CMUT driving method

Abstract

A method (10) for improving the receive sensitivity of a cMUT transducer element (32) by dynamically adjusting a bias voltage (20, 26) between a transmit phase (12) and a receive phase (16) of a drive cycle while keeping the cMUT always in a collapsed mode of operation.

Description

Technical Field

[0001] The present invention relates to a driving method for a cMUT cell, particularly in the context of harmonic ultrasound imaging. Background Art

[0002] Capacitive micromachined ultrasonic transducers (cMUTs) typically combine mechanical and electronic components in a very small package. The mechanical and electronic components work together to convert mechanical energy into electrical energy, and vice versa. Because cMUTs are typically very small and have both mechanical and electronic components, they are often referred to as micro-electromechanical systems ("MEMS") devices. Due to their small size, cMUTs can be used in numerous applications in many different technology fields, including medical device technology.

[0003] One application of cMUT in the field of medical devices is soft tissue imaging. Tissue harmonic imaging becomes important in medical ultrasound imaging because it provides unique information about the imaged tissue. In harmonic imaging, ultrasonic energy is transmitted from the imaging array to the tissue at a center frequency (f0) during transmission. This ultrasonic energy (especially at high amplitude levels) interacts with the tissue in a nonlinear manner and generates ultrasonic energy at higher harmonics of the input frequency (e.g., 2f0). These harmonic signals are then received by the imaging array and an image is formed. In order to achieve a good signal-to-noise ratio during harmonic imaging, the ultrasonic transducers in the imaging array will preferably be sensitive near both the fundamental frequency f0 and the first harmonic frequency 2f0.

[0004] Conventional ultrasonic transducers cannot operate in this manner. For example, piezoelectric transducers are not suitable for harmonic imaging applications because these transducers tend to be effective only at the fundamental frequency (f0) and its odd harmonics (3f0, 5f0, etc.). In order to compensate for the odd harmonic efficiency of piezoelectric transducers, the transducers are typically damped and several matching layers are used to create a broadband (~90% fractional bandwidth) transducer. However, this approach requires a trade-off between sensitivity and bandwidth because the backing layer and matching layers cause a large amount of energy to be lost. In addition, conventional piezoelectric transducers and manufacturing methods do not allow equipment manufacturers to control or adjust the vibration harmonics of conventional piezoelectric transducers.

[0005] The cMUT transducer is suitable for harmonic imaging purposes.

[0006] These cMUT transducers can be operated to exploit multiple vibration modes of the cMUT membrane and allow for adjustable vibration modes and / or controllable vibration harmonics. Harmonic imaging cMUTs are designed to achieve higher sensitivity over a wide bandwidth and are suitable for exploiting multiple vibration modes of the cMUT membrane.

[0007] Therefore, for harmonic mode imaging, the excitation vibration of the cMUT is at a lower frequency than in receive mode, because in receive mode the cMUT samples multiples of the frequency in the excitation mode for image formation. This means that an increase in receive sensitivity at higher frequencies will also result in improved performance.

[0008] It is well known that increasing the bias voltage of a cMUT element improves the sensitivity to higher frequencies.

[0009] However, there is a maximum electric field and a corresponding maximum voltage that can be applied, above which the dielectric layer in the device will break down. This is called the breakdown voltage. The sum of the bias voltage and the RF voltage must not exceed the breakdown voltage. In fact, in practice, cMUTs are usually operated at a voltage far below the breakdown voltage to avoid electron tunneling through the dielectric. For example, for a cMUT with a breakdown electric field of about 7-9MV / cm, a typical upper operating limit may be set to a voltage corresponding to an electric field of about 4.5-5MV / cm (where voltage = electric field * total thickness of the cMUT dielectric). The cMUT may typically include at least one dielectric layer between the lower electrode and the cMUT cavity and another dielectric layer between the upper electrode and the cavity. The breakdown voltage depends on the dielectric thickness, which is equal to the total thickness of all dielectric layers between the electrodes. The breakdown voltage may typically be in the range of 150-200V, alternatively in the range of 70-100V, alternatively in the range of 60-80V.

[0010] U.S. Patent US10313027B2 discloses a broadband through-body communication system suitable for transmitting data through the body ultrasonically, wherein a cMUT transmitter operating in a collapsed mode is configured to transmit ultrasonic data signals within a broadband operating frequency through the body and to a similarly configured cMUT receiver for decoding and processing. Summary of the invention

[0011] The invention is defined by the claims.

[0012] According to an example according to one aspect of the present invention, a method for driving a cMUT device with a driving cycle including a transmission period and a reception period is provided. The method includes: in the transmission period, driving a cMUT element of the cMUT device with a first bias voltage and an RF voltage; and in the reception period, driving the cMUT element with a second bias voltage and without an RF voltage. The second bias voltage is higher than the first bias voltage, and the combined RF voltage and the first bias voltage cause the cMUT element to operate in a collapsed mode during the transmission period. The second bias voltage causes the cMUT element to operate in a collapsed mode during the reception period.

[0013] Therefore, the idea proposed by the inventor is to change the bias voltage between the transmission cycle phase and the reception cycle phase. After the ultrasonic wave is generated in the transmission period, the bias voltage can be safely increased during the reception period due to the removal of the applied RF voltage. In this way, it is possible to improve the sensitivity in the reception period. Then, before the next generation of ultrasonic wave in the next transmission period, the bias voltage can be reduced again. By driving the cMUT element with a second bias voltage higher than the first bias voltage in the reception period, the reception sensitivity of the cMUT element is higher at a frequency higher than the frequency of the RF voltage.

[0014] Therefore, it is proposed to use the time separation principle to adjust the bias voltage level. However, importantly, it is proposed to configure the bias voltage and the RF voltage level so that the cMUT is always in collapsed mode during the transmit period and the receive period. This avoids the known problems in the prior art (where artifacts may appear) and also reduces the reaction time of the membrane (because the required movement is lower).

