Patent Description:
The heart of any ultrasound imaging system is the transducer which converts electrical energy into acoustic energy and vice versa. Traditionally, these transducers are made from piezo-electric crystals arranged in linear (1D) transducer arrays, and operating at frequencies up to <NUM>. However, the trend towards matrix (2D) transducer arrays and the drive towards miniaturization to integrate ultrasound imaging functionality into catheters and guide wires has resulted in the development of so-called capacitive micromachined ultrasound transducers (CMUT). A CMUT comprises a membrane (or diaphragm), a cavity underneath the membrane, and electrodes forming a capacitor. For receiving ultrasound waves, ultrasound waves cause the membrane to move or vibrate, wherein the variation in capacitance between the electrodes can be detected. Thereby, the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes cause the membrane to move or vibrate and thereby transmitting ultrasound waves.

Currently CMUTs are being investigated for use in medical diagnostic and therapeutic ultrasound. Of particular interest is the integration of CMUTs and application-specific integrated circuitry (ASIC), as it is feasible to directly fabricate CMUTs on the surface of an ASIC to provide complete transducer functionality through a monolithic allsilicon process.

Furthermore, it is advantageous to fill the available area of such a device as completely as possible with active CMUTs. This results in the best overall performance. Devices have been fabricated with square as well as hexagonal patterns of CMUTs, in which the CMUTs themselves are circular or hexagonal. From the point of view of optimal filling of area with CMUTs, the hexagonal pattern is the most efficient. Typical cell dimensions of such hexagonal CMUT cells are <NUM>-<NUM> microns from flat edge to flat edge on the hexagon.

<CIT> discloses an example of such a hexagonal pattern of CMUT cells. The therein provided integrated circuit comprises a substrate comprising a hexagonal arrangement of CMOS cells, wherein every other column of said CMOS cells is offset from adjoining columns by a distance equal to one-half of the cell dimension in said column direction, and the width of each cell is selected such that the CMOS cells line up with respective micromachined elements. A hexagonal arrangement of CMUT cells overlays the substrate, wherein the CMUT and the CMOS cells are arranged in a one-to-one correspondence.

<CIT> mainly refers to the fabrication of such a micromachined hexagonal array of CMUT cells on top of a substrate. It is however silent about the way beamforming is carried out on such a hexagonal array. There is therefore still room for improvement. An interesting implementation of the integration of CMUTs and ASIC circuitry is in the case where the ASIC performs part or all of the acoustic beamforming function. Specifically, the ASIC includes circuits to perform the functions of transmit, receive, delay and summation. What is needed is a way to implement a hexagonal pattern of CMUTs with an efficient ASIC layout suitable for a microbeamformer.

<CIT> discloses an integrated switch matrix for reconfiguring subelements of a mosaic sensor array to form elements. The configuration of the switch matrix is fully programmable. The switch matrix includes access switches that connect subelements to bus lines and matrix switches that connect subelements to subelements. Each subelemenet has a unit switch cell comprising at least one access switch, at least one matrix switch, a respective memory element for storing the future state of each switch, and a respective control circuit for each switch. The access and matrix switches are of a type having the ability to memorize control data representing the current switch state of the switch, which control data includes a data bit input to turn-on/off circuits incorporated in the control circuit.

<CIT> discloses a reconfigurable ultrasound array that allows groups of subelements to be connected together dynamically so that the shape of the resulting element can be made to match the shape of the wave front. Vertically running bus lines are disposed within the hexagonal array.

The invention relates to an integrated circuit arrangement as defined in claim <NUM>.

Further aspects of the invention are set forth in the dependent claims.

The improved integrated circuit arrangement of the present invention allows to implement a hexagonal pattern of CMUTs with an efficient ASIC layout suitable for a microbeamformer. Particularly, the intelligent design of such an integrated circuit arrangement enables easy and efficient microbeamforming in such a hexagonal array of CMUTs.

