An MRI (Magnetic Resonance Imaging) of the sacroiliac joints is a non-invasive imaging test used to evaluate the bones, cartilage, and soft tissues of the sacroiliac joints, which connect the lower spine (sacrum) to the pelvis (ilium). This test helps detect inflammation, injury, degenerative changes, or conditions such as sacroiliitis, ankylosing spondylitis, or other causes of lower back and pelvic pain.