Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1968. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliography.

Thesis (M.S.)--University of Wisconsin--Madison, 1966. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Introduction: Electrical Impedance Tomography (EIT) is an emerging clinical imaging technique. Functional EIT by Evoked Response (fEITER) was developed at the University of Manchester as a high-speed, functional brain imaging device for use at the bedside. This 32-electrode EIT system applies an injection frequency of 10kHz and captures data using a 10ms temporal resolution. This thesis reports on the first volunteer and patient trials undertaken using fEITER for the following conditions: (a) flashing visual sequence - 14 awake volunteers; (b) a voluntary Valsalva manoeuvre (VM) - 15 awake volunteers and (c) during the induction of anaesthesia - 16 elective surgical patients. Aims: The research presented in this thesis was undertaken to differentiate between noise and physiological changes in raw fEITER data signals. Methods: SNR was determined for fEITER. Raw fEITER signals were pre-processed to reduce noise and dominant trends before multiple comparisons between reference and stimulus data were undertaken. Histograms and ROC curves were produced to illustrate the difference between reference and stimulus fEITER data. AUC values for single-subject and pooled ROC curves were calculated to determine whether fEITER data can be reliably differentiated between reference and stimulus conditions. Approximate Entropy (ApEn) was applied to evaluate the regularity of high frequency components within fEITER data for each trial condition. Results: Average SNR values for fEITER acquired using mesh and physical phantoms ranged from 62.94dB to 63.58dB, and 28.29dB to 31.45dB respectively. The following AUC values were acquired: Visual stimulus-frontal electrode pairs and electrode pairs overlying the visual cortex 0.520 and 0.505 respectively; VM: 0.658; and induction of anaesthesia: 0.547. The VM induced the greatest difference between pooled reference and stimulus data. Visual stimulation and induction of anaesthesia data showed poor distinction between pooled reference and stimulus data, although some single subject data did show a significant response. No significant differences were acquired for the comparison of ApEn-reference and ApEn-stimulus data for all trial conditions using a Wilcoxon's signed ranks test (visual stimulus-frontal electrode pairs: upper p = 0.998, visual stimulus-electrode overlying the visual cortex: upper p = 0.980; the VM: upper p = 0.976, and induction of anaesthesia: p = 0.912). Discussion: Although single-subject and pooled fEITER data recorded during the VM produced the greatest differences between reference and stimulus measurements, stimuli such as visual flashes and induction of anaesthesia may not be large enough to induce quantifiable changes between reference and stimulus data recorded from single electrode pairs. Collectively, these results provide little evidence to show that pre-processing of raw fEITER data amplifies features in fEITER waveforms which may be representative of physiological changes induced by an applied stimulus.

Electrical bio-impedance can be considered as a physiological measurement of significant value in research and clinical applications. This is because certain structures of living organisms and the human body such as tissues and membranes are characterized by their specific impedance, which can be easily measured with existing technology. In addition, many body functions (physiological processes) involve changes in their ionic and electrolytic content and size, features that are followed by changes in their electrical characteristics indicated by their impedance values. The use of surface electrodes for measuring impedance in humans makes the method more attractive. Thus, during the 2nd half of the last century, impedance techniques have been developed to measure the blood flow in limbs, to form cross-sectional images of human body sections and to define body composition. The imaging techniques suffer from poor resolution due to the anisotropic electrical properties of the tissues. Epigastrography, based on determination of electrical impedance changes, is a simple, inexpensive, radiation free technique, which can be repeated many times without any trauma to the patient and is a valuable research tool. This work involves the generation, analysis and interpretation of epigastric electrical impedance signals. The signals recorded represent the epigastric impedance of a fasting volunteer before, during the oral intake of a liquid meal of typically 450 mL and for periods between 45 minutes to 2 hours post-prandially. The half emptying times (T50s) were calculated for a variety of test meals, and the statistically significant differences between the T50s were found for meals which varied in calorific content in a total of 7 studies with at least 9 subjects per study. Significant differences between T50s were found also when testing the same meal but under different conditions, namely, intravenous infusion of peptides (GLP-1, loxiglumide) or placebo (physiological saline) and the release in the gastric cavity of amino acids, free or bound with an orally taken gelatine capsule. However, the impedance T50s calculated were found to be considerably shorter than half emptying times presented in the literature using other techniques. Simultaneous application of scintigraphy confirmed that the T50s based on impedance were in comparison shorter. Similarly when experiments were carried out, simultaneously with the octanoic acid breath test and application of paracetamol absorption the same shorter, in comparison, values of T50s were obtained. The comparison with scintigraphy strengthened the suspicion that impedance values were being strongly influenced by the presence of gastric acid. Gastric acid studies undertaken for a number of conditions supported the considerable influence that gastric acid played. In vitro measurements of conductivity and pH of mixtures of test meals with gastric juice aspirations from subjects showed the acid influence. Finally, the physiology of the gastric mucosa from the resting state to secretion leads to the hypothesis that epigastric impedance is in the main controlled by the conductivity of the gastric content rather than by the volume of the meal content in the stomach and is supported by the laws of physics underlying the electrical impedance of a bulk object. In conclusion, both experimental and clinical results from the studies and investigations undertaken in this research convinced the author that the epigastric impedance reflects the conductivity and the acidity of the gastric content. Further by carrying out signal processing of the epigastric impedance signals employing fast Fourier transformation information was extracted about the gastric contractions in the range of 2.5 to 4.5 cycles per minute, confirming the usefulness of the technique in the study of gastric motility. In addition studies are necessary, to establish the validity of the results in the clinical environment and to answer conclusively the question "whether epigastric impedance can be developed as an external monitoring technique of gastric acidity and motility".

