Annular array transducer

Abstract

A ring array of transducers for volumetric ultrasound imaging. The ring array includes a set of first channels, each first channel including at least one outer CMUT drum and at least one inner CMUT drum connected via a radio frequency (RF) line. For each first channel, a bias system is used to apply a bias voltage to at least one outer CMUT drum or at least one inner CMUT drum of the first channel. The annular array also includes a set of second channels, each second channel including only at least one outer CMUT drum. The at least one outer CMUT drum of both the first channel and the second channel forms an outer ring having an outer diameter, and the at least one inner CMUT drum of the first channel forms an inner ring having a smaller inner diameter.

Description

Technical Field

[0001] The present invention relates to the field of ultrasonic transducers, and in particular to the field of annular array transducers. Background Art

[0002] Compared to conventional two-dimensional (2D) array transducers, annular array transducers enable volumetric imaging while using a relatively small number of channels. This means fewer cables must be connected to the transducer. More generally, annular arrays are considered the simplest transducer geometry for combining three-dimensional (3D) imaging with a small number of channels. Thus, it has been demonstrated that 3D images can be acquired using, for example, only 64 channels, as the computational load decreases with the number of elements.

[0003] For example, in Choe et al., “GPU-based real-time volumetric ultrasound image reconstruction for a ring array,” IEEE Transactions on Medical Imaging, March 18, 2013, 32(7): 1258-1264, DOI: 10.1109 / tmi.2013.2253117, a ring array with 64 elements capable of 3D imaging is shown. The ring array shown has a radius of 1.16 mm, with an imaging depth of 2 mm to 15 mm.

[0004] However, such imaging depth is generally insufficient for external imaging / monitoring applications. Furthermore, it has been recognized that the focal area of ​​such rings is limited and may not meet the requirements of external applications such as fetal monitoring.

[0005] US Pat. No. 7,257,051 B2 discloses an integrated switch matrix for reconfiguring the subelements of a mosaic sensor array to form an element. The switch matrix configuration is fully programmable and includes access switches that connect the subelements to a bus and matrix switches that connect the subelements to each other. Each subelement has a unit switch cell comprising at least one access switch, at least one matrix switch, a corresponding memory element for storing the future state of each switch, and corresponding control circuitry for each switch. The access switches and matrix switches are of a type capable of retaining control data representing the current switch state of the switches, the control data comprising data bits input to turn on / off circuitry incorporated into the control circuitry. Summary of the Invention

[0006] The present invention is defined by the claims.

[0007] According to an example according to one aspect of the present invention, there is provided a transducer annular array for volume ultrasound imaging, the annular array comprising:

[0008] a set of first channels, wherein each first channel comprises an outer CMUT drum and an inner CMUT drum connected via a radio frequency (RF) line;

[0009] a bias system for each first channel for applying a bias voltage to the outer CMUT drum or the inner CMUT drum; and

[0010] a set of second channels, wherein each second channel comprises an outer CMUT drum,

[0011] The outer CMUT drums of both the first channel and the second channel form an outer ring with the same outer diameter, and the inner CMUT drum of the first channel forms an inner ring with a smaller inner diameter.

[0012] Annular arrays enable 3D ultrasound imaging with a small number of channels. However, the focal area of ​​the annulus is limited and often does not meet the requirements of certain applications (e.g., fetal monitoring).

[0013] Therefore, we propose creating multi-ring arrays while still maintaining a small number of channels. For dual-ring arrays, this can be achieved by having two different types of channels in the array. The first channel has at least one outer CMUT drum and at least one inner CMUT drum. The second channel has only outer CMUT drums. In other words, each second channel includes at least one outer CMUT drum but no inner CMUT drum.

[0014] The outer CMUT drums of both the first and second channels form an outer ring (e.g., similar to a single-ring array), while the inner CMUT drum of the first channel forms a second inner ring. A bias system is used to activate either the outer CMUT drum (for distant imaging) or the inner CMUT drum (for nearby imaging). Thus, the focal area of ​​the annular array is increased without adding additional channels.

