| 1. |
- Grönberg, Fredrik
(författare)
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Spectral Photon-Counting Computed Tomography with Silicon Detectors: New Models and Applications
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- X-ray computed tomography (CT) is a widely used imaging modality that enables visualization of nearly every part of the human body. It is used for diagnosis of disease and injury as well as medical treatment planning. The vast majority of CT scanners in clinical use today have energy-integrating x-ray detectors, which measure the total incident energy in a given measurement.Spectral photon-counting detectors operate by counting individual photons and measuring their energy, and are expected to yield the next major advance in CT, with improvements in spatial resolution, dose efficiency, material differentiation and quantitative imaging capabilities compared to the current state-of-the-art.In this Thesis, a set of new models and applications for a spectral photon-counting silicon detector developed for CT is investigated. The first part of the Thesis is dedicated to the modeling of spectral photon-counting silicon detectors. A new statistical model for the effects of pulse pileup is presented. Also, the effects on image quality from intra-detector Compton scatter in silicon detectors are investigated via spatio-energetic modeling. In the second part of the Thesis, potential applications for spectral photon-counting detectors are investigated. An experimental study of ex vivo CT imaging of an excised human heart with calcified plaque is presented. It demonstrates the feasibility of unconstrained projection-based three-material decomposition with iodine as a third basis material and explores the potential improvements in spatial resolution and material differentiation that can be achieved with a spectral photon-counting silicon detector compared to a conventional dual-energy CTsystem. Two other applications are investigated with simulations: a method for reconstructing CT images from spectral photon-counting CT data that accurately mimic conventional CT images; and a method for estimating iron concentration in mixtures of liver and adipose tissue when using three basis functions instead of only two to describe the linear attenuation coefficient of tissues in the human body. Although the methods presented in this Thesis have been specifically developed for a spectral photon-counting silicon detector, they are also applicable for other types of photon-counting detectors.
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| 2. |
- Ritoša, Andrea
(författare)
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Measurement of child engagement in early childhood education and care
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Children's engagement is a widely studied concept in the field of education, early interventions, and disability research. High engagement among children is consistently associated with desired academic, social, and emotional outcomes. However, the engagement of young children in Early Childhood Education and Care (ECEC) settings has received less systematic attention. The aim of this thesis is to investigate the measurement of child engagement in ECEC and related conceptualizations of the construct. The thesis includes two empirical studies and two literature reviews on child engagement in ECEC.The first empirical study validates the Engagement Versus Disaffection with Learning: Teacher Report questionnaire in a Swedish preschool class. The relationship between a questionnaire of school engagement and questionnaire of child engagement was also investigated. Second study is a scoping literature review exploring how child engagement is conceptualized and operationalized in ECEC settings. For the third study, a subset of the identified studies using two or more measures of child engagement was included in an in-depth review exploring how multimethod measurement of child engagement is implemented in ECEC settings and what are the associations between different measures of child engagement. Lastly, a profile analysis of observed momentary engagement and global engagement was performed among a sample of preschool children in Sweden to investigate typical and atypical engagement profiles.Findings show that observations are the dominant method for measuring young children’s engagement in ECEC, while teacher questionnaires are mostly used for assessing academic engagement in kindergarten classes in US. Self-reports where young children can report about their own engagement are extremely rare. Child engagement can be rated as low and high in value, as a category that either is or is not present, or as a variable that can be qualitatively described and coded on mutually exclusive categories, even within a same study.Although we discovered a strong correlation between child engagement and school engagement in the Swedish preschool class, suggesting that these constructs are highly similar, literature review indicates that the conceptualization and measurement of school engagement and engagement of young children in ECEC differ in several aspects. Child engagement is dominantly associated with behaviors and seen as contextual, whereas school engagement includes internal aspects and can be seen as a stable tendency or even a trait of the child. Results from empirical studies and the in-depth literature review show that teacher questionnaires of child engagement, even if they nominally assess different aspects of engagement, tend to correlate higher than questionnaires and observations of child engagement. This indicates that questionnaires and observations of child engagement tap into qualitatively different aspects of what is considered engagement. Low global engagement in children is rare and probably more indicative of problems in functioning than low observed engagement. On the other hand, high observed engagement can indicate child’s potential for high engagement within a certain context, partly independent of child’s global engagement. Observations of child engagement are more sensitive to changes induced by interventions, whereas teacher-rated global engagement serves as a stronger predictor of future outcome.
