MRI is useful for identifying various spinal conditions. It can show dehydration and changes in the discs and facet joints in spondylosis. Disc herniations can be central, posterolateral, or lateral, and may migrate. Lumbar disc herniations most commonly occur posterolaterally at L4-L5 and L5-S1. MRI can identify disc herniations compressing the spinal cord or nerve roots. It is also ideal for demonstrating spinal stenosis by showing the size and shape of the spinal canal. Ankylosing spondylitis principally affects the sacroiliac joints and spine, causing erosions and bony ankylosis.