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What Are X-Rays?

In 1895, physicist Wilhelm Roentgen was intrigued by glowing cathode tubes and decided to see what they could do. He found that the rays they emitted could pass through certain solid objects and leave a shadowy image of that object on a fluorescent screen. He was even more amazed to find that when the rays passed through body parts, such as his hand, the bones beneath the skin became clearly visible on the screen. Because he didn’t know exactly what was causing this phenomenon, he labeled the rays ” X,” which is the mathematical symbol for anything that is unknown.

Scientists today know that X-rays are a form of energy that travels in waves. X-rays can enter solid objects, where they either are absorbed or continue to pass through the object. The denser the material X-rays enter, the more they are absorbed and the less they are able to pass through.

Teeth and bone are very dense, so they absorb X-rays, but gums and cheeks are much less dense, so X-rays pass through more easily. That’s why cheeks and gums appear dark and without detail on the X-ray film, but teeth show up much lighter. And fillings, which are even denser than bone, will show up as a solid, bright white area. Dental caries (cavities) will show up on an X-ray as a darker patch in a light tooth.

How Are X-rays Used?

X-ray images, also called dental radiographs, are among the most valuable tools a dentist has for keeping your mouth and teeth healthy. By understanding what the structures of the mouth look like normally on an X-ray film, dentists can diagnose problems in the teeth and jaws. For adults, radiographs can:

  • Show areas of decay that your dentist may not be able to see with just a visual examination, such as tiny pits of decay that might occur between teeth
  • Find decay that is developing underneath an existing filling
  • Find cracks or other damage in an existing filling
  • Alert the dentist to possible bone loss associated with periodontal (gum) disease
  • Reveal problems in the root canal, such as infection or death of the nerve
  • Help your dentist plan, prepare and place tooth implants, orthodontic treatments, dentures or other dental work.
  • Reveal other abnormalities such as cysts, cancer and changes associated with metabolic and systemic diseases (such as Paget’s disease and lymphoma.)
  • For children, radiographs are used to watch for decay and to monitor tooth growth and development. Dentists will use periodic X-rays to see whether a space in the mouth to fit all the new teeth, whether primary teeth are being lost quickly enough to allow permanent teeth to erupt properly, whether extra (supernumerary) teeth are developing or whether any teeth are impacted (unable to emerge through the gums). Often, major problems can be prevented by catching small developmental problems early and then making accommodations.

How Often Should Your Teeth Be X-Rayed?

Even though no X-ray can be considered routine, many people require X-rays on a regular basis so that their dental condition can be monitored. Exactly how often this happens will depend on your medical and dental history and current condition. Some people may need X-rays as often as every six months. For others, X-rays may not be needed for as long as two years. In patients with no recent dental or gum disease and who visit the dentist regularly for check-ups, X-rays may be taken only every five years or so.

Who needs more frequent or regular radiographs? They include:

  • Children Many children need X-rays every six months to one year, depending on age, because they are highly likely to develop caries. X-rays also help monitor tooth development.
  • Adults with extensive restoration work, including fillings All the conditions that helped create the caries to begin with continue, making it necessary to check for decay beneath existing fillings or in new locations.
  • Anyone who drinks sugary sodas, chocolate milk or coffee or tea with sugar Even mildly sugary beverages create an environment in the mouth that’s perfect for decay, so anyone who drinks these beverages regularly will need to have more regular X-rays.
  • People with periodontal (gum) disease Periodontal treatments may need to be stepped up if there are significant or continuing signs of bone loss.
  • People who are taking medications that lead to dry mouth, also called xerostomia Saliva helps keep the acid levels (pH) in the mouth stable. In a dry mouth, the pH decreases, causing the minerals in the teeth to break down, leaving them prone to caries. Medications that can decrease saliva are those prescribed for hypertension, antidepressants, antianxiety drugs, antihistamines, diuretics, narcotics, anticonvulsants and anticholinergics.
  • People who have dry mouth because of disease, such as Sjögren’s syndrome, or because of medical treatments that damaged the salivary glands, such as radiation to the head and neck for cancer treatment.
  • Smokers, because smoking increases the risk of periodontal disease.

Types Of X-Rays

X-rays are divided into two main categories: intraoral, which means that the X-ray film is inside the mouth; and extraoral, which means that the film is outside the mouth.

