X-Ray, MRI, CT Scan Explained

by Food and Drug Administration (FDA)

Within a year of German scientist Wilhelm Roentgen's discovery of x-rays in 1895, people throughout the world knew about Roentgen's work and had seen his first x-ray picture — his wife Bertha's hand, showing her bones, wedding ring, and all. Even before Roentgen was awarded the first Nobel Prize in physics in 1901 for his discovery, x-ray studios were popping up that sold bone portraits for display in the home.

As their popularity grew, some publications contained inflated claims about x-rays — they could restore vision to the blind, they could raise the dead. Other people expressed a far more skeptical view: "I can see no future in the field," the head of one x-ray clinic reportedly proclaimed. "All the bones of the body and foreign bodies have been demonstrated."

But x-ray was far from a dead-end technology. Instead, it marked the start of a revolution in medical diagnosis. Like other medical imaging technologies that followed, including ultrasound, computed tomography (or CT) scanning, and magnetic resonance imaging (or MRI), x-ray can help doctors narrow down the causes of a patient's symptoms without surgery and sometimes diagnose an illness before symptoms even appear. While it can't help a blind person see again, used appropriately, medical imaging can be a useful first step in treating a range of problems, from a simple broken bone to a cancerous tumor.

Using medical imaging appropriately, explains William Sacks, M.D., a medical officer in the Food and Drug Administration's radiology branch, means always considering the risks from a device along with its benefits. X-rays and some other imaging tests use radiation, after all, which can have serious health consequences if used improperly. FDA looks at both the risk and benefit sides of the equation to decide whether to allow marketing of a device, Sacks says. And doctors judge the risks versus the benefits in deciding if a test is medically necessary.

FDA, the U.S. Environmental Protection Agency, and other federal and state agencies share the responsibility for protecting the public from unnecessary radiation. For its part, FDA regulates x-ray equipment and all other electronic radiation-emitting products (including nonmedical consumer products, such as microwave ovens) under the Radiation Control for Health and Safety Act. For all electronic imaging devices, the agency develops and enforces standards to ensure that only safe and effective devices are allowed to be marketed.

"Nothing is entirely safe, of course, including walking down the sidewalk," Sacks says. "The question to ask is, 'In balance, do the benefits of x-ray outweigh the safety concerns?' The benefit of making bone portraits for display, like they did at the beginning of the century, is near zero. Now that we know the health risks from certain doses of radiation, we don't order x-rays willy-nilly, but only if there is a health reason to find out something imaging is capable of telling us."

Black-and-White Photo

Roentgen labeled the rays he discovered with the scientific symbol "X," meaning unknown, because he didn't understand their makeup at first. x-rays are actually electromagnetic waves. When they are passed through a patient's body to a photographic film on the other side, they create a picture of internal body structures called a radiograph.

Chest radiographs, which are among the most common imaging tests, can reveal abnormalities of the lungs (such as pneumonia, tumor or fluid), heart (such as congestive heart failure or enlarged heart), and rib cage (such as broken or abnormal bones).

Other common types of x-ray examinations include dental studies to detect cavities and other tooth and gum problems; abdominal studies, which can reveal abnormalities of not just the abdomen, but also the liver, spleen, gallbladder, and kidneys; gastrointestinal studies of the upper or lower GI tract; studies of the joints to assess things like arthritis and sports injuries; and mammograms, which can help detect breast cancer with the use of special x-ray equipment. (See "FDA Sets Higher Standards for Mammography" in this issue of FDA Consumer.)

Getting a radiograph takes only a few minutes, at a doctor's office or a radiology unit of a hospital or separate location. After positioning the patient with the body part to be examined between the unit that emits the rays and an x-ray film cassette, the doctor or technician steps away from the area and presses a button or otherwise activates the x-ray machine to take the picture.

The less dense a structure of the body is, the more radiation passes through it and reaches the film. The x-rays expose the film, changing its color after it is developed to gray or black, much like light would darken photographic film.

Bones, as well as tumors, are more dense than soft tissues. They appear white or light on the x-ray film because they absorb much of the radiation, leaving the film only slightly exposed. Structures that are less solid than bone, such as skin, fat, muscles, blood vessels, and the lungs, intestines, and other organs, appear darker on the film because they let more of the x-rays pass through. Likewise, a break in a bone allows the x-ray beams to pass through, so the break appears as a dark line in the otherwise white bone.

To make certain organs stand out more clearly, a "contrast medium" — a substance that blocks x-rays rather than transmitting any — can be introduced into the body, in the form of a drink or injection. Barium sulfate is commonly used to study the gastrointestinal tract, while iodine-containing dyes are often used to provide information about the gallbladder, kidneys, blood vessels (using a technique called angiography), or the cavities of the heart.

FDA regulates these contrast agent drugs to make sure they are safe for patients and helpful in diagnosing their medical condition.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.