| 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. |
- Gårdestig, Magnus, 1973-, et al.
(författare)
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Estimations of effective dose in X-ray examinations derived from information stored in PACS
- 2005
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Ingår i: Radiological Protection in Transition. - Stockholm : Statens Strålskyddsinstitut. ; , s. 175-178
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Information about each X-ray examination, in a modern digitized X-ray department is generated and stored in a PACS. Appropriate conversion factors, e.g. E/DAP, can be applied to separate projections and summed to the total effective dose for each examination. The objectives of the work were (i) to investigate the accuracy and precision in the calculated effective dose (ii) to identify data for registration of (1) patient dose, (2) exposure data, and (3) patient information (iii) to make it possible to derive dose statistics on patient level for documentation of diagnostic standard doses, optimizations, constancy checks, and future epidemiological studies. The effective doses were calculated using Monte Carlo based computer programs or by using tabulations. Conversion factors were calculated for different levels of information and the individual effective dose was compared to the most precise estimation. The results suggest that the accuracy in the estimations of effective dose increases by added information about the patient (gender, size) and how the examination was performed.
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| 3. |
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| 4. |
- Helmrot, Ebba, et al.
(författare)
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Estimation of the dose to the unborn child at diagnostic X-ray examinations based on data registrerad in RIS/PACS
- 2007
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Ingår i: European Radiology. - : Springer Science and Business Media LLC. - 0938-7994 .- 1432-1084. ; 17:1, s. 205-209
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this work was to determine mean absorbed doses to the unborn child in common conventional X-ray and computed tomography (CT) examinations and to find an approach for estimating foetal dose based on data registered in the Radiological Information System/Picture Archive and Communication System (RIS/PACS). The kerma-area product (KAP) and CT dose index (CTDIvol) in common examinations were registered using a human-shaped female dosimetry phantom. Foetal doses, Df, were measured using thermoluminescent dosimeters placed inside the phantom and compared with calculated values. Measured foetal doses were given in relation to the KAP and the CTDIvol values, respectively. Conversion factor Df/KAP varies between 0.01 and 3.8 mGy/Gycm2, depending on primary beam position, foetus age and beam quality (tube voltage and filtration). Conversion factors Df/CTDIvol are in the range 0.02 – 1.2 mGy/mGy, in which the foetus is outside or within the primary beam. We conclude that dose conversion factors based on KAP or CTDIvol values automatically generated by the RIS/PACS system can be used for rapid estimations of foetal dose for common examination techniques.
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| 5. |
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| 6. |
- Helmrot, Ebba, et al.
(författare)
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Influence of scattered radiation and tube potential on radiographic contrast: comparison of two different dental X-ray films
- 1991
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Ingår i: Dento-Maxillo-Facial Radiology. - 0250-832X .- 1476-542X. ; 20:3, s. 135-146
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Tidskriftsartikel (refereegranskat)abstract
- The fundamental concept in image quality of contrast has been analysed in terms of its elements; film, radiation and object contrast, and the theoretical formula to describe their interrelationship have been evaluated. Experiments were designed to investigate the dependence of radiographic contrast on the kV, the type of generator and dental film used (D and E speed). An ivory wedge was used as the object, both alone and within a polymethyl methacrylate phantom as scattering medium. Precise definition and control of the X-ray generators were achieved by means of measurements of the primary X-ray spectra using a Compton spectrometer. D speed was found to have higher film contrast than E speed when compared at the same optical density, due to its lower base and fog and lower level of saturation in these experiments. On the other hand, E speed was found to have wider latitude. The experimental object was reproduced with the highest radiographic contrast using D-speed film and, with a given type of generator, this increased when the kV was decreased. While no difference in scatter/primary ratios was observed using the two different films, a weak dependence on kV in the range from 36 to 77 kV was found and confirmed by Monte Carlo calculations. The results indicate that the D and E speed films used had equal energy absorption properties; the difference in radiographic performance is due to their different film characteristics. The importance of controlling the physical parameters (photon energy spectrum, base and fog and optical density level) when comparing image qualities is clearly demonstrated.
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| 7. |
- Helmrot, Ebba, et al.
(författare)
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Measurement of radiation dose in dental radiology.
