Integrated ultrasound imaging systems based on capacitive micromachined ultrasonic transducer arrays
31 Oct 2005-pp 704-707
TL;DR: In this article, the authors present two CMUT-based imaging systems, one for use within a 5-mm endoscopic channel and the other for use in catheterbased imaging applications.
Abstract: Capacitive micromachined ultrasonic transducers (CMUTs) overcome many limitations of existing ultrasound transducer technologies enabling new applications of ultrasound, especially for medical imaging and treatment. Some of the most important of these advancements are the ability to fabricate transducer arrays with two dimensional geometries and high operating frequencies. Over the past decade, extensive research has been conducted on the fabrication, characterization, and modelling of CMUTs. Current research efforts focus on the integration of CMUTs in systems for new medical imaging tools. This paper briefly reviews CMUT technology and presents imaging results from two CMUT-based imaging systems. The first system is designed for use within a 5-mm endoscopic channel and is based on a two dimensional, 16-element times 16-element, 5-MHz CMUT array. To provide a means of integrating the CMUT array with electronics, each element of the array is connected to a flip-chip bond pad on the back side of the array via a through-wafer interconnect. The array is flip-chip bonded to a custom-designed integrated circuit (IC) that comprises the frontend circuitry for the transducer elements. The array and IC are connected to an FPGA-based data acquisition system that can acquire volumetric imaging data in real time. Volumetric pulse-echo and photoacoustic images obtained with this system are presented. The second system is based on a 64-element, 20-MHz, 2-mm diameter CMUT ring array. This array is also designed for use in catheter-based imaging applications. Ring arrays have the advantage of providing space in the center for a guidewire or other catheter-based instrument. Volumetric images obtained with the ring-array system are also presented
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01 Nov 2010TL;DR: In this article, a design and a fabrication method for an intravascular imaging and therapeutic catheters for combined ultrasound, photoacoustic, and elasticity imaging and for optical and/or acoustic therapy of hollow organs and diseased blood vessels and tissues are disclosed.
Abstract: A design and a fabrication method for an intravascular imaging and therapeutic catheters for combined ultrasound, photoacoustic, and elasticity imaging and for optical and/or acoustic therapy of hollow organs and diseased blood vessels and tissues are disclosed in the present invention. The invention comprises both a device—optical fiber-based intravascular catheter designs for combined IVUS/IVPA, and elasticity imaging and for acoustic and/or optical therapy—and a method of combined ultrasound, photoacoustic, and elasticity imaging and optical and/or acoustic therapy. The designs of the catheters are based on single-element catheter-based ultrasound transducers or on ultrasound array-based units coupled with optical fiber, fiber bundles or a combination thereof with specially designed light delivery systems. One approach uses the side fire fiber, similar to the one utilized for biomedical optical spectroscopy. The second catheter design uses the micro-optics in the manner of a probe for optical coherent tomography.
80 citations
TL;DR: A 40-MHz, 64-element phased-array transducer packaged in a 2.5 × 3.1 mm endoscopic form factor was developed and ex vivo tissue images were generated of porcine brain tissue.
Abstract: We have developed a 40-MHz, 64-element phased-array transducer packaged in a 2.5 × 3.1 mm endoscopic form factor. The array is a forward-looking semi-kerfed design based on a 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-32%PT) single-crystal wafer with an element-to-element pitch of 38 μm. To achieve a miniaturized form factor, a novel technique of wire bonding the array elements to a polyimide flexible circuit board oriented parallel to the forwardlooking ultrasound beam and perpendicular to the array was developed. A technique of partially dicing into the back of the array was also implemented to improve the directivity of the array elements. The array was fabricated with a single-layer P(VDF-TrFE)-copolymer matching layer and a polymethylpentene (TPX) lens for passive elevation focusing to a depth of 7 mm. The two-way-6-dB pulse bandwidth was measured to be 55% and the average electromechanical coupling (keff) for the individual elements was measured to be 0.62. The one-way -6-dB directivities from several array elements were measured to be ±20°, which was shown to be an improvement over an identical kerfless array. The -3-dB elevation focus resulting from the TPX lens was measured to be 152 μm at the focal depth, and the focused lateral resolution was measured to be 80 μm at a steering angle of 0°. To generate beam profiles and images, the probe was connected to a commercial ultrasound imaging platform which was reprogrammed to allow for phased array transmit beamforming and receive data collection. The collected RF data were then processed offline using a numerical computing script to generate sector images. The radiation pattern for the beamformed transmit pulse was collected along with images of wire phantoms in water and tissue-equivalent medium with a dynamic range of 60 dB. Finally, ex vivo tissue images were generated of porcine brain tissue.
67 citations
Cites background from "Integrated ultrasound imaging syste..."
...I recent years, there has been significant progress in developing fully sampled, forward-looking high-frequency linear-array transducers [1]–[12] with some groups developing complete imaging systems [13]–[18]....
