What is an MRI scan?
An MRI examination is a special machine (big magnet) that uses strong magnetic fields and radio waves to produce detailed images of the knee. MRI is the imaging of choice when there is significant knee joint trauma with suspicion of multiple injuries to the ligaments and internal structures.
Knee MRI images showing fracture (arrow) to the outer aspect of the femur. The white colour is fluid and bleeding around the fracture, which makes picking up subtle fractures easy on MRI
What does a knee MRI scan involve?
A knee MRI scan involves you staying still for approx. 20-30 minutes inside a special machine (small tunnel) while the images are taken. It can be slightly noisy inside the machine; therefore, headphones (usually with some music) will be used inside the scanner. The technician will see you before the scan to ensure that you do not have any metallic objects before going into the scanner. This can be dangerous as the scanner has a strong magnet, and any small metallic object can become a projectile.
Is it safe to have a knee MRI scan?
An MRI scan is very safe and routinely performed nowadays with very few contraindications. The technician will ask you all the relevant questions to make sure that it is safe for you to undergo an MRI before your appointment. Generally, if you have a metallic object inside your body, then an MRI scan might not be safe for you, as it may result in the movement of the object, which can cause tissue damage if it is in a sensitive area (like in the brain or eye). Some examples of conditions where you should not go inside an MRI machine scanner include:
· If you have a metallic foreign body within your eye. This is especially seen in welders. An x-ray might be needed before the examination to ensure no significant metallic foreign body near your eyes.
· Surgical clips inside your brain.
· Cochlear implant
· A heart pacemaker or defibrillator
· A recent surgery (within six weeks from the scan), especially if it involves inserting a sizeable metallic object into the body (like a joint replacement).
What are the different types of MRI machines, and which one is better?
The newer generations of MRI are called (3T or 3 Tesla) while the older ones are called 1.5T scanners. This refers to the strength of the main magnet of the scanner. Stronger magnets usually result in better images and, therefore, can provide a superior assessment of the imaged body part. Scanners can also be divided according to the body part they can image into a body or extremity scanners. The latter are smaller, with weaker magnets and can only image the extremities. Body scanners are larger, with stronger magnets and image all body parts. The images produced by body scanners are usually superior (because they have stronger magnets and the imaging field is usually more homogenous). These are usually preferred when you are getting a knee MRI.
What type of knee problems will be picked up on an MRI scan?
Generally, MRI is a very good scan to diagnose knee problems. It needs to be interpreted by a specialist musculoskeletal radiologist. The knee conditions that can be assessed on an MRI examination include:
What are the disadvantages of an MRI examination?
A knee MRI examination has a few drawbacks
· Usually, they are costly compared to other types of scans.
· They require you to stay still for 20-30 minutes; otherwise, the images will not be clear.
· People who are not comfortable with small spaces (claustrophobic) may find it difficult to have an MRI scan
· It may be unsafe to perform in a few situations, as explained above.
How much does a knee MRI cost?
This varies depending on the institution. In the UK, the price can range from £300 to £800.
Knee MRI vs Knee x-ray and ultrasound scan
A knee X-ray (or knee radiograph) is a black-and-white image produced by sending an X-ray beam through the area and getting the images on a film. It is usually the first type of imaging done when there is a knee problem, particularly if there is suspicion of knee arthritis or fracture around the knee. Unlike MRI, X-rays cannot assess the soft tissues. To find out more, please see our article about knee X-rays.
Ultrasound is high-frequency sound waves produced by a special machine and can provide useful images of the tissues within the body. To find out more, please see our article about knee ultrasound scans. Ultrasound is the preferred imaging choice when a knee injection is required. Ultrasound allows for direct visualisation of the needle to ensure medicines are injected into the exact targeted site. There is significant evidence that ultrasound-guided injections provide better levels of pain relief and longer effects compared with injections done without imaging guidance. A knee ultrasound examination is very useful to assess the ligaments and tendons close to the skin surface (the patellar tendon, the pes anserine tendon and the collateral ligaments). It is also very useful for assessing swellings around the knee joint (like Baker’s cyst, joint effusion, and pre-patellar bursitis). It cannot assess the deep structures inside the knee joint (the menisci, the cruciate ligaments and the articular cartilage). To find out more, please see our article about the different types of scans used for knee problems