- 2005
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Ingår i: Radiation Protection Dosimetry. - : Oxford University Press (OUP). - 0144-8420 .- 1742-3406. ; 114, s. 168-171
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Tidskriftsartikel (refereegranskat)abstract
- Patient dose audit is an important tool for quality control and it is important to have a well-defined and easy to use method for dose measurements. In dental radiology, the most commonly used dose parameters for the setting of diagnostic reference levels (DRLs) are the entrance surface air kerma (ESAK) for intraoral examinations and dose width product (DWP) for panoramic examinations. DWP is the air kerma at the front side of the secondary collimator integrated over the collimator width and an exposure cycle. ESAK or DWP is usually measured in the absence of the patient but with the same settings of tube voltage (kV), tube current (mA) and exposure time as with the patient present. Neither of these methods is easy to use, and, in addition, DWP is not a risk related quantity. A better method of monitoring patient dose would be to use a dose area product (DAP) meter for all types of dental examinations. In this study, measurements with a DAP meter are reported for intraoral and panoramic examinations. The DWP is also measured with a pencil ionisation chamber and the product of DWP and the height that it is feasible to measure DAP using a DAP meter for both intraoral and panoramic examinations. The DAP is therefore recommended for the setting of DRLs. H (DWP H) of the secondary collimator (measured using film) was compared to DAP. The results show that it is feasible to measure DAP using a DAP meter for both intraoral and panoramic examinations. The DAP is therefore recommended for the setting of DRLs.
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| 8. |
- Helmrot, Ebba, et al.
(författare)
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METHODS FOR MONITORING PATIENT DOSEIN DENTAL RADIOLOGY
- 2010
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Ingår i: Radiation Protection Dosimetry. - Oxford : Oxford University Press. - 0144-8420 .- 1742-3406. ; 139:1-3, s. 303-305
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Tidskriftsartikel (refereegranskat)abstract
- Different types of X-ray equipment are used in dental radiology, such as intra-oral, panoramic, cephalometric, cone-beam computed tomography (CBCT) and multi-slice computed tomography (MSCT) units. Digital receptors have replaced film and screen-film systems and other technical developments have been made. The radiation doses arising from different types of examination are sparsely documented and often expressed in different radiation quantities. In order to allow the comparison of radiation doses using conventional techniques, i.e. intra-oral, panoramic and cephalometric units, with those obtained using, CBCT or MSCT techniques, the same units of dose must be used. Dose determination should be straightforward and reproducible, and data should be stored for each image and clinical examination. It is suggested here that air kerma-area product (PKA) values be used to monitor the radiation doses used in all types of dental examinations including CBCT and MSCT. However, for the CBCT and MSCT techniques, the estimation of dose must be more thoroughly investigated. The values recorded can be used to determine diagnostic standard doses and to set diagnostic reference levels for each type of clinical examination and equipment used. It should also be possible to use these values for the estimation and documentation of organ or effective doses.
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| 9. |
- 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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| 10. |
- Helmrot, Ebba
(författare)
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Systematic analysis of a radiological diagnostic system : A method for application in the effective use of x-rays in intraoral radiology
- 1996
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The effective use of an imaging system in diagnostic radiology implies an optimisation process. This demands knowledge and relevant descriptions of the various components, the X -ray units, the objects and receptors. Thus, in this work photon energy spectra have been measured, phantomsresembling human tissue constructed and receptor characteristics investigated. Essential image quality parameters (contrast, signal-to-noise ratio) and a radiation risk indicator for the patient (energyimparted) have been defined and their variation with photon energy spectrum determined for different objects and details. Results are presented as basic infonnation to be used by radiologists in collaboration with physicists in optimising examinations according to the patient and the particular diagnostic task. For intraoral radiology with Ultra-speed fihn, it is shown that to achieve equalised radiographic contrasts of an ivory wedge, kV-settings have to be decreased by 5-8 kV when single pulse X-ray generators are replaced by high-frequency constant potential ones. When imaging the ivory wedge in a PMMA phantom with Ektaspeed and Ultra-speed films at equal contrast, 7-9 kV lower kV-setting must be used with the Ektaspeed film. Ektaspeed film then gives a 35-40% decrease in the energy imparted tothe patient which can be compared to 45-55% decrease observed if the loss of contrast is not compensated for by lowering the kV-setting. Comparison of the contrasts of both films shows that Ultra-speed has higher contrast than Ektaspeed, but the latter has a wider dynamic range and higher values of base and fog optical densities, which contribute to its lower contrast at low optical densities. The radiographic contrast of details in the object is the product of object and film contrast. Objectcontrast depends on photon energy and is the same for both films. The energy imparted to the patient is calculated using conversion factors derived in this work. To simulate the large variety of anatomical structures encountered in intraoral radiology, a multimaterial compound hard tissue phantom was constructed. The constituent elements and their fractions by weight were carefully determined so as to allow computational methods to be used tocomplement experimental data. Different types of imaging system imply different optimal photon energy spectra. Strategies of optimising photon energy spectra with respect to image quality parameters and patient dose are described for both fihn and a digital system (Digora) using an imaging plate. In the digital system, the characteristics of the receptor affect image acquisition similarly to film but digitalized image formation and display may limit image quality (contrast resolution).
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