1. Introduction Three-dimensional gel dosimetry is a novel method for radiation measurement motivated by the need to verify experimentally the doses produced by stereotactic and conformal techniques in radiotherapy and by other applications of radiation dosimetry such as radiobiology and food sterilisation. Until recently, Magnetic Resonance Imaging (MRI) was the only technique to enable “readout” of a spatially-varying radiation dose distribution in a non-destructive and non-invasive fashion, with both high resolution and the ability to make time-course measurements [1]. However, this dosimetric method suffers from the same limitations that affect MRI generally, namely low signal-to-noise ratio, an often lengthy imaging time and the need for highly sophisticated and expensive equipment. In the search for a readout technique that overcame the above drawbacks Gore et al. [2] developed a novel technique in which radiation dose measurements can be achieved using an optical tomographic densitometer. The sample used to detect the radiation and “fix” its pattern in space was a poly(acrylamide) gel as previously introduced for MRI studies [3]. Here we report on a more powerful approach to optical tomography, made possible by the use of a novel phantom material. Results from the prototype scanner Figure 3(a) shows a selected 2-D slice from the 3-D image data-set obtained from an FXG gel phantom exposed to a cone-beam of X-rays. The silver X-ray tube target was operated at 50kVp, 0.6mA and the exposure time was about 30 minutes. 128 projections, each consisting of a plane of 128  128 pixels, were acquired in just 20 minutes, as the sample rotated through 180 . Projections were reconstructed onto a 128 3 matrix using a Shepp-Logan filtered back- projection routine, leading to a 3-D image with isotropic resolution of 0.3 mm. This system is already competitive with a corresponding MRI scan. However, there are many improvements that can be made to the prototype scanner. For example, the source of the prominent ring artifacts is known, but the correction method (already implemented on our X-ray microtomography system) requires a more sophisticated motor system, which we are currently designing. Faster optical tomography with parallel-beam “white” light for 3-D dosimetry Acknowledgements The authors are indebted to Dr. E. J. Morton for his support of the project and to BBSRC for the grant under which much of the tomography control software and reconstruction were written. Advantages of the new gel system The goal of this work was to construct an optical absorption tomography scanner using a polychromatic light source and to use it for 3D-radiation dosimetry employing a new colour- change radiation-sensitive gel system, Ferrous Xylenol Gelatin (FXG). In addition to its good sensitivity, this gel is transparent. This is a major advantage over the poly(acrylamide) system, since the lack of scatter allows us to operate in a parallel beam configuration, with consequent increase in imaging speed. Secondly, the absorption spectrum (shown below in Fig. 1) has maxima centred at 430nm, and 530nm [4] and the relative sensitivity at each of these locations is different. This means that FXG may be used to perform multi-spectral optical imaging, with the potential to obtain data over a much wider range of doses than would be possible at a single wavelength. This will be key to brachytherapy dosimetry, where current gel dosimetry methods fail. References [1] M.J. Day, Phys. Med. Biol. 35, 1605, 1990 [2] J.C. Gore et al., Phys. Med. Biol. 41, 2695, 1996 [3] M.J. Maryanski et al., Magn. Res. Im. 11, 253, 1993 [4] M.A. Bero et al., Nucl. Inst. Meth. A422, 617,1999 The optical tomography scanner The optical CT scanner illustrated in Figure 2, uses an incoherent light source and an interference filter that can be selected to cover the range of wavelengths at which the scanner is operated. The key novel feature of our scanning system is to employ a parallel beam of light, obtained using a suitable lens. This beam is directed onto the gel sample, which is surrounded by a matching solution of suitable refractive index, and a shadow projection of the irradiated gel sample, consisting of a complete plane, is taken on a ground glass screen. The light intensity is then recorded on a CCD-camera. The sample motion is controlled by a stepper-motor and data acquisition and processing are performed using in-house software. Figure 3: (a) Representative 2-D optical absorbance map, extracted from the complete 128-slice 3-D data-set for an irradiated cylindrical FXG phantom; (b) absorbance profile across the sample that showing the characteristic build-up and exponential decay regions. The data are currently contaminated with a serious ring artifact, but this can be corrected by improved acquisition. Figure 2: (a) Schematic diagram of the optical tomography scanner. Our current scanner is designed for small-scale imaging, but we are currently modifying the design to enable us to image head-sized phantoms to make the system truly competitive with MRI dosimetry. (b) The CCD camera element at the heart of the scanner. Conclusions We have shown that it is possible to obtain 3D-dose distribution with a rapidity unmatched by either MRI or previous optical methods. We use a novel parallel-beam optical tomography system that is convenient, easy to construct and uses low cost and widely available components. “White” light offers the possibility of making multi-spectral images, that have potential applications in extending the range of doses that may be measured. S M. A. Bero, P.M. Jenneson, W. B. Gilboy, P.M. Glover and S.J. Doran Department of Physics, University of Surrey, Guildford, GU2 5XH, UK Figure 1: (a) FXG absorbance spectra, for doses up to ~12Gy; (b) dose response relations for four different wavelengths. (a)(b) (a)(b) 3 cm