Ring Array Transducer

Abstract

A transducer ring array for volumetric ultrasound imaging. The ring array includes a plurality of first channels, each including an outer CMUT drum and an inner CMUT drum connected via a radio frequency (RF) line. A bias system for each first channel is used to apply a bias voltage to either the outer CMUT drum or the inner CMUT drum of that channel. The ring array also includes a plurality of second channels, each including an outer CMUT drum. The outer CMUT drums of both the first and second channels form an outer ring having an outer diameter, and the 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 ultrasound transducers, and more particularly to the field of ring array transducers. [Background technology]

[0002] Ring array transducers enable volumetric imaging with a relatively small number of channels compared to conventional two-dimensional (2D) array transducers. This means fewer cables need to be connected to the transducer. More generally, ring arrays are considered the simplest transducer shape that combines three-dimensional (3D) imaging with a small number of channels. Therefore, a smaller number of elements reduces the computational load, making it possible to acquire 3D images with, for example, just 64 channels.

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

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

[0005] U.S. Patent No. 7,257,051 B2 discloses an integrated switch matrix for reconfiguring subelements of a mosaic sensor array to form elements. The switch matrix configuration is fully programmable and includes access switches connecting the subelements to bus lines and matrix switches connecting the subelements. Each subelement has a unit switch cell that includes 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 switches and matrix switches are capable of storing control data representing the current state of the switch, the control data including a data bit input to an on / off circuit incorporated in the control circuit. Summary of the Invention

[0006] The invention is defined by the claims.

[0007] In an example according to one aspect of the present invention, there is provided a transducer ring array for volumetric ultrasound imaging, the ring array comprising: a plurality of first channels, each of the first channels comprising an outer CMUT drum and an inner CMUT drum connected via a radio frequency (RF) line; a bias system for applying a bias voltage to either the outer CMUT drum or the inner CMUT drum for each first channel; a plurality of second channels, each second channel comprising at least one outer CMUT drum only; The outer CMUT drums of both the first and second channels form outer rings with the same outer diameter, and the inner CMUT drum of the first channel forms an inner ring with a smaller inner diameter.

[0008] Ring arrays enable 3D ultrasound imaging with a low channel count, but their focal region is limited and often does not meet the requirements of certain applications (e.g., fetal monitoring).

[0009] It has been proposed to create multi-ring arrays while keeping the channel count low. In the case of double-ring arrays, this can be achieved by providing two types of channels within 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. That is, each second channel has at least one outer CMUT drum but no inner CMUT drums.

[0010] The outer CMUT drums of both the first and second channels form an outer ring (e.g., similar to a single-ring array), and 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 long-distance imaging) or the inner CMUT drum (for close-distance imaging). This expands the focal region of the ring array without adding additional channels.

[0011] The first channel can be used to transmit and receive ultrasonic signals from either the outer CMUT drum or the inner CMUT drum.

[0012] The CMUT drum may be disposed on a substrate (e.g., a printed circuit board (PCB)). The substrate may be planar, rigid, or flexible. When disposed on a flexible substrate, the ring array may be disposed on a curved structure.

[0013] The first channel may include two or more outer CMUT drums, in which case the outer CMUT drum further away from the at least one inner CMUT drum may have a larger diameter than the outer CMUT drum closer to the at least one inner CMUT drum.

[0014] The packing density of the ring array can be increased by increasing the diameter of the CMUT drum towards the outer edge of the ring array, which is possible because the center frequency of the CMUT is approximately independent of the diameter as long as other parameters (e.g., bias voltage) remain constant.

[0015] The first channel may comprise two or more inner CMUT drums, in which case the inner CMUT drums closer to the outer CMUT drum may have a larger diameter than the inner CMUT drums further from the outer CMUT drum.

[0016] 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 or more outer CMUT drums.

[0017] Having three or more CMUT drums per channel in both the inner and outer rings increases the transmit and receive signal strength, resulting in improved quality of the acquired ultrasound data.