[0015] It is advantageous to operate in collapsed mode compared to non-collapsed mode. The cMUT has a higher emission pressure in collapsed mode. In order to obtain a high emission pressure in non-collapsed mode, it is necessary to operate the cMUT at a bias voltage close to the collapse point. The resulting device will show a more nonlinear behavior, which is very disadvantageous for the purpose of harmonic imaging. In addition, switching between collapsed mode and non-collapsed mode will cause wear and reduce the lifetime (reliability) of the cMUT. Due to the sudden change in capacitance, acoustic artifacts may also occur when the membrane moves out of the collapsed state and when the membrane enters the collapsed state, which translates into artifacts in the signal that need to be filtered out. Therefore, remaining in collapsed mode avoids the need to filter these artifacts and increases the device lifetime.

[0016] In the prior art, a solution to the receive sensitivity problem is to use dynamic gain control, where the amplification level of the signal is adjusted over time during the receive phase of the ultrasound probe. Therefore, embodiments of the present invention provide additional and / or alternative ways to increase the received signal before any amplification. Therefore, this provides a new way to improve performance. In addition, boosting the received signal before amplification is also beneficial for the signal-to-noise ratio (SNR).

[0017] As discussed, the proposed method has particularly advantageous application for harmonic ultrasound imaging (e.g. using a cMUT adapted for harmonic ultrasound imaging). The drive period may be a harmonic imaging period. However, this general principle can be applied to any type of cMUT used for harmonic or non-harmonic imaging, since in all cases an increase in receive sensitivity is achieved.

[0018] A cMUT device includes one or more cMUT transducer elements.

[0019] For the avoidance of doubt, in the context of this application, RF voltage means alternating current voltage. Bias voltage means DC voltage.

[0020] The sum of the RF voltage and the first bias voltage, and the second bias voltage alone, should each always not exceed a predefined maximum voltage, which represents the breakdown voltage of the cMUT element. In practice, the voltages in both modes can be configured so that they remain below an upper limit, which is a defined margin below the breakdown voltage. For example, for a cMUT with a breakdown electric field of approximately 7-9 MV / cm, a typical upper operating limit may be set at a voltage corresponding to an electric field of approximately 4.5-5 MV / cm (wherein the breakdown electric field [V / cm] = breakdown voltage / total thickness of the dielectric layer of the cMUT element). Therefore, preferably, the maximum voltage in both the transmit period and the receive period remains below a predefined upper limit, which is lower than the breakdown voltage and is selected (as a manufacturing option) according to the lifetime requirements of the cMUT device.

[0021] When the voltage applied to the cMUT (the combined first bias voltage and RF voltage, or the second bias voltage) exceeds the collapse voltage of the cMUT element, the cMUT operates in the collapse mode. In a preferred embodiment, the bias voltages in both the transmit period and the receive period are set above the collapse voltage. This is beneficial to the life of the cMUT transducer.

[0022] According to some embodiments, the difference between the second bias voltage and the first bias voltage is equal to the voltage amplitude of the RF voltage. This means that the increase in bias voltage is exactly matched to the magnitude of the RF voltage. For example, if the RF+ bias in transmit mode is at or close to the maximum operating voltage, this feature ensures that the maximum possible bias voltage increase is obtained in receive mode without exceeding the maximum operating voltage.

[0023] In some embodiments, the method further includes sampling the cMUT element during the receiving period to obtain a received signal.

[0024] In some embodiments, the transition from the transmit period to the receive period of the imaging cycle comprises ramping up the first bias voltage to the second bias voltage according to a first ramp function. In some embodiments, the transition from the receive period to the transmit period of the cycle comprises ramping down the second bias voltage to the first bias voltage according to a second ramp function.

[0025] In some embodiments, the first ramp function and the second ramp function are controllable.

[0026] In some embodiments, each of the first ramp function and the second ramp function is a smooth linear function.

[0027] In some embodiments, the method further comprises sampling the cMUT element during the receive period to obtain a receive signal, and wherein the sampling comprises sampling only between an end of the ramp-up of the bias voltage and a start of the ramp-down of the bias voltage.

[0028] In some embodiments, the method further comprises: obtaining an indication of one or more target acoustic frequencies to be sampled during the receiving period, and determining a value of the second bias voltage in dependence on the one or more target acoustic frequencies. In other words, in accordance with this set of embodiments, it is proposed to tune the bias voltage in dependence on the harmonic frequencies to be measured. This may for example utilize a predefined mapping function or lookup table that associates target frequencies with optimal bias voltages for sampling those frequencies.

[0029] In some embodiments, the method includes determining the one or more target acoustic frequencies, and wherein the one or more target acoustic frequencies are each a harmonic of the frequency of the RF voltage applied during the transmit period. In some embodiments, the one or more target acoustic frequencies include a third harmonic of the frequency of the RF voltage applied during the transmit period.

[0030] The present invention can also be realized in the form of hardware.

[0031] In particular, another aspect of the present invention is a cMUT device, the cMUT device comprising a cMUT element and drive electronics, the drive electronics being adapted to drive the cMUT device with a drive cycle comprising a transmit period and a receive period. The drive electronics are adapted to: drive the cMUT element with a first bias voltage and an RF voltage during the transmit period; drive the cMUT element with a second bias voltage and without an RF voltage during the receive period; wherein the second bias voltage is higher than the first bias voltage; and wherein the combined RF voltage and the first bias voltage cause the cMUT element to operate in a collapsed mode during the transmit period, and wherein the second bias voltage causes the cMUT element to operate in a collapsed mode during the receive period.

[0032] The apparatus may further include signal sampling electronics adapted to sample the cMUT element during the receive period to obtain a receive signal.

[0033] In some embodiments, the transition from the transmit period to the receive period of the imaging cycle includes ramping up the first bias voltage to the second bias voltage according to a first ramp function; and wherein the transition from the receive period to the transmit period of the cycle includes ramping down the second bias voltage to the first bias voltage according to a second ramp function.