Intraoral radiographs

Intraoral X-rays are the most common radiographs made. If you’re like most people who visit the dentist, you’ve had many sets of intraoral radiographs in your life and you’ll likely have many more. Because they give a high level of detail, these are the X-rays that allow dentists to find caries, look at the tooth roots, check the health of the bony area surrounding the tooth, see the status of developing teeth, and otherwise monitor good tooth health.
The various types of intraoral X-rays show different aspects of the teeth:

  • Bite-wing X-rays highlight the crowns of the teeth. On each radiograph, the upper and lower teeth in one portion of the mouth are shown, from the crown to about the level of the jaw.
  • Periapical X-rays highlight the entire tooth. On each radiograph, the teeth from either the upper or lower jaw in one portion of the mouth are shown. The difference from bitewings is that in a periapical X-ray, the whole tooth is shown, from the crown down past the end of the root to the part of the jaw where the tooth is anchored.
  • Periodically, a dentist may recommend a “full-mouth radiographic survey,” or FMX. This means that every tooth, from crown to root to supporting structures, will be X-rayed using both bitewing and periapical radiographs.
  • Occlusal X-rays are larger and highlight tooth development and placement. On each radiograph, nearly the full arch of teeth in either the upper or lower jaw is shown. These X-rays are taken with the X-ray machine either pointing straight down from near the nose (to take pictures of the upper jaw and teeth), or straight up from under the chin (to take pictures of the lower jaw and teeth).
  • Digital radiographs are one of the newest X-ray techniques around. Because it is so new and because the machines can be so expensive, your dentist may not have it yet; but watch for this process to become standard in the future. With digital radiographs, film is replaced with a flat electronic pad or sensor. The X-rays hit the pad the same way they hit the film. But instead of developing the film in a dark room, the image is electronically sent directly to a computer where the image appears on the screen. The image can then be stored on the computer or printed out. One of the great advantages of this process is that radiographs can be digitally compared to previous radiographs in a process called subtraction radiography. The computer can digitally compare the two images, subtract out everything that is the same and give a clear image of anything that is different. This means that tiny changes that may not be noticeable with the naked eye can be caught earlier and more clearly with digital-subtraction radiography. Subtraction radiography requires a specialized projection technique and additional software.

Extraoral radiograph

Extraoral X-rays are made with the film outside the mouth. These can be considered the ” big picture” X-rays. They show teeth, but their main focus is on the jaw or skull. Extraoral radiographs are used for monitoring growth and development, looking at the status of impacted teeth, examining the relationships between teeth and jaws and examining the temporomandibular joint or other bones of the face. Extraoral X-rays are less detailed than intraoral X-rays, so they are not used for detecting caries or flaws in individual teeth.

  • Panoramic radiographs show the entire mouth area all teeth on both upper and lower jaws on a single X-ray. This type of X-ray requires a special panoramic X-ray machine. The tube head that emits the X-rays circles behind the patient’s head, while the film simultaneously circles across the front. That way, the full, broad view of the jaws is captured on one film. Because the machine moves in a set path, the patient has to be positioned very carefully. And, because the beam and the film are both moving, any movement from the patient will blur the image on the screen. That’s why such care is taken to keep the patient’s head absolutely still in exactly the right position. The machines may have chin rests, forehead rests, and side head positioners, plus bite-blocks that patients will be asked to close their teeth around. All this may look and feel intimidating, but the process is very safe and often uses less radiation than intraoral radiographs.
  • Tomograms are a special type of radiograph in which the dentist can focus in on one particular layer, or slice, of anatomy while blurring out all other layers. This allows dentists to see structures that may be difficult to see with standard X-rays. For example, the temporomandibular joint can be difficult to see. The condyle that makes up part of the joint is in the middle of a dense cranial base, so it is extremely difficult to X-ray. But by using a tomography technique called a temporomandibular joint projection, a straight ” slice” that’s lined up with the condyle shows that area more clearly.
  • Cephalometric projections are X-rays taken of the entire side of the head. They are used to look at the teeth in relation to the jaw and the profile of the individual. Orthodontists use cephalometric projections to plan their treatments. They will look at the entire face to determine the best way to get the teeth aligned in the right way for that particular person, according to the size of their teeth and jaws.
  • Sialography is a way of visualizing the salivary glands on a radiograph. Soft tissues, like gums and salivary glands, can’t usually be seen on an X-ray because they are not dense enough to absorb enough X-rays to appear clearly on film. With sialography, the dentist injects a radiopaque contrast material directly into the salivary glands. This material shows up easily on film, allowing dentists to diagnose salivary gland problems, such as blockages or Sjö gren’s disease.
  • Computed tomography, or CT scanning, usually is performed in a hospital, not the dentist’s office, although a dentist may refer a patient for this test. With this process, the patient lies still in the CT machine while the X-ray beam rotates around. From the X-ray information, a computer creates a three-dimensional image of the interior structures. It is used to identify problems in the bones of the face, such as tumors or fractures.

