O Mukdadi - Academia.edu (original) (raw)
Papers by O Mukdadi
Materials Research Society Symposium Proceedings, 2008
2007 IEEE Ultrasonics Symposium Proceedings, 2007
Abstract This work is motivated by the lack of current imaging modalities to accurately predict t... more Abstract This work is motivated by the lack of current imaging modalities to accurately predict the mechanical properties and defects in jawbone. Ultrasonic guided waves are sensitive to changes in microstructural properties and thus have been widely used for non-invasive material characterization. Guided waves propagating along the mandibles may exhibit dispersion behavior which depends on material properties, geometry and embedded cavities. In this work, we present the first theoretical and experimental study for the ...
Volume 2: Biomedical and Biotechnology, 2012
ABSTRACT Gingivitis is the most common gingival inflammation in the oral cavity, and the most pre... more ABSTRACT Gingivitis is the most common gingival inflammation in the oral cavity, and the most prevalent periodontal disease affecting 90% of the population in all age groups. Recently, a few research groups have investigated the possibility of using ultrasound in dentistry, particularly in diagnosing bony destruction in the more severe form of periodontal disease called periodontitis. This work investigates the feasibility of using ultrasound imaging to quantitatively assess gingival tissue inflammation. Signal and image processing of ultrasound data have been performed to quantitatively assess gingival tissue. A number of gingival scans were conducted in vitro to render ultrasound images of high-spatial and contrast resolutions. For each sample the B-mode images were matched with almost the same slices in histology. Results show that ultrasound scans for tissues with gingivitis exhibited low intensity of reflections (hypo echoic) at the inflamed tissues, while healthy dense epithelium layers exhibited higher reflections (hyper echoic). Histological diagnosis revealed good agreement with the ultrasound results indicating the usefulness of such ultrasound imaging in diagnosing gingivitis. In addition, a new design for an intraoral linear array ultrasound probe is demonstrated and utilized in our clinic in vivo. Analysis of the echogenicity patterns of the resultant images demonstrates the potential of using such a new probe in gingival health assessment, which would be feasible and clinical relevant for patient evaluations clinically.
ASME 2007 Summer Bioengineering Conference, 2007
Ultrasound technology has been widely used in medical imaging. Techniques using phased array tran... more Ultrasound technology has been widely used in medical imaging. Techniques using phased array transducers use an array of transducer elements to transmit a focused beam into the body, and each element then becomes a receiver to collect the echoes. The received echoes from each element are dynamically focused to form an image. These systems assume a constant acoustic velocity in the tissue of 1540 m/s while steering and focusing the beam. However, soft tissues have a range of acoustic velocities that vary from 1470 m/s for fat to 1665 m/s for collagen [1]. The acoustic wavefront propagation through a region with locally different acoustic velocities will be phase shifted relative to the rest of the wavefront. This effect is known as phase aberration. The effects of phase aberration include broadening of the system point spread function that deteriorate the image resolution, and increasing the off-axis response leading to multiple images for the target [2].
2004 2nd IEEE International Symposium on Biomedical Imaging: Macro to Nano (IEEE Cat No. 04EX821)
Accurate measurement of velocity profile, multiple velocity vectors and local shear stress in art... more 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
Biomedical sciences instrumentation, 2004
Nonlinear wave propagation in tissue can be employed for tissue harmonic imaging, ultrasound surg... more 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 ...