[0034] In particular, other aspects of the invention relate to an ultrasound probe comprising a cMUT device as defined above, and to an ultrasound imaging system comprising such an ultrasound probe.

[0035] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] For a better understanding of the invention and to show more clearly how it may be put into practice, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0037] Figure 1 shows the structure of the cMUT element;

[0038] Figure 2 An example cMUT element operating in a collapsed mode is shown;

[0039] Figure 3 The steps of an example method according to one or more embodiments of the present invention are outlined;

[0040] Figure 4 illustrates components of an example apparatus according to one or more embodiments;

[0041] Figure 5-8 illustrates example voltage signal characteristics during transmit and receive periods according to one or more embodiments; and

[0042] Fig. 9 An example ultrasound imaging system having receive or sampling electronics and drive electronics is shown. DETAILED DESCRIPTION

[0043] The present invention will be described with reference to the accompanying drawings.

[0044] It should be understood that the detailed description and specific examples, although indicating exemplary embodiments of the apparatus, system and method, are for illustrative purposes only and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the apparatus, system and method of the present invention will become better understood based on the following description, the appended claims and the accompanying drawings. It should be understood that the drawings are merely schematic and are not drawn to scale. It should also be understood that in all drawings, the same reference numerals are used to indicate the same or similar parts.

[0045] The present invention provides a method for improving the receive sensitivity of a cMUT transducer element by dynamically adjusting the bias voltage between the transmit and receive phases of a drive cycle while keeping the cMUT in a collapsed mode of operation. The bias voltage is increased in the receive mode to improve sensitivity.

[0046] To aid understanding, some brief background description regarding collapsed mode operation of cMUT elements will be provided.

[0047] As described in the article "Micromachined Ultrasonic Transducers" (IEEE Trans UFFC, Vol. 50, No. 9, 2003), for a conventional capacitive micromachined ultrasonic transducer (cMUT) to be operated in a collapsed mode, the flexible membrane of the cMUT is typically stimulated with a voltage that causes a portion of the membrane to collapse onto the corresponding cMUT substrate. The voltage applied to the membrane is then reduced to a certain threshold voltage (which is usually characterized as the Cmut "snapback voltage"), which usually causes the membrane to lift up from the substrate and return to an equilibrium position. In contrast, to the extent that the voltage applied to the previously collapsed membrane is maintained above the snapback voltage, a fairly linear and efficient output of the device can usually be achieved.

[0048] Figure 1 A conventional cMUT structure is shown. More specifically, Figure 1 A schematic cross section of a cMUT 100 is shown, comprising: a substrate 102 having a pocket or cavity 104 formed therein, and a flexible membrane 106 mounted to the substrate 102 across the cavity 104. A first electrode 112 is positioned atop the membrane 106, and a second electrode 114 is positioned below the cavity. A first dielectric layer 122 may be disposed between the first electrode 112 and the cavity 104. A second dielectric layer 124 is disposed between the second electrode 114 and the cavity 104. Thus, this forms an upper layer stack suspended above the cavity 104, the upper layer stack comprising the membrane 106 disposed atop the first electrode 112, disposed atop the first dielectric layer 122, and a second layer stack at the base of the cavity 104, the second layer stack comprising the second dielectric layer 124 disposed atop the second electrode 114.

[0049] The total dielectric thickness of the cMUT is equal to the sum of the thicknesses of the individual dielectric layers. The breakdown voltage refers to the voltage that causes these dielectric layers to break down. At the breakdown voltage point, current begins to flow from electrode 112 to electrode 114, effectively destroying the capacitor structure and causing it to behave as a resistor. As a result, the device heats up rapidly and may burn through.

[0050] With the bias voltage applied between the electrodes set at a relatively low voltage or zero volts, the cMUT 100 will typically present a gap within the cavity 104 between the flexible membrane 106 and the substrate 102 .

[0051] Reference now Figure 2 In operation, when the bias voltage applied between the first electrode 112 and the second electrode 114 changes from Figure 1 When the relatively low or zero level associated with the configuration of the cMUT 100 shown increases by a sufficient amount, the flexible membrane 106 will tend to collapse downward into the cavity 104 and toward the substrate 102. This collapse of the flexible membrane 106 can substantially eliminate the gap between the flexible membrane 106 and the substrate 102 ( Figure 1 ), so that the downward-facing surface 200 of the upper stack 106, 112, 122 is at least temporarily in physical contact with the corresponding upward-facing surface 202 of the lower stack 114, 124. Once this collapsed condition of the flexible membrane 106 relative to the substrate 102 is achieved, this collapsed condition can be maintained by continuously applying a voltage across the flexible membrane 106 and the substrate 102 that exceeds a certain minimum level (this voltage is generally referred to as the collapse voltage or snapback voltage).

[0052] Embodiments of the present invention have particularly powerful applications in the context of harmonic ultrasound imaging. As explained earlier in this document, for harmonic ultrasound imaging, it is necessary for the cMUT transducer to sense echoes at multiples of the transmit frequency (i.e., higher harmonics of the transmit frequency). This requires higher sensitivity to higher frequencies. Collapse mode operation (by keeping the bias voltage above the collapse voltage) increases the sensitivity of the cMUT to higher frequencies without increasing the bias voltage too close to the operating limit. In other words, in order to obtain higher receive sensitivity in non-collapse mode, it is necessary to operate the cMUT in a regime close to the dielectric breakdown voltage, which results in more nonlinear receive behavior, which is particularly bad for harmonic imaging.

[0053] Embodiments of the present invention facilitate providing harmonic imaging operation with improved receive sensitivity at harmonic frequencies of a center fundamental frequency.

[0054] Embodiments of the present invention are based on the insight that the bias voltage level of (one or more) cMUT elements included in a cMUT device is adjusted using time separation during a transmit / receive drive cycle to provide a dynamic bias voltage control, wherein after the generation of a transmit ultrasound wave, the bias voltage is increased with the value of the applied RF voltage to increase the sensitivity of the receive stage to higher (harmonic) frequencies, and then the bias voltage is decreased again before the next transmit event. During the entire cycle, the (one or more) cMUT elements are always in a collapsed mode.