[0015] The first channel may be used to transmit / receive ultrasonic signals from the outer CMUT drum or the inner CMUT drum.

[0016] The CMUT drum can be placed on a substrate (e.g., a printed circuit board PCB). The substrate can be a planar rigid substrate or a flexible substrate. When placed on a flexible substrate, the annular array can be arranged / placed on a curved structure.

[0017] The first channel may include two or more outer CMUT drums, and an outer CMUT drum farther away from the at least one inner CMUT drum may have a larger diameter than an outer CMUT drum closer to the at least one inner CMUT drum.

[0018] Increasing the diameter of the CMUT drum towards the outer edge of the annular array improves the packing density of the annular array. This is possible because the center frequency of the CMUT can be approximately independent of the diameter, provided that other parameters (e.g., bias voltage) are constant.

[0019] The first channel may include two or more inner CMUT drums, and an inner CMUT drum closer to an outer CMUT drum may have a larger diameter than an inner CMUT drum farther from the outer CMUT drum.

[0020] The first channel may include three or more outer CMUT drums and three or more inner CMUT drums, and the second channel may include three outer CMUT drums of the more outer CMUT drums.

[0021] Having three or more CMUT drums per channel in both the inner and outer loops increases the transmit and receive signal strength, resulting in an increase in the quality of the acquired ultrasound data.

[0022] The biasing system may comprise two individually addressable connections for each first channel, and one connection for the at least one outer CMUT drum and one connection for the at least one inner CMUT drum.

[0023] The bias system may include a bias switching system on each first channel for switching between applying a bias voltage to at least one outer CMUT drum and at least one inner CMUT drum of the first channel.

[0024] The present invention also provides an ultrasonic imaging system, comprising:

[0025] a transducer annular array; and

[0026] A controller configured to:

[0027] obtaining proximal ultrasound data from at least one inner CMUT drum (e.g., one or more inner CMUT drums) of a first channel by applying a bias to the at least one inner CMUT drum;

[0028] obtaining remote ultrasound data from at least one outer CMUT drum (e.g., one or more outer CMUT drums) of the first channel and at least one outer CMUT drum (e.g., one or more outer CMUT drums) of the second channel by applying a bias to the at least one outer CMUT drum of the first channel; and

[0029] A composite three-dimensional ultrasound image is generated by combining the nearby ultrasound data and the distant ultrasound data.

[0030] Obtaining remote ultrasound data may further include applying a bias to an outer CMUT drum of the second channel.

[0031] The present invention also provides a method for manufacturing a transducer annular array, the method comprising:

[0032] generating an annular array portion on a silicon wafer, the annular array portion comprising at least one first channel and at least one second channel, each first channel comprising at least one outer CMUT drum and at least one inner CMUT drum, and each second channel comprising only at least one outer CMUT drum; and

[0033] Two or more annular array sections are combined to create a transducer annular array.

[0034] It has been recognised that manufacturing a ring shape results in low yields and high silicon usage. Therefore, it has been proposed to manufacture parts of the ring array and then combine these parts. This means less silicon is used overall, and any defective parts can be discarded individually, rather than having to discard the entire ring array.

[0035] Each second channel comprises only at least one outer CMUT drum, such as an outer CMUT drum or more than one outer CMUT drum.However, each second channel does not comprise any inner CMUT drum.

[0036] The method may further include placing the annular array portion in an annular pattern on a substrate, the substrate including: an RF line for each first channel, the RF line for connecting the outer CMUT drum and the inner CMUT drum of the corresponding first channel; and a bias system for each of the first channels, the bias system for applying a bias voltage to the outer CMUT drum or the inner CMUT drum of each first channel.

[0037] The substrate may further include an RF line for each second channel, the RF line connected to the outer CMUT drum of the corresponding second channel; and a bias system for each of the second channels, the bias system for applying a bias voltage to the outer CMUT drum of the corresponding second channel.

[0038] The method may further include combining four or more annular array sections to generate a transducer annular array.

