1. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Optimisation of X-ray micro-tomography to perform low-dose imaging of highly- dosed gels P.M.Jenneson, E.C.Atkinson, P.Wai and S.J.Doran School of Electronics and Physical Sciences, Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, UK p.jenneson@surrey.ac.uk

2. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Density change dose mapping Radiation induced changes include, liberation of ions and induced colour changes (optical CT), polymer cross-linking effects (MRI), changes in density (MRI and X-ray CT). Density change of a polyacrylamide gel (PAG) exposed to 50 Gy is 1.021 g/cm 3 to 1.035 g/cm 3. An X-ray micro-tomography system has been designed that is optimised for low dose (~0.054 Gy) imaging of PAGs with a spatial resolution of <100  m.

3. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Proton Eye Radiotherapy 50-70 MeV protons are used to treat tumours on the rear of the eye (Clatterbridge Centre for Oncology, UK has a 62 MeV proton beam). The lateral spread and ionisation of 50 MeV protons can be modelled using SRIM 2003. Bragg peak is at 22mm and the lateral spread of ions is also show. The deposited dose is a complex three- dimensional distribution.

4. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK X-ray micro-tomography High-spatial resolution (<100um) necessary for examining 3-D dose distributions in proton therapy. The system is low in image artefact, such as refraction and diffraction which affects optical tomographic methods. Apparatus is compact, bench top size, and relatively low cost. Ease of sample preparation and analysis, using PAG or MAGIC gel dosemeters.

5. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK X-ray micro-tomography A mini-focus X-ray source (Oxford XTF5011 :- Ag target, 50kVp 0.5mA, 100um focal spot or an Oxford series 1500 :- W target, 80kVp, 0.25mA, 33um focal spot) is used in conjunction with a 100mm diameter X- ray image intensifier (Hamamatsu C7336). The sample can be rotated and moved perpendicular to the beam axis.

6. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Image quality versus imaging dose The optimum energy for the reconstruction of tomographic images can be found using the “sensitivity factor” as a guide, D = object diameter  = linear attenuation coefficient This relationship is formed by combining the equations for Nyquist angular sampling criteria and the total theoretical time for a tomographic acquisition. The most accurate images (i.e. when the “sensitivity factor” is at a minimum) are obtained between 1 to 6 Mean Free Path lengths for the X-rays in the sample and optimum at 3 MFP.

7. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Image quality versus imaging dose The mean free path radiation length (the inverse of the linear attenuation coefficient) versus energy for several thicknesses of gelatin where plotted For 25mm of gelatin the optimum source energy would be 13 keV and 27 keV.

8. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Image quality versus imaging dose By using a mini-focus X-ray tube with a silver-target operated at 50kVp and using a 50um palladium, a quasi-monoenergtic X-ray source can be generated. This reduces the imaging dose and any “beam-hardening” artefact by removing unsuitable energies.

9. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Image quality versus imaging dose The dose to the centre of a sample which is needed to form a tomographic image is, To image a 25mm diameter PAG with a 0.1% contrast a dose of 0.054 Gy to the centre of a sample is needed. This gives the ability to resolve a change in density from an unirradiated PAG (density = 1.021 g/cm 3 ) of 0.001 g/cm 3, corresponding to an irradiation dose contrast of 3.5 Gy. E x = beam energy  = detector efficiency  = plane pixel size h = slice thickness f c = Compton factor  = density of the sample SNR = signal-to-noise ratio d = diameter of reconstruction

10. Uni S School of Electronics and Physical Sciences Department of Physics University of Surrey Guildford Surrey GU2 7XH, UK Future plans Use X-ray micro-tomography to analyse PAG, MAGIC (and alternative?) polymer gel dosimeters optimised for radiation induced density changes at high doses. To study the dose response of optimised gels to X- ray, gamma and hadron beam irradiations. To map a three-dimensional dose distribution in a “human eye” gel dosimeter phantom for hadron beam radiotherapy treatments.