Dopamine appears to play a critical role in regulating spatial working memory (SWM) in non-human primates, and SWM deficits are observed in patients with Parkinson�s disease and schizophrenia. Unfortunately, the current experimental literature in humans is marred by inconsistent behavioural findings, and there is a void in neuroimaging studies examining dopaminergic manipulation of SWM-related brain activity. The present thesis used a combination of behavioural neurocognitive testing and brain imaging to further examine dopaminergic manipulation of SWM in healthy humans, using two pharmacological challenges: 1) acute tyrosine depletion (TPD) (to acutely deplete tonic dopamine), and 2) D1/D2 receptor activation using the dopamine receptor agonist pergolide (to stimulate dopamine neurotransmission) under conditions of TPD. The effects of TPD on behavioural performance were examined using three SWM tasks: 1) a delayed-recognition task previously impaired by TPD (Experiment 1) and 2) two delayed-response tasks designed to vary only in response requirements (Experiment 2). The findings demonstrated an apparent failure of TPD to impair performance on any of the tasks. Further, the effects of TPD on SWM-related brain activity during a SWM n-back task were examined using regional Cerebral Blood Flow (rCBF) measured by H2 150 Positron Emission Tomography (Experiment 2), and Steady State Visually Evoked Potentials (SSVEP) measured by Steady State Probe Topography (Experiment 4). TPD failed to produce discernable effects on either neural networks (task-related rCBF) or temporal electrophysiological activity (SSVEP) associated with the SWM n-back task. In contrast, D1/D2 receptor stimulation under dopamine depleted conditions impaired performance on both a SWM delayed-response task (Experiment 1) and SWM n-back task (Experiment 2), and resulted in task-related increases in fronto-temporal SSVEP latency (suggestive of increased inhibition) and decreases in parieto-occipital SSVEP amplitude (suggestive of increased activation) during the early delay period of the SWM n-back task (Experiment 4). These changes are consistent with the undisputed role of frontal and parietal regions in n-back task performance, and with previous evidence of dopaminergic modulation of these regions in animals and humans. In summary, TPD did not modulate SWM behavioural performance on four different SWM tasks with differing task demands, and failed to produce measurable changes to either SWM-related neural networks (task-related rCBF) or cortical electrophysiological activity (SSVEP) associated with the SWM n-back task. The implication of these findings, when taken together with previous studies, is that the degree of dopaminergic depletion achieved with TPD may be insufficient to consistently and robustly modulate SWM networks in healthy humans, questioning the utility of TPD as a probe of dopaminergic function. In addition, these findings demonstrate the complexity of stimulating D1/D2 receptors under dopamine depleted conditions, and highlight the critical importance of baseline dopamine levels in influencing the effects of acute dopamine challenge on SWM performance.

