| 381. |
- Janek Strååt, Sara, 1977-
(författare)
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Verification of high energy photon therapy based on PET/CT imaging of photonuclear reactions
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For classical and intensity modulated radiation therapy of deep-seated tumors, high-energy photons are the optimal radiation modality from an integral dose point of view. By using narrow scanned beams the treatment outcome can be improved substantially by delivering biologically optimized intensity modulated distributions often with sharp dose gradients. This requires using photons with energies well above 15 MV enabling verification of the treatment delivery in vivo by PET/CT imaging in a manner not previously possible. This new technique is based on the production of positron emitting radionuclides when the incoming high-energy photons interact through photonuclear reactions with the body tissues. The produced radionuclides, commonly 11C, 15O and 13N can then be monitored by PET and the distribution of activated nuclei show exactly where the radiation has penetrated the patient. In the subcutaneous fat, present in all humans, a high induced activity produces a perfect visualization of the location and even the intensity modulation of the incident beams. The reason for this is the high carbon content in combination with a low biological perfusion in fat tissues. Errors in the patient positioning such as setup errors or misplacement of the beams will thus show up in the PET images as a deviation from the actual radiation treatment plan. Interestingly, the imaged activity distribution from the subcutaneous fat also visualizes how the dose delivery can be deformed when the patient is erroneously positioned on the treatment couch as seen on the cover figure. Furthermore, the different half-lives of the produced radionuclides (20 min, 2 min, and 10 min, for 11C, 15O and 13N, respectively) allows for analysis of the dynamic behavior of tissue activity with the possibility of retrieving information such as tissue composition, biological and physical half-lives. The present thesis shows that considerable clinical information regarding the treatment delivery with high-energy photon beams can be obtained using PET/CT imaging. Although the study is based on the use of 50 MV photons the method may apply for beams with energies > 20 MV at higher doses.
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| 382. |
- Jansson, Mats, et al.
(författare)
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Bioavailable phosphorus in humic headwater streams in boreal Sweden
- 2012
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Ingår i: Limnology and Oceanography. - : Wiley. - 1939-5590 .- 0024-3590. ; 57:4, s. 1161-1170
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Tidskriftsartikel (refereegranskat)abstract
- Bioavailable phosphorus (BAP) concentrations were determined nine times between April and October 2010 in two humic boreal headwater streams draining forest-and mire-dominated catchments. BAP was analyzed in a bioassay in which natural P-limited bacterioplankton grew with natural P as the sole P source. In both streams, approximately 90% of the BAP occurred as dissolved species (passing a 0.2-mu m filter), consisting partly of low-molecular-weight forms (passing a filter with nominal cutoff at 1 kDa) and partly of high-molecular-weight forms (passing a 0.2-mu m filter but not a 1-kDa filter). Concentrations of total dissolved BAP varied between 1 mu g L-1 and 14 mu g L-1, with the highest values in the middle of the summer. Compared to the forest stream, BAP concentrations were generally higher in the mire stream, where it occasionally amounted to nearly 50% of total P. Molybdate reactive phosphorus overestimated BAP considerably. Most of the BAP was in forms other than free orthophosphate. Temporal BAP variations showed no relationships with dissolved organic carbon (C) or iron but were positively related to air temperature and negatively related to the absorbance ratio (a254 : a365) of organic compounds in the water, indicating connections between terrestrial export of BAP and temperature-dependent terrestrial C metabolism. Concentrations of BAP can relieve stream bacteria from P limitation, and a significant share of BAP exported to streams can reach and be used in downstream lakes.
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| 383. |
- Jansson, Mats, et al.
(författare)
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Biomass and structure of planktonic communities along an air temperature gradient in subarctic Sweden
- 2010
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Ingår i: Freshwater Biology. - : Wiley. - 0046-5070 .- 1365-2427. ; 55:3, s. 691-700
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Tidskriftsartikel (refereegranskat)abstract
- 1. Air temperature will probably have pronounced effects on the composition of plankton communities in northern lake ecosystems, either via indirect effects on the export of essential elements from catchments or through direct effects of water temperature and the ice-free period on the behaviour of planktonic organisms.2. We assessed the role of temperature by comparing planktonic communities in 15 lakes along a 6 °C air temperature gradient in subarctic Sweden.3. We found that the biomass of phytoplankton, bacterioplankton and the total planktonic biomass were positively related to air temperature, probably as a result of climatic controls on the export of nitrogen from the catchment (which affects phytoplankton biomass) and dissolved organic carbon (affecting bacterioplankton biomass).4. The structure of the zooplankton community, and top down effects on phytoplankton, were apparently not related to temperature but mainly to trophic interactions ultimately dependent on the presence of fish in the lakes.5. Our results suggest that air temperature regimes and long-term warming can have strong effects on the planktonic biomass in high latitude lakes. Effects of temperature on the structure of the planktonic community might be less evident unless warming permits the invasion of fish into previous fishless lakes.
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| 384. |
- Jansson, Mats, et al.
(författare)
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Carbon dioxide supersaturation promotes primary production in lakes
- 2012
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Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 15:6, s. 527-532
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Tidskriftsartikel (refereegranskat)abstract
- Ecology Letters (2012) Abstract A majority of the worlds lakes are supersaturated with respect to carbon dioxide (CO2). By experimental manipulation of the CO2 concentration in supersaturated boreal lakes, we demonstrate that phytoplankton primary production was up to 10 times higher in supersaturated lake water in comparison with water with CO2 at equilibrium concentrations and that CO2, together with nutrients, explained most of the variation in pelagic primary production and phytoplankton biomass over a wide variety of unproductive lakes. These results suggest that phytoplankton can be co-limited by CO2 and nutrients in unproductive lakes. As import of terrestrial organic carbon and its subsequent microbial mineralisation in lakes is a driving force of CO2-supersaturation our results suggest that lake productivity and carbon cycling may respond to variations in terrestrial organic carbon export, (e.g. caused by land use or climate change) in ways not described before.
