About a sacrum MRI scan
The sacrum is the large, triangular bone at the base of the spine, made of five vertebrae that fuse together in adulthood. It sits between the two pelvic bones, joining them at the sacroiliac joints, with the coccyx (tailbone) below it. The sacral nerves, which supply the legs, bladder, bowel and sexual organs, pass through openings in the bone. A sacrum MRI uses a strong magnetic field and radio waves, not X-rays, to show the bone, nerves and soft tissues in detail.
A common reason for this scan in older people is a sacral insufficiency fracture – a crack in the sacrum caused by thinning bones (osteoporosis), often with no fall or only a minor one. These fractures cause low back, buttock or groin pain and are easily missed on X-rays, but MRI can show them at an early stage. The scan is also used to look at stress fractures in runners, nerve problems such as sciatica coming from the lowest part of the spine, Tarlov (perineural) cysts, infection, and lumps or tumours of the sacrum.
For back pain from the discs between the lumbar vertebrae, a lumbar spine MRI is usually the right scan.
What happens during the scan
You lie on your back on the scanner bed with the coil under or 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. Contrast dye is not usually needed, but may be used to look at a lump or infection; where contrast is needed, it is charged separately.
What the scan can show
A sacrum MRI can help to look at:
- insufficiency fractures caused by osteoporosis
- stress fractures and injuries after a fall
- problems with the sacral nerves, including sciatica
- Tarlov (perineural) cysts
- infection and inflammation
- lumps and tumours of the bone and nearby tissues
When to get urgent help: call 999 or go to A&E if you have back pain with numbness around your genitals or bottom, new problems controlling your bladder or bowel, or pain, numbness or weakness in both legs. These can be signs of cauda equina syndrome, which needs emergency treatment.
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.
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 I have a fracture without falling?
Yes. When bones are thinned by osteoporosis, the sacrum can crack under normal everyday stress. These insufficiency fractures often do not show on an X-ray, but MRI can detect them.
Should I have a sacrum or lumbar spine scan?
A lumbar spine MRI is usually best for back pain and sciatica from the discs. A sacrum MRI focuses on the bone at the base of the spine. Your referrer will say which area to scan; if you are unsure, contact us before booking.
What if lying still is painful?
Tell the radiographer. They can use cushions and pads to make you more comfortable. Taking your usual painkillers beforehand can also help.
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
- Radiopaedia – Sacral insufficiency fracture
- Osteoporotic sacral insufficiency fracture: an easily neglected disease in elderly patients (PMC)
- NHS – Sciatica
- RadiologyInfo.org – MRI of the Spine
- 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.