88 Part I Chapter 3 Application of 18F-FDG-labeled leukocytes (FDG-WBC) has been investigated by several groups, where it showed important limitations of inefficient labelling of leukocytes with [18F]-FDG when compared to [99mTc]-HMPAO or [111In]-oxine . The short half-life of Fluorine-18 (110 minutes) makes imaging later than 4-6 h after injection impossible, and especially in PJI this is an imperative shortcoming as leukocytes accumulate slowly towards infected sites . The role of PET/CT in suspected PJI may increase by implementing new radiotracers such as Gallium-68 and Copper-64 and also NaF, but it may take several years before these tracers have proven their benefit in prospective studies [47, 60, 65]. Therefore, until now, PET/CT does not have a central role in the management of individual patients with persistent pain after TKA . FDG-PET/CT is useful however, in suspected seeding of infectious foci in a proven infected TKA . Conclusions As demonstrated in this review, bone-SPECT/CT is a proven useful modality in addition to conventional radiographs and MRI in patients with a painful postoperative knee, amongst others in OA (e.g. after UKA), osteochondral defects, pain after ACL reconstruction or pain after corrective osteotomy. After TKA the role of bone-SPECT/CT is even more essential. It benefits from artefact-free SPECT-images and only limited artefacts on the CT part and remains an important modality, despite recently improving MARS-protocols for MRI. After the orthopaedic surgeon has first discriminatedwhether the pain originates from the knee by means of detailed history taking and clinical examination, conventional radiographs of theknee (weight bearingAP, lateral, skyline viewand long leg radiographs) are indicated to assess structural changes and this alsogives a first impressionof possible mal-alignment, albeit with limited accuracy. Inunexplainedpersistingpain, bone-SPECT/ CT is warranted, including, if possible, determination of component position using 3D volume rendering of the CT-part. By using structured uptake patterns and information from structural changes and alignment data, a discrimination between loosening of a TKA component, varus position of the tibial component or patellar overloading, synovitis and complications such as periprosthetic fracture, can bemade [7, 29]. The diagnosis of suspected infected TKA consists of a complex combination of clinical clues fromhistory taking and clinical examination, laboratory tests, microbiology cultures, radiological images, repeated clinical examinations and radionuclide imaging . Conflict of interest: The authors declare that they have no conflicts of interest with regards to this publication.