[0039] The method may further include separating the annular array portion from the silicon wafer by etching the silicon wafer using deep reactive ion etching (DRIE) and grinding the etched silicon wafer to release the annular array portion.

[0040] This is particularly advantageous when the annular array portion is non-rectangular (eg, when the annular array portion is curved).

[0041] Of course, if the annular array portion is, for example, rectangular, the annular array portion can be obtained from a silicon wafer via dicing.

[0042] The method may also include testing the annular array channels to identify acceptable annular array portions and combining two or more of the acceptable annular portions to generate the transducer annular array.

[0043] Combining two or more annular array portions may include placing the annular array portions in an annular pattern.

[0044] Combining the two or more annular array sections may include placing the annular array section on a substrate, the substrate including an RF line connecting the at least one outer CMUT drum and the at least one inner CMUT drum of each first channel; and a bias system for each of the first channels, the bias system for applying a bias voltage to the at least one outer CMUT drum or the at least one inner CMUT drum of each first channel.

[0045] Depending on the end application requirements, the substrate may be a flexible or non-flexible (eg, flat) printed circuit board (PCB). For example, the substrate may be flexible so that it can be wrapped around a curved shape.

[0046] The biasing system may comprise two individually addressable connections for each first channel, and one connection for the at least one outer CMUT drum and one connection for the at least one inner CMUT drum.

[0047] The bias system may include a bias switching system on each first channel for switching between applying a bias voltage to at least one outer CMUT drum and at least one inner CMUT drum of the first channel.

[0048] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] For a better understanding of the invention, and in order to show more clearly how it may be put into practice, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0050] Figure 1 A sketch of a single ring array is shown;

[0051] Figure 2 The outlines of the focal region of a single ring array with different diameters are shown;

[0052] Figure 3 The concept of a multi-ring array is shown;

[0053] Figure 4 A double ring array with 64 elements is shown;

[0054] Figure 5 A portion of an annular array having a densely packed annular array geometry is shown;

[0055] Figure 6 A silicon wafer having a ring element is shown; and

[0056] Figure 7 An assembled annular array having four annular array sections is shown. DETAILED DESCRIPTION

[0057] The present invention will be described with reference to the accompanying drawings.

[0058] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, system, and method, are intended for illustrative purposes only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, system, and method of the present invention will be better understood from the following description, the appended claims, and the accompanying drawings. It should be understood that the drawings are merely schematic and not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings to indicate the same or similar parts.

[0059] The present invention provides an annular transducer array for volumetric ultrasound imaging. The annular array includes a set of first channels, each of which includes an outer CMUT drum and an inner CMUT drum connected via a radio frequency (RF) line. For each first channel, a bias system is used to apply a bias voltage to either the outer CMUT drum or the inner CMUT drum of the first channel. The annular array also includes a set of second channels, each of which includes an outer CMUT drum. The outer CMUT drums of both the first and second channels form an outer ring with an outer diameter, while the inner CMUT drum of the first channel forms an inner ring with a smaller inner diameter.

[0060] Figure 1 There is shown a sketch of a single ring array 100. The single ring array 100 has 64 channels 102, where each channel 102 has five CMUT drums 10 connected to a radio frequency RF line 106. Therefore, only 64 channels need to be analyzed.

[0061] There is interest in solutions for monitoring pregnancies. In particular, there is interest in solutions for monitoring high-risk pregnancies with pre-existing complications.

[0062] However, it has been found that the single-ring array 100 has a limited focal area, making it unsuitable for fetal imaging / monitoring. Based on experience, the average spacing between the CMUT drums 104 limits the maximum center frequency of the ring array. It has been found that for a single-ring array with a focal area of ​​up to approximately 280 mm, a ring diameter of approximately 12 mm is required. Keeping the number of channels 102 at 64 means that the maximum center frequency is limited to approximately 2.2 MHz (assuming double-pulse excitation and a 40% bandwidth—acceptable for monitoring purposes).

