cMUT driving method

Abstract

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

Description

[Technical Field]

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

[0002] Capacitive micromachined ultrasonic transducers (cMUTs) typically integrate mechanical and electronic components into a very small package. These 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 electrical parts, they are commonly referred to as micro-electromechanical systems ("MEMS") devices. Because of their small size, cMUTs can be used in many applications in many different technology fields, including medical device technology.

[0003] One application of cMUTs within the field of medical devices is soft tissue imaging. Harmonic imaging of tissue has become important in medical ultrasound imaging because it provides unique information about the tissue being imaged. In harmonic imaging, ultrasonic energy is transmitted from the imaging array to the tissue at a center frequency (f0) during transmission. This ultrasonic energy interacts with the tissue nonlinearly, especially at high amplitude levels, generating ultrasonic energy at higher harmonics of the input frequency, such as 2f0. These harmonic signals are then received by the imaging array, and an image is formed. To achieve a good signal-to-noise ratio during harmonic imaging, it is desirable for the ultrasonic transducers in the imaging array to be sensitive to both the fundamental frequency f0 and the first harmonic frequency 2f0.

[0004] Conventional ultrasound transducers are unable to operate in this manner. For example, piezoelectric transducers tend to be efficient only at the fundamental frequency (f0) and its odd harmonics (3f0, 5f0, etc.), making them unsuitable for harmonic imaging applications. To correct for the odd harmonic efficiency of piezoelectric transducers, the transducer is typically damped and multiple matching layers are used to create a broadband (approximately 90% fractional bandwidth) transducer. However, this approach requires a trade-off between sensitivity and bandwidth due to significant energy losses caused by the backing and matching layers. Furthermore, conventional piezoelectric transducers and manufacturing methods do not allow device manufacturers to control or adjust the vibration harmonics of conventional piezoelectric transducers.

[0005] cMUT transducers are suitable for use in harmonic imaging. These transducers can be operated to utilize multiple vibration modes of the cMUT membrane and to allow for tunable vibration modes and / or controllable vibration harmonics. Harmonic imaging cMUTs are designed to achieve higher sensitivity over a wide bandwidth and are configured to utilize multiple vibration modes of the cMUT membrane.

[0006] Therefore, in the case of harmonic mode imaging, the excitation vibration of the cMUT is lower frequency than the vibration in the receive mode, because in the receive mode, the cMUT samples a multiple of the excitation mode frequency for image formation, resulting in increased receive sensitivity at higher frequencies, which also translates into improved performance.

[0007] It is known that increasing the bias of a cMUT element increases its sensitivity at higher frequencies.

[0008] However, there exists a maximum electric field and corresponding maximum voltage that can be applied, exceeding which will cause breakdown of the dielectric layers in the device. This is known as the breakdown voltage. The combination of the bias voltage and RF voltage must not exceed this breakdown voltage. In practice, cMUTs are typically operated well below the breakdown voltage to avoid electron tunneling through the dielectric. For example, for a cMUT with a breakdown electric field of approximately 7-9 MV / cm, a typical upper operational limit may be set at a voltage corresponding to an electric field of approximately 4.5-5 MV / cm (voltage = electric field * total thickness of the cMUT dielectric). Typically, a cMUT may include at least one dielectric layer located between the bottom electrode and the cMUT cavity and another dielectric layer located between the top electrode and the cavity. The breakdown voltage depends on the thickness of the dielectric, which corresponds to the total thickness of all dielectric layers located between the electrodes. The breakdown voltage is typically in the range of 150-200 V, alternatively in the range of 70-100 V, or alternatively in the range of 60-80 V.

[0009] Patent Document 1 discloses a wideband intrabody communication system configured for transmitting data through the body by ultrasound, in which a cMUT transmitter operated in a collapsed mode is configured to transmit ultrasound data signals within a wideband operating frequency range through the body to a similarly configured cMUT receiver for decoding and processing. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 10,313,027 [Patent Document 2] U.S. Patent No. 5,997,479 [Patent Document 3] U.S. Patent No. 6,013,032 [Patent Document 4] U.S. Patent No. 6,623,432 [Patent Document 5] U.S. Patent No. 6,283,919 [Patent Document 6] U.S. Patent No. 6,458,083 [Patent Document 7] U.S. Patent No. 6,443,896 [Patent Document 8] U.S. Patent No. 6,530,885 [Non-patent literature]

[0011] [Non-Patent Document 1] "Micromachined Ultrasonic Transducers", IEEE Trans UFFC, Vol.50, No.9(2003) Summary of the Invention [Means for solving the problem]

[0012] The invention is defined by the claims.

[0013] According to an example according to one aspect of the present invention, there is provided a method for driving a cMUT device in a driving cycle including a transmit period and a receive period. The method includes driving a cMUT element of the cMUT device with a first bias voltage and an RF voltage during the transmit period, and driving the cMUT element with a second bias voltage without the RF voltage during the receive period. The second bias voltage is higher than the first bias voltage, and the combination of the RF voltage and the first bias voltage is such that the cMUT element operates in a collapse mode during the transmit period. The second bias voltage is such that the cMUT element operates in a collapse mode during the receive period.

[0014] Therefore, the concept proposed by the inventors is to change the bias voltage between the transmit cycle phase and the receive cycle phase. After generating ultrasound waves in the transmit period, the applied RF voltage is removed, allowing the bias voltage to be safely increased during the receive period. In this way, the sensitivity in the receive period can be improved. The bias voltage can then be decreased again, after which ultrasound waves are generated in the next transmit period. By driving the cMUT element with a second bias voltage higher than the first bias voltage in the receive period, the receive sensitivity of the cMUT element is increased at frequencies higher than the frequency of the RF voltage.