In a hexagonal array of CMUTs the array can be viewed as the superposition of two rectangular arrays, with the location of alternate columns of CMUTs offset by one-half the vertical dimension (herein denoted as dimension in the column direction). In a microbeamforming ASIC, the ASIC contains circuitry under each CMUT cell that performs partial beamforming, i.e. delay and summing functions. In a hexagonal configuration, the vertical dependent delay function (delay function dependent on the position in column direction) should be different for alternating columns, due to the half-pitch vertical offset. Without this adaptation the image quality would be affected and the resulting image could be at least partially blurred.

One of the central aspects of the present invention is the accommodation of the geometrical offset between alternate columns in the hexagonal array of CMUT cells within the beamforming control. The presented beamforming control aggregates the even and odd columns, meaning that even and odd columns are preferably controlled separately. In other words, beamforming of even columns is preferably handled separately from the beamforming of the odd columns. This is realized by an intelligent hardware design on the ASIC, which is herein in general denoted as offset regulator that is configured to provide different beamforming delays to even and odd columns of the hexagonal array of CMUT cells to account for the offset in the column direction (vertical direction) between alternate columns.

The offset regulator therefore eliminates the natural geometrical offset of alternate columns in a hexagonal arrangement. Since this offset regulation is preferably hardware-implemented, the dynamic beamforming control, which is dependent on the position of each CMUT cell within the transducer array and on the desired steering angle of the resulting beam, may be realized in a "regular" manner, as if the CMUT cells were realized as rectangular or quadratic cells in an evenly distributed array without geometrical offsets between alternate columns or rows.

Separately handling the beamforming of even and odd columns furthermore has the advantage that standard components may be used for providing the beamforming delays to the even columns, and also standard components can be used for providing the beamforming delays to the odd columns. This extremely simplifies the fabrication and therefore saves production costs. Apart from that, it allows to save space on the ASIC which is left over for the CMUT and the TR cells. This again means that image resolution can be improved. In other words, area utilization of the surface is maximized for CMUT coverage.

The offset regulator comprises two separate hardware units, a first hardware unit for controlling the beamforming delays of the TR cells of the even columns and a second hardware unit for controlling the beamforming delays of the TR cells of the odd columns.

Due to the hardware-implemented elimination of the geometrical offset between even and odd columns, these hardware units may be standard components that are usually used for rectangular transducer arrays (with no geometrical offset). Such a solution, of course, saves costs for the components and therefore also over all production costs.

In other words, the preferred embodiment for a microbeamformer having a hexagonal arrangement of CMUTs is to provide two copies of equal hardware units which are responsible for providing the vertical dependent delay functions (that depend on the vertical steering angle in the column direction) to the cells.

The beamforming delays provided by the first hardware unit and the beamforming delays provided by the second hardware unit at least differ in a time-constant delay function that depends on the geometrical offset between the even and the odd columns in the column direction. The dynamic beamforming delays that are used to steer the beam into a given direction may however be the same and only vary as a function of the vertical steering angle (in the column direction).

The first hardware unit comprises a first set of buses, and the second hardware unit comprises a second set of buses. In other words, the delay functions used for microbeamforming are communicated on two separate sets of buses which run horizontally, i.e. transverse or perpendicular to the column direction, across the ASIC. The first set of buses is used to control the TR cells of the even numbered columns, and the second set of buses is used to control the TR cells of the odd numbered columns.

The first set of buses is connected to the TR cells of the even columns but unconnected to the TR cells of the odd columns, and the second set of buses is connected to the TR cells of the odd columns but unconnected to the TR cells of the even columns. However, it shall be noted that this only refers to the direct connections. It is generally conceivable that the first set of buses is at least connected to the TR cells of the odd columns in an indirect manner. Similarly, the second set of buses may also be connected to the TR cells of the even columns in an indirect manner.

