When to get an MRI scan of your knee

Should I Get an MRI Scan on My Knee? Key Considerations

A knee MRI scan is a detailed 3D medical scan of your knee joint. It can help uncover the cause of knee pain, discomfort, weakness and swelling.

As MRI scans capture the most detailed images of the inside of your body, they can detect subtle changes in your tissues, which may be caused by disease or injury. Here, we will help answer the question, “When should I get an MRI on my knee?” so you can better understand whether a knee MRI is right for you.

We will explore the symptoms that suggest you may need a knee MRI, how to prepare for your scan and what your results may show, as well as alternative scans to investigate knee symptoms.

First, it helps to understand how an MRI scan works.

What is an MRI Scan and How Does it Work?

MRI or magnetic resonance imaging uses strong magnets and radio waves to capture high-resolution, detailed images of the inside of your body.

The magnets create a magnetic field around your body, which affects the water molecules in your tissues. Every water molecule is made up of 2 hydrogen molecules and one oxygen molecule. MRI affects the hydrogen molecules; in particular, a small part of the hydrogen molecules called protons.

During an MRI scan (sometimes referred to as an MRI exam), the protons in hydrogen molecules all line up in the same direction due to the magnetic field. Pulses of radio waves released by the MRI machine cause the protons to fall out of alignment.

When the radio waves stop, the magnetic field causes the protons to line back up in the same direction. This process releases energy that is detected by the MRI machine. How quickly the protons line up and how much energy they release varies between and within tissues.

These differences in the signals detected from different tissues and within tissues result in different parts of your body appearing as varying shades of grey on an MRI scan. White areas are tissues that produce very high-intensity signals. Black areas are tissues that produce very low-intensity signals.

When the MRI images are stitched together, the result is a detailed 3D image of the inside of your body. In certain cases, when there is a need to produce even clearer images, a contrast agent will be given before your MRI exam.

When is a Contrast Agent Needed for an MRI Scan?

If you are having an MRI to closely examine your bones, ligaments or tendons, you are unlikely to need to have a contrast agent. This is because these structures appear with enough clarity on a standard MRI scan.

However, if your MRI is performed to examine soft tissues, a contrast agent is more likely to be used. This helps highlight subtle differences within and between soft tissues that may be the result of a disease process. This can help assess how far a disease has progressed and its severity.

A contrast agent can also help detect very small tumours in soft tissues that may be missed on a standard MRI exam.

What Contrast Agent is Used in MRI Scans?

The rare earth metal gadolinium is used to improve the contrast of MRI scans. It alters the magnetic properties of the protons in hydrogen molecules that comprise the water in your tissues.

Adverse reactions to gadolinium are rare but you will nonetheless be told to wait at the scanning facility for 30 minutes after your scan. This is to ensure that if you develop an adverse reaction, which is usually immediate, help will be at hand.

Around one in every 1000 individuals who have a gadolinium contrast agent will develop a mild rash that will go away on its own within a few hours. Your care team may recommend allergy testing after your scan if you develop a rash.

If you have healthy kidneys, any gadolinium used during your MRI will pass out of your body via your urine within 24 hours. You will be advised to drink 2-3 litres of water during the first 24 hours after your MRI to help pass the contrast agent out of your body.

Signs You May Need a Knee MRI Scan

Putting aside pre-operative planning for knee surgery, you may need a knee MRI if you are experiencing knee pain, discomfort, redness or swelling in or around your knee.

Other symptoms that may need investigating with a knee MRI include:

hearing popping or crunching sounds when you move your knee
a reduced range of motion
recurring knee injuries
weakness or instability eg it feels like your knee will buckle if you try to bear weight on it

If you notice any of these symptoms, you can see your GP or book a private knee MRI – without the need for a referral – at Vista Health.

Preparing For Your Knee MRI Scan

A knee MRI usually takes around 20 minutes and is a safe, painless and non-invasive way to capture images of the inside of your knee. This includes the bones, cartilage, ligaments, tendons, muscles and blood supply.

