FDG-PET/MR in paediatric pyrexia of unknown origin: early detection of Takayasu arteritis-a single-centre experience.

Pyrexia of unknown origin (PUO) in children presents a significant diagnostic challenge owing to its broad and heterogeneous aetiology. Among autoimmune causes, childhood-onset Takayasu arteritis (cTAK) is rare but clinically critical, typically characterised by non-specific constitutional symptoms and diagnosed late. Conventional angiographic imaging depicts structural vascular changes and may fail to identify the disease in its early, pre-stenotic inflammatory phase. We evaluated the role of 18F-fluorodeoxyglucose positron emission tomography/magnetic resonance imaging (18F-FDG-PET/MR) in the early detection of pre-stenotic cTAK in children presenting with PUO and its impact on management and short-term outcomes. In this single-centre, retrospective descriptive case series conducted over 2 years, children (≤18 years) presenting with PUO who underwent 18F-FDG-PET/MR and were subsequently diagnosed with pre-stenotic Takayasu arteritis (paediatric European League Against Rheumatism/Paediatric Rheumatology International Trials Organisation/Paediatric Rheumatology European Society 2008 criteria) were analysed. During the study period, 28 children with PUO underwent 18F-FDG-PET/MR; 7 (25%) were diagnosed with pre-stenotic cTAK (mean age 13.9 years; 5 girls), and of the remaining 21, 20 received alternative diagnoses. All had constitutional symptoms and markedly elevated inflammatory markers (mean ESR of 104 mm/h; mean C-reactive protein of 128.4 mg/L), with infrequent overt vascular signs. 18F-FDG-PET/MR demonstrated FDG-avid wall thickening of the aorta and its major branches in every patient (maximum standardised uptake value 2.7-7.4; median 4.8), without haemodynamically significant stenosis at baseline. All children received pulsed methylprednisolone, oral prednisolone, and mycophenolate mofetil; three required tocilizumab and one adalimumab. Over a median follow-up of 20 months (range 9-36), one child developed a short-segment left subclavian stenosis, while the remainder showed no new structural progression, occlusion, or pulseless features. 18F-FDG-PET/MR detected metabolically active large-vessel inflammation in the pre-stenotic phase, enabling early diagnosis and treatment. These preliminary, hypothesis-generating findings support further investigation of 18F-FDG-PET/MR in selected children with PUO and persistent inflammation; prospective, multicentre validation with standardised paediatric protocols and longer follow-up is required before routine integration into diagnostic pathways.