Masters Research - Master of Medical Science / Introduction: Between 6-10% of women attending breast screening are recalled to investigate an unclear area on the mammogram. Image quality is known to affect image interpretation and it has been suggested that the number of recalls could be reduced with improved image quality. Aim: This study aimed to investigate the effect image quality has on recall rates, to assess reader consistency using the PGMI classification system and to establish factors leading to recall. Materials and Methods: A six member panel assessed 904 sets of images (698 recalled; 206 non-recalled) through a BreastScreening Program during three separate phases (pilot, main and non-recall). The pilot study was conducted without additional training in PGMI. Levels of agreement and Kappa statistics were calculated to assess intra- and inter-consistency. The percentage of and reasons for inadequate images was calculated; while a case-control study was conducted to establish factors increasing the likelihood of a client being recalled. Results: The level of agreement between panel members significantly increased from the pilot to the main study (45.5% to 57.7%) before decreasing slightly for the non-recall (57.7% to 52.2%). Overall, 3.3% of the 904 sets of images were classed as inadequate; the most common PGMI reason was exposure (31%); the left MLO was considered the most common inadequate projection (30%), with more privately produced (66%) images considered inadequate compared to public images (34%). Inadequate image quality did not hinder the cancer detection rates. The case-control component demonstrated current and previous HRT use, increased breast density, better image quality and images being taken at a public site all contributed to a client being recalled. Conclusion: The results of this study demonstrated that inadequate image quality was not a major factor leading to recall; although twice the number of recalled images were considered inadequate compared to the non-recalled images. The use of the PGMI classification system is highly subjective, with low levels of agreement amongst users. The use of HRT, breast density, imaging site and image quality all contribute to a client being recalled.

Halftones are reproductions of images created through the process of halftoning. The goal of halftones is to create a replica of an image which, at a distance, looks nearly identical to the original. Several different methods for producing these halftones are available, three of which are error diffusion, DBS and IMCDP. To check whether a halftone would be perceived as of high quality there are two options: Subjective image quality assessments (IQA’s) and objective image quality (IQ) measurements. As subjective IQA’s often take too much time and resources, objective IQ measurements are preferred. But as there is no standard for which metric should be used when working with halftones, this brings the question of which one to use. For this project both online and on-location subjective testing was performed where observers were tasked with ranking halftoned images based on perceived image quality, the images themselves being chosen specifically to show a wide range of characteristics such as brightness and level of detail. The results of these tests were compiled and then compared to that of eight different objective metrics, the list of which is the following: MSE, PSNR, S-CIELAB, SSIM, BlurMetric, BRISQUE, NIQE and PIQE. The subjective and objective results were compared using Z-scores and showed that SSIM and NIQE were the objective metrics which most closely resembled the subjective results. The online and on-location subjective tests differed greatly for dark colour halftones and colour halftones containing smooth transitions, with a smaller variation for the other categories chosen. What did not change was the clear preference for DBS by both the observers and the objective IQ metrics, making it the better of the three methods tested. / <p>Examensarbetet är utfört vid Institutionen för teknik och naturvetenskap (ITN) vid Tekniska fakulteten, Linköpings universitet</p>

In the year 2000 John Pendry described a new kind of lens that could focus both the propagating and evanescent components of light. This ‘super’ lens, which took the form of a thin slab of silver with a negative effective index of refraction under certain conditions, had the ability to reproduce images much smaller than the wavelength of light, seemingly in violation of the diffraction limit that governed the performance of conventional optics. Despite significant controversy regarding the purported operation of such superlenses, the first experimental samples were fabricated in 2005, with features as small as 63 nm successfully imaged with 365 nm light. These results put to rest disbelief in the feasibility of superlenses and ushered in an era of intense interest in near-field phenomena and negative index materials (NIMs). Despite sustained effort, progress on the practical implementation of superlenses was slow, with a further five years passing before improved experimental results were published. In the meantime, a proliferation of analytical and modelling studies appeared on the behaviour and properties of superlenses, as well as numerous suggestions for improved physical designs, very few of which had accompanying experimental evidence. The primary aim of this thesis arose from these many proposals, namely, to reconcile predictions made about the behaviour of superlenses with observed experimental results. The measurement of the theoretical and practical behaviour of superlenses is addressed in this thesis by the development of a set of characterisation metrics that can be used to describe the imaging performance of a number of near-field imaging systems. These metrics are initially calculated via transfer matrix modelling (TMM), which is a one-dimensional analytical technique traditionally used to find the transmission and reflection coefficients of planar structures. Two families of metrics are derived; one that describes imaging systems in terms of their abilities in generic situations and the other that gives the suitability of an imaging system for application to a given class of object. Transfer functions, bandwidth and peak wavenumber measurements form this first group of characterisation functions, while contrast, pseudo-contrast and correlation coefficients are used to assess the quality of imaging systems when exposed to well-defined input profiles. Both sets of metrics show that the performance of superlenses is highly application-specific, with the fidelity or otherwise of a generated image dependent more on the construction of the superlens than on the maximum spatial frequencies present in the object. The results from the characterisation metrics are also used to guide the design of hypothetical superlens structures; these suggest that sub-diffraction limited resolution may still be available with almost a full wavelength separation between object and image. The quantitative accuracy of the TMM method is assessed by comparison to full-field vector simulations performed via finite element modelling (FEM), these reveal systematic inadequacies in the application of the TMM technique to superlensing applications. These inadequacies stem from near-field mask-lens interactions that are present in superlens experiments but are not accounted for in TMM calculations. A new technique, based on a modified transfer matrix model (M-TMM), is proposed that accounts for the effects between masks and superlenses by approximating masks as solid slabs of known thickness. Results generated via M-TMM are shown to be in better agreement with FEM models than similar TMM data, even when the duty cycle of the actual mask becomes significant and the approximation in M-TMM is at its most coarse. Finally, experiments are designed and executed that directly measure the transfer functions of superlenses and other near-field imaging techniques. The problem of intimate contact between optics components, which normally hinders any such attempts to perform lithography in the near-field, is mitigated by including a flexible layer of poly (dimethylsiloxane) (PDMS) between various components in the mask:lens:resist stack. Furthermore, high spatial frequency data corresponding to low nanometre-scale features are retrieved from masks with periodic, micron-scale patterns, greatly easing the requirements on mask construction for these experiments. The end results show good agreement with FEM and M-TMM data and satisfy the aim of this thesis, which was to bridge the divide between the performance expected and experienced from silver superlenses.

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