An extended-resolution confocal image acquisition instrument (with applications to digital radiography)

Abstract

This thesis documents the development of a new computed radiography (CR) readout technique. The current commercial storage phosphor based X-ray imaging readout technique based on raster scanned laser beam is evaluated. It is argued that a readout speed threshold has been reached due to the inherent serial readout mode. Furthermore, the resolving ability of the instrument will not support the intrinsic high spatial resolution of the next generation (glass-ceramic) storage phosphor imaging plates (IPs). Three alternative techniques for image readout of storage phosphor imaging plates are proposed based on CCD technology, with design emphasis placed on speed and resolving capabilities. One is selected for implementation. This new readout instrument features confocal optics for plate illumination and imaging using a CCD for image detection. The design permits image tiling for resolution enhancement, though with a trade-off between resolution and readout speed. For high resolutions, the instrument is translated relative to the imaging plate, capturing a grid of sub-image tiles. The tiles are stitched together offline through a column-row neighbouring tiling scheme, using the normalised correlation coefficient (NCC) technique. The column-row scheme is found to display inaccurate stitch results in the tiling process when adjacent image tiles do not share common feature edges. Gaussian decomposition blending is used to remove seam artefacts present in the bi-image about the stitch point. Blending is still found to display discernable artefacts, probably due to alignment mismatches in stimulation and imaged areas, resulting in partial image loss. Two unique techniques have been developed that enhance the accuracy of the standard normalised correlation coefficient for image recognition applications. Preliminary images comparing the resolution of commercially available IPs and a new neutron imaging plate are presented. Finally, X-ray images are presented, which demonstrate the present capabilities of the instrument.