MRI vs. Ultrasound: Which Do You Need?

When you go see a healthcare provider for a medical issue you’re experiencing, they’ll usually diagnose you by performing physical exams, reviewing your medical history or even ordering blood tests. And in some cases, they may need to zoom in a little (well, a lot) closer to see what’s really going on.

There are several ways of looking inside your body. Two common techniques you’ve likely heard of include magnetic resonance imaging (MRI) scans and ultrasound scans. Both are non-radiographic imaging techniques — meaning, they don’t use radiation — that providers use to help identify issues with your bones, tissue and organs.

But is one better than the other? Not exactly. It all depends on what these tests are being used for and what questions your provider is trying to get answered. While ultrasounds are slightly easier exams to undergo, MRIs can give a clearer image of larger parts of your body. Sometimes, you may even be asked to have both, depending on your condition or injury.

Diagnostic radiologist Michael Forney, MD, breaks down the differences between the two tests and answers common questions about them.

MRI vs. ultrasound

The biggest similarity between an MRI and an ultrasound? Unlike X-rays, neither uses radiation. So, many people prefer this form of testing. And both help identify certain issues in your body, but in different ways, notes Dr. Forney.

• MRIs use a powerful magnet and radio frequency waves to produce clear, 3D images of organs and overall structures inside your body. With a muscle injury, for example, MRI images often show a bright signal indicating that there’s more water in the muscle, which is a sign of injury.
• Ultrasounds use high-frequency sound waves to look at organs and structures inside your body. It’s a high-spatial-resolution method of imaging that provides a high level of detail, especially when the tissue isn’t too deep from your skin’s surface. This means an ultrasound can detect soft tissues in your body such as organs, layers of skin and muscles.

The biggest difference between these two tests is that an ultrasound can only show soft tissues and not what is inside joints or bones. An MRI can show bones, cartilage and other structures within and around your joints. An MRI is also better at evaluating large or deeper areas of your body.

“An ultrasound is like a flashlight, whereas an MRI is like a floodlight,” illustrates Dr. Forney. “That’s why there are times when an ultrasound may not be able to make the diagnosis.” He adds that findings from an ultrasound — even if they’re inconclusive — can serve as a jumping-off point for further testing.

When you need an ultrasound

You may only associate ultrasounds with pregnancy, but they can be used for many different medical situations. Ultrasounds can show pictures in real-time, like a movie, and therefore, can demonstrate abnormalities only seen with motion.

“Ultrasound has many advantages as an imaging technique,” Dr. Forney says. These can help identify and, in some cases, diagnose certain conditions.

The main types of ultrasounds are:

• Pregnancy ultrasound. During a prenatal ultrasound, your healthcare provider can create an image of your uterus to determine if you’re pregnant or not. It can also help determine the size and age of the fetus.
• Diagnostic ultrasound. These can help identify and, in some cases, diagnose certain conditions like blood clots and joint injuries or gallstones and abnormal growths.
• Guided ultrasound for medical procedures. Ultrasound isn’t just a diagnostic tool. When some types of injection treatment are needed, such as a cortisone shot into your joint, ultrasound can be used to ensure the needle is placed in the correct location. Ultrasounds can even work as a focal therapy technique to treat prostate cancer.

Ultrasounds are also sometimes used when you feel anxious or uncomfortable being in a closed space during an MRI scan.

Some conditions an ultrasound can help evaluate, diagnose and monitor include:

• Abnormal growths or tumors
• Joint injuries
• Tendonitis
• Ectopic pregnancy
• Kidney or bladder stones
• Gallstones or gallbladder inflammation
• Blood clots
• Heart function

Dr. Forney adds that in some cases, ultrasounds can be a preferred way of examining certain injuries, especially if your body structure isn’t very deep. For example, the tendons in your fingers are often seen in greater detail with an ultrasound compared to an MRI.

When you need an MRI

There are situations when an MRI is needed instead. For example, an ultrasound doesn’t show the structure inside your joints. “We can only see the soft tissues outside or around the joint,” Dr. Forney shares. 

If you need to evaluate damage to cartilage, bone or other structures inside and/or around a joint, MRI is the better choice. MRI is also preferred for conditions that impact deep or large areas, as an ultrasound can only evaluate a small area at a time. But often, a doctor may start with an ultrasound and X-rays for musculoskeletal issues.

Different types of MRIs can zoom in on various parts of your body, including:

• Brain and spinal cord MRI. This is used for conditions like brain aneurysms, stroke, multiple sclerosis, spinal cord injuries and pinched nerves.
• Cardiovascular MRI. This can examine the function of your heart chambers and heart valves, and the blood flow in your major vessels. It can also help diagnose various cardiovascular conditions like coronary artery disease, tumors or the effects after a heart attack.
• Body MRI. This is used for conditions like liver disease, cirrhosis, Crohn’s disease and tumors in your chest, abdomen and pelvis.
• Bone and joint MRI. These MRIs can help determine bone tumors, bone infections and joint issues caused by injuries.

MRI and ultrasound: How they compare

MRIs and ultrasounds are both valuable tools for diagnosing and monitoring health issues and injuries. While ultrasounds can focus in on smaller areas, MRIs can give a clearer, wider picture. Depending on your injury or condition, either test may be beneficial — but it all comes down to location and medical severity.

Be sure to talk to your healthcare provider to learn more about why certain imaging tests may be ordered for you.