Intermittent severe lower back pain and left leg radiating pain that worsens with rest and improves with activity. x-ray showed lumbar spine facet degenerative changes. is that normal in 40-year-old who is active and normal weight? is mri needed?

MRI would give information regarding disc herniation and nerve compression. You dont mention if it is worse with lying down ,sitting , driving which usually occurs with lumbar disc disease. May be piriformis syndrome -compression of sciatic nerve by muscle, rarely mass in pelvis. Recent 1 week? or several months?. New onset may be Lyme or early shingles F/u/ w/ MD

Usually the mri is used for operative planning. If you have tried injections therapy and other non surgical measures still have symptoms that affect activity and are considering surgery get mri