Selection of X-ray versus magnetic resonance imaging as a first-line imaging modality for diagnosing axial spondyloarthritis.

The best cut-off values for age and symptom duration for predicting radiographic sacroiliitis are 33.5 and 4.1 years, respectively. It is reasonable to use X-ray as a first-line imaging modality in patients older than 33.5 years with a symptom duration longer than 4.1 years, and use MRI as a first-line imaging in patients younger than 33.5 years with a symptom duration less than 4.1 years.