Ultrasound in Medicine & Biology, 2005
Accurate measurement of velocity profiles, multiple velocity vectors and local shear stress in ar... more Accurate measurement of velocity profiles, multiple velocity vectors and local shear stress in arteries is very important for a variety of cardiovascular diseases. We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (echo PIV). This method takes advantage of the nonlinear backscatter characteristics of ultrasound contrast microbubbles when exposed to certain ultrasonic forcing conditions. Preliminary in vitro, animal and clinical studies have shown significant promise of this method for measuring multiple velocity components with good temporal (up to 2 ms) and spatial (<1 mm) resolution. However, there is still difficulty in maximizing the nonlinearity of bubble backscatter using conventional Gaussian-pulse excitation techniques because: (1) significant harmonic components may not be produced at modest pressure amplitudes; and (2) the higher incident pressure amplitudes required to induce nonlinear behavior may cause bubble destruction. We present here a potential solution to this problem through the use of multifrequency excitation, where rectangular and triangular pulses with four harmonics are used to drive the bubble. The nonlinear behavior of the microbubble, as well as fragility and backscatter, were studied through numerical modeling via a modified Rayleigh-Plesset equation. Results show that the rectangular wave is effective in improving the visibility of microbubbles, with effective scattering cross-section area significantly higher (up to 35 times) than the widely-used Gaussian waveform. However, velocity and acceleration analysis of the bubble wall shows that the rectangular wave may threaten bubble stability. Due to lower wall velocity and acceleration, the triangular wave should decrease the potential for bubble destruction yet maintain relatively high second harmonic backscatter components. The impact of higher harmonics was studied by examining backscatter differences from incident rectangular and triangular pulses with four and two harmonics. Results indicate that a two-frequency excitation (which may be easier to implement practically) may be sufficient to induce nonlinear behavior of the microbubbles at modest incident pressures. These predictions provide support for the use of multifrequency driving to enhance echo PIV applications.
Physics in Medicine and Biology, 2006
Submicron ultrasound contrast agents have aroused attention for their significant promise in ultr... more Submicron ultrasound contrast agents have aroused attention for their significant promise in ultrasonic contrast/molecular imaging, targeted therapy and echo particle imaging velocimetry. However, nonlinear acoustic properties of submicron encapsulated gas bubbles for ultrasonic applications are still not clearly understood. In this paper, nonlinear acoustic emission characteristics from submicron bubbles were examined using a numerical study. The modified RP equation incorporating viscosity, acoustic radiation, thermal effects and encapsulated shell was used to study single bubble dynamics. Further, a size integration method, shown previously to be useful in prediction of backscatter spectra from groups of bubbles, was applied to analyse response from a bubble population. We show that bubbles with radii (200-500 nm) produce significant subharmonic and ultraharmonic components of the backscatter spectrum, while smaller bubbles (<200 nm) provide substantial second harmonic components. Additionally, nanoscale bubbles (<100 nm) produce very low backscatter amplitudes and thus may not be useful with the use of current ultrasound technology. Analysing optimal ultrasound driving pressures and bubbles size ranges for maximal subharmonic and ultraharmonic signals showed that sub and ultraharmonic mode nonlinear imaging methods may be potentially competitive for larger size bubbles (>200 nm) in providing proper contrast-to-tissue signal ratios.
MRS Proceedings, 2009
Over the past few decades, there has been considerable research and advancement in surface acoust... more Over the past few decades, there has been considerable research and advancement in surface acoustic wave (SAW) technology. At present, SAW devices have been highly successful as frequency band pass filters for the mobile telecommunications and electronics industries. In addition to their inherent frequency selectivity, SAW devices are also highly sensitive to surface perturbations. This sensitivity, along with a relative ease of manufacture, makes SAW devices ideally suited for many sensing applications including mass, pressure, temperature, and biosensors. In the area of biosensing, surface plasmon resonance (SPR) and quartz crystal microbalances (QCM) are still in the forefront of research and development, but advancement in SAW sensors could prove to have significant advantages over these technologies. This study investigates the advantages of using aluminum nitride (AlN) as a material for SAW sensors. AlN retains its piezoelectric properties at relatively high temperatures when ...
Exact and approximate analyses of ultrasonic guided wave propagation in thin orthotropic layers a... more Exact and approximate analyses of ultrasonic guided wave propagation in thin orthotropic layers are presented in this work. Exact solutions to the equations governing the dependence of guided wave propagation speeds on the elastic constants characterizing the anisotropic properties of the layers are presented and compared with the predictions of first order approximate theories for extensional and flexural waves in thin plates. Comparison with available experimental results for dispersion of these waves in thin sheets of different types of papers leads to the confirmation or modification of the elastic constants and density reported for these papers. A particular focus of this study is the coupling of three types of guided waves (extensional (S), flexural (A), and shear-horizontal (SH)) due to anisotropy of the material. It is shown that there are significant changes in the dispersion characteristics of these modes at certain frequencies, which can be exploited to measure the in-plane elastic properties of thin layers. Another focus is to study the limitations of approximate results when compared with exact solutions for wave propagation in different directions. In general good agreements are found at low frequencies.
