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Title: Spatial frequency analysis of the perceptual features involved in pulmonary nodule detection and recognition from posterior-anterior chest radiographs
Author: Pietrzyk, Mariusz W.
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2009
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RATIONALE AND OBJECTIVES: Radiological error due to the incorrect interpretation of medical images still occurs in current practice, and continues to be reported both in laboratory and clinical experimental conditions. In general radiological practice error rates range from 3 - 5%. However, that scale reaches up to 30% for detection of some early pulmonary cancers. Computer-Aided Detection (CAD) algorithms have been proposed to support human observers in verifying their choices. Although CAD systems might help in certain situations, its general implementation in clinical practice is still controversial. Perceptual studies involving psychophysical approaches to the error problem may give some insight into the gap between advances in image processing and the nature of radiological expertise. Moreover, some neuroscientific evidence underlines the importance of processing spatial frequency properties of visual stimuli that is carried out by the Human Visual System (HVS). This has provided the inspiration for Spatial Frequency Analysis of certain Regions of Interest (ROI) selected by human observers in medical image interpretation in a number of studies. Such studies have been conducted in mammography focusing on the relationship between the physical properties, the type of radiological outcomes and the dwelling time. The spatial frequency features in mammograms have very specific features however, and this leads to the question of whether the results for mammography could be generalised to other medical images. RESEARCH AIMS: This study aims to investigate the perceptual criteria used in decision-making processes in pulmonary lung nodule detection from Posterior-Anterior (PA) Chest Radiographs (CxR). Moreover, the development in radiological expertise has been taken into account by comparing the results obtained from subjects with different levels of experience in the field. MATERIALS AND METHODOLOGY: Ten participant observers were selected from each of the following groups:  radiologists and reporting radiographers (experts),  radiography students (two levels of novices) and those  without any relevant experience (naive). 11 ~ •• ------------~=== Subjects participated in the eye tracking experiment during lung nodule detection from a set of PA radiographs with a 50% prevalence of pathology. Twenty radiological cases were included in the data bank., where ten contained one to five nodules. The assessment of performance for each individual was calculated based on Jack-knife Alternative Free-response Receiver Operating Characteristic Figure of Merit (JAFROC FOM). Eye tracking data was used to divide images into areas of foveal visual attention distribution from the most dwelled to totally ignored Regions of Interest (ROI). These selected sites were analysed in terms of spatial frequency properties using 2D Stationary Wavelet Packets Transform (SWPT) frames by Dubieties functions up to three levels of decomposition. The logarithm of energy carried by each wavelet coefficient represents the amount of visual information coded by the spatial frequency range ω1 = f(ωx/ωy) in a particular orientation θi = g(tag(ωx/ωy)) and is called Spatial Frequency Band (SFBj). A reduction procedure was applied to eliminate redundancy in information coding by a set of SFB. Thus, 84 bands obtained from the third level of decomposition were reduced to twenty nine bands. The degree of dissimilarity in spatial frequency domains between selected regions was explored by statistical analysis on wavelet representations at the sites of subjects' responses. The locality of selected sites was limited by the foveal Field of View (FOV). The dissimilarities between wavelet representations were measured according to the number of non-redundant SFBj within which significant differences (p<0.05) were found according to an analysis of variance (ANOV A) with post hoc test. The statistical analysis embraced subject-related factors (expertise level, JAFROC FOM, dwell time) and image-based features (nodule detectability, conspicuity, localization and spatial frequency description). These factors were considered as independent variables in visual attention distribution and decision-sites studies. RESULTS: The correctness of the second or higher order responses were highly correlated with the category of first decision-outcomes made on the case. That correlation shows the probability of accurate end-point decisions related to the first decision. Experts are more accurate in dedicating visual attention to the more relevant areas containing pulmonary nodules. Significant differences were found in the spatial frequency domain between nodule-containing regions which have been fixated and those which were left without focal attention. The JAFROC FOM calculation based on overall performance characterizes the more experienced subjects as being more accurate in decision-making and less variable in FOM value within a group. Moreover, the high accuracy of subject performance was correlated with the allocation of visual attention in normal regions which are more similar to the nodule-containing sites,.in terms of the spatial frequency features. The experts' ability in distinguishing the most attractive True- Negative (TN) from True-Positive (TP) avoiding False-Positive (FP) was proven with differences at the spatial frequency level. High correlation between the correctness of a first overt decision made on certain cases and the .quality performance was found significant (r=D. 75), The category of the first response effects the perceptual criteria applied to form the final decision outcome. CONCLUSIONS: The main contribution to knowledge of this work is that for the first time the SF A was conducted on a radiological task other than mammography. The work lends significant weight to the argument that spatial frequency channels coded through a wavelet paradigm are a characterising feature of visual perception and that this is phenomenon is generalisable to areas of radiology other than breast imaging, where mammographs are quite unique in terms of image-based features compare to pictures obtained from other medical irnaging modalities. Also, this work contributes an extension to previous studies on non-expert groups through investigation into trends in development of radiological experience. There is some agreement with the conclusions presented by others who suggest that experts may use specific neural connections - a set of spatial frequency channels tuned to specific object detection - during visual searching in a radiological task.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available