 This study is a prospective cohort study examining the effectiveness of ultrasound-guided SIJ sensory block and RFA of the PSN, compared to fluoroscopic guided SIJ RFA. Participants will first undergo a local anesthetic block under ultrasound guidance to determine the predictive value of this test for SIJ RFA; they will then undergo the SIJ RFA procedure. For some patients in this study, this will be a repeat RFA procedure as they would have had a previous fluoroscopic-guided SIJ RFA with success, but subsequent return of their pain. 