In this paper, we present the model study of capacitive micromachined ultrasonic transducers (CMU... more In this paper, we present the model study of capacitive micromachined ultrasonic transducers (CMUTs) fabricated by 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 membrane thickness, 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. Simulation results show that these systems have the potential for excellent sensitivity and decreased power requirements. Work to test the fabricated elements is currently underway.
Non-invasive in vivo medical ultrasound imaging using contrast agents requires further physical u... more 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.
We have recently developed an ultrasound based velocimetry technique, termed echo particle image ... more 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.
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000
The elastic properties of many materials in sheet or plate form can be approximated with orthotro... more The elastic properties of many materials in sheet or plate form can be approximated with orthotropic symmetry. In many sheet material manufacturing industries (e.g., the paper industry), manufacturers desire knowledge of certain anisotropic elastic properties in the sheet for handling and quality issues. Ultrasonic wave propagation in plate materials forms a method to determine the anisotropic elastic properties in a nondestructive manner. This work explores exact and approximate analysis methods of ultrasonic guided wave propagation in thin layers, explicitly dealing with orthotropic symmetry and propagation off-axis with respect to the manufacturing direction. Recent advances in full-field ultrasonic imaging methods, based on dynamic holography, allow simultaneous measurement of the plate wave motion in all planar directions within a single image. Results from this laser ultrasonic imaging approach are presented that record the lowest anti-symmetric (flexural) mode wavefront in a single image without scanning. Specific numerical predictions for flexural wave propagation in two distinctly different types of paper are presented and compared with direct imaging measurements. Very good agreement is obtained for the lowest anti-symmetric plate mode using paper properties independently determined by a third party. Complete determination of the elastic modulus tensor for orthotropic layers requires measurement of other modes in addition to the lowest anti-symmetric. Theoretical predictions are presented for other guided wave modes [extensional (S), flexural (A), and shear-horizontal (SH)] in orthotropic plates with emphasis on propagation in all planar directions. It is shown that there are significant changes in the dispersion characterization of these modes at certain frequencies (including off-axis mode coupling) that can be exploited to measure additional in-plane elastic moduli of thin layers. At present, the sensitivity of the imaging measurement approach limits experimental investigation to relatively large amplitudes easily produced by flexural wave motion (> 0.1 nm). Extension of the measurement range and application to other plate wave modes are in progress and shall be reported in future work.
Review of progress in …, 2001
Exact and approximate theoretical analysis of the wavelengths of plate wave mode propagation in a... more Exact and approximate theoretical analysis of the wavelengths of plate wave mode propagation in all planar directions for the dispersive antisymmetric Lamb wave mode are compared with measurements from a laser ultrasonic imaging approach that records the out of plane ultrasonic motion over a large area in a single frame without scanning. Good agreement is demonstrated, based on independent determination of the elastic constants, for two different types of paper.
The Journal of the …, 2002
Guided waves in a layered elastic plate of rectangular cross section ͑finite width and thickness͒... more Guided waves in a layered elastic plate of rectangular cross section ͑finite width and thickness͒ has been studied in this paper. A semianalytical finite element method in which the deformation of the cross section is modeled by two-dimensional finite elements and analytical representation of propagating waves along the length of the plate has been used. The method is applicable to arbitrary number of layers and general anisotropic material properties of each layer, and is similar to the stiffness method used earlier to study guided waves in a laminated composite plate of infinite width. Numerical results showing the effect of varying the width of the plate on the dispersion of guided waves are presented and are compared with those for an infinite plate. In addition, effect of thin anisotropic coating or interface layers on the guided waves is investigated.
