X-rays are a type of radiation called electromagnetic waves. X-ray imaging creates pictures of the inside of your body. The images show the parts of your body in different shades of black and white. This is because different tissues absorb different amounts of radiation. Calcium in bones absorbs x-rays the most, so bones look white. Fat and other soft tissues absorb less, and look gray. Air absorbs the least, so lungs look black.

The most familiar use of x-rays is checking for broken bones, but x-rays are also used in other ways. For example, chest x-rays can spot pneumonia. Mammograms use x-rays to look for breast cancer.

When you have an x-ray, you may wear a lead apron to protect certain parts of your body. The amount of radiation you get from an x-ray is small. For example, a chest x-ray gives out a radiation dose similar to the amount of radiation you're naturally exposed to from the environment over 10 days.

What are medical x-rays?

X-rays are a form of electromagnetic radiation, similar to visible light. Unlike light, however, x-rays have higher energy and can pass through most objects, including the body. Medical x-rays are used to generate images of tissues and structures inside the body. If x-rays travelling through the body also pass through an x-ray detector on the other side of the patient, an image will be formed that represents the “shadows” formed by the objects inside the body.

One type of x-ray detector is photographic film, but there are many other types of detectors that are used to produce digital images. The x-ray images that result from this process are called radiographs.

How do medical x-rays work?

To create a radiograph, a patient is positioned so that the part of the body being imaged is located between an x-ray source and an x-ray detector. When the machine is turned on, x-raystravel through the body and are absorbed in different amounts by different tissues, depending on the radiological density of the tissues they pass through. Radiological density is determined by both the density and the atomic number (the number of protons in an atom’s nucleus) of the materials being imaged. For example, structures such as bone contain calcium, which has a higher atomic number than most tissues. Because of this property, bones readily absorb x-rays and, thus, produce high contrast on the x-ray detector. As a result, bony structures appear whiter than other tissues against the black background of a radiograph. Conversely, x-raystravel more easily through less radiologically dense tissues such as fat and muscle, as well as through air-filled cavities such as the lungs. These structures are displayed in shades of gray on a radiograph.

When are medical x-rays used?

Listed below are examples of examinations and procedures that use x-ray technology to either diagnose or treat disease:

Diagnostic

X-ray radiography: Detects bone fractures, certain tumors and other abnormal masses, pneumonia, some types of injuries, calcifications, foreign objects, dental problems, etc.

Mammography: A radiograph of the breast that is used for cancer detection and diagnosis. Tumors tend to appear as regular or irregular-shaped masses that are somewhat brighter than the background on the radiograph (i.e., whiter on a black background or blacker on a white background). Mammograms can also detect tiny bits of calcium, called microcalcifications, which show up as very bright specks on a mammogram. While usually benign, microcalcifications may occasionally indicate the presence of a specific type of cancer.

CT (computed tomography): Combines traditional x-ray technology with computer processing to generate a series of cross-sectional images of the body that can later be combined to form a three-dimensional x-ray image. CT images are more detailed than plain radiographs and give doctors the ability to view structures within the body from many different angles.

Fluoroscopy: Uses x-rays and and a fluorescent screen to obtain real-time images of movement within the body or to view diagnostic processes, such as following the path of an injected or swallowed contrast agent. For example, fluoroscopy is used to view the movement of the beating heart, and, with the aid of radiographic contrast agents, to view blood flow to the heart muscle as well as through blood vessels and organs. This technology is also used with a radiographic contrast agent to guide an internally threaded catheter during cardiac angioplasty, which is a minimally invasive procedure for opening clogged arteries that supply blood to the heart.

Therapeutic

Radiation therapy in cancer treatment: X-rays and other types of high-energy radiation can be used to destroy cancerous tumors and cells by damaging their DNA. The radiation dose used for treating cancer is much higher than the radiation dose used for diagnostic imaging. Therapeutic radiation can come from a machine outside of the body or from a radioactive material that is placed in the body, inside or near tumor cells, or injected into the blood stream.
Click here for more information on radiation therapy for cancer.

Are there risks?

When used appropriately, the diagnostic benefits of x-ray scans significantly outweigh the risks. X-ray scans can diagnose possibly life-threatening conditions such as blocked blood vessels, bone cancer, and infections. However, x-rays produce ionizing radiation—a form of radiation that has the potential to harm living tissue. This is a risk that increases with the number of exposures added up over the life of the individual. However, the risk of developing cancer from radiation exposure is generally small.

An x-ray in a pregnant woman poses no known risks to the baby if the area of the body being imaged isn’t the abdomen or pelvis. In general, if imaging of the abdomen and pelvis is needed, doctors prefer to use exams that do not use radiation, such as MRI or ultrasound. However, if neither of those can provide the answers needed, or there is an emergency or other time constraint, an x-ray may be an acceptable alternative imaging option.

Children are more sensitive to ionizing radiation and have a longer life expectancy and, thus, a higher relative risk for developing cancer than adults. Parents may want to ask the technologist or doctor if their machine settings have been adjusted for children.

Medical imaging has led to improvements in the diagnosis and treatment of numerous medical conditions in children and adults.

There are many types - or modalities - of medical imaging procedures, each of which uses different technologies and techniques. Computed tomography (CT), fluoroscopy, and radiography ("conventional X-ray" including mammography) all use ionizing radiation to generate images of the body. Ionizing radiation is a form of radiation that has enough energy to potentially cause damage to DNA and may elevate a person’s lifetime risk of developing cancer.

