Foot and ankle imaging: X-rays, CT, MRI and ultrasound

The questions imaging answers

Imaging can show whether a bone is broken, how the bones are aligned, whether a joint has arthritis, and whether a tendon, ligament or cartilage is damaged. It can also show a stress fracture or infection before it is obvious. The right test depends on the suspected problem, and professional guidelines organize the choice of imaging by clinical scenario, for example for chronic foot pain and chronic ankle pain.^[1,2]

X-rays

X-rays (radiographs) show bone, and they are the usual starting point. X-rays are available on site at our practice, so they can often be taken and reviewed at the same visit. The details matter:

• Weight-bearing X-rays, taken while you stand, show how the bones line up when the foot carries your weight. They are essential for judging alignment and for planning surgery for problems such as a bunion, flatfoot or arthritis.^[3,4,7]
• After an injury, X-rays are needed when specific findings are present. A checklist called the Ottawa ankle and midfoot rules is very sensitive for ruling out a fracture, though not very specific.^[8]
• In a midfoot injury, standing X-rays can reveal ligament injury that isn’t visible when the foot is not bearing weight.^[9]
• Limits: early stress fractures often don’t show up on the first X-ray; the crack may not become visible until several weeks later.^[6]

CT scans

Computed tomography (CT) shows bone in three dimensions, and it is used for complex fractures, for joint injuries, and to plan surgery.^[10,9] Weight-bearing cone-beam CT is a newer technology that scans the foot and ankle while you stand, so the alignment and joint spaces can be measured under load. Its scan time is short, and its radiation dose may not be greater than that of standard X-rays. It is increasingly used, particularly for deformity, arthritis and the flatfoot classification, although access varies.^[11,12]

MRI

Magnetic resonance imaging (MRI) gives detailed images of soft tissues without radiation. In the foot and ankle it is used to look at tendons, ligaments and cartilage, and to find stress fractures and bone bruises that X-rays miss, as well as infection of bone.^[5,6] For chronic ankle pain that hasn’t been explained by X-rays, MRI can help distinguish between causes such as impingement, an osteochondral lesion or a tendon injury.^[2] MRI may not be suitable if you have certain implants or devices, so tell us if you have a pacemaker, a neurostimulator or metal in your body.

Ultrasound

Ultrasound uses sound waves and has no radiation. It can show tendons, the plantar fascia and other soft tissues, and it can be done while the foot moves. It also helps guide injections. It is limited in how well it can see inside a joint or in bone. Diagnostic ultrasound is available on site at our practice, so when a tendon or soft-tissue question comes up it can often be answered at the visit.

Bone scans and other tests

A bone scan can detect a stress fracture or an area of increased activity in a bone earlier than X-rays.^[6] Blood tests, and sometimes a sample of tissue or fluid, are used when infection, gout or inflammatory arthritis are suspected. Tests of circulation and nerve function may be needed for people with diabetes or numbness. See diabetic foot ulcers and infections.

Reading the images sensibly

Images are only one piece of the picture. Some changes, like a bone spur or mild arthritis, can be present in feet that don’t hurt. Others, like an early stress fracture, can be absent on the first X-ray. Your surgeon will interpret imaging together with your history and examination, and will explain what the findings mean for your treatment. Ask for copies of your images and reports, and bring them if you seek a second opinion. See second opinions before foot or ankle surgery.

When to seek care