About a knee CT scan
The knee is the joint where the thigh bone (femur) meets the shin bone (tibia), with the kneecap (patella) at the front. Ligaments, cartilage and the menisci (two C-shaped pads of cartilage) hold it together and cushion it. A CT (computed tomography) scan uses X-rays and a computer to make detailed cross-section pictures of the bones, which can be turned into 3D images.
Most knee problems, such as ligament and meniscus tears, are best seen on an MRI scan of the knee. A knee CT is used when the question is about the bones, for example:
- complex fractures, especially tibial plateau fractures (breaks in the top of the shin bone that enter the joint), where CT shows how the joint surface has broken and helps surgeons plan an operation
- kneecap instability, where the kneecap keeps slipping out of place; CT can measure the position of the kneecap and its groove (such as the TT–TG distance) before surgery
- knee replacements, to check the position and rotation of the parts, look for loosening, and plan revision surgery
- how a fracture is healing, arthritis, loose bodies, and bone lumps and cysts
- knee problems when MRI is not possible, for example if you have a pacemaker or other device that is not safe for MRI
When you book, choose the left or right side.
What happens during the scan
You lie on your back on the scanner bed, usually feet first, with your knee supported by pads so it stays still. The bed moves through a short, open ring while your knee is scanned. For some measurements, the scan also includes your hip and ankle. If you have a brace or cast, it can usually stay on. The radiographer controls the scanner from the next room, but can see, hear and talk to you throughout. The scan itself takes only a few minutes. Contrast dye is not usually needed; where contrast is needed, it is charged separately.
Is CT safe?
CT uses X-rays, which are a type of ionising radiation. A knee CT gives a very low dose, because the area scanned is small and far from the organs most sensitive to radiation. The risk from a single scan is very small, and every CT scan is checked by a radiologist or radiographer to make sure the benefit of the scan outweighs the risk, as required by the Ionising Radiation (Medical Exposure) Regulations. If you have had other CT scans recently, tell us when you book.
What the scan can show
A knee CT can help to look at:
- tibial plateau and other complex fractures
- kneecap fractures and alignment
- kneecap instability and TT–TG measurement
- knee replacement position, rotation and loosening
- fracture healing
- osteoarthritis and bone spurs
- loose bodies and bone lumps
When to get urgent help: go to A&E if you cannot put weight on your leg after an injury, your knee looks misshapen, or it swells up quickly after an injury. Get help the same day if your knee becomes hot, red and swollen and you feel unwell, especially if you have a knee replacement.
Preparing for your scan
No special preparation is needed, and you can eat and drink normally. Wear clothing that lets you easily uncover your knee, or you may be asked to change into a gown. Upload any previous X-ray or scan reports and letters from your surgeon when you book, and remember to choose the correct side.
Wear loose, comfortable clothing without metal zips, poppers or buttons, and leave jewellery at home, as metal in the scan area can affect the pictures. You may be asked to change into a gown. Unlike MRI, CT is not affected by pacemakers or most metal implants, and the scanner is a short open ring rather than a tunnel, so it is usually easier for people who feel anxious in enclosed spaces.
Tell the team before your scan if you are, or might be, pregnant or breastfeeding, if you have kidney problems, diabetes (especially if you take metformin), asthma, thyroid problems or allergies, or if you have had a reaction to contrast dye before. If you need contrast, you may be asked for a recent kidney blood test result.
Frequently asked questions
Is CT or MRI better for my knee?
MRI is better for most knee problems, including ligament, meniscus and cartilage injuries. CT is best for the bones, such as complex fractures, kneecap alignment and knee replacements. Your referrer will say which is needed; if you are unsure, contact us before booking.
I have a knee replacement. Can I have a CT?
Yes. CT is often used to check knee replacements, including their rotation, and modern scanners reduce the effect of the metal on the pictures. Tell us about your implant when you book.
Do I need to choose a side?
Yes. Choose the left or right knee when you book. If both knees need scanning, contact us so the centre can plan the appointment.
Do I need a referral?
Because CT uses radiation, each scan has to be justified by a qualified professional before it can go ahead. Upload a referral from your GP or consultant when you book if you have one. If you do not, select “no referral” and the scanning centre will review your request and confirm the next steps with you, which may include arranging a referral.
Sources
- Comparison between CT and MRI in the clinical diagnosis and treatment of tibial plateau fractures (PMC)
- Factors affecting tibial tuberosity–trochlear groove distance in recurrent patellar dislocation (PMC)
- NHS – Knee pain
- NHS – Knee replacement
- NHS – CT scan
- UKHSA – Patient dose information: guidance
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.