<p>Radioactive substances are used <i>in vivo</i> to localize and characterize malignant tumours, generally by scintigraphic methods. In this context positron emission tomography (PET) in combination with radiolabelled monoclonal antibodies (mAbs) may provide a sensitive and specific method for detection of cancer. Individual dose calculations, based on such PET measurements, may be carried out to predict the possible use of mAbs labelled with therapeutic nuclides. The positron emitter ⁷⁶Br, with a half-life of 16 h, is a well-suited candidate for radiolabelling and PET imaging. One drawback of radiobromine is that bromide, the ultimate catabolite after degradation of brominated mAb, is only tardily excreted from the body and is evenly distributed throughout the extracellular space, thereby increasing the background radioactivity. The aim of this work was to produce ⁷⁶Br-mAb preparations with high accumulation and retention in tumour tissue together with a quick clearance of ⁷⁶Br-labelled catabolites. Furthermore, the possibility to use brominated or iodinated mAbs in combination with PET to predict ²¹¹At-mAb dosimetry was evaluated.</p><p>Monoclonal Abs directed against colorectal cancer were labelled with ⁷⁶Br using the direct Chloramine-T-method or indirectly by labelling the precursor molecule N-succinimidyl para-(tri-methylstannyl) benzoate with ⁷⁶Br, which was subsequently conjugated to the mAbs. Monoclonal Ab A33 labelled with ⁷⁶Br using the two labelling protocols was characterized in vitro and in vivo in a rat tumour xenograft model. The mAb A33 was also labelled with ¹²⁵I for comparison. In addition, mAb A33 was labelled with ²¹¹At, ¹²⁵I and ⁷⁶Br using the indirect labelling protocol and the mAb pharmacokinetics was studied in normal rats in order to estimate if data from brominated or iodinated mAb could be used for dosimetry of ²¹¹At in healthy organs and tissue.</p><p>In conclusion, both direct and indirect labelling resulted in high yields and mAbs with preserved immunoreactivity. <i>In vivo</i> characterization of ⁷⁶Br-brominated mAb A33 showed that the indirect labelling method makes ⁷⁶Br-brominated mAb A33 a promising candidate for tumour imaging with PET due to the faster excretion of radiolabelled catabolites compared with direct bromination. Finally, mAb A33 labelled with ⁷⁶Br and ^124/125I can be used to predict the ²¹¹At dose of astatinated mAb A33 in most organs given that a correction factor is applied for organs with varying uptake.</p>

<p>Stroke is the third leading cause of morbidity and mortality in the industrialized world. Subarachnoid hemorrhage (SAH), the least common form of stroke, is one of the most demanding diseases treated in neurointensive care units. Cerebral ischemia may develop rapidly, and has a major influence on outcome.To be able to save parts of the brain that are at risk for ischemic brain damage, there is a need for reliable monitoring techniques. Understanding the pathophysiology of cerebral ischemia is a prerequisite both for the correct treatment of these diseases and for the development of new monitoring techniques and treatment modalities. The main aim of this thesis was to gain insight into the mechanisms of cerebral ischemia by studying early hemodynamic and metabolic changes with positron emission tomography and neurochemical changes with microdialysis. A secondary aim was to evaluate the potential of these techniques for detecting ischemia and predicting the degree of reversibility of ischemic changes.</p><p>Early changes in cerebral blood flow (CBF) and metabolism (CMRO₂) were studied with repeated positron emission tomography in an experimental model (MCAO) of transient focal ischemia, and in SAH patients. CMRO₂ was superior to CBF in discriminating between tissue with irreversible damage and tissue with the potential for survival in the experimental model. A metabolic threshold of ischemia was found. Neurochemical changes in the ischemic regions were studied simultaneously with microdialysis. Extracellular concentrations of glucose, lactate, hypoxanthine, glutamate and glycerol were measured, and the lactate/pyruvate (LP) and lactate/glucose ratios were calculated. Changes in all the microdialysis parameters were related to the degree of ischemia (severe ischemia or penumbra). Especially the LP ratio and glycerol were found to be robust and specific markers of ischemia. In the patients, hemodynamic and metabolic changes were common, but diverse in the acute phase of SAH, and it was suggested that these changes may contribute to an increased vulnerability for secondary events and the development of secondary ischemic brain damage.</p>

