Showing papers by "F. Levent Degertekin published in 2015"

Modal and transient analysis of membrane acoustic metasurfaces

Abstract: Dispersive surface waves on an acoustic 2D metamaterial, a metasurface consisting of membranes on a rigid surface, have certain propagation characteristics with potential applications for resonance based sensing and subwavelength imaging. The trapped modes of the system that is responsible for the dispersive properties of these acoustic waves are analyzed through modal analysis for a small linear membrane array to obtain the mode shapes, resonant frequencies, quality factors, and wavenumbers. Transient analysis is used for larger arrays to obtain the dispersive properties of the traveling waves and is compared to the modal analysis. Equifrequency contours of the 2D metasurface illustrate interesting features of the metasurface at different frequency regimes around the membrane resonance. These features include anisotropic wave propagation, directional band gap, negative refraction, and complete band gap. Effects of membrane pitch, randomness of resonance, and aperiodic membrane spacing on dispersion, band gaps, and quality factor of the trapped modes on the metasurface are investigated as they relate to realistic implementations for different applications.

Real-time imaging system using a 12-MHz forward-looking catheter with single chip CMUT-on-CMOS array

Abstract: Forward looking (FL) imaging catheters would be an important tool for several intravascular ultrasound (IVUS) and intracardiac echocardiography (ICE) applications. Single chip capacitive micromachined ultrasonic transducer (CMUT) arrays fabricated on front-end CMOS electronics with simplified electrical interconnect have been previously developed for highly flexible and compact catheters. In this study, we present a custom built real time imaging system utilizing catheters with single chip CMUT-on-CMOS arrays and show initial imaging results. The fabricated array has a dual-ring structure with 64 transmit (Tx) and 56 receive (Rx) elements. The CMUT arrays fit on a 2.1 mm diameter circular region with all the required front-end electronics. The device operates at 12 MHz center frequency and has around 20 V collapse voltage. The single-chip system requires 13 external connections including 4 Rx channels and power lines. The electrical connections to micro cables in the catheter are made from the top side of the chip using polyimide flex tapes. The device is placed on a 6-Fr catheter shaft and secured with a medical grade silicon rubber. For real time data acquisition, we developed a custom design FPGA based imaging platform to generate digital control sequences for the chip and collect RF data from Rx outputs. We performed imaging experiments using wire phantoms immersed in water to test the real time imaging system. The system has the potential to generate images at 32 fps rate with the particular catheter. The overall system is fully functional and shows promising image performance.

A nonlinear lumped model for ultrasound systems using CMUT arrays

Abstract: We present a nonlinear lumped model that predicts the electrical input-output behavior of an ultrasonic system using CMUTs with arbitrary array/membrane/electrode geometry in different transmit-receive configurations and drive signals. The receive-only operation, where the electrical output signal of the CMUT array in response to incident pressure field is calculated, is included by modifying the boundary elementbased vibroacoustic formulation for a CMUT array in rigid baffle. Along with the accurate large signal transmit model, this formulation covers pitch-catch and pulse-echo operation when transmit and receive signals can be separated in time. In cases when this separation is not valid, such as CMUTs used in continuous wave transmit-receive mode, pulse-echo mode with a nearby hard or soft wall or in a bounded space such as in a microfluidic channel, an efficient formulation based on the method of images is used. Some of these particular applications and the overall modeling approach have been validated through comparison with finite element analysis on specific examples including CMUTs with multiple electrodes. To further demonstrate the capability of the model for imaging applications, the two-way response of a partial dual-ring intravascular ultrasound array is simulated using a parallel computing cluster, where the output currents of individual array elements are calculated for given input pulse and compared with experimental results. With its versatility, the presented model can be a useful tool for rapid iterative CMUT-based system design and simulation for a broad range of ultrasonic applications.

