| 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. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- Daniel, Maxime, et al.
(författare)
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Charge collection efficiency of CdTe detectors : Impact of charge collection time and polarisation
- 2023
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Ingår i: Medical Imaging 2023. - : SPIE-Intl Soc Optical Eng.
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Konferensbidrag (refereegranskat)abstract
- Cadmium telluride (CdTe) is one of the materials used in photon-counting detectors for x-ray computed tomography. One challenge with this material is that it is susceptible to polarisation due to holes being trapped in impurities in the material. This can potentially lead to the buildup of bulk charge in the semiconductor, causing decreased charge collection efficiency and degraded energy resolution. In this work, we develop a simulation model of CdTe detectors with polarisation and use it to study the effect of polarisation on the measured energy spectrum for different charge collection times. To this end, we use a theoretical model of charge buildup to find the critical charge in the detector's bulk above which the detector can be considered completely polarised. We then simulate a 320-by-270-by-1600 μm CdTe detector used in CT clinical imaging, for varying degrees of polarisation (ratio between the actual charge and the critical charge) and charge collection time. Our results show that the measured spectrum gets heavily distorted for large degrees of polarisation or for short charge collection time. We also put these results in context by discussing how they relate to the critical fluence rate and the time of flight of the charge carriers. These results can lead to improved simulation models of CdTe detectors and a better understanding the factors affecting their imaging performance.
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| 8. |
- Danielsson, Mats, Professor, et al.
(författare)
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Photon-counting x-ray detectors for CT
- 2021
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 66:3, s. 03TR01-
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Forskningsöversikt (refereegranskat)abstract
- The introduction of photon-counting detectors is expected to be the next major breakthrough in clinical x-ray CT. During the last decade, there has been considerable research activity in the field of photon-counting CT, in terms of both hardware development and theoretical understanding of the factors affecting image quality. In this article, we review the recent progress in this field with the intent of highlighting the relationship between detector design considerations and the resulting image quality. We discuss detector design choices such as converter material, pixel size, and readout electronics design, and then elucidate their impact on detector performance in terms of dose efficiency, spatial resolution, and energy resolution. Furthermore, we give an overview of data processing, reconstruction methods and metrics of imaging performance; outline clinical applications; and discuss potential future developments.
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| 9. |
- Danielsson, Mats, Professor, et al.
(författare)
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Razor blade re-coating
- 2023
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Patent (populärvet., debatt m.m.)
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| 10. |
- Jin, Zihui, et al.
(författare)
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First experimental evaluation of count-rate performance for micrometre resolution deep silicon detector
- 2024
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 69:3
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Tidskriftsartikel (refereegranskat)abstract
- Objective. An ultra-fine-pitch deep silicon detector has been developed for clinical photon-counting computed tomography (CT). With a small pixel size of 14 × 650 μm2, it has shown potential to reach micrometre spatial resolution in previous simulation studies. A detector prototype with such geometry has been manufactured, and we report on the first experimental evaluation of its count-rate performance. Approach. The measurement was carried out at MAX IV synchrotron laboratory with 35 keV monochromatic x-rays. By inserting tungsten attenuators of 50, 75, 100, 150, 200, 225, 325 μ m-thicknesses into the beam, the response of the detector to fluence rates from 3.3 × 107 to 1.3 × 1011 mm−2 s−1 was characterized. Main results. The measurement result showed that the detector exhibited count rate linearity up to 6.66 × 108 mm−2 s−1 with 13% count loss and was still functional at count rate up to 2.9 × 1010 mm−2 s−1. A semi-nonparalyzable dead-time model was fitted to the count-rate behaviour of the detector, showing great agreement with the measured data, with an estimated nonparalyzable dead time of 2.9 ns. Significance. This is the first experimental evaluation of the count-rate performance for a deep silicon detector with such small pixel geometry. The results suggest that this type of detector shows the potential to be used at fluence rates encountered in clinical CT with little count loss due to pile-up.
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