[0055] Embodiments of the invention are based on the insight that the bias voltage can be safely increased during the receive phase because the additional RF voltage applied during the transmit phase is not needed during the receive phase. This leaves room for increasing the bias voltage without the risk of exceeding a safe upper operating limit of the total applied voltage, which is typically a certain fraction (e.g. between 50-80%) of the breakdown voltage of the transducer.

[0056] Figure 3 The steps of an example method according to one or more embodiments are summarized in block diagram form. These steps will be summarized before further explanation in the form of an example embodiment.

[0057] A method 10 is provided for driving a cMUT device including one or more cMUT elements in a driving cycle including a transmission period 12 and a reception period 16. The method includes: in the transmission period 12, driving the cMUT elements of the cMUT device with a first bias voltage 20 and an RF voltage 22. The method includes: in the reception period 16, driving the cMUT elements with a second bias voltage 26 and without an RF voltage 28. The second bias voltage 26 is higher than the first bias voltage 20. The combined RF voltage 22 and the first bias voltage 20 cause the cMUT elements to operate in a collapsed mode during the transmission period (always), and the second bias voltage 26 causes the cMUT elements to operate in a collapsed mode during the reception period.

[0058] As mentioned above, the method can also be implemented in hardware.

[0059] refer to Figure 4Another aspect of the present invention is a cMUT device 30. The cMUT device includes a cMUT apparatus including at least one cMUT element 32. The device also includes drive electronics 34, the drive electronics 34 being adapted to drive the cMUT element with a drive cycle including a transmit period and a receive period. The drive electronics are adapted to: drive the cMUT element with a first bias voltage and an RF voltage during the transmit period; drive the cMUT element with a second bias voltage without using an RF voltage during the receive period; wherein the second bias voltage is higher than the first bias voltage; and wherein the combined RF voltage and the first bias voltage cause the cMUT element to operate in a collapsed mode during the transmit period, and wherein the second bias voltage causes the cMUT element to operate in a collapsed mode during the receive period.

[0060] Regarding the drive electronics, they are arranged to drive at least one cMUT transducer element during the transmit mode (directly or via a microbeamformer). The drive electronics may also include a transmit / receive (T / R) switch for switching at least one cMUT element from the transmit mode to the receive mode.

[0061] In some embodiments, the apparatus may further include signal sampling electronics adapted to sample the cMUT element during a receive period to obtain a receive signal.

[0062] As to the cMUT element itself, the structure of such an element is well known and has been described above with reference to Figure 1 and Figure 2 For operation, an additional pair of electrodes is provided, one electrode is applied to the membrane and the other electrode is coupled on top of the substrate, or just below the cavity. In the transmit mode, the bias voltage is applied to both electrodes together with the RF voltage. In the receive mode, the bias voltage is applied to both electrodes.

[0063] According to some embodiments, standard downstream processing electronics and software for generating images may also be provided.For example, a processing device may be provided that is configured to process signals received from the cMUT to generate a harmonic image data set.

[0064] An important feature of the proposed concept is the dynamic adjustment of the bias voltage between the transmit phase and the receive phase, but in both phases the cMUT is operated in collapsed mode. Collapsed mode operation is known to those skilled in the art and has been described above with reference to Figure 2 Described.

[0065] Figure 5 The diagram shows the voltage characteristics during the transmission period and the receiving period of the driving cycle according to at least one set of embodiments of the present invention. The transmission period and the receiving period are respectively determined by Figure 5 The duty cycle waveforms 56 and 58 are used to indicate this.

[0066] As indicated, the bias voltage 54 increases during the receive period, transitioning from a first bias voltage in the transmit period to a second (higher) bias voltage in the receive period. Once the receive period is over, it is reduced back to the first bias voltage in preparation for the next transmit period.

[0067] The bias voltage in the transmit and receive periods causes the cMUT element to operate in the collapsed mode, i.e., it exceeds the collapse voltage of the cMUT. For any cMUT, the collapse voltage can be easily identified because it is the minimum applied bias voltage at which the membrane switches to its collapsed state, as described above with reference to Figure 2 discussed.

[0068] Preferably, if Figure 5 As shown in the example of , the transition from the transmit period to the receive period of the drive cycle includes ramping up the first bias voltage to the second bias voltage according to the first ramp function 62. Preferably, the transition from the receive period to the transmit period of the drive cycle includes ramping down the second bias voltage to the first bias voltage according to the second ramp function 64. Using a ramp function instead of a step change avoids bias voltage changes that result in the generation of transmit pulses (which is not the intended effect).

[0069] With respect to specific values ​​of the first bias voltage, the second bias voltage, and the RF voltage, these can be configured according to the preferences or requirements of the specific hardware and the specific application, as long as the constraints already discussed are met.

[0070] A further constraint that should typically be met is that the applied voltage(s) in both transmit and receive modes should not exceed an upper operating limit, which is typically chosen to be a certain fraction of the breakdown voltage (eg between 50-80%).

[0071] The breakdown voltage is the voltage above which the dielectric layer in the device will break down. This voltage level of any cMUT element can be easily tested by gradually increasing the applied voltage V and simultaneously monitoring the current I. Plotting the IV curve allows the voltage value at which device breakdown (destruction) occurs to be determined. This can be identified as the voltage point in the IV curve where an abrupt inflection point occurs. In particular, this can be identified as the voltage at which the current increases significantly (i.e., at a faster rate than the previous IV curve). Tunneling scenarios can also be identified before breakdown occurs.

[0072] The combination of the bias voltage and the RF voltage (if applied) must never exceed the breakdown voltage. Since the RF voltage cycles between higher and lower amplitudes, more precisely, the sum of the maximum amplitude of the RF voltage plus the first bias voltage should not exceed the breakdown voltage, and the second bias voltage itself should not exceed the breakdown voltage.

