This thesis quantitatively investigates gaseous electron-ion recombination in an environmental scanning electron microscope (ESEM) at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector (GSED) images immediately after a region of enhanced secondary electron (SE) emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron-ion recombination that takes into consideration transients caused by the time constant of the GSED electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks is then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients, and (iii) electron drift velocities, as well as absolute values of the total time constant of the detection system, to be determined as a function of microscope operating parameters. Results reveal the exact dependence that the effects of SE-ion recombination on signal formation have on reduced electric field intensity and time in ESEM. Furthermore, the model implicitly demonstrates that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought. Following that the generation and detection of gaseous scintillation and electro- luminescence produced via electron-gas molecule excitation reactions in ESEM is investigated. Here a novel gaseous scintillation detection (GSD) system is developed to efficiently detect photons produced. Images acquired using GSD are compared to those obtained using conventional GSED detection, and demonstrate that images rich in SE contrast can be achieved using such systems. A theoretical model is developed that describes the generation of photon signals by cascading SEs, high energy backscattered electrons (BSEs) and primary beam electrons (PEs). Photon amplification, or the total number of photons produced per sample emissive electron, is then investigated, and compared to conventional electronic amplification, over a wide range of microscope operating parameters, imaging gases and photon collection geometries. The main findings of the investigation revealed that detected electroluminescent signals exhibit larger SE signal-to-background levels than that of conventional electronic signals detected via GSED. Also, dragging the electron cascade towards the light pipe assemblage of GSD systems, or electrostatic focusing, dramatically increases photon collection efficiencies. The attainment of such an improvement being a direct consequence of increasing the `effective' solid angle for photon collection. Finally, in attempt to characterize the scintillating wavelengths arising from sample emissive SEs, PEs, BSEs, and their respective cascaded electrons, such that future photon filtering techniques can be employed to extract nominated GSD imaging signals, the emission spectra of commonly utilized electroluminescent gases in ESEM, such as argon (Ar) and nitrogen (N2), were collected and investigated. Spectra of Ar and N2 reveal several major emission lines that occur in the ultraviolet (UV) to near infrared (NIR) regions of the electromagnetic spectrum. The major photon emissions discovered in Ar are attributed to occur via atomic de-excitation transitions of neutral Ar (Ar I), whilst for N2, major emissions are attributed to be a consequence of second positive band vibrational de-excitation reactions. Major wavelength intensity versus gas pressure data, for both Ar and N2, illustrate that wavelength intensities increase with decreasing pressure. This phenomenon strongly suggesting that quenching effects and reductions in excitation mean free paths increase with imaging gas pressure.

This thesis quantitatively investigates gaseous electron-ion recombination in an environmental scanning electron microscope (ESEM) at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector (GSED) images immediately after a region of enhanced secondary electron (SE) emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron-ion recombination that takes into consideration transients caused by the time constant of the GSED electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks is then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients, and (iii) electron drift velocities, as well as absolute values of the total time constant of the detection system, to be determined as a function of microscope operating parameters. Results reveal the exact dependence that the effects of SE-ion recombination on signal formation have on reduced electric field intensity and time in ESEM. Furthermore, the model implicitly demonstrates that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought. Following that the generation and detection of gaseous scintillation and electro- luminescence produced via electron-gas molecule excitation reactions in ESEM is investigated. Here a novel gaseous scintillation detection (GSD) system is developed to efficiently detect photons produced. Images acquired using GSD are compared to those obtained using conventional GSED detection, and demonstrate that images rich in SE contrast can be achieved using such systems. A theoretical model is developed that describes the generation of photon signals by cascading SEs, high energy backscattered electrons (BSEs) and primary beam electrons (PEs). Photon amplification, or the total number of photons produced per sample emissive electron, is then investigated, and compared to conventional electronic amplification, over a wide range of microscope operating parameters, imaging gases and photon collection geometries. The main findings of the investigation revealed that detected electroluminescent signals exhibit larger SE signal-to-background levels than that of conventional electronic signals detected via GSED. Also, dragging the electron cascade towards the light pipe assemblage of GSD systems, or electrostatic focusing, dramatically increases photon collection efficiencies. The attainment of such an improvement being a direct consequence of increasing the `effective' solid angle for photon collection. Finally, in attempt to characterize the scintillating wavelengths arising from sample emissive SEs, PEs, BSEs, and their respective cascaded electrons, such that future photon filtering techniques can be employed to extract nominated GSD imaging signals, the emission spectra of commonly utilized electroluminescent gases in ESEM, such as argon (Ar) and nitrogen (N2), were collected and investigated. Spectra of Ar and N2 reveal several major emission lines that occur in the ultraviolet (UV) to near infrared (NIR) regions of the electromagnetic spectrum. The major photon emissions discovered in Ar are attributed to occur via atomic de-excitation transitions of neutral Ar (Ar I), whilst for N2, major emissions are attributed to be a consequence of second positive band vibrational de-excitation reactions. Major wavelength intensity versus gas pressure data, for both Ar and N2, illustrate that wavelength intensities increase with decreasing pressure. This phenomenon strongly suggesting that quenching effects and reductions in excitation mean free paths increase with imaging gas pressure.

