| 1. |
- Alm Carlsson, Gudrun, et al.
(författare)
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Monte Carlo metoden : ett verktyg inom strålningsfysiken
- 1995
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Detta kompendium är tänkt att användas som ett propedeutiskt kursmaterial för kursdeltagare i kursen "Monte Carlo simulering av foton- och elektrontransport vid diagnostiska och radioterapeutiska strålkvaliteter". Först följer en kort repetition av den grundläggande statistik som utnyt1jas i beräkningarna. Därefter följer en beskrivning av slumptal. det fundament som metoden bygger på. Vidare beskrivs val ur olika frekvensfunktioner. Valet kan även göras ur så kallade falska fördelningar för att reducera variansen i den skattade storheten. Metoderna belyses i ett avsnitt om problemlösningsmetodik. först i allmänna termer för att sen gå in på ett specifikt problem (Buffons nålproblem) där en analys och strukturering av problemet görs varefter flödesschema och kodning exemplifieras. Så följer två moment där en beskrivning görs av färderna av fotoner respektive elektroner genom materia. För elektronfärderna gör man en indelning i klass 1- och klass II-färder. Vad detta innebär och hur deltapartiklar tas om hand beskrivs i ett kapitel. Till sist kommer en kort introduktion till de tre laborationerna med laborationshandledningar. Speciell vikt har lagts vid att initiera laboranten att fundera på fysiken i de simulerade experimenten. Detta kompendium har tillkommit som examinationsarbete vid en kurs i "Monte Carlo simulering av foton- och elektrontransport vid diagnostiska och radioterapeutiska strålkvaliteter", med andra ord den kurs du själv nu ämnar studera. Författarna önskar dig lycka till med kursen och hoppas att du kommer att få glädje av den. Speciellt hoppas vi att denna skrift ska underlätta för dig att tillgodogöra dig informationen vid föreläsningarna och under laborationerna.
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| 2. |
- Dangtip, Somsak, et al.
(författare)
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Description of the Medley Code : Monte Carlo Simulation of the Medley Setup
- 1998
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Neutron-induced charged-particle production, i.e., reactions like (n,xp), (n,xd), (n,xt), (n,x3He) and (n,xa), yields a large - and relatively poorly known - contribution to the dose delivered in fast-neutron cancer therapy. At the The Svedberg Laboratory (TSL) in Uppsala, a project is underway to measure these cross sections with a precision required for clinical use. For this purpose, an experimental facility, MEDLEY, is under commissioning. It consists of eight detector telescopes, each being a Si-Si-CsI detector combination. This general design has been selected because it provides reasonable performance over the very wide dynamic range required to detect particles ranging from 5 MeV a particles to 100 MeV protons. A general problem in this kind of experiments is to characterize the response of the detection system. The MEDLEY code has been developed for this purpose. Experimental studies of these kinds of charged-particle reactions show specific features. Some of these need to be optimized by means of, for instance, computer codes, prior to the measurement if good data are to be achieved. Basically, charged particles loose energy along their paths by interactions with the electrons of the material. Particles with low energy or with high specific energy loss are easily absorbed. Systems, which use thick charged-particle production targets to gain desirable count rate, can then detect only charged particles with enough energy to escape the target. Thus, using a thick target results in a degraded energy resolution, and particle losses. Thin targets are required to provide better resolution, but at the cost of low count rates. Registration of the entire energy of the particles reaching the detection system is also an ultimate goal. However, charged particles can interact with detection materials via nuclear reactions, which could result in other species of particles. From the detection point of view, the primary particles are lost and replaced by new types of particles, which may behave differently from their predecessors. It is well known that charged particles traveling in a medium are deflected by many small-angle scatterings. This so-called multiple scattering can be described with a statistical distribution. The fluctuations in energy loss per step, called energy-loss straggling, are modeled in the same way, i.e., assuming a statistical behavior. To get an acceptable neutron beam intensity, a rather thick neutron production target (2-8 mm) is required. This causes an energy spread of the incident neutron beam. In our case, the spread after a 4 mm thick 7Li target for neutron production is of the order of about 2 MeV. To analyze the data and determine the true double-differential cross sections, the above mentioned effects have to be taken into consideration. We have therefore developed a Monte Carlo code, MEDLEY, in FORTRAN language, to simulate the experimental setup taking all relevant physical characteristics into account. In the MEDLEY code, particles, chosen from a given distribution, are followed through the detection system. The particle distribution is obtained by applying a stripping method to the measured spectrum supplied by a user. When the result from the MEDLEY code is in good agreement with the experimental data, the true double-differential cross sections is obtained. If needed, the correction procedure can be iterated. This iteration is performed until the above condition is satisfied. This report presents the features included in the code, and some results. We compare ourresults with those from others where available.
