Elastography-based imaging techniques have received substantial attention in recent years for non-invasive assessment of tissue mechanical properties. These techniques take advantage of changed soft tissue elasticity in various pathologies to yield qualitative and quantitative information that can be used for diagnostic purposes. Measurements are acquired in specialized imaging modes that can detect tissue stiffness in response to an applied mechanical force (compression or shear wave). Ultrasound-based methods are of particular interest due to its many inherent advantages, such as wide availability including at the bedside and relatively low cost. Several ultrasound elastography techniques using different excitation methods have been developed. In general, these can be classified into strain imaging methods that use internal or external compression stimuli, and shear wave imaging that use ultrasound-generated traveling shear wave stimuli. While ultrasound elastography has shown promising results for non-invasive assessment of liver fibrosis, new applications in breast, thyroid, prostate, kidney and lymph node imaging are emerging. Here, we review the basic principles, foundation physics, and limitations of ultrasound elastography and summarize its current clinical use and ongoing developments in various clinical applications.

Ultrasound Elastography: Review of Techniques and Clinical Applications

Photoacoustic clinical imaging

Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated front-end electronic circuits we developed and their use for 2D and 3D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a micro-electro-mechanical systems technology for many medical diagnostic and therapeutic applications.

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Capacitive micromachined ultrasonic transducers for medical imaging and therapy

Capacitive micromachined ultrasonic transducer (MUT) technology is a prime candidate for next generation imaging systems. Medical and underwater imaging and the nondestructive evaluation (NDE) societies have expressed growing interest in cMUTs over the years. Capacitive micromachined ultrasonic transducer technology is expected to make a strong impact on imaging technologies, especially volumetric imaging, and to appear in commercial products in the near future. This paper focuses on fabrication technologies for cMUTs and reviews and compares variations in the production processes. We have developed two main approaches to the fabrication of cMUTs: the sacrificial release process and the recently introduced wafer-bonding method. This paper gives a thorough review of the sacrificial release processes, and it describes the new wafer-bonding method in detail. Process variations are compared qualitatively and quantitatively whenever possible. Through these comparisons, it was concluded that wafer-bonded cMUT technology was superior in terms of process control, yield, and uniformity. Because the number of steps and consequent process time were reduced (from six-mask process to four-mask process), turn-around time was improved significantly.

Capacitive micromachined ultrasonic transducers: fabrication technology

For three-dimensional (3D) ultrasound imaging, connecting elements of a two-dimensional (2D) transducer array to the imaging system's front-end electronics is a challenge because of the large number of array elements and the small element size. To compactly connect the transducer array with electronics, we flip-chip bond a 2D 16 times 16-element capacitive micromachined ultrasonic transducer (CMUT) array to a custom-designed integrated circuit (IC). Through-wafer interconnects are used to connect the CMUT elements on the top side of the array with flip-chip bond pads on the back side. The IC provides a 25-V pulser and a transimpedance preamplifier to each element of the array. For each of three characterized devices, the element yield is excellent (99 to 100% of the elements are functional). Center frequencies range from 2.6 MHz to 5.1 MHz. For pulse-echo operation, the average -6-dB fractional bandwidth is as high as 125%. Transmit pressures normalized to the face of the transducer are as high as 339 kPa and input-referred receiver noise is typically 1.2 to 2.1 rnPa/ radicHz. The flip-chip bonded devices were used to acquire 3D synthetic aperture images of a wire-target phantom. Combining the transducer array and IC, as shown in this paper, allows for better utilization of large arrays, improves receive sensitivity, and may lead to new imaging techniques that depend on transducer arrays that are closely coupled to IC electronics.

Integration of 2D CMUT arrays with front-end electronics for volumetric ultrasound imaging

Ultrasound has become more prevalent as a medical tool for real-time volumetric imaging. Sparse 2D CMUT ring arrays generate high resolution volumetric images with fewer elements than a fully populated 2D array, allow for integration with electronics in an endoscope form factor, and provide a central lumen for simultaneous use of additional diagnostic and therapeutic tools, such as HIFU or photoacoustic fibers, without increasing the overall package size. We have previously fabricated QuadRing capacitive micromachined ultrasound transducers (CMUTs). Each of the four independent, concentric rings in the array contains 128 elements and operates at a different center frequency. In this work, we use one of the four concentric rings at 4MHz for a fully integrated endoscope assembly. Custom charge-amplifier ASICs were used in these assemblies rather than transimpedance amplifiers, reducing the noise figure of the system. The CMUT arrays are flip chip bonded to a custom 8-leg flexible PCB (flex) that provides electrical connections between the CMUT array, ASICs, and Verasonics imaging system. The flip chip bonded assembly is integrated with a custom 3D printed tip that encases and mechanically supports the assembly, and provides a convenient built-in reference for the passivation layer. Additionally, one flex version flips the CMUT bias, grounding the top electrode without additional circuitry by level-shifting the IC supplies. This feature is particularly desirable for clinical applications, as it shields the patient from the CMUT bias voltage. This new 128-element forward-facing CMUT endoscope has been used for real-time 3D imaging on the bench and expands the toolkit beyond previous work in several ways: high quality images are obtained using a relatively sparse array; new ASICs have shown improvements in SNR; and the array size has enabled use of new 3D-printed, highly customizable assembly tools. We have validated operation with a grounded CMUT top electrode, a critical step towards clinical use. Furthermore, a large lumen increases the breadth of tools that can be used in conjunction with the imaging array.

