Definition and Overview
MRI (magnetic resonance imaging) of the knees is a non-invasive diagnostic imaging test performed to analyze the condition of the various parts of the knees, such as the tissues, tendons, joints, muscles, bones, and ligaments.
The knee is a joint that connects the two major bones in the leg. These are the tibia (shin bone), which is in the lower half of the leg, and the femur (thigh bone), which is the longest bone in the body. As a joint, the knee contributes significantly to a person’s mobility, making walking, jumping, and standing, etc. possible.
The knee itself is composed of 4 parts: tendons, ligaments, bones, and cartilages. There are three bones that make up the knees, including the patella (knee cap). The cartilages, on the other hand, are meniscus, which is rubbery and serves as a cushion for the bones, and articular, which is watery and allows the bones to glide smoothly when they move. The ligaments, on the other hand, are the ones that help bring together these bones. The tendons attach the muscles to the bones.
Who Should Undergo and Expected Results
Below are knee conditions that typically require an MRI:
Fracture – Fracture is a traumatic injury and is often a result of an accident, including a fall or slip. A fracture is more than just a dislocation of a bone; it means that the bone is broken and it needs to be brought back to its original state. Otherwise, more severe complications can arise.
Knee pain – Knee pain is more of a symptom than a disease, and it can mean a lot of things such as inflammation brought by osteoarthritis, internal bleeding of the tissues, general wear, or even tumor growth, which can be benign or malignant (cancerous).
Degeneration – The knees can show significant signs of wear and tear, especially since they are used frequently. However, for people who have been diagnosed with degenerative conditions such as arthritis, the wear and tear can be very painful and debilitating. An MRI can be used to detect the extent of the wear and possibly predict the progression of the disease.
Decreased mobility – This is another symptom that commonly affects the knee. A person with decreased mobility may find it difficult to stand or climb a flight of stairs without exerting more effort or feeling some pain in the knee.
Swelling – Knees can experience swelling for many different reasons including the buildup of fluid. The knees can become sore, tender to touch, and appear red in colour.
Normally, it takes at least 30 minutes to complete an MRI session, but sometimes it can extend to an hour or even longer. At the end of the session, the images are forwarded to the doctor, who will discuss the results with the patient.
The MRI captures detailed images of the various parts of the knee, but there will still be times when the diagnosis is unclear. When this happens, the doctor may request for another MRI or an entirely new imaging test such as a PET or CT scan.
How Does the Procedure Work?
Many tests can be used to diagnose, treat, or manage a condition that affects the knees. One of these is the MRI, which is often carried out if the other non-invasive and less expensive tests such as an ultrasound or X-ray cannot provide a definitive finding.
An MRI is an imaging test that involves the use of a magnetic field and a radiofrequency waves to generate images of the parts of the knees. It is based on the assumption that tissues in the body are responsive to the magnetic field, which then help create the necessary images.
MRI should not be confused with CT and PET. Unlike other similar tests, MRI doesn’t use ionizing agents (such as contrast dye, which may be injected or drank) to create the images needed unless it’s an MR arthrogram, which is a specific type of MRI. It also has a different function. PET can be used to analyze complex body functions such as metabolism or absorption of sugar. CT scan is often utilized to diagnose tumor masses as long as they are not smaller than 2cm.
In general, there’s no special preparation needed, although the doctor will provide the patient with specific instructions prior to the procedure.
Often, wearing of jewelry is not allowed. The patient may be allowed to wear personal clothing as long as it’s loose fitting, but usually the patient has to change to a hospital gown. Patients who already have metal implants can still proceed with the MRI as long as these implants are not cochlear, clips to treat a brain aneurysm, defibrillators, or pacemakers.
Children and infants can also undergo MRI, but they are usually sedated to prevent them from moving and being frightened once they enter into the “tunnel-like” structure.
If contrast dye has to be used, an IV line will be attached before the actual procedure to deliver the agent into the bloodstream. It may take a few more minutes before the test begins to allow the body to absorb the dye.
The MRI is composed of a large “dome” and a table that goes inside it. The patient lies on the table with feet first. The knees may be supported with some pillows to prevent them from moving and keep them comfortable while still.
The technician operates the scanner and captures images in another room. The patient cannot see the technician but can hear the voice in the examination room.
Possible Risks and Complications
In general, MRI is a safe procedure for most people, including infants and children. Pregnant women can also undergo it on a case-to-case basis. Those who are in their first trimester are not allowed, and the test is recommended only when it’s absolutely necessary.
A patient who’s claustrophobic may feel panic because of the equipment’s structure and thus may have to be sedated before the exam. If contrast dye has to be provided, it’s essential the patient informs the technician if he or she is allergic to the agent.
In a very rare case, MRI can cause nephrogenic systemic fibrosis, which happens when a person with an existing impaired kidney function is given a high-dose contrast dye.
Wilkinson ID, Paley MNJ. Magnetic resonance imaging: basic principles. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 5.
DeLee JC, Drez D Jr, Miller MD, eds. DeLee and Drez’s Orthopaedic Sports Medicine. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2009:chap 24.
Grainger RG, Thomsen HS, Morcos SK, Koh DM, Roditi G. Intravascular contrast media for radiology, CT, and MRI. In: Adam A, Dixon AK, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 2.