Cerebral blood flow and glucose metabolism in healthy volunteers measured using a high-resolution PET scanner

BACKGROUND: Positron emission tomography (PET) allows for the measurement of cerebral blood flow (CBF; based on [(15)O]H(2)O) and cerebral metabolic rate of glucose utilization (CMR(glu); based on [(18) F]-2-fluoro-2-deoxy-d-glucose ([(18) F]FDG)). By using kinetic modeling, quantitative CBF and CMR(glu) values can be obtained. However, hardware limitations led to the development of semiquantitive calculation schemes which are still widely used. In this paper, the analysis of CMR(glu) and CBF scans, acquired on a current state-of-the-art PET brain scanner, is presented. In particular, the correspondence between nonlinear as well as linearized methods for the determination of CBF and CMR(glu) is investigated. As a further step towards widespread clinical applicability, the use of an image-derived input function (IDIF) is investigated. METHODS: Thirteen healthy male volunteers were included in this study. Each subject had one scanning session in the fasting state, consisting of a dynamic [(15)O]H(2)O scan and a dynamic [(18) F]FDG PET scan, acquired at a high-resolution research tomograph. Time-activity curves (TACs) were generated for automatically delineated and for manually drawn gray matter (GM) and white matter regions. Input functions were derived using on-line arterial blood sampling (blood sampler derived input function (BSIF)). Additionally, the possibility of using carotid artery IDIFs was investigated. Data were analyzed using nonlinear regression (NLR) of regional TACs and parametric methods. RESULTS: After quality control, 9 CMR(glu) and 11 CBF scans were available for analysis. Average GM CMR(glu) values were 0.33 ± 0.04 μmol/cm(3) per minute, and average CBF values were 0.43 ± 0.09 mL/cm(3) per minute. Good correlation between NLR and parametric CMR(glu) measurements was obtained as well as between NLR and parametric CBF values. For CMR(glu) Patlak linearization, BSIF and IDIF derived results were similar. The use of an IDIF, however, did not provide reliable CBF estimates. CONCLUSION: Nonlinear regression analysis, allowing for the derivation of regional CBF and CMR(glu) values, can be applied to data acquired with high-spatial resolution current state-of-the-art PET brain scanners. Linearized models, applied to the voxel level, resulted in comparable values. CMR(glu) measurements do not require invasive arterial sampling to define the input function. TRIAL REGISTRATION: ClinicalTrials.gov NCT0062608

Kinetics of protein-based in vivo Imaging tracers for positron emission tomography

Within the framework of the “Sel-tag imaging project”, a novel method was used to rapidly label protein tracers and the in vivo targeting abilities of these tracers were studied in animal models of cancer using a preclinical positron emission tomography (PET) camera. To first evaluate and optimize preclinically the use of PET tracers can facilitate their translation to and implementation in human patient studies. The ultimate goal of the different projects within the Sel-tag imaging project was to find imaging biomarkers that could potentially be used for individualizing cancer treatment and thereby improve the therapeutic results. This thesis focuses on methods employed to describe the distribution of these protein-based tracers in human xenografts. Many of the techniques used had been developed for other imaging circumstances. Therefore verification for these imaging applications was an important aspect of these papers. Paper I examined the distribution in a tumour of a medium-sized AnnexinA5-based tracer that targeted phosphatidylserine externalised during cell death in tumours in two cases; first, with no pre-treatment (baseline) and, second, after pre-treatment with a chemotherapeutic agent. Small differences between tracer uptakes in the two cases required a macro parameter analysis method for quantifications. Evaluations of the influence of the enhanced permeability and retention effect by using a size-matched control were introduced. The AnnexinA5 results were compared to those of the metabolic tracer [18F]FDG and complemented with circulating serum markers to increase sensitivity. Paper II extended the analysis in paper I to incorporate more verifications that were also more thorough. The choice of input (blood or reference tissue) and the statistical significance of intergroup comparisons when using conventional uptake measurements and the more involved macro parameter analyses like in paper I were compared. We also proposed that distribution volume ratio was a more appropriate quantification parameter concept for these protein-based tracers with relatively large non-specific uptake. Paper III assessed the smaller Affibody™ tracer ZHER2:342 as an imaging biomarker for human epidermal growth factor 2 (HER2), whose overexpressions are associated with a poor prognosis for breast cancer patients. In order to demonstrate specific binding to HER2, pre-treatment of the tumour with unlabelled protein and uptake in xenografts with low HER2 expression was evaluated. Ex vivo immunohistochemistry of expression levels supported the imaging results. Paper IV examined a radiopharmaceutical that targeted the epidermal growth factor receptor (EGFR), whose overexposure in tumours is associated with a negative prognosis. Again an Affibody™ molecule, (ZEGFR:2377), was used and, as in in paper I, a size-matched control was also used to estimate the non-specific uptake. Uptakes, quantified by conventional uptake methods, varied in tumours with different EGFR expression levels. Ex vivo analyses of expression levels were also performed. Paper V addressed the non-uniform (heterogeneous) uptake of different tracers in a tumour tissue. An algorithm was written that aimed at incorporating all relevant aspects that will influence non-uniformity. Histograms were generated that visualized how the frequency and spread of deviations contributed to the heterogeneity. These aspects could not always be attended in a direct manner, but instead had to be handled in an indirect way. The effect of varying imaging parameters was examined as part of the validation procedure. The method developed is a robust, user-friendly tool for comparing heterogeneity in similar volume preclinical tumor tissues