Materials Research Society Symposium Proceedings, 2008
2007 IEEE Ultrasonics Symposium Proceedings, 2007
Abstract This work is motivated by the lack of current imaging modalities to accurately predict t... more Abstract This work is motivated by the lack of current imaging modalities to accurately predict the mechanical properties and defects in jawbone. Ultrasonic guided waves are sensitive to changes in microstructural properties and thus have been widely used for non-invasive material characterization. Guided waves propagating along the mandibles may exhibit dispersion behavior which depends on material properties, geometry and embedded cavities. In this work, we present the first theoretical and experimental study for the ...
Volume 2: Biomedical and Biotechnology, 2012
ABSTRACT Gingivitis is the most common gingival inflammation in the oral cavity, and the most pre... more ABSTRACT Gingivitis is the most common gingival inflammation in the oral cavity, and the most prevalent periodontal disease affecting 90% of the population in all age groups. Recently, a few research groups have investigated the possibility of using ultrasound in dentistry, particularly in diagnosing bony destruction in the more severe form of periodontal disease called periodontitis. This work investigates the feasibility of using ultrasound imaging to quantitatively assess gingival tissue inflammation. Signal and image processing of ultrasound data have been performed to quantitatively assess gingival tissue. A number of gingival scans were conducted in vitro to render ultrasound images of high-spatial and contrast resolutions. For each sample the B-mode images were matched with almost the same slices in histology. Results show that ultrasound scans for tissues with gingivitis exhibited low intensity of reflections (hypo echoic) at the inflamed tissues, while healthy dense epithelium layers exhibited higher reflections (hyper echoic). Histological diagnosis revealed good agreement with the ultrasound results indicating the usefulness of such ultrasound imaging in diagnosing gingivitis. In addition, a new design for an intraoral linear array ultrasound probe is demonstrated and utilized in our clinic in vivo. Analysis of the echogenicity patterns of the resultant images demonstrates the potential of using such a new probe in gingival health assessment, which would be feasible and clinical relevant for patient evaluations clinically.
ASME 2007 Summer Bioengineering Conference, 2007
Ultrasound technology has been widely used in medical imaging. Techniques using phased array tran... more Ultrasound technology has been widely used in medical imaging. Techniques using phased array transducers use an array of transducer elements to transmit a focused beam into the body, and each element then becomes a receiver to collect the echoes. The received echoes from each element are dynamically focused to form an image. These systems assume a constant acoustic velocity in the tissue of 1540 m/s while steering and focusing the beam. However, soft tissues have a range of acoustic velocities that vary from 1470 m/s for fat to 1665 m/s for collagen [1]. The acoustic wavefront propagation through a region with locally different acoustic velocities will be phase shifted relative to the rest of the wavefront. This effect is known as phase aberration. The effects of phase aberration include broadening of the system point spread function that deteriorate the image resolution, and increasing the off-axis response leading to multiple images for the target [2].
2004 2nd IEEE International Symposium on Biomedical Imaging: Macro to Nano (IEEE Cat No. 04EX821)
Accurate measurement of velocity profile, multiple velocity vectors and local shear stress in art... more 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
Biomedical sciences instrumentation, 2004
Nonlinear wave propagation in tissue can be employed for tissue harmonic imaging, ultrasound surg... more 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 ...
