Temperature dependence of the LabPET small-animal PET scanner

INTRODUCTION In quantitative PET imaging it is important to correct for all image-degrading effects, for example detector efficiency variation. Detector efficiency variation depends on the stability of detector efficiency when operating conditions vary within normal limits. As the efficiency of APD-based light detection strongly depends on ambient temperature, temperature-dependent detector efficiency normalization may be needed in APD-based PET scanners. We have investigated the temperature dependence of the LabPET APD-based small-animal PET scanner. MATERIALS AND METHODS First a simulation study was performed to evaluate the effect of different APD temperature coefficients on the temperature dependence of scanner sensitivity. Five experiments were also performed. First the immediate effect of temperature changes on scanner sensitivity was evaluated. Second, the effect of temperature changes that have stabilized for a few hours was investigated. In a third experiment the axial sensitivity profile was acquired at 21 degrees C and 24 degrees C. Next, two acquisitions of the NEMA image quality phantom (at 21 degrees C and 23 degrees C) were performed and absolute quantification was done based on normalization scans acquired at the correct and incorrect temperature. Finally, the feasibility of maintaining a constant room temperature and the stability of the scanner sensitivity under constant room temperature was evaluated. RESULTS Simulations showed that the relation between temperature-dependent APD gain changes and scanner sensitivity is quite complex. A temperature deviation leading to a 1 % change in APD gain corresponds to a much larger change in scanner sensitivity due to the shape of the energy histogram. In the first and second experiment a strong correlation between temperature and scanner sensitivity was observed. Changes of 2.24 kcps/MBq and 1.64 kcps/MBq per degrees C were seen for immediate and stabilized temperature changes respectively. The NEMA axial sensitivity profile also showed a decrease in sensitivity at higher temperature. The quantification experiment showed that a larger quantification error (up to 13%) results when a normalization scan acquired at the incorrect temperature is used. In the last experiment, temperature variability was 0.19 degrees C and counts varied by 10.2 Mcts (1.33%). CONCLUSION The sensitivity of the LabPET small-animal PET scanner strongly depends on room temperature. Therefore, room temperature should be kept as stable as possible and temperature-dependent detector efficiency normalization should be used. However, with constant room temperature excellent scanner stability is observed. Temperature should be kept constant within 0.5 degrees C and weekly normalization scans are recommended

Studying in vivo dynamics of xylem-transported 11CO2 using positron emission tomography

Respired CO2 in woody tissues can build up in the xylem and dissolve in the sap solution to be transported through the plant. From the sap, a fraction of the CO2 can either be radially diffuse to the atmosphere or be assimilated in chloroplasts present in woody tissues. These processes occur simultaneously in stems and branches, making it difficult to study their specific dynamics. Therefore, an 11C-enriched aqueous solution was administered to young branches of Populus tremula L., which were subsequently imaged by positron emission tomography (PET). This approach allows in vivo visualization of the internal movement of CO2 inside branches at high spatial and temporal resolution, and enables direct measurement of the transport speed of xylem-transported CO2 (vCO2). Through compartmental modeling of the dynamic data obtained from the PET images, we (i) quantified vCO2 and (ii) proposed a new method to assess the fate of xylem-transported 11CO2 within the branches. It was found that a fraction of 0.49 min−1 of CO2 present in the xylem was transported upwards. A fraction of 0.38 min−1 diffused radially from the sap to the surrounding parenchyma and apoplastic spaces (CO2,PA) to be assimilated by woody tissue photosynthesis. Another 0.12 min−1 of the xylem-transported CO2 diffused to the atmosphere via efflux. The remaining CO2 (i.e., 0.01 min−1) was stored as CO2,PA, representing the build-up within parenchyma and apoplastic spaces to be assimilated or directed to the atmosphere. Here, we demonstrate the outstanding potential of 11CO2-based plant-PET in combination with compartmental modeling to advance our understanding of internal CO2 movement and the respiratory physiology within woody tissues