No description available.

Electrical impedance tomography (EIT) is an imaging modality with many possible practical applications. It is mainly used for geophysical applications, for which it is called electrical resistivity tomography. There have also been many proposed medical applications such as respiratory monitoring and breast tumour screening. Although there have been many uniqueness and stability results published over the last few decades, most of the results are in the context of the theoretical continuous problem. In practice however, we almost always have to solve a discretised problem for which very few theoretical results exist. In this thesis we aim to bridge the gap between the continuous and discrete problems. The first problem we solve is the three-dimensional triangulation problem of uniquely embedding a tetrahedral mesh in R3. We parameterise the problem in terms of dihedral angles and we provide a constructive procedure for identifying the independent angles and the independent set of constraints that the dependent angles must satisfy. We then use the implicit function theorem to prove that the embedding is locally unique. We also present a numerical example to illustrate that the result works in practice. Without the understanding of the geometric constraints involved in embedding a three-dimensional triangulation, we cannot solve more complex problems involving embeddings of finite element meshes. We next investigate the discrete EIT problem for anisotropic conductivity. It is well known that the entries of the finite element system matrix for piecewise linear potential and piecewise constant conductivity are equivalent to conductance values of resistors defined on the edges of the finite element mesh. We attempt to tackle the problem of embedding a finite element mesh in R3, such that it is consistent with some known edge conductance values. It is a well known result that for the anisotropic conductivity problem, the boundary data is invariant under diffeomorphisms that fix the boundary. Before investigating this effect on the discrete case, we define the linear map from conductivities to edge conductances and investigate the injectivity of this map for a simplistic example. This provides an illustrative example of how a poor choice of finite element mesh can result in a non-unique solution to the discrete inverse problem of EIT. We then extend the investigation to finding interior vertex positions and conductivity distributions that are consistent with the known edge conductances. The results show that if the total number of interior vertex coordinates and anisotropic conductivity variables is larger than the number of edges in the mesh, then there exist discrete diffeomorphisms that perturb the vertices and conductivities such that no change in the edge conductances is observed. We also show that the non-uniqueness caused by the non-injectivity of the linear map has a larger effect than the non-uniqueness caused by diffeomorphism invariance.