Radioactive substances are used in vivo to localize and characterize malignant tumours, generally by scintigraphic methods. In this context positron emission tomography (PET) in combination with radiolabelled monoclonal antibodies (mAbs) may provide a sensitive and specific method for detection of cancer. Individual dose calculations, based on such PET measurements, may be carried out to predict the possible use of mAbs labelled with therapeutic nuclides. The positron emitter 76Br, with a half-life of 16 h, is a well-suited candidate for radiolabelling and PET imaging. One drawback of radiobromine is that bromide, the ultimate catabolite after degradation of brominated mAb, is only tardily excreted from the body and is evenly distributed throughout the extracellular space, thereby increasing the background radioactivity. The aim of this work was to produce 76Br-mAb preparations with high accumulation and retention in tumour tissue together with a quick clearance of 76Br-labelled catabolites. Furthermore, the possibility to use brominated or iodinated mAbs in combination with PET to predict 211At-mAb dosimetry was evaluated. Monoclonal Abs directed against colorectal cancer were labelled with 76Br using the direct Chloramine-T-method or indirectly by labelling the precursor molecule N-succinimidyl para-(tri-methylstannyl) benzoate with 76Br, which was subsequently conjugated to the mAbs. Monoclonal Ab A33 labelled with 76Br using the two labelling protocols was characterized in vitro and in vivo in a rat tumour xenograft model. The mAb A33 was also labelled with 125I for comparison. In addition, mAb A33 was labelled with 211At, 125I and 76Br using the indirect labelling protocol and the mAb pharmacokinetics was studied in normal rats in order to estimate if data from brominated or iodinated mAb could be used for dosimetry of 211At in healthy organs and tissue. In conclusion, both direct and indirect labelling resulted in high yields and mAbs with preserved immunoreactivity. In vivo characterization of 76Br-brominated mAb A33 showed that the indirect labelling method makes 76Br-brominated mAb A33 a promising candidate for tumour imaging with PET due to the faster excretion of radiolabelled catabolites compared with direct bromination. Finally, mAb A33 labelled with 76Br and 124/125I can be used to predict the 211At dose of astatinated mAb A33 in most organs given that a correction factor is applied for organs with varying uptake.

Stroke is the third leading cause of morbidity and mortality in the industrialized world. Subarachnoid hemorrhage (SAH), the least common form of stroke, is one of the most demanding diseases treated in neurointensive care units. Cerebral ischemia may develop rapidly, and has a major influence on outcome.To be able to save parts of the brain that are at risk for ischemic brain damage, there is a need for reliable monitoring techniques. Understanding the pathophysiology of cerebral ischemia is a prerequisite both for the correct treatment of these diseases and for the development of new monitoring techniques and treatment modalities. The main aim of this thesis was to gain insight into the mechanisms of cerebral ischemia by studying early hemodynamic and metabolic changes with positron emission tomography and neurochemical changes with microdialysis. A secondary aim was to evaluate the potential of these techniques for detecting ischemia and predicting the degree of reversibility of ischemic changes. Early changes in cerebral blood flow (CBF) and metabolism (CMRO2) were studied with repeated positron emission tomography in an experimental model (MCAO) of transient focal ischemia, and in SAH patients. CMRO2 was superior to CBF in discriminating between tissue with irreversible damage and tissue with the potential for survival in the experimental model. A metabolic threshold of ischemia was found. Neurochemical changes in the ischemic regions were studied simultaneously with microdialysis. Extracellular concentrations of glucose, lactate, hypoxanthine, glutamate and glycerol were measured, and the lactate/pyruvate (LP) and lactate/glucose ratios were calculated. Changes in all the microdialysis parameters were related to the degree of ischemia (severe ischemia or penumbra). Especially the LP ratio and glycerol were found to be robust and specific markers of ischemia. In the patients, hemodynamic and metabolic changes were common, but diverse in the acute phase of SAH, and it was suggested that these changes may contribute to an increased vulnerability for secondary events and the development of secondary ischemic brain damage.

