Row-Column Capacitive Micromachined Ultrasonic Transducers for Medical Imaging

Ultrasound imaging plays an important role in modern medical diagnosis. Recent progress in real-time 3-D ultrasound imaging can offer critical information such as the accurate estimation of organ, cyst, or tumour volumes. However, compared to conventional 2-D ultrasound imaging, the large amount of data and circuit complexity found in 3-D ultrasound imaging results in very expensive systems. Therefore, a simplification scheme for 3-D ultrasound imaging technology is needed for a more wide-spread use and to advance clinical development of volumetric ultrasound. Row-column addressing 2-D array is one particular simplification scheme that requires only N + N addressing lines to activate each element in an N × N array. As a result, the fabrication, circuit, and processing complexity dramatically decrease. Capacitive micromachined ultrasonic transducer (CMUT) technology was chosen to fabricate the array as it offers micro-precision fabrication and a wide bandwidth, which make it an attractive transducer technology. The objective of this thesis is to investigate and demonstrate the imaging potential of row-column CMUT arrays for RT3D imaging. First, the motivation, physics, and modelling of both CMUTs and row-column arrays are described, followed by the demonstration of a customized row-column CMUT pseudo-real-time 3-D imaging system. One particular limitation about row-column arrays discovered as part of this dissertation work is the limited field-of-view of the row-column arrays’ imaging performance. A curved row-column CMUT array was proposed to improve the field-of-view, and the resulting modelling of the acoustic field and simulated reconstructed image are presented. Furthermore, a new fabrication process was proposed to construct a curved row-column CMUT array. The resulting device was tested to demonstrate its flexibility to achieve the necessary curvature. Finally, a new wafer bonding process is introduced to tackle the next generation of RC-CMUT fabrication. Many of the new fabrication techniques reported in this work are useful for CMUT fabrication engineers. The analysis on row-column array also provides additional insights for 2-D array simplification research

Capacitive Micromachined Ultrasound Transducers for Non-Destructive Testing Applications

The need for using ultrasound non-destructive testing (NDT) to characterize, test and detect flaws within metals, led us to utilize Capacitive Micromachined Ultrasound Transducers (CMUTs) in the ultrasound NDT field. This is due to CMUT's large bandwidths and high receive sensitivity, to be a suitable substitute for piezoelectric (PZT) transducers in NDT applications. The basic operational test of CMUTs, conducted in this research, was carried out based on a pulse-echo technique by propagating acoustic pulses into an object and analyzing the reflected signals. Thus, characterizing the tested material, measuring its dimension, and detecting flaws within it can be achieved. Throughout the course of this research, the fundamental parameters of CMUT including pull-in voltage and resonance frequency were initially calculated analytically and using Finite Element Analysis (FEA). Afterward, the CMUT was fabricated out of two mechanically bonded wafers. The device's movable membrane (top electrode) and stationary electrode (bottom electrode) were made out of Boron-doped Silicon. The two electrodes were electrically isolated by an insulation layer containing a sealed gap. The CMUT was then tested and characterized to analyze its performance for NDT applications. In-immersion characterization revealed that the 2.22 MHz CMUT obtained a -6 dB fractional bandwidth of 189%, and a receive sensitivity of 31.15 mV/kPa, compared to 45% and 4.83 mV/kPa of the PZT probe. A pulse-echo test, performed to examine an aluminum block with and without flaws, showed success in distinguishing the surfaces and the flaws of the tested sample

MEMS Technology for Biomedical Imaging Applications

Biomedical imaging is the key technique and process to create informative images of the human body or other organic structures for clinical purposes or medical science. Micro-electro-mechanical systems (MEMS) technology has demonstrated enormous potential in biomedical imaging applications due to its outstanding advantages of, for instance, miniaturization, high speed, higher resolution, and convenience of batch fabrication. There are many advancements and breakthroughs developing in the academic community, and there are a few challenges raised accordingly upon the designs, structures, fabrication, integration, and applications of MEMS for all kinds of biomedical imaging. This Special Issue aims to collate and showcase research papers, short commutations, perspectives, and insightful review articles from esteemed colleagues that demonstrate: (1) original works on the topic of MEMS components or devices based on various kinds of mechanisms for biomedical imaging; and (2) new developments and potentials of applying MEMS technology of any kind in biomedical imaging. The objective of this special session is to provide insightful information regarding the technological advancements for the researchers in the community

