Scattered photons—shortly scatter—are generated by interaction processes when photon beams interact with matter. In diagnostic radiology, they deteriorate image quality since they add an undesirable signal that lowers the contrast in projection radiography and causes cupping and streak artefacts in computed tomography (CT). Scatter is one of the most detrimental factors in cone beam CT owing to irradiation geometries using wide beams. It cannot be fully eliminated, nevertheless its amount can be lowered via scatter reduction techniques (air gaps, antiscatter grids, collimators) and its effect on medical images can be suppressed via scatter correction algorithms. Aim: Develop a tool—a virtual tomograph—that simulates projections and performs image reconstructions similarly to a real CT scanner. Use this tool to evaluate the effect of scatter on projections and reconstructed images in cone beam CT. Propose improvements in CT scanner design and image reconstruction algorithms. Methods: A software toolkit (CTmod) based on the application development framework ROOT was written to simulate primary and scatter projections using analytic and Monte Carlo methods, respectively. It was used to calculate the amount of scatter in cone beam CT for anthropomorphic voxel phantoms and water cylinders. Configurations with and without bowtie filters, antiscatter grids, and beam hardening corrections were investigated. Filtered back-projection was used to reconstruct images. Automatic threshold segmentation of volumetric CT data of anthropomorphic phantoms with known tissue compositions was tested to evaluate its usability in an iterative image reconstruction algorithm capable of performing scatter correction. Results: It was found that computer speed was the limiting factor for the deployment of this method in clinical CT scanners. It took several hours to calculate a single projection depending on the complexity of the geometry, number of simulated detector elements, and statistical precision. Data calculated using the CTmod code confirmed the already known facts that the amount of scatter is almost linearly proportional to the beam width, the scatter-to-primary ratio (SPR) can be larger than 1 for body-size objects, and bowtie filters can decrease the SPR in certain regions of projections. Ideal antiscatter grids significantly lowered the amount of scatter. The beneficial effect of classical antiscatter grids in cone beam CT with flat panel imagers was not confirmed by other researchers nevertheless new grid designs are still being tested. A simple formula estimating the effect of scatter on the quality of reconstructed images was suggested and tested. Conclusions: It was shown that computer simulations could calculate the amount of scatter in diagnostic radiology. The Monte Carlo method was too slow for a routine use in contemporary clinical practice nevertheless it could be used to optimize CT scanner design and, with some enhancements, it could become a part of an image reconstruction algorithm that performs scatter correction.

There is a need for a repeatable method of measuring polyethylene wear in total knee re-placement (TKA). Model-based radiostereometric analysis (MBRSA) is a high accuracy diagnostic tool with the potential to meet this need; however, a number of unknowns must be examined. This research examines optimized patient positioning for MBRSA imaging as well as in vitro validation of a wear measurement technique using a loaded TKA phantom to mimic patient physiology. Imaging along the antero-posterior and medio-lateral axes of the patient achieved op-timal MBRSA accuracy of better than 0.035 mm (in-plane) and 0.12° (in-plane). The de-veloped MBRSA technique underestimated TKA wear on previously-worn polyethylene bearings; however, a number of factors can be corrected to improve this discrepancy. This research provides an overall methodology for measuring TKA wear, but additional experimentation and an improved physiological phantom are needed to obtain accurate wear measurement.

Thesis--PlanB (M.S.)--University of Wisconsin--Stout, 1998. / Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1968. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

In the past few years, the collection of positron emission tomography (PET) data without inter-plane shielding has become a widely accepted technique for significantly increasing the sensitivity of multi-ring scanners. However, the resultant increase in the registration of counts due to scattered events is undesirable for quantitative studies, since it reduces contrast and confounds the linear response of the scanner to activity concentration. This thesis describes the development, implementation and evaluation of a correction for scattered photons based on the simultaneous acquisition of emission data in two energy windows. Initial experiments were performed to characterize the distribution of of scattered photons in data collected with a commercial PET brain scanner operated without inter-plane shielding (septa). In this mode of acquisition, termed 3-D, coincidences between all rings of detectors are accepted. This is in contrast to the conventional 2-D acquisition mode, where data is acquired with a more restricted range of inter-ring combinations. The fraction of scattered photons under standard operating conditions was measured as 35 (+/-2) % for a line source in a 20 cm diameter water-filled cylinder, and the scatter response function found to be shift-variant. A 20% gain in counts from events that do not scatter in the object but in the detectors themselves was achieved by lowering the energy threshold as far as reasonably possible. The correction developed for scattered photons relies on parameters relating two energy windows which were selected to maximize counting statistics and minimize spatial variations. The ratio functions for the selected windows were found to be shift-invariant, and showed little variation with object size. The parameters were however found to be very susceptible to changes in detector efficiency, showing up to 10% variations over time. The correction was implemented with constant values taken for the ratio functions, and integrated into the routine reconstruction sequence with pre-processing steps taken to minimize noise propagation. When evaluated in a range of standard and customized test objects, the correction restored contrast in inactive areas to within 5% of the true value. Relative activity concentrations in different sized phantoms were restored to better than 6%. A means of calibrating the data corrected for scattering was implemented and quantification in a range of activity distributions was accurate to within 7%. The correction method was tested in a phantom which simulates the activity distribution in a human brain. Applying the method to human data confirmed the potential of using this method routinely for quantification in vivo. A limitation of the method for dynamic scanning was identified: high count rate pile-up effects introduce global spatial and spectral distortions which are enhanced in the dual energy window correction for scattering. However, in multi-time frame scanning of test phantoms, the correction consistently restored contrast and maintained linearity. The data acquisition, correction for scattering and reconstruction regimes that have been developed in this work have, thus far, allowed the routine collection of several hundred dynamic ligand studies in patients and normal volunteers, which have been analyzed as part of clinical research projects in a fully quantitative manner.

