Currently, x-ray rooms are designed by employing measured scatter and transmission data obtained nearly thirty years ago. Subsequently, there is a clear need to reevaluate this information as the use of modern equipment is becoming increasingly common. For example, x-ray tubes with medium frequency generators are replacing single phase units. Recent literature has re-evaluated transmission data. However, the scatter found in x-ray rooms has not been studied in detail. Two programs using Monte Carlo techniques have been developed for this purpose. The first program models the x-ray room including a slab phantom simulating the patient, a DAP meter, air and the room walls. The second program implements a realistic model of a patient in the imaging system. The results of the two programs have been validated by comparison with each other and by measurements. The results confirm that the patient is the primary source of scatter in a x-ray room. However, large variations in the scatter values are observed for different x-ray examinations and when the imaging parameters are changed. The results also show that there is a significant scatter contribution from other materials in the primary xray beam including the air, DAP meter and x-ray collimators. The first program has been used to calculate the scatter at a door positioned behind a protective screen at the radiographer's console. It predicts that no shielding in the door is required for the imaging system investigated. Therefore, a valuable computational tool has been developed which will aid the decision process in the design of clinical x-ray rooms.

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Positron Emission Tomography (PET) images reconstructed without adequate scatter corrections introduce noise and degrade image contrast. In commercial imaging systems, misalignment between computed tomography (CT) and PET images can introduce biases in the activity distribution. Recently, several reconstruction algorithms have been proposed, which made direct use of single scattered photons in the activity reconstruction. However, the realistic dataset contains single and higher order scattered photons, and current scatter reconstruction methods do not distinguish them. In this study, a novel reconstruction algorithm that is capable of processing higher order scattered photons was developed. A restricted attenuation correction method was created to avoid overcorrecting for scattered photons. The simulation outcomes have shown that the proposed methods can, under ideal energy resolution, reconstruct images that are qualitatively and quantitatively better than those obtained using existing algorithms, and that the methods show promise for use under more realistic clinical conditions. / October 2016

<p>Diss. (sammanfattning) Umeå : Umeå universitet, 1995, härtill 5 uppsatser.</p> / digitalisering@umu

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Positron emission tomography (PET) and single photon emission tomography (SPECT) are two nuclear emission-imaging modalities that rely on the detection of high-energy photons emitted from radiotracers administered to the subject. The majority of these photons are attenuated (absorbed or scattered) in the body, resulting in count losses or deviations from true detection, which in turn degrades the accuracy of images. In clinical emission tomography, sophisticated correction methods are often required employing additional x-ray CT or radionuclide transmission scans. Having proven their potential in both clinical and research areas, both PET and SPECT are being adapted for small animal imaging. However, despite the growing interest in small animal emission tomography, little scientific information exists about the accuracy of these correction methods on smaller size objects, and what level of correction is required. The purpose of this work is to determine the role of attenuation and scatter corrections as a function of object size through simulations. The simulations were performed using Interactive Data Language (IDL) and a Monte Carlo based package, Geant4 application for emission tomography (GATE). In IDL simulations, PET and SPECT data acquisition were modeled in the presence of attenuation. A mathematical emission and attenuation phantom approximating a thorax slice and slices from real PET/CT data were scaled to 5 different sizes (i.e., human, dog, rabbit, rat and mouse). The simulated emission data collected from these objects were reconstructed. The reconstructed images, with and without attenuation correction, were compared to the ideal (i.e., non-attenuated) reconstruction. Next, using GATE, scatter fraction values (the ratio of the scatter counts to the total counts) of PET and SPECT scanners were measured for various sizes of NEMA (cylindrical phantoms representing small animals and human), MOBY (realistic mouse/rat model) and XCAT (realistic human model) digital phantoms. In addition, PET projection files for different sizes of MOBY phantoms were reconstructed in 6 different conditions including attenuation and scatter corrections. Selected regions were analyzed for these different reconstruction conditions and object sizes. Finally, real mouse data from the real version of the same small animal PET scanner we modeled in our simulations were analyzed for similar reconstruction conditions. Both our IDL and GATE simulations showed that, for small animal PET and SPECT, even the smallest size objects (~2 cm diameter) showed ~15% error when both attenuation and scatter were not corrected. However, a simple attenuation correction using a uniform attenuation map and object boundary obtained from emission data significantly reduces this error (~1% for smallest size and ~6% for largest size, in non-lung regions). In addition, we did not observe any significant improvement between the uses of uniform or actual attenuation map (e.g., only ~0.5% for largest size in PET studies). The scatter correction was not significant for smaller size objects, but became increasingly important for larger sizes objects. These results suggest that for all mouse sizes and most rat sizes, uniform attenuation correction can be performed using emission data only. For smaller sizes up to ~ 4 cm, scatter correction is not required even in lung regions. For larger sizes if accurate quantization needed, additional transmission scan may be required to estimate an accurate attenuation map for both attenuation and scatter corrections.