Depending on your symptoms, you may need to have a contrast agent injected into your knee joint before your scan to create clearer images. When a contrast agent is injected into a joint to enhance an MRI scan, it is called a magnetic resonance (MR) arthrogram.

If you need to have an MR arthrogram of your knee, you will need to fast for 4 hours preceding your scan. This means you should not eat any food but can drink clear liquids, such as water, juice, black tea and black coffee.

Even though you are being admitted for a scan of your lower body, when you arrive for your scan, you may be asked to change into a hospital gown. So make sure you wear clothes that are easy to change out of.

You will also be asked to remove any metal items from your body. This could include clothes with metal parts (zips, clasps or buttons), jewellery, watches, piercings, hair accessories and medical devices, such as glucose monitors and tethered insulin pumps.

As magnetic resonance imaging machines use strong magnets, metal objects can interfere with the MRI scanner, and metal implants in your body can potentially shift and/or overheat.

Try to avoid bringing any unnecessary valuable items with you to your scan. Valuable items that you need to carry with you can be stored in a secure locker at the scanning facility or, if you are bringing a friend or family member, they can hold onto them for you.

If you are feeling anxious about your scan, it is a good idea to bring a friend or family member with you to help calm your nerves. You should tell your care team too so they can take steps to make you feel as comfortable as possible. You can also practice techniques such as mindfulness, visualisation and breathing exercises to ease any anxiety.

Understanding your knee MRI results

A doctor who specialises in diagnosing and treating health conditions through medical imaging (a radiologist) will carefully examine your knee scan results. They will look for telltale signs of injury or disease and include these insights in a report detailing your scan results.

This report will be sent to you, your GP and any specialist doctor involved in your care. During a follow-up appointment, a doctor will explain your results and their significance. If appropriate, they will also discuss whether further tests are needed to confirm your diagnosis and, if treatment is needed, what your options are.

What Conditions Can a Knee MRI Scan Detect?

Knee injuries

Common knee injuries that can be detected with a knee MRI include:

torn ligaments – the knee contains 4 ligaments and the most common ligament injury of the knee is a torn ACL (anterior cruciate ligament)
a torn meniscus – the meniscus is a C-shaped cushion of cartilage that sits in your knee joint and acts as a shock absorber
inflamed tendons – also known as tendonitis, this is usually caused by repetitive exercises that strain the knee or sudden, sharp knee movements, such as pivoting
ruptured tendons – tendonitis that is left untreated increases the likelihood of the tendon tearing (rupturing); a tendon can also tear due to a sudden, sharp movement
a partial or fully dislocated knee – when the bones in your knee become misaligned; this can happen as a result of trauma thrusting your bones out of position
knee bone fractures – also known as a patella fracture, which involves damage to the kneecap bone.

A knee MRI can also detect knee infections. These are most often caused by cellulitis when bacteria enter your knee via a cut.

Diseases of the knee

Diseases that affect the knee can also be detected with a knee MRI. This includes:

bone cancer – the most common type of bone cancer that affects the knee is osteosarcoma; however, osteosarcoma is very rare
osteoarthritis – osteoarthritis of the knee is common in individuals aged over 50 and occurs due to wear and tear of the knee joint over time that causes the cartilage covering the joint surfaces to deteriorate
bursitis – your knee contains 5 fluid-filled sacs (bursae) that allow for smooth joint movement; these sacs can become inflamed due to overuse or frequent kneeling and, therefore, cause knee bursitis

Is a Knee MRI Scan Right for You?

If you have knee pain or knee symptoms that are getting worse, affecting your quality of life or causing you concern, it is important to have them investigated.

With a private knee MRI, you don’t need to wait on a referral from a healthcare professional. This means you can get insights into the health of your knee as quickly as possible.

A knee MRI is a low-risk type of imaging as it does not use any ionising radiation, unlike an X-ray. Therefore, MRI exams are considered safe for most people, including pregnant women in their second and third trimesters.