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| 3. |
- Almberg, Wah-Sui
(författare)
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Modelling Epidemic Influenza : A Novel Approach
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modellering av dynamiska förlopp i socio-biologiska komplexa system är långt ifrån trivialt. Behovet av fördjupad förståelse av sådana förlopp är många gånger stort. Vi har presenterat ett oortodoxt förslag på hur spridning av influensa i mänsklig population kan beskrivas och simuleras. Traditionellt använder man sig i huvudsak av två typer av modeller. Den ena bygger på matematiska strukturer där dynamiken beskrivs av kontinuerliga differentialförlopp mellan ett fåtal grovt uppdelade sektioner av populationen. Den andra bygger på diskreta förlopp inom visualiserade kontaktnätverk där varje enskild individ i populationen är representerad. Båda dessa metoder har styrkor såväl som svagheter. Bland svagheterna kan man framförallt lyfta fram svårigheterna med att förklara de flesta av influensans observerade epidemiologiska karaktärsdrag. Ett flertal idéer har framlagts för att förklara dessa anomalier. Bland dessa finns en hypotes som trots sin enkelhet skulle kunna förklara en majoritet av de epidemiologiska karaktärsdragen. Problemet med denna hypotes är dock att man inte kunnat hitta empiriska belägg för den. Vi tror att man kan nå närmare en lösning genom att omtolka signifikansen hos denna hypotes och den epidemiologiska karaktäristikan inom ramen för en annorlunda naturvetenskaplig ontologi. Denna omtolkning visar på en informationsbaserad såväl som fysiskt baserad verklighet. Vi menar då inte det traditionella informationsbegreppet som Claude Shannons teoretiserande bygger på, utan David Bohms `aktiva information', där informationen utgör en objektiv aktiv del av naturen, något som existerar oberoende av ett vetande medvetande eller en observatör. Bohm har gett den aktiva informationen en formell struktur i hans teori om det han kallar för `the implicate order'. Tillsammans med Basil Hiley visade han även att kvantmekaniken kan tolkas på ett sådant sätt att aktiv information utgör en fundamental aspekt av teorin, och att den därigenom även förenklas. Baserat på observationer som gjorts och våra resonemang har vi pekat på att en förståelse av epidemiologiska förlopp kanske kan nås genom att använda ett teoretiskt ramverk grundat på aktiv information. I en planerad kommande doktorsavhandling avser vi att presentera en matematisk modell som ytterligare kan underbygga vår hypotes.
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| 4. |
- Araújo, Marco, 1982-
(författare)
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Infrastructure deployment in unprofitable areas : A techno-economic model for multiple criteria decision analysis under the European Union telecommunications regulatory framework
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the European Union significant asymmetries prevail among member states in the most diverse areas. The telecom sector is no exception to this, with some members being much more advanced than others. Within the member states asymmetries also occur. And if we extrapolate this to a broader audience, for example the OECD, the European Union has been far away from some other members in what concerns asymmetries within own countries, with Japan and South Korea being the most evident. In 2016 the European Commission proposed a new regulatory framework for the telecom sector which was adopted in late 2018 by the European Parliament and must be transposed to national laws by member-states by late 2020. As we move towards the digital revolution, the so-called industry 4.0, the new EU regulatory framework has the aim of removing most of these asymmetries, by guaranteeing that most citizens will have access to a very fast Internet connection regardless of where they live. This regulatory framework provides some guidance and goals and funding objectives, but for operators and governments this is just not sufficient per se. Against this background, the overall research issue of this thesis is a methodological approach to how to determine the best access technology from a multi-criteria and multi-stakeholder perspective. In particular, to provide coverage in a certain region who lacks service a priori (therefore unprofitable by nature), problematics to be addressed include: i) what are exactly the new regulatory framework guidelines; ii) which strategic operational model to use; iii) what is the most appropriate technology to achieve these objectives; iv) how to handle the pricing incognita and all the financial component; v) how risky it is to make these objectives a reality.