Carbon nanotubes (CNTs) have been successfully grafted onto the surfaces of spherical silica gels via a floating-catalyst chemical vapour deposition method. Two types of silica gels were used as substrates; SG6 (6 – 8 nm pore size) and SG26 (26 – 34 nm pore size). The optimum growth conditions were found to be 760 °C growth temperature, 3 hours growth time, and 5 wt .% of ferrocene catalyst (dissolved in toluene) injected into the furnace at a rate of 0.04 ml/min. Under these conditions, CNTs coated the exterior surface of the gels with growth occurring from both the pores and from the surface. The geometry and porosity of the silica gel were also found to influence the alignment and density of the grafted CNTs, with SG6 producing the best quality hybrid particle under the above conditions (labelled SG6_3). Thermal Gravimetric Analysis showed the yield of CNTs grown under these optimum condition was 33.6 + 0.37 wt. %. As the CNTs remained strongly attached to the surface of the gel, the grafting process produced excellent dispersion of the CNTs within polymer matrices as CNT bundle formation was prevented. The inclusion of micron size silica introduced a large excluded volume within polymer matrices, with the dispersion of CNTs restricted to a region along the silica surface. This arrangement aided the formation of an electrically conductive network in a poly (vinyl) alcohol matrix, where the critical percolation threshold (pc) was calculated as 0.62 wt. % of SG6_3, equivalent to a CNT content of 0.2 wt. %.The inclusion of SG6_3 into an epoxy resin suspension did not alter the rheological behaviour of the resin up to the highest loading employed (5 wt. %, ≈1.65 wt. % of CNTs). Within the linear viscoelastic region the viscoelastic moduli of the neat resin remained within the same magnitude, whilst the addition of 1.65 wt. % of commercially available CNTs into the same resin increased the storage and loss moduli by up to seven and three orders of magnitude, respectively. The complex viscosity of the suspension remained unchanged regardless of the amount of SG6_3 added; whereas an increase in viscosity of up to five orders of magnitude resulted from the addition of 1.65 wt. % non – grafted CNTs.Grafting of CNTs onto the surface of micron size spherical silica gel has been shown to provide a means of incorporating CNTs into a polymer without increasing viscosity. In addition, the SG6_3 also formed an electrically-conductive percolated network in an epoxy resin composite at low levels of addition, with pc = 0.16 wt. % of CNTs (i.e. 0.5 wt. % SG6_3).

Electrical impedance tomography is an imaging method with applications to geophysics and medical imaging. A new approximation is presented based on Nachman's 2-dimensional construction for closed domains. It improves upon existing approximations by extending the range of application from resolving 2 times the surface conductivity to imaging perfect conductors and insulators. With perfect knowledge of boundary data, this approximation exactly resolves a single conductive disc embedded in a homogenous domain. The problem, however, is ill-posed, and imaging performance degrades quickly as the distance from the boundary increases. The key to the approximation lies in (a) approximating Fadeev's Green's function (b) pre-processing measured voltages based on a boundary-integral equation (c) solving a linearized inverse problem (d) solving a d-bar equation, and (e) scaling the resulting image based on analytical results for a disc. In the development of the approximation, a new formula for Fadeev's Green's function is presented in terms of the Exponential Integral function. Also, new comparisons are made between reconstructions with and without solving the d-bar equation, showing that the added computational expense of solving the d-bar equation is not justified for radial problems. There is no discernible improvement in image quality. As a result, the approximation converts the inverse conductivity problem into a novel one-step linear problem with pre-conditioning of boundary data and scaling of the resulting image. Several extensions to this work are possible. The approximation is implemented for a circular domain with unit conductivity near the boundary, and extensions to other domains, bounded and unbounded should be possible, with non-constant conductivity near the boundary requiring further approximation. Ultimately, further research is required to ascertain whether it is possible to extend these techniques to imaging problems in three dimensions. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

This thesis is concerned with the problem of estimating broadband acoustic impedance from normal incidence reflection seismograms. This topic is covered by following the linear inverse formalisms described by Parker (1977) and Oldenburg (1980). The measured seismogram is modelled as a convolution of subsurface reflectivity with a source wavelet. Then an appraisal of the seismogram is performed to obtain unique bandlimited reflectivity information. This bandlimited reflecitivity information is then utilized in two different construction algorithms which provide a broadband estimate of reflectivity; from which a broadband impedance function may be computed. The first construction method is a maximum entropy method which uses an autoregressive representation of a small portion of the reflectivity spectrum to predict spectral values outside that small portion. The second and most versatile construction method is the linear programming approach of Levy and Fullagar (1981) which utilizes the unique bandlimited spectral information obtained from an appraisal and provides a broadband reflectivity function which has a minimum 1( norm. Both methods have been tested on synthetic and real seismic data and have shown good success at recovering interpretable broadband impedance models. Errors in the data and the uniqueness of constructed reflectivity models play important roles in estimating the impedance function and in assessing its uniqueness. The Karhunen-Loeve transformation is discussed and applied on real data to stabilize the construction results in the presence of noise. The generally accepted idea that low frequency impedance information must be supplied from well log or velocity analyses because of the bandlimited nature of seismic data has been challenged. When accurate, bandlimited reflectivity information can be recovered from the seismic trace, then an interpretable, broadband impedance model may be recovered using the two construction algorithms presented in this thesis. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate