| 1. |
- Carlsson, Fredrik, et al.
(författare)
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Using eigenstructure of the Hessian to reduce the dimension of the intensity modulated radiation therapy optimization problem
- 2006
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Ingår i: Annals of Operations Research. - : Springer Science and Business Media LLC. - 0254-5330 .- 1572-9338. ; 148:1, s. 81-94
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Tidskriftsartikel (refereegranskat)abstract
- Optimization is of vital importance when performing intensity modulated radiation therapy to treat cancer tumors. The optimization problem is typically large-scale with a nonlinear objective function and bounds on the variables, and we solve it using a quasi-Newton sequential quadratic programming method. This study investigates the effect on the optimal solution, and hence treatment outcome, when solving an approximate optimization problem of lower dimension. Through a spectral decompostion, eigenvectors and eigenvalues of an approximation to the Hessian are computed. An approximate optimization problem of reduced dimension is formulated by introducing eigenvector weights as optimization parameters, where only eigenvectors corresponding to large eigenvalues are included. The approach is evaluated on a clinical prostate case. Compared to bixel weight optimization, eigenvector weight optimization with few parameters results in faster initial decline in the objective function, but with inferior final solution. Another approach, which combines eigenvector weights and bixel weights as variables, gives lower final objective values than what bixel weight optimization does. However, this advantage comes at the expense of the pre-computational time for the spectral decomposition.
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| 2. |
- Rehbinder, Henrik, et al.
(författare)
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Scheduling of a limited communication channel for optimal control
- 2004
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 40:3, s. 491-500
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Tidskriftsartikel (refereegranskat)abstract
- In this paper a method for optimal off-line scheduling of a limited resource used for control purposes is presented. For various reasons, real-time communication channels are prone to have-limited bandwidth. To overcome this obstacle, the rate of actions must be chosen accordingly at design time, both with respect to the limitation of the resource and to control performance. A resulting off-line schedule implements the rate of actions as a repetitive sequence of communication instants. Periodic control theory is used to define a cost functional for LQ-control, that measures the performance of a sampled-data system in relation to a desired continuous time performance. In contrast to uniform sampling, the communication sequence is here allowed to be time-varying. This approach results in a complex combinatorial optimization problem, whose solution gives the optimal off-line schedule, i.e., the sequence in which the actions should take place. The optimization problem is solved by a neighborhood search method where a heuristic method is used to generate initial guesses close to the optimum. The optimal schedule is typically such that the sampling is non-uniform, but the resulting LQ-control law is time-varying and takes this non-uniform sampling into account.
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| 3. |
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| 4. |
- Ridderström, Christian, et al.
(författare)
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The basic design of the quadruped robot WARP1
- 2000
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Ingår i: International Conference on Climbing and Walking Robots.
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the basic design of a walking robot platform, in terms of its mechanics,electronics and control. The four-legged robot weighs about 60 kilograms, has three actuatorsin each leg and uses a distributed control system over six CAN busses. Basic strength and speedis demonstrated by experiments where the robot performs walking motions and goes down onits knees and then up again. A brief discussion of experiences from designing and implementingthe platform is included.
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| 5. |
- Wedenberg, Mina, et al.
(författare)
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Analytical description of the LET dependence of cell survival using the repairable-conditionally repairable damage model
- 2010
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Ingår i: Radiation Research. - 0033-7587 .- 1938-5404. ; 96, s. S534-S534
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Tidskriftsartikel (refereegranskat)abstract
- In light-ion radiation therapy, both the dose and the local energy spectrum, which is often characterized with the linear energy transfer (LET), must be considered. In treatment optimization, it is advantageous to use a radiobiological model that analytically accounts for both dose and LET for the ion type of interest. With such a model the biological effect can also be estimated for dose and LET combinations for which there are no observations in the underlying experimental data. In this study, the repairable-conditionally repairable (RCR) damage model was extended by expressing its parameters as functions of LET to provide a radiobiological model that accounts for both the dose and the LET for a given ion type and cell line. This LET-parameterized RCR model was fitted to published cell survival data for HSG and V79 cells irradiated with carbon ions and for T1 cells irradiated with helium ions. To test the robustness of the model, fittings to only a subset of the data were performed. Good agreement with the cell survival data was obtained, including survival data for LET values not used for model fitting, opening up the possibility of using the model in treatment planning for light ions.
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