CT, radiography, and fluoroscopy all work on the same basic principle: an X-ray beam is passed through the body where a portion of the X-rays are either absorbed or scattered by the internal structures, and the remaining X-ray pattern is transmitted to a detector (e.g., film or a computer screen) for recording or further processing by a computer. These exams differ in their purpose:

• Radiography - a single image is recorded for later evaluation. Mammography is a special type of radiography to image the internal structures of breasts.
• Fluoroscopy - a continuous X-ray image is displayed on a monitor, allowing for real-time monitoring of a procedure or passage of a contrast agent (“dye”) through the body. Fluoroscopy can result in relatively high radiation doses, especially for complex interventional procedures (such as placing stents or other devices inside the body) which require fluoroscopy be administered for a long period of time.
• CT - many X-ray images are recorded as the detector moves around the patient's body. A computer reconstructs all the individual images into cross-sectional images or “slices” of internal organs and tissues. A CT exam involves a higher radiation dose than conventional radiography because the CT image is reconstructed from many individual X-ray projections.

Benefits of X-Ray

The discovery of X-rays and the invention of CT represented major advances in medicine. X-ray imaging exams are recognized as a valuable medical tool for a wide variety of examinations and procedures. They are used to:

• noninvasively and painlessly help to diagnose disease and monitor therapy;
• support medical and surgical treatment planning; and
• guide medical personnel as they insert catheters, stents, or other devices inside the body, treat tumors, or remove blood clots or other blockages.

Risks of X-Ray

As in many aspects of medicine, there are risks associated with the use of X-ray imaging, which uses ionizing radiation to generate images of the body. Ionizing radiation is a form of radiation that has enough energy to potentially cause damage to DNA. Risks from exposure to ionizing radiation include:

• a small increase in the possibility that a person exposed to X-rays will develop cancer later in life. (General information for patients and health care providers on cancer detection and treatment is available from the National Cancer Institute.)
• tissue effects such as cataracts, skin reddening, and hair loss, which occur at relatively high levels of radiation exposure and are rare for many types of imaging exams. For example, the typical use of a CT scanner or conventional radiography equipment should not result in tissue effects, but the dose to the skin from some long, complex interventional fluoroscopy procedures might, in some circumstances, be high enough to result in such effects.

Another risk of X-ray imaging is possible reactions associated with an intravenously injected contrast agent, or “dye”, that is sometimes used to improve visualization.

The risk of developing cancer from medical imaging radiation exposure is generally very small, and it depends on:

• radiation dose - The lifetime risk of cancer increases the larger the dose and the more X-ray exams a patient undergoes.
• patient’s age - The lifetime risk of cancer is larger for a patient who receives X-rays at a younger age than for one who receives them at an older age.
• patient’s sex - Women are at a somewhat higher lifetime risk than men for developing radiation-associated cancer after receiving the same exposures at the same ages.
• body region - Some organs are more radiosensitive than others.

The above statements are generalizations based on scientific analyses of large population data sets, such as survivors exposed to radiation from the atomic bomb. One of the reports of such analyses is Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 (Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council). While specific individuals or cases may not fit into such generalizations, they are still useful in developing an overall approach to medical imaging radiation safety by identifying at-risk populations or higher-risk procedures.

Because radiation risks are dependent on exposure to radiation, an awareness of the typical radiation exposures involved in different imaging exams is useful for communication between the physician and patient.

The medical community has emphasized radiation dose reduction in CT because of the relatively high radiation dose for CT exams (as compared to radiography) and their increased use, as reported in the National Council on Radiation Protection and Measurements (NCRP) Report No. 160. Because tissue effects are extremely rare for typical use of many X-ray imaging devices (including CT), the primary radiation risk concern for most imaging studies is cancer; however, the long exposure times needed for complex interventional fluoroscopy exams and resulting high skin doses may result in tissue effects, even when the equipment is used appropriately. For more information about risks associated with particular types of X-ray imaging studies, please see the CT, Fluoroscopy, Radiography, and Mammography web pages.

Balancing benefits and risks

While the benefit of a clinically appropriate X-ray imaging exam generally far outweighs the risk, efforts should be made to minimize this risk by reducing unnecessary exposure to ionizing radiation. To help reduce risk to the patient, all exams using ionizing radiation should be performed only when necessary to answer a medical question, treat a disease, or guide a procedure. If there is a medical need for a particular imaging procedure and other exams using no or less radiation are less appropriate, then the benefits exceed the risks, and radiation risk considerations should not influence the physician’s decision to perform the study or the patient's decision to have the procedure. However, the "As Low as Reasonably Achievable" (ALARA) principle should always be followed when choosing equipment settings to minimize radiation exposure to the patient.

Patient factors are important to consider in this balance of benefits and risks. For example:

• Because younger patients are more sensitive to radiation, special care should be taken in reducing radiation exposure to pediatric patients for all types of X-ray imaging exams (see the Pediatric X-ray Imaging webpage).
• Special care should also be taken in imaging pregnant patients due to possible effects of radiation exposure to the developing fetus.
• The benefit of possible disease detection should be carefully balanced against the risks of an imaging screening study on healthy, asymptomatic patients (more information on CT screening is available on the CT webpage).