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01 Nov 2008
TL;DR: This paper discusses design of CMOS-ASICs for monolithic integration of CMUT arrays by post-CMOS fabrication, and presents initial characterization of the CMOS electronics and pulse-echo measurements obtained post- CMOS fabricated CMUT elements.
Abstract: This paper discusses design of CMOS-ASICs for monolithic integration of CMUT arrays by post-CMOS fabrication. We describe design strategies for monolithic integration and demonstrate the advantages of CMUT-on-CMOS approach. On the same wafer, separate sets of IC cells are designed to interface different types of CMUT arrays for IVUS and ICE applications. Circuit topologies include resistive feedback transimpedance amplifiers on the receiver side, along with multiplexers and buffers. Gains and bandwidths of receiving amplifiers are optimized separately to fit different array specifications such as number of elements, element size and operation bandwidth. To drive CMUTs a high voltage pulser array is designed in the same 3.3V unmodified CMOS technology by combining existing technological layers in an unconventional way. CMUT arrays are then built on top of the custom made 8" wafer containing these circuits fabricated in a 0.35 mum standard CMOS process. We present initial characterization of the CMOS electronics and pulse-echo measurements obtained post-CMOS fabricated CMUT elements.
52 citations
Additional excerpts
...Hybrid integration of CMUT array and CMOS electronics through flip-chip bonding is demonstrated in [1]....
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TL;DR: This paper explores the advantages of the CMUT larger bandwidth in PA imaging by carrying out an experimental comparative study using various CMUT and PZT probes from different research laboratories and manufacturers.
48 citations
TL;DR: In this paper, an analytical capacitive micromachined ultrasonic transducer (CMUT) model is proposed that couples three physical domains using plate vibration and radiation theories, and the model accounts for the spatial distribution of the normal deflection over the CMUT cell membrane.
Abstract: This paper proposes an analytical capacitive micromachined ultrasonic transducer (CMUT) model that couples three physical domains using plate vibration and radiation theories. The model accounts for the spatial distribution of the normal deflection over the CMUT cell membrane. A mode superposition expression is used to capture the complicated flexural-plate-like movement. We compare the results of analytical calculations using the proposed plate model with those of finite-element modeling, together with the analytical results of the piston-like models. This approach shows promise for more detailed and convenient physical modeling of CMUTs, which could lead to new ways of designing and controlling them.
16 citations
References
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TL;DR: A theoretical model is proposed that agrees well with observed transducer behavior and is used to demonstrate that microfabricated ultrasonic transducers constitute an attractive alternative to piezoelectric transducers in many applications.
Abstract: The current state of novel technology, surface microfabricated ultrasonic transducers, is reported. Experiments demonstrating both air and water transmission are presented. Air-coupled longitudinal wave transmission through aluminum is demonstrated, implying a 110 dB dynamic range for transducers at 2.3 MHz in air. Water transmission experiments from 1 to 20 MHz are performed, with a measured 60 dB SNR at 3 MHz. A theoretical model is proposed that agrees well with observed transducer behavior. Most significantly, the model is used to demonstrate that microfabricated ultrasonic transducers constitute an attractive alternative to piezoelectric transducers in many applications.
616 citations
"Integrated ultrasound imaging syste..." refers methods in this paper
...Fabrication of CMUTs using the waferbonding technique begins with two wafers: a prime quality silicon wafer and an SOI wafer....
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...This paper briefly reviews the design, modelling, and fabrication of CMUTs and then presents imaging results from two transducer arrays: a 256-element 2D array with integrated frontend electronics and a 64-element annular ring array....
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...I. INTRODUCTION Capacitive micromachined ultrasonic transducers (CMUTs) were initially developed in the early 1990’s [1]....
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...A number of sacrificial layer, membrane, and substrate material combinations can be used to fabricate CMUTs....
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...A transducer equivalent circuit based on Mason’s model has been developed for CMUTs [7]....
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TL;DR: In this article, an electrical equivalent circuit model for electrostatic transducers based on the early work of Mason (1942) was designed and constructed for operation at 1.8 and 4.6 MHz.
Abstract: Airborne ultrasound has many applications such as, ranging, nondestructive evaluation, gas flow measurement, and acoustic microscopy. This paper investigates the generation and detection of ultrasound in air at a few MHz. Conventional plane piston lead zirconium titanate (PZT) based transducers perform poorly for this application due to the lack of proper matching layer materials. Electrostatic, or capacitive, transducers promise higher efficiency and broader bandwidth performance. The device structure in this work consists of a capacitor where one plate is a circular silicon nitride membrane coated with gold and the other is a rigid silicon substrate. By applying a voltage between the membrane and the silicon substrate, an electrostatic force is exerted on the membrane which sets it in motion, thus generating a sound wave in air. Presented here is an electrical equivalent circuit model for electrostatic transducers which is based on the early work of Mason (1942). The electrostatic transducers were designed and constructed for operation at 1.8 and 4.6 MHz. The transducers were fabricated using standard micromachining techniques. An optical interferometer was used to measure the peak displacement of the 1.8 MHz electrostatic transducer at 230 /spl Aring//V. A transmit-receive system was built using two electrostatic transducers. The system had a signal to noise ratio of 34 dB at a transducer separation of 1 cm. Each transducer had a 3-dB bandwidth of 20%, and a one-way insertion loss of 26 dB. There is excellent agreement between the measured device performance and theoretical predictions.
