Ultrasonic propagation in cancellous bone

The thesis covers two main areas of work. The first is detailed experimental work and the second is the evaluation of existing ultrasonic theories in attempt to apply them to the propagation in cancellous bone. The work is related to a new technique which uses ultrasonic attenuation to measure and predict osteoporosis, especially in the elderly population.The ultrasonic attenuation, the longitudinal ultrasonic velocity, the scattering effect and the attenuation as a function of frequency were measured on a range of cancellous bone samples, from healthy to severely osteoporotic, and also a few cortical samples. The cancellous bone was human os calces and vertebrae. The relationships between the ultrasonic propagation and the structural parameters and density of the bone were investigated, and were considered both for whole bones and separate purely cancellous samples. Image analysis of photomicrographs taken under low magnification was carried out to find the architectural parameters of the bone structure. The ultrasonic measurements were also compared with quantitative computed tomography assessment and compressive strength testing.Many theories which are currently used to evaluate ultrasonic propagation in a porous material are reviewed, and three particular ones are developed in detail and applied to models of cancellous bone. The self consistent theory (SCT), Biot's theory and the multiple scattering theory based on the work of Waterman and Truell were all assessed for their limits with repect to this particular application, and each had its own deficiencies. The Blot theory, however, proved the most successful at predicting the experimental attenuation results observed, but still only in a limited way

A numerical study of ultrasonic response of random cortical bone plates

A probabilistic study on ultrasound wave reflection and transmission from cortical bone plates is proposed. The cortical bone is  modeled by an anisotropic and heterogeneous elastic plate sandwiched between two fluids and has randomly varied elastic properties in the thickness direction. A parametric stochastic model is proposed to describe the elastic heterogeneity in the plate. Reflection and transmission coefficients are computed via the semi-analytical finite element (SAFE) method. The effect of material heterogeneity on reflected and transmitted waves is investigated from a probabilistic point of view. The parametric study highlights effects of the uncertainty of material properties on the reflection and transmission coefficients by varying the frequency, angle of incidence and bone thickness

SOLID-SHELL FINITE ELEMENT MODELS FOR EXPLICIT SIMULATIONS OF CRACK PROPAGATION IN THIN STRUCTURES

Crack propagation in thin shell structures due to cutting is conveniently simulated using explicit finite element approaches, in view of the high nonlinearity of the problem. Solidshell elements are usually preferred for the discretization in the presence of complex material behavior and degradation phenomena such as delamination, since they allow for a correct representation of the thickness geometry. However, in solid-shell elements the small thickness leads to a very high maximum eigenfrequency, which imply very small stable time-steps. A new selective mass scaling technique is proposed to increase the time-step size without affecting accuracy. New ”directional” cohesive interface elements are used in conjunction with selective mass scaling to account for the interaction with a sharp blade in cutting processes of thin ductile shells

Ultra-high-speed imaging of bubbles interacting with cells and tissue

Ultrasound contrast microbubbles are exploited in molecular imaging, where bubbles are directed to target cells and where their high-scattering cross section to ultrasound allows for the detection of pathologies at a molecular level. In therapeutic applications vibrating bubbles close to cells may alter the permeability of cell membranes, and these systems are therefore highly interesting for drug and gene delivery applications using ultrasound. In a more extreme regime bubbles are driven through shock waves to sonoporate or kill cells through intense stresses or jets following inertial bubble collapse. Here, we elucidate some of the underlying mechanisms using the 25-Mfps camera Brandaris128, resolving the bubble dynamics and its interactions with cells. We quantify acoustic microstreaming around oscillating bubbles close to rigid walls and evaluate the shear stresses on nonadherent cells. In a study on the fluid dynamical interaction of cavitation bubbles with adherent cells, we find that the nonspherical collapse of bubbles is responsible for cell detachment. We also visualized the dynamics of vibrating microbubbles in contact with endothelial cells followed by fluorescent imaging of the transport of propidium iodide, used as a membrane integrity probe, into these cells showing a direct correlation between cell deformation and cell membrane permeability

Comparison of Conventional and Bayesian Analysis for the Ultrasonic Characterization of Cancellous Bone

