity-facing surface with an additional layer (not pictured). A preferred insulating layer is an oxide-nitride-oxide (ONO) dielectric layer formed above the substrate electrode 122 and below the membrane electrode 120. The ONO- dielectric layer advantageously reduces charge accumulation on the electrodes which leads to device instability and drift and reduction in acoustic output pressure. The fabrication of ONO-dielectric layers on a CMUT is discussed in detail in European patent application no. 08305553.3 by Klootwijk et al., filed September 16, 2008 and entitled "Capacitive micromachined ultrasound transducer." Use of the ONO-dielectric layer is desirable with pre-collapsed CMUTs, which are more susceptible to charge retention than CMUTs operated with suspended membranes. The disclosed components may also be fabricated from CMOS compatible materials, e.g., Al, Ti, nitrides (e.g., silicon nitride), oxides (various grades), tetra ethyl oxysilane (TEOS), poly-silicon and the like. In a CMOS fabrication, for example, the oxide and nitride layers may be formed by chemical vapor deposition and the metallization (electrode) layer put down by a sputtering process. Suitable CMOS processes are LPCVD and PECVD, the latter having a relatively low operating temperature of less than 400 °C. Exemplary techniques for producing the disclosed cavity 118 involve defining the cavity in an initial portion of the membrane layer 114 before adding a top face of the membrane layer 114. Other fabrication details may be found in US Pat. 6,328,697 (Fraser).

In the example embodiment depicted in FIGURE 2, the diameter of the cylindrical cavity 118 is larger than the diameter of the circularly configured electrode plate 122. Electrode 120 may have the same outer diameter as the circularly configured electrode plate 122, although such conformance is not required. Thus, in an exemplary implementation of the present invention, the membrane electrode 120 is fixed relative to the top face of the membrane layer 114 so as to align with the electrode plate 122 below. The electrodes of the CMUT provide the capacitive plates of the device and the gap 118 is the dielectric between the plates of the capacitor. When the diaphragm vibrates, the changing dimension of the dielectric gap between the plates provides a changing capacitance which is sensed as the response of the CMUT to a received acoustic echo. The spacing between the electrodes is controlled by applying a DC bias voltage to the electrodes with a DC bias circuit 104. For transmission the electrodes 120, 122 are driven by an r.f. signal generator 102 whose a.c. signal causes the diaphragm to vibrate and transmit an acoustic signal. The DC bias voltage can be analogized to a carrier wave with the r.f. signal modulating the carrier in the transmission of the acoustic signal.

In accordance with the principles of the present invention, a CMUT cell of the array 10' can be operated in a conventional mode or a collapsed mode. During the conventional mode of operation the DC bias voltage applied to the electrodes 120 and 122 is kept below a threshold value that causes the membrane to touch the cell floor. This threshold value depends on the exact design of the CMUT cell. When the bias is set below the threshold value, the membrane vibrates freely above the cell floor during operation of the CMUT cell. The conventional mode of operation can be used to generate lower frequencies and intensities of ultrasound waves, in comparison with the collapsed mode.

During the collapsed mode, the DC bias voltage is operated at a value above the threshold value, thereby causing the membrane to touch the cell floor. In one implementation, a CMUT cell is set by DC bias voltage to a pre- collapsed state in which the membrane 114 is in contact with the floor of the cavity 118 as shown in FIGURE 3a. This is accomplished by applying a DC bias voltage to the two electrodes as indicated in FIGURE 2. In the illustrated collapsed mode implementation the membrane electrode 120 is formed as a ring electrode 130. Other implementations may use a continuous disk electrode which advantageously provides the pull-down force for collapse at the center of the membrane as well as peripherally. When the membrane 114 is biased to its collapsed state as shown in FIGURES 3a and 3b, the center area of the membrane is in contact with the floor of the cavity 118. As such, the center of the membrane 114 does not move during operation of the CMUT. Rather, it is the peripheral area of the membrane 114 which moves, that which is above the remaining open void of the cavity 118 and below the ring electrode. By forming the membrane electrode 130 as a ring, the charge of the upper plate of the capacitance of the device is located above the area of the CMUT which exhibits the motion and capacitive variation when the CMUT is operating as a transducer. Thus, the coupling coefficient of the CMUT transducer is improved.

As indicated above, the membrane 114 may be brought to its collapsed state in contact with the center of the floor of the cavity 118 by applying a DC bias voltage above the threshold value, which is a function of the cell diameter, the gap between the membrane and the cavity floor, and the membrane materials and thickness. As the voltage is increased, the capacitance of the CMUT cell is monitored with a capacitance meter. A sudden change in the capacitance indicates that the membrane has collapsed to the floor of the cavity. The membrane can be biased downward until it just touches the floor of the cavity as indicated in FIGURE 3 a, or can be biased further downward as shown in FIGURE 3b to increase collapse beyond that of minimal contact, such that the area of the membrane that is collapsed to the cell floor increases.