[0063] Pitch typically refers to the period of a periodic structure. In this case, the distance between adjacent CMUT drums is called the pitch. For annular arrays, the pitch (distance) is not constant: the outer CMUT elements are farther apart than the inner CMUT elements. However, assuming a "thin" annular array, we can use the average pitch.

[0064] The relationship between pitch, operating frequency, and image quality is well known, both experimentally and theoretically. For phased arrays, a general rule is that the pitch should be less than the wavelength λ to avoid grating lobes that degrade image quality. Typically, the pitch is chosen to be less than half the wavelength λ. However, slightly larger pitches are often acceptable. For example, a pitch of 0.7λ can be used.

[0065] In this case, the wavelength corresponds to the upper corner wavelength λ 上 The transducer is usually excited in a pulsed mode, which produces a spectrum with a certain width. The center frequency is called F c , the upper corner frequency is called F 上 and the lower corner frequency is called F 下 The bandwidth corresponds to F 上 and F 下 A high bandwidth means that the transducer responds to a large frequency range, which provides a good and clear image.

[0066] The relationship between frequency and wavelength is: F = λc, where c is the speed of sound. Therefore, a higher frequency means a smaller wavelength, which also means a smaller spacing is required. For water, the speed of sound is approximately 1500 m / s. Therefore, the higher the desired upper corner frequency, the smaller the required spacing.

[0067] However, due to the limited focal area, such a single ring array would be limited to a minimum depth of about 120 mm.For fetal monitoring, a penetration depth of about 280 mm to about 80 mm is required.

[0068] Figure 2The outlines 202, 204 and 206 of the focal regions of single ring arrays with different diameters are shown. The x-axis shows the lateral distance (in mm) relative to the single ring array. The z-axis shows the depth (in mm) relative to the single ring array.

[0069] The contour lines are isoamplitude lines of the absolute pressure values ​​(70% of the maximum value). The indicated 70% contour line is usually of interest, as this part is best suited for imaging. Within this contour line, the pressure is maximum.

[0070] Contour line 202 shows the focal region of a single ring array with a diameter of 28 mm at 2.5 MHz. Contour line 204 shows the focal region of a single ring array with a diameter of 20 mm at 2.5 MHz. Contour line 206 shows the focal region of a single ring array with a diameter of 12 mm at 2.5 MHz.

[0071] As can be seen, none of the outlines shown provide a large enough focal area for fetal monitoring / imaging (ie, covering about 80 mm to about 280 mm).

[0072] Focal areas for single ring arrays with diameters of 26 mm, 24 mm, 16 mm, and 14 mm were also determined (not shown). None of these single ring arrays proved adequate for fetal monitoring (i.e., had a focal area covering approximately 80 mm to approximately 280 mm).

[0073] Therefore, there is a need for an improved solution that provides a higher focal area (eg, for fetal monitoring) while keeping the number of channels to a minimum.

[0074] It is proposed to provide a multi-ring array in which the inner ring(s) of the CMUT drum(s) are connected to the same RF line as portions of the outer ring of the CMUT drum(s), such that some channels include both the inner ring(s) of the CMUT drum(s) and the outer ring of the CMUT drum(s). A biasing system is then provided to bias either the outer ring of the CMUT drum or the inner ring of the CMUT drum(s). The outer ring and the inner ring(s) will have different diameters, such that the multi-ring array essentially has two (or more) focal regions. Thus, the combination of the focal regions provides an increased total focal region.

[0075] This resulted from Figure 2 The combination of the outlines shown constitutes an effective focal area. For example, outline 202 (having a diameter of 28 mm) and outline 204 (having a diameter of 20 mm) when combined can cover a focal area of ​​about 80 mm to about 280 mm.

[0076] Figure 3 The concept of a multi-ring array is shown. In particular, Figure 3A portion of a dual-ring array is shown. Five channels are shown: three first channels 301 and two second channels 302. In this example, the first channel 301 has RF lines 304 connected to six CMUT drums: three outer CMUT drums 306 and three inner CMUT drums 308. The outer CMUT drums 306 are all connected to bias lines 310, which are used to bias the outer CMUT drums 306. The inner CMUT drums 308 are also connected to bias lines 310, which are used to bias the inner CMUT drums 308. Thus, bias can be applied to either the outer CMUT drums 306 or the inner CMUT drums 308.

