WO2015028945A2

WO2015028945A2 - Variable frequency control of collapsed mode cmut transducer

Info

Publication number: WO2015028945A2
Application number: PCT/IB2014/064079
Authority: WO
Inventors: Richard Edward DAVIDSEN; Junho Song
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-08-27
Filing date: 2014-08-27
Publication date: 2015-03-05
Also published as: WO2015028945A3

Abstract

An ultrasonic diagnostic imaging system has a CMUT transducer probe with an array of CMUT cells operated in a collapsed mode during ultrasonic signal transmission and reception. The frequency response to the CMUT cells is tailored for different clinical applications or continuously varied during echo reception by decreasing the DC bias voltage for the CMUT cells for lower frequency clinical applications, increasing the DC bias voltage for higher frequency clinical applications, or continuously decreasing the DC bias voltage as echoes are received to track the information frequency composition of the returning echo signals.

Description

VARIABLE FREQUENCY CONTROL OF

COLLAPSED MODE CMUT TRANSDUCER

This invention relates to medical diagnostic ultrasound imaging systems and, in particular, to collapsed mode CMUT transducers for ultrasound systems with controllable frequency response.

The ultrasonic transducers used for medical imaging have numerous characteristics which lead to the production of high quality diagnostic images.

Among these are broad bandwidth and 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. However the ceramic PZT

materials require manufacturing processes including dicing, matching layer bonding, fillers,

electroplating and interconnections which are

distinctly different and complex and require

extensive handling, all of which can result in transducer stack unit yields which are less than desired. Furthermore, this manufacturing complexity increases the cost of the final transducer probe. As ultrasound system mainframes have become smaller and dominated by field programmable gate arrays (FPGAs) and software for much of the signal processing functionality, the cost of system mainframes has dropped with the size of the systems. Ultrasound systems are now available in inexpensive portable, desktop and handheld form. As a result, the cost of the transducer probe is an ever-increasing percentage of the overall cost of the system, an increase which has been accelerated by the advent of higher element- count arrays used for 3D imaging. The probes used for electronic 3D imaging rely on specialized

semiconductor devices application-specific integrated circuits (ASICs) which perform microbeamforming for two-dimensional (2D) arrays of transducer elements. Accordingly it is desirable to be able to manufacture transducer arrays with improved yields and at lower cost to facilitate the need for low-cost ultrasound systems, and preferably by manufacturing processes compatible with semiconductor production.

Recent developments have led to the prospect that medical ultrasound transducers can be

manufactured by semiconductor processes. Desirably these processes should be the same ones used to produce the ASIC circuitry needed by an ultrasound probe such as a CMOS process. These developments have produced micromachined ultrasonic transducers or MUTs, the preferred form being the CMUT . CMUT transducers are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated

capacitance. For transmission the capacitive charge applied to the electrodes is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave. Since these devices are manufactured by semiconductor processes the devices generally have dimensions in the 10-200 micron range, but can range up to device diameters of 300-500 microns. Many such individual CMUTs can be connected together and operated in unison as a single transducer element. For example, four to sixteen CMUTs can be coupled together to function in unison as a single transducer element. A typical 2D transducer array can have 2000-3000 piezoelectric transducer elements. When fabricated as a CMUT array, over one million CMUT cells will be used. Surprisingly, early results have indicated that the yields on semiconductor fab CMUT arrays of this size should be markedly improved over the yields for PZT arrays of several thousand

transducer elements.

CMUTs are conventionally produced with an electrode-bearing membrane or diaphragm suspended over a substrate base carrying an opposing electrode. Referring to FIGURE 9, a typical CMUT transducer cell 110 is shown in cross-section. The CMUT transducer cell 110 is fabricated along with a plurality of similar adjacent cells on a substrate 112 such as silicon. A diaphragm or membrane 114 which may be made of silicon nitride is supported above the substrate by an insulating support 116 which may be made of silicon oxide or silicon nitride. The cavity 118 between the membrane and the substrate may be air or gas-filled or wholly or partially evacuated. A conductive film or layer 120 such as gold forms an electrode on the diaphragm, and a similar film or layer 122 forms an electrode on the substrate. These two electrodes, separated by the dielectric cavity 118, form a capacitance. When an acoustic echo signal causes the membrane 114 to vibrate the

variation in the capacitance can be detected, thereby transducing the acoustic wave into a corresponding electrical signal. Conversely, an a.c. signal applied to the electrodes 120, 122 will modulate the capacitance, causing the membrane to move and thereby transmit an acoustic signal. Due to the micron-size dimensions of a typical CMUT, numerous such CMUT cells are typically fabricated in close proximity to form a single transducer element. The individual cells can have round, rectangular, hexagonal, or other peripheral shapes.

