Positron Emission Tomography (PET) has great value in research and clinical applications from oncology to neurodegenerative disorders. However, there is a barrier in translating biological knowledge into new PET applications due in part to the lack of efficient, widely applicable methods for labelling compounds with PET radioisotopes. Herein, a generic approach to radiolabelling is presented which is direct, broadly applicable and potentially adaptable to either of the two most commonly used PET radioisotopes, 11C and 18F. This approach employs the advantages of solid phase synthesis to achieve selective release of only the desired radiolabelled product from a solid support in a single step, simplifying purification and hence improving synthetic efficiency. Polystyrene resin was functionalised with a 1,2-diol group; this allowed the covalent attachment of compounds bearing boronic acid groups via formation of a boronate ester linkage. A Suzuki-Miyaura reaction with methyl iodide was used to cleave a model compound from the resin in 61% conversion after five minutes. This reaction was adapted to develop a fully automated radiosynthesis with [11C]- methyl iodide which generated a radiolabelled model compound in 2 – 7% non-decay-corrected radiochemical yield. This provided proof of concept for the simultaneous cleavage of compounds from the resin and radiolabelling with 11C. A boronic acid precursor of the known radiotracer [11C]-M-MTEB was attached to the resin and successfully radiolabelled with 11C in 2.4% non-decay-corrected radiochemical yield and 96 – 100% radiochemical purity under the same conditions. Furthermore, the potential adaptability of this solid phase approach to 18F radiolabelling was demonstrated by treatment of the resin-bound small molecules and peptides with potassium bifluoride, which released the compounds rapidly as trifluoroborate salts.

A characteristic feature of Chronic Obstructive Pulmonary Disease (COPD) is an abnormal inflammatory response in the lungs to inhaled particles or gases. The ability to assess and monitor this response in the lungs of COPD patients is important for understanding the pathogenic mechanisms, but also provides a measure of the activity of the disease. Disease activity is more likely to relate to lung inflammation rather than the degree of airflow limitation as measured by the FEV1. Preliminary studies have shown the 18F fluorodeoxyglucose positron emission tomography (18F FDG-PET) signal, as a measure of lung inflammation, is quantifiable in the lungs and is increased in COPD patients compared to controls. However, the methodology requires standardisation and any further enhancement of the methodology would improve its application to assess inflammation in the lungs. I investigated various methods of assessing FDG uptake in the lungs and assessed the reproducibility of these methods, and particularly evaluated whether the data was reproducible or not in the COPD patients (smokers and ex-smokers). This data was then compared with a group of healthy controls to assess the role of dynamic 18F FDG-PET scanning as a surrogate marker of lung inflammation. My data showed a good reproducibility of all methods of assessing FDG lung uptake. However, using conventional Patlak analysis, the uptake was not statistically different between COPD and the control group. Encouraging results in favour of COPD patients were nonetheless shown using compartmental methods of assessing the FDG lung uptake, suggesting the need to correct for the effect of air and blood (tissue fraction effect) when assessing this in a highly vascular organ like the lungs. A prospective study analysis involving a bigger cohort of COPD patients would be desirable to investigate this further.

Reconstructed tomographic image resolution of small animal PET imaging systems is improving with advances in radiation detector development. However the trend towards higher resolution systems has come with an increase in price and system complexity. Recent developments in the area of solid-state photomultiplication devices like silicon photomultiplier arrays (SPMA) are creating opportunities for new high performance tools for PET scanner design. Imaging of excised small animal organs and tissues has been used as part of post-mortem studies in order to gain detailed, high-resolution anatomical information on sacrificed animals. However, this kind of ex-vivo specimen imaging has largely been limited to ultra-high resolution μCT. The inherent limitations to PET resolution have, to date, excluded PET imaging from these ex-vivo imaging studies. In this work, we leverage the diminishing physical size of current generation SPMA designs to create a very small, simple, and high-resolution prototype detector system targeting ex-vivo tomographic imaging of small animal organs and tissues. We investigate sensitivity, spatial resolution, and the reconstructed image quality of a prototype small animal PET scanner designed specifically for imaging of excised murine tissue and organs. We aim to demonstrate that a cost-effective silicon photomultiplier (SiPM) array based design with thin crystals (2 mm) to minimize depth of interaction errors might be able to achieve sub-millimeter resolution. We hypothesize that the substantial decrease in sensitivity associated with the thin crystals can be compensated for with increased solid angle detection, longer acquisitions, higher activity and wider acceptance energy windows (due to minimal scatter from excised organs). The constructed system has a functional field of view (FoV) of 40 mm diameter, which is adequate for most small animal specimen studies. We perform both analytical (3D-FBP) and iterative (ML-EM) methods in order to reconstruct tomographic images. Results demonstrate good agreement between the simulation and the prototype. Our detector system with pixelated crystals is able to separate small objects as close as 1.25 mm apart, whereas spatial resolution converges to the theoretical limit of 1.6 mm (half the size of the smallest detecting element), which is to comparable to the spatial resolution of the existing commercial small animal PET systems. Better system spatial resolution is achievable with new generation SiPM detector boards with 1 mm x 1 mm cell dimensions. We demonstrate through Monte Carlo simulations that it is possible to achieve sub-millimeter spatial image resolution (0.7 mm for our scanner) in complex objects using monolithic crystals and exploiting the light-sharing mechanism among the neighboring detector cells. Results also suggest that scanner (or object) rotation minimizes artifacts arising from poor angular sampling, which is even more significant in smaller PET designs as the gaps between the sensitive regions of the detector have a more exaggerated effect on the overall reconstructed image quality when the design is more compact. Sensitivity of the system, on the other hand, can be doubled by adding two additional detector heads resulting in a, fully closed, 4π geometry.

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.