[0077] The second channel 302 has three outer CMUT drums 306, all connected to a bias line 310 for applying a bias to the outer CMUT drums 306. An outer ring 312 is formed by the outer CMUT drums 306 of both the first channel 301 and the second channel 302, and an inner ring 314 is formed by the inner CMUT drum 308 of the first channel 301.

[0078] The outer ring 312 having an outer diameter will have a first focal region, and the inner ring 314 having a smaller inner diameter will have a second focal region. Thus, ultrasound data obtained from the two rings can be combined to provide ultrasound data for a larger combined focal region.

[0079] Either of the CMUT drums can be used in either collapsed or non-collapsed mode.

[0080] Bias switching between the rings can be used to select the appropriate ring. The data can then be combined into a single composite image. Alternatively, separate bias lines can be used for the outer CMUT drum 306 and the inner CMUT drum 308. It should be noted that bias switching may require fewer connections than having separate bias lines.

[0081] A bias system using bias switching involves a circuit capable of providing bias to only the outer CMUT drum 306 or the inner CMUT drum 308 of the first channel 301 .

[0082] Thus, a dual-ring or multi-ring array can provide both near imaging (using the inner ring 314) and far imaging (using the outer ring 312). For near imaging, the bias is turned on for the inner CMUT drum 308 and off for the outer CMUT drum 306. In other words, during near imaging, it is as if only the CMUT drum of the inner ring 314 is active and the CMUT drum of the outer ring 312 does not exist at all.

[0083] For distant imaging, the bias is turned off for the inner CMUT drum 308 and on only for the outer CMUT drum 306. Thus, composite ultrasound data can be obtained by combining ultrasound data obtained from both nearby and distant imaging. For example, a composite 3D image can be generated by combining images from nearby and distant imaging. Thus, the composite ultrasound data / image will be able to cover the focal regions of both the outer ring 312 and the inner ring 314.

[0084] Thus, the dual or multi-ring array solves the focal region problem. Additionally, the diameters of the outer and inner rings 312, 314 can be selected to cover the entire penetration depth, for example, from 50 mm to 250 mm, without increasing the complexity of the electronics (i.e., the number of channels remains low).

[0085] Figure 4 A dual-ring array 400 with 64 elements is shown. The dual-ring array 400 has 32 first channels 401 and 32 second channels 402. Each of the first channels 401 has five outer CMUT drums 406 and three inner CMUT drums 408 connected via RF lines. Each of the second channels 402 has five outer CMUT drums 406 connected via RF lines 404. A biasing system (not shown) is used that can bias either the outer CMUT drums 406 or the inner CMUT drums 408 of the first channels 401. Thus, the dual-ring array 400 can switch between using the outer ring (i.e., the outer CMUT drums 406 of both the first and second channels 401, 402) or the inner ring (i.e., the inner CMUT drum 308 of the first channel 401).

[0086] The dual-loop array 400 includes an integration of scanning CMUT MEMS ultrasound transducers. Because a single US transducer plane cannot cover the entire uterus, multiple US transducers are used (e.g., distributed across the abdomen for fetal monitoring) and integrated into a conformal patch and / or smart fabric. Thus, the dual-loop array can be used to identify fetal movement, position, and anatomy, and detect both maternal and fetal heart rates due to its larger overall focal range.

[0087] As previously discussed, a single-ring array cannot cover the focal depth required for fetal monitoring. This problem is addressed by using a dual-ring array, where either the inner or outer ring is activated by applying a bias voltage. Thus, a greater depth of focus is achieved. Note that the number of channels is not increased and is determined entirely by the outer ring.

[0088] Note that for Figure 4 The hollow circles represent individual CMUT drums (e.g., with a typical diameter of about 350 microns for a low-frequency CMUT drum of about 2.5 MHz). The squares on each channel represent bonding pads. Not all components (e.g., bias lines) are shown.