[0018] The bias system may comprise two individually addressable connections for each first channel, one for at least one outer CMUT drum and one for at least one inner CMUT drum.

[0019] The bias system may include a bias switching system on each first channel for switching application of a bias voltage between the at least one outer CMUT drum and the at least one inner CMUT drum of the first channel.

[0020] The present invention also provides an ultrasound imaging system, the system comprising: a transducer ring array; a controller, the controller comprising: applying a bias to at least one inner CMUT drum (e.g., one or more inner CMUT drums) of the first channel to acquire near-field ultrasonic data from the drum (e.g., the at least one inner CMUT drum of the first channel); applying a bias to at least one outer CMUT drum (e.g., one or more outer CMUT drums) of a first channel to acquire long-range ultrasonic data from the drum (e.g., the at least one outer CMUT drum of the first channel) and from at least one outer CMUT drum (e.g., the one or more outer CMUT drums) of a second channel; The near-field ultrasound data and the far-field ultrasound data are configured to combine to generate a composite three-dimensional ultrasound image.

[0021] Acquiring long-range ultrasonic data may also include applying a bias to the outer CMUT drum of the second channel.

[0022] The present invention also provides a method of manufacturing a transducer ring array, the method comprising: producing a ring array portion on a silicon wafer, the ring 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; combining two or more ring array portions to create a transducer ring array.

[0023] It is recognized that fabricating in a ring configuration results in lower yields and increased silicon usage, so it is proposed to fabricate several sections of the ring array and then combine them, which reduces the overall silicon usage and allows for individual discard of defective sections rather than the entire ring array.

[0024] Each second channel comprises only at least one outer CMUT drum, i.e. one or more outer CMUT drums, etc. However, each second channel does not include any inner CMUT drums.

[0025] The method may further include arranging the ring array portions in a ring pattern on a substrate, the substrate comprising, for each first channel, an RF line for connecting its corresponding outer CMUT drum and inner CMUT drum, and a bias system for applying a bias voltage to either the outer CMUT drum or the inner CMUT drum of each first channel.

[0026] The substrate may also include an RF line connected to the outer CMUT drum of the corresponding second channel for each second channel, and a bias system for applying a bias voltage to the outer CMUT drum of the corresponding second channel for each second channel.

[0027] The method may further include combining four or more ring array portions to create a transducer ring array.

[0028] The method may further include separating the ring array portion from the silicon wafer by etching the silicon wafer using Deep Reactive Ion Etching (DRIE) and then grinding the etched silicon wafer to separate the ring array portion.

[0029] This is particularly advantageous when the ring array portion is non-rectangular (eg, curved).

[0030] Of course, if the ring array portion is, for example, rectangular, it is obtained from a silicon wafer by dicing.

[0031] The method may further include testing channels of the ring array to identify acceptable ring array portions and combining two or more of the acceptable ring array portions to create a transducer ring array.

[0032] Combining two or more ring array portions may include arranging them in a ring pattern.

[0033] The step of combining two or more ring array portions may include a step of placing the ring array portions on a substrate, the substrate including RF lines connecting at least one outer CMUT drum and at least one inner CMUT drum of each first channel, and a bias system for each first channel for applying a bias voltage to either the at least one outer CMUT drum or the at least one inner CMUT drum of the first channel.

[0034] The substrate may be a flexible or non-flexible (e.g., planar) printed circuit board (PCB) depending on the application requirements of the final product, for example, the substrate may be flexible so that it can be wrapped into a curved shape.

[0035] The bias system may comprise two individually addressable connections for each first channel, one for its at least one outer CMUT drum and another for its at least one inner CMUT drum.

[0036] The bias system may comprise a bias switching system on each first channel for switching application of a bias voltage between the at least one outer CMUT drum and the at least one inner CMUT drum.