[0015] Therefore, it is proposed to utilize the time separation principle to adjust the bias voltage level. However, importantly, it is proposed to set the bias voltage and RF voltage levels so that the cMUT stays in collapse mode all the time during both transmit and receive periods. This avoids known problems in the state of the art that can cause artifacts and further shortens the membrane response time by reducing the required movement.

[0016] Operating in the collapsed mode has advantages compared to the non-collapsed mode. The cMUT has a higher transmit pressure in the collapsed mode. To achieve a high transmit pressure in the non-collapsed mode, it may be necessary to operate the cMUT at a bias voltage near the collapse point. In that case, the resulting device would exhibit more nonlinear behavior, which is highly detrimental for harmonic imaging purposes. Furthermore, switching between the collapsed and non-collapsed modes results in wear and tear, reducing the useful life (reliability) of the cMUT. Also, acoustic artifacts can occur when the membrane moves from the collapsed state to the collapsed state due to rapid changes in capacitance, resulting in artifacts in the signal that require filtering. Therefore, remaining in the collapsed mode avoids the need to filter these artifacts and extends the useful life of the device.

[0017] A state-of-the-art approach to addressing receiver sensitivity issues is to utilize dynamic gain control, where the signal amplification level is adjusted as a function of time during the receive phase of an ultrasound probe. Accordingly, some embodiments of the present invention provide additional and / or alternative ways to enhance the received signal before amplification, thus providing a novel method for improving performance. Furthermore, boosting the received signal before amplification can also be beneficial to the signal-to-noise ratio (SNR).

[0018] As discussed, the proposed method can be particularly advantageously applied to harmonic ultrasound imaging, such as with a cMUT configured for harmonic ultrasound imaging. The drive cycle can be a harmonic imaging cycle. However, the general principles can be applied to any type of cMUT for harmonic or non-harmonic imaging, since an increase in receive sensitivity is achieved in either case.

[0019] The cMUT device comprises one or more cMUT transducer elements.

[0020] For the avoidance of doubt, in the context of this application, RF voltage means AC voltage, and bias voltage means DC voltage.

[0021] The sum of the RF voltage and the first bias voltage, and the second bias voltage alone, must not exceed a predefined maximum voltage corresponding to the breakdown voltage of the cMUT element at any time. In practice, the voltages for these two modes may be set below an upper limit set by a predetermined margin below the breakdown voltage. For example, for a cMUT with a breakdown field of approximately 7-9 MV / cm, a typical upper operational limit may be set to a voltage corresponding to an electric field of approximately 4.5-5 MV / cm (where breakdown field [V / cm] = breakdown voltage / total thickness of the dielectric layers of the cMUT element). Therefore, the maximum voltage during both the transmit and receive periods is preferably kept below the breakdown voltage and below a predefined upper limit selected as a manufacturing choice depending on the service life requirements of the cMUT device.

[0022] The cMUT operates in collapse mode when the applied voltage (the combination of the first bias voltage and the RF voltage, or the second bias) exceeds the collapse voltage of the cMUT element. In a preferred embodiment, the bias voltage in both the transmit and receive periods is set to exceed the collapse voltage. This is beneficial to the service life of the cMUT transducer.

[0023] According to some embodiments, the difference between the second bias voltage and the first bias voltage may be equal to the voltage amplitude of the RF voltage. This means that the bias voltage step-up closely matches the magnitude of the RF voltage. For example, when the RF+ bias in transmit mode is at or near the maximum operating voltage, this feature ensures that the maximum possible bias voltage increase is achieved without exceeding the maximum operating voltage.

[0024] In some embodiments, the method further includes sampling the cMUT element during the receive period to obtain a received signal.

[0025] In some embodiments, the transition from the transmit period to the receive period of the imaging cycle includes a ramp-up from the first bias voltage to the second bias voltage according to a first ramp function, and in some embodiments, the transition from the receive period to the transmit period of the cycle includes a ramp-down from the second bias voltage to the first bias voltage according to a second ramp function.

[0026] In some embodiments, the first ramp function and the second ramp function are controllable.

[0027] In some embodiments, each of the first ramp function and the second ramp function is a smoothed linear function.

[0028] In some embodiments, the method further includes sampling the cMUT element during the receiving period to obtain a received signal, and the sampling step includes sampling only between the end of the bias voltage ramp-up and the start of the bias voltage ramp-down.

[0029] In some embodiments, the method further comprises obtaining an indication of one or more target acoustic frequencies to be sampled during the receive period, and determining the value of the second bias voltage depending on the one or more target acoustic frequencies. In other words, according to this set of embodiments, it is proposed to adjust the bias voltage depending on the harmonic frequencies to be measured. This may, for example, utilize a predefined mapping function or look-up table relating target frequencies to optimal bias voltages for sampling those frequencies.

[0030] In some embodiments, the method includes determining one or more target acoustic frequencies, the one or more target acoustic frequencies being respective harmonics of a frequency of an RF voltage applied during a transmit period, hi some embodiments, the one or more target acoustic frequencies include a third harmonic of a frequency of an RF voltage applied during a transmit period.

[0031] The present invention can also be embodied in the form of hardware.

[0032] In particular, another aspect of the present invention is a cMUT device comprising a cMUT element and a driver circuit configured to drive the cMUT device in a drive cycle including a transmit period and a receive period. The driver circuit is configured to drive the cMUT element with a first bias voltage and an RF voltage during the transmit period, and to drive the cMUT element with a second bias voltage without an RF voltage during the receive period. The second bias voltage is higher than the first bias voltage. The combination of the RF voltage and the first bias voltage causes the cMUT element to operate in a collapse mode during the transmit period, and the second bias voltage causes the cMUT element to operate in a collapse mode during the receive period.