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| 5. |
- Berggren, Karl, 1989-
(författare)
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Spectral image quality and applications in breast tomosynthesis
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the 1970s, it was determined that screening mammography is an efficient tool in fighting the increasing number of women dying from breast cancer, and many countries have established screening programs since then. Mammography systems have improved substantially over the years with one of the major advances being the transition from x-ray film to digital x-ray detectors. Following this development, the number of women dying from breast cancer has decreased, but there is still much room for improvement. One technology that is changing the breast imaging landscape is breast tomosynthesis; tomographic imaging with in-plane resolution similar to that of mammography, albeit limited height resolution. Breast tomosynthesis is commonly implemented with flat-panel detectors, but line detectors in a slit-scanning geometry can also be used. The latter configuration allows for more complex detector technologies, such as spectral photon-counting detectors that enable single-shot spectral imaging. The combination of spectral imaging and tomosynthesis opens up for a range of new applications, but the slit scanning geometry, which differs substantially from that of flat-panel tomosynthesis systems, and the factors affecting image quality have not been well understood. This thesis aims at filling this gap. Image quality and the parameters that influence image quality in spectral photon-counting slit-scanning breast tomosynthesis are characterized and analyzed using cascaded-systems modelling and linear image quality metrics. In addition, the thesis goes into characterizing the x-ray properties of breast tissue, an important input parameter for accurate material decomposition of in-vivo tissue. Material decomposition with spectral imaging opens up a range of applications, such as accurate measurement of volumetric breast density and spectral lesion characterization for decision support as part of mammography screening, and contrast-enhanced K-edge imaging for diagnostics. Tomosynthesis combined with material decomposition has the potential to improve these methods further by, for instance, separating lesions or regions of interest from surrounding fibro-glandular tissue in quantitative 3D maps of breast tissue.
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| 6. |
- Brunskog, Rickard, et al.
(författare)
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Experimental Evaluation of a Micron-Resolution CT Detector
- 2024
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Ingår i: Medical Imaging 2024: Physics of Medical Imaging. - : SPIE-Intl Soc Optical Eng.
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Konferensbidrag (refereegranskat)abstract
- Purpose: Current photon-counting detectors are limited to a pixel size of 0.3 mm-1 mm, as decreasing the pixel size further generally introduces degraded dose efficiency and energy resolution from excessive charge sharing. In this work, we present experimental measurements of the first photon-counting detector prototype designed to leverage the charge sharing to estimate the photon interaction position, where simulations indicate a theoretical resolution of around 1 µm using a similar geometry. The goal of the measurements is to validate our Monte-Carlo simulation for further development. Approach: DAC sweeps are performed with an X-ray beam at specified locations on the sensor front, with the beam at 20 keV and 35 keV, as well as with different sensor biases with the beam at 35 keV. The experimental data are then compared to a Monte Carlo simulation combined with a charge transport model. In this first prototype wire bonds are used, and as such only a few channels are connected. Results: The experimental data agree generally well with the simulated data with the beam close to the electrodes, with the simulated data diverging from the experiments with the beam further away from the electrodes. The induced charge cloud signal exhibits a fairly linear dependency on the beam position, indicating that any estimation techniques will yield more precise position when the photon interacts further away from the electrodes, rather than closer. Conclusions: With the experimental data and the simulations agreeing generally well, together with the same software previously indicating a resolution of around 1 µm, we expect an ultra-high-resolution detector to be in reach, and are encouraged to continue development.
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| 7. |
- Brunskog, Rickard, et al.
(författare)
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First experimental evaluation of a high-resolution deep silicon photon-counting sensor
- 2024
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Ingår i: Journal of Medical Imaging. - : SPIE-Intl Soc Optical Eng. - 2329-4302 .- 2329-4310. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Current photon-counting computed tomography detectors are limited to a pixel size of around 0.3 to 0.5 mm due to excessive charge sharing degrading the dose efficiency and energy resolution as the pixels become smaller. In this work, we present measurements of a prototype photon-counting detector that leverages the charge sharing to reach a theoretical sub-pixel resolution in the order of 1 μm. The goal of the study is to validate our Monte-Carlo simulation using measurements, enabling further development. Approach: We measure the channel response at the MAX IV Lab, in the DanMAX beamline, with a 35 keV photon beam, and compare the measurements with a 2D Monte Carlo simulation combined with a charge transport model. Only a few channels on the prototype are connected to keep the number of wire bonds low. Results: The measurements agree generally well with the simulations with the beam close to the electrodes but diverge as the beam is moved further away. The induced charge cloud signals also seem to increase linearly as the beam is moved away from the electrodes. Conclusions: The agreement between measurements and simulations indicates that the Monte-Carlo simulation can accurately model the channel response of the detector with the photon interactions close to the electrodes, which indicates that the unconnected electrodes introduce unwanted effects that need to be further explored. With the same Monte-Carlo simulation previously indicating a resolution of around 1 μm with similar geometry, the results are promising that an ultra-high resolution detector is not far in the future.
