Practical aspects of in situ O-16 (gamma, n) O-15 activation using a conventional medical accelerator for the purpose of perfusion imaging

We report investigations into the feasibility of generating radioactive oxy gen (O-15, a positron emitter, with half-life 2.05 min) using a tuned Elekt a SL25 accelerator, for the end purpose of imaging tumor perfusion. O-15 is produced by the "gamma, neutron," (gamma ,n) reaction between high-energy photons and normal oxygen (O-16) in the body. As most in vivo O-16 is bound in water molecules the O-15 radio-marker is produced in proportion to wate r content in tissue. Imaging the washout of the O-15 distribution using sen sitive positron-emission-tomography (PET) technology can yield spatial info rmation about blood perfusion in the tissue. The aim of this article was to determine the amount of O-15 activity that could be produced by the tuned medical accelerator. A further aim was to model the activation process usin g Monte Carlo and to investigate ways to optimize the amount of O-15 that c ould be generated. Increased activation was achieved by (i) tuning the beam to give higher-energy electrons incident on the target of the accelerator, (ii) increasing dose rate by removing the conventional filtration in the b eam and reducing the source to object distance, and (iii) reducing low-ener gy photons by means of a carbon block absorber. The activity per-unit-dose produced by the tuned beam was measured by irradiating spheres of water to known doses and placing the spheres in a calibrated coincidence-counting ap paratus. Peak energy of the tuned bremsstrahlung beam was estimated at 29 M eV, and generated activity up to 0.24 mu Ci/cc/3Gy in water. The measured a mount of O-15 agreed to within 10% of the prediction from the Monte-Carlo-c omputed spectrum, indicating reasonable ability to model the activation pro cess. The optimal thickness of the carbon absorber was found to be about 25 cm. The insertion of a carbon absorber improved spectral quality for activ ation purposes but at the cost of reduced dose rate. In conclusion, the via bility of generating O-15 with an Elekta SL25 has been demonstrated. In con junction with recent advances in high-sensitivity portable PET imaging devi ces, real potential exists for imaging in situ activated O-15 washout as a surrogate measurement of macroscopic tumor perfusion. (C) 2001 American Ass ociation of Physicists in Medicine.