X-ray imaging (CT, fluoroscopy, and radiography) exams should be performed only after careful consideration of the patient's health needs. They should be performed only when the referring physician judges them to be necessary to answer a clinical question or to guide treatment of a disease. The clinical benefit of a medically appropriate X-ray imaging exam outweighs the small radiation risk. However, efforts should be made to help minimize this risk.

Questions to ask your health care provider

Patients and parents of children undergoing X-ray imaging exams should be well informed and prepared by:

• Keeping track of medical-imaging histories as part of a discussion with the referring physician when a new exam is recommended.
• Informing their physician if they are pregnant or think they might be pregnant.
• Asking the referring physician about the benefits and risks of imaging procedures, such as:
  • How will the results of the exam be used to evaluate my condition or guide my treatment (or that of my child)?
  • Are there alternative exams that do not use ionizing radiation that are equally useful?
• Asking the imaging facility:
  • If it uses techniques to reduce radiation dose, especially to sensitive populations such as children.
  • About any additional steps that may be necessary to perform the imaging study (e.g., administration of oral or intravenous contrast agent to improve visualization, sedation, or advanced preparation).
  • If the facility is accredited. (Accreditation may only be available for specific types of X-ray imaging such as CT.)

Description

Medical radiography is a broad term that covers several types of studies that require the visualization of the internal parts of the body using x-ray techniques. For the purposes of this page radiography means a technique for generating and recording an x-ray pattern for the purpose of providing the user with a static image(s) after termination of the exposure. It is differentiated from fluoroscopy, mammography, and computed tomography which are discussed elsewhere. Radiography may also be used during the planning of radiation therapy treatment.

It is used to diagnose or treat patients by recording images of the internal structure of the body to assess the presence or absence of disease, foreign objects, and structural damage or anomaly.

During a radiographic procedure, an x-ray beam is passed through the body. A portion of the x-rays are absorbed or scattered by the internal structure and the remaining x-ray pattern is transmitted to a detector so that an image may be recorded for later evaluation. The recoding of the pattern may occur on film or through electronic means.

Uses

Radiography is used in many types of examinations and procedures where a record of a static image is desired. Some examples include

• Dental examination
• Verification of correct placement of surgical markers prior to invasive procedures
• Mammography
• Orthopedic evaluations
• Spot film or static recording during fluoroscopy
• Chiropractic examinations

Risks/Benefits

Radiography is a type of x-ray procedure, and it carries the same types of risks as other x-ray procedures. The radiation dose the patient receives varies depending on the individual procedure, but is generally less than that received during fluoroscopy and computed tomography procedures.

The major risks associated with radiography are the small possibilities of

• developing a radiation-induced cancer or cataracts some time later in life, and
• causing a disturbance in the growth or development of an embryo or fetus (teratogenic defect) when performed on a pregnant patient or one of childbearing age.

When an individual has a medical need, the benefit of radiography far exceeds the small cancer risk associated with the procedure. Even when radiography is medically necessary, it should use the lowest possible exposure and the minimum number of images. In most cases many of the possible risks can be reduced or eliminated with proper shielding.

Description

Computed tomography (CT), sometimes called "computerized tomography" or "computed axial tomography" (CAT), is a noninvasive medical examination or procedure that uses specialized X-ray equipment to produce cross-sectional images of the body. Each cross-sectional image represents a “slice” of the person being imaged, like the slices in a loaf of bread. These cross-sectional images are used for a variety of diagnostic and therapeutic purposes.

CT scans can be performed on every region of the body for a variety of reasons (e.g., diagnostic, treatment planning, interventional, or screening). Most CT scans are performed as outpatient procedures.

How a CT system works:

• A motorized table moves the patient through a circular opening in the CT imaging system.
• While the patient is inside the opening, an X-ray source and a detector assembly within the system rotate around the patient. A single rotation typically takes a second or less. During rotation the X-ray source produces a narrow, fan-shaped beam of X-rays that passes through a section of the patient's body.
• Detectors in rows opposite the X-ray source register the X-rays that pass through the patient's body as a snapshot in the process of creating an image. Many different "snapshots" (at many angles through the patient) are collected during one complete rotation.
• For each rotation of the X-ray source and detector assembly, the image data are sent to a computer to reconstruct all of the individual "snapshots" into one or multiple cross-sectional images (slices) of the internal organs and tissues.

CT images of internal organs, bones, soft tissue, and blood vessels provide greater clarity and more details than conventional X-ray images, such as a chest X-Ray (see Figures 3 and 4).

Uses

CT is a valuable medical tool that can help a physician:

• Diagnose disease, trauma or abnormality
• Plan and guide interventional or therapeutic procedures
• Monitor the effectiveness of therapy (e.g., cancer treatment)

Benefits/Risks

When used appropriately, the benefits of a CT scan far exceed the risks. CT scans can provide detailed information to diagnose, plan treatment for, and evaluate many conditions in adults and children. Additionally, the detailed images provided by CT scans may eliminate the need for exploratory surgery.

Concerns about CT scans include the risks from exposure to ionizing radiation and possible reactions to the intravenous contrast agent, or dye, which may be used to improve visualization. The exposure to ionizing radiation may cause a small increase in a person’s lifetime risk of developing cancer. Exposure to ionizing radiation is of particular concern in pediatric patients because the cancer risk per unit dose of ionizing radiation is higher for younger patients than adults, and younger patients have a longer lifetime for the effects of radiation exposure to manifest as cancer.

