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Technegas (TcG) was produced using a commercially available TcG generator. The structure and size distribution of TcG particles were examined by means of the following methods: (a) transmission electron microscopy (TEM), range: 1 nm-100 microns; (b) time-of-flight mass spectroscopy (MS), range: < 3 nm; (c) photon correlation spectroscopy (PCS), range: 3 nm-3 microns. The TEM images showed graphite particles (size 7-23 nm) agglomerated into larger secondary aggregates. The size of these aggregates, determined from PCS and TEM measurements, showed a log-normal distribution and typically ranged from 60 to 160 nm. No isolated particles smaller than 3 nm could be detected by MS. We conclude that TcG particles consist of technetium-99m labelled agglomerated graphite particles and have an average size of 97 nm (geometrice standard deviation (GSD): 1.55).