What’s the Difference between an X-ray, Ultrasound, MRI and CT Medical Imaging?

If you are like most people, you have undergone some type of medical imaging in your life already, such as an x-ray, ultrasound, MRI or CT. Medical imaging creates pictures of tissues, bones and organs inside the human body. These imaging technologies help doctors diagnose and assess diseases.

All of these tests are non-invasive, which means they do not involve general anesthesia or stitches. Some types of tests do include the use of an intravenous (IV) needle. These imaging tests are pain-free and provide a wealth of information.

But what exactly are the differences between these imaging techniques?

Types of Diagnostic Imaging


X-rays use radiation to create images. During an x-ray, radiation passes through the body onto an x-ray film. Radiation passes right through fluid and thin tissues to cause a dark area on the x-ray film. Bones and other dense tissues stop the x-rays from passing, so these areas show up as light areas on the x-ray film. Health care professionals use x-rays to diagnose bone disease, fractures, dislocations, infections and tumors.

X-rays usually entail standing or lying between two pieces of equipment – one piece of equipment sends the radiation out and another contains the x-ray film.


Ultrasound uses high-frequency sound waves to create images. Like x-rays, the ultrasound beams pass through fluid and less-dense tissue. The sound waves bounce off bones and denser tissues. The ultrasound machine uses the echo of the sound waves to create the image. Health care professionals use ultrasound to diagnose problems affecting soft tissues, such as muscles, blood vessels, the heart and other organs.

To perform an ultrasound, the technician places a handheld wand against your skin. The wand, known as a transducer, emits the sound waves and detects their echoes. A computer converts information from the echoes to create an image.


Computed tomography (CT or CAT scan) combines x-rays and a computer to create 360-degree pictures of the spine and of internal organs. During a CT, the technician takes a series of x-ray images from different angles. Then the computer processes and assembles the series of images to create cross-sectional images, or slices, of the bones, blood vessels and soft tissues. CTs create images that are more detailed than traditional x-rays.

A CT is a large box-like machine with a hole or tunnel in the center. You lie on a table, which passes through the hole during the test. Special dye, known as contrast, is sometimes administered through an IV.  


Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves to create images of the body that cannot be seen as well with x-rays, ultrasounds or CT scans. MRIs use magnets to create images.

The human body is made mostly of water, which has magnetic polarity. In other words, fluid in the body contains atoms that spin at a certain rate. Atoms in some tissues spin faster than atoms in other tissues. In some cases, the atoms in diseased tissue spin at a different rate than in healthy tissue. MRIs use magnets to spin the atoms and radio waves that detect how fast the atoms spin. A computer transforms the information into images.

Health care professionals use MRI to assess joints, muscles, cartilage, ligaments and tendons. This makes MRIs helpful for diagnosing sports injuries. MRI is also helpful for diagnosing strokes, tumors, aneurysms, multiple sclerosis, spinal cord injuries, and inner ear or eye problems.

An MRI machine is similar to a CT scan in that it involves a large box-like device with a hole in the center. MRIs tend to be very loud, though.

Nuclear medicine

Nuclear medicine is a branch of medical imaging that allows for early detection of cancer and other serious illnesses. In this imaging procedure, health care professionals inject a small amount of radioactive material through an IV. The radioactive material, known as radiotracers, accumulates in cancer tumors and other types of unhealthy tissue. The radiotracers make it easy to see the unhealthy tissue with a type of imaging test known as a positron emission tomography (PET) scan. Health care professionals can even overlay the results of PET scans on top of CT or MRI images to produce special images.

For more information on diagnostic imaging, and the differences between the various tests, consult with your health care professional. The more you know about diagnostic imaging, the more control you have over your own health.

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