In 3-dimensional positron emission tomography (PET), the proportion of the detected photon coincidences arising from scatter can be very high, up to 60%. A standard approach to scatter correction in 3D PET is to estimate the scattered coincidences and subtract them from the list of recorded events. More recently, novel techniques have been proposed in which the information carried by the scattered photons was extracted and directly used in the image reconstruction step. In this work, the algorithms were extended for use in non-TOF 3D PET systems and were qualitatively and quantitatively evaluated. Despite some promising initial outcomes, standard performance metrics of images reconstructed with each technique, representing the quality of images, was shown to be deteriorated under specified conditions. Further work is required to investigate potential benefits of the proposed algorithms for both ideal and clinical conditions.

Stem cell therapy is emerging as a possible method for treating many diseases and disorders, such as cardiovascular disease. In particular, stem cells may be able to revive the dead tissue caused by acute myocardial infarction (heart attack). Adipose-derived stem cells were labelled with 18F-fluorodeoxyglucose (FDG) and superparamagnetic iron oxide (SPIO) particles, for imaging with positron emission tomography (PET) and magnetic resonance imaging (MRI), respectively, and injected into several rats following induced myocardial infarction. Stem cell retention in the heart was investigated following three injection sites; two within the heart (intramyocardial and left intraventricular), and one easily accessible vein (tail vein). The PET and MR images were registered and the initial distributions analyzed using region of interest (ROI) analysis, to determine which injection method would result in the highest stem cell retention in the infarcted heart. The ROI results determined that the intramyocardial injection had the highest % injected dose (%ID) in the heart with 14 +/- 4%, followed by left intraventricular and tail vein with %IDs of 3.6 +/- 0.8% and 1.2 +/- 0.6%, respectively. The results indicate that stem cell delivery via intramyocardial injection should be utilized for optimal retention in the heart.

Vacuum microelectronic (VME) devices are of interest for the development of flat-screen displays and microwave devices. In many cases, their operation depends on the field emission of electrons from micron-sized cathodes (semiconductor or metal), into a vacuum. Major challenges to be met before these devices can be fully exploited include obtaining - low operating voltages, high maximum emission currents, uniform emission characteristics, and long-term emission stability. The research in this thesis concerns the production of silicon field emitters and the improvement of their emission properties by the process of anodisation. Anodisation was carried out for short times, in order to form a very thin layer of porous silicon (PS) at the surface of both p and p⁺-type silicon emitters. The aim in doing this was to form a high density of asperities over the surface of the emitters. It was the intention that these asperities, rather than the "macroscopic" apex of the emitter, would control emission. This was the first work of its kind to be carried out. Transmission electron microscopy was used to characterise the morphology of p and p⁺-type silicon emitters before and after anodisation. Both the structure and arrangement of the surface fibrils, the thickness of the PS layers at the apex and nature of PS cross-sections were studied. The morphology was correlated to subsequent field emission measurements. Field emission characteristics, before and after anodisation, were obtained using a scanning electron microscope adapted for field emission measurements, and a field emission microscope. Extensive measurements showed that, following anodisation, there was substantial improvement in emission behaviour. After anodisation, the following was found to be true: i) The starting voltage was reduced by up to 50% (with p<sup>+</sup -type PS emitters exhibiting a greater reduction in starting voltage than p-type PS emitters). ii) Number of emitting tips per array was increased. iii) Higher maximum currents (up to 3 times higher) were obtained before tips underwent destruction. iv) The resistive effect of the PS layer at the apex was important in determining the maximum current obtained from a tip. In addition, both field emission and field ion microscopy were used to identify the emission source following anodisation. It was shown that individual fibrils on the emission surface caused an increase in field enhancement over a flat plane, leading to emission at lower voltage. Overall, porous silicon appears to be a very promising material for field emission displays.

