CT vs MRI for foot and ankle pain
Do I need an X-ray, ultrasound, CT or MRI for foot and ankle pain?
If your foot or ankle hurts, it is frustrating to have test after test and still not know what is wrong. Imaging can help us understand what is happening, but different scans are useful for different reasons. As a foot and ankle surgeon, the key is to choose the scan that is most likely to give us the information we need.
CT vs MRI vs ultrasound vs X-ray
What scan do I actually need for foot or ankle pain?
If you have been told you need imaging, it is not always obvious why your doctor recommends an X-ray, ultrasound, CT scan or MRI. Each one gives us a different type of information, so the scan we use depends on what we are trying to understand.
The table below shows some common foot and ankle problems, which type of imaging may be used first, and why another scan may sometimes be needed.
| What is going on? | Imaging that may be useful first | When another scan may help | Why? |
|---|---|---|---|
| Possible broken ankle or foot | X-ray | CT if the break is complex | X-rays can show many fractures and whether the bones have moved. CT may give more detail when the fracture is complex or not clearly shown on X-ray. |
| Ankle sprain | Sometimes no scan, or X-ray unless there is 1. Tenderness at the inside or outside part of the ankle (malleoli), 2. Tenderness at the last 6cm at back of the tibia or fibula bone down to the ankle, or 3. Inability to weight bear for 4 steps. (Ottawa criteria) | MRI if pain continues or another injury is suspected | MRI can show ligaments and cartilage that are not seen clearly on X-ray. |
| Ankle keeps rolling or feels unstable | X-ray may be useful looking for bone fragments | MRI if ligament or cartilage damage is suspected | MRI may help show ligament damage or an injury inside the ankle joint. |
| X-ray looks normal but foot or ankle pain continues | X-ray has already been done | MRI may be useful if the cause is still unclear. A CT if an occult fracture is suspected | Some tendon, ligament, cartilage and bone injuries may not be obvious on X-ray. |
| Achilles or other tendon problem | Ultrasound may be useful | MRI if more detail is needed | Ultrasound can look at the tendon. MRI may give more detail if the diagnosis is unclear. |
| Foot or ankle arthritis | Usually a weight-bearing X-ray | CT defines the anatomy but an MRI may be needed to fully assess the cartilage wear in the joint | A weight-bearing X-ray can show joint wear and how the bones line up under body weight. |
| Planning ankle replacement or complex bone surgery | X-rays are often reviewed first | CT for detailed surgical planning | CT can give a much more detailed view of the bones and their alignment and help with surgical planning. |
X-ray for foot or ankle pain
What an X-ray shows
Think mainly about bones, joints and alignment.
An X-ray may help when we are looking for a fracture, arthritis, a bunion or another change in the position of the bones. Weight-bearing X-rays, taken while you are standing, can also show how your foot or ankle lines up when you put weight through it.
A normal X-ray does not always explain ongoing pain. Ligaments, tendons and cartilage are not shown in the same detail, so another scan may be considered if your symptoms suggest one of these structures could be involved.
An X-ray can answer many important questions, but the type of imaging still needs to match the problem we are investigating.
CT for foot or ankle pain
What a CT scan shows
Think of CT as a more detailed look at the bones.
A CT scan uses X-rays taken from different angles to create detailed images, i.e. it gives a 3-D view that X-rays cannot. It may be useful for a complex fracture, when a joint is involved, or when more detail is needed before certain operations.
CT can also be used for ankle replacement planning. The images may be used to create a three-dimensional model to help plan the procedure.
How is a CT different from an X-ray?
An X-ray gives us a quick, two-dimensional picture of the bones. It is often enough to show a straightforward fracture, arthritis or changes in the way the bones line up.
A CT scan goes further. It takes many X-ray images from different angles and combines them, so we can look at the bones in much more detail and from different views. This can be useful when a fracture is complex, several bones are involved, or we need a clearer picture before surgery.
So, an X-ray gives us the overview, while a CT can give us a more detailed map of the bones. The CT does involve more radiation than a standard xray although this is much less with the latest CT scanners.
Does a CT scan have more radiation than an X-ray?
Yes. Both X-rays and CT scans use ionising radiation to create images. A CT scan usually involves more radiation than a standard X-ray because it takes many X-ray images from different angles to build a more detailed picture.
This does not mean a CT scan is unsafe or should be avoided when it is needed. We consider the expected benefit of the scan against the radiation exposure and only recommend CT when the extra detail may be useful for your diagnosis or treatment planning.
MRI for foot or ankle pain
What an MRI shows
MRI is useful when we need to look more closely at structures such as ligaments, tendons and cartilage.
For example, MRI may be considered if we suspect a torn ankle ligament, Achilles tendon damage, cartilage injury or another problem that has not been explained by an X-ray.
MRI can also show some changes inside bone that may not be obvious on a standard X-ray. Whether it is appropriate depends on your symptoms and examination.
The most useful scan is the one that is most likely to answer the clinical question. Sometimes that is an X-ray, sometimes a CT, and sometimes an MRI.
Why not just MRI everybody?
Because MRI is not automatically more useful for every problem.
If we want to assess a fracture, arthritis or bone alignment, an X-ray or CT may provide the information we need. If we suspect a ligament, tendon or cartilage injury, MRI may be more helpful.
There is no general rule that every patient must have an X-ray before an MRI. The order depends on what is suspected, what has already been done and whether another scan is likely to add useful information.
Does an MRI use radiation?
No. MRI does not use ionising radiation. It also produces detailed images of structures such as ligaments, tendons, cartilage and bone marrow, which can make it useful for certain foot and ankle problems. However, MRI is not suitable for everyone, so we will always check things such as metal implants, pacemakers or other medical devices before recommending it.
CT vs MRI for foot and ankle pain
Getting the right answer
Imaging is only one part of diagnosis. Your symptoms, examination, previous injuries and how your foot or ankle is functioning all matter.
The aim is not to order more scans than you need. It is to choose imaging that may help clarify what is causing the problem and guide the next step in your care.
FAQs
What do these imaging scans cost in Perth?
Costs vary between radiology providers and depend on the exact scan, Medicare eligibility and your referral.
As a broad guide, some Medicare-rebatable X-rays and CT scans may be bulk billed or involve a small out-of-pocket cost. Eligible MRI scans may also be bulk billed, while an MRI without a Medicare rebate can cost several hundred dollars.
Before booking, ask the imaging provider what your expected out-of-pocket cost will be.
CT vs MRI: why might I still need a weight-bearing X-ray?
A weight-bearing X-ray, taken while you are standing, shows how the bones and joints line up when your body weight is going through the foot or ankle. This can be useful for problems such as arthritis, bunions, flat feet and other changes in foot alignment.
CT and MRI answer different questions. CT gives more detail about the bones, while MRI can show structures such as ligaments, tendons and cartilage. Depending on what we are investigating, a weight-bearing X-ray may still provide information that CT or MRI does not.
CT vs MRI: do I need another scan if I have already had imaging?
Not necessarily. We will usually review any recent X-rays, CT scans or MRI scans you already have before deciding whether more imaging may be useful. It is important that we have an up to date scan (generally in the last 12m).
Another scan may be considered if your symptoms have changed, the previous imaging does not answer the question we are investigating, or more detail is needed to plan treatment or surgery. The aim is not to repeat imaging unnecessarily, but to make sure we have the information needed to understand your foot or ankle problem.