Photon attenuation and scatter in myocardial perfusion SPECT are major sources of diagnostic uncertainty because they affect both the qualitative and quantitive characteristics of patient images. Recent studies have demonstrated improved performance in the diagnostic accuracy of Sestamibi myocardial perfusion SPECT images when using attenuation corrected images [1]. One technique for correcting for attenuation uses a short (65 cm) focal length fan beam collimator and a line Methods for correcting images for scatter using adjacent energy windows have also been proposed. Though methods for correcting for these problems exist, implementation in a clinical setting has been difficult. We have developed a clinical protocol for performing attenuation and scatter correction. The system uses a fan-beam collimator and a line source of activity on a PRISM 3000, triple head camera system for acquiring transmission data. For acquiring emission data, parallel hole collimators are placed on the two other camera heads to determine: (1) the photo-peak image, (2) the scatter contribution to the photo-peak image, and (3) a large scatter window for estimating the body outline. The body outline is used as a spatially varying gamma prior for correcting the transmission image for the effects of object truncation. Presented herein is also a method of automatically selecting a spatially varying Gibbs prior to control the noise in the iterative reconstruction. The method of Ordered Subsets-Expectation Maximization is also used to accelerate the reconstruction of the images [2]. We observed in phantom studies that correcting images for attenuation and scatter will reduce the effects of these artifacts but may emphasize other effects, i.e. detector response, patient motion, etc. Improvements in uniformity of myocardial images were observed for Sestamibi phantom studies using a breast attenuator which partially obscured the heart. However, less significant improvements were observed using 201Tl where the heart was completely obscured by the breast attenuator. This protocol demonstrated that clinical implementation of attenuation and scatter correction is possible without significant adjustment of clinical routine. However, because of the residual artifacts present in the corrected images, it is necessary to obtain simultaneously data for reconstructing images using traditional FBP protocols. in conjunction with correction protocols.

The International Commission of Radiological Protection (ICRP) advises that in principle Diagnostic Reference Levels (DRL) could be used in fluoroscopically guided interventional procedures to avoid unnecessary stochastic radiation risk. The increase in complexity of interventional procedures, combined with a lack of specialist training on radiation techniques, poses a significant risk to patients. These risks have not gone unnoticed by government authorities worldwide and in 2015 the South African Department of Health: Directorate Radiation Control issued requirements to license holders of interventional fluoroscopy units, requiring that a medical physicist optimize their radiation usage using DRLs. The Dose Area Product (DAP) quantity measured for each patient represents a dosimetry index, the value of which for the purpose of improvement should be optimized against the DRL. In this dissertation, I aim to establish if DRLs in the South African private healthcare interventional theatres are high compared to international levels and whether DRLs will optimize the doses used.