[0089] Multi-ring arrays can be used in both professional and non-professional settings (eg, for remote monitoring in the home).

[0090] Notice, Figure 4 A flat double ring array is shown. However, the ring array can be placed on a flexible substrate so that it can be wrapped around a curved structure.

[0091] A “densely packed annular array” geometry is further proposed to improve the performance.

[0092] Figure 5 A portion of an annular array with a densely packed annular array geometry is shown. The packing density of an annular array can be improved by having CMUT diameters that increase toward the edges of the array. In other words, the CMUT drums 502 farther from the center of the annulus have a larger diameter than the CMUT drums 504 closer to the center of the annulus.

[0093] Approximately, the center frequency of a CMUT is independent of the diameter, provided all other parameters, including the bias voltage, are constant. This is true for both a drum CMUT operated in standard collapsed mode and an annular ring CMUT operated in collapsed mode.

[0094] Additionally, the shape of the channel can be varied to improve image quality. For example, it is known that using a spiral shape can reduce the number of artifacts in the resulting data, thereby providing improved image quality. More generally, the shape of the channel can be curved, the diameter of the CMUT can be variable, and / or the length of the channel can be variable.

[0095] See, e.g., Li, X., Gachagan, A., and Murray, P. (2020), Design of 2D sparsearray transducers for anomaly detection in medical phantoms, Sensors 2020, 20(18),

[5370] , https: / / doi.org / 10.3390 / s20185370 , where various array configurations / shapes are discussed.

[0096] It is preferred to have more than 3 CMUT drums per channel, as this provides higher signal strength per channel for both transmit and receive.

[0097] Manufacturing large outer rings with diameters of up to 30 mm can lead to yield losses. Therefore, a manufacturing method has been proposed to remedy this situation, in which the annular array sections are preferentially etched out of the wafer (DRIE) or separated by means of, for example, a dicing saw or stealth dicing. The tested and selected annular array sections can then be assembled onto a substrate using pick-and-place technology.

[0098] Figure 6 A silicon wafer 602 with an annular element 604 is shown. The annular array portion 604 is fabricated in a densely packed manner on the silicon wafer 602. The areas between segments can be filled with "dummy CMUT structures" to improve processing uniformity. For example, the area 604 can be filled with a dummy CMUT structure or with a CMUT drum in the area containing the annular array portion 604. The dummy CMUT structure may not have connections to other CMUT structures or may not have electrodes.

[0099] For example, DRIE etching is used to separate the segments so that the curved annular array portion 604 can be obtained. DRIE etching can be combined with back grinding to separate the annular array portions on the silicon wafer. After testing, the annular array portions are combined on a substrate by a "pick and place" system to assemble the annular array.

[0100] Figure 7 An assembled annular array 700 is shown having four annular array sections 604. The substrate is not shown. To improve packaging of the annular array sections 604 on a silicon wafer, more than four annular arrays may be preferred.

[0101] Disadvantages of manufacturing annular arrays typically include low yields and high silicon usage.

[0102] The proposed assembly overcomes the typical drawbacks of manufacturing annular arrays because the annular array elements 604 are individually picked and placed in an annular pattern. This increases yield while reducing silicon usage.

[0103] This arrangement may for example use square wafer segments.Complete circular windows may be placed on the annular array portion 604 to form a uniform surface of the portion to be imaged.

[0104] The substrate (eg, PCB) can be made of a non-flexible or flexible substrate, depending on the end application requirements.

[0105] It should be understood that the fabrication method can be used for both single-ring arrays and multi-ring arrays (eg, dual-ring arrays).

[0106] Variations on the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, by studying the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0107] 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.

[0108] The term CMUT used in this application stands for Capacitive Micromachined Ultrasonic Transducer.

[0109] If the term "adapted to" is used in the claims or the specification, it should be noted that the term "adapted to" is intended to be equivalent to the term "configured to."

