| 1. |
- Brockstedt, Sara, et al.
(författare)
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Use of an enhanced gradient system for diffusion MR imaging with motion-artifact reduction
- 1995
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Ingår i: Acta Radiologica. - 1600-0455. ; 36:6, s. 662-670
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: A spin-echo diffusion-sensitized pulse sequence using high gradients (23 mT/m) is introduced. MATERIAL AND METHODS: In order to minimize motion artefacts, velocity-compensating gradients, ECG-triggering and post-processing with phase correction and raw data averaging using navigator echoes was performed. The in vitro ratio of diffusion coefficients for water and acetone was determined and the water self-diffusion coefficient at different temperatures was evaluated. The pulse sequence was tested in 7 healthy volunteers and in 2 tumour patients with astrocytomas of grades I-II and III-IV. Both single-slice and multi-slice techniques were used. RESULTS: The incorporation of phase correction clearly improved the quality of both diffusion-encoded images and the calculated diffusion maps. Mean values of the diffusion coefficients in vivo were for CSF 2.66 x 10(-9) m2/s and for white and grey matter 0.69 x 10(-9) m2/s and 0.87 x 10(-9) m2/s, respectively. CONCLUSION: Velocity-compensating gradients in combination with a high gradient strength were shown to be useful for in vivo diffusion MR imaging.
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| 2. |
- Lundh, Olle, et al.
(författare)
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Comparison of measured with calculated dose distribution from a 120-MeV electron beam from a laser-plasma accelerator
- 2012
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 39:6, s. 3501-3508
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To evaluate the dose distribution of a 120-MeV laser-plasma accelerated electron beam which may be of potential interest for high-energy electron radiation therapy. Methods: In the interaction between an intense laser pulse and a helium gas jet, a well collimated electron beam with very high energy is produced. A secondary laser beam is used to optically control and to tune the electron beam energy and charge. The potential use of this beam for radiation treatment is evaluated experimentally by measurements of dose deposition in a polystyrene phantom. The results are compared to Monte Carlo simulations using the GEANT4 code. Results: It has been shown that the laser-plasma accelerated electron beam can deliver a peak dose of more than 1 Gy at the entrance of the phantom in a single laser shot by direct irradiation, without the use of intermediate magnetic transport or focusing. The dose distribution is peaked on axis, with narrow lateral penumbra. Monte Carlo simulations of electron beam propagation and dose deposition indicate that the propagation of the intense electron beam (with large self-fields) can be described by standard models that exclude collective effects in the response of the material. Conclusions: The measurements show that the high-energy electron beams produced by an optically injected laser-plasma accelerator can deliver high enough dose at penetration depths of interest for electron beam radiotherapy of deep-seated tumors. Many engineering issues must be resolved before laser-accelerated electrons can be used for cancer therapy, but they also represent exciting challenges for future research. (C) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.47199621
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| 3. |
- Martens, C, et al.
(författare)
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Underdosage of the upper-airway mucosa for small fields as used in intensity-modulated radiation therapy: A comparison between radiochromic film measurements, Monte Carlo simulations, and collapsed cone convolution calculations
- 2002
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 29:7, s. 1528-1535
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Tidskriftsartikel (refereegranskat)abstract
- Head-and-neck tumors are often situated at an air-tissue interface what may result in an underdosage of part of the tumor in radiotherapy treatments using megavoltage photons. especially for small fields. In addition to effects of transient electronic disequilibrium, for these small fields, an increased lateral electron range in air will result in an important extra reduction of the central axis (lose beyond the cavity. Therefore dose calculation algorithms need to model electron transport accurately. We simulated the trachea by a 2 cm diameter cylindrical air cavity with the rin) situated 2 cm beneath the phantom surface. A 6 MV photon beam from an Elekta SLi plus linear accelerator, equipped with the standard multileaf collimator (MLC), was assessed. A 10 x 2 cm(2) and a 10 K 1 cm(2) field, both widthwise collimated by the MLC, were applied with their long side parallel to the cylinder axis. Central axis dose rebuild-up was studied. Radiochromic film measurements were performed in an in-house manufactured polystyrene phantom with the films oriented either along or perpendicular to the beam axis. Monte Carlo simulations were performed with BEAM and EGSnrc. Calculations were also performed using the pencil beam (PB) algorithm and the collapsed cone convolution (CCC) algorithm of Helax-TMS (MDS Nordion, Kanata. Canada) version 6.0.2 and using the CCC algorithm of Pinnacle (ADAC Laboratories, Milpitas. CA, USA) version 4.2. A very good agreement between the film measurements and the Monte Carlo simulations was found. The CCC algorithms were not able to predict the interface dose accurately when lateral electronic disequilibrium occurs, but were shown to be a considerable improvement compared to the PB algorithm. The CCC algorithms overestimate the dose in the rebuild-up region. The interface dose was overestimated by a maximum of 31% or 54%, depending on the implementation of the CCC algorithm. At a depth of I rum, the maximum dose overestimation was 14% or 24%.
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| 4. |
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| 5. |
- Martens, L., et al.
(författare)
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Electromagnetic field calculations used for exposure experiments on small animals in TEM-cells
- 1993
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Ingår i: Bioelectrochemistry and Bioenergetics. - : Elsevier BV. - 0302-4598. ; 30:C, s. 73-81
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional electromagnetic calculations for loaded transverse electromagnetic (TEM) transmission cells are presented. Based on those calculations a prediction of the perturbation of the standard uniform field in the TEM-cell, due to the scattering by inhomogeneous structures placed inside the cell cavity, is given. The influence of the dimensions of a lossy structure and its position in the TEM-cell on its absorption of the electromagnetic fields is presented. Knowing the perturbation of the uniform field is important for good interpretation of the biological experimental results.
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| 6. |
- Bohsung, J, et al.
(författare)
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IMRT treatment planning - A comparative inter-system and intor-centre planning exercise of the ESTRO QUASIMODO group
- 2005
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Ingår i: Radiotherapy and Oncology. - : Elsevier BV. - 1879-0887 .- 0167-8140. ; 76:3, s. 354-361
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Tidskriftsartikel (refereegranskat)abstract
- Background and purpose: The purpose of this work was a comparison of realistic IMRT plans based on the same CT-image data set and a common predefined set of dose objectives for the planning target volume and the organs at risk. This work was part of the larger European QUASIMODO IMRT verification project. Materials and methods: Eleven IMRT. plans were produced by nine different European groups, each applying a representative set of. clinically used IMRT treatment planning systems. The plans produced were to be deliverable in a clinically acceptable treatment time with the local technical equipment. All plans were characterized using a set of different quality measures such as dose-volume histograms, number of monitor units and treatment time. Results: Only one plan was able to fulfil all dose objectives strictly; six plans failed some of the objectives but were still considered to be clinically acceptable; four plans were not able to reach the objectives. Additional quality scores such as the number of monitor units and treatment time showed large variations, which mainly depend on the delivery technique. Conclusion: The presented planning study showed that with nearly all presently available IMRT planning and delivery systems comparable dose distributions could be achieved if the planning goals are clearly defined in advance.
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