| 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. |
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| 3. |
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| 4. |
- Dance, David, et al.
(författare)
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Optimisation of the design of antiscatter grids by computer modelling
- 1995
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Ingår i: Radiation Protection Dosimetry. - 0144-8420 .- 1742-3406. ; 57:1, s. 207-210
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Tidskriftsartikel (refereegranskat)abstract
- A Monte Carlo computer program has been developed to model diagnostic radiological examinations, and has been used to study and optimise the design of antiscatter grids. This is important because the use of an inappropriate or poorly designed grid can lead to increased patient dose. Optimal grid parameters may be different for large and small scattering volumes. The program treats the patient as a rectangular block of tissue and takes account of the grid and image receptor. Image quality is measured in terms of contrast and signal-to-noise ratio and patient risk in terms of mean absorbed dose. Test objects of appropriate size and composition are used in the calculation of these image quality parameters. A new performance comparison and optimisation procedure has been developed, and the program has been used to study grid design in screen-film and digital radiology for small, medium and large scattering volumes.
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| 5. |
- Dance, David, et al.
(författare)
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The use of carbon fibre material in radiographic cassettes: estimation of the dose and contrast advantages
- 1997
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Ingår i: British Journal of Radiology. - 0007-1285 .- 1748-880X. ; 70, s. 383-390
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Tidskriftsartikel (refereegranskat)abstract
- A Monte Carlo simulation has been used to estimate the dose and contrast advantages of replacing radiographic cassette fronts fabricated from aluminium with cassette fronts fabricated from low atomic number material (carbon fibre). The simulation used a realistic imaging geometry and calculations were made both with and without an anti-scatter grid. Account was taken of the scatter generated in the cassette front and the effect of beam hardening on primary contrast. Dose and contrast were evaluated for a range of cassette front thicknesses and tube potentials (60-150 kV) as well as for four examinations representative of situations with varying amounts of scatter. The results with an anti-scatter grid show a clear dose and contrast advantage in all cases when an aluminium cassette front is replaced with a low attenuation cassette front. The contrast advantage is dependent upon the examination and is generally greater for imaging bony structures than for imaging soft tissue. If a 1.74 mm aluminium cassette front is compared with a 1.1 mm carbon fibre cassette front, then the dose advantages are 16%, 9%, 8% and 6% and the contrast advantages are 10%, 7%, 4% and 5% for the AP paediatric pelvis examination at 60 kV, the anteroposterior (AP) lumbar spine examination at 80 kV, the lateral lumbar spine examination at 100 kV and the posteroanterior (PA) chest examination at 150 kV, respectively. The results without an anti-scatter grid show an increased dose advantage when a low attenuation cassette front is used, but the contrast advantage is small and in some situations negative.
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| 6. |
- Hammersberg, P., et al.
(författare)
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A theoretical model for determination of the optimal irradiation conditions for computerised tomography
- 1995
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Ingår i: Insight. - 1354-2575 .- 1754-4904. ; 37:12, s. 978-985
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Tidskriftsartikel (refereegranskat)abstract
- Image quality in Computerised Tomography (CT) depends strongly on the quality of the CT-projection data. These depend on sample composition and geometry, contrasting details within the sample and the equipment used, i.e. X-ray spectra, filtration, detector response and geometry. This paper focuses on the problem of selecting the optimal physical parameters to maximise the signal-to-noise in CT projection data (SNRCT) between a contrasting detail and the surrounding sample for CT-scanners equipped with poly-energetic X-ray sources (conventional X-ray tubes) and energy-integrating detector systems (image intensifier and optical video chain). The work includes the derivation and verification ofa theoretical model for SNRCT which can be used for predicting the optimal physical parameters for specific imaging tasks. It is shown that simplified calculations valid for mono-energetic X-ray sources and/or photon counting detectors do not correctly predict the optimal settings. This study also includes measurements of the actual X-ray source and photon transport Monte Carlo simulations of the response of the detector system.
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| 7. |
- Helmrot, Ebba, et al.
(författare)
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Scientific instrument for a controlled choice of optimal photon energy in intra-oral radiography
- 1998
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Ingår i: Radiation Protection Dosimetry. - 0144-8420 .- 1742-3406. ; 80:1, s. 321-325
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Tidskriftsartikel (refereegranskat)abstract
- Basic performance parameters are defined and analysed in order to optimise physical image quality in relation to the energy imparted to the patient in dental radiology. Air cavities were embedded in well-defined multimaterial, hard tissue phantoms to represent various objects in dento-maxillo-facial examinations. Basic performance parameters were: object contrast (C), energy imparted (_) to the patient, signal-to-noise ration (SNR), C2/_ (film) and (SNR)2/_ (digital imaging system) as functions of HVL (half-value layer), used to describe the photon energy spectrum. For the film receptor, the performance index C2/_ is maximum (optimal) at HVL values of 1.5-1.7 mm Al in the simulated Incisive, Premolar and Molar examinations. Other imaging tasks (examinations), not simulated here, may require other optimal HVL. For the digital imaging system (Digora) the performance index (SNR)2/_, theoretically calculated, indicates that a lower value of HVL is optimal than with film as receptor. However, due to the limited number of bits (8 bits) in the analogue to digital converter (ADC) contrast resolution is degraded and calls for use of higher photon energies (HVL). Customised optimisations with proper concern for patient category, type of examination, diagnostic task is the ultimate goal of this work. The conclusions stated above give some general advice on the appropriate choice of photon energy spectrum (HVL). In particular situations, it may be necessary to use more dose demanding kV settings (lower HVL) in order to get sufficient image quality for the diagnostic task.
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| 8. |
- Larsson, Peter, et al.
(författare)
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Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration
- 1996
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 41:11, s. 2381-2398
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Tidskriftsartikel (refereegranskat)abstract
- Kerma - area product meters (KAP meters) are frequently used in diagnostic radiology to measure the integral of air-collision kerma over an area perpendicular to the x-ray beam. In this work, a precise method for calibrating a KAP meter to measure is described and calibration factors determined for a broad range of tube potentials (40 - 200 kV). The integral is determined using a large number of TL dosimeters spread over and outside the nominal field area defined as the area within 50% of maximum . The method is compared to a simplified calibration method which approximates the integral by multiplying the kerma in the centre of the field by the nominal field area . While the calibration factor using the precise method is independent of field area and distance from the source, that using the simplified method depends on both. This can be accounted for by field inhomogeneities caused by the heel effect, extrafocal radiation and scattered radiation from the KAP meter. The deviations between the calibration factors were as large as for collimator apertures of and distances from the source of 50 - 160 cm. The uncertainty in the calibration factor using the precise method was carefully evaluated and the expanded relative uncertainty estimated to be with a confidence level of 95%.
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| 9. |
- Mångård, Måns, et al.
(författare)
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Modelling of an X-ray image-intensifier-based radiography system
- 1998
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Ingår i: Journal of X-Ray Science and Technology. - 0895-3996 .- 1095-9114. ; 8:1, s. 31-50
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Tidskriftsartikel (refereegranskat)abstract
- A model will be proposed for predicting the expected value and variance of the measured signal-level in collected radiographic images obtained with an image-intensifier-based X-ray radiography system. The model parameters are determined from both theoretical and experimental data and incorporate all parameters that can be varied by the system operator, except CCD-camera readout rate. The proposed model predicts the expected value and variance of the grey-level in the output image with high accuracy. It is also shown that it is very important to compensate for the inhomogeneous pixel sensitivity when comparing the variance of the signal-level in a pixel from sequentially collected images with the variance determined in a single image.
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| 10. |
- 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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