However, in children and adults, the risk from a medically necessary imaging exam is quite small when compared to the benefit of accurate diagnosis or intervention. It is especially important to make sure that CT scans in children are performed with appropriate exposure factors, as use of exposure settings designed for adults can result in a larger radiation dose than necessary to produce a useful image for a pediatric patient.

Information for Patients and Parents

If a physician recommends a CT scan for you or your child, the FDA encourages you to discuss the benefits and risks of the CT scan, as well as any past X-ray procedures you or your child have had, with your physician. A CT scan should always be performed if it is medically necessary and other exams using no or less radiation are unsuitable. At this time, the FDA does not see a benefit to whole-body scanning of individuals without symptoms.

Description

Cone-beam computed tomography systems (CBCT) are a variation of traditional computed tomography (CT) systems. The CBCT systems used by dental professionals rotate around the patient, capturing data using a cone-shaped X-ray beam. These data are used to reconstruct a three-dimensional (3D) image of the following regions of the patient’s anatomy: dental (teeth); oral and maxillofacial region (mouth, jaw, and neck); and ears, nose, and throat (“ENT”).

Cone-beam computed tomography system.

Uses

Dental CBCT systems have been sold in the United States since the early 2000s and are increasingly used by radiologists and dental professionals for various clinical applications including dental implant planning, visualization of abnormal teeth, evaluation of the jaws and face, cleft palate assessment, diagnosis of dental caries (cavities), endodontic (root canal) diagnosis, and diagnosis of dental trauma.

Benefits/Risks

X-ray imaging, including dental CBCT, provides a fast, non-invasive way of answering a number of clinical questions. Dental CBCT images provide three-dimensional (3-D) information, rather than the two-dimensional (2-D) information provided by a conventional X-ray image. This may help with the diagnosis, treatment planning and evaluation of certain conditions.

Although the radiation doses from dental CBCT exams are generally lower than other CT exams, dental CBCT exams typically deliver more radiation than conventional dental X-ray exams. Concerns about radiation exposure are greater for younger patients because they are more sensitive to radiation (i.e., estimates of their lifetime risk for cancer incidence and mortality per unit dose of ionizing radiation are higher) and they have a longer lifetime for ill effects to develop.

The FDA has launched a pediatric X-ray imaging website that provides specific recommendations for parents and health care providers to help reduce unnecessary radiation exposure to children. The FDA’s Center for Devices and Radiological Health defines the ages of the pediatric population as birth through 21 years.

Information for Patients and Parents

The American Dental Association (ADA) and the FDA recommend that clinicians perform dental X-ray examinations, including dental CBCT, only when necessary for the diagnosis or treatment of disease. The clinical benefit of a medically appropriate X-ray imaging exam outweighs the small radiation risk. However, efforts should be made to help minimize this risk.

The FDA also recommends that for all X-ray imaging procedures, including dental CBCT, patients and parents of children should:

• Talk with their health care provider.
  • Review the benefits and risks of the procedure before it is performed.
  • Discuss if the imaging exam is necessary and if there are equally useful alternative exams that use no or less ionizing radiation.
  • Ask if the facility uses radiation reduction techniques such as size-based protocols for children.
• Keep a record of your family’s medical-imaging histories.

Information for Dental Professionals

Dental CBCT should be performed only when necessary to provide clinical information that cannot be provided using other imaging modalities.

As stated in FDA’s Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging, the FDA recommends imaging professionals follow the principles of justification and optimization in the protection of patients undergoing radiological examinations. The FDA recommends that dental professionals:

• Discuss the rationale for the examination with the patient and/or parent to ensure a clear understanding of benefits and risks.
• Reduce the number of inappropriate referrals (i.e., justify X-ray imaging exams) by:
  • determining if the examination is needed to answer a clinical question,
  • considering alternate exams that use less or no radiation exposure, and
  • reviewing the patient's medical imaging history to avoid duplicate exams.
• Use exposure settings for dental CBCT exams that are optimized to provide the lowest radiation dose that yields an image quality adequate for diagnosis (i.e., radiation doses should be "As Low as Reasonably Achievable"). The technique factors used should be chosen based on the clinical indication, patient size, and anatomical area scanned, and the equipment should be properly maintained and tested.

Fluoroscopy is a type of medical imaging that shows a continuous X-ray image on a monitor, much like an X-ray movie. During a fluoroscopy procedure, an X-ray beam is passed through the body. The image is transmitted to a monitor so the movement of a body part or of an instrument or contrast agent (“X-ray dye”) through the body can be seen in detail.

Benefits/Risks

Fluoroscopy is used in a wide variety of examinations and procedures to diagnose or treat patients. Some examples are:

• Barium X-rays and enemas (to view the gastrointestinal tract)
• Catheter insertion and manipulation (to direct the movement of a catheter through blood vessels, bile ducts or the urinary system)
• Placement of devices within the body, such as stents (to open narrowed or blocked blood vessels)
• Angiograms (to visualize blood vessels and organs)
• Orthopedic surgery (to guide joint replacements and treatment of fractures)

Fluoroscopy carries some risks, as do other X-ray procedures. The radiation dose the patient receives varies depending on the individual procedure. Fluoroscopy can result in relatively high radiation doses, especially for complex interventional procedures (such as placing stents or other devices inside the body) which require fluoroscopy be administered for a long period of time. Radiation-related risks associated with fluoroscopy include:

• radiation-induced injuries to the skin and underlying tissues (“burns”), which occur shortly after the exposure, and
• radiation-induced cancers, which may occur some time later in life.

