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- Hildeman, Anders, et al.
(författare)
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Hildeman, A., Bolin, D., Wallin, J., Johansson, A., Nyholm, T., Asklund, T., and Yu, J. Whole-brain substitute CT generation using Markov random field mixture models.
- 2016
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Computed tomography (CT) equivalent information is needed for attenuation correction in PET imaging and for dose planning in radiotherapy. Prior work has shown that Gaussian mixture models can be used to generate a substitute CT (s-CT) image from a specific set of MRI modalities. This work introduces a more flexible class of mixture models for s-CT generation, that incorporates spatial dependency in the data through a Markov random field prior on the latent field of class memberships associated with a mixture model. Furthermore, the mixture distributions are extended from Gaussian to normal inverse Gaussian (NIG), allowing heavier tails and skewness. The amount of data needed to train a model for s-CT generation is of the order of 10^8 voxels. The computational efficiency of the parameter estimationand prediction methods are hence paramount, especially when spatial dependency is included in the models. A stochastic Expectation Maximization (EM) gradient algorithm is proposed in order to tackle this challenge. The advantages of the spatial model and NIG distributions are evaluated with a cross-validation study based ondata from 14 patients. The study show that the proposed model enhances the predictive quality of the s-CT images by reducing the mean absolute error with 17.9%. Also, the distribution of CT values conditioned on the MR images are better explainedby the proposed model as evaluated using continuous ranked probability scores.
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| 4. |
- Kautsky, Hans, et al.
(författare)
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Improvement of Baltic Sea coastal ecosystems indicated by increased distribution of Fucus vesiculosus L. since 1984
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Although there are several reports of continued eutrophication in the open Baltic Sea, we found increased distribution and depth penetration of Fucus vesiculosus in the coastal, phytobenthic system, indicating recovery. After a decline in depth penetration of Fucus in the Åland Sea by 3 m, from a maximum of 11.5 m in the 1940s to 8.5 m in 1984, the Fucus plants had again increased their depth distribution in 1992 and 1996, and in 2006 being back to the depths of the 1940s. Also, in the Askö area, data from the national monitoring programme show an increase of the Fucus maximum depth by 1 m between 1993 and 2009. We used Generalized Additive Models (GAM) to predict the change in percent cover of Fucus in the Askö area from 1993 to 2009. The largest change in Fucus coverage was predicted to occur in the inner parts of the archipelago, decreasing further out, thus being related to reductions in nutrient inputs from land sources. The field data however, showed the largest changes in the middle part since coverage and depth penetration of Fucus in the inner parts are limited by the quick change with depth from hard to soft substrates. In the Askö area the results in the inner archipelago could be linked to an increase in the spring Secchi depth since the beginning of the 1990s. The Secchi depth change was in accordance with the trends of decreasing spring primary production and chlorophyll-a concentration. As the most pronounced change in Secchi depth during the time period was a decrease in July and August, the summer conditions seem to have little influence on the depth distribution of Fucus.
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- Wallin, Anders, et al.
(författare)
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Spatially dependent relationships between environmental factors and phytobenthic communities along the Swedish coast of the Baltic Sea, a numeric model approach
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The phytobenthic plant and animal communities are important components in the Baltic Sea ecosystem. In order to properly manage and monitor these communities the knowledge of structuring factors is important. In this study we used multivariate analyses to test the relative importance of environmental variables structuring the phytobenthic communities along the Swedish Baltic Sea coast, how the importance of these factors change with spatial scale and between the three main Baltic Sea sub-basins, the Baltic proper, Bothnian Sea and Bothnian Bay. We also studied the effect of these environmental factors for different species groups. The environmental factors included were salinity, wave exposure, substrate slope, depth and substrate type. For the analysis a large dataset of 1362 diving transects performed with comparable methods was used, describing the phytobenthic plant and animal species depth distribution and coverage. The environmental factors changed in importance at the different scales. The community-environment relationships also differed between the sub-basins, especially in the Bothnian Bay compared to the Baltic proper and Bothnian Sea. In the calculated best combination of factors correlating with the phytobenthic community the depth and the substrate were included in a majority of the analyses, both at different scales and in the different sub-basins. Differences in the correlation between the phytobenthic community and the environmental factors were also found between species groups. The differences between the spatial scales, the sub-basins and the species groups indicate that the criteria for environmental status, and stratification of sampling during local monitoring assessments, have to be area specific.
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