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| 385. |
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| 386. |
- Jansson, Mats, et al.
(författare)
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Impact of allochthonous organic carbon on microbial food web carbon dynamics and structure in Lake Örträsket
- 1999
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Ingår i: Archiv für Hydrobiologie. - 0003-9136. ; 144:4, s. 409-428
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Tidskriftsartikel (refereegranskat)abstract
- Bacterial utilization of allochthonous organic carbon and the subsequent fate of the bacterial production in the pelagic food web were estimated in Lake Örträsket, a large humic lake in northern Sweden. Bacterial production relied mainly on allochthonous DOC and exceeded primary production in the epilimnion. Bacterial productivity was clearly stimulated during high flow episodes, increasing the input of bacteria degradable organic material to the lake. Bacterioplankton were exploited mainly by mixotrophic flagellates which probably used bacteria as a source of carbon, phosphorus and nitrogen. The possibly extremely low availability of inorganic P during periods with high bacterial production may have allowed the mixotrophs to outcompete obligate autotrophs and help them become dominant phytoplankters during large parts of the summer. The results from Lake Örträsket indicated that the total production depended on bacterial energy mobilization from allochthonous organic carbon compounds and that heterotrophic mobilized energy was linked via mixotrophs to higher levels in the food chain.
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| 387. |
- Jansson, Mats, et al.
(författare)
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Links between terrestrial primary production and bacterial production and respiration in lakes in a climate gradient in subarctic Sweden
- 2008
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Ingår i: Ecosystems (New York. Print). - : Springer Science and Business Media LLC. - 1432-9840 .- 1435-0629. ; 11, s. 367-376
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Tidskriftsartikel (refereegranskat)abstract
- We compared terrestrial net primary production (NPP) and terrestrial export of dissolved organic carbon (DOC) with lake water heterotrophic bacterial activity in 12 headwater lake catchments along an altitude gradient in subarctic Sweden. Modelled NPP declined strongly with altitude and annual air temperature decreases along the altitude gradient (6ºC between the warmest and the coldest catchment). Estimated terrestrial DOC export to the lakes was closely correlated to NPP. Heterotrophic bacterial production (BP) and respiration (BR) were mainly based on terrestrial organic carbon and strongly correlated with the terrestrial DOC export. Excess respiration over PP of the pelagic system was similar to net emission of CO2 in the lakes. BR and CO2 emission made up considerably higher shares of the terrestrial DOC input in warm lakes than in cold lakes, implying that respiration and the degree of net heterotrophy in the lakes were dependant not only on terrestrial export of DOC, but also on characteristics in the lakes which changed along the gradient and affected the bacterial metabolization of allochthonous DOC. The study showed close links between terrestrial primary production, terrestrial DOC export and bacterial activity in lakes and how these relationships were dependant on air temperature. Increases in air temperature in high latitude unproductive systems might have considerable consequences for lake water productivity and release of CO2 to the atmosphere, which are ultimately determined by terrestrial primary production.
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| 388. |
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| 389. |
- Jansson, Mats, et al.
(författare)
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Nutrient limitation of bacterioplankton, autotrophic and mixotrophic phytoplankton, and heterotrophic nanoflagellates in Lake Örträsket
- 1996
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Ingår i: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 41:7, s. 1552-1559
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Tidskriftsartikel (refereegranskat)abstract
- Enrichment experiments with P and N were conducted in humic Lake Örträsket in northern Sweden. The composition of the microplankton community showed a dominance by bacterioplankton, followed by mixotrophic sind potentially mixotrophic phytoplankton, heterotrophic nanoflagellates, and autotrophic phytoplankton. Bacterioplankton was P limited for most of the ice-free period, and phytoplankton biomass and primary production mostly increased after enrichment with N, but not with P. The dominant group of phytoplankton, the mixotrophic flagellates, was stimulated by N bur not by P, while obligate autotrophic species were stimulated only by P+N. It is suggested that N limitation in mixotrophic species is induced by grazing of P-rich bacteria. The results suggest that primary productivity in humic lakes can be limited by N and indicate the importance of phagocytosis as a means of nutrition in phytoplankton. A link is suggested to exist in humic lakes whereby heterotrophic bacterioplankton, which use humic compounds as their principal energy source, can transfer energy and nutrients to potentially autotrophic organisms, with subsequent utilization by other components of the food web.
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| 390. |
- Jarfors, Anders E.W. 1963-, et al.
(författare)
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An a Priori Discussion of the Fill Front Stability in Semisolid Casting
- 2022
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Ingår i: Technologies. - : MDPI AG. - 2227-7080. ; 10:3, s. 67-
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Tidskriftsartikel (refereegranskat)abstract
- Metal casting is an industrially important manufacturing process offering a superior combination of design flexibility, productivity and cost-effectiveness, but has limitations due to filling related defects. Several semisolid casting processes are available capable of casting at a range of solid fractions to overcome this. The current communication aims to review the filling front behaviour and give a new perspective to the gate design in semisolid processing compared to conventional high-pressure die-casting. It is shown that solid fraction and gate widths are critical to avoid instability and spraying.
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