The probability that a person will experience these effects from a fluoroscopic procedure is statistically very small. Therefore, if the procedure is medically needed, the radiation risks are outweighed by the benefit to the patient. In fact, the radiation risk is usually far less than other risks not associated with radiation, such as anesthesia or sedation, or risks from the treatment itself. To minimize the radiation risk, fluoroscopy should always be performed with the lowest acceptable exposure for the shortest time necessary.

Information for Patients

Fluoroscopy procedures are performed to help diagnose disease, or to guide physicians during certain treatment procedures. Some fluoroscopy procedures may be performed as outpatient procedures while the patient is awake – for example, upper gastrointestinal series to examine the esophagus, stomach and small intestine, or a barium enema to examine the colon.

Other procedures are performed as same-day hospital procedures or sometimes as inpatient procedures, typically while the patient is sedated – for example, cardiac catheterization to examine the heart and the coronary arteries that supply blood to the heart muscle. Still other fluoroscopy procedures may be performed under general anesthesia during surgery – for example to help align and fix fractured bones.

The clinical benefit of a medically appropriate X-ray imaging exam outweighs the small radiation risk.

Description

Mammography is a type of medical imaging that uses x-rays to capture images (mammograms) of the internal structures of the breasts. Quality mammography can help detect breast cancer in its earliest, most treatable stages; when it is too small to be felt or detected by any other method.

Procedures

The two types of imaging currently used for mammography are

• Screen-film mammography where x-rays are beamed through the breast to a cassette containing a screen and film that must be developed. The image is commonly referred to as a mammogram.

• Full field digital mammography where x-rays are beamed through the breast to an image receptor. A scanner converts the information to a digital picture which is sent to a digital monitor and/or a printer.

Risks/Benefits

Mammography uses x-rays to produce an image of the breast, and the patient is exposed to a small dose of radiation. The Mammography Quality Standards Act (MQSA) established baseline standards for radiation dose, personnel, equipment, and image quality.

The benefits of mammography in detecting breast cancer at an early stage outweigh the risks of radiation exposure. In some cases, early detection of a breast lump may mean that chemotherapy is unnecessary.

From broken bones to life threatening illnesses, x-rays help doctors find and treat medical conditions. Specially trained doctors can read these images to diagnose medical conditions or injuries. Different imaging procedures expose patients to different amounts of radiation. For the average American, medical x-rays are their single largest source of man-made radiation exposure.

About Radiation and Medical X-rays

Medical x-rays are used to help doctors see what is happening inside the body. X-rays pass through objects, including internal organs, body tissue and clothing. The x-rays project a picture onto film or send a digital image to a computer. Bones appear white on x-ray images because denser objects absorb more radiation. Less dense parts of the body, such as skin and muscles, remain dark on x-ray images because the radiation passes through them. A medical x-ray produces a picture that can help find broken bones, tumors and foreign objects in the body. X-rays are also used in other types of examinations and procedures, including CT scans, mammograms and fluoroscopy. Medical x-rays, dental x-rays, and mammograms use relatively low amounts of radiation. CT scans and fluoroscopic procedures result in higher radiation doses due to the need for multiple images and/or a longer exposure time. However, medical imaging is a very powerful and valuable technique that can provide important and lifesaving information.

Dental X-rays

Dental x-rays are pictures of your teeth from the crown to the roots. They allow the dentist to see inside and between your teeth and check the overall condition of the bones of your jaw and face. During a dental x-ray, radiation passes through your cheek and gums and creates an image using the special x-ray film clamped between your teeth. Some x-ray machines create a digital image instead of using film. Conventional dental x-rays use a small amount of radiation to take the pictures. More detailed images may be needed in certain circumstances, such as planning for orthodontics or dental implants. In these cases, cone beam computed tomography may be used, which results in higher doses than other dental examinations (for more information, see CT Scans below).

Mammography

A mammogram is an x-ray picture of breast tissue that is used to detect breast cancer. There are two kinds of mammograms, screening and diagnostic. Screening mammograms are used to check healthy women with no signs of disease. Screening mammograms use very small doses of x-ray radiation. Diagnostic mammograms are helpful when there are some symptoms of breast cancer. Diagnostic mammograms often include more than a single x-ray. X-rays from multiple angles are needed, therefore patients who have diagnostic mammograms are exposed to more radiation. The risk of not having a needed mammogram is greater than the risks from radiation. Mammograms are a valuable tool in the early detection of breast cancer.

CT Scans

Computed tomography scans (also known as CT scans, CAT scans, or computed axial tomography scans) are x-ray procedures that create cross-sectional views and three-dimensional images of a patient's internal organs. When a person has a CT scan, many x-rays are taken at nearly the same time. CT scans create very clear images of internal organs. These detailed images help doctors diagnose problems inside the body, such as tumors or organ damage. CT scans can also provide surgeons a map of the inside of the patient that they can follow when operating. When a person has a CT scan, they are being exposed to up to several hundred times more radiation than a conventional x-ray. Like any medical test, the beneficial information gained from a CT scan should outweigh the risk of radiation exposure from the test performed. When needed, CT scans are a very powerful and valuable technique that can provide important and lifesaving information.