According to a further embodiment, the TR cells have a rectangular shape. Even though TR cells of hexagonal shape could be aligned with the hexagonal CMUT cells in an easier manner, such microbeamformer TR cells are preferably designed on a rectangular grid, as this facilitates both the design of the ASIC as well as the sawing of the crystal material into individual elements on top of the ASIC. This again saves production costs.

Despite the geometrical differences of the CMUT cells and the TR cells, it is still possible to accomplish a one-to-one correspondence, meaning that each CMUT cell is assigned to a single TR cell on the ASIC. The CMUT cells are laid out on top of the rectangular array of microbeamformer TR cells. By adjusting the basic dimensions of the CMUTs and the rectangular period spacing of the ASIC TR cells, it is possible to achieve the one-to-one correspondence between the CMUT cells and the TR cells, such that interconnections can be readily made. The interconnections can still be designed as short as possible.

In a further preferred embodiment, the TR cells are arranged in parallel rows running transverse to the column direction, wherein the TR cells in each row are arranged in line with each other.

This means that the TR cells that are assigned to CMUT cells in even columns may be arranged directly below the respective CMUT cells (having the same geometrical center), whereas the TR cells that are connected to CMUT cells in odd columns are arranged slightly offset of the respective CMUT cells. Nevertheless, the interconnections between these TR cells and the CMUT cells in odd columns can still be kept quite short.

In a further embodiment, a dimension of the TR cells in the column direction (vertical dimension) is smaller than the dimension of a CMUT cell in the column direction (vertical direction).

Thus, there is space left on the ASIC in between the TR cell rows. This extra space may be used for the arrangement of the communication lines of the microbeamforming control.

According to a preferred embodiment, the first and the second set of buses each comprise several horizontal bus lines which are arranged in the gaps between the parallel TR cell rows running transverse to the column direction. Using these gaps for the arrangement of the horizontal bus lines is a very intelligent solution, since these bus lines do then not interfere with the TR cells, while there is still enough room for the TR cells themselves.

While the foregoing discussion was mainly focusing on the horizontally arranged bus lines to account for the vertical dependent delay functions, it is clear that the horizontal dependent delay functions are preferably communicated via further bus lines that run vertically (in the column direction) across the ASIC.

According to a preferred embodiment, the first and the second set of buses each further comprise separate vertical bus lines running in column direction for providing different beamforming delays to even and odd columns of the hexagonal array of CMUT cells to account for offsets of different CMUT cells in a direction transverse to the column direction. However, the vertical bus lines do not necessarily have to be separated in the same manner as the horizontal bus lines. The horizontal dependent delays may also be controlled by a single bus system that does not differentiate between even and odd columns.

<FIG> and <FIG> illustrate the principle design of an ultrasound imaging system and the principle design of a transducer array of such an ultrasound system. These figures are used to explain the background and the working principle of ultrasound imaging. It shall be understood that the claimed integrated circuit arrangement, the claimed ultrasound transducer as well as the claimed ultrasound imaging system are not restricted to such kind of systems.

The ultrasound imaging system in <FIG> is generally denoted with reference numeral <NUM>. The ultrasound imaging system <NUM> is used for scanning an area or volume of the body, e.g. of a patient <NUM>. It is to be understood that the ultrasound system <NUM> may also be used for scanning other areas or volumes, e.g. body parts of animals or other living beings.

For scanning the patient <NUM>, an ultrasound probe <NUM> may be provided. In the embodiment shown, the ultrasound probe <NUM> is connected to a console device <NUM>. The console device <NUM> is shown in <FIG> as a mobile console. This console <NUM> may, however, also be realized as a stationary device. The console device <NUM> is connected to the probe <NUM> via an interface <NUM> formed in a wired manner. Further, it is contemplated that the console device <NUM> may also be connected to the probe <NUM> in a wireless manner, for example using UWB transmission technology. The console device <NUM> may further comprise an input device <NUM>. The input device <NUM> may have buttons, a key pad and/or a touchscreen to provide an input mechanism to a user of the ultrasound imaging system <NUM>. Additionally or alternatively, other mechanisms may be present in the input device <NUM> to enable a user to control the ultrasound imaging system <NUM>.

