Doctor of Philosophy / Radiographic film/screen (F/S) images have a narrow latitude or dynamic range. The film’s ability to record and view all the anatomy within the x-ray field is limited by this narrow dynamic range. The advent of digital radiographic means of storing and displaying radiographic images has improved the ability to record and visualise all of the anatomy. The problem still exists in digital radiography (DR) when radiographic examinations of certain anatomical regions are undertaken. In this work, the value of anatomically shaped radiographic contrast-enhancement masks (RCMs) in improving image contrast and reducing the dynamic range of images in DR was examined. Radiographic contrast-enhancement masks are digital masks that alter the radiographic contrast in DR images. The shape of these masks can be altered by the user. Anatomically shaped RCMs have been modelled on tissue compensation filters (TCFs) commonly used in F/S radiographic examinations. The prime purpose of a TCF is to reduce the dynamic range of photons reaching the image receptor and hence improve radiographic contrast in the resultant image. RCMs affect the dynamic range of the image rather than the energy source of the image, that of the x-ray photons. The research consisted of three distinct phases. The first phase was to examine physical TCFs and their effects on F/S radiographic images. Physical TCFs are used in radiographic F/S examinations to attenuate the x-ray beam to compensate for varying patient tissue thicknesses and/or densities. The effect of the TCF is to reduce resultant radiographic optical density variations in the image, allowing the viewer to observe a range of densities within the image which would otherwise not be visualised. Physical TCFs are commonly aluminium- or lead-based materials that attenuate the x-ray beam. A TCF has varying physical thickness to differentially attenuate the iii beam and is shaped for specific anatomical situations. During this project, various commonly used physical TCFs were examined. Measurements of size and thickness were made. Characteristics of linear attenuation coefficients and half-value thicknesses were delineated for various TCF materials and at various energies. The second phase of the research was to model the physical TCFs in a digital environment and apply the RCMs to DR images. The digital RCMs were created with similar characteristics to mimic the shapes to the physical TCFs. The RCM characteristics can be adjusted by the viewer of the image to suit the anatomy being imaged. Anatomically shaped RCMs were designed to assist in overcoming a limitation when viewing digital radiographic images, that of the dynamic range of the image. Anatomically shaped RCMs differ from other means of controlling the dynamic range of a digital radiographic image. It has been shown that RCMs can reduce the range of optical densities within images with a large dynamic range, to facilitate visualisation of all anatomy within the image. Physical TCFs are used within a specific range of radiographic F/S examinations. Digital radiographic images from this range of examinations were collected from various clinical radiological centres. Anatomically shaped RCMs were applied to the images to improve radiographic contrast of the images. The third phase of the research was to ascertain the benefits of the use of RCMs. Various other methods are currently in use to reduce the dynamic range of digital radiographic images. It is generally accepted that these methods also introduce noise into the image and hence reduce image quality. Quantitative comparisons of noise within the image were undertaken. The anatomically shaped RCMs introduced less noise than current methods designed to reduce the dynamic range of digital radiographic images. It was shown that RCM methods do not affect image quality. Radiographers make subjective assessment of digital radiographic image quality as part of their professional practice. To assess the subjective quality of images enhanced with anatomically shaped RCMs, a survey of radiographers and other iv qualified people was undertaken to ascertain any improvement in RCM-modified images compared to the original images. Participants were provided with eight pairs of image to compare. Questions were asked in the survey as to which image had the better range of optical densities; in which image the anatomy was easiest to visualise; which image had the simplest contrast and density manipulation for optimal visualisation; and which image had the overall highest image quality. Responses from 123 participants were received and analysed. The statistical analysis showed a higher preference by radiographers for the digital radiographic images in which the RCMs had been applied. Comparisons were made between anatomical regions and between patient-related factors of size, age and whether pathology was present in the image or not. The conclusion was drawn that digital RCMs correctly applied to digital radiographic images decrease the dynamic range of the image, allowing the entire anatomy to be visualised in one image. Radiographic contrast in the image can be maximised whilst maintaining image quality. Using RCMs in some digital radiographic examinations, radiographers will be able to present optimised images to referring clinicians. It is envisaged that correctly applied RCMs in certain radiographic examinations will enhance radiographic image quality and possibly lead to improved diagnosis from these images.
| Identifer | oai:union.ndltd.org:ADTP/232769 |
| Date | January 2006 |
| Creators | Davidson, Robert Andrew |
| Publisher | University of Sydney., School of Medical Radiation Sciences, Faculty of Health Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | The author retains copyright of this thesis., http://www.library.usyd.edu.au/copyright.html |
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