[0073] In fact, in practice, it is preferred that the cMUT be operated well below the breakdown voltage to avoid electron tunneling through the dielectric. For example, for a cMUT with a breakdown voltage of 170-200 V, a typical upper operating limit might be set at about 150 V-180 V. However, the specific choice of the upper operating limit as a fraction of the breakdown voltage can be a manufacturing choice; it represents a balance between device lifetime (the lower the voltage limit, the longer the lifetime) and device sensitivity (the higher the voltage limit, the higher the sensitivity).

[0074] During receive mode, the frequency to which the cMUT element is sensitive is a function of the applied bias voltage, wherein the greater the bias voltage, the greater the frequency to which the cMUT is sensitive in receive mode.

[0075] For harmonic imaging, the imaging principle relies on sensing higher harmonics of the transmit center frequency. Therefore, the higher the target harmonic, the higher the bias voltage in receive mode must be relative to the bias voltage in transmit mode; that is, the difference between the first bias voltage and the second bias voltage discussed above must be higher.

[0076] Following this logic, in some embodiments, to further increase the difference between the transmit frequency and the frequency to which the cMUT is sensitive during receive mode, the first bias voltage can be reduced (during the transmit period) to further increase the difference between the transmit center frequency and the receive frequency sensitivity. Figure 6 This situation is illustrated in the figure. Figure 6 It shows that: Figure 5 Compared to the example shown, the first bias voltage is reduced during the transmit period. Therefore, this provides a way to increase the difference between the transmit frequency and the receive frequency without further increasing the bias voltage in the receive mode (which may risk exceeding the maximum operating level). However, this comes at the expense of slightly reducing the transmit pressure due to the lower bias voltage during the transmit period. Therefore, this is an optional variable that can be optimized according to manufacturing preferences.

[0077] To illustrate the concept and not to limit the general scope of the invention, by way of example, the lower limit of a suitable cMUT device transmit center frequency may be about 1.8 MHz. This means that with increased receive sensitivity, the third harmonic of the center transmit frequency may be detected.

[0078] To further illustrate, consider an example where the combination of the first bias voltage and the RF voltage during the transmit phase is (at the maximum value of the RF cycle) 180 V. Any combination of bias voltage level and RF voltage amplitude can be selected as long as the bias voltage level during the transmit phase is greater than the collapse voltage (e.g., on the order of about 60 V).

[0079] For example, for this particular example under consideration, some illustrative combinations would include the following:

[0080] Transmit bias 140V, RF 40V, receive bias 180V.

[0081] Transmit bias 80V, RF 100V, receive bias 180V.

[0082] Transmit bias 120V, RF 40V, receive bias 180V.

[0083] For example, in the above case, the breakdown voltage may be around 200 V, so that in all cases the total applied voltage is below the breakdown voltage by a certain margin.

[0084] Optionally, the difference between the second bias voltage and the first bias voltage may be equal to the (maximum) voltage amplitude of the RF voltage. This means that the increase in bias voltage exactly matches the magnitude of the RF voltage at the maximum point of the RF cycle. For example, if the RF+ bias in transmit mode is at or close to the maximum operating voltage, this feature ensures that the maximum possible bias voltage increase is obtained in receive mode without exceeding the maximum operating voltage.

[0085] As described above, the transition from the transmit period to the receive period of the drive cycle may include ramping up the first bias voltage to the second bias voltage according to the first ramp function 62, and the transition from the receive period to the transmit period of the drive cycle may include ramping down the second bias voltage to the first bias voltage according to the second ramp function 64.

[0086] In some embodiments, Figure 7 As shown, the timing of the receive period 58 can be adjusted so that the receive period starts only at the end of the ramp up 62 and ends before the ramp down 64 begins. This therefore avoids interference with the receive electronics that may be caused by the ramp phase. Figure 7 The adjusted timing of the receive period is indicated in the circled area in . The solid line indicates the timing of the receive period before adjustment, and the dashed line indicates the proposed adjustment of the timing so that the receive period starts after the ramp-up is completed and ends before the ramp-down begins.

[0087] In other words, in some embodiments, the method further includes sampling the cMUT element during the receiving period to obtain a received signal, and wherein the sampling includes sampling only between an end of a ramp-up of the bias voltage and a start of a ramp-down of the bias voltage.

[0088] Additionally, in some embodiments, the shape and timing of the ramp-up function 62 and the ramp-down function 64 of the bias voltage change can be modified, such as Figure 8 The shallower ramp-up and ramp-down functions (given by Figure 8 In particular, a steep ramp up or ramp down may result in the emission of ultrasound (which is not the intended effect of adjusting the bias voltage of the receiving stage).

[0089] In other words, in some embodiments, the first ramp function 62 and the second ramp function 64 are controllable.

[0090] In some embodiments, each of the first ramp function 62 and the second ramp function 64 is a smooth linear function. The slope or gradient of the ramp-up function and / or the ramp-down function may be adjustable. However, other shapes of functions can also be used.

[0091] As described above, in some embodiments, the method further includes sampling the cMUT element during the receiving period to obtain a received signal.

[0092] One particularly advantageous application of embodiments of the present invention is harmonic imaging.

[0093] To optimize the method and apparatus for harmonic imaging, in some embodiments, the method may also optionally include obtaining an indication of one or more target acoustic frequencies to be sampled during the receive period, and determining a value of the second bias voltage based on the one or more target acoustic frequencies.

[0094] In other words, the method may comprise the step of tuning the bias voltage in dependence on the harmonic frequencies to be measured.This may for example utilize a predefined mapping function or lookup table that relates target frequencies to optimal bias voltages for sampling those frequencies.

[0095] The method may further comprise the step of determining or identifying one or more target acoustic frequencies to be measured, and wherein the one or more target acoustic frequencies are each a harmonic of the frequency of the RF voltage applied during the transmit period. In other words, if the center transmit frequency is known (e.g., this may be identified from a register entry in a processor register), then the target acoustic frequencies can be determined as those frequencies that are predefined harmonics (e.g., first or second or third harmonics) of the center transmit frequency.

