Li, Xiang. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 161-184). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract and appendix also in Chinese.

"The goal of this thesis work has been to investigate improved diagnostic methods for both detecting osteoporosis and estimating fracture risk non-invasively, by assessing bone mass density (BMD) and bone microstructure. It evaluates new approaches for analyzing through-transmission ultrasound signals using coral samples as models for trabecular bone. The results of BUA, impulse response of coral samples and the angular decorrelation function of coral samples indicate that these ultrasound parameters may be useful in detecting changes in both bone mineral density (BMD) and the presence of dominant trabecular bone structure axis. "

A level set-based segmentation procedure has been implemented to identify target object boundaries from 3D medical ultrasound images. Several test images (simulated, scanned phantoms, clinical) were subjected to various preprocessing methods and segmented. Two metrics of segmentation accuracy were used to compare the segmentation results to ground truth models and determine which preprocessing methods resulted in the best segmentations. It was found that by using an anisotropic diffusion filtering method to reduce speckle type noise with a 3D active contour segmentation routine using the level set method resulted in semi-automated segmentation on par with medical doctors hand-outlining the same images.

Thesis (M.Eng. (Hons.)) -- University of Western Sydney, 2001. / Bibliography : leaves [185]-195 (v. 1).

<p>Non-destructive characterization of multi-layered structures that can be accessed from only a single side is important for applications such as well-bore integrity inspection. Collimated beam ultrasound systems are a technology for imaging inside multi-layered structures such as geothermal wells. These systems work by using a collimated narrow-band ultrasound transmitter that can penetrate through multiple layers of heterogeneous material. A series of measurements can then be made at multiple transmit frequencies. However, commonly used reconstruction algorithms such as Synthetic Aperture Focusing Technique (SAFT) tend to produce poor quality reconstructions for these systems both because they do not model collimated beam systems and  they do not jointly reconstruct the multiple frequencies.</p> <p><br></p> <p>In this thesis, we first propose a multi-frequency ultrasound model-based iterative reconstruction (UMBIR) algorithm designed for multi-frequency collimated beam ultrasound systems. The combined system targets reflective imaging of heterogeneous, multi-layered structures. For each transmitted frequency band, we introduce a physics-based forward model to accurately account for the propagation of the collimated narrow-band ultrasonic beam through the multi-layered media. We then show how the joint multi-frequency UMBIR reconstruction can be computed by modeling the direct arrival signals, detector noise, and incorporating a spatially varying image prior.</p> <p><br></p> <p>We also propose a ringing artifact reduction method for ultrasound image reconstruction that uses a multi-agent consensus equilibrium (RARE-MACE) framework. Our approach integrates a physics-based forward model that accounts for the propagation of a collimated ultrasonic beam in multi-layered media, a spatially varying image prior, and a denoiser designed to suppress the ringing artifacts that are characteristic of reconstructions from high-fractional bandwidth ultrasound sensor data.</p> <p><br></p> <p>Results using both simulated and experimental data indicate that multi-frequency UMBIR reconstruction yields much higher reconstruction quality than either single frequency UMBIR or SAFT. In addition, our results demonstrate the capability of our RARE-MACE method to suppress ringing artifacts and substantially improve the image quality over single frequency UMBIR and SAFT.</p>

Thesis (Ph. D.)--Ohio State University, 2006. / Available online via OhioLINK's ETD Center; full text release delayed at author's request until 2007 Jun 16

The development of low-frequency SONAR systems, using a network of autonomous systems in unmanned vehicles, provides a practical means for bistatic measurements (i.e. when the source and receiver are widely separated, thus allowing multiple viewpoints of a target). Furthermore, time-frequency analysis, in particular Wigner-Ville analysis, takes advantage of the evolution of the time dependent echo spectrum to differentiate a man-made target (e.g. an elastic spherical shell, or cylinder) from a natural one of the similar shape (e.g. a rock). Indeed, key energetic features of man-made objects can aid in identification and classification in the presence of clutter and noise. For example, in a fluid-loaded thin spherical shell, an energetic feature is the mid-frequency enhancement echoes (MFE) that result from antisymmetric Lamb waves propagating around the circumference of the shell, which have been shown to be an acoustic feature useful in this pursuit. This research investigates the enhancement and benefits of bistatic measurements using the Wigner-Ville analysis along with acoustic imaging methods. Additionally, the advantage of joint space-time-frequency coherent processing is investigated for optimal array processing to enhance the detection of non-stationary signals across an array. The proposed methodology is tested using both numerical simulations and experimental data for spherical shells and solid cylinders. This research was conducted as part of the Shallow Water Autonomous Mine Sensing Initiative (SWAMSI) sponsored by ONR.