[0037] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. [Brief explanation of the drawings]

[0038] For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: [Figure 1] Figure 1 is a schematic diagram of a single ring array. [Figure 2] FIG. 2 shows the contours of the focal regions of a single ring array with different diameters. [Figure 3] FIG. 3 is a diagram showing the concept of a multi-ring array. [Figure 4] FIG. 4 shows a double ring array with 64 elements. [Figure 5] FIG. 5 shows a portion of a ring array having a densely packed ring array configuration. [Figure 6] FIG. 6 shows a silicon wafer having ring elements. [Figure 7] FIG. 7 shows a ring array assembled from four ring array sections. DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention will now be described with reference to the drawings.

[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 better understood from the following description, appended claims, and accompanying drawings. It should be understood that the drawings are 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.

[0041] The present invention provides a transducer ring array for volumetric ultrasound imaging. The ring array includes a plurality of first channels, each including an outer CMUT drum and an inner CMUT drum connected via a radio frequency (RF) line. A bias system for each first channel is used to apply a bias voltage to either the outer CMUT drum or the inner CMUT drum of that first channel. The ring array also includes a plurality of second channels, each including an outer CMUT drum. The outer CMUT drums of both the first and second channels form an outer ring having an outer diameter, and the inner CMUT drum of the first channel forms an inner ring having a smaller inner diameter.

[0042] 1 is a schematic diagram of a single ring array 100. This single ring array 100 has 64 channels 102, each having five CMUT drums 104 connected to radio frequency (RF) lines 106. Therefore, only 64 channels need to be analyzed.

[0043] There is growing interest in pregnancy monitoring solutions, especially for high-risk pregnancies with pre-existing complications.

[0044] However, a single ring array 100 has proven unsuitable for fetal imaging / monitoring due to its limited focal region. As a rule of thumb, the average pitch between the CMUT drums 104 limits the maximum center frequency of the ring array. It has been found that a single ring array with a focal region of up to approximately 280 mm requires a ring diameter of approximately 12 mm. If the number of channels 102 is kept at 64, the maximum center frequency is limited to approximately 2.2 MHz (assuming two-pulse excitation and 40% bandwidth, which is acceptable for monitoring applications).

[0045] Pitch generally refers to the period of a periodic structure. In this case, the distance between adjacent CMUT drums is called the pitch. In the case of a ring array, the pitch (distance) is not constant; the outer CMUT elements are located farther apart than the inner ones. However, assuming a "thin" ring array, the average pitch can be used.

[0046] The relationship between pitch, operating frequency, and image quality is well known from both experimental and theoretical points of view. For phased arrays, the general rule is that the pitch should be smaller than the wavelength λ to avoid grating lobes that degrade image quality. Typically, the pitch is chosen to be smaller than half the wavelength λ. However, pitches slightly larger than this are sometimes acceptable. For example, pitches of 0.7λ have been used.

[0047] In this case, the wavelength is the upper limit wavelength λ up The transducer is often excited in a pulsed mode, and its frequency spectrum has a certain width. The center frequency is F c , the upper frequency limit is F up , the lower limit frequency is F low The bandwidth is called F up and F low A wide bandwidth means that the transducer responds to a wide frequency range, providing a good, clear image.

[0048] The relationship between frequency and wavelength is F = λc (c is the speed of sound). Therefore, the higher the frequency, the shorter the wavelength, and the smaller the pitch required. The speed of sound in water is approximately 1500 m / s. Therefore, the higher the desired upper frequency limit, the smaller the pitch required.

[0049] However, due to the limited focal region, the minimum depth of such a single ring array would be limited to about 120 mm. For fetal monitoring, a penetration depth ranging from about 280 mm to about 80 mm is required.

[0050] 2 shows contours 202, 204, 206 of the focal regions of single ring arrays of different diameters, with the x-axis representing lateral distance (mm) relative to the single ring array and the z-axis representing depth (mm) relative to the single ring array.

[0051] These contours are lines of equal amplitude (70% of the maximum) of the absolute pressure. The 70% contour shown is generally of interest because this is the area most suitable for imaging. Inside this contour, the pressure is at its maximum.

