SUMMARYUnravelling the factors determining the allocation of carbon to various plant organs is one of the great challenges of modern plant biology. Studying allocation under close to natural conditions requires noninvasive methods, which are now becoming available for measuring plants on a par with those developed for humans. By combining magnetic resonance imaging (MRI) and positron emission tomography (PET), we investigated three contrasting root/shoot systems growing in sand or soil, with respect to their structures, transport routes and the translocation dynamics of recently fixed photoassimilates labelled with the shortlived radioactive carbon isotope 11 C. Storage organs of sugar beet (Beta vulgaris) and radish plants (Raphanus sativus) were assessed using MRI, providing images of the internal structures of the organs with high spatial resolution, and while species-specific transport sectoralities, properties of assimilate allocation and unloading characteristics were measured using PET. Growth and carbon allocation within complex root systems were monitored in maize plants (Zea mays), and the results may be used to identify factors affecting root growth in natural substrates or in competition with roots of other plants. MRI-PET co-registration opens the door for non-invasive analysis of plant structures and transport processes that may change in response to genomic, developmental or environmental challenges. It is our aim to make the methods applicable for quantitative analyses of plant traits in phenotyping as well as in understanding the dynamics of key processes that are essential to plant performance.

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Positron emitters such as (11)C, (13)N and (18)F and their labelled compounds are widely used in clinical diagnosis and animal studies, but can also be used to study metabolic and physiological functions in plants dynamically and in vivo. A very particular tracer molecule is (11)CO(2) since it can be applied to a leaf as a gas. We have developed a Plant Tomographic Imaging System (PlanTIS), a high-resolution PET scanner for plant studies. Detectors, front-end electronics and data acquisition architecture of the scanner are based on the ClearPET system. The detectors consist of LSO and LuYAP crystals in phoswich configuration which are coupled to position-sensitive photomultiplier tubes. Signals are continuously sampled by free running ADCs, and data are stored in a list mode format. The detectors are arranged in a horizontal plane to allow the plants to be measured in the natural upright position. Two groups of four detector modules stand face-to-face and rotate around the field-of-view. This special system geometry requires dedicated image reconstruction and normalization procedures. We present the initial performance of the detector system and first phantom and plant measurements.

The metabotrophic subtype 5 glutamate receptor (mGluR5) plays a critical role in synaptic plasticity besides its involvement in numerous neurological disorders, such as depression. As mGluR5 availability in humans is altered in sleep deprivation, we hypothesized that mGluR5 availability underlies a circadian variation. To investigate whether mGluR5 underlies potential circadian changes we measured its density in a randomized fashion at six different daytimes in 11 adult Sprague-Dawley rats. mGluR5 density was quantified by positron emission tomography (PET) using the radioactive ligand [ C]ABP688. [ C]ABP688 uptake was quantified in nine regions of interest with a reference tissue model. Significant differences in the binding potential (BP ) and therefore mGluR5 availability between the different circadian times were found in cortex, cingulate cortex, amygdala, caudate putamen and nucleus accumbens. Further post-hoc statistical analysis (Tukey-Kramer test) of the different time-points revealed significant changes in BP between 07:00 hours (start of light-on phase) and 15:00 hours (last time-point of the light-on phase) in the caudate putamen. This study shows that mGluR5 availability is increased during the light-on, or sleep phase, of rodents by approximately 10%. Given that altered mGluR5 densities play a role in psychiatric disorders, further investigation is warranted to evaluate their circadian involvement in mood changes in humans.

In vivo imaging of the A 1 adenosine receptor (A 1 AR) using 18 F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ( 18 F-CPFPX) and PET has become an important tool for studying physiologic and pathologic states of the human brain. However, dedicated experimental settings for small-animal studies are still lacking. The aim of the present study was therefore to develop and evaluate suitable pharmacokinetic models for the quantification of the cerebral A 1 AR in high-resolution PET. Methods: On a dedicated animal PET scanner, 15 rats underwent 18 F-CPFPX PET scans of 120-min duration. In all animals, arterial blood samples were drawn and corrected for metabolites. The radioligand was injected either as a bolus or as a bolus plus constant infusion. For the definition of unspecific binding, the A 1 AR selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was applied. After PET, the brains of 9 animals were dissected and in vitro saturation binding was performed using high-resolution 3 H-DPCPX autoradiography. Results: The kinetics of 18 F-CPFPX were well described by either compartmental or noncompartmental models based on arterial input function. The resulting distribution volume ratio correlated with a low bias toward identity with the binding potential derived from a reference region (olfactory bulb) approach. Furthermore, PET quantification correlated significantly with autoradiographic in vitro data. Blockade of the A 1 AR with DPCPX identified specific binding of about 45% in the reference region olfactory bulb. Conclusion: The present study provides evidence that 18 F-CPFPX PET based on a reference tissue approach can be performed quantitatively in rodents in selected applications. Specific binding in the reference region needs careful consideration for quantitative investigations. The A 1 adenosine receptor (A 1 AR) is a G-protein-coupled receptor that modulates synaptic transmission and neuronal excitability (1). The A 1 AR has been proposed to contribute to various neurologic and psychiatric disorders (2), physiologic processes such as caffeine-induced neurostimulation, and sleep-wake regulation (3).These observations created a high interest in imaging cerebral A 1 AR activity. So far, 2 A 1 AR-specific radioligands have been applied in humans. PET with 18 F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ( 18 F-CPFPX) and [1-methyl-11 C]8-dicyclopropylmethyl-1-methyl-3-propylxanthine has been used to visualize and quantify the A 1 AR in vivo (4). Both compounds are xanthine-type antagonists of the A 1 AR.In previous reports we have described the quantification of 18 F-CPFPX PET in humans using arterial input and a noninvasive reference region approach (5). Clinical applications and studies in neuroscience (4) proved the suitability and innovative potential of this new imaging method. Currently, numerous agonists, antagonists, and allosteric modulators are under development to explore the therapeutic potential of adenosine receptors (6). Experimental animal imaging could streamline the evalua...