[0033] The apparatus may further comprise a signal sampling circuit configured to sample the cMUT element during the receive period to obtain a received signal.

[0034] In some embodiments, the transition from the transmit period to the receive period of the imaging cycle includes a ramp-up from the first bias voltage to the second bias voltage according to a first ramp function, and the transition from the receive period to the transmit period of the cycle includes a ramp-down from the second bias voltage to the first bias voltage according to a second ramp function.

[0035] In particular, further aspects of the present invention relate to an ultrasound probe comprising a cMUT device as defined above, and to an ultrasound imaging system comprising such an ultrasound probe.

[0036] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0037] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: [Brief explanation of the drawings]

[0038] [Figure 1] FIG. 1 is a diagram showing the structure of a cMUT element. [Figure 2] FIG. 10 shows an example of a cMUT element operated in collapse mode. [Figure 3] FIG. 1 illustrates an overview of example method steps in accordance with one or more embodiments of the present invention. [Figure 4] FIG. 1 illustrates components of an example device according to one or more embodiments. [Figure 5] FIG. 10 illustrates example voltage signal characteristics during transmit and receive periods in accordance with one or more embodiments. [Figure 6] FIG. 10 illustrates example voltage signal characteristics during transmit and receive periods in accordance with one or more embodiments. [Figure 7] FIG. 10 illustrates example voltage signal characteristics during transmit and receive periods in accordance with one or more embodiments. [Figure 8] FIG. 10 illustrates example voltage signal characteristics during transmit and receive periods in accordance with one or more embodiments. [Figure 9] FIG. 1 illustrates an example ultrasound imaging system having receive or sampling electronics and drive electronics. DETAILED DESCRIPTION OF THE INVENTION

[0039] The invention will be described with reference to these figures.

[0040] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems, and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the devices, systems, and methods of the present invention will become more fully understood from the following description, appended claims, and accompanying drawings, which should be understood to be schematic representations only and are not drawn to scale. It should also be understood that the same reference numerals will be used throughout the drawings to refer to the same or similar parts.

[0041] 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 the drive cycle while always maintaining the cMUT in a collapsed mode of operation, in which the bias voltage is increased, thereby increasing the sensitivity.

[0042] To facilitate understanding, a brief background on the collapse mode operation of cMUT elements is provided.

[0043] As described in Non-Patent Document 1, for a conventional capacitive micromachined ultrasonic transducer (cMUT) to be operated in a collapse mode, the flexible membrane of the cMUT is typically excited with a voltage that causes a portion of the membrane to collapse and contact the corresponding cMUT substrate. Subsequently, by reducing the applied voltage to the membrane to a certain threshold voltage, commonly referred to as the cMUT's "snapback voltage," the membrane is typically pulled upward from the substrate and returned to its equilibrium position. In contrast, as long as the applied voltage to the previously collapsed membrane is maintained above the snapback voltage, a sufficiently linear and efficient output of the device can generally be achieved.

[0044] FIG. 1 illustrates a conventional cMUT structure. More specifically, FIG. 1 illustrates, in a schematic cross-sectional view, a cMUT 100 comprising a substrate 102 having a pocket or cavity 104 formed therein and a flexible membrane 106 attached to the substrate 102 over the cavity 104. A first electrode 112 is positioned on 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. This thus forms an upper layer stack spanning above the cavity 104, comprising a membrane 106 disposed on a first electrode 112 disposed on a first dielectric layer 122, and a second layer stack located at the bottom of the cavity 104, comprising a second dielectric layer 124 disposed on a second electrode 114.

[0045] The total dielectric thickness of a cMUT corresponds to the sum of the thicknesses of the dielectric layers. The breakdown voltage refers to the voltage that leads to breakdown of these dielectric layers. At the breakdown voltage point, current begins to flow from electrode 112 to electrode 114, essentially destroying the capacitor structure, causing it to behave as a resistor. This can result in the device heating up rapidly and potentially burning.

[0046] In a situation where the bias voltage applied between these electrodes is set to a relatively low voltage or zero volts, the cMUT 100 will typically have a gap in the cavity 104 between the flexible membrane 106 and the substrate 102.

[0047] 2, in operation, when the voltage bias applied between the first electrode 112 and the second electrode 114 is increased by a sufficient amount from a relatively low or zero level corresponding to the configuration of the cMUT 100 shown in FIG. 1, the flexible membrane 106 will tend to collapse into the cavity 104 and downward toward the substrate 102. Such collapse of the flexible membrane 106 can substantially eliminate the gap (FIG. 1) between the flexible membrane 106 and the substrate 102, placing the downward-facing surfaces 200 of the upper layer stacks 106, 112, 122 in at least temporary physical contact with the corresponding upward-facing surfaces 202 of the lower layer stacks 114, 124. Once achieved, this collapsed state of the flexible membrane 106 relative to the substrate 102 can be maintained by continuously applying a voltage across the flexible membrane 106 and the substrate 102 that exceeds a certain minimum level, commonly referred to as the collapse voltage or snapback voltage.

[0048] Some embodiments of the present invention are particularly advantageous for use in the context of harmonic ultrasound imaging. As previously described herein, harmonic ultrasound imaging requires a cMUT transducer to sense echo waves at multiples of the transmit frequency, i.e., at higher harmonics of the transmit frequency. This necessitates high sensitivity to higher frequencies. Collapse-mode operation (by maintaining the bias voltage above the collapse voltage) increases the cMUT's sensitivity to high frequencies without the need to increase the bias voltage too close to its operating limit. In other words, achieving higher receive sensitivity in non-collapse mode requires operating the cMUT near its dielectric breakdown voltage, which results in more nonlinear receive behavior and is therefore less optimal, particularly for harmonic imaging.

