Design and implementation of intravascular hifu catheter ablation system

High-intensity focused ultrasound is an energy-based thermal therapy for noninvasive or minimally invasive treatment of wide range of medical disorders including solid cancer tumors, brain surgery, atrial fibrillation (AF) and other cardiac arrhythmias. Conventional HIFU is extracorporeally administered but in applications where a small lesion or more precise energy localization in shorter time is required, catheter-based HIFU devices which are positioned directly within or adjacent to the target may be the best solution. Available HIFU catheters use array of piezoelectric transducers with complex external high-voltage (HV) and high-frequency amplifiers, a cooling system and several coaxial cables within the catheter. In this study, a HV transmitter IC has been designed, manufactured and integrated with an 8-element capacitive micromachined ultrasound transducer (CMUT) on a prototype HIFU probe appropriate for a 6-Fr catheter. The transmitter IC fabricated in 0.35 μm HV CMOS process and comprises eight continuouswave HV buffers (10.9 ns and 9.4 ns rise and fall times at 20 Vpp output into a 15 pF), an eight-channel transmit beamformer (8-12 MHz output frequency with 11.25 º phase accuracy) and a phase locked loop with an integrated VCO as a tunable clock source (128–192 MHz). The chip occupies 1.85×1.8 mm2 area including input and output (I/O) pads. Electrical measurements, IR thermography and Ex-vivo experiment results reveal that the presented HIFU system can elevate the temperature of the target region of tissue around 19 ºC by delivering 600 CEM43 equivalent thermal dose while surface temperature of the probe rises less than 5 º

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

PMMA-based wafer-bonded capacitive micromachined ultrasonic transducer for underwater applications

This article presents a new wafer-bonding fabrication technique for Capacitive Micromachined Ultrasonic Transducers (CMUTs) using polymethyl methacrylate (PMMA). The PMMA-based single-mask and single-dry-etch step-bonding device is much simpler, and reduces process steps and cost as compared to other wafer-bonding methods and sacrificial-layer processes. A low-temperature (<180 ◦ C) bonding process was carried out in a purpose-built bonding tool to minimize the involvement of expensive laboratory equipment. A single-element CMUT comprising 16 cells of 2.5 mm radius and 800 nm cavity was fabricated. The center frequency of the device was set to 200 kHz for underwater communication purposes. Characterization of the device was carried out in immersion, and results were subsequently validated with data from Finite Element Analysis (FEA). Results show the feasibility of the fabricated CMUTs as receivers for underwater applications

Capacitive micromachined ultrasound transducer (CMUT) design and fabrication for intracardiac echocardiography

The objective of this research is to develop capacitive micromachined ultrasonic transducer (CMUT) arrays with novel geometry for intracardiac echocardiography (ICE) imaging along with a novel reliable CMUT fabrication process to improve the system performance. We used custom CMOS electronics and monolithically integrated our CMUT arrays to CMOS chips. The arrays are designed for 9-Fr (<3mm) ICE catheters over a total area of about 2.6x11 mm2 at around 7MHz center frequency with ~80% fractional bandwidth in both 1-D and 2-D configurations. The 1-D array transducer includes 64 channels with beam-steering capabilities for cross sectional ICE imaging application at distance range of about 5-cm. The ICE image with 40dB dynamic range from 7 metal wires has been obtained. Several 2-D (sparse) arrays are designed based on signal-to-noise ratio (SNR) optimization capable of generating volumetric images. The CMUT-on-CMOS technique is used for arrays integration with our ASICs using vias for top and bottom electrode connections to the related electronics pads. A 60V pulse is optimized during transmit operation and 2MPa surface pressure has been achieved that is in agreement with our simulation results. We also developed an improved CMOS compatible low temperature sacrificial layer fabrication process for CMUTs. The process adds the fabrication step of silicon oxide evaporation which is followed by a lift-off step to define the membrane support area without a need for an extra mask. The parasitic capacitance is reduced about 15% and device long-term test demonstrates 72-hours stable output pressure showing no significant degradation on performance. We have also developed a new energy-based calculation method for CMUT performance evaluation that is valid during both small and large signal operation since well-known frequency and capacitance based coupling coefficients definitions are not valid for large signal and nonlinear operation regimes. The quantitative modeling results show that CMUTs do not need DC bias to achieve high efficiency large signal transduction: AC only signals at half the operation frequency with amplitudes beyond the collapse voltage can provide energy conversion ratio (ECR) above 0.9 with harmonic content below -25dB. The overall modeling approach is also qualitatively validated by experiments.Ph.D

