ORIGINAL ARTICLE Effect of the positron range of 18 F, 68 Ga and 124 I on PET/CT in lung-equivalent materials Gerrit J. Kemerink & Mariëlle G. W. Visser & Renee Franssen & Emiel Beijer & Mariangela Zamburlini & Servé G. E. A. Halders & Boudewijn Brans & Felix M. Mottaghy & Gerrit J. J. Teule Received: 8 September 2010 / Accepted: 4 January 2011 / Published online: 2 February 2011 # Springer-Verlag 2011 Abstract Purpose The aim of this study was to investigate the effect of positron range on visualization and quantification in 18 F, 68 Ga and 124 I positron emission tomography (PET)/CT of lung-like tissue. Methods Different sources were measured in air, in lung- equivalent foams and in water, using a clinical PET/CT and a microPET system. Intensity profiles and curves with the cumulative number of annihilations were derived and numerically characterized. Results 68 Ga and 124 I gave similar results. Their intensity profiles in lung-like foam had a peak similar to that for 18 F, and tails of very low intensity, but extending over distances of centimetres and containing a large fraction of all annihilations. For 90% recovery, volumes of interest with diameters up to 50 mm were required, and recovery within the 10% intensity isocontour was as low as 30%. In contrast, tailing was minor for 18 F. Conclusion Lung lesions containing 18 F, 68 Ga or 124 I will be visualized similarly, and at least as sharp as in soft tissue. Nevertheless, for quantification of 68 Ga and 124 I large volumes of interest are needed for complete activity recovery. For clinical studies containing noise and background, new quantification approaches may have to be developed. Keywords High-energy positrons . 68 Ga . 124 I . Lung PET . Quantification Introduction Many studies addressed potential consequences of the finite positron range in positron emission tomography (PET) imaging [1–8], like loss of spatial resolution and incomplete recovery of activity, some including techniques to correct for it [9–11]. For 124 I the additional effects of cascading gamma radiation and spurious coincidences were investi- gated [4–8]. The distribution of annihilations around a positron- emitting point source is cusp-shaped with tails of relatively low intensity [1–3]. On clinical scanners, and in an environment of water or soft tissue, such a distribution was shown to lead to differences in spatial resolution of about 1 mm between 18 F and the high-energy positron emitter 124 I[4–8]. The situation is worse in an environment of low density, as was shown by Sánchez-Crespo et al. [2] who performed Monte Carlo simulations for point sources in various tissues. In the lung they found substantial broadening of the entire distribution, in particular for the high-energy positron emitters. In principle this could lead to severe blurring of lesions and the need for large volumes of interest to recover all activity. These considerations led to the following clinically relevant questions: (1) How are aqueous sources of different size containing 18 F, 68 Ga and 124 I visualized in an environment of low density as found in the lung? and (2) How is quantification affected by the finite positron range? Since these questions have not been addressed to our knowledge, we will try to answer them in this study. For the former question we will focus on the perceived width of the Electronic supplementary material The online version of this article (doi:10.1007/s00259-011-1732-1) contains supplementary material, which is available to authorized users. G. J. Kemerink (*) : M. G. W. Visser : R. Franssen : E. Beijer : M. Zamburlini : S. G. E. A. Halders : B. Brans : F. M. Mottaghy : G. J. J. Teule Department of Nuclear Medicine, Maastricht University Medical Center, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands e-mail: gerrit.kemerink@mumc.nl Eur J Nucl Med Mol Imaging (2011) 38:940–948 DOI 10.1007/s00259-011-1732-1