In tissue engineering, the continuous monitoring of cell and tissue cultures in vitro is crucial to assess their functional status over time. However, these constructs can be large, thick and non-transparent. Medical imaging techniques can allow real-time in situ monitoring of cell and tissue cultures in thick solid scaffolds. Here, human endothelial cells were embedded in fibrin gels that were continuously perfused by a culture medium. Positron emission tomography (PET) imaging was used to assess cell viability non-destructively over periods extending up to a few weeks. PET imaging protocols were adapted and validated to measure culture perfusion and cell metabolism using [F-18]-fluorodeoxyglucose ((18)FDG). Cell densities down to 100,000 cells/mL were detectable after 12 h of culture and cell structures were localized within the fibrin gels after 1-2 weeks of culture. PET is a promising tool to investigate a wide range of cellular properties and reveal information on tissue development. (C) 2013 Elsevier Ltd. All rights reserved.