THE EXPLORATION OF THE VALIDITY OF QUANTITATIVE 2-DEOXY-2-[FLUORINE-18] FLUORO-D-GLUCOSE (18F-FDG) POSITRON EMISSION TOMOGRAPHY/COMPUTED TOMOGRAPHY (PET/CT) TO ASSESS LUNG INFLAMMATION

Lung diseases are one of the leading causes of death in the UK; responsible for 20% of all deaths each year. Inflammation is thought to be an important driver of the pathogenesis and progression of several lung diseases. Positron emission tomography/computed tomography (PET/CT) is an imaging modality capable of providing functional and molecular imaging through detection of trace quantities of a radioactive tracer. 18F-FDG is the most widely available tracer and in several small studies has been used to investigate diffuse lung diseases such as chronic obstructive pulmonary disease (COPD). These studies rely on quantification of the PET image. Static acquisitions provide information on the biodistribution of the tracer at a single time point after administration, the standardised uptake value (SUV) is the most widely used measure of 18F-FDG uptake. Dynamic acquisitions provide information on the spatial and temporal distribution of the tracer; linear and non-linear modelling techniques allow estimation of the metabolic rate of 18F-FDG in the lung. Previous studies have used a linear method called Patlak graphical analysis, whilst non-linear compartmental models have been used more recently to estimate metabolism. However, these contending measures of pulmonary 18F-FDG uptake, which are putative biomarkers of lung inflammation, have so far been disparately applied. Further, there is nascent understanding that these imaging endpoints are affected by pulmonary air and blood volume; the importance of this effect will likely depend on the disease and its severity. These issues are exacerbated by the presence of respiratory motion and the low signal- to-noise ratio achieved in PET studies. This has led to questions regarding the utility of quantitative 18F-FDG PET to assess lung inflammation. In this work, prospective and retrospective clinical studies were used to assess the clinical, biological and technical validity of 18F-FDG PET imaging endpoints in several clinically relevant diseases. The central hypothesis was that pulmonary inflammation can be assessed using quantitative 18F-FDG. Using retrospective data from two complementary imaging studies, pulmonary 18F- FDG uptake was investigated in COPD patients, α1ATD patients, smokers without COPD and heathy non-smokers. The results demonstrate that the different 18F-FDG imaging endpoints produce disparate findings, and this is exacerbated by the presence of varying blood and air volumes due to emphysema. Nevertheless, measures derived from Patlak analysis revealed elevated uptake consistent with the pathophysiologi- cal understanding of the disease process and further demonstrated correlation with other putative markers of inflammation hence, one could speculate that it relates to inflammation. Further, 18F-FDG imaging outcomes assessed using Patlak analysis were shown to be more reliable than compartmental modelling outcomes. However, the Patlak outcomes are composite measures, not only driven by inflammation but also by pulmonary blood and air. In circumstances where differences in pulmonary blood and air volume between subjects are substantial, it may not be a suitable biomarker of inflammation, but it could be a useful marker of disease activity. Further tissue validation and independent measures of pulmonary blood are required to support its role as a marker of inflammation. In a prospective study, dynamic 18F-FDG PET scans were used to evaluate pulmonary inflammation in sarcoidosis patients and healthy controls. The results show that 18F-FDG uptake was increased in sarcoidosis using Patlak analysis, whilst no difference was found using SUV or compartmental models. Preliminary findings suggest that the signal relates to inflammatory cell counts (macrophages and lymphocytes were most numerous) rather than any one specific cell line; however, further evidence is required to determine if 18F-FDG uptake is driven by underlying inflammation. Consistent with previous findings, mismatch in CT and PET lung images has substantial effect on the quantitative 18F-FDG outcomes; notably, Patlak outcomes were less influenced than compartmental modelling. In summary, the observations made in this work demonstrate the substantial challenges of using 18F-FDG PET/CT to assess diffuse lung disease. Given the incongruity between the different imaging outcomes, these data highlight that future studies should be carefully planned with particular justification of the acquisition and analysis methods. The results of this work suggest that Patlak measures may have the most utility in diffuse lung disease. However, differences in Patlak measures should be interpreted judiciously, as they may be driven by differences in pulmonary blood and air along with inflammation; further study is required to determine if it may be a useful marker of disease activity. In contrast, no differences in pulmonary 18F-FDG uptake between patients and controls were found using compartmental modelling across all studies. Equally, the SUV was found to have poor utility across studies. Future efforts to expedite the use of novel tracers that are more specific to inflammation combined with the development of improved noise reduction techniques may improve the utility of quantitative PET/CT in the context of lung inflammation