Further, the console device <NUM> comprises a display <NUM> to display display data generated by the ultrasound imaging system <NUM> to the user. By this, the volume within the patient <NUM> that is scanned via the ultrasound probe <NUM> can be viewed on the console device <NUM> by the user of the ultrasound system <NUM>.

<FIG> shows a block diagram illustrating the typical operations of a 2D or 3D ultrasound imaging system <NUM>. An ultrasound transducer <NUM> emits ultrasound signals which generate a response from the volume <NUM> back to the transducer <NUM>. The received signals from the volume <NUM> are transformed by the transducer <NUM> into electrical signals. These electrical signals will then be beamformed by several microbeamformers <NUM> and finally by a main or master beamformer <NUM>, as this will be explained in more detail below. The main beamformer <NUM> provides an image signal to a signal processor <NUM>. The signal processor <NUM> in turn generates detected acoustic data - the so-called image data - therefrom. An image processor <NUM> converts the image data into display data to be displayed on the display <NUM>. The image processor <NUM> may prepare 2D tomographic slices of the volume <NUM> to be displayed or may convert or render the image data into a 3D image that is displayed on the display <NUM>.

<FIG> is a schematic detailed view of the transducer array <NUM>, the microbeamformers <NUM> and the main beamformer <NUM>. The transducer array <NUM> is usually formed of a plurality of acoustic elements, which are herein denoted as transducer elements <NUM>. According to the present invention, these transducer elements <NUM> are realized by capacitive micro-machined ultrasound transducer (CMUT) cells <NUM>, which are arranged in a hexagonal matrix array, as this will be explained in more detail with reference to <FIG>. The transducer elements <NUM> transmit the ultrasound signals and receive the generated responses. A transducer array <NUM> may comprise thousands of transducer elements <NUM> forming a multitude of sub-arrays <NUM>, <NUM>'. For illustrative purposes, merely two sub-arrays <NUM>, <NUM>' are shown in <FIG>. However, the number of sub-arrays <NUM>, <NUM>' may also be greater than <NUM>, e.g. <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, etc..

The transducer array <NUM> may have a plurality of microbeamformers <NUM>, which control both transmission and reception of acoustic pulses through the transducer elements <NUM>, and combine the acoustic responses generated by the scanned medium in order to form sub-array summed acoustic signals, which are then transferred from the transducer array <NUM> through signal lines to the main beamformer <NUM>. In the shown example, the two sub-arrays <NUM>, <NUM>' are each connected to four different microbeamformers <NUM>. However, the number of microbeamformers <NUM> in each group may also be different from <NUM>, e.g. <NUM>, <NUM>, <NUM>, <NUM>, etc. Each signal line within a sub-array <NUM>, <NUM>' may emanate from one microbeamformer <NUM> and is joint with the other signals of that sub-array <NUM>, <NUM>' to form a sub-array group output. The sub-array group output is then connected to the main beamformer <NUM> as described below.