Integrated Electronics for Wireless Imaging Microsystems with CMUT Arrays

Integration of transducer arrays with interface electronics in the form of single-chip CMUT-on-CMOS has emerged into the field of medical ultrasound imaging and is transforming this field. It has already been used in several commercial products such as handheld full-body imagers and it is being implemented by commercial and academic groups for Intravascular Ultrasound and Intracardiac Echocardiography. However, large attenuation of ultrasonic waves transmitted through the skull has prevented ultrasound imaging of the brain. This research is a prime step toward implantable wireless microsystems that use ultrasound to image the brain by bypassing the skull. These microsystems offer autonomous scanning (beam steering and focusing) of the brain and transferring data out of the brain for further processing and image reconstruction. The objective of the presented research is to develop building blocks of an integrated electronics architecture for CMUT based wireless ultrasound imaging systems while providing a fundamental study on interfacing CMUT arrays with their associated integrated electronics in terms of electrical power transfer and acoustic reflection which would potentially lead to more efficient and high-performance systems. A fully wireless architecture for ultrasound imaging is demonstrated for the first time. An on-chip programmable transmit (TX) beamformer enables phased array focusing and steering of ultrasound waves in the transmit mode while its on-chip bandpass noise shaping digitizer followed by an ultra-wideband (UWB) uplink transmitter minimizes the effect of path loss on the transmitted image data out of the brain. A single-chip application-specific integrated circuit (ASIC) is de- signed to realize the wireless architecture and interface with array elements, each of which includes a transceiver (TRX) front-end with a high-voltage (HV) pulser, a high-voltage T/R switch, and a low-noise amplifier (LNA). Novel design techniques are implemented in the system to enhance the performance of its building blocks. Apart from imaging capability, the implantable wireless microsystems can include a pressure sensing readout to measure intracranial pressure. To do so, a power-efficient readout for pressure sensing is presented. It uses pseudo-pseudo differential readout topology to cut down the static power consumption of the sensor for further power savings in wireless microsystems. In addition, the effect of matching and electrical termination on CMUT array elements is explored leading to new interface structures to improve bandwidth and sensitivity of CMUT arrays in different operation regions. Comprehensive analysis, modeling, and simulation methodologies are presented for further investigation.Ph.D

Capacitive Micromachined Ultrasonic Transducers for Non-destructive Testing Applications

Ultrasound is a popular technique for industrial non-destructive testing (NDT) applications. By sending ultrasonic waves into an object and observing the amplitude and the delay of the reflected or transmitted waves, one can characterize the material, measure the thickness of the object, and detect discontinuities (flaws) as well as the size, location, and orientation of the defects in the object. Traditionally, ultrasonic transducers for NDT are made with piezoelectric crystals. Meanwhile, another class of ultrasonic transducers known as capacitive micromachined ultrasonic transducers (CMUTs) have become popular in medical ultrasound research because of their large bandwidths and other attributes that allow them to be integrated into the tip of a catheter. However, CMUTs have not been widely adopted in ultrasonic NDT applications. In this thesis, three important CMUTs characteristics that could potentially make them attractive for NDT applications are introduced and demonstrated. First, CMUTs can be beneficial to NDT because the fabrication techniques of CMUTs can easily be used to implement high-frequency, high-density phased arrays, which are essential for high resolution scanning. Surface scanning using a 2-D row-column addressed CMUT array was demonstrated. Secondly, CMUTs can be integrated with supporting microelectronic circuits, thus one can implement a highly integrated transducer system, which can be useful in structural health monitoring NDT applications. Front-end microelectronic circuits that include a transmit pulser and a receive amplifier were designed, tested, and characterized. Thirdly, CMUTs are suitable for air-coupled applications because of their low acoustic impedance at resonance. Air-coupled CMUTs fabricated in a standard RF-MEMS process were characterized and tested. This thesis concludes with an analysis of the potential usefulness of CMUTs for ultrasonic NDT. While many ultrasonic NDT applications are better off being performed using conventional piezoelectric transducers, CMUTs can and should be used in certain NDT applications that can take advantage of the beneficial characteristics of this exciting transducer technology