Absolute quantification for small-animal PET

Quantification is important in preclinical PET studies. To achieve absolute quantification, an accurate reconstruction algorithm is necessary. Such an algorithm includes corrections for different effects such as geometric sensitivity of the scanner, detection efficiency, attenuation, scatter and random coincidences. In this work we present a method for performing absolute quantification on the LabPET system. All acquisitions were done on a GE Triumph system. This tri-modality system consists of a micro-PET (LabPET), micro-CT (X-O) and micro-SPECT (X-SPECT) scanner. Three PET scans were done. In the first scan 5 vials with different activity concentrations of F-18-FDG were scanned. The total activity inside the scanner was 80 MBq. The second scan was performed after 4 hours when the total activity in the scanner had decayed to 20 MBq. In the third scan 3 vials and 1 sphere were scanned with a total activity of 20 MBq. Before each PET scan a micro-CT scan was acquired. Point sources with a known activity were placed inside the field of view. The counts obtained in these point sources are used to obtain a correction factor for absolute sensitivity. Reconstruction was done using a 3D ML-EM reconstruction with micro-CT based attenuation correction. VOIs were drawn over the vials and the sphere in the reconstructed images. The total activity in the VOIs was calculated using the correction factor for absolute sensitivity. It was compared to the activity measured in a dose calibrator. The average quantification error was 56 %, 6.4 % and 0.6 % for the first, second and third scan. The high error in the first scan is explained by count rate effects, as 80 MBq can be considered a high activity level for this system. The feasibility of absolute quantification on the LabPET system was demonstrated. When the count rate is below 20 MBq absolute quantification is possible with an average quantification error smaller than 6.4 %

Accurate molecular imaging of small animals taking into account animal models, handling, anaesthesia, quality control and imaging system performance

Small-animal imaging has become an important technique for the development of new radiotracers, drugs and therapies. Many laboratories have now a combination of different small-animal imaging systems, which are being used by biologists, pharmacists, medical doctors and physicists. The aim of this paper is to give an overview of the important factors in the design of a small animal, nuclear medicine and imaging experiment. Different experts summarize one specific aspect important for a good design of a small-animal experiment

An Analysis of the Relation between Personality and the Attractiveness of Total Rewards Components