[0096] In some advantageous embodiments, the one or more target acoustic frequencies may include a third harmonic of the frequency of the RF voltage applied during the transmit period.

[0097] Certain embodiments utilize drive electronics and / or receiving or sampling electronics. By way of further detailed explanation, reference will now be made to Fig. 9 The general operation of an exemplary ultrasound imaging system, including drive electronics, receive / sampling electronics, and image forming components, is described below.

[0098] System 302 includes an ultrasound probe, in particular an array transducer probe 304 having a transducer array 306 for transmitting ultrasound waves and receiving echo information. Transducer array 306 includes cMUT transducers. In this example, transducer array 306 is a two-dimensional array of transducers 308 capable of scanning a 2D plane or a three-dimensional volume of a region of interest. In another example, the transducer array can be a 1D array.

[0099] The transducer array 306 may be coupled to a microbeamformer 312 that controls the reception of signals by the transducer elements. The microbeamformer is capable of at least partially beamforming signals received by subarrays (often referred to as "groups" or "patches") of transducers as described in U.S. Pat. Nos. 5,997,479 (Savord et al.), 6,013,032 (Savord), and 6,623,432 (Powers et al.).

[0100] It should be noted that the microbeamformer is generally entirely optional. In addition, the system includes a transmit / receive (T / R) switch 316 to which the microbeamformer 312 can be coupled and which switches the array between transmit and receive modes and protects the main beamformer from high energy transmit signals when the microbeamformer is not used and the transducer array is operated directly by the main system beamformer 320. Transmit operations for transmitting ultrasound beams from the transducer array 306 are directed by a transducer controller 318, which is coupled to the microbeamformer and a main transmit beamformer (not shown) through the T / R switch 316, which can receive user-operated input from a user interface or control panel 338. The controller 318 can include transmit circuitry arranged to drive the transducer elements of the array 306 during transmit mode (either directly or via the microbeamformer).

[0101] According to an embodiment of the present invention, an ultrasound controller unit can facilitate the functionality of the control panel 338 in this example system.

[0102] In a typical line-by-line imaging sequence, the beamforming system within the probe may operate as follows. During transmit, the beamformer (which may be a microbeamformer or a main system beamformer, depending on the implementation) activates a transducer array or a subaperture of a transducer array. A subaperture may be a one-dimensional transducer line or a two-dimensional transducer patch within a larger array. In transmit mode, the focusing and steering of the ultrasound beam generated by the array or a subaperture of the array is controlled as described below.

[0103] When a backscattered echo signal is received from the object, the received signal is subjected to receive beamforming (described below) in order to align the received signal and, in the case of a subaperture, then shift the subaperture, for example, by one transducer element. The shifted subaperture is then activated, and the process is repeated until all transducer elements of the transducer array have been activated.

[0104] For each line (or sub-aperture), the total received signal for the associated line used to form the final ultrasound image will be the sum of the voltage signals measured by the transducer elements of the given sub-aperture during the receive period. After the beamforming process below, the resulting line signal is generally referred to as radio frequency (RF) data. Each line signal (RF data set) generated by the various sub-apertures is then subjected to additional processing to generate the lines of the final ultrasound image. The change in the amplitude of the line signal over time will contribute to the change in the brightness of the ultrasound image with depth, where high amplitude peaks will correspond to bright pixels (or sets of pixels) in the final image. Peaks that appear near the beginning of the line signal will represent echoes from shallow structures, while peaks that gradually appear later in the line signal will represent echoes from structures at increasing depths within the object.

[0105] One of the functions controlled by the transducer controller 318 is the direction in which the beam is steered and focused. The beam can be steered straight ahead (orthogonal to the transducer array) from the transducer array, or at different angles for a wider field of view. The steering and focusing of the transmit beam can be controlled based on the transducer element actuation time.

[0106] In general ultrasound data acquisition, two methods are distinguished: plane wave imaging and "beam steering" imaging. The two methods are distinguished by the presence of beam forming in the transmit mode ("beam steering" imaging) and / or the presence of beam forming in the receive mode (plane wave imaging and "beam steering" imaging).

[0107] First, let's look at the focusing function. By activating all transducer elements simultaneously, the transducer array generates a plane wave that diverges as it travels through the object. In this case, the beam of ultrasound waves remains unfocused. By introducing a position-dependent time delay into the activation of the transducer, the wavefront of the beam can be made to converge at a desired point, known as the focal zone. The focal zone is defined as a point where the lateral beam width is less than half the transmit beam width. In this way, the lateral resolution of the final ultrasound image is improved.

[0108] For example, if the time delay causes the transducer elements to be activated in series, starting with the outermost elements and finishing at the center element(s) of the transducer array, then the focal zone will be formed at a given distance away from the probe in line with the center element(s). The distance of the focal zone from the probe will vary depending on the time delay between each subsequent round of transducer element activation. After the beam passes through the focal zone, the beam will begin to diverge, forming a far-field imaging region. It should be noted that for a focal zone positioned close to the transducer array, the ultrasound beam will diverge rapidly in the far field, resulting in beam width artifacts in the final image. Typically, the near field located between the transducer array and the focal zone shows little detail due to the large overlap in the ultrasound beam. Therefore, changing the position of the focal zone can significantly change the quality of the final image.

[0109] It should be noted that in transmit mode, unless the ultrasound image is divided into multiple focal zones (each of which may have a different transmit focus), only one focus may be defined.

[0110] Additionally, upon receiving echo signals from within the object, the inverse of the above process may be performed to perform receive focusing. In other words, the incoming signal may be received by the transducer elements and subjected to an electronic time delay before being passed into the system for signal processing. The simplest example of this is known as delay-and-sum beamforming. The receive focus of the transducer array may be adjusted dynamically based on time.