Evaluation of novel positron emission tomography radiotracers in humans: tissue distribution kinetics and potential for cancer diagnosis and staging

Positron emission tomography (PET) imaging has emerged as an important decision-making tool in oncology with respect to diagnosis, staging, and assessment of treatment response. We proposed to investigate the ligand binding and retention kinetics of two novel PET/CT tracers in human tumours that do not normally exhibit high [18F]fluorodeoxyglucose ([18F]FDG) uptake, and a third tracer in the context of specific death mechanism. Biological validation of the imaging endpoint included histological correlation with PET/CT data and establishment of an optimum PET/CT methodologies for the probe for implementation into clinical practice. The internal dosimetry and receptor-mediated tumour localisation of the ‘click’ labelled [18F]fluoroethyl triazole octreotate analogue, [18F]FET-βAG-TOCA, in neuroendocrine tumours (NETs) were investigated for the first time in humans. The biomarker demonstrated favourable dosimetry, biodistribution and safety. The calculated effective dose over all subjects (mean ± SD) was 0.029 ± 0.004 mSv/MBq. Regarding staging, [18F]FET-βAG-TOCA PET/CT showed high tumoural uptake with high sensitivity (per lesion) compared with [68Ga]DOTATATE PET/CT (92.8% vs 87.5%). Tissue retention kinetics of the novel choline analogue, [18F]fluoromethyl-[1,2-2H4]- choline ([18F]D4-FCH) were investigated in the staging of muscle invasive bladder cancer (MIBC) and non-small cell lung cancer (NSCLC). The biomarker showed high contrast in lung cancer but poor contrast in bladder cancer. In lung tumours, [18F]D4-FCH uptake was quantitatively lower than [18F]FDG. Pharmacokinetic modelling revealed net tracer influx in tumour consistent with radiotracer phosphorylation via choline kinase, however choline kinase-alpha expression did not correlate with PET parameters. Beyond staging, we evaluated for the first time a caspase-3/7 imaging biomarker, [18F](S)- 1-((1-(2-fluoroethyl)-1H-[1,2,3]-triazol-4-yl)methyl)-5-(2(2,4-difluorophenoxymethyl)- 8 pyrrolidine-1-sulfonyl) ([18F]ICMT-11), for imaging apoptosis and/or necrosis in patients; [18F]FDG-PET is not a marker of caspase-3/7 activation. In breast cancer, lung cancer and lymphoma patients receiving first-line chemotherapy treatment, [18F]ICMT-11 and cytokeratin-18 analysis (blood) were performed. [18F]ICMT-11 showed low uptake pre- and post-chemotherapy in all tumours consistent with unremarkable changes in M30/M60 cytokeratin-18 cleavage products in the breast cohort suggesting a lack of predominantly apoptotic cell death mechanism in responding patients. In lung cancer, multi-parametric [18F]ICMT-11 PET/CT, diffusion weighted (DW-MRI) and dynamic contrast enhanced-MRI (DCE-MRI) showed that PET changes were concordant with cell death in the absence of significant perfusion changes. Thus, tumour response could occur in the absence of predominant chemotherapy-induced caspase-3/7 activation measured non-invasively across entire tumour lesions. In conclusion, the optimal clinical context whereby the [18F]ICMT-11 PET endpoint critically determines the outcome of therapy remains to be established.Open Acces