[0052] Contour line 202 shows the focal region for a single ring array with a diameter of 28 mm operating at a frequency of 2.5 MHz. Contour line 204 shows the focal region for a 20 mm diameter ring array at a frequency of 2.5 MHz. Contour line 206 shows the focal region for a 12 mm diameter ring array at a frequency of 2.5 MHz.

[0053] As can be seen, none of the contour lines shown provide a focal area large enough for fetal monitoring / imaging (ie, an area covering from about 80 mm to about 280 mm).

[0054] The focal areas of single ring arrays with diameters of 26 mm, 24 mm, 16 mm, and 14 mm were also measured (not shown), but none were sufficient for fetal monitoring (i.e., they did not have a focal area covering approximately 80 mm to approximately 280 mm).

[0055] Therefore, there is a need for an improved solution that provides a wider focal area (eg, for fetal monitoring) while minimizing the number of channels.

[0056] It is proposed to provide a multi-ring array in which the CMUT drums in the inner ring are connected to the same RF line as some of the CMUT drums in the outer ring, with some channels including both the inner and outer ring CMUT drums. A bias system is then provided to apply a bias to either the outer ring CMUT drum or the inner ring CMUT drum. Because the outer and inner rings have different diameters, this multi-ring array effectively has two (or more) focal regions. Combining the focal regions in this way increases the overall focal region.

[0057] This results in an effective focal region that combines the contour lines shown in Figure 2. For example, combining contour line 202 (28 mm diameter) with contour line 204 (20 mm diameter) may potentially cover a focal region from approximately 80 mm to approximately 280 mm.

[0058] Figure 3 is a conceptual diagram of a multi-ring array. In particular, Figure 3 shows a portion of a double-ring array. Five channels are shown, consisting of three first channels 301 and two second channels 302. In this example, the first channel 301 has an RF line 304 connected to six CMUT drums (three outer CMUT drums 306 and three inner CMUT drums 308). All of the outer CMUT drums 306 are connected to a bias line 310 for applying a bias to these drums. The inner CMUT drums 308 are also connected to a bias line 310 for applying a bias to these drums. Therefore, it is possible to apply a bias to either the outer CMUT drums 306 or the inner CMUT drums 308.

[0059] The second channel 302 has three outer CMUT drums 306, all connected to bias lines 310 for applying a bias to the outer CMUT drums 306. An outer ring 312 is formed from the outer CMUT drums 306 of both the first channel 301 and the second channel 302, and an inner ring 314 is formed from the inner CMUT drum 308 of the first channel 301.

[0060] The outer ring 312, which has an outer diameter, has a first focal region, and the inner ring 314, which has a smaller inner diameter, has a second focal region, so that ultrasound data from both rings can be combined to provide a larger composite focal region of ultrasound data.

[0061] Both CMUT drums can be used in collapsed or non-collapsed mode.

[0062] 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. Note that bias switching may require fewer connections than having separate bias lines.

[0063] The bias system using bias switching includes circuitry that can supply bias to only one of the outer CMUT drum 306 or the inner CMUT drum 308 of the first channel 301 .

[0064] Thus, a double or multi-ring array can provide near-field imaging (via the inner ring 314) and far-field imaging (via the outer ring 312). During near-field imaging, the bias is on for the inner CMUT drum 308 and off for the outer CMUT drum 306. In other words, during near-field imaging, it operates as if only the CMUT drum in the inner ring 314 is active and the CMUT drum in the outer ring 312 is not present at all.

[0065] During long-distance imaging, the bias is off for the inner CMUT drum 308 and on only for the outer CMUT drum 306. Thus, ultrasound data obtained from near-field and long-distance imaging can be combined to obtain composite ultrasound data. For example, near-field and long-distance images can be combined to generate a composite 3D image. This composite ultrasound data / image can cover the focal regions of both the outer ring 312 and the inner ring 314.

[0066] Thus, double or multi-ring arrays solve the focal region problem. Furthermore, the diameters of the outer ring 312 and inner ring 314 can be selected to cover the entire penetration depth, e.g., from 50 mm to 250 mm, without increasing the electronic complexity, i.e., keeping the channel count low.

