Septic Facet Arthritis

53-year-old male with recent ablation of the left L4 and L5 nerve roots for persistent back pain. There is widening of the left L5-S1 facet joint. There is loss of T1 signal within the inferior articular process of L5 on the left and superior articular process of S1 on the left. Marrow edema and an effusion centered at the L5-S1 facet are demonstrated on the STIR image. On the post contrast axial T1-weighted image, there is a thick rind of peripheral post contrast enhancement. The findings are compatible with septic arthritis of the left L5-S1 facet joint. Hematogenous infection of the facet joints is rare. Iatrogenic cases are much more common. MRI is an excellent exam of choice as it can better assess spread of infection to the spinal canal and paravertebral regions.