Fluoroscopy

Fluoroscopy uses x-rays to show movement in real-time. It can show the movement of a body part, like the beating of a heart, or the path taken by a medical instrument or dye (contrast agent) as it moves through the body. Unlike conventional x-rays, fluoroscopy uses an intermittent, pulsed x-ray beam that is passed through the body. The images are sent to a monitor where doctors can see the body part and its motion in real-time. The total radiation exposure from x-rays depends on the length of time of the fluoroscopic procedure and how often the x-ray beam is used. Fluoroscopy is used in many types of examinations and procedures, including:

• Viewing movement of materials through the stomach and intestines
• Directing the placement of a catheter during heart surgery
• Visualizing blood flow to organs
• Helping doctors properly set broken bones

What You Can Do

• Ask your doctor how an x-ray will help. How will an x-ray, CT scan, or fluoroscopy procedure help find out what is wrong or determine treatment? Ask if there are other test that might use less or no radiation.
• Don't refuse an x-ray that is needed. If your healthcare provider explains why an x-ray is medically needed, then do not refuse the test. The risk of not having a needed x-ray is greater than the risk from radiation.
• Don't insist on an x-ray. If your doctor tells you that an x-ray is not needed, then do not demand one.
• Tell the x-ray technologist if you are, or might be, pregnant. This will allow the doctor or technician to take steps to limit radiation to the fetus.
• Ask if a protective shield can be used. If you or your children are getting an x-ray, ask whether a lead apron or other shield is appropriate.
• Ask your dentist about the type of x-ray film used. Ask if your dentist uses the faster E or F speed film for x-rays. It costs about the same as the conventional D speed film and offers similar quality images with a lower radiation dose. Many dentists now use digital x-ray imaging instead of film, which further reduces radiation dose as long as image retakes are minimized.
• Know your x-ray history. Keep a list of your imaging records, including dental x-rays. When an x-ray is taken, fill out the card with the date and type of exam, referring doctor, and the name and address of where the images are kept. Maintaining and referring to this record can help you avoid unnecessary duplicate x-rays. Keep a record card for everyone in your family.

Ionizing radiation  has sufficient energy to affect the atoms in living cells and thereby damage their genetic material (DNA). Fortunately, the cells in our bodies are extremely efficient at repairing this damage. However, if the damage is not repaired correctly, a cell may die or eventually become cancerous.

Exposure to very high levels of radiation, such as being close to an atomic blast, can cause acute health effects such as skin burns and acute radiation syndrome (“radiation sickness"). It can also result in long-term health effects such as cancer and cardiovascular disease. Exposure to low levels of radiation encountered in the environment does not cause immediate health effects, but is a minor contributor to our overall cancer risk.

Acute Radiation Syndrome from Large Exposures

A very high level of radiation exposure delivered over a short period of time can cause symptoms such as nausea and vomiting within hours and can sometimes result in death over the following days or weeks. This is known as acute radiation syndrome, commonly known as “radiation sickness.”

It takes a very high radiation exposure to cause acute radiation syndrome—more than 0.75  gray  (75  rad)  in a short time span (minutes to hours). This level of radiation would be like getting the radiation from 18,000 chest x-rays distributed over your entire body in this short period. Acute radiation syndrome is rare, and comes from extreme events like a nuclear explosion or accidental handling or rupture of a highly radioactive source.

Radiation Exposure and Cancer Risk

Exposure to low-levels of radiation does not cause immediate health effects, but can cause a small increase in the  risk  of cancer over a lifetime. There are studies that keep track of groups of people who have been exposed to radiation, including atomic bomb survivors and radiation industry workers. These studies show that radiation exposure increases the chance of getting cancer, and the risk increases as the dose increases: the higher the dose, the greater the risk. Conversely, cancer risk from radiation exposure declines as the dose falls: the lower the dose, the lower the risk.

Radiation doses are commonly expressed in millisieverts (international units) or  rem  (U.S. units). A dose can be determined from a one-time radiation exposure, or from accumulated exposures over time. About 99 percent of individuals would not get cancer as a result of a one-time uniform whole-body exposure of 100 millisieverts (10 rem) or lower. At this dose, it would be extremely difficult to identify an excess in cancers caused by radiation when about 40 percent of men and women in the U.S. will be diagnosed with cancer at some point during their lifetime.

Risks that are low for an individual could still result in unacceptable numbers of additional cancers in a large population over time. For example, in a population of one million people, an average one-percent increase in lifetime cancer risk for individuals could result in 10,000 additional cancers. The EPA sets regulatory limits and recommends emergency response guidelines well below 100 millisieverts (10 rem) to protect the U.S. population, including sensitive groups such as children, from increased cancer risks from accumulated radiation dose over a lifetime.

Exposure Pathways

Understanding the type of radiation received, the way a person is exposed (external vs. internal), and for how long a person is exposed are all important in estimating health effects.

The risk from exposure to a particular  radionuclide  depends on:

• The energy of the radiation it emits.
• The type of radiation (alpha, beta, gamma, x-rays).
• Its activity (how often it emits radiation).
• Whether exposure is external or internal:
  • External exposure is when the radioactive source is outside of your body. X-rays and gamma rays can pass through your body, depositing energy as they go.
  • Internal exposure is when radioactive material gets inside the body by eating, drinking, breathing or injection (from certain medical procedures). Radionuclides may pose a serious health threat if significant quantities are inhaled or ingested.
• The rate at which the body metabolizes and eliminates the radionuclide following ingestion or inhalation.
• Where the radionuclide concentrates in the body and how long it stays there.