The threaded fasteners are one of the most versatile methods for assembly of structural components. For example, in bridges large bolts are used to fix base columns and small bolts are used to support access ladders. Naturally not all bolts are critical for the operation of the structure. Fasteners loaded with small forces and present in large quantities do not receive the same treatment as the critical bolts. Typical maintenance operations such tension measurements, internal stress checking or monitoring of crack development are not practical due to cost and time constrains. Although failure of a single non-critical fastener is not a significant threat to the structure's stability, massive malfunction may cause structural problem such as insufficient stiffness or excessive vibrations. The health of bolted joints is defined by a single parameter: the clamping force (CF). The CF is the force that holds the elements of the joint together. If the CF is too low, separation and bolt fatigue may occur. On the other hand, excessive CF may produce damages in the structural members such as excessive distortion or breakage. The CF is generated by the superposition of the individual tension of the bolts. The bolt tension, also referred as bolt preload, is the actual force that is stretching the bolt body. Maintaining the appropriate tension in bolts ensures a proper CF and hence a good health of the joint. In this thesis, a novel methodology for estimating the tension in bolts using surface acoustic waves (SAWs) is investigated. The tension is estimated by using the reflection of SAWs created by the bolt head interference. Increments in the bolt tension raise the points of interaction between the waves and the bolt head (real area of contact), and hence the position of the reflective boundaries. The variations are estimated using the "conventional linear synthetic array" imaging technique. A singular transducer is actuated from predefined positions to produce an array of signals that are subsequently arranged and added to construct an acoustic image. Three sets of experiment are presented in this research for validating the proposed concept: tension estimation of a ¼ inch stainless steel bolt, a ½ inch stainless steel bolt and ¼ inch grade 8 bolt. Acoustic images of the surface of the clamped plate illustrate a clear trend in the position of the reflective boundary when torque is changed. In all cases, the torque increments increase the real area of contact and therefore the position of the reflective boundary. As expected, the real area of contact grew from the bolt head center to the perimeter, which causes an effect of apparent movement of the boundary. This research proves the potential of the ultrasonic imaging methodology to measure applied tension. The result showed that the system can be used to successfully inspect tension in bolts of ½ and ¼ inches. The methodology investigated in this thesis is the first steps towards the development of bolt tension sensor based on surface acoustic waves.

The objective of this research is to develop novel capacitive micromachined ultrasonic transducer (CMUT) arrays for intravascular ultrasonic (IVUS) imaging along with the fabrication processes to allow for monolithic integration of CMUTs with custom CMOS electronics for improved performance. The IVUS imaging arrays include dual-ring arrays for forward-looking volumetric imaging in coronary arteries and annular-ring arrays with dynamic focusing capabilities for side-looking cross sectional imaging applications. Both are capable of integration into an IVUS catheter 1-2 mm in diameter. The research aim of monolithic integration of CMUTs with custom CMOS electronics has been realized mainly through the use of sloped sidewall vias less than 5 µm in diameter, with only one additional masking layer as compared to regular CMUT fabrication. Fabrication of CMUTs has been accomplished with a copper sacrificial layer reducing isolation layers by 50%. Modeling techniques for computational efficient analysis of CMUT arrays were developed for arbitrary geometries and further expanded for use with larger signal analysis. Dual-ring CMUT arrays for forward-looking volumetric imaging have been fabricated with diameters of less than 2 mm with center frequencies at 10 MHz and 20 MHz, respectively, for an imaging range from 1 mm to 1 cm. These arrays, successfully integrated with custom CMOS electronics, have generated 3D volumetric images with only 13 cables necessary. Performance from optimized fabrication has reduced the bias required for a dual-ring array element from 80 V to 42 V and in conjunction with a full electrode transmit array, it was shown that the SNR can be improved by 14 dB. Simulations were shown to be in agreement with experimental characterization indicated transmit surface pressure in excess of 8 MPa. For side-looking IVUS, three versions of annular CMUT arrays with dynamic focusing capabilities have been fabricated for imaging 1 mm to 6 mm in tissue. These arrays are 840 µm in diameter membranes linked to form 8 ring elements with areas that deviate by less than 25 %. Through modeling and simulation undesirable acoustic cross between ring elements was reduced from -13 dB to -22 dB.