This dissertation investigates the physics underlying the propagation of ultrasonic waves in cancellous bone. Although quantitative ultrasound has the potential to evaluate bone quality even better than the current gold standard X-ray based modality, its clinical utility has been hampered by the incomplete understanding of the mechanisms governing the interaction between ultrasound and bone. Therefore, studies that extend the understanding of the fundamental physics of the relationship between ultrasound and trabecular bone tissue may result in improved clinical capabilities. Ultrasonic measurements were carried out on excised human calcaneal specimens in order to study the effects of overlapping fast and slow compressional mode waves on the ultrasonic parameters of attenuation and velocity. Conventional analysis methods were applied to received sample signals that appeared to contain only a single wave mode. The same signals were also analyzed using a Bayesian parameter estimation technique that showed that the signals, which appeared to be only a single wave, could be separated into fast and slow wave components. Results demonstrated that analyzing the data under the assumption that only a single wave mode is present, instead of two interfering waves, yielded a phase velocity that lay between the fast and slow wave velocities and a broadband ultrasound attenuation that was much larger than the ultrasound attenuations of the individual fast and slow waves. The fast and slow wave ultrasonic parameters were found to correlate with microstructural parameters, including porosity, determined by microCT measurements. Simulations of fast and slow wave propagation in cancellous bone were carried out to demonstrate the plausibility of a proposed explanation for an anticipated sample-thickness dependence of the apparent attenuation in bovine bone. The results showed that an apparent sample-thickness dependence could arise if the fast and slow waves are not separated sufficiently and if frequency-domain analysis is not performed on broadband data. The sample-thickness dependence of the ultrasonic parameters was explored further using experimental data acquired on an equine cancellous bone specimen that was systematically shortened. The thickness of the sample varied the degree to which the fast and slow waves overlapped, permitting the use of conventional analysis methods for sufficiently long sample lengths. Bayesian parameter estimation was performed successfully on data from all sample lengths. The ultrasonic parameters obtained by both conventional and Bayesian analysis methods were found unexpectedly to display small, systematic variations with sample thickness. A very thorough and systematic series of studies were carried out on one-mode Lexan phantoms to investigate the potential cause of the observed sample-thickness dependence. These studies ruled out a series of potential contributors to the sample-thickness dependence, but yielded no clear cause. Although the clinical implications of the small but systematic sample-thickness dependence may be negligible, these studies may provide additional insights into the propagation of ultrasonic waves in cancellous bone and how to maximize the quality of information obtained

Physics of ultrasonic wave propagation in bone and heart characterized using Bayesian parameter estimation

This Dissertation explores the physics underlying the propagation of ultrasonic waves in bone and in heart tissue through the use of Bayesian probability theory. Quantitative ultrasound is a noninvasive modality used for clinical detection, characterization, and evaluation of bone quality and cardiovascular disease. Approaches that extend the state of knowledge of the physics underpinning the interaction of ultrasound with inherently inhomogeneous and isotropic tissue have the potential to enhance its clinical utility. Simulations of fast and slow compressional wave propagation in cancellous bone were carried out to demonstrate the plausibility of a proposed explanation for the widely reported anomalous negative dispersion in cancellous bone. The results showed that negative dispersion could arise from analysis that proceeded under the assumption that the data consist of only a single ultrasonic wave, when in fact two overlapping and interfering waves are present. The confounding effect of overlapping fast and slow waves was addressed by applying Bayesian parameter estimation to simulated data, to experimental data acquired on bone-mimicking phantoms, and to data acquired in vitro on cancellous bone. The Bayesian approach successfully estimated the properties of the individual fast and slow waves even when they strongly overlapped in the acquired data. The Bayesian parameter estimation technique was further applied to an investigation of the anisotropy of ultrasonic properties in cancellous bone. The degree to which fast and slow waves overlap is partially determined by the angle of insonation of ultrasound relative to the predominant direction of trabecular orientation. In the past, studies of anisotropy have been limited by interference between fast and slow waves over a portion of the range of insonation angles. Bayesian analysis estimated attenuation, velocity, and amplitude parameters over the entire range of insonation angles, allowing a more complete characterization of anisotropy. A novel piecewise linear model for the cyclic variation of ultrasonic backscatter from myocardium was proposed. Models of cyclic variation for 100 type 2 diabetes patients and 43 normal control sub jects were constructed using Bayesian parameter estimation. Parameters determined from the model, specifically rise time and slew rate, were found to be more reliable in differentiating between sub ject groups than the previously employed magnitude parameter