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| 3. |
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| 4. |
- Larsson, Peter, et al.
(författare)
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Evaluation of the uncertainties in KAP-meter calibrations
- 1996
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This report was prepared in order to give more details to the uncertainty evaluation of the Kerma area product meter calibration procedure described in the paper: Larsson J P Persliden J Sandborg S and Alm Carlsson G 1996 Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration. Phys. Med. Biol. 41 2381-2398. Figures and equations referred to in this report will be found in the paperabove. For convenience, however, the equations in the paper that are used in the uncertainty analysis are retyped on the next two sides, see text in section 2.7. in the paper for further details. The numbering of the equations are kept as in the paper.
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| 5. |
- Olsson, Sara
(författare)
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Calibration of an alanine/agarose gel
- 1998
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In brachy therapy treatment, as well as in treatment with external beams, it is of crucial importance to thoroughly determine the absorbed dose in the tumour, in surrounding normal tissue and in risk organs. Several kinds of gel dosimeters have been, or are about to be, developed in order to get a three dimensional dosimeter, which would be very useful, especially in the context of brachy therapy. The need for high spatial resolution is raised by the fact that the absorbed dose decreases very fast with the distance from a brachy therapy source. The steep dose gradient also requires a dosimeter material with a wide dose range and no signal diffusion. Fe(II)/Fe(III) gel and polymer gels such as BANANA and BANG. These systems are analysed with magnetic resonance imaging (MRI) which gives a detailed picture with very high resolution (~0.5 mm) without the need to cut out samples and thereby destroy the geometry of the gel. One of the drawbacks for MRI-gels is that inhomogeneities in the magnetic field make it difficult to calibrate the gel in absolute values of absorbed dose. The Fe(II)/Fe(III) gel is the most well known of the gel dosimeters mentioned. The working principle is that it contains Fe(II)-ions that are oxidised to Fe(III)-ions when irradiated. The differences in paramagnetic properties between the ions can then be used to make an MRIimage of the dose distribution. The dose response is linear up to 40 Gy. The problem with this dosimeter type is the rapid diffusion of the Fe-ions which makes it necessary to image the gel immediately after irradiation to maintain the high resolution. In 1993 Maryanski et al. (Maryanski, 1993) reported a tissue-equivalent gel based on agarose, acrylamide and N,N´-methylene-bis-acrylamide (bis) in a de-aerated aqueous solution. The gel is called BANANA and works as a dosimeter due to the radiation induced polymerisation of the monomers acrylamide and bis. Later on, the agarose was replaced by gelatin because of its lower background signal. It is also more transparent which makes it easy to optically see the dose distribution since the polymerised gel volume changes to a white colour. This new gel is called BANG, and when further improved by substituting the acrylamide with acrylic acid it got the name BANG-2. The BANG-2 gel can measure doses down to 0.1 Gy which is much below the limit for both alanine gel and Fe(II/III) gel, but the dose response is only linear up to 6 Gy. Another drawback is the difficulties in preparing the gel. The preparation has to be made absolutely oxygen free since oxygen inhibits the polymerisation, and the gel must be stored in glass containers since most plastics are oxygen permeable. This puts great requirements on the preparation equipment, or the gel has to be bought, already cast in a predetermined shape. The glass container might also give some dosimetric effects since it has a higher atomic number than the gel itself. We have instead used a stiff agarose gel, heavily doped with alanine. The gel is heated and over saturated with alanine which recrystallizes when the gel is cooled down. When crystalline alanine is irradiated, radicals are formed which can be detected by means of electron spin resonance (ESR) spectroscopy. A signal proportional to the amount of radicals is then obtained. Since the amount of radicals is proportional to the absorbed dose, the substance may serve as a dosimeter material. The radicals in alanine are unusually