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| 8. |
- Bujila, Robert, 1983-
(författare)
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Methods and utilities to assist in the optimization of image quality and radiation dose in X-ray Computed Tomography
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- X-ray Computed Tomography (CT) is a highly utilized tool in diagnos- tic radiology. CT provides radiologists with unobstructed views of patient anatomy. However, image quality in CT is highly dependent on the imaging task, scan technique, beam quality and radiation dose. The size of the patient must be considered and the radiation dose must be subsequently adapted to achieve consistent image quality. As CT is a modality that utilizes radia- tion to generate images, the purpose of this thesis was to explore tools and methodologies that can be utilized to optimize image quality with respect to radiation dose.In Paper I, fundamental image quality metrics were modeled whereby the effect of radiation dose/beam quality on these metrics could be estimated. The applications of these models were demonstrated by estimating the de- tectability of low contrast details across different radiation doses/beam qual- ities using a mathematical model observer.Dose indices are reported by CT scanners to indicate the level of radiation that has been utilized during a scan. As the level of radiation dose is a central aspect in optimization work, methods were deployed to verify the accuracy of CT dose indices for wide beam CT scanners in Paper II.CT scanners employ so called Automatic Tube Current Modulation (ATCM) to adapt the level of radiation dose to obtain consistent image quality across varying sized patients. In Paper III, a comprehensive study was executed to better understand the complex nature of ATCM as it relates to different vendor implementations as ATCM plays a central role in the optimization of CT scans.As the beam quality used during an examination has a central role for both dosimetry and image quality, a toolkit was developed that can be used to model x-ray spectra emerging from an x-ray tube in Paper IV. The toolkit underwent a rigorous validation and has since been made available online.In conclusion, this thesis has resulted in a number of methods and utilities that can help unfold the complex relationship between image quality and radiation dose on a practical level in CT.
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| 9. |
- da Silva, Joakim, et al.
(författare)
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Resolution characterization of a silicon-based, photon-counting computed tomography prototype capable of patient scanning
- 2019
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Ingår i: Journal of Medical Imaging. - USA : SPIE - International Society for Optical Engineering. - 2329-4302 .- 2329-4310. ; 6:4
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Tidskriftsartikel (refereegranskat)abstract
- Photon-counting detectors are expected to bring a range of improvements to patient imaging with x-ray computed tomography (CT). One is higher spatial resolution. We demonstrate the resolution obtained using a commercial CT scanner where the original energy-integrating detector has been replaced by a single-slice, silicon-based, photon-counting detector. This prototype constitutes the first full-field-of-view silicon-based CT scanner capable of patient scanning. First, the pixel response function and focal spot profile are measured and, combining the two, the system modulation transfer function is calculated. Second, the prototype is used to scan a resolution phantom and a skull phantom. The resolution images are compared to images from a state-of-the-art CT scanner. The comparison shows that for the prototype 19 lp∕cm are detectable with the same clarity as 14 lp∕cm on the reference scanner at equal dose and reconstruction grid, with more line pairs visible with increasing dose and decreasing image pixel size. The high spatial resolution remains evident in the anatomy of the skull phantom and is comparable to that of other photon-counting CT prototypes present in the literature. We conclude that the deep silicon-based detector used in our study could provide improved spatial resolution in patient imaging without increasing the x-ray dose.
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| 10. |
- Dahlman, Nils, et al.
(författare)
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Evaluation of photon-counting spectral breast tomosynthesis
- 2011
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Ingår i: Medical Imaging 2011. - : SPIE.
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Konferensbidrag (refereegranskat)abstract
- We have designed a mammography system that for the first time combines photon-counting spectral imaging with tomosynthesis. The present study is a comprehensive physical evaluation of the system; tomosynthesis, spectral imaging, and the combination of both are compared using an ideal-observer model that takes anatomical noise into account. Predictions of signal and noise transfer through the system are verified by contrast measurements on a tissue phantom and 3D measurements of MTF and NPS. Clinical images acquired with the system are discussed in view of the model predictions.
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