Sensitive Populations

Children and fetuses are especially sensitive to radiation exposure. The cells in children and fetuses divide rapidly, providing more opportunity for radiation to disrupt the process and cause cell damage. EPA considers differences in sensitivity due to age and sex when revising radiation protection standards.

Most people have had an X-ray taken at some time during their lives — perhaps checking for a possible broken bone or during a visit to the dentist. X-ray exams provide important information to physicians about how to treat their patients. However, X-rays use ionizing radiation, and these imaging exams must be carefully and judiciously used on pediatric patients.

While the level of risk from the radiation associated with X-rays is small, especially when compared with the benefits of an accurate diagnosis, health care professionals must be especially sensitive to their appropriate use in children. Pediatric patients generally require less radiation than adults to obtain a quality image from an X-ray exam, so doctors must take extra care to “child size” the radiation dose.

FDA’s Role

The FDA's Center for Devices and Radiological Health (CDRH) regulates medical imaging devices. Among its responsibilities is keeping consumers and health care professionals informed about the importance of minimizing unnecessary radiation exposure during medical procedures.

The level of ionizing radiation from X-ray imaging is generally very low, but can contribute to an increased risk of cancer. Because children have longer expected lifetimes ahead of them for potential effects to appear and the risk for cancer is not fully understood, it’s important to use the lowest radiation dose necessary to provide a diagnostic exam.

The FDA is committed to protecting the health of children by providing guidance to manufacturers and users of imaging devices to help lower the exposure to radiation from X-ray exams. The FDA has regulatory oversight of X-ray imaging devices and the companies that make them. To better address radiation safety concerns, the FDA has been encouraging both equipment improvements and better user information.

The FDA also promotes the adoption of improved radiation safety guidelines by professional organizations for both facilities and personnel.

Recommendations

In a new guidance FDA recommends that medical X-ray imaging exams be optimized to use the lowest radiation dose needed. These exams, which include computed tomography (CT), fluoroscopy, dental, and conventional X-rays, should be performed on children and younger patients only when the health care provider believes they are necessary to answer a clinical question or to guide treatment.

The FDA defines the pediatric population as birth through 21 years old. However, the optimization of image quality and radiation dose in X-ray imaging depends more on a patient’s size than their age. Smaller patients require less radiation to obtain a medically useful image. Technically, the patient’s body thickness (the distance an X-ray travels through the body to create the image) is the most important consideration when “child-sizing” an image protocol.

Unnecessary radiation exposure during medical procedures should be avoided. However, X-rays and CT scans should never be withheld from a child or adult who has a medical condition where the exam could provide important health care information that may aid in the diagnosis or treatment of a serious or even life-threatening illness.

What Parents Can Do

The FDA encourages parents and caregivers to talk to their child’s health care provider about X-rays and suggests:

• Keeping track of their child's medical-imaging histories
• Asking the referring physician about the benefits and risks of imaging procedures, such as: How will the exam improve my child's health care? Are there alternative exams to X-rays that are equally useful?
• Asking the imaging facility: How does the facility use reduced radiation techniques for children? Is there any advanced preparation necessary? Report any adverse events to the FDA.

The Role of Health Care Professionals

Health care professionals are responsible for ensuring there is justification for all X-ray imaging exams performed on pediatric patients. They should also consider whether another type of imaging exam that does not expose the patient to ionizing radiation, such as ultrasound or magnetic resonance imaging, could be used to obtain the same result.

The FDA encourages health care professionals and hospital administrators to refer to guidelines and instructions provided at the agency’s Pediatric X-ray Imaging web portal.

For thousands of years, fear of the dead and legal sanctions limited the ability of anatomists and physicians to study the internal structures of the human body. An inability to control bleeding, infection, and pain made surgeries infrequent, and those that were performed—such as wound suturing, amputations, tooth and tumor removals, skull drilling, and cesarean births—did not greatly advance knowledge about internal anatomy. Theories about the function of the body and about disease were therefore largely based on external observations and imagination. During the fourteenth and fifteenth centuries, however, the detailed anatomical drawings of Italian artist and anatomist Leonardo da Vinci and Flemish anatomist Andreas Vesalius were published, and interest in human anatomy began to increase. Medical schools began to teach anatomy using human dissection; although some resorted to grave robbing to obtain corpses. Laws were eventually passed that enabled students to dissect the corpses of criminals and those who donated their bodies for research. Still, it was not until the late nineteenth century that medical researchers discovered non-surgical methods to look inside the living body.

X-Rays

German physicist Wilhelm Röntgen (1845–1923) was experimenting with electrical current when he discovered that a mysterious and invisible “ray” would pass through his flesh but leave an outline of his bones on a screen coated with a metal compound. In 1895, Röntgen made the first durable record of the internal parts of a living human: an “X-ray” image (as it came to be called) of his wife’s hand. Scientists around the world quickly began their own experiments with X-rays, and by 1900, X-rays were widely used to detect a variety of injuries and diseases. In 1901, Röntgen was awarded the first Nobel Prize for physics for his work in this field.

The X-ray is a form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing gases. As they are used in medicine, X-rays are emitted from an X-ray machine and directed toward a specially treated metallic plate placed behind the patient’s body. The beam of radiation results in darkening of the X-ray plate. X-rays are slightly impeded by soft tissues, which show up as gray on the X-ray plate, whereas hard tissues, such as bone, largely block the rays, producing a light-toned “shadow.” Thus, X-rays are best used to visualize hard body structures such as teeth and bones (Figure). Like many forms of high energy radiation, however, X-rays are capable of damaging cells and initiating changes that can lead to cancer. This danger of excessive exposure to X-rays was not fully appreciated for many years after their widespread use.