[0049] Embodiments of the present invention facilitate achieving harmonic imaging operation with improved receive sensitivity at harmonic frequencies of a central base frequency.

[0050] Embodiments of the present invention are based on the insight of utilizing time separation to adjust the bias voltage levels of the cMUT elements included in a cMUT device during a transmit / receive drive cycle, thereby enabling dynamic bias voltage control, in which the bias voltage increases with the applied RF voltage value after the occurrence of a transmit ultrasound wave to improve sensitivity in the receive phase to higher (harmonic) frequencies, and then this bias voltage decreases again before the next transmit event, during which the cMUT elements remain constantly in collapse mode.

[0051] Embodiments of the present 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 required during the receive phase. This leaves room to increase the bias voltage without risking exceeding a safe operating limit for the total applied voltage. This safe operating limit typically corresponds to a percentage of the breakdown voltage of the transducer, e.g., between 50-80% of the breakdown voltage of the transducer.

[0052] 3 illustrates, in block diagram form, steps of an example method according to one or more embodiments. These steps are illustrated generally and then further described in the context of exemplary embodiments.

[0053] A method 10 is provided for driving a cMUT device having one or more cMUT elements in a driving cycle including a transmit period 12 and a receive period 16. The method includes driving the cMUT elements of the cMUT device with a first bias voltage 20 and an RF voltage 22 during the transmit period 12. The method includes driving the cMUT elements with a second bias voltage 26 without an RF voltage 28 during the receive period 16. The second bias voltage 26 is higher than the first bias voltage 20. The combination of the RF voltage 22 and the first bias voltage 20 is such that the cMUT elements operate in a collapse mode (always) during the transmit period, and the second bias voltage 26 is such that the cMUT elements operate in a collapse mode during the receive period.

[0054] As previously mentioned, this method may also be implemented in hardware form.

[0055] Referring to FIG. 4, another embodiment of the present invention is a cMUT apparatus 30. The cMUT apparatus comprises a cMUT device having at least one cMUT element 32. The apparatus further comprises a drive circuit 34 configured to drive the cMUT element in a drive cycle including a transmit period and a receive period. The drive circuit is configured to drive the cMUT element with a first bias voltage and an RF voltage during the transmit period, and to drive the cMUT element with a second bias voltage without an RF voltage during the receive period. In this case, the second bias voltage is higher than the first bias voltage, and the combination of the RF voltage and the first bias voltage causes the cMUT element to operate in a collapse mode during the transmit period, and the second bias voltage causes the cMUT element to operate in a collapse mode during the receive period.

[0056] With regard to the drive circuit, the circuit is configured to drive at least one cMUT transducer element (either directly or via a microbeamformer) during a transmit mode, and the drive circuit may further comprise a transmit / receive (T / R) switch for switching the at least one cMUT element from the transmit mode to the receive mode.

[0057] In some embodiments, the apparatus may further comprise a signal sampling circuit configured to obtain a received signal by sampling the cMUT element during a receive period.

[0058] Regarding the cMUT element itself, the structure of such an element is well known and has already been described with reference to Figures 1 and 2. For operation, a pair of electrodes is additionally provided, one applied to the membrane and the other coupled to the substrate or directly below the cavity. In transmit mode, a bias voltage and an RF voltage are applied across these two electrodes. In receive mode, a bias voltage is applied across these two electrodes.

[0059] According to some embodiments, standard downstream processing circuitry and software for generating images may also be provided, for example, a processing device configured to process received signals from the cMUT to generate a harmonic image dataset.

[0060] A key feature of the proposed concept is the dynamic adjustment of the bias voltage between the transmit and receive phases, such that the cMUT operates in collapse mode during both phases, which should be known to those skilled in the art and has already been described above with reference to FIG.

[0061] 5 illustrates voltage characteristics for the transmit and receive periods of a drive cycle in accordance with at least one embodiment set of the present invention. The transmit and receive period timings are illustrated in FIG. 5 by duty cycle waveforms 56 and 58, respectively.

[0062] As shown, the bias voltage 54 rises during the receive period, transitioning from a first bias voltage during the transmit period to a second (higher) bias voltage during the receive period, and then drops back to the first bias voltage once the receive period ends, ready for the next transmit period.

[0063] The bias voltages in both the transmit and receive periods are such that the cMUT element operates in the collapse mode, i.e., exceed the collapse voltage of the cMUT, which can be easily identified for any cMUT because it is the minimum applied bias voltage at which the membrane switches to the collapsed state, as discussed above with reference to FIG.

[0064] Preferably, as shown in the example of Figure 5, the transition from the transmit period to the receive period of the drive cycle includes a ramp-up from the first bias voltage to the second bias voltage according to a first ramp function 62. Preferably, the transition from the receive period to the transmit period of the drive cycle includes a ramp-down from the second bias voltage to the first bias voltage according to a second ramp function 64. The use of a ramp function rather than a step change avoids a change in bias voltage that would cause a transmit pulse, which is not the intended effect.

[0065] With regard to the specific values ​​of the first bias voltage, the second bias voltage, and the RF voltage, these can be set according to the preferences or requirements of the particular hardware and the particular application, so long as they meet the constraints already discussed.

[0066] A further constraint that must generally be met is that the applied voltage in both transmit and receive modes should not exceed an upper operating limit, which is usually chosen as a percentage of the breakdown voltage, for example 50-80% of the breakdown voltage.

