1. Hg lamp Cylindrical lens, pinhole and filter  pseudo point-source Lens  parallel beam Scanning tank with matching medium Exposed gel Unexposed gel Diffuser screen on which real shadow image forms CCD detector Standard 50mm camera lens PC with frame- grabber card Turntable controlled by acquisition computer via stepper motors Introduction Three-dimensional gel dosimetry has been demonstrated over recent years to be an important tool for verifying experimentally complex 3-D dose distributions created in radiotherapy. During much of the 1990’s, researchers focussed on MRI as the data readout technique. However, this method suffers a number of limitations, chief among which is the need for expensive equipment, not always available on a routine basis to radiotherapy treatment planning departments. In the search for a less expensive readout technique, Gore et al. [1] developed a novel technique in which radiation dose measurements are made using optical computed tomography (OCT). However, the laser scanning technique developed by Gore et al. is relatively slow (approximately 15 minutes per slice). We have developed a new type of OCT scanner, faster by two orders of magnitude, the schematic diagram for which is shown in Fig. 1. It is based on a CCD detector which obtains a 2-D projection images at every rotation step, acquiring the data for a full 3-D dose map in under 40 minutes. The total cost of the scanner illustrated was less than £5000, including the PC used for data acquisition. Technical development of a high-resolution CCD-based scanner : 1. Scanner construction Acknowledgements KKK was supported by the Socrates exchange programme of the European Union. The authors thank Mr C Bunton for technical help in construction of the scanner. Key components of the scanner system An earlier prototype of this scanner was presented at DOSGEL ‘99 [2]. Since that time, a number of key components have been investigated in detail: light source and collimation; scanning tank and turntable; detection system. The CCD-tomography approach adopted requires the creation of a parallel beam of light. Creating a beam that is both highly parallel and has a large cross-sectional area is difficult. We require (a) a high intensity point source with its output concentrated in the desired spectral lines and (b) a large diameter converging lens. Various options were examined to create the point source, including a beam-expanded laser and the output of an optical fibre, but the solution eventually chosen was to use a mercury vapour discharge tube as the light source. Its elongated shape requires the insertion of a cylindrical lens in front of the pinhole — see Fig. 2. A photograph of the actual device is shown in Fig. 3. References [1] JC Gore et al., Phys. Med. Biol. 41, 2695, 1996 [2] M A Bero et al. Proc. DOSGEL ’99 p. 136 [3] J G Wolodzko et al. Med. Phys. 26, 2508, 1999 Previous CCD-based OCT scanners have had relatively primitive turntable control. The proof of principle by Wolodzko et al. [3] used a turntable positioned by hand, whilst our own previous prototype used a stepper motor attached to the turntable by a belt drive. The current system is more sophisticated, with a rotation table that can be positioned, under computer control, with a precision of 0.05°. This is attached to the turntable through the bottom of the tank with a specialised watertight seal. The detection system consists of a projection screen (recently changed from engineering tracing film to opal white perspex to reduce its “granularity” and hence reduce artefact level in the images) and a specialist CCD detector (recently upgraded to Pulnix model 62-EX) connected to a standard 50 mm camera lens. The signal is digitised using a 10-bit framegrabber card (Matrox Pulsar) for high dynamic range measurements. Figure 2: Purpose-built optics to turn the extended mercury vapour discharge tube into a pseudo point source Conclusions We have described the construction of a high-resolution scanner for 3-D gel dosimetry. The 3-D images may be obtained on timescales that are similar to 2-D acquisitions using the laser based optical method. Our acquisition data matrix size of up to (768  536 pixels) spans a field-of-view whose minimum size is limited only by the minimum focal length of the camera lens and we thus have the potential to acquire ultra-high resolution images. S S J Doran, K K Koerkamp*, M A Bero, P Jenneson E J Morton and W B Gilboy Department of Physics, University of Surrey, Guildford, GU2 7XH, UK Department of Applied Physics, University of Twente, Enschede, The Netherlands * Short dimension of discharge tube Circular converging lens Collimating aperture Light source (elongated discharge tube) Cylindrical lens, focussing dimension Long dimension of discharge tube Filter Cylindrical lens, non-focussing dimension Cylindrical lens Pinhole Filter 7 cm Figure 3: Photograph of the optics for creating the pseudo point source Figure 3: Photograph of the OCT scanner (excluding acquisition computer and stepper motor controller) Figure 1: Schematic diagram of the new optical computed tomography scanner