Novel aspects of age-protection by spermidine supplementation are associated with preserved telomere length

Ageing provokes a plethora of molecular, cellular and physiological deteriorations, including heart failure, neurodegeneration, metabolic maladaptation, telomere attrition and hair loss. Interestingly, on the molecular level, the capacity to induce autophagy, a cellular recycling and cleaning process, declines with age across a large spectrum of model organisms and is thought to be responsible for a subset of age-induced changes. Here, we show that a 6-month administration of the natural autophagy inducer spermidine in the drinking water to aged mice is sufficient to significantly attenuate distinct age-associated phenotypes. These include modulation of brain glucose metabolism, suppression of distinct cardiac inflammation parameters, decreased number of pathological sights in kidney and liver and decrease of age-induced hair loss. Interestingly, spermidine-mediated age protection was associated with decreased telomere attrition, arguing in favour of a novel cellular mechanism behind the anti-ageing effects of spermidine administration

PET/MR imaging of hypoxic atherosclerotic plaque using 64Cu-ATSM

ABSTRACT OF THE DISSERTATION PET/MR Imaging of Hypoxic Atherosclerotic Plaque Using 64Cu-ATSM by Xingyu Nie Doctor of Philosophy in Biomedical Engineering Washington University in St. Louis, 2017 Professor Pamela K. Woodard, Chair Professor Suzanne Lapi, Co-Chair It is important to accurately identify the factors involved in the progression of atherosclerosis because advanced atherosclerotic lesions are prone to rupture, leading to disability or death. Hypoxic areas have been known to be present in human atherosclerotic lesions, and lesion progression is associated with the formation of lipid-loaded macrophages and increased local inflammation which are potential major factors in the formation of vulnerable plaque. This dissertation work represents a comprehensive investigation of non-invasive identification of hypoxic atherosclerotic plaque in animal models and human subjects using the PET hypoxia imaging agent 64Cu-ATSM. We first demonstrated the feasibility of 64Cu-ATSM for the identification of hypoxic atherosclerotic plaque and evaluated the relative effects of diet and genetics on hypoxia progression in atherosclerotic plaque in a genetically-altered mouse model. We then fully validated the feasibility of using 64Cu-ATSM to image the extent of hypoxia in a rabbit model with atherosclerotic-like plaque using a simultaneous PET-MR system. We also proceeded with a pilot clinical trial to determine whether 64Cu-ATSM MR/PET scanning is capable of detecting hypoxic carotid atherosclerosis in human subjects. In order to improve the 64Cu-ATSM PET image quality, we investigated the Siemens HD (high-definition) PET software and 4 partial volume correction methods to correct for partial volume effects. In addition, we incorporated the attenuation effect of the carotid surface coil into the MR attenuation correction _-map to correct for photon attention. In the long term, this imaging strategy has the potential to help identify patients at risk for cardiovascular events, guide therapy, and add to the understanding of plaque biology in human patients