When ultrasonic waves pass through tissue on both transmit and receive, they 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. It is for this reason that higher frequency ultrasound is used for shallow, more superficial imaging while lower frequencies are used when imaging at greater depths. One effort to tailor the response of the ultrasound system to this

phenomenon is what is known as a tracking filter as shown in US Pat. 4,016,750 (Green) . The passband of the ultrasound system is set to a high frequency band as echoes are initially received from shallow depths, then moves to lower center frequency bands as echoes are received from increasing depths. While a

tracking filter adapts the response of the ultrasound system to depth-dependent frequency attenuation, it would also be desirable to adapt the response of the transducer probe in the same way.

One effort to do this is described in U.S. Pat. 6,795,374 (Barnes et al . ) In this patent Barnes et al . describe control of the bias voltage of a CMUT to vary its frequency response. CMUTs use a DC bias voltage to control the spacing between the diaphragm and the substrate: the higher the bias voltage, the greater the electrostatic attraction between the diaphragm and substrate electrodes, and the closer the diaphragm is pulled toward the substrate. It is desirable to operate the CMUT with the diaphragm operating as close to the substrate as possible as this results in the greatest electromechanical coupling coefficient of the device; a small vibration from a returning acoustic signal will have a large effect on the variation of the capacitance of the two electrodes. This is where the CMUT is most sensitive to weak echo signals. However, there is a hazard in operating the CMUT in this manner. If the diaphragm touches the substrate it can become stuck to the floor of the CMUT cell by VanderWals forces,

rendering the CMUT inoperable, as described in US Pat. 6, 328, 696 (Fraser) . Barnes et al . caution against doing this in their patent and make the standard accommodation of the bias voltage for the expected vibration of the diaphragm, using a lower bias

voltage and greater spacing between the diaphragm and substrate for strong transmission vibration of the diaphragm, and a higher bias voltage and lesser spacing when the small vibrations of echo signals are being received. In addition, they propose to augment this control with a lower bias voltage as high

frequency echoes are received initially, then

increase the bias voltage as echoes from deeper depths are received. This variation utilizes a phenomenon known as "spring softening", which has an effect on the center frequency of the CMUT transducer, shifting it from a higher frequency to a lower

frequency as the bias voltage is varied from a low initial voltage to a higher ending voltage during echo reception. Care must be taken to limit the high ending voltage so that VanderWals sticking of the diaphragm is not accidentally caused. Barnes et al . are thus employing an inverse relationship between the bias voltage variation and the frequency response.

The present inventors have found that this spring softening effect is negligible in practice, and the resultant sensitivity due to the effect is poor. Accordingly it is desirable to be able to effect frequency control of CMUT transducers with high resultant sensitivity over the range of

frequencies used in ultrasonic imaging. Furthermore, it is desirable to be able to effect such frequency control without the hazard of accidentally disabling the CMUT cell by VanderWals sticking.

In accordance with the principles of the present invention, a CMUT transducer is controlled to exhibit a variable frequency response. The CMUT transducer is operated in a collapsed mode with the diaphragm of the cell in contact with the floor of the cell during operation. A DC bias voltage is controlled to vary the frequency response of the collapsed mode CMUT in a direct relationship between the bias voltage and the frequency response. As the bias voltage is decreased during echo reception, the passband of the transducer moves to progressively lower bands of frequencies. Effecting frequency control in this manner has been found to improve the sensitivity of the CMUT by an order of magnitude as compared to the frequency control techniques of the prior art.

In the drawings :

FIGURE 1 illustrates in block diagram form an ultrasonic diagnostic imaging system with a

frequency-controlled CMUT transducer probe

constructed in accordance with the principles of the present invention.

FIGURE 2 illustrates a standard 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.

FIGURES 6a and 6b illustrate the variation of the passband of a collapsed mode CMUT transducer in accordance with the present invention when varied by the PEN/GEN/RES control of an ultrasound system.

FIGURE 7 illustrates the change in frequency of returning echo signals as a function of time and depth .

FIGURE 8 illustrates the variation of the DC bias voltage used to respond to the changing

frequencies of returning echo signals shown in FIGURE 7.