Patients suffering from different forms of neurodegenerative diseases, such as: Creutzfeldt Jacob Disease (CJD), Alzheimer disease (AD), mild cognitive impairment (MCI), frontotemporal dementia and Parkinson’s disease (PD) were examined with Positron Emission Tomography (PET) and the combination of different radiotracers: 15O-water, N-[11C-methyl]-L-deuterodeprenyl (DED), [18F] 2-fluorodeoxyglucose: (FDG), N-methyl-[11C]2-(4-methylaminophenyl)-6-hydroxybenzothiazole (PIB) and L-[11C]-3,4-dihydroxiphenyl-alanine (DOPA). The radiotracers and the combinations of different radiotracers were selected with the intention to detect, in the brain, patterns of neuronal dysfunction, astrocytosis, axon degeneration or protein aggregation (amyloid), in the brain which are pathognomonic for specific diseases and may contribute to improve clinical differential diagnoses. Examinations in healthy volunteers were performed to allow comparisons with patients. In addition, animal studies were conducted to complement the information. In some cases, the PET findings could be compared with the results of autopsies. In contrast to the micropathology, in which only a limited part of a tissue (obtained post-mortem or by biopsy) is inspected, one PET acquisition provides an image of the whole system (e.g.: the brain and the cerebellum). This form of imaging pathology is “in vivo”, where the examination is innocuous for the patient. This thesis is an attempt to stimulate the development of new tracers, new tracer combinations and methods that directly or indirectly describe the anatomo-physiopathological changes produced in the brain in neurodegenerative diseases. A better description of different diseases can be obtained, confirming or questioning the clinical diagnoses and widening our understanding of the mechanisms underlying neurodegeneration. Different pathologies can produce similar symptoms and thus causing confusion regarding clinical diagnosis. The used PET combinations improved the accuracy of the diagnoses. The incipient knowledge emerging from a new neuroimaging pathology in combination with other disciplines may open the way to new classifications of dementias and neurodegenerative diseases based on an “in vivo” pathology.

The use of positron emission tomography (PET) has so far mainly been limited to a few nuclides with short half-lives such as 11C and 18F. Certain applications require nuclides with longer half-lives, such as 76Br and 52Fe. In radionuclide therapy positron emitting analogues of therapeutic nuclides, such as 110mIn, or daughter nuclides, such as 134La, can enable improved dosimetry with the use of PET. A challenge associated with the use of these positron emitters is that they emit gamma radiation in cascade with positrons, which complicates quantitative PET imaging. Other possible problems are the high energies of the emitted positrons, and the decay of 52Fe to the short-lived positron emitter 52mMn. Performance measurements were made to investigate the effects of these decay characteristics on the quantitative accuracy, spatial resolution, and other parameters of PET. The distribution of gamma radiation coincidences in PET data was studied and correction methods were implemented and evaluated. PET resolution degrades with 1-2 mm for the studied nuclides in comparison with 18F. The implemented sinogram tail fit and delayed coincidence based gamma radiation coincidence correction methods lead to a quantitative accuracy similar as for 18F and to improved image contrast. Standard dead time corrections are not adequate for gamma-emitting nuclides. Noise equivalent count rates are considerably lower for 76Br than for 18F at clinically relevant radioactivity concentrations. A method to correct 52Fe patient data for the contribution of 52mMn is discussed. The use of 110mIn is evaluated in a patient study and compared to SPECT imaging with 111In. A dosimetric and PET evaluation of the use of 134Ce/134La for radionuclide therapy and dosimetry is presented. Dosimetry of 76Br-labelled antibodies is studied in a pig model. Finally, the possibility to use PET for dosimetry during radionuclide therapy is studied and a nonuniform dose calculation program is presented.

Positron-Emission-Tomography (PET) is a modern imaging technique in nuclear medicine providing quantitative 3D distribution of a radioactive tracer substance in the human body. The gamma-detector is the first link in the chain of components that constitutes a PET. It converts incoming radiation into optical light pulses, which are detected by photo multiplier tubes. Here the light is converted into electric pulses, to be further processed by the acquisition electronics. Improving detector sensitivity and resolution is of great value in research and in clinical practice. The focus of this work is to improve the detector to give it time-of-flight (TOF) capabilities, in order to further improve sensitivity, which in turn leads to increased image quality, faster scan time and/or reduced dose exposure for the patient. Image quality has improved over the years, but losses in image quality have been reported for heavy patients, due to increased attenuation, and more dispersed counts over a larger volume. Instrumentation limits are still significant in heavy patient images, but the incorporation of TOF information promises to alleviate some of the limitations. In order to improve the timing resolution of the detector fast photo-multipliers and a novel scheme to extract the event timing trigger from a detector by using the summed dynode signal were investigated. When designing new PET detectors, it is important to have detailed understanding and control of the light sharing mechanisms in the crystal arrays. Therefore it was necessary to perform optical simulations and single crystal light output measurements to derive a model for an LSO block detector. Another way to improve the image quality is to use the depth-of-interaction (DOI) of the gamma ray within the detector. It is shown that a multi-layer phoswich detector comprised of LSO with different decay times and TOF capability, combines the benefits of TOF and DOI in one detector, maximizing the effective sensitivity gain. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 7: Submitted.