[0067] The breakdown voltage is the voltage above which the dielectric layer in the device breaks down. This voltage level can be easily tested for any cMUT element by stepping up the applied voltage V while simultaneously monitoring the current I. By plotting the IV curve, it is possible to determine the voltage value at which device breakdown occurs. This can be identified as the voltage point at which a sharp inflection occurs in the IV curve. Specifically, this can be identified as the voltage at which the current rises significantly (i.e., rises faster than in the previous IV curve). A tunneling region can also be identified before breakdown occurs.

[0068] The combination of bias voltage and RF voltage (if applied) must not exceed this breakdown voltage at any time. More precisely, as the RF voltage cycles between upper and lower amplitude limits, the maximum amplitude of the RF voltage plus the first bias voltage must not exceed the breakdown voltage, and the second bias voltage alone must not exceed the breakdown voltage.

[0069] In practice, cMUTs are preferably 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 may be set at approximately 150-180 V. However, specifically selecting the upper operating limit as a percentage of the breakdown voltage may be a manufacturing choice. This corresponds to a balance between device lifetime (lower upper limit voltages result in longer lifetimes) and device sensitivity (higher upper limit voltage levels result in higher sensitivity).

[0070] The frequency at which the cMUT element is sensitive during receive mode is a function of the applied bias voltage; as the bias voltage increases, the frequency at which the cMUT is sensitive in receive mode also increases.

[0071] In the case of 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 needs to be than the bias voltage in transmit mode, i.e., the larger the difference between the first and second bias voltages discussed above.

[0072] Following this logic, in some embodiments, the difference between the transmit center frequency and the receive frequency sensitivity can be further increased by further reducing the first bias voltage (during the transmit period) to further increase the difference between the transmit frequency and the frequency to which the cMUT is sensitive during the receive mode. This is illustrated in FIG. 6, where the first bias voltage during the transmit period can be seen to be reduced compared to the example shown in FIG. 5. This therefore provides a way to increase the difference between the transmit and receive frequencies without further increasing the bias voltage in the receive mode (which may carry the risk of exceeding the maximum operating level). However, this comes at the expense of a slight reduction in transmit pressure due to the relatively lower bias voltage during the transmit period. Therefore, this is an optional variable that can be optimized according to manufacturing preferences.

[0073] For purposes of illustrating the concept, and without limiting the general scope of the invention, by way of example, the lower limit of the transmit center frequency of a suitable cMUT device may be approximately 1.8 MHz, which means that with increased receive sensitivity, the third harmonic of the center transmit frequency may be detected.

[0074] For further illustrative purposes, consider an example in which the first bias voltage and RF voltage combination during the transmit phase is 180 volts (at the maximum value of the RF cycle). 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 higher than the breakdown voltage (e.g., on the order of about 60 volts).

[0075] For example, in the particular example under consideration, some exemplary combinations include: Transmit bias 140V, RF 40V, receive bias 180V. Transmit bias 80V, RF 100V, receive bias 180V. Transmit bias 120V, RF 40V, receive bias 180V.

[0076] For example, in the above case, the breakdown voltage can be about 200 V, and therefore, in either case, the total applied voltage is set to be lower than the breakdown voltage by some margin.

[0077] 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 bias voltage step-up closely matches the size of the RF voltage at the maximum point of the RF cycle. For example, if the RF+bias in transmit mode is at or near the maximum operating voltage, this feature ensures that the largest possible bias voltage increase is achieved in receive mode without exceeding the maximum operating voltage.

[0078] As described above, the transition from the transmit period to the receive period of the drive cycle may include a ramp-up from the first bias voltage to the second bias voltage according to a first ramp function 62, and the transition from the receive period to the transmit period of the drive cycle may include a ramp-down from the second bias voltage to the first bias voltage according to a second ramp function 64.

[0079] In some embodiments, as shown in FIG. 7, the timing of the receive period 58 may be adjusted so that the receive period begins only when ramp-up 62 ends and ends before ramp-down 64 begins. This therefore avoids disturbances to the receive circuitry that may be caused by the ramp phase. In FIG. 7, the adjusted timing of the receive period is shown within the circled area. The solid line shows the timing of the receive period before the adjustment is made, and the dotted line shows the proposed timing adjustment. With this adjustment, the receive period begins after ramp-up ends and ends before ramp-down begins.

[0080] In other words, in some embodiments, the method further includes sampling the cMUT element during the receiving period to obtain a received signal, and the sampling includes sampling only between the end of the bias voltage ramp-up and the start of the bias voltage ramp-down.

[0081] Additionally, in some embodiments, it is possible to modify the shape and timing of the bias voltage change ramp-up function 62 and ramp-down function 64, as shown in Figure 8. Shallower ramp-up and ramp-down functions (shown by the dotted lines in Figure 8) may reduce electronic and ultrasonic effects. In particular, a steep ramp-up or ramp-down may cause ultrasonic transmissions, which is not the intended effect when adjusting the bias voltage of the receive phase.

[0082] In other words, in some embodiments, the first ramp function 62 and the second ramp function 64 are controllable.

[0083] In some embodiments, the first ramp-up function 62 and the second ramp-up function 64 are each smoothed linear functions. The slope or gradient of the ramp-up and / or ramp-down functions may be adjustable. However, functions of other shapes may also be used.

[0084] As discussed above, in some embodiments, the method further includes obtaining a received signal by sampling the cMUT element during a receive period.

[0085] One particularly advantageous application for embodiments of the present invention is harmonic imaging.

[0086] To optimize the method and apparatus for harmonic imaging, in some embodiments, the method may optionally further include obtaining an indication of one or more target acoustic frequencies sampled during the receive period and determining the value of the second bias voltage based on the one or more target acoustic frequencies.

[0087] In other words, the method may include adjusting the bias voltage according to the harmonic frequencies measured, which may, for example, utilize a predefined mapping function or look-up table that associates target frequencies with optimal bias voltages for sampling those frequencies.

