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.

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).