There are two main phases of beamforming, namely transmit and receive. During transmittance, acoustic pulses are generated from the transducer elements <NUM> of the transducer array <NUM>. During the receive phase, echoes from those pulses in the volume <NUM> are received by the acoustic elements of the transducer array <NUM>, amplified and combined. For beamforming in the transmit phase, transmit delay pulsers generate delayed high voltage pulses. The acoustic pulses are transmitted by the transducer elements <NUM>. The acoustic pulses are timed relative to each other to generate a focus in the three-dimensional space of the insonified medium. In the receive phase, the acoustic pulses previously transmitted are echoed by structures in the volume <NUM>. Between the time that the acoustic pulses are transmitted and the generated pulse echoes are received by the transducer elements <NUM>, so-called T-R (transmit/receive) switches switch to the receive position. Acoustic pulses are received by the transducer elements <NUM> from many points on the body, and receive samplers take periodic samples of the resulting acoustic wave to generate analog samples, which are small voltages. The analog samples are then delayed by receive delays. The receive delays may be static delays, meaning they are unchanged during the course of acoustic reception. The receive delays may also be programmed and thereby modified dynamically during the receive phase so as to maintain a constant array focus as the transmitted pulses propagate into the medium and create echoes from successively deeper locations in the medium. The separately delayed receive signals are summed together by summers, and after summing, variable gain amplifiers perform time gain compensation. Time variable gain is required because the signals received by the transducer elements <NUM> from later times correspond to deeper depths of the body, and are therefore attenuated. The variable gain amplifiers compensate for this attenuation by increasing output. The sub-array summed acoustic signals are transmitted by the signal lines.

Hence, the beamforming control provides dynamic and/or static beamforming to generate a plurality of sub-arrays summed acoustic signals, which are received by a further static and/or dynamic beamformer in a main beamformer <NUM>. The main beamformer <NUM> performs static and/or dynamic beamforming to generate a set of fully beamformed image signals. Hence, one main or master beamformer <NUM> sub-groups a multitude of microbeamformers <NUM>. By this, the number of signals from the beamformer <NUM> to the signal processor <NUM> may be significantly reduced compared to the number of transducer elements <NUM>.

A CMUT cell <NUM> that is preferably used as a transducer element <NUM> is shown in <FIG> in a schematic cross-section. Such a CMUT cell <NUM> usually comprises a membrane <NUM>, a cavity <NUM> underneath the membrane <NUM>, and electrodes <NUM>, <NUM> which form a capacitor. For receiving ultrasound waves, ultrasound waves cause the membrane <NUM> to move or vibrate and the variation in capacitance between the electrodes <NUM>, <NUM> can be detected. Thereby, the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrode <NUM>, <NUM> causes the membrane <NUM> to move or vibrate and thereby transmitting ultrasound waves. The membrane <NUM> itself may, for example, be made of silicone nitrate. The CMUT cell <NUM> is preferably fabricated on a substrate <NUM> which may comprise heavily doped silicone. The electrodes <NUM>, <NUM> are preferably made of a conductive material, such as a metal alloy. In order to guarantee elastic vibrations of the membrane <NUM>, the cavity <NUM> is preferably evacuated. However, it may also be filled with any suitable gas. It shall be understood that the CMUT cells used in the integrated circuit arrangement according to the present invention may differ in its design from the CMUT cell <NUM> that is schematically shown in <FIG> is herein only used for illustrative purposes and to explain the general working principle of such a CMUT cell.

<FIG> schematically shows a part of the integrated circuit arrangement according to the present invention. It comprises a plurality of CMUT cells <NUM> which are arranged in a hexagonal pattern. The hexagonal pattern of CMUT cells <NUM> is overlaid on top of an ASIC <NUM>. The ASIC <NUM> comprises a plurality of transmit-receive (TR) cells <NUM>. These TR cells <NUM> include parts or all of the microbeamforming circuitry, such that each TR cell <NUM> forms a part of a microbeamformer <NUM>. Specifically, the ASIC <NUM> includes circuits to perform the functions of microbeamforming and/or beamforming, i.e. transmit, receive, delay, and summation. Typically, such microbeamforming TR cells <NUM> are designed on a rectangular grid, as this facilitates both the design of the ASIC <NUM> as well as the sawing of the crystal material into individual elements on top of the ASIC <NUM>.

