Ultrasounds vs. CT scans: differences and uses

Ultrasounds vs CT scans — what’s the difference? If you’re wondering that, you’re not alone. It’s easy to confuse the two. 

Both ultrasounds and CT scans are imaging technologies. Radiologists (doctors who specialize in medical imaging) and other healthcare professionals use them to get a better look at the inside of your body. While they have some similarities, ultrasounds and CT scans work in different ways. 

Each also comes with unique uses, benefits, and risks. 

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An ultrasound is an imaging technology that uses high-frequency sound waves—but not the kind you can hear. 

The ultrasound technology sends those sound waves into your body. It also “listens” to the echoes those sound waves make as they reflect off structures or tissues inside you (NIBIB-a, 2016). 

Healthcare providers perform an ultrasound using a handheld device called a transducer. The transducer sends out soundwaves and also converts the echoes into electrical signals. Those signals go to the ultrasound scanner, which can turn them into an image. This ultrasound image is also called a sonogram. In some cases, specialized ultrasound transducers—also known as “probes”—may need to go inside your body. For example, transducers are sometimes placed in the vagina, GI tract, or blood vessels. But this is less common than a standard ultrasound (Zimmerman, 2021; NIBIB-a, 2016). 

There are two different types of ultrasound. They are (NIBIB-a, 2016):

• Diagnostic ultrasound: This is the most common type. It’s used to create images of your body that medical professionals can identify or rule out health problems.

• Functional ultrasound: This less-common type measures or creates images of processes happening inside your body. For example, functional ultrasounds can reveal how blood is flowing in your heart or vessels. They can also look at the health of your tissues. 

The “CT” stands for computed tomography. Tomography is another name for an image made up of different sections or slices (NIBIB-b, n.d.). 

Using X-ray technology, a CT scanner rotates around your body and takes a series of pictures. The CT device saves each of these pictures. Later, it can assemble them into a detailed 3D image with more specificity than a traditional X-ray (Patel, 2021).

Medical professionals can use both the individual pictures and the assembled 3D image to better understand what’s going on inside you (NIBIB-b, n.d.).

The two imaging tests use different technologies to create pictures of your body. They also produce different types of pictures. 

Like MRIs (magnetic resonance imaging), Ultrasounds are good at imaging soft tissues and other non-bony parts of your body (NIBIB-a, 2016).  On the other hand, CT scans are often more helpful when doctors need to look at complex fractures, worn-down joints, or other problems concerning your skeleton (Patel, 2021; NIBIB-b, n.d.).

The images are also collected in very different ways. During an ultrasound, a medical professional will typically drag the transducer across parts of your bare skin to collect real-time images of your insides from different angles. For example, during a fetal ultrasound, the transducer is usually applied to a pregnant woman’s abdomen (Kurzweil, 2021).

A CT scan, on the other hand, requires that you lay down on a motorized bed. That bed moves slowly through a donut-shaped X-ray tube called a gantry (Patel, 2021).

Ultrasound and CT-scan tend to have different uses. 

Some common uses of ultrasound imaging include (NIBIB-a, 2016):

• Fetal imaging during pregnancy

• Imaging of the heart or internal organs

• Blood vessel imaging

• Eye imaging

• Brain imaging

• Breast or testicle imaging

• Organ imaging

• Tumor imaging

• Identifying the sources of abdominal pain 

• Identifying gallstones (Benarroch-Gampel, 2011)

Some common uses for CT scan include (NIBIB-b, n.d.):

• Imaging complex bone fractures 

• Imaging the skeleton and other body structures

• Imaging eroded joints or bone tumors

• Imaging the abdomen to identify tumors or lesions

• Imaging the heart to detect abnormalities 

• Imaging the head to detect tumors or other problems

• Imaging the lungs or vascular system to detect blood clots, fluid buildup, or other problems

While ultrasound and CT scans often have different uses, doctors usually perform an ultrasound first before ordering a CT scan. For example, a CT scan can help examine the female pelvis in cases where an ultrasound doesn't lead to a clear picture or diagnosis (Patel, 2021; Benacerraf, 2015). 

Like any other imaging test, ultrasounds have advantages and disadvantages.

Advantages of ultrasound

Ultrasounds are inexpensive compared to CT scans and some other imaging techniques. They’re often quick and easy. You typically don’t need to prepare ahead of time for an ultrasound. It produces helpful and detailed images instantaneously. And when used according to safety guidelines, standard ultrasounds come with virtually no health risks (Levine, 2001; NIBIB-a, 2016). 

Drawbacks or risks of ultrasound

The images that ultrasound produces are in some ways not as detailed as those taken with other imaging technologies. Some researchers have found that when medical professionals rely only on ultrasound, they may miss things. For this reason, your doctor may order follow-up imaging using another technology (Levine, 2001; Sotiriadis, 2019). 

Also, the quality of the images that ultrasound produces depends on the skill and experience of the medical professional who uses it. This can potentially open up room for human error (Reddy, 2008). 

CT scans come with a few advantages and disadvantages as well.

 Advantages of CT scans

CT scans produce detailed images that can provide life-saving information. Cancer, brain hemorrhages, and blood clots are serious medical issues that CT scans can help identify (NIBIB-b, n.d.). 

Drawbacks or risks of CT scans

All X-ray imaging, including CT scans, exposes your body to ionizing radiation. This radiation exposure is associated with an elevated risk for cancer because it can damage your cells and lead to DNA mutations. The risk is usually small, but every radiation dose adds up and increases the risk (Patel, 2021; NIBIB-b, n.d.). 

For this reason, women who are pregnant usually won’t undergo pelvis or abdominal CT scans unless it’s absolutely necessary. Likewise, doctors usually won’t order CT scans for kids unless they have no other options because they are more sensitive to ionizing radiation (Patel, 2021; NIBIB-b, n.d.).

Apart from the radiation risks, some CT scans require the use of contrast agents. These are liquids you swallow or that a medical professional injects into your body that help the CT scan create more accurate images. The contrast agents can sometimes cause allergic reactions. They can also be risky for people who have kidney problems (NIBIB-b, n.d.). 

Finally, CT scans are costlier and take longer than ultrasounds (Benacerraf, 2015).

Ultrasounds and CT scans are both helpful and are commonly used medical imaging technologies. Each has advantages and disadvantages. The bottom line is that either one of them could save your life. 

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Current version

September 14, 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.