In accordance with the principles of the present invention, the frequency response of a collapsed mode CMUT is varied by adjusting the DC bias voltage applied to the CMUT electrodes after collapse. As a result, the resonant frequency of the CMUT cell increases as higher DC bias is applied to the electrodes. The principles behind this phenomenon are illustrated in FIGURES 3a-3d. The cross-sectional views of FIGURES 3a and 3c illustrate this one dimensionally by the distances Di and D2 between the outer support of the membrane 114 and the point where the membrane begins to touch the floor of the cavity 118 in each illustration. It can be seen that the distance Di is a relatively long distance in FIGURE 3 a when a relatively low bias voltage is applied after collapse, and the distance D2 in FIGURE 3c is a much shorter distance when a higher bias voltage is applied. These distances can be analogized to long and short strings which are held by the ends and then plucked. The long, relaxed string will vibrate at a much lower frequency when plucked than will the shorter, tighter string. Analogously, the resonant frequency of the CMUT cell in FIGURE 3 a will be lower than the resonant frequency of the CMUT cell in FIGURE 3c which is subject to the higher DC pulldown bias voltage.

The phenomenon can also be appreciated from the two dimensional illustrations of FIGURES 3b and 3d, as it is in actuality a function of the effective operating area of the CMUT membrane. When the membrane 114 just touches the floor of the CMUT cell as shown in FIGURE 3a, the effective vibrating area Ai of the noncontacting (free vibrating) portion of the cell membrane 114 is large as shown in FIGURE 3b. The small hole in the center 17 represents the center contact region of the membrane. The large area membrane will vibrate at a relatively low frequency. This area 17 is an area of the membrane 114, which is collapsed to the floor of the CMUT cell. But when the membrane is pulled into deeper collapse by a higher bias voltage as in FIGURE 3c, the greater central contact area 17' results in a lesser free vibrating area A2 as shown in FIGURE 3d. This lesser area A2 will vibrate at a higher frequency than the larger Ai area. Thus, as the DC bias voltage is decreased the frequency response of the collapsed CMUT cell decreases, and when the DC bias voltage increases the frequency response of the collapsed CMUT cell increases.

The variation of the DC bias voltage of a collapsed CMUT can be optimized for the transducer for a particular desired frequency of operation that corresponds to a selected imaging application. For example, for deeper imaging applications, such as abdominal imaging, the bias voltage can be set to operate the CMUT cells at a lower frequency for transmit and receive. For shallower imaging applications, such as carotid imaging, the bias voltage can be set to operate the CMUT cells at a higher frequency for transmit and receive.

FIGURE 4 illustrates a frequency response curve 54 for a CMUT transducer with a fixed DC bias being operated in the collapsed mode, which has a nominal center frequency of around 6 MHz. When the transducer probe is operated with signals at 6 MHz it is seen that the response curve of signals around 6 MHz exhibits good sensitivity, as it is operating in the center of the passband. But when the probe is operated with signals at a low band such as 4 MHz, it is seen that a band 52 of signals in this range rolls off because the band 52 is at the lower end of the response curve 54 and is down around 4 dB below peak.

Similarly, when operated around 8 MHz as shown by band 56, the high frequency rolloff of the transducer passband 54 attenuates signals down by 6 dB below peak. But when the DC bias voltage is varied to optimize the transducer for the desired frequency band of operation, this passband skirt attenuation is avoided. As FIGURE 5 illustrates, when a DC bias of 70 volts is used for low band operation, 90 volts is used for mid-band operation, and 120 volts is used for high band operation in this example, the desired passbands 52', 54' and 56' are in the center of the shifted resonant transducer passband in each case, resulting in little or no side skirt frequency rolloff.

Referring to FIGURE 6, the imaging system described herein can also vary configurations of groups of CMUT cells in the array transducer. Many of the individual CMUT cells can be connected together and operated in unison as an individual acoustic element. The array of CMUT cells 10' is coupled to a multiplexer 11, which is adapted to select the groups of CMUT cells that will be operated as individual acoustic elements. The multiplexer 11 is further coupled to a microbeamformer 12 via outputs that correspond to the number of acoustic elements activated in the array. The expanded inset indicated by the dashed lines shows grey CMUT cells 60 that are grouped together as an individual acoustic element 62 having dimensions X and Y. Depending on the desired imaging application, the multiplexer 11 can selectively operate the CMUT cells in the array to produce individual acoustic elements having a variety of dimensions. Here, for example, the acoustic element includes thirty-six CMUT cells in a 6 x 6 arrangement (X=6, Y=6). It can be easily envisioned that other configurations of individual acoustic element can be produced using the multiplexer. For instance, configurations of an individual acoustic element could include a 10 x 10 arrangement, a 5 x 10 arrangement, a 1 x 10

