Showing papers by "Osama M. Mukdadi published in 2004"

Transient Ultrasonic Guided Waves in Bi‐Layered Anisotropic Plates with Rectangular Cross Section

Abstract: Transient ultrasonic guided waves in anisotropic bi‐layered plates with finite‐width are investigated in this paper Composite bi‐layered plates consisting of GaAs substrate coated with Nb sheath is considered as an example because of its application to electronics and calorimetry The purpose is to investigate the acoustic mode coupling (“pinching”) phenomena for phonon transport A semi‐analytical finite element (SAFE) method is adopted to study the guided wave dispersion behavior in finite‐width elastic plates Nine‐noded quadrilateral elements are used to model the cross section of the finite‐width plate Propagation in the axial direction is modeled by analytical wave functions Elastodynamic Green’s functions are derived using modal summation in the frequency‐wavenumber and time‐space domains Results for dispersion and transient analysis of guided waves in finite‐width plates are presented and compared for different aspect ratios Group velocities are calculated and wave arrival times are computed for different plate cross sections as well as different excitation frequency Numerical results show significant influence of the plate aspect ratio on the dispersion and transient wave response Complex nature of quasi‐mode dispersion and propagation due to pinching phenomena in anisotropic plates require such quantitative analysis to afford easy interpretation These results would be important for nondestructive material evaluation and for characterization of phonon transport in anisotropic bi‐layered plates

A novel method for fabricating capacitive micromachined ultrasonic transducers with ultra-thin membranes

Abstract: Production of ultra-thin membranes facilitates the development of miniature capacitive micromachined ultrasonic transducers (CMUTs), which have great potential in biomedical imaging applications. We introduce a novel process, incorporating atomic layer deposition (ALD) and diffusion bonding, for the fabrication of CMUTs with ultra-thin membranes. First, an Al/sub 2/O/sub 3/ layer is deposited on an upper silicon wafer by ALD. Next, a gold layer is deposited on the Al/sub 2/O/sub 3/ layer and patterned to create circular cavities. Then the whole structure is transferred to a bottom wafer by diffusion bonding and the upper silicon wafer is etched away to release the Al/sub 2/O/sub 3/ membrane. Finally, another gold layer is deposited on the membrane for wiring and membrane excitation. Initial results show high quality membranes can be produced using this process with highly conformal surface qualities and extremely thin dimensions (<300 nm). Based on the dimensional characteristics created by this process, we simulate the performance of these transducers using equivalent circuit analysis. The results show that this new fabrication method provides another avenue for optimizing CMUT performance, especially in power savings, sensitivity and potentially increased reliability. Work to test the fabricated elements is currently under way.

Atomic layer deposition for fabricating capacitive micromachined ultrasonic transducers: initial characterization

Abstract: In this paper, we examine the utility of a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs). The method is based on atomic layer deposition (ALD) technology, which uses a self-limiting binary reaction process to produce ultra-thin membranes. Advantages of ALD include precise control of dimensions including gap-width between the capacitor plates, membrane thickness and radius, lower cost due to a reduction in the number of fabrication steps, the potential to use a large variety of materials, and increased reliability due to the enhanced surface quality of the membranes. These capabilities promise fabrication of transducers with superior operating characteristics. However, no study has yet documented sensitivity and power requirements for CMUTs created using ALD. We present here a first-order mechanical and equivalent circuit analysis along with a fabrication process to create and characterize CMUTs using ALD. Results show that these systems have the potential for excellent sensitivity and decreased power requirements. Work to test the fabricated elements is currently underway.

Advantages in using multi-frequency driving ultrasound for optimizing echo particle image velocimetry techniques.

Abstract: We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (echo PIV). This method takes advantage of the non-linear backscatter characteristics of ultrasound contrast microbubbles when exposed to certain ultrasonic field. Preliminary in vitro, animal and clinical studies have shown significant promise of this method for measuring multiple velocity components with good temporal and spatial resolution. However, there is still difficulty in maximizing the non-linearity of bubble backscatter using conventional Gaussian-pulse excitation techniques because significant harmonic components may not be produced at modest pressure amplitudes and the higher incident pressure amplitudes required to induce non-linear behavior may cause bubble destruction. We present here a potential solution to this problem through the use of multi-frequency excitation. A rectangular pulse with multiple harmonics is used to drive the bubble. The backscatter process is studied through a modified Rayleigh-Plesset equation. Results show that the rectangular wave is effective in improving the visibility of microbubbles with ultrasound backscattered efficiency significantly higher than the widely used Gaussian waveform. Use of rectangular pulses with 4 and 2 harmonics showed no significant difference in bubble backscatter behavior, indicating that a two-frequency excitation may be sufficient to induce non-linear behavior of the microbubbles practically at modest incident pressures.

Ultrasound wave propagation in tissue and scattering from microbubbles for echo particle image velocimetry technique.