Optimum switch sizing for class DE amplifier

Recently, integrated class DE ampli fiers without matching networks have been proposed as a compact solution to drive a multi-element piezoelectric ultrasound transducer array for high-intensity focused ultrasound (HIFU) therapy. These transducers produce acoustic energy that translates into heat for tissue ablation. In order to steer the focal zone, each element in the transducer array is driven at a different phase. Hence, there's a need for the power amplifi er with a digital control unit in this application. Since each element in the transducer array has a different electrical characteristic and they have to be driven at the same frequency, it is a challenge to drive all transducers in the array at their optimum conditions. This work introduces strategies to determine efficient driving parameters for an entire transducer array. In addition. a method to improve the power efficiency of the class DE amplifi er by choosing the optimum size for switching MOSFETs is also proposed. During the operation of a class DE ampli fier, losses are caused by the ON resistance and the drivers of the MOSFET gate capacitances. These parameters are directly dependent on the size of the switching MOSFETs. A wider MOSFET will have a higher gate capacitance, but lower ON resistance. With the correct sizing, these losses can be greatly reduced to improve power efficiency and prevent excessive heating. The challenge with this method is the wide selection of transducers with varying impedance. As the load impedance changes, the MOSFET size also needs to be changed to maintain the maximum power efficiency. Also, the proposed design must deliver at least 1 W output power to the transducer in order to produce enough acoustic pressure. This output requirement will limit the available technology that can be used to design the amplifi er. In addition, this work also proposes a new driving circuit that consumes less power to operate, and also allows a full 0-360 degree phase shift. The design is simulated with Spectre simulator using 0.35 m 50V CMOS process data available from Austria Micro Systems. The proposed design can deliver 1422mW of average power to 6-elements transducer array, and achieve up to 91% power efficiency

Fabrication of CMUTS based on PMMA adhesive wafer bonding

Capacitive Micromachined Ultrasonic Transducers (CMUTs) are the potential alternatives for the conventional piezoelectric ultrasonic transducers. CMUTs have been under an extensive research and development since their first development in the mid- 1990s. Initially developed for air-coupled applications, CMUTs have shown far better acceptability in immersion-based applications (i.e. medical ultrasonic imaging, medical therapy, and underwater imaging) when compared to the piezoelectric ultrasonic transducers. CMUTs are parallel-plate capacitors fabricated using the Micro Electro Mechanical Systems (MEMS) technology. Despite of the fact that various CMUT fabrication methods have been reported in the literature, there are still many challenges to address in CMUTs design and fabrication. Standard fabrication techniques are further sub-divided into the Sacrificial Layer Release Process and the Wafer Bonding methods. A number of complications are associated with these techniques, such as optimization of the design parameters, process complexity, sacrificial layer material with the corresponding etchant selection, wafer cost and selection. In particular, the sacrificial release methods consist of complex fabrication steps. Furthermore, structural parameters like gap height and radius have optimization issues during the sacrificial release process. On the other hand, the wafer bonding techniques for the CMUTs fabrication are simple and have a great control over the structure parameters in contrast to the sacrificial release methods. At the same time, the wafer-bonded CMUTs require very high quality wafer surface and have a very high contamination sensitivity. For this purpose, this dissertation aims to develop a simple, low cost and lower constraint thermocompression-based technique for the CMUT fabrication. The proposed wafer bonding technique for the CMUT fabrication in the dissertation uses Polymethyl methacrylate (PMMA) adhesive as an intermediate layer for the thermocompression wafer bonding. The advantages associated with the PMMA adhesivebased wafer bonding over the other wafer bonding methods include low process temperature (usually 200 C or less), high wafer surface defects and contamination tolerance, high surface energy and low bonding stresses. These factors will add cost effectiveness and simplicity to the CMUTs fabrication process. Furthermore, the achieved receive sensitivity with the reported CMUT is found comparable to the commercially available ultrasonic transducer

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

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

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