BACKGROUND: Intraocular scatter is an important factor when considering the performance of the human eye as it can negatively affect visual performances (e.g. glare). However, and in contrast to other optical factors that also affect vision such as high order aberrations, there is currently no efficient method to measure accurately and objectively the amount and the angular distribution of forward light scatter in the eye. Various methods and instruments exist to assess forward light scatter (FLS) but the relation between these methods has rarely been quantified. In addition, FLS measurements obtained with existing instruments cannot be related to any physiological factors due to the absence of a valid model. PURPOSE: To investigate the relations between some of the main methods to measure forward light scatter, and to develop an experimental set -up for the objective measurement of forward light scatter that could be ideally related to physiological parameters. METHODS: After a short review of intraocular light scatter, the three main methods used to assess forward light scattering are compared. In this sense, the C-quant (CQ) straylight meter is compared to the van den Berg (VDB) straylight meter and the Hartmann-Shack spot pattern analysis obtained from the Hartmann-Shack aberrometer. The potential of the new Oculus Pentacam functionalities for providing information on backward light scatter (BLS) are also investigated. Finally, an innovative prototype for objective assessment of intraocular light scattering together with a scatter model of the eye is presented. RESULTS and DISCUSSION: Although no significant relationship was found between the different instruments considered (VDB straylight meter, CQ, Pentacam), our results allowed us to clarify some possible confusion introduced by previously published results and to illustrate the fact that existing commercial instruments such as aberrometers and the Pentacam cannot be used to measure FLS without at least some major modifications (hardware or software). Preliminary results with the prototype built in this study suggest that it could be used for the objective measurement of intraocular light scatter. Relating this measurement to physiological parameters stays however elusive, a fact that widens the future scope of this research.

Abstract This dissertation presents new incoherent scatter radar measurement techniques and data analysis methods. The measurements used in the study were collected by connecting a computer-based receiver to the EISCAT (European Incoherent SCATter) radar on Svalbard. This hardware consists of a spectrum analyzer, a PCI-bus-based programmable digital I/O card and a desktop computer with a large-capacity hard disk. It takes in the 70-MHz signal from the ESR (Eiscat Svalbard Radar) signal path and carries out down-conversion, AD conversion, quadrature detection, and finally stores the output samples effective sampling rate is 1 MHz, large enough to span all the frequency channels used in the experiment. Hence the total multichannel signal was stored instead of separate lagged products for each frequency channel, which is the procedure in the standard hardware. This solution has some benefits including elimination of ground clutter with only a small loss in statistical accuracy. The capability of our hardware in storing the incoherent scatter radar signals directly allows us to use very flexible and versatile signal processing methods, which include clutter suppression, filtering, decoding, lag prole calculation, inversion and optimal height integration. The performance of these incoherent scatter radar measurement techniques and data analysis methods are demonstrated by employing an incoherent scatter experiment that applies a new binary phase code. Each bit of this code has been further coded by a 5-bit Barker code. In the analysis, stochastic inversion has been used for the first time in decoding Barker-coded incoherent scatter measurements, and this method takes care of the ambiguity problems associated with the measurements. Finally, we present new binary phase codes with corresponding sidelobe-free decoding filters that maximize the signal-to-noise ratio (SNR) and at the same time eliminate unwanted sidelobes completely. / Original papers The original papers are not included in the electronic version of the dissertation. Lehtinen, M., Markkanen, J., Väänänen, A., Huuskonen, A., Damtie, B., Nygrén, T., &amp; Rahkola, J. (2002). A new incoherent scatter technique in the EISCAT Svalbard Radar. Radio Science, 37(4), 3-1-3–14. https://doi.org/10.1029/2001rs002518 Damtie, B., Nygrén, T., Lehtinen, M. S., &amp; Huuskonen, A. (2002). High resolution observations of sporadic-E layers within the polar cap ionosphere using a new incoherent scatter radar experiment. Annales Geophysicae, 20(9), 1429–1438. https://doi.org/10.5194/angeo-20-1429-2002 Damtie, B., Lehtinen, M. S., &amp; Nygrén, T. (2004). Decoding of Barker-coded incoherent scatter measurements by means of mathematical inversion. Annales Geophysicae, 22(1), 3–13. https://doi.org/10.5194/angeo-22-3-2004 Lehtinen, M. S., Damtie, B., &amp; Nygrén, T. (2004). Optimal binary phase codes and sidelobe-free decoding filters with application to incoherent scatter radar. Annales Geophysicae, 22(5), 1623–1632. https://doi.org/10.5194/angeo-22-1623-2004

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