The Generation and Control of Ultrasonic Waves in Nonlinear Media

The objective of this thesis is to utilise modern open-design ultrasound research platforms to develop new and advance several existing techniques that incorporate nonlinear phenomena. Acoustically, nonlinearity refers to changes in speed of sound, attenuation or elasticity that vary with frequency, temperature or pressure. These effects cannot be linearised by the wave equation and require fluid dynamics and elasticity equations to be fully understood. While this is a hindrance and source of error in many areas of ultrasound such as high-intensity focused ultrasound (HIFU) and medical imaging, nonlinearities do have uses in non-destructive guided wave (GW) testing. These effects are influenced greatly by the transducer surface pressure, and so precise control of the excitation is necessary to achieve the desired nonlinear effect, if any, in the medium. In this thesis, aided by the use of two new research platforms, several new ultrasound techniques were developed. It was shown the frequency content in the electrical waveform is pertinent and so distortion must be minimised. This requirement conflicts with several hardware limitations, however. Accordingly, a genetic algorithm was applied to find novel switched waveform designs. It was found to achieve a 2% granularity in amplitude control with harmonic reduction, where existing waveform designs could not produce any. This fine amplitude control is a requirement for array applications. Following this, a technique to control the direction of GWs without knowledge of the waveguide was devised. Recordings of a propagating GW, induced by the first element of an array transducer, were re-transmitted in a recursive fashion. The effect was that the transducer's transmissions constructively interfered with the transverse wave, causing most of the guided wave energy to travel in the direction of the transducer's spatial influence. Experimental results show a 34 dB enhancement in one direction compared with the other. GWs were then applied to bone for two purposes: for assessment of osteoporosis and for measurement of skull properties to assist transcranial therapy. It was shown that existing methods for obtaining dispersion curves are ineffectual due to limitations in the available sampling area. A signal processing scheme was devised to temporally align transverse dispersive waves so that beamforming style techniques could be applied to prove or disprove the existence of certain modes. The technique in combination with multiplication was applied to numerical, ex vivo and in vivo experiments. It was found to improve the contrast of the higher order modes. The technique could improve the reliability of osteoporosis diagnosis with ultrasound, but may also prove useful for acquiring dispersion images in NDT. Numerically the technique was shown to improve the S3 and A3 mode intensity by 6 dB and 13 dB respectively compared with an existing Fourier method. In skull, a relationship was found between the curved therapeutic array geometry and the delay profile necessary to form GWs in skull. Several numerical models were tested and it was shown that the thickness could be obtained from the group velocity. The estimated maximum error using this technique was 0.2 mm. Since the data is co-registered with the therapeutic elements, this method could be used to improve the accuracy of thermal treatments in the brain. Finally, the application of switched excitation for HIFU was considered. To improve on cost, efficiency and size, alternative excitation methods have the potential to replace the linear amplifier circuitry currently used in HIFU. In this final study, harmonic reduction pulse width modulation (HRPWM) was proposed as an algorithmic solution to the design of switched waveforms. Its appropriateness for HIFU was assessed by design of a high power 5 level unfiltered amplifier and subsequent thermal-only lesioning of ex vivo chicken breast. HRPWM produced symmetric, thermal-only lesions that were the same size as their linear amplifier equivalents (p > 0.05). These results demonstrate that HRPWM can minimise HIFU drive circuity size without the need for filters to remove harmonics or adjustable power supplies to achieve array apodisation. Overall it has been shown in this thesis that precise control of the nonlinear wave phenomena can be afforded when using open-platform ultrasound research hardware. The methods described within may reduce the cost and increase the efficacy of future commercial systems