ASAS sacroiliitis classification system
First published in 2009 with a revised consensus in 2016, the Assessment in SpondyloArthritis International Society (ASAS) classification system utilizes imaging features of the sacroiliac joints on MR imaging to assist in characterizing the presence of sacroiliitis.
The ASAS classification is meant to supplement the existing modified New York radiographic grading system, especially in cases where the grading is equivocal.
Background
The utility of MR in sacroiliitis assessment lies in its ability to characterize active inflammation, rather than just the consequences of it, as CR and CT do, and is thus particularly useful for assessment of axial spondyloarthridites.
Classification
There is no grading system per se, however, the term "active sacroiliitis on MRI" can be used if there are features highly suggestive of inflammation. These features can be broken down into two categories:
Required MRI features
- bone marrow edema (BMO) on a T2-weighted sequence sensitive for free water (such as short tau inversion recovery (STIR) or T2FS) or bone marrow contrast enhancement on a T1-weighted sequence (such as T1FS post-Gd)
- BMO must be present on either (i) two consecutive slices, or (ii) multiple BMO lesions on a single slice
- inflammation must be clearly present and located in a typical anatomical area (subchondral bone)
- MRI appearance must be highly suggestive of a spondyloarthropathy
Not required MRI features
Other findings related to sacroiliitis may be observed on MRI but are not required to fulfill the imaging criterion of "active sacroiliitis on MRI":
- the sole presence of other inflammatory lesions such as synovitis, enthesitis or capsulitis without concomitant BMO is not sufficient for the definition of "active sacroiliitis on MRI"
- in the absence of MRI signs of BMO, the presence of structural lesions such as fat metaplasia, sclerosis, erosion or ankylosis does not meet the definition of "active sacroiliitis on MRI", but can be considered chronic lesions