arrangement, or a 10 x 100 arrangement of CMUT cells. Similarly, any desired number of individual acoustic elements can be configured by the multiplexer. For example, a 64-element or 128-element transducer can be configured by grouping some or all of the CMUT cells as 64 or 128 acoustic elements in the array. The acoustic elements can be shaped as lines, squares, rectangles, ellipsoids, and irregular shapes. FIGURE 7 shows an example of using hexagonally-shaped CMUT cells that can be activated in single individual lines for high frequency imaging with a relatively tight pitch 66 or in pair of lines for lower frequency imaging exhibiting an increased pitch 64.

The configurations of the groups of CMUT cells depend on various factors, such as the size of the CMUT cells as well as a desired pitch for the acoustic elements of a transducer probe. The transducer element pitch is less than or equal to λ/2 for phased arrays or λ for linear arrays to minimize grating lobes. Accordingly, the pitch of a transducer is determined by the highest intended imaging frequency of a particular application. The frequency control capability of the CMUT cells coupled with their inherent flexibility in layout allows for quick and easy configuration for optimized imaging over different frequency ranges and for different clinical applications. Here, the bias control allows the frequency of the elements to be modified based on the clinical application. Also, because CMUTs do not require dicing like piezoceramic elements, there is great flexibility in element layout than can directly benefit the agility of the transducer probe.

FIGURE 8 illustrates the concept of a partially summing

microbeamformer that is coupled to the multiplexer 11 used to activate individual acoustic element groups of CMUTs in the array to produce a desired frequency and pitch according to a selected imaging application. The drawing of FIGURE 8 is sectioned into three areas by dashed lines 70 and 72.

Components of the probe 10 are shown to the left of line 70, components of the system mainframe are shown to the right of line 72, and the cable 74 is shown between the two lines. As shown in FIGURE 6, the CMUT array 10' of the probe can be configured to activate a desired configuration of individual acoustic elements including groups of CMUT cells. In this example, five sets of individual acoustic elements of the array 10' are shown in the drawing, each set including nine neighboring elements. Each element comprises a commonly operated group of CMUT cells. The microbeamformer channels for sets of individual acoustic elements 12a, 12c, and 12e are shown in the drawing. The nine acoustic elements of set 12a are coupled to nine delay lines of the microbeamformer indicated at DLL Similarly the nine acoustic elements of sets 12c and 12e are coupled to the delay lines indicated at DL2 and DL3. The delays imparted by these delay lines are a function of numerous variables such as the size of the array, the element pitch, the range of beam steering, and other considerations. The delay line groups DL1, DL2, and DL3 each delay the signals from the acoustic elements of their respective set to a common time reference for the set of acoustic elements. The nine delayed signals from each group of delay lines are then combined by a respective summer∑ to form a partial sum signal of the array from the patch of elements. Each partial sum signal is put on a separate bus 15a, 15b, and 15c, each of which is coupled to a conductor of the cable 16, which conducts the partial sum signals to the system mainframe. In the system mainframe each partial sum signal is applied to a delay line 22a, 22b, 22c of the system beamformer 22. These delay lines focus the partial sum signals into a common beam at the output of the system beamformer summer 22s. The fully formed beam is then forwarded to the signal and image processor for further processing and display. While the example of FIGURE 8 is shown with 9-element sets, it will be appreciated that a constructed microbeamformer system will generally have sets of acoustic elements with larger numbers of elements such as 12, 20, 48, or 70 elements or more. The elements of a set can be adjacent to each other, be spaced apart, or even intermingled in a checkerboard pattern, with "odd" numbered elements combined in one patch and "even" numbered elements combined in another.

FIGURE 9 is a flow chart showing the workflow 76 of an

implementation of the present invention. This workflow 76 begins with a step 78 of providing an ultrasound imaging system of the present invention, which includes a CMUT transducer probe that can be configured to transmit and receive at a desired frequency and have an optimized pitch of individual acoustic elements corresponding to the desired frequency of operation. In step 80, the ultrasound imaging system receives a user input for a selected imaging application. As an example, a sonographer may want to perform deep abdominal imaging on a patient. The sonographer can then set a control on the control panel, select an icon on a touchscreen or use a mouse to select the abdominal imaging application on a display. In accordance with steps 82 and 84, upon the user selection of the desired imaging application, the system uses presets that will set the DC bias voltage to a center frequency and set the configuration of CMUT cells for individual acoustic elements exhibiting an associated pitch that is optimal for the selected imaging application. Once the configuring of the probe is complete, ultrasound imaging (e.g., deep abdominal imaging) can be performed with the configured probe to generate optimized ultrasound images.