Capacitive Micromachined Ultrasonic Transducers for Non-destructive Testing Applications

Ultrasound is a popular technique for industrial non-destructive testing (NDT) applications. By sending ultrasonic waves into an object and observing the amplitude and the delay of the reflected or transmitted waves, one can characterize the material, measure the thickness of the object, and detect discontinuities (flaws) as well as the size, location, and orientation of the defects in the object. Traditionally, ultrasonic transducers for NDT are made with piezoelectric crystals. Meanwhile, another class of ultrasonic transducers known as capacitive micromachined ultrasonic transducers (CMUTs) have become popular in medical ultrasound research because of their large bandwidths and other attributes that allow them to be integrated into the tip of a catheter. However, CMUTs have not been widely adopted in ultrasonic NDT applications. In this thesis, three important CMUTs characteristics that could potentially make them attractive for NDT applications are introduced and demonstrated. First, CMUTs can be beneficial to NDT because the fabrication techniques of CMUTs can easily be used to implement high-frequency, high-density phased arrays, which are essential for high resolution scanning. Surface scanning using a 2-D row-column addressed CMUT array was demonstrated. Secondly, CMUTs can be integrated with supporting microelectronic circuits, thus one can implement a highly integrated transducer system, which can be useful in structural health monitoring NDT applications. Front-end microelectronic circuits that include a transmit pulser and a receive amplifier were designed, tested, and characterized. Thirdly, CMUTs are suitable for air-coupled applications because of their low acoustic impedance at resonance. Air-coupled CMUTs fabricated in a standard RF-MEMS process were characterized and tested. This thesis concludes with an analysis of the potential usefulness of CMUTs for ultrasonic NDT. While many ultrasonic NDT applications are better off being performed using conventional piezoelectric transducers, CMUTs can and should be used in certain NDT applications that can take advantage of the beneficial characteristics of this exciting transducer technology

Beamforming for 3D Transesophageal Echocardiography

In this thesis, we study beamforming techniques that offer opportunities for 3D transesophageal echocardiography imaging, especially to achieve higher frame rates. In 3D TEE with a matrix transducer, two main challenges are to connect a large number of elements to a standard ultrasound system and to achieve a high volume rate (>200 Hz). We develop a prototype miniaturized matrix transducer for pediatric patients with micro-beamforming to reduce the channel count. Initially, we propose two dual stage beamforming techniques for 1D arrays to produce high-quality images with reduced channel count: one using fixed focused receive and another with a simple summation in receive (no delays). Because of their inapplicability to the prototype transducer, we propose multiline 3D ultrasound beamforming schemes that utilize the micro-beamforming capabilities. The proposed beamforming schemes use an angle-weighted combination of the neighboring overlapping sub-volumes to suppress the crossover artifacts that are typical for parallel beamforming and produce high-quality images at a high volume rate (~300 Hz). A similar beamforming scheme adapted for a newly designed prototype matrix adult TEE probe is used for in vivo 3D imaging of the heart of a healthy adult pig to produce good quality 3D images at a high frame rate. The proposed 3D beamforming scheme can easily be adapted for matrix probes with micro-beamforming capabilities to produce good quality volume images at a high volume rate, even for a very different layout of the transmit and receive arrays

