The Investigators Will Evaluate the Diagnostic Performance of [18F]-AlF-FAPI-74 PET/CT in Inflammatory Disorders and Compare it With the Current Gold Standard for Inflammation, FDG PET/CT, in Three Patient Cohort: Patients Presenting With Fever of Unknown Origin, IgG4-RD and AxSpA.

Inclusion Criteria FUO: * An illness of more than 3 weeks' duration, * Temperature exceeding 38.3°C on \> 3 occasions, or elevated inflammatory markers on \> 3 occasions * Diagnosis uncertain despite appropriate first-line investigations. Inclusion criteria IgG4-RD: * High clinical suspicion of IgG4-RD by an experienced clinician of Internal Medicine, based on anamnesis, physical examination, first-line investigations and blood tests. * Patients with a new diagnosis of IgG4-RD on histopathology and need for further assessment of disease extent with PET/CT, * Relapse of diagnosed IgG4-RD on histopathology according to an experienced clinician of Internal Medicine Inclusion criteria AxSpA: * High clinical suspicion of axial spondyloarthropahy according to an experienced rheumatology (based on inflammary back pain \> 3 months, insidious onset, morning stifness, improvement with exercise and worsening in rest, pain worse at night, age at onset \<45 years of age, imaging findings). * Patient has persisting inflammatory back pain after 2 different types of NSAID's (tried for over 2-4 weeks), and is therefore eligible for biological DMARDS therapy. Exclusion Criteria: * Participant is mentally or legally incapacitated, doesn't understand the study design or is not willing or capable to undergo all study-specific procedures. * Any disorder or condition, which in the Investigator's opinion might jeopardise the participant's safety or compliance with the protocol. * Any prior or…