| 1. |
- Kubancak, J., et al.
(författare)
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Liulin silicon semiconductor spectrometers as cosmic ray monitors at the high mountain observatories Jungfraujoch and Lomnický štít
- 2014
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Ingår i: Journal of Instrumentation. - : Institute of Physics Publishing (IOPP). - 1748-0221. ; 9:P07018
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Tidskriftsartikel (refereegranskat)abstract
- Currently, most cosmic ray data are obtained by detectors on satellites, aircraft, high-altitude balloons and ground (neutron monitors). In our work, we examined whether Liulin semiconductor spectrometers (simple silicon planar diode detectors with spectrometric properties) located at high mountain observatories could contribute new information to the monitoring of cosmic rays by analyzing data from selected solar events between 2005 and 2013. The decision thresholds and detection limits of these detectors placed at Jungfraujoch (Switzerland; 3475 m a.s.l.; vertical cut-off rigidity 4.5 GV) and Lomnicky. stit (Slovakia; 2633 m a.s.l.; vertical cut-off rigidity 3.84 GV) highmountain observatories were determined. The data showed that only the strongest variations of the cosmic ray flux in this period were detectable. The main limitation in the performance of these detectors is their small sensitive volume and low sensitivity of the PIN photodiode to neutrons.
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| 2. |
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| 3. |
- Magnusson, Maria, et al.
(författare)
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Iterative Reconstruction for QuantitativeTissue Decomposition in Dual-Energy CT
- 2011
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Ingår i: Proceedings of the 17th Scandinavian Conference, SCIA 2011, Ystad, Sweden, May 2011.. - Berlin, Heidelberg : Springer Berlin/Heidelberg. - 9783642212260 - 9783642212277 ; , s. 479-488
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Konferensbidrag (refereegranskat)abstract
- Quantitative tissue classification using dual-energy CT has the potential to improve accuracy in radiation therapy dose planning as it provides more information about material composition of scanned objects than the currently used methods based on single-energy CT. One problem that hinders successful application of both single-and dualenergy CT is the presence of beam hardening and scatter artifacts in reconstructed data. Current pre-and post-correction methods used for image reconstruction often bias CT numbers and thus limit their applicability for quantitative tissue classification. Here we demonstrate simulation studies with a novel iterative algorithm that decomposes every soft tissue voxel into three base materials: water, protein and adipose. The results demonstrate that beam hardening artifacts can effectively be removed and accurate estimation of mass fractions of all base materials can be achieved. In the future, the algorithm may be developed further to include segmentation of soft and bone tissue and subsequent bone decomposition, extension from 2-D to 3-D and scatter correction.
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| 4. |
- Malusek, Alexandr, et al.
(författare)
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Analysis of the tandem calibration method for kerma area meters vis Monte Carlo simulations
- 2011
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Ingår i: Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS). Proceedings of an International Symposium held in Vienna, Austria 9-12 November 2010 (2 Volumes). - : IAEA. - 9789201162106 ; , s. 129-136
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Konferensbidrag (refereegranskat)abstract
- The IAEA recommends that uncertainties of dosimetric measurements in diagnostic radiology for risk assessment and quality assurance should be less than 7% on the confidence level of 95%. This accuracy is difficult to achieve with kerma area product (KAP) meters currently used in clinics. The reasons range from the high energy dependence of KAP meters to the wide variety of configurations in which KAP meters are used and calibrated. The tandem calibration method introduced by Pöyry, Komppa and Kosunen in 2005 has the potential to make the calibration procedure simpler and more accurate compared to the traditional beam-area method. In this method, two positions of the reference KAP meter are of interest: (a) a position close to the field KAP meter and (b) a position 20 cm above the couch. In the close position, the distance between the two KAP meters should be at least 30 cm to reduce the effect of back scatter. For the other position, which is recommended for the beam-area calibration method, the distance of 70 cm between the KAP meters was used in this study. The aim of this work was to complement existing experimental data comparing the two configurations with Monte Carlo (MC) simulations. In a geometry consisting of a simplified model of the VacuTec 70157 type KAP meter, the MCNP code was used to simulate the kerma area product, PKA, for the two (close and distant) reference planes. It was found that PKA values for the tube voltage of 40 kV were about 2.5% lower for the distant plane than for the close one. For higher tube voltages, the difference was smaller. The difference was mainly caused by attenuation of the X ray beam in air. Since the problem with high uncertainties in PKA measurements is also caused by the current design of X ray machines, possible solutions are discussed.