The objective of this study was to fully characterise drift phenomena in inductively coupled plasma atomic emission spectroscopy (ICP-AES) in order to develop novel correction procedures to aid routine analysis. Long-term drift of the analytical signal continues to be a potential disadvantage when using ICP-AES and often necessitates regular recalibration. The long-term stability of three commercially available Instruments was studied using In each case a range of analyte and intrinsic plasma emission lines. Long-term fluctuations were observed which generated drift bias of up to 20% on the initial values. The drift pafterns were characterised and found to be qualitatively reproducible. In most cases, similar long-term fluctuations were observed independent of the analyte or nature of the emission line. In addition, high inter-element correlation was observed on the long-term fluctuations even when sequential acquisition was employed. In order to study the fundamental causes of drift, the effect of two key instrumental parameters, i.e. the RF power and the nebutiser gas flow rate were studied with respect to the stability of the signal. Different drift patterns were found depending on the working conditions. Classical statistical methods and a multi-way approach. PARAFAC. were then employed to describe the system. The use of intemal standards to correct for drift has also been investigated, but found to be of benefit only under certain defined conditions (i.e. robust conditions, high RF power and low nebuliserflow rate). At soft conditions, low RF power and medium to high nebuliser flow rate, the system Is very unstable and intemal standardisation is not fully effective as a correction method. For such conditions, a novel correction procedure has been developed, which employs the drift pattem of one intrinsic plasma line (i.e. an argon line) and a correction factor which is specific for each emission line. The drift values were reduced from around 20% before correction to better than ±2% following the described protocol. Finally, the effects of chemical matrices on the long-term stability of the emission signals have been evaluated. Three synthetic matrices were prepared simulating nitric, soil and water matrices. The stability of the instrument when working wrth these matrices at both robust and soft conditions was found to be poor, especially when the solution was matched with the soil matrix. The use of more robust conditions did not improve the long-temi stability of the emission signals. The outcome of this study proved to be a better understanding of drift phenomena and a novel method for drift correction.

No description available.

Volcanic gases carry valuable information about processes occurring at active volcanoes, and so their accurate measurement and analysis are highly desirable. Masaya is a low-lying persistently active basaltic volcano, which is renowned for strong passive gas emission unaccompanied by lava extrusion, although it has also undergone plinian eruptions in the past. Its consistent behaviour sustains a reliable tropospheric plume, making it an ideal location at which to study volcanic degassing. During February-March 1998 and March 1999, Masaya's gas plume composition was investigated using the new ground-based remote sensing technique of open-path Fourier transform infrared spectroscopy. This technique meets criteria for safe and accurate quantitative characterisation of volcanic gases and on a temporal resolution previously impracticable. Flexibility of operation is the chief merit of OP-FTIR, since a range of infrared sources are available (e.g. an active lamp, the sun or hot volcanic vents). Laboratory calibration experiments using primary gas standards confirmed that the instrument and subsequent spectral analysis provide highly accurate concentration measurements for volcanic species, with errors typically around 5 %. Results showed that Masaya's plume composition was little changed between the field seasons. Average molar ratios for S0₂/HCl, HCl/HF, C0₂/S0₂ and H₂0/S0₂ were 1.6, 4.9, 2.2 and 72, respectively, in 1998, and 1.6, 5.1, 2.3 and 66 in 1999. These ratios, coupled with simultaneous COSPEC-derived SO₂ data, indicated that emission rates, especially of HCl and HF, were high compared with other passively-degassing volcanoes. Measurements using the spectrometer with different infrared sources and at different locations downwind demonstrated that tropospheric scavenging processes had little discernible effect on plume composition. in the light of these new OP-FTIR gas data, possible physical mechanisms for the degassing behaviour at Masaya Volcano have been discussed. In order to account for all the observations, an integrated model, in which gas emission rates are primarily controlled by degassing-driven convection in the conduit, is proposed.

No description available.

Stem cell therapy is emerging as a possible method for treating many diseases and disorders, such as cardiovascular disease. In particular, stem cells may be able to revive the dead tissue caused by acute myocardial infarction (heart attack). Adipose-derived stem cells were labelled with 18F-fluorodeoxyglucose (FDG) and superparamagnetic iron oxide (SPIO) particles, for imaging with positron emission tomography (PET) and magnetic resonance imaging (MRI), respectively, and injected into several rats following induced myocardial infarction. Stem cell retention in the heart was investigated following three injection sites; two within the heart (intramyocardial and left intraventricular), and one easily accessible vein (tail vein). The PET and MR images were registered and the initial distributions analyzed using region of interest (ROI) analysis, to determine which injection method would result in the highest stem cell retention in the infarcted heart. The ROI results determined that the intramyocardial injection had the highest % injected dose (%ID) in the heart with 14 +/- 4%, followed by left intraventricular and tail vein with %IDs of 3.6 +/- 0.8% and 1.2 +/- 0.6%, respectively. The results indicate that stem cell delivery via intramyocardial injection should be utilized for optimal retention in the heart.

Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 5 uppsatser.

Diss. (sammanfattning) Uppsala : Univ., 2001. / Härtill 6 uppsatser.