Ultrasound in Medicine & Biology, 2005
Accurate measurement of velocity profiles, multiple velocity vectors and local shear stress in ar... more Accurate measurement of velocity profiles, multiple velocity vectors and local shear stress in arteries is very important for a variety of cardiovascular diseases. We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (echo PIV). This method takes advantage of the nonlinear backscatter characteristics of ultrasound contrast microbubbles when exposed to certain ultrasonic forcing conditions. Preliminary in vitro, animal and clinical studies have shown significant promise of this method for measuring multiple velocity components with good temporal (up to 2 ms) and spatial (<1 mm) resolution. However, there is still difficulty in maximizing the nonlinearity of bubble backscatter using conventional Gaussian-pulse excitation techniques because: (1) significant harmonic components may not be produced at modest pressure amplitudes; and (2) the higher incident pressure amplitudes required to induce nonlinear behavior may cause bubble destruction. We present here a potential solution to this problem through the use of multifrequency excitation, where rectangular and triangular pulses with four harmonics are used to drive the bubble. The nonlinear behavior of the microbubble, as well as fragility and backscatter, were studied through numerical modeling via a modified Rayleigh-Plesset equation. Results show that the rectangular wave is effective in improving the visibility of microbubbles, with effective scattering cross-section area significantly higher (up to 35 times) than the widely-used Gaussian waveform. However, velocity and acceleration analysis of the bubble wall shows that the rectangular wave may threaten bubble stability. Due to lower wall velocity and acceleration, the triangular wave should decrease the potential for bubble destruction yet maintain relatively high second harmonic backscatter components. The impact of higher harmonics was studied by examining backscatter differences from incident rectangular and triangular pulses with four and two harmonics. Results indicate that a two-frequency excitation (which may be easier to implement practically) may be sufficient to induce nonlinear behavior of the microbubbles at modest incident pressures. These predictions provide support for the use of multifrequency driving to enhance echo PIV applications.
Physics in Medicine and Biology, 2006
Submicron ultrasound contrast agents have aroused attention for their significant promise in ultr... more Submicron ultrasound contrast agents have aroused attention for their significant promise in ultrasonic contrast/molecular imaging, targeted therapy and echo particle imaging velocimetry. However, nonlinear acoustic properties of submicron encapsulated gas bubbles for ultrasonic applications are still not clearly understood. In this paper, nonlinear acoustic emission characteristics from submicron bubbles were examined using a numerical study. The modified RP equation incorporating viscosity, acoustic radiation, thermal effects and encapsulated shell was used to study single bubble dynamics. Further, a size integration method, shown previously to be useful in prediction of backscatter spectra from groups of bubbles, was applied to analyse response from a bubble population. We show that bubbles with radii (200-500 nm) produce significant subharmonic and ultraharmonic components of the backscatter spectrum, while smaller bubbles (<200 nm) provide substantial second harmonic components. Additionally, nanoscale bubbles (<100 nm) produce very low backscatter amplitudes and thus may not be useful with the use of current ultrasound technology. Analysing optimal ultrasound driving pressures and bubbles size ranges for maximal subharmonic and ultraharmonic signals showed that sub and ultraharmonic mode nonlinear imaging methods may be potentially competitive for larger size bubbles (>200 nm) in providing proper contrast-to-tissue signal ratios.
MRS Proceedings, 2009
Over the past few decades, there has been considerable research and advancement in surface acoust... more Over the past few decades, there has been considerable research and advancement in surface acoustic wave (SAW) technology. At present, SAW devices have been highly successful as frequency band pass filters for the mobile telecommunications and electronics industries. In addition to their inherent frequency selectivity, SAW devices are also highly sensitive to surface perturbations. This sensitivity, along with a relative ease of manufacture, makes SAW devices ideally suited for many sensing applications including mass, pressure, temperature, and biosensors. In the area of biosensing, surface plasmon resonance (SPR) and quartz crystal microbalances (QCM) are still in the forefront of research and development, but advancement in SAW sensors could prove to have significant advantages over these technologies. This study investigates the advantages of using aluminum nitride (AlN) as a material for SAW sensors. AlN retains its piezoelectric properties at relatively high temperatures when ...
Exact and approximate analyses of ultrasonic guided wave propagation in thin orthotropic layers a... more Exact and approximate analyses of ultrasonic guided wave propagation in thin orthotropic layers are presented in this work. Exact solutions to the equations governing the dependence of guided wave propagation speeds on the elastic constants characterizing the anisotropic properties of the layers are presented and compared with the predictions of first order approximate theories for extensional and flexural waves in thin plates. Comparison with available experimental results for dispersion of these waves in thin sheets of different types of papers leads to the confirmation or modification of the elastic constants and density reported for these papers. A particular focus of this study is the coupling of three types of guided waves (extensional (S), flexural (A), and shear-horizontal (SH)) due to anisotropy of the material. It is shown that there are significant changes in the dispersion characteristics of these modes at certain frequencies, which can be exploited to measure the in-plane elastic properties of thin layers. Another focus is to study the limitations of approximate results when compared with exact solutions for wave propagation in different directions. In general good agreements are found at low frequencies.
