How can inflammatory sacroiliitis be distinguished from normal physiologic growth in pediatric patients using magnetic resonance imaging (MRI) and computed tomography (CT)?

Differentiating Inflammatory Sacroiliitis from Physiological Growth in Pediatric Patients

MRI is the imaging modality of choice for distinguishing inflammatory sacroiliitis from normal physiological changes in children, but requires pediatric-specific interpretation criteria that account for normal developmental features including periarticular marrow hyperintensity, cortical irregularities, and intra-articular ossification centers that can mimic disease. 1, 2, 3

Key Imaging Protocol Requirements

MRI Protocol (Preferred Modality)

The optimal MRI protocol for pediatric sacroiliac joints must include 4, 2:

• Semicoronal sequences parallel to the dorsal cortex of S2:

  • Fluid-sensitive sequences (STIR or T2-weighted fat-suppressed) for detecting active inflammation
  • T1-weighted sequences for structural damage assessment
  • High-resolution sequence targeting the bone-cartilage interface for erosion detection

• Semiaxial fluid-sensitive sequence perpendicular to the semicoronal plane for spatial confirmation

Contrast is generally not required for initial evaluation, as gadolinium has not been shown to significantly increase diagnostic accuracy for sacroiliitis 1, 5

CT Protocol (Alternative When MRI Unavailable)

CT, particularly low-dose CT, can identify subtle erosions and structural changes but lacks sensitivity for direct inflammatory changes 1, 2. Dual-energy CT with virtual non-calcium images can depict bone marrow edema 2.

Critical Pediatric-Specific Pitfalls

Normal Developmental Features That Mimic Disease

1. Periarticular Marrow Signal Changes 6, 7, 8, 9

• Increased T2 signal in sacral marrow adjacent to sacroiliac joints is normal in skeletally immature patients
• This physiologic hyperintensity is significantly greater in adolescents compared to skeletal maturity
• Peak signal occurs at age 10 in girls and age 12 in boys
• Must be distinguished from pathologic bone marrow edema by:
  • Location: Physiologic signal is typically symmetric and follows vascular primary spongiosa distribution
  • Intensity: Should be less intense than fluid (compare to vessels/CSF)
  • Pattern: Diffuse rather than focal/patchy

2. Joint Space Fluid and Enhancement 7

• Mildly increased STIR signal in the joint space occurs in 74.7% of normal pediatric sacroiliac joints
• Thin rim-like enhancement on post-contrast T1 images is nearly universal in children (related to cartilage)
• Pathologic findings require:
  • Signal intensity as bright as fluid (score 2-3)
  • Focal, thick, or intense enhancement (not thin symmetric rim)

3. Cortical Irregularities and Flaring 10, 8, 9

• Subchondral irregularities and flaring are common normal developmental findings
• Joint facet defects >3 mm occur in 17% of normal children/adolescents
• Ossified nuclei antero-lateral at S1 or intra-articular occur in 77% of individuals ≥13 years
• These should not be mistaken for erosions

4. Sacral Fusion Patterns 10

• S1/S2 fusion: Starts at age 6, complete after age 13 (girls) or 14 (boys)
• S2/S3 fusion: Starts at age 9, can remain incomplete until age 18
• Transitional intersegmental fusion interfaces can simulate disease

Diagnostic Criteria for True Inflammatory Sacroiliitis

Active Inflammatory Lesions on MRI 4, 11, 3

Bone Marrow Edema (BME)/Osteitis:

• Ill-defined low signal on T1, high signal on STIR/T2FS
• Located in subchondral bone marrow adjacent to sacroiliac joints
• Must be clearly pathologic and in typical anatomical region
• Use sacral interforaminal bone marrow as internal reference for normal signal

Additional Active Lesions to Report:

• Capsulitis: Hyperintense signal with diffuse thickening of anterior superior capsule
• Enthesitis: Increased signal in bone marrow and/or adjacent soft tissues at ligament insertions
• Synovitis: Enhancement and thickening of synovial lining

Structural Lesions 4, 11

Must be reported semiquantitatively (mild/moderate/severe) with location specified:

• Erosions: Loss of cortical definition with adjacent marrow signal abnormality
• Fat lesions: High T1 signal in subchondral bone (indicates prior inflammation)
• Sclerosis, ankylosis, bone buds

Critical distinction: In children, normal developmental ossification centers are well-corticated, whereas true erosions show cortical disruption 10, 8

Algorithmic Approach to Interpretation

Step 1: Assess Patient Age and Skeletal Maturity

• Girls: Consider skeletally mature at ≥15 years
• Boys: Consider skeletally mature at ≥17 years
• Younger patients have more prominent physiologic marrow signal

Step 2: Evaluate Marrow Signal Pattern

If increased T2 signal present:

• Compare intensity to fluid (vessels/CSF)
• Assess distribution: Symmetric/diffuse vs. focal/patchy
• Check location: Periarticular vascular zones vs. subchondral bone
• Physiologic if: Less intense than fluid, symmetric, follows vascular distribution
• Pathologic if: As bright as fluid, focal, subchondral location

Step 3: Assess Joint Space

If joint fluid/enhancement present:

• Thin symmetric rim = likely physiologic
• Focal, thick, or intense = concerning for inflammation
• Correlate with bone marrow findings

Step 4: Evaluate Structural Features

For any cortical irregularity:

• Well-corticated with smooth margins = likely developmental ossification center
• Cortical disruption with adjacent marrow edema = erosion
• Check for bilateral symmetry (developmental features often bilateral)

Step 5: Integration with Clinical Context 12

Red flags for true sacroiliitis:

• Chronic back pain (especially inflammatory pattern)
• HLA-B27 positivity
• Clinical features of juvenile spondyloarthritis
• Unilateral findings (consider infection)

Common Pitfalls to Avoid

1. Applying adult MRI criteria directly to children - This leads to false positives 3, 8

2. Interpreting physiologic periarticular marrow hyperintensity as BME - Always compare to skeletal maturity and use internal references 6

3. Mistaking developmental ossification centers for erosions - Look for cortication and smooth margins 10, 8

4. Over-relying on joint space fluid - Present in 74.7% of normal children 7

5. Ignoring NSAID use - NSAIDs can suppress inflammatory findings; consider 1-2 week washout if clinically feasible 13

Reporting Recommendations

The radiology report must explicitly state 11:

• Presence or absence of BME, erosions, and fat lesions with semiquantitative assessment
• Location of findings (specify if restricted to mechanical load zones)
• Whether findings are bilateral or unilateral
• Presence of developmental features that may simulate disease
• Recommendation for rheumatology referral if inflammatory findings present

For follow-up examinations: Document changes in active and structural lesions to assess treatment response 11