Radiography:

• Conventional radiographic diagnosis insensitive (= delay in diagnosis by 8–11 years)

Benefit: helpful if positive

Disadvantage: low intra- and interobserver agreement

• uniform narrowing of joint space
• erosions
• intraarticular ankylosis
• osteophytes
• pneumocysts
• articular “vacuum” sign

Grading:

• 0 normal
• 1 incipient sclerosis + ↓ focal joint width
• 2 minimally abnormal with loss of definition of articular margins, subchondral osteoporosis, areas of reactive sclerosis
• 3 unequivocally abnormal with subchondral sclerosis of sacral + iliac articular margins, erosions, reduced width, widening of joint space, incipient ankylosis
• 4 complete ankylosis + residual sclerosis

CT: (more sensitive than conventional radiography)

Grading:

• IA SI joint space >4 mm
• IB SI joint space <2 mm
• IIA joint contour irregularities
• IIB erosions on iliac >sacral side
• IIIA significant subchondral sclerosis
• IIIB spur formation
• IVA transarticular bone bridge
• IVB total ankylosis

MR: (cardinal tool + biomarker for disease activity)

• bone marrow edema (first to appear)
• subchondral sclerosis = low SI bands extending >5 mm from joint space (DDx: physiologic sclerosis)
• erosions = T1-hypointense + T2-hyperintense bone defect at margins of cartilaginous compartment
• periarticular fat deposits (= previous inflammation)
• ankylosis (= fusion of bone buds)

CEMR:

• vasoactive bone marrow edema
• synovitis = enhancement of synovium
• capsulitis = enhancing anterior + posterior joint capsule
• enthesitis = enhancing ligamentous attachments

DDx:

1. Osteoarthritis (anterior osteophytes)
2. Septic sacroiliitis (joint effusion, bone marrow edema, intramuscular abscess)
3. Sacral insufficiency fracture
4. Osteitis condensans ilii (middle-aged woman, mainly iliac bone sclerosis, normal joint space)
5. Bone tumor