stable because of the crystalline form, and in pure dry crystals the signal remains almost unchanged for several years. When the alanine is added to an agarose gel, the crystals are trapped in the gel which prevents signal diffusion. After irradiation, samples can be cut out at positions of interest. For the gel composition used in this work a sample weight of ~0.16 g is needed, which corresponds to a volume of ~0.12 cm3 (density: 1.28 g/ cm3). The shape of the sample can be chosen as convenient for the situation. The ESR analysis does not destroy the signal and thereby repeated read-outs of one sample are allowed. Alanine has a linear dose response from well below 1 Gy up to 104 Gy. The sensitivity when used in a gel allows doses down to ~3 Gy, as will be shown later on in this report. To make absolute dose measurements possible, as well as relative, the alanine/agarose gel requirescalibration. Absolute dose measurements are for example needed to verify Monte Carlo calculations experimentally. Dose planning systems used today do not take into account scattering effects at interfaces between materials of different atomic numbers, or scattering effects in a larger volume due to inserted shielding material. To verify that these simplifications do not set the outcome of the treatment at risk, and if possible to correct for the introduced errors, experimental measurements in such critical situations are needed. The aim of this report is to indicate a way of calibrating the alanine/agarose gel, and to examine the radical stability in the obtained calibration samples.
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| 6. |
- Persliden, Jan
(författare)
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Analys av filmkassation : Ett SSI-projekt
- 1996
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Reject analysis in a radiology department can play an important role in the quality assurance process. Reject analysis was performed in the Department of Radiology, University Hospital in Linköping, Sweden, during 22 weeks 1992 and 5 weeks 1994. Between the two occasions, an education and training program was carried through. The rejected films were classified acording to 6 criteria. The reject frequency was 9.9% before and 8.5% after. Faulty exposure and faulty positioning of the patient contributing with 53 % of all rejected films. It was shown that reject analysis can easily be carried through. Compared to reports from the literature, the level found here was neither high nor low. The decrease in reject frequency after the training program was low and probably not significant. Reducing the rejections results in reduced patient doses and lower costs for the films. However, too low frequencies may be an indication of accepting bad image quality and reduced diagnostic accuracy. In the future when digital equipment is more frequently introduced the rejection of films will decrease, but not necessarily the retakes. Here, measurements of mean absorbed doses to the patients may provide a better toal for quality assurance of the radiology department.
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| 7. |
- Sandborg, Michael
(författare)
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Calculation and analysis of DQE for some image detectors in mammography
- 1998
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The development and clinical testing of digital detector designs for mammography are making rapid progress and there is widespread interest in comparing the performance of these new detectors to that of analogue screen-film mammography systems. In this report, Monte Carlo calculations of the x-ray absorption characteristics (single-event distribution), the quantum absorption, Aq, and detective quantum, DQEq, efficiencies are made and compared to results from the literature. Detectors of CsI and Si of various thicknesses are compared to a state-ofthe art analogue, screen-film system (Gd2O2S) in the energy range 1-35 keV. The results show that 1.5 mm thick Si detectors will have the same DQEq as commonly used Gd2O2S fluorescent screens and that a CsI phosphor of 80 mm has similar DQEq as 1.0 mm Si. The total DQE (including added noise and inherent detector unsharpness) of fluorescent screen-film systems will be significantly reduced from this value due to the light scatter, film noise and the inherent limitations caused by the film characteristic curve. This indicates that also thinner Si detectors (0.3-0.5 mm), which do not suffer from these limitations but from a comparably low Aq may have a total performance DQE(f) comparable to that of traditional screen-film based, image detectors.
