Spondyloarthropathies (SpAs) are chronic inflammatory diseases encompassing ankylosing spondylitis, psoriatic arthritis, reactive arthritis, enteropathic arthropathy, and undifferentiated SpA.

In 2001, the estimated prevalence of SpA was 1.5% worldwide. Sacroiliitis is a condition caused by inflammation within the sacroiliac joint. It is the most frequent damage of SpA depicted at imaging evaluation.

Conventional radiography (X-ray) is usually used to depict the structural changes associated with sacroiliitis.

However further evaluation often requires additionnal computed tomography (CT).

Tomosynthesis is an Xray-based imaging technology which allows reconstruction of multiple section images from a set of projection images acquired as the x-ray tube moves along a prescribed path. The advantagee of tomosynthesis is the significant reduction of radiation dose exposure compared to CT Tomosynthesis is currently used in the field of breast imaging and pneumology. Very few studies have examined the value of tomosynthesis for osteoarticular imaging.

The study aims at evaluating the diagnostic performances of tomosynthesis as compared to standard X-ray and CT, in patients with a clinical suspicion of sacroiliitis.

the investigators hypothesize that tomosynthesis is superior to conventional radiography for detection of sacroiliitis and is at least equal to CT with lower irradiation.

Consecutive patients referred to CT for exploration of sacrollitis will be included and investigated with standard radiography, CT and tomosynthesis of SI joints.

Imaging examinations will be performed at day of enrollment as follow : X-ray and CT as standard management and tomosynthesis as additional evaluation. CT will be considered as the reference standard Sacroiliitis will be assessed in a centralized, blind and randomized retrospective reading of all imaging modalities by two independent radiologists.

Diagnostic performances and radiation doses of tomosynthesis will be compared to those of standard CT and X-ray examinations.

Inclusion Criteria:

• Patients aged 18 and over referred for suspected sacroiliitis.
• Patients who have entered into their participation agreement by signing the informed consent form.
• Patient affiliated to a social security scheme or beneficiary.

Exclusion Criteria:

• Pregnant or lactating women.
• Vulnerable people according.
• Major people placed under guardianship or curatorship.