This study examines the links between personality and the relative attraction of various total rewards components. A survey approach is adopted, with 967 individuals completing a questionnaire. These individuals are currently employed. Results show that, after controlling for the effects of several demographic variables, “Big-Five” personality traits do affect individuals’ attraction to the following total rewards components: quality of work and of social relationships, development and career opportunities, variable pay, indirect pay, flexibility of working conditions, and prestige. Among Big-Five personality traits, openness to experience best predicts the relative importance employees give to the various total rewards components.L’attraction du personnel représente un défi croissant pour les employeurs en raison des multiples changements environnementaux. Dans un tel contexte de concurrence sur le marché de l’emploi, la gestion de la rémunération constitue un levier important d’attraction des employés (Ermel et Bohl, 1997). En effet, pour mieux concurrencer sur le marché de l’emploi, les employeurs doivent proposer des conditions de travail plus compétitives, plus distinctives et plus susceptibles de répondre aux attentes du personnel recherché. Pour relever le défi de l’attraction du personnel, un nombre croissant d’employeurs cherchent a identifier, déployer et communiquer une stratégie ou une philosophie de « rémunération totale » (total rewards) prenant en considération une diversité de composantes comme le salaire, les avantages sociaux, les possibilités de formation et de carricre, les aménagements flexibles, etc. (Mercer, 2007). Du coté des employés, des enquztes confirment que les personnes prennent leurs décisions d’emploi en considérant diverses composantes de la rémunération totale : salaire, défis, développement des compétences, conciliation travail-famille, contenu du travail, avantages sociaux, climat de travail, qualité de supervision, etc. (p. ex., Feldman et Arnold, 1978; Foote, 1998; IPSOS-Reid, 2000).S’appuyant sur la perspective des dispositions personnelles, la présente recherche innove en explorant les liens entre la personnalité et l’attrait de diverses composantes de la rémunération totale. Plus précisément, cette recherche a pour objectif de répondre a la question de recherche suivante : Comment les traits de personnalité des individus influencent-ils l’importance qu’ils accordent a diverses composantes de la rémunération totale lors du choix d’un emploi ? Au cours des dernicres années, les études ont confirmé des liens entres les traits de personnalité des employés et leurs attitudes et comportements, particulicrement la performance au travail (Mount et Barrick, 1995). Ainsi, et tel qu’exprimé par Barber et Bretz (2000), étant donné l’importance de cibler les différences individuelles qui ont un effet sur les organisations, il apparait approprié d’en savoir plus sur la manicre dont différentes composantes de la rémunération peuvent faciliter ou empzcher l’attraction de personnes ayant des traits de personnalité particuliers. Plusieurs auteurs ont précisément exprimé le besoin d’analyser l’incidence des traits de personnalité sur les préférences a l’égard des caractéristiques des emplois (p. ex., Schneider, 1987; Schneider, Goldstein et Smith, 1995).Une enquzte en ligne comprenant les items de l’étude a eu lieu en mai et juin 2006 auprcs d’un échantillon aléatoire représentant 50 % des affiliés de l’Ordre des conseillers en ressources humaines et en relations industrielles agréés du Québec, soit 2 945 personnes. Au total, 967 personnes ont complété le questionnaire, soit un taux de réponse de 32,84 %. Les participants étaient invités a indiquer jusqu’a quel point les différentes caractéristiques présentées avaient joué un role dans leur décision d’accepter l’emploi qu’ils occupaient actuellement. Une analyse factorielle sur l’importance accordée a 30 composantes de la rémunération a permis de regrouper ces dernicres en huit grands facteurs : la qualité du travail et des relations, la rémunération variable, la rémunération indirecte, les possibilités de développement et de carricre, la flexibilité des conditions de travail, le prestige de l’emploi, les gratifications et la charge de travail. Le modcle du Big-Five a été utilisé pour mesurer les cinq traits de personnalité fondamentaux : méticulosité (conscientiousness), extraversion, conscience des autres (agreeableness), équilibre émotionnel (emotional stability) vs. névrotisme, et ouverture a l’expérience (openness to experience). Nos résultats montrent qu’une fois l’effet des huit variables de controle pris en compte (age, sexe, nombre de personnes a charge, expérience de travail, le niveau de scolarité ou le dernier diplome, salaire, ancienneté dans l’organisation, taille de l’organisation), les traits de personnalité des individus contribuent a expliquer une partie significative supplémentaire de la variance de l’importance accordée a six des huit composantes de la rémunération totale étudiées et ce, dans les proportions suivantes : le contenu du travail et les relations interpersonnelles (Δ R2 = 7 %), les possibilités de développement et de carricre (Δ R2 = 4 %), la rémunération variable (Δ R2 = 2 %), la rémunération indirecte (Δ R2 = 2 %), la flexibilité des conditions de travail (Δ R2 = 2 %), et le prestige (Δ R2 = 2 %).Notre étude montre que l’ouverture a l’expérience est le trait de personnalité le plus (positivement) lié a l’importance accordée aux diverses composantes de la rémunération totale, soit la flexibilité des conditions de travail, les possibilités de développement et de carricre, la rémunération variable, le contenu du travail et les relations interpersonnelles, et les gratifications. La « conscience des autres » comme trait de personnalité est significativement liée a l’importance accordée a deux composantes de la rémunération totale : la qualité du travail et des relations et la rémunération indirecte. Les trois autres traits de personnalité ne sont liés chacun qu’a l’importance d’une seule composante de la rémunération totale : la « méticulosité » avec la rémunération indirecte, l’« extraversion » avec la qualité du travail et des relations et le « névrotisme » avec le prestige de l’emploi.Finalement, la faible contribution des traits de personnalité a l’attractivité pereue des composantes de la rémunération totale alimente le débat actuel sur l’usage des tests de personnalité, certains auteurs doutant de leurs retombées pratiques dans la prédiction de la performance au travail, alors que d’autres persistent a les considérer pertinents a cette fin. Aprcs avoir traité des limites de l’étude, des avenues de recherche sont proposées aux chercheurs désireux de mieux comprendre l’attrait envers les composantes de la rémunération totale ou encore, de mieux gérer les composantes de la rémunération pour attirer les meilleurs talents.Este estudio innova al analizar los vínculos entre la personalidad de los individuos y su respectivo interés por los diferentes componentes de la remuneración total. Los datos compilados por cuestionario aplicado a 976 personas muestran que, después haber controlado por el efecto de varias variables demográficas, los rasgos de personalidad del “Big-Five” contribuyen a explicar una parte significativa de la varianza de la importancia acordada a los componentes de la remuneración total : calidad del trabajo y de las relaciones, posibilidades de desarrollo y de carrera, remuneración variable, remuneración indirecta, flexibilidad de las condiciones de trabajo y prestigio. La abertura a experimentar constituye el rasgo de personalidad du “Big-Five” que permite de predecir mejor la importancia acordada a los componentes de la remuneración total estudiados

