| 1. |
- Karlsson, Magnus G, et al.
(författare)
-
Semi-conductor detectors in output factor measurements
- 1997
-
Ingår i: Radiotherapy and Oncology. - 0167-8140 .- 1879-0887. ; 42:3, s. 293-296
-
Tidskriftsartikel (refereegranskat)abstract
- Background and purpose: Output factors are generally measured with cylindrical ionization chambers. It was investigated if Si-diodes of p-type instead could be used. The advantage would be the small detector size and the robust construction of the detector. Materials and methods: Two types of diodes were studied, one with a shielding layer of tungsten specially made to reduce the excess response for scattered photons and one standard diode without any extra shielding. The measurements were performed at accelerating potentials between 4 and 50 MV and beam sizes between 4 cm x 4 cm and 40 cm x 40 cm. Results: The results showed that both types of diodes are suitable for measurements of head scatter factors in mini-phantoms. However, the diodes were found inappropriate for measurement of output factors for large fields in extended water phantoms. For small fields (<10 cm x 10 cm) a small detector is advantageous and no errors due to the scatter contribution were seen. Conclusions: An cylindrical ionization chamber is the best choice for output factor measurements in extended water phantoms for large field sizes while diodes are an alternative in small fields. There were negligible differences between the detectors in head scatter measurements in mini phantoms.
|
|
| 2. |
- Karlsson, M G, et al.
(författare)
-
Treatment head design for multileaf collimated high-energy electrons
- 1999
-
Ingår i: Medical physics (Lancaster). - 0094-2405. ; 26:10, s. 2161-2167
-
Tidskriftsartikel (refereegranskat)abstract
- This paper describes how a conventional treatment head can be modified for use of multileaf collimated electron beams. Automatic and dynamic beam delivery are possible for both electrons and photons by using the computer controlled multileaf collimator (MLC) for both photon and electron beams. Thereby, the electron beams can be mixed more freely into the treatment to take advantage of the specific depth modulation characteristics of electrons. The investigation was based on Monte Carlo calculations using the software package BEAM. The physical parameters used in this optimization were the beam penumbra and the virtual/effective point source position. These parameters are essential for shaping beams, beam matching and for dosimetry calculations. The optimization was carried out by modifying a number of parameters: replacing the air atmosphere in the treatment head with helium, adding a helium bag below the MLC, changing the position of the scattering foils, modifying the monitor chamber, and adjusting the position of the MLC. The beam characteristics for some of these designs were found to fulfil our criteria for clinically useful beams down to at least 9 MeV. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00610-0].
|
|
| 3. |
|
|
| 4. |
- Sjogren, R, et al.
(författare)
-
Methods for the determination of effective monitor chamber thickness
- 1999
-
Ingår i: Medical physics (Lancaster). - 0094-2405. ; 26:9, s. 1871-1873
-
Tidskriftsartikel (refereegranskat)abstract
- There are a number of models, both analytical and Monte Carlo, which are used to describe the fluence from the treatment head of accelerators. One common problem in these simulations is to find relevant information about details in the treatment head. A complex unit in the treatment head for which reliable data is seldom given is the monitor chamber. In this work two methods are described for obtaining this information by analyzing the increased scattering of an electron beam when the monitor chamber is introduced in the beam. It was found that the effective thickness of the electrodes in a monitor chamber can be determined with sufficient accuracy by using experimental results combined with Fermi-Eyges theory or Monte Carlo simulations. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01009-3].
|
|
| 5. |
- Sjögren, Rickard, et al.
(författare)
-
Depth for dose calibration in high energy photon beams
- 1997
-
Ingår i: Radiotherapy and Oncology. - 0167-8140 .- 1879-0887. ; 43:3, s. 311-313
-
Tidskriftsartikel (refereegranskat)abstract
- Background and purpose: The normalisation depth for determination of output factors in photon fields has frequently been the depth of dose maximum. At high energies the contribution from contaminating electrons is significant at dose maximum and is critically dependent on the beam geometry parameters, which is why a larger depth should be preferred. Materials and methods: The effect of electron contamination was studied using a purging magnet to remove charged particles from the treatment head and a helium bag to minimise production between the head and the phantom. Results: A depth of 10 cm was found to be beyond the range of the contaminating electrons for photon energies up to 20 MV (TPR1020 = 0.772). However, at 50 MV (TPR1020 = 0.810) contaminating electrons contribute 2-3% to the absorbed dose at 10 cm depth. Conclusions: 10 cm is recommended as both reference and normalisation depth for all megavoltage photon beam qualities, i.e. Co-60 and X-rays from accelerators up to 50 MV. (C) 1997 Elsevier Science Ireland Ltd.
|
|
| 6. |
- Andreani, Pietro, et al.
(författare)
-
A chip for linearization of RF power amplifiers using digital predistortion with a bit-parallel complex multiplier
- 1999
-
Ingår i: Proceedings of the 1999 IEEE International Symposium on Circuits and Systems, 1999. ISCAS '99.. - 0780354710 ; 1, s. 346-349
-
Konferensbidrag (refereegranskat)abstract
- This paper presents a custom chip for linearization of RF power amplifiers using digital predistortion. The chip has been implemented in a standard digital 0.8 μm CMOS process with standard static cells and single-phase clocking. A systolic complex multiplier based on distributed arithmetic constitutes the core of the chip. The nonlinear function is realized with a look-up table containing complex gain factors applied to the complex multiplier. The maximum clock frequency was found by means of simulation to be 105 MHz corresponding to 21 Msamples/s throughput with 3 W power consumption using 5 V supply voltage. The fabricated chip is fully functional and has been measured up to 60 MHz clock frequency with 825 mW power consumption with 3.3 V supply voltage. Operation at 1.5 V supply voltage allows 10 MHz clock frequency with 35 mW power consumption
|
|
| 7. |
- Blomquist, M, et al.