Shape evolution of multiple interacting droplets in inkjet deposition

Abstract: Purpose – The aim of this paper is to advance the understanding of the droplet deposition process to better predict and control the manufacturing results for ink-jet deposition. Design/methodology/approach – As material interface has both geometric and physical significance to manufacturing, the approach the authors take is to study the interface evolution during the material joining process in ink-jet deposition using a novel shape metric and a previously developed powerful simulation tool. This tool is an experimentally validated numerical solver based on the combination of the lattice Boltzmann method and the phase-field model that enabled efficient simulation of multiple-droplet interactions in three dimensions. Findings – The underlying physics of two-droplet interaction is carefully examined, which provides deep insights into the effects of the printing conditions on the interface evolution of multiple-droplet interaction. By studying line printing, it is found that increasing impact velocity or dec...

Time-division multiplexing for cable reduction in ultrasound imaging catheters

Abstract: In ultrasound imaging catheter applications, gathering the data from multi-element transducer arrays is difficult as there is a restriction on cable count due to the diameter of the catheter. In such applications, CMUT-on-CMOS technology allows for 2D arrays with many elements to be designed and bonded directly onto CMOS circuitry. This allows for complex electronics to be placed at the tip of the catheter which leads to the possibility to include electronic multiplexing techniques to greatly reduce the cable count required for a large element array. Current approaches to cable reduction tend to rely on area and power hungry circuits to function, making them unsuitable for use in catheters. Furthermore the length requirement for catheters and lack of power available to on-chip cable drivers leads to limited signal strength at the receiver end. In this paper an alternative approach using Analogue Time Division Multiplexing (TDM) is presented, which addresses the cable restrictions of the catheter and, using a novel digital demultiplexing technique, allows for a reduction in the number of analogue signal processing stages required.

Ultrasonic subwavelength focusing above two dimensional membrane metasurface using time reversal

Abstract: Surface acoustic waves can propagate above immersed membrane arrays, such as of capacitive micromachined ultrasonic transducers (CMUTs). Similar waves on metamaterials and metasurfaces with rigid structures (typically in the kHz range) have been studied and used for tunable band gaps, negative refraction, and subwavelength focusing and imaging. This work demonstrates through simulation and experiments that a 2D membrane array can be used for subwavelength focusing utilizing a time reversal method. The studied structure consisted of the focusing region, which is a dense grid of 7x7 membranes (6.6 MHz resonance) that support the slow surface acoustic waves. Eight additional membranes are located on the same surface outside the focusing region. Subwavelength focusing was performed by using a time reversal method in which the external eight membranes were used as excitation transducers. Modeling results were verified with experiments that were performed with the membranes being actuated electrostatically and the membrane displacements were measured with a laser Doppler vibrometer. Subwavelength focusing (lambda/5) was achieved on the metasurface while a modal decomposition of the spatial focus from an iterative time reversal method was done to illustrate that optimal focusing resolution requires efficient excitation of the mode shapes containing subwavelength features.

On-chip reduced wire transceiver for high frequency CMUT imaging system

Abstract: A system-on-a-chip (SoC) for interfacing with a high frequency capacitive micromachined ultrasound transducer (CMUT) imaging array is presented at the post-layout simulation level. The SoC receives high and low voltage supply and clock from external sources and transfers the RF echo signal directly through a single wire. It occupies a 1.26 mm2 chip area in a 0.18-μm high voltage CMOS process and consumes 11.2 mW and 4.5 mW from the 1.8 V and 46 V supplies on the receiver (Rx) and transmitter (Tx) sides, respectively. The Tx generates a unipolar pulse with 12.5 ns width and 44 V swing in each firing. The Rx has a 95 kΩ resistive feedback transimpedance amplifier (TIA) that amplifies the echo signal with 296 μV noise integrated over 20–50 MHz band. High voltage regulators provide 44 V, 40 V, and 4.5 V supplies for the Tx driving circuits with 98.2%, 6.98%, and 1% efficiencies, respectively.