Conclusions Despite being able to acquire successfully high resolution images of radiation dose, image quality is limited by hardware problems. Upgrading the detector screen, the CCD and the gel container are expected to enhance significantly the final image quality. Introduction The imaging of radiation dose patterns using optical computed tomography (OCT) has previously been demonstrated successfully using a laser-based “first generation, pencil beam” scanner [1]. Approaches using 2-D charge-coupled device (CCD) detectors have the potential to acquire data several orders of magnitude quicker [2]. However, the use of this type of system presents a number of problems not encountered in the laser-based method. In this poster, we examine: the effects of the detection screen and the inhomogeneous detector response; the effect of imperfections in container walls. Simulation of the effect of sample-container interfaces in 3-D dosimetry using OCT Acknowledgements KKK was supported by the EU Socrates exchange programme. The authors thank C Bunton for technical help. References [1] Gore et al., Phys. Med. Biol. 41, 2695 (1996) [2] Doran et al. Phys. Med. Biol. 46 (in press) S Simon J DoranDepartment of Physics, University of Surrey, Guildford, GU2 7XH, UK Artifacts caused by the signal detection The output of the available CCD detector was measured with no light input. “Dark field” images were obtained using the PC-based framegrabber card and averaging different numbers of frames (1 – 400). A typical image is shown in Fig. 1(a). Ideally, one would expect it to be featureless, except for random noise, and the noise standard deviation to be inversely proportional to the square root of the number of frames averaged. However, this is not the case in practice and the dark current of the detector shows significant structure. Fig. 1(b) quantifies the effect this has on efforts to improve projection signal -to- noise ratio by averaging frames. Fig. 1(c) shows an x-profile of our light field with no sample in place. Noise in the data comes from a combination of non-uniform signal detection and granularity in the projection screen as well as the usual random processes. Figure 2: Results of a study investigating the “ring artifact”: (a) back-projection reconstruction using ideal data; (b) reconstruction using data with coherent noise added (i.e., each projection has the same noise added, based on Fig. 1(c)); (c) experimental image of a uniform phantom; (d) experimental image obtain using a technique we call wobbling, in which the sample is moved relative to the detector at each rotation. Imperfections in the container walls During this work, it proved difficult to obtain adequate containers for the gel samples, with optical properties that were well defined. A brief survey demonstrated that many vessels of the size required present considerable inhomogeneity in refractive index. Plastic containers are often made by extrusion, leading to characteristic variations in optical properties. The plastics used were also susceptible to scratching. By contrast optical quality glass containers are not easy to obtain in the size required. Surface imperfections cause spurious reflections and refractions, leading to light and dark features in the projections. As the sample is rotated, these features move and when the data are reformatted into sinograms, we see characteristic diagonal tracks, as shown in Fig. 3(a). By using a “correction scan” Fig. 3(b), in which data for an irradiated container is divided by data for the same container before irradiation, these problems can be ameliorated. However, residual image artifacts still occur when the data are reconstructed. True position of outer edge of container wall Incorrect position of inner edge of container wall Bright signal outside container Bright fringe inside container Water Gel Water This edge of image affected by tape joining two ends of film. Surface of matching liquid True position of outer edge of container wall Incorrect position of inner edge of container wall ab x / mm y / mm xixi xoxo Matching medium, n 1 Container, n 2 Gel, n 3 End wall, n 4 r1r1 r2r2 y / mm x / mm bca r 1 = 41.5 mm, r 2 = 43.0 mm r 1 = 42.9 mm, r 2 = 43.0 mm a b r 1 = 42.9 mm