[0111] Turning now to the function of beam steering, by properly applying time delays to the transducer elements, a desired angle can be imposed on the ultrasound beam as it leaves the transducer array. For example, by activating a transducer on a first side of the transducer array, followed by activating the remaining transducers in a sequence ending at the opposite side of the array, the wavefront of the beam will be tilted toward the second side. The magnitude of the steering angle relative to the normal to the transducer array depends on the magnitude of the time delay between subsequent transducer element activations.

[0112] Additionally, the steered beam can be focused, where the total time delay applied to each transducer element is the sum of the focusing time delay and the steering time delay. In this case, the transducer array is called a phased array.

[0113] To provide a DC bias voltage for the cMUT transducers, the transducer controller 318 can be coupled to control a DC bias control 345 for the transducer array. The DC bias control 345 sets the DC bias voltage(s) to be applied to the CMUT transducer elements.

[0114] For each transducer element of the transducer array, an analog ultrasound signal (commonly referred to as channel data) enters the system through a receive channel. In the receive channel, a partially beamformed signal is generated by a microbeamformer 312 based on the channel data and then passed to a main receive beamformer 320 where the partially beamformed signals from the individual transducer tiles are combined into a fully beamformed signal (referred to as radio frequency (RF) data). The beamforming performed at each stage may be performed as described above or may include additional functionality. For example, the main beamformer 320 may have 128 channels, each of which receives a partially beamformed signal from a tile of dozens or hundreds of transducer elements. In this way, the signals received by thousands of transducers of the transducer array can effectively contribute to a single beamformed signal.

[0115] The beamformed received signals are coupled to a signal processor 322. The signal processor 322 can process the received echo signals in various ways, such as: bandpass filtering; decimation; separation of the I component and the Q component; and harmonic signal separation, which is used to separate linear signals from nonlinear signals so that nonlinear (higher harmonics of the fundamental frequency) echo signals returned from tissue and microbubbles can be identified. This facilitates, for example, harmonic imaging. The signal processor can 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 whose passband slides from a higher frequency band to a lower frequency band as the echo signal is received from an increasing depth, thereby rejecting noise at higher frequencies (which come from greater depths where there is usually no anatomical information).

[0116] The beamformer for transmission and the beamformer for reception are implemented in different hardware and can have different functions. Of course, the receiver beamformer is designed to take into account the characteristics of the transmit beamformer. Fig. 9 In the illustration, for simplicity, only receiver beamformer 312, receiver beamformer 320 are shown. In a complete system, there would also be a transmit chain with a transmit microbeamformer and a main transmit beamformer.

[0117] The function of the microbeamformer 312 is to provide initial combining of the signals in order to reduce the number of analog signal paths. This is typically performed in the analog domain.

[0118] Final beamforming is done in the main beamformer 320 and is typically done after digitization.

[0119] The transmit and receive channels use the same transducer array 306 with fixed frequency bands. However, the bandwidth occupied by the transmit pulses can vary depending on the transmit beamforming used. The receive channel can capture the entire transducer bandwidth (this is the classical approach), or by using bandpass processing, the receive channel can extract only the bandwidth containing the desired information (e.g., harmonics of the main harmonic).

[0120] The RF signal can then be coupled to a B-mode (i.e., brightness mode or 2D imaging mode) processor 326 and a Doppler processor 328. The B-mode processor 326 performs amplitude detection on the received ultrasound signal to image structures in the body (e.g., organ tissue and blood vessels). In the case of line-by-line imaging, each line (beam) is represented by an associated RF signal, whose amplitude is used to generate a brightness value to be assigned to a pixel in the B-mode image. The exact position of the pixel within the image is determined by the position measured along the associated amplitude of the RF signal and the number of lines (beams) of the RF signal. B-mode images of such structures can be formed in a harmonic image mode or a fundamental image mode or a combination of the two, as described in U.S. Pat. No. 6,283,919 (Roundhill et al.) and U.S. Pat. No. 6,458,083 (Jago et al.). The Doppler processor 328 processes the temporally different signals caused by tissue movement and blood flow to detect moving matter (e.g., the flow of blood cells in the image field). The Doppler processor 328 typically includes a wall filter having parameters set to pass or reject echoes returning from selected types of material within the body.

[0121] The structural signals and motion signals generated by the B-mode processor and the Doppler processor are coupled to the scan converter 332 and the multi-plane reformatter 344. The scan converter 332 arranges the echo signals in the spatial relationship from which they are received in the desired image format. In other words, the scan converter is used to convert the RF data from a cylindrical coordinate system to a Cartesian coordinate system suitable for displaying an ultrasound image on the image display 340. In the case of B-mode imaging, the brightness of the pixel at a given coordinate is proportional to the amplitude of the RF signal received from that position. For example, the scan converter can arrange the echo signals into a two-dimensional (2D) sector format or a pyramid three-dimensional (3D) image. The scan converter is capable of superimposing the B-mode structural image with a color corresponding to the motion at a point in the image field, where the Doppler estimated velocity is used to generate a given color. The combined B-mode structural image and the color Doppler image depict tissue motion and blood flow within the structural image field. The multi-planar reformatter converts echoes received from points in a common plane in a volumetric region of the body into an ultrasound image of that plane, as described in U.S. Pat. No. 6,443,896 (Detmer). The volume renderer 342 converts the echo signals of a 3D data set into a projected 3D image as viewed from a given reference point, as described in U.S. Pat. No. 6,530,885 (Entrekin et al.).

[0122] The 2D or 3D image is coupled from the scan converter 332, the multi-planar reformatter 344, and the volume renderer 342 to the image processor 330 for further enhancement, buffering, and temporary storage, optionally for display on the image display 340. The imaging processor may be adapted to remove certain imaging artifacts (e.g., acoustic shadowing (e.g., caused by strong attenuators or refraction), post-enhancement (e.g., caused by weak attenuators), reverberation artifacts (e.g., where multiple highly reflective tissue interfaces are located in close proximity), etc.) from the final ultrasound image. In addition, the image processor may be adapted to process certain speckle reduction functions to improve the contrast of the final ultrasound image.