Each CMUT cell <NUM> is assigned and connected to a single TR cell <NUM> in a one-to-one correspondence. The CMUT cells <NUM> are therein arranged in a hexagonal pattern. This hexagonal pattern allows to arrange the plurality of CMUT cells <NUM> in a densest possible packing manner. A hexagonal arrangement as shown in <FIG> therefore maximizes the area utilization of the surface for CMUT coverage. If the side dimension of a CMUT cell <NUM> is denoted by s, the spacing between two centers of adjacent CMU'T cells <NUM> is in any of the three major access of the hexagonal pattern only √<NUM> ≈ <NUM>. The fill factor that can be achieved with the usually circular membranes <NUM> in such a hexagonal pattern is around <NUM>%, which follows from the following dependency: <MAT> <MAT> <MAT>.

From the point of view of optimal filling of area with CMUT cells <NUM>, the hexagonal pattern is the most efficient. The hexagonal arrangement furthermore has the advantage that the effective acoustic pitch in the x+<NUM>° and the y-direction is reduced below the inherent hexagonal pitch spacing of the CMUTs <NUM> and below the y-pitch of the TR cells <NUM>. This increases the frequency at which grating lobes become a significant source of artifacts for a given steering angle.

It can be further seen in <FIG> that due to the hexagonal arrangement of the CMUT cells <NUM>, they appear to be arranged in different straight columns <NUM>, <NUM>', wherein CMUT cells in one column <NUM>, <NUM>' are all aligned with each other. However, as indicated in the upper part of <FIG>, CMUT cells <NUM> of adjacent columns <NUM>, <NUM>' are arranged offset to each other by one-half the vertical pitch of a hexagonal CMUT cell <NUM>. This offset is indicated in <FIG> by d, wherein: <MAT>.

In other words, the hexagonal array of CMUT cells <NUM> comprises a plurality of alternating even columns <NUM> and odd columns <NUM>' being parallel to a column direction y, wherein the odd columns <NUM>' are arranged offset to the even column <NUM> by d, seen in column direction y. Since the TR cells <NUM> are preferably arranged in parallel rows running in x-direction (perpendicular to the column direction y), this means that TR cells <NUM> which are assigned to CMUT cells <NUM> in odd columns <NUM>' are also arranged slightly offset from them. Connection pads <NUM>' that are used to connect the TR cells <NUM> to the CMUTs <NUM> are in odd columns <NUM>' therefore slightly larger than in even columns <NUM> (compare reference numerals <NUM> and <NUM>').

More important is however that the vertical dependent delay function (in column direction y) must be different for alternating columns, due to the half-pitch vertical offset d. This vertical dependent delay function offset varies as a function of the vertical steering angle.

According to the present invention this is solved by providing an offset regulator <NUM> on the ASIC <NUM> for providing different beamforming delays to even and odd columns <NUM>, <NUM>' of the hexagonal array of CMUT cells <NUM> in order to account for the offset d. The geometrical offset between alternate columns <NUM>, <NUM>' is thereby preferably eliminated by a hardware-implemented solution. The presented beamforming control aggregates the even and odd columns <NUM>, <NUM>', meaning that even and odd columns <NUM>, <NUM>' are preferably controlled separately. According to a preferred embodiment this is realized by providing two copies of the hardware which is responsible for providing the vertical dependent delay functions to the TR cells <NUM>. These functions are communicated on two separate sets of buses <NUM>, <NUM>.

A first set of buses <NUM> is used to control even numbered columns <NUM> and a second set of buses <NUM> is used to control odd numbered columns <NUM>'. Separately handling the beamforming of even and odd columns <NUM>, <NUM>' has the advantage that standard components may be used for providing the beamforming delays to the even and odd columns <NUM>, <NUM>' separately. If only one communication bus was used for all columns <NUM>, <NUM>' together, the delay values that would have to be applied to each CMUT cell <NUM> in the alternating columns <NUM>, <NUM>' would not only depend on the steering angle and the position of each CMUT cell on the array, but also the offset between alternate columns <NUM>, <NUM>' would have to be taken into account. In the presented solution the beamforming delays provided by the first set of buses <NUM> and the beamforming delays provided by the second set of buses <NUM> are however implemented to differ in a time-constant delay function that depends on the offset d between even and odd columns <NUM>, <NUM>' in the column direction y. Since this static difference is thereby already accounted for, the two separate sets of buses <NUM>, <NUM> may control the delays as if the CMUT cells <NUM> were realized as rectangular or quadratic cells in an evenly distributed array without geometrical offsets between alternate columns or rows. This extremely facilitates the microbeamforming control of the system.