Claims

CLAIMS:
1. An ultrasonic diagnostic imaging system comprising:
an ultrasound imaging control system comprising a user control for user selection of an imaging procedure;
a transducer probe comprising capacitive micromachined ultrasonic transducer (CMUT) cells;
a multiplexer coupled to the CMUT cells and responsive to user selection of the imaging procedure so as to couple a plurality of groups of CMUT cells each for common operation as individual acoustic elements in the transducer probe;
a microbeamformer having a plurality of delays, each delay being coupled to receive signals produced by a corresponding individual acoustic element in the transducer probe;
a DC bias voltage control that is responsive to user selection of the imaging procedure so as to apply a DC bias voltage to control a frequency response of the groups of CMUT cells,
wherein the imaging control system is adapted to (1) vary the DC bias voltage applied to the individual CMUT cells so as to set the frequency of the groups of CMUT cells according to the selected imaging procedure and (2) vary the configuration of the groups of CMUT cells according to the selected imaging procedure.
2. The ultrasonic diagnostic imaging system of claim 1, wherein the DC bias voltage, the configuration of the groups of CMUT cells, or a combination thereof are selectable for different clinical applications.
3. The ultrasonic diagnostic imaging system of claim 1, wherein the imaging control system is adapted to vary the DC bias voltage to tune a center frequency of the CMUT cells to a frequency used for a selected clinical application.
4. The ultrasonic diagnostic imaging system of claim 1, wherein the imaging control system is adapted to vary dimensions of the commonly operated groups of CMUT cells to configure the pitch of the individual acoustic elements.
5. The ultrasonic diagnostic imaging system of claim 1, wherein during operation of the transducer probe an increase in dimensions of the individual acoustic elements results in an increased pitch, and a decrease in dimensions of the individual acoustic elements results in a decreased pitch.
6. The ultrasonic diagnostic imaging system of claim 1, wherein the system is adapted to set dimensions of the individual acoustic elements to provide a pitch equal to about half a wavelength of sound waves generated by the probe in transmit mode.
7. The ultrasonic diagnostic system of claim 3, wherein an increase in the DC bias voltage results in an increase in the frequency response of the groups of CMUT cells during the operation in the collapsed mode, and a decrease in the DC bias voltage results in a decrease in the frequency response of the groups of CMUT cells during the operation in the collapsed mode.
8. The ultrasonic diagnostic imaging system of claim 1, wherein the system is adapted to operate at least some of the CMUT cells in conventional mode, collapsed mode, or a combination thereof.
9. The ultrasonic diagnostic imaging system of claim 1, wherein at least some of the CMUT cells in the array differ in diameter.
10. A method of performing ultrasonic imaging, the method comprising: providing a transducer probe coupled to an ultrasonic diagnostic imaging system, the probe comprising an array of CMUT cells;
receiving, on the ultrasonic diagnostic imaging system, a user input that selects a clinical application, wherein the user input causes the ultrasound system to perform the following steps:
vary the DC bias voltage applied to individual CMUT cells in the array to tune a center frequency of the CMUT cells to a frequency used in the selected clinical application; and
vary a configuration of individual acoustic elements to tune a pitch of the transducer probe according to the frequency used for the selected clinical application, wherein each individual acoustic element comprises a group of CMUT cells; and
performing ultrasound imaging with the transducer probe at the tuned center frequency and pitch.
11. The method of claim 10, wherein the selected clinical application is one of contrast agent imaging, enhanced image elastography, opto-acoustics or high intensity focused ultrasound.
12. The method of claim 10, wherein the DC bias voltage is varied over a voltage range for reception of echo signals.
13. The method of claim 10, wherein the dimensions of the individual acoustic elements are varied for reception of echo signals.
14. The method of claim 10, wherein the system is adapted to operate at least some of the CMUT cells in conventional mode, collapsed mode, or a combination thereof.
15. The method of claim 14, wherein an increase in the DC bias voltage results in an increase in the frequency of operation of the groups of CMUT cells during the operation in the collapsed mode, and a decrease in the DC bias voltage results in a decrease in the frequency of operation of the groups of CMUT cells during the operation in the collapsed mode.
PCT/IB2014/064085 2013-08-27 2014-08-27 A cmut-based ultrasound imaging system for wide frequency range imaging WO2015028949A2 (en)

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