This thesis presents the design and analysis for development of a Capacitive Micromachined Ultrasonic Transducer (CMUT), a novel sensor and actuator, aimed at replacing the conventional piezoelectric transducers for air-coupled ultrasonic imaging applications. These CMUTs are fabricated using the silicon micromachining technology wherein all fabrication is done on the surface of a silicon wafer by means of thin-film depositions, patterning with photolithography and etching. The main emphasis of this study is on developing analytical models that serve as effective design tools for the development of these devices. A desirable goal of such study is to create reasonable mathematical models, obtain analytical solutions, wherever possible, for various measures of transducer performance and provide design aids. A logical start is the lumped parameter modeling wherein the explicit dependence of the physical parameters on the spatial extent of the device is ignored. The system lumped parameters, such as the equivalent stiffness, the equivalent mass, and the equivalent damping are extracted from reasonable analytical or numerical models and subsequently used in the static and dynamic analysis of the device. Useful predictions are made with regard to the key transducer parameters, such as, the pull-in voltage, the static deflection, the dynamic response and the acoustic field produced. The modeling work presented embodies two main objectives: (i) it serves to provide direction in the design phase, and, (ii) it serves to aid in the extraction of critical parameters which affect the device behavior. Comparison of the results with the more rigorous FEM simulations as well as with those present in the existing literature assure that the developed models are accurate enough to serve as useful design tools. The distributed parameter modeling is presented next. Analysis of MEMS devices which rely on electrostatic actuation is complicated due to the fact that the structural deformations alter the electrostatic forces, which redistribute and modify the applied loads. Hence, it becomes imperative to consider the electro-elastic coupling aspect in the design of these devices. An approximate analytical solution for the static deflection of a thin, clamped circular plate caused by electrostatic forces which are inherently nonlinear, is presented. The model is based on the Kirchhoff-Love assumptions that the plate is thin and the deflections and slopes are small. The classical thin-plate theory is adequate when the ratio of the diameter to thickness of the plate is very large, a situation commonly prevalent in many MEMS devices, especially the CMUTs. This theory is used to determine the static deflection of the CMUT membrane due to a DC bias voltage. The thin-plate electro-elastic equation is solved using the Galerkin weighted residual technique under the assumption that the deflections are small in comparison to the thickness of the plate. The results obtained are compared to those obtained from ANSYS simulations and an excellent agreement is observed between the two. The pull-in voltage predicted by our model is close to the value predicted by ANSYS simulations. A simple analytical formula, which gives fairly accurate results (to within 3% of the value predicted by ANSYS simulations) for determination of the pull-in voltage, is also presented. As stated, this formula accounts for the elastic deflection of the membrane due to the coupled interaction with the electrostatic field. The effect of vacuum sealing the backside cavity of a CMUT is investigated in some detail. The presence or absence of air inside the cavity has a marked effect upon the system parameters, such as the natural frequency and the pull-in voltage. The possibility of using sealed CMUT cavities with air inside at ambient pressure is explored. In order to estimate the transducer loss due to the presence of air in the sealed cavity, the squeeze film forces resulting from the compression of the trapped air film are evaluated. Towards this end, the linearized Reynolds equation is solved in conjunction with the appropriate boundary conditions, taking the flexure of the membrane into account. From this analysis, it is concluded that, for a sealed CMUT cavity, the presence of air does not cause any squeeze film damping even when the flexure of the membrane is taken into account (the case of a rigid plate is already known). Although the emphasis of the study undertaken here is not on the physical realization of a working CMUT, a single cell as well as a linear array based on the design presented here, were fabricated (in a foundry elsewhere) in order to verify some of the most fundamental device parameters from experimental measurements. The fabricated devices have been characterized for their resonant frequency, quality factor, and structural integrity. These tests were conducted using the laser Doppler vibrometer and the Focused Ion Beam milling. Having investigated thoroughly the behavior of a single cell, we proceed to demonstrate how these cells can be arranged optimally in the form of an array to provide a comprehensive ultrasonic imaging system. A thorough analysis of the requirements for the array architecture is undertaken to determine the optimal configuration. The design constraints that need to be taken into account for CMUT arrays, especially for NDE applications, are presented. The main issue of designing an array consisting of a large number of CMUT cells required for producing a pressure wave of sufficient strength which is detectable upon reflection from the desired location even after suffering severe attenuation resulting from propagation in various media is addressed. A scalable annular array architecture of CMUT cells is recommended based on the analysis carried out.