[0067] 4 shows a 64-element double-ring array 400. The double-ring array 400 has 32 first channels 401 and 32 second channels 402. Each first channel 401 has five outer CMUT drums 406 and three inner CMUT drums 408 connected via RF lines. Each second channel 402 has five outer CMUT drums 406 connected via RF lines 404. A bias system (not shown) is used to bias either the outer CMUT drums 406 or the inner CMUT drums 408 of the first channel 401. This allows the double-ring array 400 to be used by switching between the outer ring (i.e., the outer CMUT drums 406 of both the first channel 401 and the second channel 402) and the inner ring (i.e., the inner CMUT drum 408 of the first channel 401).

[0068] The double ring array 400 includes an integration of scanning CMUT MEMS ultrasound transducers. Because a single ultrasound transducer surface cannot cover the entire uterus, multiple ultrasound transducers (e.g., distributed across the abdomen for fetal monitoring) are used and integrated into a conformal patch and / or smart textile. This double ring array has a wider overall focal range, allowing it to identify fetal movement, position, and anatomy, as well as detect maternal and fetal heart rates.

[0069] As mentioned above, a single ring array cannot cover the focal depth required for fetal monitoring. This problem is solved by using a double ring array, where either the inner or outer ring is activated with a bias voltage. This allows for a greater focal depth. Note that the number of channels does not increase and is entirely determined by the outer ring.

[0070] Please note that in Figure 4, the open circles represent a single CMUT drum (e.g., for a low-frequency CMUT drum (2.5 MHz), the typical diameter is about 350 μm). The squares on each channel represent bonding pads. Not all components (e.g., bias lines) are shown.

[0071] Multi-ring arrays can be used in professional settings, such as medical institutions, as well as non-professional settings (e.g., remote monitoring in the home).

[0072] It should be noted that although Figure 4 shows a planar double ring array, the ring array can also be placed on a flexible substrate and wrapped around a curved structure.

[0073] To further improve the performance, we propose a "densely packed ring array" geometry.

[0074] Figure 5 shows a portion of a ring array with a densely packed ring array geometry. The packing density of the ring array is increased by increasing the diameter of the CMUTs toward the edge of the array. In other words, the CMUT drum 502 farther from the center of the ring has a larger diameter than the CMUT drum 504 closer to the center.

[0075] To be quite precise, the center frequency of a CMUT is independent of its diameter as long as all other parameters, including the bias voltage, remain constant. This is true for both a CMUT drum operating in standard collapse mode and an annular ring CMUT operating in collapse mode.

[0076] Additionally, the channel geometry can be modified to improve image quality. For example, a spiral geometry is known to reduce artifacts in the resulting data and improve image quality. More generally, the channel geometry can be curved, or the CMUT diameter or channel length can be varied.

[0077] See, for example, Li, X., Gachagan, A., & Murray, P. (2020). Design of 2D sparse array transducers for anomaly detection in medical phantoms. Sensors 2020, 20(18),

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

[0078] Each channel preferably has three or more CMUT drums, which allows for high signal strength per channel for both transmission and reception.

[0079] The manufacturability of large outer rings with diameters up to 30 mm can lead to reduced yields. To improve this situation, a manufacturing method is proposed in which the ring array sections are preferentially etched from the wafer (DRIE) or separated by means of a dicing saw or stealth dicing. Tested and selected ring array sections can then be assembled onto the substrate using pick-and-place technology.

[0080] FIG. 6 shows a silicon wafer 602 having ring elements 604. The ring array portion 604 is fabricated on the silicon wafer 602 in a densely packed manner. The areas between the segments may be filled with "dummy CMUT structures" to improve processing uniformity. For example, the area 604 may be filled with dummy CMUT structures or CMUT drums within the area that makes up the ring array portion. The dummy CMUT structures may not have any electrodes or connections to other CMUT structures.