In this paper, we present the model study of capacitive micromachined ultrasonic transducers (CMU... more In this paper, we present the model study of capacitive micromachined ultrasonic transducers (CMUTs) fabricated by 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 membrane thickness, 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. Simulation results show that these systems have the potential for excellent sensitivity and decreased power requirements. Work to test the fabricated elements is currently underway.
Non-invasive in vivo medical ultrasound imaging using contrast agents requires further physical u... more 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.
We have recently developed an ultrasound based velocimetry technique, termed echo particle image ... more 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.
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000
The elastic properties of many materials in sheet or plate form can be approximated with orthotro... more The elastic properties of many materials in sheet or plate form can be approximated with orthotropic symmetry. In many sheet material manufacturing industries (e.g., the paper industry), manufacturers desire knowledge of certain anisotropic elastic properties in the sheet for handling and quality issues. Ultrasonic wave propagation in plate materials forms a method to determine the anisotropic elastic properties in a nondestructive manner. This work explores exact and approximate analysis methods of ultrasonic guided wave propagation in thin layers, explicitly dealing with orthotropic symmetry and propagation off-axis with respect to the manufacturing direction. Recent advances in full-field ultrasonic imaging methods, based on dynamic holography, allow simultaneous measurement of the plate wave motion in all planar directions within a single image. Results from this laser ultrasonic imaging approach are presented that record the lowest anti-symmetric (flexural) mode wavefront in a single image without scanning. Specific numerical predictions for flexural wave propagation in two distinctly different types of paper are presented and compared with direct imaging measurements. Very good agreement is obtained for the lowest anti-symmetric plate mode using paper properties independently determined by a third party. Complete determination of the elastic modulus tensor for orthotropic layers requires measurement of other modes in addition to the lowest anti-symmetric. Theoretical predictions are presented for other guided wave modes [extensional (S), flexural (A), and shear-horizontal (SH)] in orthotropic plates with emphasis on propagation in all planar directions. It is shown that there are significant changes in the dispersion characterization of these modes at certain frequencies (including off-axis mode coupling) that can be exploited to measure additional in-plane elastic moduli of thin layers. At present, the sensitivity of the imaging measurement approach limits experimental investigation to relatively large amplitudes easily produced by flexural wave motion (> 0.1 nm). Extension of the measurement range and application to other plate wave modes are in progress and shall be reported in future work.
Review of progress in …, 2001
Exact and approximate theoretical analysis of the wavelengths of plate wave mode propagation in a... more Exact and approximate theoretical analysis of the wavelengths of plate wave mode propagation in all planar directions for the dispersive antisymmetric Lamb wave mode are compared with measurements from a laser ultrasonic imaging approach that records the out of plane ultrasonic motion over a large area in a single frame without scanning. Good agreement is demonstrated, based on independent determination of the elastic constants, for two different types of paper.
The Journal of the …, 2002
Guided waves in a layered elastic plate of rectangular cross section ͑finite width and thickness͒... more Guided waves in a layered elastic plate of rectangular cross section ͑finite width and thickness͒ has been studied in this paper. A semianalytical finite element method in which the deformation of the cross section is modeled by two-dimensional finite elements and analytical representation of propagating waves along the length of the plate has been used. The method is applicable to arbitrary number of layers and general anisotropic material properties of each layer, and is similar to the stiffness method used earlier to study guided waves in a laminated composite plate of infinite width. Numerical results showing the effect of varying the width of the plate on the dispersion of guided waves are presented and are compared with those for an infinite plate. In addition, effect of thin anisotropic coating or interface layers on the guided waves is investigated.