High energy electromagnetic radiation allows the internal structures of the body, such as bones, to be seen in X-rays like these. (credit: Trace Meek/flickr)

Refinements and enhancements of X-ray techniques have continued throughout the twentieth and twenty-first centuries. Although often supplanted by more sophisticated imaging techniques, the X-ray remains a “workhorse” in medical imaging, especially for viewing fractures and for dentistry. The disadvantage of irradiation to the patient and the operator is now attenuated by proper shielding and by limiting exposure.

Modern Medical Imaging

X-rays can depict a two-dimensional image of a body region, and only from a single angle. In contrast, more recent medical imaging technologies produce data that is integrated and analyzed by computers to produce three-dimensional images or images that reveal aspects of body functioning.

Computed Tomography

Tomography refers to imaging by sections. Computed tomography (CT) is a noninvasive imaging technique that uses computers to analyze several cross-sectional X-rays in order to reveal minute details about structures in the body (Figurea). The technique was invented in the 1970s and is based on the principle that, as X-rays pass through the body, they are absorbed or reflected at different levels. In the technique, a patient lies on a motorized platform while a computerized axial tomography (CAT) scanner rotates 360 degrees around the patient, taking X-ray images. A computer combines these images into a two-dimensional view of the scanned area, or “slice.”

Medical Imaging Techniques

(a) The results of a CT scan of the head are shown as successive transverse sections. (b) An MRI machine generates a magnetic field around a patient. (c) PET scans use radiopharmaceuticals to create images of active blood flow and physiologic activity of the organ or organs being targeted. (d) Ultrasound technology is used to monitor pregnancies because it is the least invasive of imaging techniques and uses no electromagnetic radiation. (credit a: Akira Ohgaki/flickr; credit b: “Digital Cate”/flickr; credit c: “Raziel”/Wikimedia Commons; credit d: “Isis”/Wikimedia Commons)

Since 1970, the development of more powerful computers and more sophisticated software has made CT scanning routine for many types of diagnostic evaluations. It is especially useful for soft tissue scanning, such as of the brain and the thoracic and abdominal viscera. Its level of detail is so precise that it can allow physicians to measure the size of a mass down to a millimeter. The main disadvantage of CT scanning is that it exposes patients to a dose of radiation many times higher than that of X-rays. In fact, children who undergo CT scans are at increased risk of developing cancer, as are adults who have multiple CT scans.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave off different signals than normal body tissue. He applied for a patent for the first MRI scanning device, which was in use clinically by the early 1980s. The early MRI scanners were crude, but advances in digital computing and electronics led to their advancement over any other technique for precise imaging, especially to discover tumors. MRI also has the major advantage of not exposing patients to radiation.

Drawbacks of MRI scans include their much higher cost, and patient discomfort with the procedure. The MRI scanner subjects the patient to such powerful electromagnets that the scan room must be shielded. The patient must be enclosed in a metal tube-like device for the duration of the scan (see Figureb), sometimes as long as thirty minutes, which can be uncomfortable and impractical for ill patients. The device is also so noisy that, even with earplugs, patients can become anxious or even fearful. These problems have been overcome somewhat with the development of “open” MRI scanning, which does not require the patient to be entirely enclosed in the metal tube. Patients with iron-containing metallic implants (internal sutures, some prosthetic devices, and so on) cannot undergo MRI scanning because it can dislodge these implants.

Functional MRIs (fMRIs), which detect the concentration of blood flow in certain parts of the body, are increasingly being used to study the activity in parts of the brain during various body activities. This has helped scientists learn more about the locations of different brain functions and more about brain abnormalities and diseases.

Positron Emission Tomography

Positron emission tomography (PET) is a medical imaging technique involving the use of so-called radiopharmaceuticals, substances that emit radiation that is short-lived and therefore relatively safe to administer to the body. Although the first PET scanner was introduced in 1961, it took 15 more years before radiopharmaceuticals were combined with the technique and revolutionized its potential. The main advantage is that PET (see Figurec) can illustrate physiologic activity—including nutrient metabolism and blood flow—of the organ or organs being targeted, whereas CT and MRI scans can only show static images. PET is widely used to diagnose a multitude of conditions, such as heart disease, the spread of cancer, certain forms of infection, brain abnormalities, bone disease, and thyroid disease.

Ultrasonography

Ultrasonography is an imaging technique that uses the transmission of high-frequency sound waves into the body to generate an echo signal that is converted by a computer into a real-time image of anatomy and physiology (see Figured). Ultrasonography is the least invasive of all imaging techniques, and it is therefore used more freely in sensitive situations such as pregnancy. The technology was first developed in the 1940s and 1950s. Ultrasonography is used to study heart function, blood flow in the neck or extremities, certain conditions such as gallbladder disease, and fetal growth and development. The main disadvantages of ultrasonography are that the image quality is heavily operator-dependent and that it is unable to penetrate bone and gas.

Review

Detailed anatomical drawings of the human body first became available in the fifteenth and sixteenth centuries; however, it was not until the end of the nineteenth century, and the discovery of X-rays, that anatomists and physicians discovered non-surgical methods to look inside a living body. Since then, many other techniques, including CT scans, MRI scans, PET scans, and ultrasonography, have been developed, providing more accurate and detailed views of the form and function of the human body.