The purpose is to compare detection of axial inflammatory lesions in patients affected by spondyloarthritis (SpA) with sodium fluoride positron emission tomography (PET) and magnetic resonance imaging (MRI), the reference technique.

It would be interesting to show the superiority of sodium fluoride PET on MRI for diagnosis of inflammatory lesions. It could be used for diagnostic care but also therapeutic care of patients with early forms of spondyloarthritis.

Secondary purposes are:

• To evaluate performances of sodium fluoride PET in detection of SpA peripheral damage (except spine and sacroiliac joints)
• To evaluate bone inflammation (axial and peripheral) with sodium fluoride PET and compare these data to clinic-biologic parameters used in clinical practice (BASDAI, BASFI, BASMI, sedimentation rate, C-reactive protein (CRP)) and to structural evaluation (sacroiliac New York, mSASSS and BASRI scoring methods)
• To study correlation of axial and peripheral bone inflammation (spine and sacroiliac joints) evaluated with sodium fluoride PET with biologic markers of inflammation and bone remodeling (MMP-3; DKK-1, IL-6, IL-17, TNF-α).

Known biologic markers of inflammation in SpA are correlated to activity obtained with MRI. It is interesting to compare to activity obtained with PET. Moreover, it has been recently showed the disassociation of inflammation and structural progression (anti-TNF treatments do not prevent structural progression) that is associated to markers of bone remodeling whose activity is influenced by an activation/inhibition system of biologic markers of inflammation.

Inclusion Criteria:

• 18-50 year-old patients affected by axial or mixed SpA (ASAS criteria) or >18 year-old patients according to New York criteria
• Patients with bone marrow edema lesions in sacroiliac joints and/or spine detected with MRI
• No therapeutic changes between hospitalization for evaluation of SpA activity and inclusion visit
• Informed consent
• Affiliation to social security plan

Exclusion Criteria:

• Refusal or impossibility of signed informed consent
• Doubt about possible pregnancy in women of childbearing potential
• Pregnancy and breastfeeding
• History of drug addiction, alcoholism, psychological problems, severe co-morbidities
• Renal insufficiency (creatinine clearance < 60 ml/min)
• Juridical protection