Table 2

MRI grading system for acute and chronic sacroiliac inflammation

1 Activity index
Grade x:enhancement <30°= no sacroiliitis
Grade A:enhancement 30°–70°= moderate sacroiliitis
Grade B:enhancement >70°= severe sacroiliitis
In some early cases there might be no sacroiliitis detectable in the joint space but enhancement can be visible in the joint capsula or in the periarticular bone marrow. In these cases the degree of enhancement should be taken as for sacroiliitis itself.
2 Chronicity index
Grad 0:no chronic changes
Grade I:circumscribed subchondral sclerosis and/or ⩽2 erosions
Grade II:moderate subchondral sclerosis and/or >2 erosions with normal joint space width
Grad III:severe subchondral sclerosis and/or pseudodilatation of the joint and/or a minor degree of partial ankylosis (<25% of the joint)
Grad VI:definite ankylosis
  • The pathological changes are counted for each section separately. They only add up if clearly different regions are involved. Sacroiliitis can be diagnosed if at least an activity index A or B and/or a chronicity index > II is detected.