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| 8. |
- Sandborg, Michael, et al.
(författare)
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Collection and analysis of patient and image data for calibration of a voxelphantombased Monte Carlo code and for the modelling of important structures
- 1997
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The contribution of the Medical Physics Departments at Linköping University (LKP) and The Royal Marsden NHS Trust (RMH) to the joint project ‘Predictivity and Optimisation in Medical Radiation Protection’ is in modelling of the chest and lumbar spine radiographic examinations. This involves:the development of quantitative imaging requirements;an investigation of the effect of imaging technique on image quality and patientdose, andan optimisation of system design.One of the objectives for this first reporting period (0-12 months) was to collect a set of chest and lumbar spine radiographs of patients for subsequent analysis in order to establish patient doses and important features in the images. The set of radiographs and the outcome of the image feature analysis will during this project’s second year be used to calibrate our Monte Carlo computational model of the conventional chest and lumbar spine screen-film X-ray imaging systems.
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| 9. |
- Sandborg, Michael
(författare)
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Computed Tomography : Physical principles and biohazards
- 1995
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In planar projected images of the patient, important details may be hidden by over-laying tissues. By using slice-imaging techniques (tomography), selective demonstration of morphologic properties, layer by layer, may be performed. Computerised tomography, CT, is an ideal form of tomography yielding sequence images of thin consecutive slices of the patient and providing the opportunity to localise in three dimensions. Unlike conventional, classical tomography, computerised tomography does not suffer from interference from structures in the patient outside the slice being imaged. This is achieved by irradiating only thin slices of the patient with a fan-shaped beam. Transaxial images (tomograms) of the patient’s anatomy can give more selective information than conventional planar projection radiographs. Compared to planar radiography, CT images have superior contrast resolution, i.e., they are capable of distinguishing very small differences in tissue-attenuation (contrasts), but have inferior spatial resolution. An attenuation difference of 0.4% can be visualised but the smallest details in the image that can be resolved must be separated at least 0.5 mm. In conventional planar radiography, the lowest detectable contrast is larger but details of smaller size can be separated.
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| 10. |
- Sandborg, Michael, et al.
(författare)
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Implementation of unsharpness and noise into the model of the imaging system : Applications to chest and lumbar spine screen-film radiography
- 1999
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A model of the complete x-ray imaging system including the patient is a powerful tool for imaging system analysis and the optimisation of image quality and patient dose. It allows flexible variation of the system components (i.e. x-ray source, antiscatter device and image detector) and study of their effect on image quality and patient risk. Our group has developed, validated and calibrated a Monte Carlo model of the complete imaging system for chest and lumbar spine examination including voxalised human male anatomy. The Monte Carlo program calculates the contrast and signal-to-noise ratio (SNR) of various contrasting details within the voxel phantom. Important details in the images have been selected by consulting radiologist and the EU document of image quality criteria. The entrance surface dose without back-scatter and the effective dose are used as measures of patient radiation risk. The contrasts of the details are derived initially from Monte Carlo estimates of the energy imparted per unit area to the image detector beside and behind the detail. However, this ignores the effects of unsharpness in the imaging chain (such as screen-film, geometric and motion unsharpness) and the influence on contrast of the film characteristic curve. In the Monte Carlo program, SNR is calculated assuming that the noise arises from the random fluctuations in the energy imparted per unit area to the image detector only. However, other noise sources also contribute to the total noise, such as screen and film noise. Hence the model of the imaging system needs to be further developed to take these effects into account. The methods used to extend the model are described below together with illustrations of their effect on the difference in optical density, DOD, and SNR in chest and lumbar spine imaging.
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