Effects of biocompatible encapsulations on the acoustic characteristics of CMUTs

Advances in modern medicine enable the use of medical implants for the treatment of an increasing number of diseases. If different implanted systems need to communicate with each other, data transmission using ultrasound is a promising solution. In this dissertation, an encapsulation strategy, which allows the use of capacitive micromachined ultrasonic transducers (CMUTs) within conventional implant housings, was developed and evaluated for the first time. The novel encapsulation approach consists of a silicone layer for coupling the CMUT to a layer of polyether ether ketone (PEEK) or titanium. Both materials are widely used for medical implant housings. Finite element simulations, complemented by measurements in air and in immersion as well as ex vivo experiments, have shown that effective data transmission with data rates of minimum 0.8 Mbps is possible over at least 6 cm with this encapsulation strategy.Die Fortschritte in der modernen Medizin ermöglichen immer häufiger den Einsatz von medizinischen Implantaten zur Therapie. In Anwendungsfällen, die eine Kommunikation mehrerer implantierter Systeme untereinander erfordern, stellt die Datenübertragung mit Hilfe akustischer Wellen eine vielversprechende Lösung dar. Hierfür ist eine biokompatible Kapselung nötig, die eine effiziente Datenübertragung nicht verhindert. In dieser Arbeit wird erstmals eine Kapselungsstrategie entwickelt und evaluiert, die den Einsatz von kapazitiven mikromechanischen Ultraschallwandlern (CMUTs) innerhalb konventioneller Implantatgehäuse ermöglicht. Die untersuchte neuartige Kapselung besteht aus einer Silikonschicht zur Ankopplung an eine Schicht aus Polyetheretherketon (PEEK) oder Titan, zwei weitverbreitete Materialien für die Kapselung medizinischer Implantate. Finite Elemente Simulationen, Messungen in Luft und Flüssigkeit sowie ex vivo Experimente haben gezeigt, dass mit dieser Kapselungsstrategie eine effektive Datenübertragung über mindestens 6 cm möglich ist. Die in ex vivo Experimenten ermittelten Frequenzbandbreiten der gekapselten CMUTs ermöglichen Datenraten von mindestens 0.8 Mbps. Ein zusätzlicher experimenteller Vergleich mit herkömmlichen Kapselungen für CMUTs bestätigt das große Potenzial der neuartigen Kapselung aus Silikon und PEEK. Abschließend wurden zukünftige Ansatzpunkte zur Steigerung von Signalamplitude und Datenrate identifiziert und diskutiert

Variations on Acoustophoretic Microchannels: From the Facile to the Highly Capable

The lab-on-a-chip toolkit is replete with techniques for microparticle manipulation. The use of ultrasonic waves to impart forces on particles, referred to as acoustophoresis, is one particularly powerful yet gentle option. Traditionally, piezoelectric transducers have been coupled with acoustic resonators in order to sustain standing waves for aggregating, sorting, and sensing applications. However, there are several drawbacks to these setups. First, the construction techniques underpinning resonators tend to be expensive and laborious. Secondly, the acoustic impedance of piezoelectric transducers is much higher than the aqueous medium in which microparticles are suspended. This requires the use of impedance-matching layers to adequately couple acoustic energy into the medium. Finally, piezoelectric materials tend to be incompatible with traditional fabrication methods for microelectromechanical systems (MEMS). This can significantly impact the use of piezoelectric materials in more complex lab-on-a-chip type platforms where many processes beyond sample manipulation may be necessary. Over the past two decades, another type of transducer, known as the capacitive micromachined ultrasonic transducer (CMUT), has been challenging the dominance of the piezotransducer. However, this competition has been occurring primarily in the arenas of non-destructive testing and medical imaging. Given some of the unique attributes of CMUTs, it is surprising that they have yet to be fully realized in acoustophoresis, especially because acoustophoresis is popularly practiced within a microfluidic setting. We first present a highly cost-effective and cleanroom-free technique for prototyping piezo-equipped, transversal acoustophoretic resonators. Simple bifurcation and trifurcation microchannels suitable for acoustophoresis in microscopy applications are fabricated and demonstrated. The second focus of our research looks at the suitability of CMUT microarrays as platforms for acoustophoresis. We address the following: issues surrounding their implementation; modeling of potential resonator designs; and possibility of foregoing resonators altogether. Several CMUT devices are then fabricated and tested, using travelling-wave ultrasound to demonstrate aggregation within droplets as well as sequential capture and separation within flows. A unique phenomenon of bead aggregate palpation is encountered, the physical underpinnings of which are attributed to magnitude changes in lateral gradient forces with frequency. Finally, we attempt to perform standing wave manipulations of particles using the CMUT microarray, and describe the difficulties encountered in doing so