[0088] The method may further include determining or identifying one or more target acoustic frequencies to be measured, the one or more target acoustic frequencies being respective harmonics of the frequency of the RF voltage applied during the transmit period. In other words, if the center transmit frequency is known (e.g., may be identified from a register entry in a processor register), the target acoustic frequencies may be determined as frequencies that are pre-defined harmonics, such as the first, second, or third harmonic of the center transmit frequency.

[0089] 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.

[0090] In certain embodiments, driver and / or receiver or sampling circuits are used. For further details, the general operation of one exemplary ultrasound imaging system including driver electronics, receiver / sampling electronics, and further image forming components will now be described with reference to Figure 9.

[0091] The system 302 includes an ultrasound probe, specifically an array transducer probe 304. The array transducer probe 304 has a transducer array 306 that transmits ultrasound waves and receives echo information. The transducer array 306 includes cMUT transducers. In this example, the transducer array 306 is a two-dimensional array of transducers 308 capable of scanning either a 2D plane or a 3D volume of a region of interest. In another example, the transducer array may be a 1D array.

[0092] The transducer array 306 is coupled to a microbeamformer 312 that controls signal reception by the transducer elements. The microbeamformer is capable of at least partial beamforming of signals received by sub-arrays, commonly called "groups" or "patches," of the transducers, as described in U.S. Patent Nos. 6,275,999, 6,282,985, and 6,369,949.

[0093] Note that the microbeamformer is generally entirely optional. Additionally, the system includes a transmit / receive (T / R) switch 316, to which the microbeamformer 312 may be coupled, to switch the array between transmit and receive modes and to protect the main beamformer 320 from high-energy transmit signals when the microbeamformer is not used and the transducer array is driven directly by the main system beamformer. Transmission of ultrasound beams from the transducer array 306 is directed by a transducer controller 318, which is coupled to the microbeamformer by the T / R switch 316 and a main transmit beamformer (not shown). This main transmit beamformer may receive input from a user operating a user interface or control panel 338. The controller 318 may include transmit circuitry configured to drive the transducer elements of the array 306 (either directly or via the microbeamformer) during transmit mode.

[0094] The functionality of the control panel 338 in this example system may be facilitated by an ultrasound controller unit according to one embodiment of the present invention.

[0095] In a typical line-by-line imaging sequence, the beamforming system in 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 the transducer array. A subaperture may be a one-dimensional line of transducers or a two-dimensional patch of transducers within a larger array. In transmit mode, the focusing and steering of the ultrasound beam generated by the array or subaperture of the array is controlled as described below.

[0096] When backscattered echo signals from the object are received, they are aligned by undergoing receive beamforming (as described below). If a subaperture is used, the subaperture is then shifted, for example, by one transducer element. The shifted subaperture is then activated, and this process is repeated until all transducer elements of the transducer array have been activated.

[0097] For each line (or subaperture), the sum of the received signals used to form the corresponding line of the final ultrasound image is the sum of the voltage signals measured by the transducer elements of the given subaperture during the receive period. The resulting line signals after the following beamforming process are typically referred to as radio frequency (RF) data. Each line signal (RF data set) generated by the various subapertures then undergoes additional processing to generate a line of the final ultrasound image. Changes in the amplitude of the line signals over time result in changes in brightness of the ultrasound image with depth, with high amplitude peaks corresponding to bright pixels (or groups of pixels) in the final image. Peaks that appear near the beginning of the line signal represent echoes from shallow structures, while peaks that appear gradually later in the line signal represent echoes from structures located at increasing depths within the object.

[0098] One of the functions controlled by the transducer controller 318 is the direction in which the beam is steered and focused. The beam may be steered straight ahead from the transducer array (orthogonal to the transducer array) or at various angles to further increase the field of view. The steering and focusing of the transmit beam may be controlled as a function of the activation time of the transducer elements.

[0099] In general ultrasound data acquisition, two methods can be distinguished: plane wave imaging and "beam steering" imaging. These two methods are differentiated by the presence or absence of beamforming in the transmit mode ("beam steering" imaging) and / or the receive mode (plane wave imaging and "beam steering" imaging).

[0100] First, regarding the focusing function, the transducer array generates a plane wave that diverges as it moves through the object by simultaneously activating all transducer elements. In this case, the ultrasound beam remains unfocused. By introducing position-dependent time delays to the transducer activations, it is possible to focus the wavefront of the beam to a desired point, called the focal zone. This focal zone is defined as the point where the lateral beamwidth is less than half the transmit beamwidth. This improves the lateral resolution of the final ultrasound image.

[0101] For example, if the transducer elements are sequentially activated using a time delay, starting with the outermost elements of the transducer array and ending with the centrally located elements, a focal zone is formed aligned with the centrally located elements at a given distance from the probe. The distance of this focal zone from the probe will vary depending on the time delay between each subsequent round of transducer element activation. After passing through the focal zone, the beam begins to diverge to form the far-field imaging region. Note that if the focal zone is located near the transducer array, the ultrasound beam will diverge rapidly in the far field, resulting in beamwidth 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 of the ultrasound beams. Therefore, changing the position of the focal zone can result in significant changes in the quality of the final image.

[0102] It should be noted that in transmit mode, only one focal point may be defined unless the ultrasound image is divided into multiple focal zones (each of which may have a different transmit focal point).

[0103] Furthermore, upon receiving echo signals from within the object, receive focusing can be performed by performing the above process in reverse. In other words, the input signals may be subjected to an electronic time delay before being received by the transducer elements and sent to the signal processing system. The simplest example of this is called delay-and-sum beamforming. This allows the receive focusing of the transducer array to be dynamically adjusted as a function of time.

