Les nouvelles gamma-caméras à semi-conducteurs utilisant des détecteurs au CZT sont dédiées aux explorations cardiaques. Leurs sensibilité, résolution spatiale et en énergie nettement améliorées comparativement aux gamma-caméras conventionnelles sont une révolution en médecine nucléaire. Ces gamma-caméras utilisent de nouvelles géométries d’acquisition, de nouveaux algorithmes de reconstruction, et ouvrent de nouvelles perspectives dans les études simultanées en double-isotope de l’123I et du 99mTc, dont les pics énergétiques sont proches.Nous avons étudié l’impact de l’amélioration de la résolution en énergie en comparant deux modèles de gamma-caméras à détecteurs semi-conducteurs aux gamma-caméras conventionnelles. A l’aide d’études sur fantômes anthropomorphes et chez des patients porteurs d’insuffisance cardiaque, notre travail s’est concentré sur les acquisitions scintigraphiques (i) de la fonction ventriculaire gauche (99mTc) en présence d’123I, (ii) de la perfusion myocardique (99mTc) en présence d’123I (innervation), et (iii) du rapport cardiomédiastinal de la fixation d’123I- métaiodobenzylguanidine (123I-MIBG) lors d’acquisitions double-isotope (123I-MIBG/99mTc- tétrofosmine) chez les patients souffrant d’insuffisance cardiaque.Nos résultats montrent que la meilleure résolution en énergie des gamma-caméras CZT permet en étude double-isotope (i) une évaluation de la FEVG et du mouvement régional dans les différentes fenêtres d'énergie (123I ou 99mTc) et les types d'acquisition (simple- vs double-isotope), (ii) une évaluation simultanée et combinée de la perfusion (99mTc) et de l’innervation (123I) du myocarde, et (iii) l’évaluation du rapport cardiomédiastinal de la fixation d’123I-MIBG. Chacune de ces trois parties a fait l’objet d’une publication. / New dedicated-cardiac cameras using CZT detectors have dramatically transformed the routine of myocardial perfusion imaging. With a better count detection sensitivity, an improved spatial and energy resolution, they potentially enable combined assessment of myocardial innervation (123I) and perfusion (99mTc) within a single imaging session. These cameras images with different sharpness and contrast-to-noise ratios.Using two CZT cameras with anthropomorphic phantom, and clinical studies in heart failure patients, our work focused on (i) the left ventricular function assessment within the 99mTc window in presence of 123I, (ii) the evaluation of regional myocardial innervation (123I) and perfusion (99mTc) match and mismatch with single- (separate 123I and 99mTc acquisition) and simultaneous dual-isotope acquisitions, and (iii) the late heart-to-mediastinal ratio (HMR) of 123I-MIBG uptake determined using dual-isotope CZT acquisition with that determined using conventional planar imaging in patients with heart failure.Our results found no impact of the acquisition mode (single vs dual) or the type of CZT camera on 123I and 99mTc defect size and mismatch, LVEF, and HMR of 123I-MIBG uptake.This work provides a new step toward simultaneous dual-isotope acquisition for combined innervation, perfusion and ventricular function assessment.

<p>Simultaneous dual-isotope SPECT imaging has a number of applications, for exam- ple, cardiac, brain and cancer imaging. The major concern in simultaneous dual- isotope SPECT is the significant crosstalk contamination between the different isotopes used. The current study focuses on a method of crosstalk compensation between two isotopes in simultaneous dual isotope SPECT acquisition applied to cancer imaging using ^99mTc/¹¹¹In and breast SPECT using ^99mTc/¹²³I. Monte Carlo (MC), which is thought to offer the most realistic crosstalk and scatter compensation modelling, in typical implementations, has inherent long calculation times (often several hours or days) associated with it. This makes MC unsuitable for clinical applications. We have previously incorporated convolution based forced detection into SIMIND Monte Carlo (MC) program which have made MC feasible to use in clinical time frames. In order to evaluate the accuracy of our accelerated MC program a number of point source simulation results were compared to experimentally acquired data in terms of spatial resolution and detector sensitivity. We have developed an iterative MC based image reconstruction technique that simulates the photon down-scatter from one isotope into the acquisition window of a second isotope. The MC based estimation of scatter contamination contained in projection views is then used to compensate for the photon contamination during iterative reconstruction. We use a modified ordered subset expectation maximization (OSEM) alogrithm, named as simultaneous ordered subset-expectation maximization (Sim-OSEM), to perform this step. We have undertaken a number of simulation tests and phantom studies to verify this approach in case of both of the dual-isotope combinations (i.e. ^99mTc/¹¹¹In and ^99mTc/¹²³I). In breast SPECT studies three different breast sizes were simulated. For each of the breast sizes ten combinations of lesions with 3 lesions per combination, were selected randomly for acquisition and reconstruction of simulation data. The images reconstructed using Sim-OSEM showed crosstalk compensation when compared with images reconstructed using simultaneously (with crosstalk) acquired projection data using analytical attenuation based reconstruction. In case of Sim-OSEM the lesion to background ratios were much closer to actual values compared to images reconstructed for both separately (without crosstalk) and simultaneously (with crosstalk) acquired projection data using analytical attenuation based reconstruction. Activity estimation is also possible with Sim-OSEM and yielded accurate estimates of lesion activities with relatively small error compared to deposited activities. The proposed reconstruction technique also evaluated by reconstruction of experimentally acquired projection phantom data in case of ^99mTc/¹¹¹In. Reconstruction using Sim-OSEM showed very promising results in terms of crosstalk and small angle scatter compensation and uniformity of background compared to analytical attenuation based reconstruction after triple energy window (TEW) based scatter correction of projection data. In our case images obtained using Sim-OSEM showed more uniform background even when compared to the images reconstructed for separately acquired projection data using analytical attenuation based reconstruction may be due to better correction of photons scattered at small angle and got detected under photopeak.</p> / Master of Science (MSc)

Release of different nitrogen compounds can cause eutrophication in lakes and rivers whichcan lead to oxygen-free environments in bottom water and in the sediment and can in turnlead to fish-deaths. Ammonium can be toxic to biota and nitrate can in high concentrationeven be toxic to humans. WHO has released a limit for nitrate concentration in drinking waterof 10mg/l. The LKAB mine in Kiruna is the largest underground iron mine in the world and isprospecting, mining and refining iron ore, with an annual production of around 28 milliontons. Release of different nitrogen compounds as a result of the explosives used during themining operations has been a known problem for some time; however the processes affectingnitrogen during the water transport have never been fully investigated. The main objective of this MSc thesis is to determine if changes in nitrogen and oxygenisotope composition can be used as a tracer for nitrogen transformation processes in the minewater at LKABs underground mine in Kiruna. Water samples were collected at key points in the water transport system and were analyzedfor isotopic composition. Isotopic and chemical data revealed two different sources of nitrateand ammonium, undetonated explosives and leachate from waste rocks. Three differentnitrogen changing processes affecting concentration of nitrate and ammonium were found:ammonium volatilization, nitrification and sorption. It was not possible to quantify theseprocesses individually. No processes which decrease the nitrate concentration were found.