453 citations
"Integrated ultrasound imaging syste..." refers background in this paper
...Capacitive micromachined ultrasonic transducers (CMUTs) were initially developed in the early 1990’s [1]....
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TL;DR: In this paper, a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique is introduced. But the method is not suitable for large CMUTs.
Abstract: Introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 /spl mu/m). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turn-around time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage.
312 citations
"Integrated ultrasound imaging syste..." refers background or methods in this paper
...A process for CMUT fabrication based on wafer bonding also exists [3]....
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...Over the past decade, there has been extensive research exploring the fabrication [2], [3], characterization [4], and modelling [5] of CMUTs....
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...The wafer-bonding method is a relatively new CMUT fabrication technique [3] that uses a combination of bulk and surface micromachining techniques, and a different approach to cavity formation....
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TL;DR: Two types of ultrasound-mediated biophotonic imaging are reviewed–acousto-optical tomography and photo-acoustic tomography–both of which are based on non-ionizing optical and ultrasonic waves.
Abstract: This article reviews two types of ultrasound-mediated biophotonic imaging–acousto-optical tomography (AOT, also called ultrasound-modulated optical tomography) and photo-acoustic tomography (PAT, also called opto-acoustic or thermo-acoustic tomography)–both of which are based on non-ionizing optical and ultrasonic waves. The goal of these technologies is to combine the contrast advantage of the optical properties and the resolution advantage of ultrasound. In these two technologies, the imaging contrast is based primarily on the optical properties of biological tissues, and the imaging resolution is based primarily on the ultrasonic waves that either are provided externally or produced internally, within the biological tissues. In fact, ultrasonic mediation overcomes both the resolution disadvantage of pure optical imaging in thick tissues and the contrast and speckle disadvantages of pure ultrasonic imaging. In our discussion of AOT, the relationship between modulation depth and acoustic amplitude is clarified. Potential clinical applications of ultrasound-mediated biophotonic imaging include early cancer detection, functional imaging, and molecular imaging.
249 citations
"Integrated ultrasound imaging syste..." refers background in this paper
...In photoacoustic imaging, the ultrasound generated by a material illuminated by a short laser pulse is detected [12]....
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TL;DR: In this paper, several innovative steps used in fabricating surface micromachined capacitive ultrasonic immersion transducers are reported, such as membrane formation, vacuum sealing, and electrode metallization.
Abstract: In this paper, several innovative steps used in fabricating surface micromachined capacitive ultrasonic immersion transducers are reported. The investigation is focused on major steps in the device fabrication processes necessary to optimize transducer performance. Such steps include membrane formation, vacuum sealing, and electrode metallization. Three transducer membrane structures are evaluated: a nitride membrane with an oxide sacrificial layer; a polysilicon membrane with an oxide sacrificial layer; and a nitride membrane with a polysilicon sacrificial layer. Three vacuum sealing mechanisms are compared, each of which requires a different degree of lithographic sophistication, uses a particular sealing mechanism, and results in a sealed cavity. Submicrometer via sealing requires sophisticated lithography but is amenable to LPCVD nitride, LTO, and other sealing procedures. Standard g-line lithography results in vies which seal only with high sticking coefficient species, such as LTO. A novel etch channel structure, which results in lateral sealing and requires neither sophisticated lithography nor a particular sealing material, is demonstrated. Finally, the impact of electrode metallization on the impedance, bandwidth, and efficiency of the transducers is discussed. The experiments in the paper are guided by theoretical analysis and computer simulations when applicable. The new process results in optimized devices which have a broad-band 50-/spl Omega/ real part impedance in the megahertz range. A transducer dynamic range in excess of 100 dB is achieved around 4.5 MHz. An untuned transducer exhibits more than 100% bandwidth when connected to electronics with 50-/spl Omega/ input impedance. In addition, beam pattern measurement shows the immersion devices behave like uniform piston transducers and are readily suitable for array applications. The fabrication techniques and results herein reported indicate that surface micromachined ultrasonic immersion transducers are an attractive alternative to piezoelectric transducers in immersion applications.
192 citations
"Integrated ultrasound imaging syste..." refers background in this paper
...Over the past decade, there has been extensive research exploring the fabrication [2], [3], characterization [4], and modelling [5] of CMUTs....
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