MRI, X-Ray, and Ultrasound: It’s All about the Energy

There’s only so much your doctor can tell from the outside, so your health care providers may rely on imaging to see more about what’s going on inside your body. There are different kinds of imaging, but they’re all about sending energy into your body to get a better look at what’s happening inside.

This may sound strange, but if you think about it, energy goes through your body every day. The sun’s energy, light waves, and sound waves are some common examples. The chart here shows different kinds of waves of electromagnetic energy. We can see some of these — the part shown labeled “visible light.” Different kinds of visible light rays show up as different colors when we look at them.


X-ray is the oldest kind of imaging technology. X-rays use high-energy ionizing radiation. When the rays pass through your body, bones and teeth stop the rays and show up white on the radiograph, or X-ray picture. Less hard substances like muscles let more of the rays through, so they show up as gray or black. X-rays are most often used to examine bones and teeth.


Magnetic resonance imaging uses radio waves and a magnetic field to create more detailed pictures of the inside of the body. This type of imaging is used to find tumors and other abnormalities in tissues that won’t show up in X-rays. In the chart above you can see that radio waves have lower energy than visible light rays. There’s less risk from the type of energy used in MRIs than from the type used in X-rays.


Ultrasound uses sound waves. Sound waves have different patterns of energy that create different sounds — high and low sounds, for example, are made by different frequencies (roughly speaking, sizes of sound waves). Just as we can’t see the highest and lowest energy electromagnetic rays, we can’t hear the highest and lowest frequencies of sound waves. The sound waves used for ultrasound are high-frequency sound waves, not the kind of sound waves we hear.