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<p>In the human body, turbulent flow is associated with many complications. Turbulence typically occurs downstream from stenoses and heart valve prostheses and at branch points of arteries. A proper way to study turbulence may enhance the understanding of the effects of stenoses and improve the functional assessment of damaged heart valves and heart valve prostheses.</p><p>The methods of today for studying turbulence in the human body lack in either precision or speed. This thesis exploits a magnetic resonance imaging (MRI) phenomenon referred to as signal loss in order to develop a method for estimating turbulence intensity in blood flow.</p><p>MRI measurements were carried out on an appropriate flow phantom. The turbulence intensity results obtained by means of the proposed method were compared with previously known turbulence intensity results. The comparison indicates that the proposed method has great potential for estimation of turbulence intensity.</p>

In the human body, turbulent flow is associated with many complications. Turbulence typically occurs downstream from stenoses and heart valve prostheses and at branch points of arteries. A proper way to study turbulence may enhance the understanding of the effects of stenoses and improve the functional assessment of damaged heart valves and heart valve prostheses. The methods of today for studying turbulence in the human body lack in either precision or speed. This thesis exploits a magnetic resonance imaging (MRI) phenomenon referred to as signal loss in order to develop a method for estimating turbulence intensity in blood flow. MRI measurements were carried out on an appropriate flow phantom. The turbulence intensity results obtained by means of the proposed method were compared with previously known turbulence intensity results. The comparison indicates that the proposed method has great potential for estimation of turbulence intensity.

With the increase in and more diverse applications of microwave radio communications, the probability of a signal propagating through a medium of vegetation is increased. As a direct result of this demand for microwave communication systems, knowledge is required of the effects of vegetation media on the propagating microwave signal. This enables radio system planners to predict the signal loss more accurately, necessitating a detailed study of the propagation effects of vegetation. A vegetation depth attenuation model has been developed based on the International Telecommunications Union-Radio Sector model and validated against measurements conducted at two microwave frequencies of 11.2 GHz and 20 GHz. The measurements were conducted on a number of sites of differing geometries at different times of the year to obtain the two extreme states of foliage, in- and out-of-leaf. The trees found at the sites were of a number of indigenous species. A variety of species and environments were employed for the outdoor measurements as it was felt that any variation in the signal, occurring as a direct result of the species, climate, environment etc., would be reduced. A further study has been conducted in an anechoic chamber, the purpose being to investigate the depolarising effect of vegetation, to characterise and to ascertain how and to what extent the polarisation of the incident signal is changed as it passes through the vegetation without the effects of climate, location and environment affecting the resultant signal. To enable larger quantities of data to be obtained, collated and subsequently analysed and also to remove any scope for error during the collection of results, two data acquisition programs were written for the two main environments in which the measurements were to be undertaken, that is to say, outdoor and indoor (anechoic chamber) environments. In seeking to provide a model for the prediction of attenuation a radio wave will suffer as it is propagated through a body of vegetation, several models have been examined in turn and their relative merits discussed together with their applicability to the study. After examining the possible models available, the thesis provides a model which enables the prediction of additional attenuation a radiowave signal will suffer as a function of path length (depth) of the vegetation medium and frequency. The model can be recommended for use in the 10-30 GHz band. The study on the depolarisation of signals by vegetation has shown that the components of a vegetation medium e.g. tree trunks, branches and leaves, can cause considerable changes in the polarisation of the incident signal as it propagates through a volume of vegetation. The work presented in this thesis contains new measured results of the polarisation state of the radio wave as it emerges from a vegetation specimen. These results obtained in an anechoic chamber under controlled conditions have demonstrated that additional effects, other than attenuation by absorption and scatter need to be considered in order to characterise and subsequently model the overall effect of vegetation in the radio path of propagating signals.

This thesis presents a comprehensive work on computerised analysis of fetal heart rate (FHR) features, including feature extraction, feature selection, analysis of influencing factors and setting up/validation of a computerised decision support system. Firstly, a novel feature – pattern readjustment – was extracted and tested. Clinical data were used to train a Support Vector Machine (SVM) to detect pattern readjustment. Then, the association of pattern readjustment and adverse labour outcome was investigated. The validation results with clinical experts show that the pattern readjustment can be accurately detected, while the study on labour outcome shows that the feature is related to fetal acidemia at birth. Secondly, Genetic Algorithms were employed as a feature selection method to select a best subset of FHR features and to use them to predict fetal acidemia with linear and nonlinear SVM. The diagnostic power of the classifier output using selected features was tested on the total set of 7,568 cases. As the classifier output increases, there is a consistent increase of the risk of fetal acidemia. Thirdly, an important influencing factor on FHR features - signal loss – was investigated. A bivariate model was built to estimate error based on signal loss. Validation results show that the bivariate model can accurately predict the error generated by signal loss. The influence of signal loss on labour outcome classification was also investigated. Finally, a computerised decision support system to estimate the risk of fetal acidemia was set up based on the above studies. The system was validated using new retrospective data. Validation results show that the system is capable of predicting adverse labour outcome and providing timely decision support. It is the first time an intrapartum computerised FHR decision support system has been built and validated on this size of dataset. With further improvements, such a system could be implemented clinically in the long term.