Abstract: Nonlinear wave propagation in tissue can be employed for tissue harmonic imaging, ultrasound surgery, and more effective tissue ablation for high intensity focused ultrasound (HIFU). Wave propagation in soft tissue and scattering from microbubbles (ultrasound contrast agents) are modeled to improve detectability, signal-to-noise ratio, and contrast harmonic imaging used for echo particle image velocimetry (Echo-PIV) technique. The wave motion in nonlinear material (tissue) is studied using KZK-type parabolic evolution equation. This model considers ultrasound beam diffraction, attenuation, and tissue nonlinearity. Time-domain numerical model is based on that originally developed by Lee and Hamilton [J. Acoust. Soc. Am 97:906-917 (1995)] for axi-symmetric acoustic field. The initial acoustic waveform emitted from the transducer is assumed to be a broadband wave modulated by Gaussian envelope. Scattering from microbubbles seeded in the blood stream is characterized. Hence, we compute the pressure field impinges the wall of a coated microbubble; the dynamics of oscillating microbubble can be modeled using Rayleigh-Plesset-type equation. Here, the continuity and the radial-momentum equation of encapsulated microbubbles are used to account for the lipid layer surrounding the microbubble. Numerical results show the effects of tissue and microbubble nonlinearities on the propagating pressure wave field. These nonlinearities have a strong influence on the waveform distortion and harmonic generation of the propagating and scattering waves. Results also show that microbubbles have stronger nonlinearity than tissue, and thus improves S/N ratio. These theoretical predictions of wave phenomena provide further understanding of biomedical imaging technique and provide better system design.

Use of a composite nonlinear model including microbubbles and tissue to examine effects for ultrasonic propagation and backscatter in complex tissues: nonlinear ultrasonic waves in composite tissues

Abstract: Several numerical modeling studies have examined nonlinear propagation of ultrasound in tissue (KZK equation). Similarly, other investigations have studied nonlinear backscatter characteristics from ultrasound contrast microbubbles (RP equation). However, very few reports on combined tissue-bubble models are available, especially in the area of nonlinear contribution of microbubbles to ultrasonic propagation and backscatter in complex tissue where both blood vessels and surrounding soft tissue are incorporated. Results from such studies should contribute further to the continuing optimization of harmonic imaging techniques. In this study, the well-known KZK equation is adapted to include tissue and blood vessels with microbubbles. A composite approach utilizing the equivalent nonlinearity and attenuation parameters for the blood vessel with varying amount of contrast (volume fraction of bubbles to blood: 10/sup -8/ to 10/sup -6/) provides the resultant equation which is then solved using a time-domain finite difference technique. Harmonic analysis of tissue propagation shows that the microbubbles induce significant nonlinearity in the propagating wave, which influences both harmonic generation and waveform distortion by causing spatial shifts of the harmonic focal points (2/sup nd/ and 3/sup rd/ harmonics) and production of higher amplitude harmonic wave components. Wave-diffraction patterns of the multi-harmonic components varied mainly as a function of bubble concentration within the local blood vessel. These results provide a means for understanding harmonic effects in a realistic composite tissue-blood vessel model and should provide pathways to improve signal-to-noise ratio in harmonic imaging.

Advantages in using multi-frequency driving to enhance ultrasound contrast microbubble non-linearities for optimizing echo particle image velocimetry techniques

Abstract: Accurate measurement of velocity profile, multiple velocity vectors and local shear stress in arteries is important for a variety of cardiovascular diseases. We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (Echo-PIV). Preliminary in vitro, animal and clinical studies have shown significant promise of this method for measuring multiple velocity components with good temporal and spatial resolution. However, there is still difficulty in maximizing the non-linearity of bubble backscatter using conventional Gaussian-pulse excitation techniques at modest pressure amplitudes. We present here a potential solution to this problem through the use of multifrequency excitation. Results show that a rectangular wave is effective in improving the visibility of microbubbles. Use of rectangular pulses with 4 and 2 harmonics showed no significant difference in backscatter behavior, indicating that a two-frequency excitation may be sufficient to induce nonlinear behavior of the microbubbles at modest incident pressures.

Numerical modeling of ultrasound imaging using contrast agents for particle image velocimetry in vivo

Abstract: Non-invasive in vivo medical ultrasound imaging using contrast agents requires further physical understanding of ultrasound wave propagation phenomenon in tissue and scattering from microbubbles. Cumulative nonlinearity exhibited by wave motion in tissue and local nonlinearity by microbubble dynamics are strongly influence the imaging technique and microbubble detectability. The wave propagation in tissue is studied using KZK-type parabolic evolution equation. This model considers ultrasound beam diffraction, attenuation, and tissue nonlinearity. Pressure-wave scattering from microbubbles, seeded in the blood stream, is modeled using Rayleigh-Plesset-type equation. The continuity and the radial-momentum equations of encapsulated microbubbles are employed to account for the lipid layer surrounding the microbubble. Numerical results show the effects of tissue and microbubble nonlinearities on pressure-wave propagation and scattering. These nonlinearities have a strong influence on the waveform distortion and harmonic generation. Results also show that microbubbles have stronger nonlinearity than that of tissue, and thus improves signal-to-noise ratio.