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| 5. |
- Malusek, Alexandr, et al.
(författare)
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In-situ calibration of clinical built-in KAP meters with traceability to a primary standard using a reference KAP meter
- 2014
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing: Hybrid Open Access. - 0031-9155 .- 1361-6560. ; 59:23, s. 7195-7210
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Tidskriftsartikel (refereegranskat)abstract
- The air kerma-area product (KAP) is used for settings of diagnostic reference levels. The International Atomic Energy Agency (IAEA) recommends that doses in diagnostic radiology (including the KAP values) be estimated with an accuracy of at least +/- 7% (k = 2). Industry standards defined by the International Electrotechnical Commission (IEC) specify that the uncertainty of KAP meter measurements should be less than +/- 25% (k = 2). Medical physicists willing to comply with the IAEAs recommendation need to apply correction factors to KAP values reported by x-ray units. The aim of this work is to present and evaluate a calibration method for built-in KAP meters on clinical x-ray units. The method is based on (i) a tandem calibration method, which uses a reference KAP meter calibrated to measure the incident radiation, (ii) measurements using an energy-independent ionization chamber to correct for the energy dependence of the reference KAP meter, and (iii) Monte Carlo simulations of the beam quality correction factors that correct for differences between beam qualities at a standard laboratory and the clinic. The method was applied to the KAP meter in a Siemens Aristos FX plus unit. It was found that values reported by the built-in KAP meter differed from the more accurate values measured by the reference KAP meter by more than 25% for high tube voltages (more than 140 kV) and heavily filtered beams (0.3 mm Cu). Associated uncertainties were too high to claim that the IECs limit of 25% was exceeded. Nevertheless the differences were high enough to justify the need for a more accurate calibration of built-in KAP meters.
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| 6. |
- Malusek, Alexandr, et al.
(författare)
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Patient-specific kerma-area product as an exposure estimator in computed tomography: the concept and typical values
- 2011
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Ingår i: IAEA, International Symposium on Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS). 9-12 november 2010, Vienna, Austria. Book of extended synopses. IAEA-CN-182. - Vienna : IAEA. - 9789201162106 ; , s. 83-92
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Konferensbidrag (refereegranskat)abstract
- Monitoring of exposure levels in computed tomography is important from the radiation safety point of view. In this article, the concept suggested by Huda X[1]X of using the patient-specific kerma-area product as an exposure estimator is extended by providing both a rigorous definition of this quantity and a method for its evaluation. The method was demonstrated on an axial scan of the standard CT dosimetry head phantom taken with a Siemens Somatom Open CT scanner. The resulting patient-specific kerma-area product was 0.25 Gy cm2 for the x-ray tube voltage of 120 kV, tube current of 100 mA, scanning time of 1 s, and beam width at the iso-center of 1.2 cm. To implement this method, the CT scanner must be equipped with a KAP meter, and the calculation procedure must be added to the scanner's software. Alternatively, the patient-specific kerma-area product can be calculated by the CT scanner without using a KAP meter. In this case, however, the extra safety feature provided by the direct monitoring of the x-ray beam by the KAP meter is lost.
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| 7. |
- Malusek, Alexandr, et al.
(författare)
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Prostate tissue decomposition via DECT using the modelbased iterative image reconstruction algorithm DIRA
- 2014
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Ingår i: Medical Imaging 2014. - : SPIE - International Society for Optical Engineering. ; , s. Art.nr. 90333H-
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Konferensbidrag (refereegranskat)abstract
- Better knowledge of elemental composition of patient tissues may improve the accuracy of absorbed dose delivery in brachytherapy. Deficiencies of water-based protocols have been recognized and work is ongoing to implement patient-specific radiation treatment protocols. A model based iterative image reconstruction algorithm DIRA has been developed by the authors to automatically decompose patient tissues to two or three base components via dual-energy computed tomography. Performance of an updated version of DIRA was evaluated for the determination of prostate calcification. A computer simulation using an anthropomorphic phantom showed that the mass fraction of calcium in the prostate tissue was determined with accuracy better than 9%. The calculated mass fraction was little affected by the choice of the material triplet for the surrounding soft tissue. Relative differences between true and approximated values of linear attenuation coefficient and mass energy absorption coefficient for the prostate tissue were less than 6% for photon energies from 1 keV to 2 MeV. The results indicate that DIRA has the potential to improve the accuracy of dose delivery in brachytherapy despite the fact that base material triplets only approximate surrounding soft tissues.
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| 8. |
- Malusek, Alexandr, et al.