FIGURE 9 illustrates in cross-section a typical CMUT cell of the prior art.

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 one- or a two-dimensional array of

transducer elements capable of scanning in a 2D plane or in three dimensions for 3D imaging. The

transducer array is coupled to a microbeamformer 12 in the probe which controls transmission and

reception of signals by the CMUT array cells.

Microbeamformers are capable of at least partial beamforming of the signals received by groups or

"patches" of transducer elements as described in US Pats. 5,997,479 (Savord et 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 when a microbeamformer is not used and the transducer array is operated directly by the main system beamformer. 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, which receives input from the user's operation of the user interface or control panel 38. One of the functions controlled by the transducer controller is the

direction in which beams are steered. Beams may be steered straight ahead from (orthogonal to) the transducer array, or at different angles for a wider field of view.

The partially beamformed signals produced by the microbeamformer 12 on receive are coupled to a main beamformer 20 where partially beamformed signals from individual patches of transducer elements are

combined into a fully beamformed signal. For example, the main beamformer 20 may have 128 channels, each of which receives a partially beamformed signal from a patch of dozens or hundreds of CMUT transducer cells. In this way the signals received by thousands of transducer elements 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 can process 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 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 as described above, 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 information.

The processed signals are coupled to a B mode processor 26 and a Doppler processor 28. The B mode processor 26 employs amplitude detection 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 mode or the fundamental 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 and/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 measure 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 also 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. 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 elements of the transducer array 10' comprise CMUT cells. FIGURE 2 shows a conventional 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. In this

example, the 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 18 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. 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 precollapsed CMUTs, which are more susceptible to charge retention than CMUTs operated with suspended membranes. The disclosed components may 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 fab, 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 exemplary 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 vibrate, 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 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 the CMUT cells of the array 10' are

operated in a collapsed mode. In the employment of the collapsed mode in a preferred implementation, the CMUT cell is biased to a precollapsed 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 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.

The membrane 114 may be brought to its collapsed state in contact with the center of the floor of the cavity 118 by applying the necessary bias voltage, 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 3a, or can be biased further downward as shown in FIGURE 3b to increased collapse beyond that of minimal contact.

If it is desired to precollapse the membrane at the time of manufacture, one way to bring the

membrane 114 to its precollapsed state is to apply pressure to the top of the membrane. When the cavity is formed in a partial or complete vacuum, it has been found that the application of atmospheric pressure of 1 Bar is sufficient to precollapse the membrane 114 to contact with the floor of the cavity

118. It is also possible to use a combination of pressure differential and bias voltage to

controllably precollapse the membrane 14, which is effective with smaller devices that may have a high atmospheric collapse pressure (e.g., 10 Bar.) Once the membrane has been precollapsed to the floor of the CMUT cell, it can be retained in this state by fabricating a structural member over the cell to physically retain the cell in the precollapsed state. As explained in US pat. pub. no. 2012/0010538 (Dirksen) , this overlaying structural member can also be constructed to act as an acoustic lens for the CMUT transducer.

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 3a 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 3a 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 portion of the cell membrane 114 is large as shown in FIGURE 3b. The small hole in the center represents the center contact region of the membrane. The large area membrane will vibrate at a relatively low frequency. But when the membrane is pulled into deeper collapse by a higher bias voltage as in FIGURE 3c, the greater central contact area results in a lesser noncontacting vibration 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.

FIGURES 4 and 5 illustrate how variation of the DC bias voltage of a collapsed CMUT can optimize the transducer for a particular desired frequency of operation. FIGURE 4 illustrates a frequency response curve 54 for a CMUT transducer with a fixed DC bias 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 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 .

An ultrasound system generally provides the operating clinician with the ability to set the frequency band of operation for a particular clinical application. Typically, the clinician can adjust a user control on the system control panel 38 to excite the transducer at lower frequencies for better penetration (PEN mode 52), higher frequencies for better resolution (RES mode 56) , or a range of intermediate frequencies for general applications requiring both good penetration and good resolution (GEN mode 54) as illustrated in FIGURE 6a. When only a single DC bias setting is used, a compromised band of CMUT transducer operation must be used for all three system setting. But with the ability to vary the CMUT transducer frequency response band in correspondence with the clinical application setting, a lower band 52' can be used in the PEN mode, an intermediate band 54' used in the GEN mode, and a high band 56' used in the RES mode as shown in FIGURE 6b. The PEN and RES bands 52^' and 56^' are seen to exhibit improved sensitivity as compared to the lower response of bands 52 and 56 when a fixed DC bias optimized for the center GEN band is used. Thus, the frequency response of the variable band collapsed mode CMUT transducer probe is tailored to the needs of a particular clinical application.