[0104] Turning now to the beam steering function, by precisely applying time delays to the transducer elements, it is possible to impart a desired angle to the ultrasound beam as it exits the transducer array. For example, by activating transducers located on a first side of the transducer array, followed by activating the remaining transducers in a sequence ending on the opposite side of the array, the wavefront of the beam will be angled toward the second side. The size of the steering angle relative to the normal to the transducer array depends on the size of the time delay between the activation of subsequent transducer elements.

[0105] Furthermore, the steered beam can be focused, in which case the total time delay applied to each transducer element is the sum of both the focusing and steering time delays, in which case the transducer array is called a phased array.

[0106] To provide a DC bias voltage to the cMUT transducers, the transducer controller 118 may be coupled to control a DC bias control 345 for the transducer array, which sets the DC bias voltage applied to the cMUT transducer elements.

[0107] For each transducer element in the transducer array, an analog ultrasound signal, typically referred to as channel data, enters the system via a receive channel. In the receive channel, a partial beamformed signal is generated from the channel data by the microbeamformer 312 and then sent to the main receive beamformer 320, which combines the partial beamformed signals from individual transducer patches 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 functions. For example, the main beamformer 320 may have 328 channels, each receiving partial beamformed signals from a patch of tens or hundreds of transducer elements. In this manner, signals received by thousands of transducers in the transducer array can effectively result in a single beamformed signal.

[0108] 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 I and Q components, and harmonic signal separation. This harmonic signal separation separates linear and nonlinear signals, thereby enabling the identification of nonlinear (higher harmonics of the fundamental frequency) echo signals returning from tissue and microbubbles. This facilitates, for example, harmonic imaging. Furthermore, the signal processor can perform additional signal enhancements, such as speckle reduction, signal combining, and noise removal. The bandpass filter of the signal processor can be a tracking filter. The passband of this tracking filter slides from high to low frequencies as the echo signal reception depth increases, thereby removing higher-frequency noise from deeper regions that typically do not contain anatomical information.

[0109] The transmit and receive beamformers can be implemented with different hardware and have different functions. Naturally, the receive beamformer is designed taking into account the characteristics of the transmit beamformer. For simplicity, only receive beamformers 312, 320 are shown in Figure 9. In a complete system, there are further transmit chains with transmit microbeamformers and a main transmit beamformer.

[0110] The function of the microbeamformer 312 is to perform the initial combining of signals to reduce the number of analog signal paths, which is typically performed in the analog domain.

[0111] Final beamforming is performed in the main beamformer 320, typically after digitization.

[0112] The transmit and receive channels use the same transducer array 306 with a fixed frequency band. However, the bandwidth occupied by the transmit pulses can differ depending on the transmit beamforming used. The receive channels can capture the entire transducer bandwidth (which is the classical approach) or can use bandpass processing to extract only the bandwidth containing the desired information (e.g., harmonics of the main harmonic).

[0113] The RF signals may then be coupled to a B-mode (i.e., intensity mode or 2D imaging mode) processor 326 and a Doppler processor 328. The B-mode processor 326 performs amplitude detection on the received ultrasound signals to image internal structures, such as organ tissues and blood vessels. In line-by-line imaging, each line (beam) is represented by an associated RF signal, the amplitude of which is used to generate an intensity value assigned to a pixel in the B-mode image. The exact location of a pixel in the image is determined by the position of the corresponding amplitude measurement along the RF signal and the line (beam) number of the RF signal. B-mode images of such structures may be formed in harmonic imaging mode or fundamental imaging mode, or in a combination of both, as described in U.S. Patent Nos. 5,629,997 and 5,629,997. The Doppler processor 328 processes temporally distinct signals resulting from tissue motion and blood flow to detect moving objects, such as blood cell flow, within the image field. The Doppler processor 328 typically includes a wall filter with parameters set to pass or reject echoes returning from selected types of internal objects.

[0114] The structural and motion signals generated by the B-mode and Doppler processors are coupled to the scan converter 332 and multiplane reformatter 344. The scan converter 332 arranges these echo signals into the desired image format based on the spatial relationship in which they were received. In other words, the scan converter converts the RF data from a cylindrical coordinate system to a Cartesian coordinate system suitable for displaying ultrasound images on the image display 340. In B-mode imaging, the brightness of a pixel located at a given coordinate is proportional to the amplitude of the RF signal received from that location. For example, the scan converter may arrange the echo signals into a two-dimensional (2D) sector format or a pyramidal three-dimensional (3D) image. The scan converter can overlay a color on the B-mode structural image that corresponds to the motion of points within the image field, the given color being generated from the Doppler estimated velocity. The combination of the B-mode structural image and the color Doppler image shows the motion of tissue and blood flow within the structural image field. The multiplane reformatter converts echoes received from points in a common plane within a volumetric region of the body into an ultrasound image of that plane, as described in U.S. Patent No. 6,277,999. The volume renderer 342 converts the echo signals of the 3D data set into a projected 3D image as seen from a given reference point, as described in U.S. Patent No. 6,277,999.

[0115] The 2D or 3D images are coupled from the scan converter 332, multiplane reformatter 344, and volume renderer 342 to an image processor 330 for further enhancement, buffering, and temporary storage for optional display on an image display 340. The imaging processor may be configured to remove certain imaging artifacts from the final ultrasound image, such as acoustic shadowing caused by, for example, strong attenuators or refraction, back enhancement caused by, for example, weak attenuators, and reverberation artifacts when highly reflective tissue interfaces are located nearby. Additionally, the image processor may be configured to process certain speckle reduction functions to improve the contrast of the final ultrasound image.