(författare)
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The potential of dual-energy computed tomography for quantitative decomposition of soft tissues to water, protein and lipid in brachytherapy
- 2013
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Ingår i: Physics in Medicine and Biology. - : Institute of Physics. - 0031-9155 .- 1361-6560. ; 58:4, s. 771-785
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Tidskriftsartikel (refereegranskat)abstract
- Dosimetric accuracy of radiation treatment planning in brachytherapy depends on knowledge of tissue composition. It has been speculated that soft tissues can be decomposed to water, lipid and protein. The aim of our work is to evaluate the accuracy of such tissue decomposition. Selected abdominal soft tissues, whose average elemental compositions were taken from literature, were decomposed using dual energy computed tomography to water, lipid and protein via the three-material decomposition method. The quality of the decomposition was assessed using relative differences between (i) mass energy absorption and (ii) mass energy attenuation coefficients of the analyzed and approximated tissues. It was found that the relative differences were less than 2% for photon energies larger than 10 keV. The differences were notably smaller than the ones for water as the transport and dose scoring medium. The choice of the water, protein and lipid triplet resulted in negative elemental mass fractions for some analyzed tissues. As negative elemental mass fractions cannot be used in general purpose particle transport computer codes using the Monte Carlo method, other triplets should be used for the decomposition. These triplets may further improve the accuracy of the approximation as the differences were mainly caused by the lack of high-Z materials in the water, protein and lipid triplet.
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| 9. |
- Mukhopadhyay, Nitai D, et al.
(författare)
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Estimating statistical uncertainty of Monte Carlo efficiency-gain in the context of a correlated sampling Monte Carlo code for brachytherapy treatment planning with non-normal dose distribution
- 2012
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Ingår i: Applied Radiation and Isotopes. - UK : Pergamon. - 0969-8043 .- 1872-9800. ; 70:1, s. 315-323
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Tidskriftsartikel (refereegranskat)abstract
- Correlated sampling Monte Carlo methods can shorten computing times in brachytherapy treatment planning. Monte Carlo efficiency is typically estimated via efficiency gain, defined as the reduction in computing time by correlated sampling relative to conventional Monte Carlo methods when equal statistical uncertainties have been achieved. The determination of the efficiency gain uncertainty arising from random effects, however, is not a straightforward task specially when the error distribution is non-normal. The purpose of this study is to evaluate the applicability of the F distribution and standardized uncertainty propagation methods (widely used in metrology to estimate uncertainty of physical measurements) for predicting confidence intervals about efficiency gain estimates derived from single Monte Carlo runs using fixed-collision correlated sampling in a simplified brachytherapy geometry. A bootstrap based algorithm was used to simulate the probability distribution of the efficiency gain estimates and the shortest 95% confidence interval was estimated from this distribution. It was found that the corresponding relative uncertainty was as large as 37% for this particular problem. The uncertainty propagation framework predicted confidence intervals reasonably well; however its main disadvantage was that uncertainties of input quantities had to be calculated in a separate run via a Monte Carlo method. The F distribution noticeably underestimated the confidence interval. These discrepancies were influenced by several photons with large statistical weights which made extremely large contributions to the scored absorbed dose difference. The mechanism of acquiring high statistical weights in the fixed-collision correlated sampling method was explained and a mitigation strategy was proposed.
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| 10. |
- Pachnerova Brabcová, Katerina, 1978, et al.
(författare)
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Uncertainties in linear energy transfer spectra measured with track-etched detectors in space
- 2013
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 713, s. 5-10
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Tidskriftsartikel (refereegranskat)abstract
- Polyallyldiglycol carbonate-based track-etched detectors can measure linear energy transfer (LET) spectra of charged particles. Accuracy of the spectra is affected by many factors whose effects are difficult to quantify. Typically, only uncertainty arising from the randomness of particle detection is reported in scientific literature. The aim of this paper is to classify the sources of uncertainties of an LET spectrum measurement and provide a simple model for the calculation of the combined uncertainty. The model was used for a spectrum measured with the track-etched detector (Harzlas TD-1) on board of the International Space Station from May-October 2009. For some spectrum bins the largest contribution to the combined uncertainty came from the uncertainty arising from the randomness of particle detection. For other bins it came from the uncertainty of the calibration curve. Contribution from the cross talk between bins was small for most of the bins as the width of the bins was relatively large compared to the intrinsic resolution of the track-etched detector. The analysis showed that sources of uncertainties other than the randomness of particle detection should not, in general, be neglected.
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