The aim of the study to evaluate the performance of new [18F]-AlF-FAPI-74 PET/CT in three inflammatory disorders (fever of unknown origin, IgG4-related disease and axial spondyloarthritis) and compare with the current stand-of-care [18F]-FDG PET/CT

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