[0081] For example, DRIE etching is used to separate the segments to obtain curved ring array portions 604. DRIE etching, combined with back-grinding, can separate the ring array portions on the silicon wafer. After testing, the ring array portions are assembled on the substrate by a "pick and place" system, and the ring array is assembled.

[0082] 7 shows a ring array 700 assembled with four ring array portions 604. The substrate is not shown. More than four ring arrays may be preferred to improve packing of the ring array portions 604 on the silicon wafer.

[0083] Ring array fabrication has typical drawbacks such as low yield and high silicon usage.

[0084] The proposed assembly method overcomes these typical drawbacks of ring array fabrication by individually picking and placing the ring array elements 604 in a ring pattern, thereby improving yield while reducing silicon usage.

[0085] This arrangement may use, for example, a square wafer section. Placing a perfect circular window over the ring array portion 604 provides a uniform surface for the imaging target.

[0086] Substrates (e.g., PCBs) can be made of rigid or flexible materials depending on the application requirements of the final product.

[0087] It will be appreciated that this fabrication method is applicable to both single ring arrays and multi-ring arrays (eg, double ring arrays).

[0088] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the 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.

[0089] 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.

[0090] The term CMUT as used herein stands for Capacitive Micromachined Ultrasonic Transducer.

[0091] It should be noted that when the term "adapted to" is used in the claims or specification, it is intended to be equivalent to the term "configured to."

[0092] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. 1. A transducer ring array for volumetric ultrasound imaging, comprising: a plurality of first channels (301), each of which 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 applying a bias voltage to either the at least one outer CMUT drum or the at least one inner CMUT drum for each first channel; a plurality of second channels (302), each second channel comprising at least one outer CMUT drum only; A transducer ring array, 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. 2. The transducer ring array of claim 1, wherein the first channel comprises two or more outer CMUT drums, and an outer CMUT drum further away from the at least one inner CMUT drum has a larger diameter than an outer CMUT drum closer to the at least one inner CMUT drum.

3. 3. The transducer ring array of claim 2, wherein the first channel comprises two or more inner CMUT drums, the inner CMUT drums closer to the outer CMUT drum having a larger diameter than the inner CMUT drums further from the outer CMUT drum.

4. The transducer ring array of any one of claims 1 to 3, wherein the first channel comprises three or more outer CMUT drums and three or more inner CMUT drums, and the second channel comprises three or more outer CMUT drums.

5. 5. The transducer ring array of claim 1, wherein the bias system comprises two individually addressable connections for each first channel, one for the at least one outer CMUT drum and the other for the at least one inner CMUT drum.

6. The transducer ring array of any one of claims 1 to 4, wherein the bias system comprises a bias switching system on each first channel for switching application of a bias voltage between the at least one outer CMUT drum and the at least one inner CMUT drum of the first channel.

7. A transducer ring array according to any one of claims 1 to 6; a controller, the controller comprising: acquiring near-field ultrasonic 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; applying a bias to the at least one outer CMUT drum of the first channel to acquire long-range ultrasonic 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; An ultrasound imaging system configured to combine the near-field ultrasound data and the far-field ultrasound data to generate a composite three-dimensional ultrasound image.

8. 1. A method of manufacturing a transducer ring array, comprising: producing a ring array portion (604) on a silicon wafer (602), the ring 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; combining two or more of said ring array portions to create said transducer ring array.

9. The method of claim 8 , further comprising combining four or more of the ring array portions to create the transducer ring array.

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

11. 11. The method of claim 8, further comprising testing channels of the ring array to identify acceptable ring array portions, and combining two or more of the acceptable ring array portions to create the transducer ring array.

12. The method of any one of claims 8 to 11, wherein combining the two or more ring array portions comprises arranging the ring array portions in a ring pattern.

13. 13. The method of claim 8, wherein the step of combining the two or more ring array portions includes a step of disposing the ring array portions on a substrate, the substrate including: 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 first channel that applies a bias voltage to either the at least one outer CMUT drum or the at least one inner CMUT drum of each first channel.

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

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

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