Growing evidence from animal and clinical studies suggests that cardiac cell therapy can restore perfusion and improve function in the ischemic/infarcted myocardium. However, cell therapy is hindered by insufficient cell numbers, inefficient cell homing and engraftment, and inadequate cellular interactions. Furthermore, the biological mechanisms and local effects of transplanted cells have not been well-elucidated. The research presented herein attempts to address some of these issues. In manuscript #1, a new subpopulation of circulating progenitor cells (CPCs), termed derived CD133+ cells, was generated from the CD133- fraction of human peripheral blood. The derived CD133+ progenitors appeared to have superior vasculogenic potential in vitro, which may prove to be beneficial in inducing vasculogenesis in ischemic tissues. Positron emission tomography (PET) with direct cell labeling and reporter gene techniques were employed to assess the fate of transplanted human CPCs in vivo at different subjects of investigation, and different stages of cell transplantation. In manuscript #2, PET imaging with 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG) direct cell labeling was used to demonstrate that collagen-based matrices improve the early homing and retention of delivered CPCs in a rat ischemic hindlimb model. This mechanism conferred by the matrix may have implications on cell therapy at the early stages after transplantation. In manuscript #3, a more efficient, stable and accurate labeling method, hexadecyl-4-[18F]fluorobenzoate (18F-HFB) direct cell labeling, was developed to quantify cell distribution of transplanted CPCs in a rat myocardial infarction model. PET imaging of 18F-HFB-CPCs revealed significant cell washout from the myocardium immediately after intramyocardial injection, with only a small proportion of transplanted CPCs remaining in the target area in the first 4 hours after delivery. In manuscript #4, human CPCs transduced with lentiviral vectors showed stable expression of PET reporter genes. This reporter gene based-cell labeling technique can be developed for noninvasive tracking cells within a bioengineered matrix by PET, while preserving cell phenotype, viability and function. These studies contribute important insights into the biology and physiology of transplanted stem cells and the ability of delivery matrices to improve transplanted cell engraftment, survival, and function. I believe with further refinement, cell expansion, tissue engineering and PET imaging could facilitate the clinical applications of cell therapies in years to come.

Cardiac hypertrophy is a compensatory response to increased work load or stress on the heart, but over time can lead to heart failure and death. The molecular mechanisms underlying this disease are still not completely understood, however the Rho/Rho kinase pathway has been shown to play a role. N-[11C]-methyl-hydroxyfasudil, a PET radiotracer, binds to active Rho kinase and could be a possible tracer for hypertrophy. Hypertrophy was induced in vitro using the β-adrenergic receptor agonist isoproterenol to evaluate optimal Rho kinase activity. Rho kinase activity data was correlated to N-[11C]-methyl-hydroxyfasudil binding. Cardiac hypertrophy was verified with an increase in nuclear size (1.74 fold) and cell size (~2 fold), activation of hypertrophic signalling pathways, and increased Rho kinase activity (1.64 fold). This correlated to a 10.3% increase in N-[11C]-methyl-hydroxyfasudil binding. This data suggests that N-[11C]-methyl-hydroxyfasudil may be useful as a radiotracer for detecting cardiac hypertrophy and merits further in vivo investigation.