About a sacroiliac joint MRI scan
The sacroiliac (SI) joints sit on each side of the lower back, where the sacrum (the triangular bone at the base of the spine) joins the two pelvic bones. They are strong, stiff joints that pass the weight of the upper body to the hips and legs. An SI joint MRI uses a strong magnetic field and radio waves, not X-rays, to show the joints and the bone on either side of them.
The main reason for this scan is to look for inflammation of the SI joints (sacroiliitis), which is often the first sign of axial spondyloarthritis, including ankylosing spondylitis. This condition usually starts in teenagers and young adults, causing lower back pain that lasts more than 3 months, does not ease with rest, may wake you in the night, and improves with exercise. MRI can show active inflammation in the bone next to the joints long before any damage appears on an X-ray, which can help a rheumatologist make the diagnosis earlier. Some people with this condition have a normal X-ray but changes on MRI (non-radiographic axial spondyloarthritis).
MRI is also used to look for infection in the joint, stress fractures and other causes of buttock and lower back pain. A normal MRI does not completely rule out axial spondyloarthritis, and inflammation on MRI can have other causes, so the results are always read alongside your symptoms and blood tests. For pain from the discs and nerves, see lumbar spine MRI.
What happens during the scan
You lie on your back on the scanner bed with a coil placed over your lower back and pelvis. A cushion under your knees can make this more comfortable. The scanner is loud, so you will wear ear plugs or headphones, and the radiographer can talk to you throughout. You need to keep still while the pictures are taken. Special fat-suppressed pictures are used to show inflammation, so contrast dye is not usually needed; where contrast is needed, it is charged separately.
What the scan can show
A sacroiliac joint MRI can help to look at:
- inflammation of the SI joints (sacroiliitis)
- axial spondyloarthritis, including ankylosing spondylitis
- erosions and other damage to the joints
- infection in the joint
- stress and insufficiency fractures of the sacrum
- other causes of buttock and lower back pain
Preparing for your scan
No special preparation is needed, and you can eat and drink normally. If lying flat is painful, you may want to take your usual painkillers before your appointment. Upload any previous X-ray, scan or blood test reports when you book.
If you are being investigated for axial spondyloarthritis, bring or upload any blood test results (such as CRP or HLA-B27) and previous X-rays of the pelvis.
You can usually eat, drink and take your normal medicines unless the scanning centre tells you otherwise. Wear loose, comfortable clothing without metal zips or buttons, and leave jewellery at home. You will be asked to remove metal items such as hair clips, hearing aids, coins and keys, and you may be asked to change into a gown.
Before the scan you will complete a safety questionnaire. Tell the team if you have a pacemaker, any metal implant, surgical clips or other device, if you have kidney problems or allergies, or if you are, or might be, pregnant or breastfeeding. If you feel anxious in enclosed spaces, let us know when you book. Some centres can offer a relaxant, and if you take one you will need someone to take you home and must not drive for 24 hours.
Frequently asked questions
Can an MRI diagnose ankylosing spondylitis?
An MRI can show inflammation in the SI joints, often before an X-ray shows any change. A rheumatologist makes the diagnosis using the scan together with your symptoms, examination and blood tests.
My X-ray was normal. Why do I need an MRI?
X-rays only show damage that has already happened to the bone, which can take years to appear. MRI can show active inflammation at an earlier stage.
Should I stop my anti-inflammatory tablets before the scan?
Do not change any medicine without speaking to your doctor first. If you are unsure, ask the doctor who requested the scan.
Do I need a referral?
You can tell us if you have a referral from your GP, physiotherapist or consultant and upload it when you book. If you do not have one, select “no referral” when you book and the scanning centre will confirm the next steps with you.
Sources
- NHS – Ankylosing spondylitis: diagnosis
- NHS – Ankylosing spondylitis
- Diagnostic and Interventional Radiology – MRI of sacroiliitis in axial spondyloarthritis
- RadiologyInfo.org – Musculoskeletal MRI
- NHS – MRI scan
The information on this page has been gathered from various sources and is provided for general information only. It is not medical advice. TOP MRI is not liable for the accuracy or completeness of this information. Please speak to your doctor or the scanning centre about your own situation.