[0116] In addition to being used for imaging purposes, the blood flow values ​​produced by the Doppler processor 328 and the tissue structure information produced by the B-mode processor 126 are coupled to a quantification processor 334, which produces measurements of various flow conditions, such as blood volumetric flow rate, as well as structural measurements, such as organ size and gestational age. The quantification processor may receive input from a user control panel 338, such as the point on the anatomical structure on the image where the measurement is to be taken.

[0117] Output data from the quantification processor is coupled to a graphics processor 336 for reproducing measurement graphics and measurements along with the image on a display 340, as well as for audio output from the display device 340. The graphics processor 336 is further capable of generating graphic overlays for display along with the ultrasound images. These graphic overlays may include standard identifying information, such as the patient's name, the date and time of the image, and imaging parameters. To this end, the graphics processor receives input, such as the patient's name, from a user interface 338. The user interface is further coupled to a transmit controller 318 for controlling the generation of ultrasound signals from the transducer array 306 and, therefore, the images generated by the transducer array and the ultrasound imaging system. The transmit control function of the controller 318 is only one of the functions performed. Furthermore, the controller 318 takes into account the operating mode (as specified by the user) and the corresponding required transmitter configuration and receiver analog-to-digital converter bandpass configuration. The controller 318 may be a state machine having a finite number of states.

[0118] Additionally, the user interface is coupled to a multiplane reformatter 344 for selection and control of multiple multiplane reformatted (MPR) image planes, which may be used to perform quantitative measurements within the image field of the MPR image.

[0119] Those skilled in the art will understand and effect variations of the disclosed embodiments in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0120] A single processor or other unit may fulfill the functions of several items recited in the claims.

[0121] 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.

[0122] The computer program may be stored / distributed on a suitable medium, such as an optical storage medium or a solid-state medium, provided together with or as part of other hardware, but may also be distributed in other forms, such as over the Internet or other wired or wireless communication systems.

[0123] Please note that when the term "adapted to" is used in the claims or the description, this term "adapted to" is intended as equivalent to the term "configured to."

[0124] Any reference signs in the claims should not be construed as limiting the scope. [Explanation of symbols]

[0125] 10 ways 12 Transmission Period 16 Reception period 20 First bias voltage 22 RF Voltage 26 Second bias voltage 28 RF Voltage 30 cMUT equipment 34 Drive circuit 54 Bias Voltage 56 Duty Cycle Waveforms 58 Reception Period 62 First Ramp Function 64 Second Ramp Function 100 cMUT 102 Circuit Board 104 Cavity 106 Flexible Membrane 112 first electrode 114 Second electrode 118 Transducer Controller 122 first dielectric layer 124 second dielectric layer 126 B-mode processor 200 Downward Surface 202 upward surface 302 System 304 Array Transducer Probe 306 Transducer Array 308 Transducer 312 Microbeamformer 316 Transmit / Receive (T / R) Switch 318 Transducer controller, transmission controller 320 Main Beamformer 322 Signal Processor 326 B-mode processor 328 Doppler Processor 330 Image Processor 332 scan converter 334 Quantification Processor 336 graphics processor 338 User Interface, Control Panel, User Control Panel 340 Display devices, image displays 342 Volume Renderer 344 Multiplane Reformatter 345 DC bias control section

Claims

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

2. The method (10) of 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) of claim 1 or 2, wherein the method further comprises sampling the cMUT element (32) during the receive period to obtain a receive signal.

4. a transition from the transmit period to the receive period of the drive cycle includes ramping up from the first bias voltage to the second bias voltage according to a first ramp function (62); 4. The method (10) of claim 1, wherein a transition from the receive period to the transmit period of the drive cycle includes a ramp down from the second bias voltage to the first bias voltage according to a second ramp function (64).

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

6. 6. The method (10) of claim 4 or 5, wherein each of the first ramp function (62) and the second ramp function (64) is a smoothed linear function.

7. 7. The method (10) of claim 4, further comprising the step of sampling the cMUT element (32) during the receiving period to acquire a received signal, wherein the sampling is performed only between the end of the ramp-up (62) of the bias voltage and the start of the ramp-down (64) of the bias voltage.

8. The method comprises: obtaining an indication of one or more target acoustic frequencies to be sampled during said receive period; determining a value of the second bias voltage based on the one or more target acoustic frequencies; The method (10) of any one of claims 1 to 7, further comprising:

9. 9. The method of claim 8, wherein the method includes determining one or more target acoustic frequencies, the one or more target acoustic frequencies being respective harmonics of a frequency of the RF voltage applied during the transmit period.

10. 10. The method of 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 comprising at least one cMUT element (32); a driving circuit (34) configured to drive the cMUT element in a driving cycle including a transmitting period (12) and a receiving period (16), Driving the cMUT element with a first bias voltage (20) and an RF voltage (22) during the transmit period; and configured to drive the cMUT element with a second bias voltage (26) without an RF voltage during the receive period; the second bias voltage (26) is higher than the first bias voltage (20); the drive circuit (34), wherein a combination of the RF voltage and the first bias voltage causes the cMUT element to operate in a collapse mode during the transmit period, and the second bias voltage causes the cMUT element to operate in a collapse mode during the receive period; A cMUT device (30) comprising:

12. The cMUT device (30) of claim 11, further comprising a signal sampling circuit configured to sample the cMUT element during the receiving period to obtain a received signal.

13. a transition from the transmit period to the receive period of the drive cycle includes ramping up from the first bias voltage to the second bias voltage according to a first ramp function (62); 13. The cMUT device (30) of claim 11 or 12, wherein a transition from the receive period to the transmit period of the drive cycle includes a ramp down from the second bias voltage to the first bias voltage based on a second ramp function (64).

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

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

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