Ultrasound vs. MRI: differences and uses

When it comes to ultrasound vs. MRI, there are a few key differences and similarities. 

Both are medical imaging technologies. That means medical professionals use them to get a better look at the inside of your body. They often do this when trying to find issues or abnormalities. Unlike some other imaging tests—such as X-rays or CT scans—ultrasound and MRI are often used to look at soft tissues or other non-bony parts of your insides.

While they have some similarities, MRI and ultrasound work in different ways. Each also comes with unique benefits and risks.  

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Ultrasound technology uses high-frequency sound waves—too high for the human ear to hear—to create real-time images of the body’s insides. To achieve this, an ultrasound involves a handheld device called a transducer. Transducers send out soundwaves and detect any echoes those soundwaves make in the body (NIBIB-a, 2016). 

During an ultrasound, a medical professional will put gel on the body part requiring imaging. The gel keeps air bubbles from messing with the ultrasound’s signals. Then, as the transducer moves over the skin, the ultrasound equipment sends and receives soundwaves and turns those into a 2D image on a screen (Kurzweil, 2021). Sometimes, ultrasound transducers or “probes” are placed inside of the body to improve the images. For example, they may be placed in the GI tract, vagina, or blood vessels (NIBIB-a, 2016). 

There are two types of ultrasound. These are diagnostic ultrasounds and functional ultrasounds (NIBIB-a, 2016):

• Diagnostic ultrasound: The most common type of ultrasound, a diagnostic ultrasound, creates images of the body that healthcare providers use to identify problems (or confirm that everything is in good shape).

• Functional ultrasound: Not as commonly used, functional ultrasound is used to measure or create images of processes in your body. 

MRI stands for magnetic resonance imaging. MRI technology uses a powerful magnet to shift the position of the protons inside the cells of the body. Basically, it makes the proteins line up with the direction of the magnet’s pull (NIBIB-b, n.d.). 

Next, using radiofrequency currents, MRI technology temporarily pushes those protons away from the magnet’s pull. By measuring the effects of this push and pull on the body’s protons, the MRI technology can create high-resolution 3D images of your insides.

An MRI machine looks like a big tube or cylinder. All or part of your body will go inside this tube during the MRI (NIBIB-b, n.d.).

Ultrasound and MRI technologies produce different types of images using different methods. For that reason, they have different uses. That said, medical professionals may sometimes use both in order to identify or treat a health issue.  

Ultrasound can be used to take images of (NIBIB-a, 2016):

• Fetus during pregnancy

• Heart or internal organs

• Blood vessels

• Eyes

• Brain (in some cases)

• Breasts or testicles

• Abdominal organs 

• Tumors

Some common uses of MRI scans include (NIBIB-b, n.d.):

• Imaging of muscles, ligaments, and tendons

• Knee, shoulder, or rotator cuff imaging

• Spinal cord imaging

• Some types of brain imaging

• Nerve imaging

• Fetal imaging during pregnancy

Apart from how they work and their uses, there are some other important differences between ultrasound and MRI.

For starters, MRI tends to produce more detailed images than ultrasound scans. While 3D or even 4D ultrasound is now available, standard ultrasound produces 2D images. Standard MRI, on the other hand, produces 3D images that have higher specificity than ultrasound (NIBIB-b, n.d.; Reddy, 2008). 

MRI images also include different information than ultrasound images. That’s especially true when it comes to the nervous system and some other parts of the body. For this reason, MRI is sometimes used following an ultrasound if medical professionals need to take a closer look—or gather more detailed information—about a person’s body (Reddy, 2008). 

For example, when monitoring the health of a fetus, ultrasound is almost always performed first. But medical professionals frequently use MRI if they need to take a closer look at a fetus’s brain or spinal cord (Reddy, 2008).  

Ultrasounds have a handful of pros, including (Levine, 2001; NIBIB-a, 2016):

• Affordable

• Quick

• Little-to-no preparation needed

• Images are produced in real-time

• Virtually no health risks

Despite their advantages, while ultrasound images are good, they’re not perfect. Some research has shown that when healthcare providers solely rely on ultrasound, misdiagnoses can occur. Because of this, if a doctor or medical technician sees something fishy on an ultrasound, they may order follow-up imaging using MRI or another technology (Levine, 2001; Sotiriadis, 2019). 

To add, because a human conducts ultrasound, human error can occur. The experience and level of skill (or lack thereof) of the technician performing the ultrasound can affect the quality of the images obtained (Reddy, 2008). 

A big advantage to MRI is that it produces very detailed images. These images can help medical professionals see more—and in some cases make more accurate diagnoses—than if they relied on ultrasound or other imaging techniques alone (Levine, 2001).

Especially when it comes to the brain, tissues, and spinal cord, MRI imaging can see things that ultrasound can’t (NIBIB-b, n.d.). 

However, compared to ultrasound, an MRI is more expensive. Also, the magnets in an MRI are super powerful—powerful enough to fling a metal wheelchair across a room. If your body has any metal in it, such as a pacemaker or other metal implant, you can’t undergo MRI (NIBIB-b, n.d.).

MRI machines can also be quite loud. And unlike a standard ultrasound, an MRI can take a lot of time—often 20 to 30 minutes or more. During that time, you must remain almost perfectly still. Especially if you’re afraid of tight spaces, spending that much time in a loud MRI machine can be difficult (NIBIB-b, n.d.; Reddy, 2008). 

Finally, you sometimes have to do some things ahead of time to prepare for an MRI. For example, you may not be able to eat or drink for several hours on the morning of an MRI that looks at your intestines. Also, you will likely have to schedule your MRI for a later date, whereas ultrasound is often performed immediately (UC-Davis, n.d.). 

Both ultrasound and MRI are helpful and popular forms of imaging technology. Don’t be surprised if your medical professional uses one of them (or both of them) to guide your care. 

If you have specific concerns about an upcoming imaging scan, your doctor can help address those concerns and answer any additional questions you may have. 

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Current version

September 24, 2021

Written by

Health Guide Team

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.