However, an MRI is generally not recommended in the first trimester of pregnancy as it is unknown how a developing baby at this stage will react to the potential warming effect of an MRI.

If you have metal implants or fragments in your body that are not compatible with MRI scanning, MRI is also not recommended. However, many modern metal implants are compatible with MRI scanning. It is, therefore, important to discuss your particular circumstances and medical history with a healthcare professional to determine if a knee MRI is possible in your case.

If you experience claustrophobia, an MRI may cause you some anxiety as you will be slid into the tunnel part of the MRI machine. However, during an MRI scan on your knee, you will be slid into the MRI machine feet first, and your torso will remain outside the machine. This can make it easier to tolerate the scan if you struggle with enclosed spaces.

Alternatives To A Knee MRI Scan

If you have knee symptoms but a knee MRI is not suitable or possible, your doctor will discuss alternative scans with you.

Knee X-ray

X-rays can be used to diagnose problems with the bones in your knee. X-rays create 2D images of the bones within your body using low doses of ionising radiation, which in high doses, can potentially cause damage to our DNA.

They can help to diagnose joint disorders, bone fractures, dislocations and other bone abnormalities.

However, unlike a knee MRI, an X-ray of your knee cannot clearly detect problems with the soft tissues of your knee, such as the tendons, ligaments, muscles or meniscus.

Although low-level, due to the use of ionising radiation, X-rays are not recommended for women during pregnancy.

Knee CT Scan

A knee CT scan can detect damage to soft tissues as well as the bones of your knee using low doses of ionising radiation in the form of X-rays. It combines X-ray technology with computer processing, which is why CT stands for computerised tomography. This allows each image layer to be stitched together using computer technology to create a 3D cross-section of any part of your body, such as the knee.

It is faster than knee MRI and has better spatial resolution, that is, the ability to show the margins of tissues. Knee MRI, on the other hand, has better contrast resolution, that is, it can show differences within and between tissues.

The speed of a knee CT scan means it is often the preferred choice in emergency situations to get a rapid, accurate diagnosis eg after a traumatic injury.

Depending on your symptoms, you may need to have a contrast agent for your knee CT scan.

If you have had an allergic reaction to the contrast agent used during MRI (ie gadolinium), this does not mean you will have an allergic reaction to the contrast agent used in a CT scan as a different, iodine-based agent is used. However, you should always inform your care team of past allergic reactions to any contrast agents.

Knee Ultrasound Scan

A knee ultrasound scan can detect damage to the soft tissues of your knee, such as the tendons, ligaments and muscles. It does not use ionising radiation and instead uses high-frequency sound waves. These sound waves are beyond the range of human hearing.

These sound waves are passed into the body and encounter tissues with different characteristics. Sound waves bounce back or are reflected whenever they encounter a boundary between different tissues eg soft tissue and fluid or bone.

The reflected sound waves or echoes are converted into electrical signals that are then translated into visual information (points of brightness) by a computer. In just one second, sound waves can be reflected over one thousand times, which allows detailed real-time images to be collected.

While you need to keep your knee still during an X-ray, MRI scan or CT scan, an ultrasound scan of your knee allows for a dynamic assessment of your knee. This means it can capture images of the inside of your knee while you move it.

Real-time images captured while you move your knee allow your doctor to better assess potential muscle and/or bone injuries that are caused by movement.

Check Your Knee Health Today

If you have knee pain, weakness or swelling and are not sure what to do, you can book a Vista Health private GP appointment to discuss your options. Alternatively, if you want your symptoms investigated straight away, you can self-refer for a private knee MRI.

With Vista Health, you can look forward to speedy results – we aim to deliver your knee MRI results within 3 working days.

To get to the bottom of your knee symptoms, so you can understand your treatment options, and make an informed decision on how to get your knee health back on track, book a private MRI scan of your knee with Vista Health now.