Iterative CT reconstruction using shearlet-based regularization

In computerized tomography, it is important to reduce the image noise without increasing the acquisition dose. Extensive research has been done into total variation minimization for image denoising and sparse-view reconstruction. However, TV minimization methods show superior denoising performance for simple images (with little texture), but result in texture information loss when applied to more complex images. Since in medical imaging, we are often confronted with textured images, it might not be beneficial to use TV. Our objective is to find a regularization term outperforming TV for sparse-view reconstruction and image denoising in general. A recent efficient solver was developed for convex problems, based on a split-Bregman approach, able to incorporate regularization terms different from TV. In this work, a proof-of-concept study demonstrates the usage of the discrete shearlet transform as a sparsifying transform within this solver for CT reconstructions. In particular, the regularization term is the 1-norm of the shearlet coefficients. We compared our newly developed shearlet approach to traditional TV on both sparse-view and on low-count simulated and measured preclinical data. Shearlet-based regularization does not outperform TV-based regularization for all datasets. Reconstructed images exhibit small aliasing artifacts in sparse-view reconstruction problems, but show no staircasing effect. This results in a slightly higher resolution than with TV-based regularization

Dual Commitment to Organization and Supervisor: A Person-centered Approach

A recent trend in commitment research has been to use person-centered analytic strategies to identify homogeneous subgroups with varying configurations of commitment mindsets (affective, normative, continuance) or targets (e.g., organization, supervisor, team). A person-centered approach takes a more holistic perspective than the traditional variable-centered approach and can reflect potentially complex interactions among commitment mindsets and/or targets. We extend application of the person-centered approach to investigate profiles of commitment to two interrelated targets, the organization and supervisor, in two studies (Ns = 481 and 264) involving Belgian university graduates. Using latent profile analyses, we found that a similar 5-profile model fit best in both studies. The mindset pattern for the two targets was similar for some profile groups, but differed for others. The groups differed on perceived organizational and supervisory support and voluntary turnover largely as expected from commitment and support theory. Implications for future research and management practice are discussed

Nurse plants, tree saplings and grazing pressure: changes in facilitation along a biotic environmental gradient

Current conceptual models predict that an increase in stress shifts interactions between plants from competitive to facilitative; hence, facilitation is expected to gain in ecological importance with increasing stress. Little is known about how facilitative interactions between plants change with increasing biotic stress, such as that incurred by consumer pressure or herbivory (i.e. disturbance sensu Grime). In grazed ecosystems, the presence of unpalatable plants is reported to protect tree saplings against cattle grazing and enhance tree establishment. In accordance with current conceptual facilitation-stress models, we hypothesised a positive relationship between facilitation and grazing pressure. We tested this hypothesis in a field experiment in which tree saplings of four different species (deciduous Fagus sylvatica, Acer pseudoplatanus and coniferous Abies alba, Picea abies) were planted either inside or outside of the canopy of the spiny nurse shrub Rosa rubiginosa in enclosures differing in grazing pressure (low and high) and in exclosures. During one grazing season we followed the survival of the different tree saplings and the level of browsing on these; we also estimated browsing damage to the nurse shrubs. Shrub damage was highest at the higher grazing pressure. Correspondingly, browsing increased and survival decreased in saplings located inside the canopy of the shrubs at the high grazing pressure compared to the low grazing pressure. Saplings of both deciduous species showed a higher survival than the evergreens, while sapling browsing did not differ between species. The relative facilitation of sapling browsing and sapling survival - i.e. the difference between saplings inside and outside the shrub canopy - decreased at high grazing pressure as the facilitative species became less protective. Interestingly, these findings do not agree with current conceptual facilitation-stress models predicting increasing facilitation with abiotic stress. We used our results to design a conceptual model of facilitation along a biotic environmental gradient. Empirical studies are needed to test the applicability of this model. In conclusion, we suggest that current conceptual facilitation models should at least consider the possibility of decreasing facilitation at high levels of stres

Five-factor model of personality and organizational commitment: The mediating role of positive and negative affective states

Using a one-year longitudinal study of four components of organizational commitment (affective, normative, continuance–sacrifices, and continuance–alternatives) on a sample of employees from multiple organizations (N = 220), we examined the relationships of employee Big-Five personality traits to employee commitment components, and the mediating role of positive and negative affective states. Personality was measured at Time 1 while affective states and commitment components were measured at Time 2, while controlling for Time 1 commitment. Extraversion and agreeableness were positively related to affective, normative, and continuance–sacrifices commitments via enhanced positive affect. Agreeableness was also positively linked to affective commitment and negatively associated with continuance–alternatives commitment through reduced negative affect. Finally, neuroticism was negatively linked to affective commitment, and positively related to continuance–alternatives commitment, through increased negative affect. The implications of these findings for our understanding of personality–commitment linkages are discussed