Radiation Physics and Chemistry 76 (2007) 351–356 Performance of triple coincidence imaging as an addition to dedicated PET Elfatih Abuelhia a,à , K. Kacperski b , S. Kafala a , N. M Spyrou a a Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, UK b Institute of Nuclear Medicine, University College London, Middlesex Hospital, London W1T 3AA, UK Abstract In this work, we investigate the performance of semiconductor detectors imaging capabilities in three-photon annihilation processes in order to combine this novel imaging modality with conventional positron emission tomography. The spatial resolution is studied as a function of detector positions and selected energy window. This was measured from different experimental arrangements and found to be in the range between 3.3–3.9 cm with a mean of 3.570.1 cm. Scatter and random events, coincidence timing resolution and count rate performance are discussed. r 2006 Elsevier Ltd. All rights reserved. 1. Introduction Positron emission tomography (PET) is a functional imaging technique to measure the concentration of radioactivity in body organs to study biological processes. Radioisotopes can label metabolically active compounds and can be used for imaging a number of metabolic processes. The quantification of functional imaging with PET, especially for small structures requires improvements in instrumentations to enhance imaging performance (Zaidi and Sossi, 2004). The challenge for advanced PET instrumentation is the optimization of the performance in terms of spatial resolution, contrast and sensitivity, at minimized fabrication and operation cost of the PET scanner (Phelps and Cherry, 1998). In gamma ray imaging applications, direct detection of g-rays by semiconductor materials offers several advantages over conventional scintillation based detectors. HPGe detectors have good energy resolution. Their use for a dedicated small animal PET system have been investigated (Philips et al., 2002). Semiconductors like CdZTe that can be operated at room temperature could improve PET imaging technol- ogy (Moses et al., 1994). 2. Theory and model Recently the usefulness of three-photon annihilation events has been demonstrated (Kacperski et al., 2004). The information gained per event was significantly higher than that in case of the two-photon annihilation. Three-photon annihilation in matter depends on the rates of positronium formation, which in turn depends on the local physical and chemical environment, particularly the existence of oxygen. Three-photons imaging could provide valuable information on the state of oxygenation in tumours (presence of hypoxia). By considering a three-photon decay event that occurs at a point r ¼ðx; y; zÞ (Fig. 1), the three annihilation photons can have energies between 0 and 511 keV, fulfilling the laws of energy and momentum conserva- tion. If the three photons of energies E 1 , E 2 , E 3 are detected in coincidence by three detectors, then from ARTICLE IN PRESS www.elsevier.com/locate/radphyschem 0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.03.066 à Corresponding author. Fax: +44 1483686781. E-mail address: e.abuelhia@survey.ac.uk (E. Abuelhia).