Includes bibliographical references / INTRODUCTION: The diagnosis of childhood pulmonary tuberculosis (TB) can be notoriously difficult. The chest X-ray (CXR) is a significant diagnostic resource in the detection of PTB in children. However, non-specific radiological features combined with variable inter-observer assessment s contribute to diagnostic uncertainty. The CXR would be of most value when used specifically to evaluate those features of childhood TB that it shows best and where expert observers agree, namely those signs indicating lymphadenopathy. AIM: To identify simple and reliable CXR features of primary TB in children by determining signs and anatomical sites of best observer agreement. METHOD: This is a retrospective descriptive study within a clinical trial performed by the South African TB Vaccine Initiative (SATVI). Healthy BCG-vaccinated newborn infants in a high TB prevalence rural area in Worcester, near Cape Town, South Africa, were followed for a minimum of two years for possible incident al pulmonary TB. Three independent, blinded, expert paediatric radiologists reported the resultant CXR images using a standardised data collection tick sheet, on which the specific anatomical sites and signs of pathology consistent with pulmonary TB were recorded. The first 200 original data collection tick sheets were sampled and recorded in a pre-compiled data spreadsheet for our study. The sampled data were t hen analysed using kappa statistics. RESULTS: The overall combined agreement for airway compression (by presumed lymphadenopathy) was 0.5%. Five % of the CXR's had soft tissue densities reflecting lymphadenopathy on the frontal view and 5% on the lateral view. The most common site reflecting lymphadenopathy through airway narrowing or displacement was the left main bronchus. The hilar region (kappa 0.27) on the frontal CXR and behind bronchus intermedius (kappa 0.18) on the lateral were the most common sites of soft tissue densities reflecting lymphadenopathy. There were no positive findings for cavitation or pleural effusion. The overall decisions reflecting PTB (lymphadenopathy or miliary) by each individual reader were 27.6% by Reader 1, 8.5% by Reader 2 and 24.6 % by Reader 3. Abnormal findings not specific for PTB were found in 3.5 % by Reader 1, 10.5% by Reader 2 and 3.5% by Reader 3.68. 3 % of the radiographs were reported as normal by Reader 1, 81.9% by Reader 2 and 66.8 % by Reader 3. Only 5% of the radiographs were found to be unreadable by one reader. The overall agreement of all three readers on PTB was 14.6 % and for normal CXR 49.2%. CONCLUSIONS: The fair degree of agreement amongst expert readers suggests that the CXR alone is not a reliable tool for detecting pulmonary TB and should be utilised in conjunction with the clinical features and/or skin tests and blood results. Soft tissue masses rather than airway compression are a more reliable sign for lymphadenopathy, with the most agreed upon sites on the frontal projection for soft tissue mass detection being the right hilar region, followed by the left hilum. Unfortunately, this study could not confirm the usefulness of the CXR in subcategorising PTB into severe and non-severe groups due to the absence of any positive features for severe PTB in the selected sample. The use of prescribed tick-sheets with specified features for detecting lymphadenopathy did not have the expected impact of promoting interobserver consensus of CXR findings in children in terms of detection of TB. The absence of a credible reference standard for lymphadenopathy remains a significant limitation.

Background: Diagnostic imaging forms an integral part of patient evaluation and its use has increased dramatically. Not only is medical imaging a source of increased radiation dose, but also poses other risks such as those related to the procedure performed, the contrast and drugs administered, acoustic and heat deposition and para-magnetic risks. While many studies have assessed doctors' knowledge of radiation risk, data regarding doctors' knowledge of the remaining risks of medical imaging and doctors' attitudes toward consenting practice for imaging is lacking. Aim: To survey and compare the levels of knowledge between referring clinicians and radiologists regarding the risks to patients undergoing medical imaging and to explore doctors' attitudes toward consenting practice. Method: A cross sectional, observational, descriptive study design was employed. The study was conducted using a non-validated, piloted, self-administered three-page questionnaire. The questionnaire was distributed to doctors in various stages of their medical careers at a tertiary level hospital. The questionnaire was constructed in sections including demographics, risks of medical imaging and consent practice. The maximum score potentially attainable was 79, with a point given for each correct answer. No points were given for incorrect, unsure or blank responses. Results: A total of 431 questionnaires were distributed but only 85 doctors (19 radiologists and 66 clinicians) returned a completed survey, yielding a response rate of 19,7%. Older respondents with more years of experience had greater levels of knowledge regarding the risks of medical imaging. There were no significant differences according to gender or university. Although the levels of knowledge of risk was poor overall, radiologists had greater levels of knowledge (mean knowledge score expressed as a percentage =79% compared to that of clinicians= 71%). The largest proportion of doctors' (49%) were of the opinion that clinicians should be responsible for obtaining consent for medical imaging. Only 18% of doctors (radiologists and clinicians) and 5% of clinicians admitted to feeling adequately prepared to obtain consent for medical imaging. Conclusion: We successfully surveyed and compared the levels of knowledge of medical imaging risks amongst doctors and determined their attitudes toward responsibility for consent. The levels of knowledge of the risks of medical imaging is inadequate among radiologists and poor amongst non-radiologists. While statutory body guidelines recommend that the performing health care provider obtain consent, there remains varying opinion as to who should obtain consent. The largest proportion of doctors' were of the opinion that clinicians should obtain consent for medical imaging - this despite clinicians' feelings of inadequacy when consenting patients to the risks of imaging. It is therefore important to take into consideration the levels of knowledge and comfort when making decisions as to who is best suited to obtain consent for medical imaging. With the increased dependence on medical imaging as part of the diagnostic work up, awareness of the risks of medical imaging is of tantamount importance. It is essential to review educational curricula and local policies in order to improve the levels of knowledge of risks of medical imaging amongst healthcare providers, thereby ensuring improved patient safety.

Includes bibliographical references (leaves 44-48). / The primary objective was to undertake a prospective study in order to determine whether abdominal lymphadenopathy detected at ultrasound examination could be used as an indicator of the presence of active extrapulmonary and/or disseminated TB.