[0123] In addition to being used for imaging, the blood flow values ​​generated by the Doppler processor 328 and the tissue structure information generated by the B-mode processor 326 are also coupled to the quantification processor 334. In addition to structural measurements (e.g., organ size and fetal age), the quantification processor can also measure different flow conditions (e.g., volume rate of blood flow). The quantification processor can receive input from the user control panel 338 (e.g., the point in the imaged anatomy to be measured).

[0124] Output data from the quantization processor is coupled to a graphics processor 336 for reproducing measurement graphs and values ​​together with the image on the display 340, and for audio output from the display device 340. The graphics processor 336 can also generate graphic overlays for display with the ultrasound image. These graphic overlays can contain standard identification information (e.g., patient name), date and time of the image, imaging parameters, etc. For these purposes, the graphics processor receives input (e.g., patient name) from a user interface 338. The user interface is also coupled to a transmit controller 318 to control the generation operation of generating ultrasound signals from the transducer array 306, and thus control the images produced by the transducer array and the ultrasound imaging system. The transmit control function of the controller 318 is only one of the functions performed. The controller 318 also takes into account the operating mode (given by the user) in the receiver analog-to-digital converter and the corresponding required transmitter configuration and bandpass configuration. The controller 318 can be a state machine with fixed states.

[0125] The user interface is also coupled to the multi-planar reformatter 344 for selecting and controlling the planes of a plurality of multi-planar reformatted (MPR) images that may be used to perform quantitative measurements in the image fields of the MPR images.

[0126] Those skilled in the art will be able to understand and implement variations to the disclosed embodiments in practicing the claimed invention by studying the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality.

[0127] A single processor or other unit may fulfill the functions of several items recited in the claims.

[0128] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0129] The computer program may be stored / distributed on suitable media, such as optical storage media or solid-state media supplied together with or as part of other hardware, but may also be distributed in other forms (e.g. via the Internet or other wired or wireless telecommunications systems).

[0130] If the term "suitable for" is used in the claims or the specification, it should be noted that the term "suitable for" is intended to be equivalent to the term "configured to".

[0131] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method (10) for driving a cMUT device with a driving cycle including a transmission period (12) and a reception period (16), the method comprising: In the transmission period (12), a cMUT element (32) of the cMUT device is driven with a first bias voltage (20) and an RF voltage (22); During the receiving period (16), the cMUT element is driven with a second bias voltage (26) and without (28) an RF voltage; wherein the second bias voltage (26) is higher than the first bias voltage (20); and Wherein the combined RF voltage and the first bias voltage cause the cMUT element to operate in a collapsed mode during the transmit period, and wherein the second bias voltage causes the cMUT element to operate in a collapsed mode during the receive period.

2. The method (10) according to claim 1, wherein: The difference between the second bias voltage (26) and the first bias voltage (20) is equal to the voltage amplitude of the RF voltage (22).

3. The method (10) according to claim 1 or 2, wherein: The method also includes sampling the cMUT element (32) during the receiving period to obtain a received signal.

4. The method (10) according to any one of claims 1 to 3, in, Transitioning from the transmit period to the receive period of the imaging cycle includes ramping up the first bias voltage to the second bias voltage according to a first ramp function (62); and wherein transitioning from the receive period to the transmit period of the cycle comprises ramping down the second bias voltage to the first bias voltage according to a second ramp function (64).

5. The method (10) according to claim 4, wherein: The first ramp function (62) and the second ramp function (64) are controllable.

6. The method (10) according to claim 4 or 5, wherein: Each of the first ramp function (62) and the second ramp function (64) is a smooth linear function.

7. The method (10) according to any one of claims 4 to 6, wherein: The method also includes sampling the cMUT element (32) during the receive period to obtain a receive signal, and wherein the sampling includes sampling only between the end of the ramp-up (62) of the bias voltage and the beginning of the ramp-down (64) of the bias voltage.

8. The method (10) according to any one of claims 1 to 7, further comprising: obtaining an indication of one or more target acoustic frequencies to be sampled during the receive period, and A value of the second bias voltage is determined according to the one or more target acoustic frequencies.

9. The method (10) according to claim 8, wherein: The method includes determining the one or more target acoustic frequencies, and wherein the one or more target acoustic frequencies are each a harmonic of a frequency of the RF voltage applied during the transmit period.

10. The method (10) according to claim 9, wherein: The one or more target acoustic frequencies include a third harmonic of a frequency of the RF voltage applied during the transmit period.

11. A cMUT device (30), comprising: A cMUT device comprising at least one cMUT element (32); as well as drive electronics (34) adapted to drive the cMUT element with a drive cycle comprising a transmit period (12) and a receive period (16), the drive electronics being adapted to: During the transmission period, driving the cMUT element with a first bias voltage (20) and an RF voltage (22); During the receiving period, the cMUT element is driven with a second bias voltage (26) and without (28) an RF voltage; wherein the second bias voltage (26) is higher than the first bias voltage (20); and Wherein the combined RF voltage and the first bias voltage cause the cMUT element to operate in a collapsed mode during the transmit period, and wherein the second bias voltage causes the cMUT element to operate in a collapsed mode during the receive period.

12. The device (30) according to claim 11, further comprising: Signal sampling electronics adapted to sample the cMUT element during the receive period to obtain a receive signal.

13. The device (30) according to claim 11 or 12, in, Transitioning from the transmit period to the receive period of the imaging cycle includes ramping up the first bias voltage to the second bias voltage according to a first ramp function (62); and wherein transitioning from the receive period to the transmit period of the cycle comprises ramping down the second bias voltage to the first bias voltage according to a second ramp function (64).

14. An ultrasound probe (304) comprising the cMUT device (30) according to any one of claims 11-13.

15. An ultrasound imaging system (302) comprising the ultrasound probe (304) according to claim 14.

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