[0110] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A transducer annular array for volumetric ultrasound imaging, the annular array comprising: a set of first channels (301), wherein each first channel comprises at least one outer CMUT drum (306) and at least one inner CMUT drum (308) connected via a radio frequency (RF) line (304); a bias system (310) for each first channel for applying a bias voltage to the at least one outer CMUT drum or the at least one inner CMUT drum; and a set of second channels (302), wherein each second channel comprises only at least one outer CMUT drum, wherein the at least one outer CMUT drum of both the first channel and the second channel forms an outer ring (312) having the same outer diameter, and the at least one inner CMUT drum of the first channel forms an inner ring (314) having a smaller inner diameter.

2. The transducer annular array of claim 1 , wherein the first channel comprises two or more outer CMUT drums, and wherein the outer CMUT drum farther away from the at least one inner CMUT drum has a larger diameter than the outer CMUT drum closer to the at least one inner CMUT drum. 3 . The transducer annular array of claim 2 , wherein the first channel comprises two or more inner CMUT drums, and wherein the inner CMUT drum closer to the outer CMUT drum has a larger diameter than the inner CMUT drum farther from the outer CMUT drum. 4 . The transducer annular array according to claim 1 , wherein the first channel comprises three or more outer CMUT drums and three or more inner CMUT drums, and wherein the second channel comprises three outer CMUT drums of the plurality of outer CMUT drums.

5. The transducer annular array according to any one of claims 1 to 4, wherein the bias system comprises two individually addressable connections for each first channel, one connection for the at least one outer CMUT drum and one connection for the at least one inner CMUT drum.

6. The transducer annular array according to any one of claims 1 to 4, wherein the bias system comprises a bias switching system on each first channel, the bias switching system being configured to switch between applying a bias voltage to the at least one outer CMUT drum and the at least one inner CMUT drum of the first channel.

7. An ultrasound imaging system, comprising: The transducer annular array according to any one of claims 1 to 6; and A controller configured to: obtaining proximate ultrasound data from the at least one inner CMUT drum of the first channel by applying a bias to the at least one inner CMUT drum; obtaining remote ultrasound data from the at least one outer CMUT drum of the first channel and the at least one outer CMUT drum of the second channel by applying a bias to the at least one outer CMUT drum of the first channel; as well as A composite three-dimensional ultrasound image is generated by combining the nearby ultrasound data and the distant ultrasound data.

8. A method for manufacturing a transducer annular array, the method comprising: Generating an annular array portion (604) on a silicon wafer (602), the annular array portion comprising at least one first channel (301) and at least one second channel (302), each first channel comprising at least one outer CMUT drum (306) and at least one inner CMUT drum (308), and each second channel comprising only at least one outer CMUT drum; as well as Two or more annular array sections are combined to generate the transducer annular array.

9. The method of claim 8, further comprising combining four or more annular array sections to generate the transducer annular array.

10. The method according to claim 8 or 9, further comprising separating the annular array portion from the silicon wafer by etching the silicon wafer using deep reactive ion etching (DRIE) and grinding the etched silicon wafer to release the annular array portion.

11. The method of any one of claims 8 to 10, further comprising testing the annular array channels to identify acceptable annular array portions and combining two or more of the acceptable annular portions to generate the transducer annular array.

12. The method of any one of claims 8 to 11, wherein combining the two or more annular array portions comprises: The annular array sections are positioned in an annular pattern.

13. The method of any one of claims 8 to 12, wherein combining the two or more annular array portions comprises: The annular array portion is placed on a substrate, the substrate comprising: an RF line (304) connecting the at least one outer CMUT drum and the at least one inner CMUT drum of each first channel; and a bias system (310) for each of the first channels, the bias system for applying a bias voltage to the at least one outer CMUT drum or the at least one inner CMUT drum of each first channel.

14. The method of claim 13, wherein the biasing system comprises two individually addressable connections for each first channel, one connection for the at least one outer CMUT drum and one connection for the at least one inner CMUT drum.

15. The method of claim 13, wherein the bias system comprises a bias switching system on each first channel, the bias switching system for switching between applying a bias voltage to the at least one outer CMUT drum and the at least one inner CMUT drum of the first channel.

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