Fully integrated 2D CMUT ring arrays for endoscopic ultrasound

We report on capacitive micromachined ultrasonic transducers (CMUTs) with vented cavities for varying their bandwidth and sensitivity in airborne applications The devices are simulated using a finite element model which incorporates viscous and thermal fluid losses in the squeeze film and the vent holes, as well as acoustic radiation and acoustic resonance in the backing Our model accurately predicts the behavior of such CMUTs The model is also validated with measurements at elevated pressure

Effect of fluid losses and acoustic resonances in CMUTs with vented cavities

Singulation of MEMS is a critical step in the transition from wafer-level to die-level devices. As is the case for capacitive micromachined ultrasound transducer (CMUT) ring arrays, an ideal singulation must protect the fragile membranes from the processing environment while maintaining a ring array geometry. The singulation process presented in this paper involves bonding a trench-patterned CMUT wafer onto a support wafer, deep reactive ion etching (DRIE) of the trenches, separating the CMUT wafer from the support wafer and de-tethering the CMUT device from the CMUT wafer. The CMUT arrays fabricated and singulated in this process were ring-shaped arrays, with inner and outer diameters of 5 mm and 10 mm, respectively. The fabricated CMUT ring arrays demonstrate the ability of this method to successfully and safely singulate the ring arrays and is applicable to any arbitrary 2D shaped MEMS device with uspended microstructures, taking advantage of the inherent planar attributes of DRIE.

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Singulation for imaging ring arrays of capacitive micromachined ultrasonic transducers

Ultrasound is increasingly in demand as a medical imaging tool and can be particularly beneficial in the field of intracardiac echocardiography (ICE). However, many challenges remain in the development of a 3D ultrasound imaging system.We have designed and fabricated a quad-ring capacitive micromachined ultrasound transducer (CMUT) for real-time, volumetric medical imaging. Each CMUT array is composed of four concentric, independent ring arrays, each operating at a different frequency, with 128 elements per ring. In this project, one ring will be used for imaging. A large (5mm diameter) lumen is available for delivering other devices, including high intensity focused ultrasound transducers for therapeutic applications or optical fibers for photoacoustic imaging.We address several challenges in developing a 3D imaging system. Through wafer vias are incorporated in the fabrication process for producing 2D CMUT arrays. Device integration with electronics is achieved through solder bumping the arrays, designing a flexible PCB, and flip chip bonding CMUT and ASICs to the flexible substrate. Finally, we describe a method for integrating the flex assembly into a catheter shaft. The package, once assembled, will be used for in-vivo open chest experiments.Copyright © 2015 by ASME

Fabrication, Packaging, and Catheter Assembly of 2D CMUT Arrays for Endoscopic Ultrasound and Cardiac Imaging

In this paper, we introduce a modified fabrication method for CMUTs with substrate-embedded springs (post-CMUTs or PCMUTs) to increase the fabrication yield. This modified fabrication process includes three additional steps from the previous process: vacuum cleaning prior to the wafer bonding step, thermal oxidation of the silicon piston top plate, and preservation of the 150-nm buried oxide (BOX) layer on top of the 250 nm Si layer of the bonded SOI wafer. These modifications increased the fabrication yield from 10% to 60%. For these newly fabricated PCMUT devices, we measured the electrical input impedance with a precision impedance analyzer, the plate displacement with a laser Doppler vibrometer (LDV), and the output pressure with a calibrated hydrophone. The measured electrical input impedance matches well with the FEA simulation results; the LDV measurement in air confirmed a non-flexural plate displacement; and the hydrophone measurement showed a peak-to-peak acoustic pressure of 27.6 kPa at a distance of 3.6 mm in immersion, corresponding to 1.05 MPa at the face of the transducer, for a particular test 2-D array element.

Amin Nikoozadeh

Papers

Fully integrated 2D CMUT ring arrays for endoscopic ultrasound

Effect of fluid losses and acoustic resonances in CMUTs with vented cavities

Singulation for imaging ring arrays of capacitive micromachined ultrasonic transducers

Fabrication, Packaging, and Catheter Assembly of 2D CMUT Arrays for Endoscopic Ultrasound and Cardiac Imaging

Non- flexural parallel piston movement across CMUT with substrate-embedded springs