The frequency response of a variable band collapsed mode CMUT transducer can also be

continually varied during echo reception, offering the same effect as a system tracking filter as shown in FIGURES 7 and 8. FIGURE 7 illustrates the progressive decline in the center frequency of echo signals 62, 64, 66 as the echoes are received from increasing depths over time as shown by the ordinate axis of the illustration. The line 60 plots the steady decline in center frequency with depth (time) As echoes are received from shallow depths and then from progressively deeper depths, the DC bias voltag of a collapsed mode CMUT is varied from a higher voltage to a lower voltage as shown by the line 70, and the center frequency of the CMUT cells declines correspondingly. The frequency response of the collapsed mode CMUT array is continually tailored to follow the depth-dependent frequency attention by this method of DC bias control

Claims

WHAT IS CLAIMED IS:

1. An ultrasonic diagnostic imaging system with an adjustable frequency CMUT transducer probe comprising:

an array of CMUT cells each having a cell membrane, membrane electrode, cell floor, substrate, and substrate electrode with the membrane collapsed to the cell floor during operation of the CMUT cell array; and

a source of DC bias voltage coupled to the membrane electrode and the substrate electrode;

wherein the DC bias voltage is variably set to different DC bias voltages for different clinical applications; and

wherein an increase in the DC bias voltage results in an increase in the center frequency of the frequency response of the CMUT cells, and a decrease in the DC bias voltage results in a decrease in the center frequency of the frequency response of the

CMUT cells.

2. The ultrasonic diagnostic imaging system of Claim 1, wherein the different clinical applications further comprise imaging at a shallow depth and imaging at a deeper depth,

wherein a relatively high DC bias voltage is applied to the cell electrodes for shallow depth imaging and a relatively lower DC bias voltage is applied to the cell electrodes for deeper depth imaging .

3. The ultrasonic diagnostic imaging system of Claim 2, wherein the DC bias voltages for the

different clinical applications are set using an ultrasound system control.

4. The ultrasonic diagnostic imaging system of Claim 3, wherein the ultrasound system control further comprises a PEN/GEN/RES control.

5. The ultrasonic diagnostic imaging system of Claim 1, wherein an increase in the DC bias voltage further results in deeper collapse of the membrane electrode to the cell floor.

6. The ultrasonic diagnostic imaging system of Claim 1, wherein each CMUT cell further comprises a cavity between the membrane and the cell floor where the membrane is not collapsed to the cell floor; and wherein an increase in the DC bias voltage further results in a decrease of the area of the membrane which is not collapsed to the cell floor.

7. The ultrasonic diagnostic imaging system of Claim 1, wherein each CMUT cell further comprises a cavity between the membrane and the cell floor where the membrane is not collapsed to the cell floor; and wherein a decrease in the DC bias voltage further results in an increase of the area of the membrane which is not collapsed to the cell floor.

8. The ultrasonic diagnostic imaging system of Claim 1, wherein each CMUT cell has a circular shape; and

wherein the membrane electrode further comprises a ring electrode.

9. The ultrasonic diagnostic imaging system of Claim 1, wherein the substrate electrode is overlaid with an insulating layer comprising the surface of the cell floor.

10. The ultrasonic diagnostic imaging system of Claim 1, wherein each CMUT cell is configured in a square or hexagonal shape.

11. The ultrasonic diagnostic imaging system of Claim 1, wherein a plurality of CMUT cells are operated together as a unitary transducer array element .

12. The ultrasonic diagnostic imaging system of Claim 1, wherein the DC bias voltage is continuously varied from a high voltage to a relatively lower voltage during reception of echo signals.

13. The ultrasonic diagnostic imaging system of Claim 12, wherein each CMUT cell further exhibits a frequency response characteristic;

wherein the frequency response characteristic is varied from a relatively high band of frequencies to a relatively lower band of frequencies as the DC bias voltage is decreased during reception of echo signals

14. The ultrasonic diagnostic imaging system of Claim 12, wherein the imaging system further

comprises a tracking filter exhibiting a variable passband which decreases during echo reception,

wherein the variation of the frequency response of the CMUT cells is varied in correspondence with the tracking filter passband variation.