1. Introduction Since 1996 [1], it has been clear that optical computed tomography (CT) represents a viable alternative to MRI scanning of dosimeter gels. Both laser- and CCD-based methods allow high-resolution images to be obtained. We are pursuing the CCD route, because it allows the generation of several hundred slices in a time similar to that required for a single slice on a laser-based system. This will allow full 3-D datasets to be obtained in a realistic scan-time. PRESAGE TM is a novel dosimeter with a number of very attractive properties and the overall aim of this project was to investigate what modifications are required to the previous prototype scanner to enable successful scanning of PRESAGE TM. This poster demonstrates the following areas of our work: the first imaging of a PRESAGE TM dosimeter using a CCD-based system; the modifications needed to the scanner to improve the image quality further; the resolution capabilities of this prototype scanner. Optical CT scanning of PRESAGE TM polyurethane samples with a CCD-based readout system Acknowledgements NK is supported by a PhD studentship from the EPSRC. The authors thank Mr C Bunton for technical help in construction of the scanner. Experiments performed using PRESAGE TM PRESAGE TM samples were supplied by Heuris Pharma in the form of cylinders of diameter 7 cm. These were irradiated using 6 MV X- rays (30 Gy) from a linac in two patterns: a square field 2.7 × 2.7 cm 2 ; a grid pattern using a purpose built lead collimator The results of these scans are displayed in Fig. 1 and show considerable grounds for optimism. In particular, the in-plane resolution in the image of the grid phantom is excellent (~7 pixels across a 2 mm spot). However, there are a number of issues that must be resolved before obtaining useful images. The limiting factor for both spatial resolution and dose sensitivity is the poor contrast / artefact ratio. This occurs for three main reasons: Ring artefacts arise from the scanner’s diffuser screen [2]; Refraction artefacts arise from the matching liquid. It is necessary to match very precisely (within 0.1%) the refractive index of the dosimeter. This is done using different ratios of two components in a mixture. If the final index is slightly wrong then bands appear at the sample edges [2]. More seriously, if the mixture is not homogeneous (incomplete mixing of the two components), then irregular light and dark fringes are seen in the projections. It is also possible that some minor non-uniformities may exist in the PRESAGE TM itself. Absolute attenuation values cannot yet be measured, because the CCD camera is not yet quantitatively calibrated. The first of these effects has already been corrected by redesigning the scanner to dispense with the need for the diffuser screen and we are optimistic that the ideas we are currently pursuing will alleviate the other problems, too. References [1] JC Gore et al., Phys. Med. Biol. 41, 2695 (1996) [2] SJ Doran et al., Phys. Med. Biol. 46, 3191–3213 (2001) Figure 1: (a) Optical CT image of a 2.7 × 2.7 cm 2 square field, 30 Gy at the top surface of the dosimeter; (b) schematic arrangement of irradiation protocol for the grid phantom, with 30 Gy at the top of the lead collimator; (c) optical-CT image of the grid phantom S Figure 2: (a) New ultra-bright LED light source; (b) New optical arrangement S J Doran, N Krstajic, J Adamovics, P Jenneson Department of Physics, University of Surrey, Guildford, GU2 7XH, UK Heuris Pharma, Skillman, NJ, USA * Cross-section X-ray beam ~30 Gy at lead surface PRESAGE TM a c b Modifications to the scanner Three key design features have been changed since DOSGEL ’01: The mercury lamp has been replaced with an ultra-bright LED (nominally 1 W) — see Fig. 2a. The focusing arrangement is changed so acquisition no longer uses a diffuser screen — see Fig. 2b The tank has been redesigned so that optical glass is used rather than perspex. * LED light source Scanning tank CCD chip camera lens ab Characterisation of the scanner From a series of experiments imaging phantoms made from thin wires (similar to the “needle phantoms” of Oldham et al.), we were able to obtain a profile through a wire of known thickness (0.25 mm) and hence, by constrained deconvolution, to make a first estimate of the modulation transfer function of our system. Note that by altering the position of the camera lens, we are able to look at a very wide range of fields-of-view and so the MTF will change. The calculation below is for a field of view of ~4 cm. Position on profile / mm Image intensity ~ optical density Line-pairs / mm MTF abc Figure 3: (a) Images of cross-section of wire phantom, with enlargement inset. The streak artefacts relate to the fact that the wires are completely opaque to light, rather than partially absorbing like a gel (for which this artefact would not be present). The thinnest wire has diameter 0.25 mm. (b) Profile through thinnest wire, with (inset) an enlargement of the central region, showing excellent resolution. (c) First measurement of MTF.