(författare)
-
Measured lung dose correction factors for 50 MV photons
- 1998
-
Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 43:11, s. 3225-3234
-
Tidskriftsartikel (refereegranskat)abstract
- Some clinically relevant measurements of lung tissue/water equivalent interfaces have been performed for a 50 MV therapeutic x-ray beam. The purpose was to investigate the severity of dose perturbation effects in lung tissue and adjacent tissues using an energy well above the common clinical practice in thoracic irradiations. The phantoms were constructed of solid water, PMMA and white polystyrene as soft tissue (water) equivalents, and cork was used as the lung tissue equivalent. Measurements were performed using radiographic film and a cylindrical ionization chamber. The results show that the degradation of the 20/80% beam penumbra in the lung region is severe, up to 2.5 times the penumbra in water for a 10 cm thick lung with a density of 0.30 x 10(3) kg m(-3). The lack of electronic equilibrium in the low-density region can cause underdosage at the lung/tumour interface of up to 30% of maximum target dose, and the build-up depth to 95% of target dose in unit density tissue behind the lung may be as large as 22 mm. It is also shown that these figures strongly depend on patient anatomy and beam size and why a careful calculation of the individual dose distribution is needed for optimal choice of photon beam energy in thoracic treatments.
|
|
| 8. |
- Blomquist, M, et al.
(författare)
-
Scanned intensity modulations for 50 MV photons
- 1998
-
Ingår i: Physics in Medicine and Biology. - 0031-9155 .- 1361-6560. ; 43:5, s. 1185-1197
-
Tidskriftsartikel (refereegranskat)abstract
- Optimization of the dose distributions by individual beam compensation is a useful tool in conformal radiation therapy. Intensity modulation by electromagnetic scanning of a narrow elementary beam allows fast dose delivery and causes little change in beam quality compared with other methods, especially for high energies such as 50 MV. Intensity modulated beams from the MM50 accelerator were measured and compared with calculations based on Monte Carlo simulations. Good agreement between measurements and calculations were found, typically within 1% for central dose profiles. The steepest wedge angle that was produced with the scanning beam technique was of 45 degrees or 3.5% cm(-1) for a 20 cm x 20 cm field, slightly varying with depth. The elementary 50 MV photon 'pencil beam' for a full range, high-z bremsstrahlung target, is a wide dose distribution at 10 cm depth in water which limits the modulation gradient and hence the complexity of the modulation by the scanning of a photon pencil beam only. Scanned wedge beam distributions were modelled in the treatment planning system and a pelvic treatment with three fields was used to illustrate a clinical application. The resulting dose volume data were compared for different radiation qualities but with similar beam portals. 'Energy modulation' by field matching with lower photon energies was performed to sharpen the penumbra towards organs at risk.
|
|
| 9. |
- Blomquist, M, et al.
(författare)
-
Test procedures for verification of an electron pencil beam algorithm implemented for treatment planning
- 1996
-
Ingår i: Radiotherapy and Oncology. - : Elsevier BV. - 0167-8140 .- 1879-0887. ; 39:3, s. 271-286
-
Tidskriftsartikel (refereegranskat)abstract
- The calculation of an electron dose distribution in a patient is a difficult problem because of the presence of tissue and surface inhomogeneities. Verification of the dose planning system is therefore essential. In this investigation, a novel method is used to evaluate a commercially available system (Helax-TMS), at electron energies between 10 and 50 MeV, both for a conventional treatment unit and an MLC-collimated scanned beam unit with a helium-filled treatment head. First, the experiments were designed to verify the local beam database and some fundamental characteristics of the electron beam calculations. Secondly, a number of generalised situations that would be encountered in the clinical treatment planning were evaluated: oblique incidence, field shaping with multi-leaf collimator, bolus edges, and air cavities. Dose distributions in two generalised anatomical phantoms simulating a neck and a nose were also analysed. The results have, when so possible, been presented as the dose ratio within the 'flattened area' for dose profiles and down to the 'treatment depth' (80% dose level) for depth doses. In the penumbra region and in the dose fall-off region, the comparison has been represented by the distance deviation between calculated and measured dose profiles or depth doses. A new tool, 'volume integration', was used to evaluate the deviations from a more clinical point of view. Most results were within +/-2% in dose for volumes larger than a sphere with a diameter of 15 mm, or +/-2 mm in position. Dose deviations were generally found for oblique incidences and below heterogeneities such as small air cavities and bolus edges in limited volumes.
|
|
| 10. |
- Bundy, Kenneth, et al.
(författare)
-
An electrochemical impedance spectroscopy method for prediction of the state of charge of a nickel-metal hydride battery at open circuit and during discharge
- 1998
-
Ingår i: Journal of Power Sources. ; 72:2, s. 118-125
-
Tidskriftsartikel (refereegranskat)abstract
- A multivariate method for predicting state of charge, from electrochemical data, of a nickel-metal hydride (NiMH)-battery is presented. Partial least square (PLS) regression is used to evaluate electrochemical impedance spectra and predict state of charge. The impedance spectra are analysed in the frequency range 239-0.6 Hz. The impedance is measured for different states of charge at open-circuit conditions and during continuous discharge at loads ranging between 0.2 C and 0.8 C. When measuring the impedance during discharge, the AC-current signal is imposed on the DC-current. The predictive capability of the method is tested by a cross validation procedure and the root mean square error of prediction is 7% when using the outlier identification capability of the PLS-regression method. The state of charge is evaluated with a single model, independently of whether the cell is subjected to open-circuit or polarised conditions. The predictive performance of the present model decreases at state of charge values less than 10%. © 1998 Elsevier Science S.A.
|
|