Of course, the first and the second set of buses <NUM>, <NUM> may also comprise separate vertical bus lines (not shown) running in column direction y for providing different beamforming delays to even and odd columns of the hexagonal array of CMUT cells <NUM> to account for offsets of different CMUT cells in the direction x. However, since there is a constant distance between adjacent CMUT cells <NUM> in x-direction, this could also be handled by only one communication bus (as usual).

A still further advantage can be seen in <FIG>. The vertical dimensions in y-direction of the TR cells <NUM> are preferably chosen to be smaller than the respective dimensions in vertical direction y of the CMUT cells <NUM>. Therefore, a small space is left between adjacent rows of TR cells <NUM>. This extra space can be used to place the horizontal bus lines of the first and second set of buses <NUM>, <NUM> on the ASIC <NUM>.

In summary, the present invention provides an intelligent combination of a hexagonal pattern of CMUTs with an efficient ASIC layout suitable for microbeamforming. The intelligent design of the provided integrated circuit arrangement allows easy and efficient micro beamforming with such a hexagonal array of CMUT cells. Due to the separate consideration of even and odd columns within the hexagonal cell array the micro beamforming technique accounts for the vertical offset of alternating columns of sensors in an easy and cost-efficient way.

Claim 1:
An integrated circuit arrangement comprising:
- a plurality of capacitive micromachined ultrasound transducer, CMUT, cells (<NUM>) arranged in a hexagonal array, wherein said hexagonal array comprises a plurality of alternating even and odd columns (<NUM>, <NUM>') of CMUT cells (<NUM>) being parallel to a column direction (y), wherein the odd columns (<NUM>') are arranged offset to the even columns (<NUM>) by one-half of a dimension of a CMUT cell (<NUM>) in said column direction (y),
- an application-specific integrated circuit, ASIC, (<NUM>) comprising a plurality of transmit-receive, TR, cells (<NUM>), wherein each CMUT cell (<NUM>) is assigned and connected to a respective TR cell (<NUM>) in a one-to-one correspondence,
wherein the ASIC (<NUM>) further comprises an offset regulator (<NUM>) configured to provide different beamforming delays to even and odd columns (<NUM>, <NUM>') of the hexagonal array of CMUT cells (<NUM>) to account for the offset in the column direction (y), wherein the offset regulator (<NUM>) comprises:
a first hardware unit for controlling the beamforming delays of the TR cells (<NUM>) of the even columns (<NUM>) comprising a first set of busses (<NUM>) connected to the TR cells (<NUM>) of the even columns (<NUM>) but unconnected to the TR cells (<NUM>) of the odd columns (<NUM>'), wherein the first set of busses is configured to provide the beamforming delays of the TR cells of the even columns; and
a second hardware unit for controlling the beamforming delays of the TR cells (<NUM>) of the odd columns (<NUM>') comprising a second set of busses (<NUM>) connected to the TR cells (<NUM>) of the odd columns (<NUM>') but unconnected to the TR cells (<NUM>) of the even columns (<NUM>), wherein the second set of busses is configured to provide the beamforming delays of the TR cells of the odd columns, and
wherein the beamforming delays provided by the first set of busses and the beamforming delays provided by the second set of busses at least differ in a time-constant delay function that depends on the offset between the even and odd columns (<NUM>, <NUM>') in the column direction (y), thereby enabling further delays to be controlled as if the CMUT cells (<NUM>) were realized as rectangular or quadratic cells in an evenly distributed array without offsets between the even and odd columns (<NUM>, <NUM>').