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 concomitant treatment(s) that might jeopardise the participant's safety or that would compromise the integrity of the Trial. * Female who is pregnant (urinary hCG test will be performed in every WOCBP), breast-feeding or intends to become pregnant or is of child-bearing potential and not using an adequate, highly effective contraceptive (with a relatively high Pearl Index: natural methods, minipill outside postpartum period, spermicides or condoms in monotherapy or no usage of contraception when sexually active are not accepted). * Participation in an interventional Trial with an investigational medicinal product (IMP) or device when the trial designs are not considered compatible by the study team. * Participation in a clinical scientific study in the last 12 months with a radiation exposure caused by the experimental procedures greater than 1 mSv. * Participant has a known hypersensitivity to \[18F\]AlF-FAPI-74 or the used excipients. * Active treatment has already commenced