X-Ray Safety

All types of radiation can cause damage to body cells. In very high doses, such as might be released during a nuclear reactor accident, the damage can be swift, leading to ” radiation burn” and other serious effects. People who receive large doses of radiation as part of their cancer treatment can also experience skin burns or damage to healthy body tissue near the cancer.

The X-rays used in dental and medical offices emit extremely small doses of radiation. However, cells can be damaged by many small doses of radiation that add up over time. Although the amount of radiation used in dental X-rays is very small, the effect is cumulative, so all radiation counts. That’s why experts recommend that X-rays be used judiciously and with precautions to help protect the patient from unnecessary radiation exposure. To keep exposure to X-rays low for their patients, dentists and regulatory agencies have done several things:

  • Reduced X-ray dose
    The single most important way dentists keep their patients safe from radiation is by limiting the beam to the small area being X-rayed and by reducing the amount of radiation that strays from that path. This is done by a process called collimation, in which the machine directs the X-rays through a lead-lined column and out a tiny opening at the end. So although an X-ray machine looks quite large, the X-rays are limited to a small area less than three inches in diameter as they come out of a small cone at the end. X-ray machines are well shielded and there is very little radiation exposure beyond the diameter of the primary beam.
  • Improved X-ray film
    The speed of films used for dental X-rays has been improved so less exposure is needed to get the same results.
  • Changed to using film holders
    Do you remember the days when dental patients had to hold X-ray film in their mouths with their fingers? Those days are long gone. Now, fingers have been replaced by holders that not only keep the film in place, but also help the dentist aim the X-ray machine. By using film holders, there is less chance of the film slipping or being held in the wrong place, which means that fewer repeat X-rays need to be taken.
  • Required regular X-ray machine checks and licensure
    Federal law requires that X-ray machines be checked for accuracy and safety every two years, and some states require more frequent checks. Once the machine passes the testing process, the dentist receives a license to operate the machine. If you have any doubts about the safety of the X-ray machine in your dentist’s office, feel free to ask to see a copy of the inspection license.
  • Recommended or required use of lead shields
    Before making radiographs, dentists will cover a patient from the neck to the knees with a lead-lined full-body apron. If the apron doesn’t extend up to the neck, a separate neck protector called a thyroid collar may also be used. These shields have been used for decades to help protect patients from radiation scatter. Many states now require lead shields to be used. Although this type of protection was very important in the old days of high-scatter machines, today the lead aprons offer more peace of mind than actual protection because stray radiation from modern dental X-ray machines is almost nonexistent.
  • Recommended that radiographs be made only when necessary for diagnosis and treatment There are no such things as necessary ” routine” radiographs the way there are required vaccination schedules for children. Instead, dentists make radiographs only when they think they are necessary to make an accurate dental assessment or diagnosis. This keeps the number of X-rays taken to the minimal needed for dental health.
  • Developed digital radiography
    A new system of taking X-rays, called digital radiography, reduces radiation by as much as 80 percent.

How Safe Are Dental X-Rays?

Nearly all medical procedures have risks and benefits and dental X-rays are no exception.

The benefits are well known: X-rays help dentists diagnose relatively common disorders such as cavities, periodontal disease and infections. Without this ability to see inside a tooth and beneath the gums, more disease would go unchecked, patients would experience more pain and discomfort and more teeth would be lost because proper treatment couldn’t be started in time. For these reasons, dentists consider X-rays one of their best tools in helping patients maintain good oral health.

But dental X-rays are not without risk. The risk is minuscule compared with the huge diagnostic benefit. Still, with technological advances, people are absorbing increasing amounts of radiation from their environment, so radiation should be limited whenever possible.

X-rays are a form of energy that’s absorbed in your body. This radiation has been shown to be associated with cancer and changes in chromosomes. In the head and neck, areas exposed to dental radiation, X-rays increase the risk of damage to or cancer of the lens of the eye, thyroid, salivary glands, bone marrow and skin. Although no one knows the exact effects of low-dose radiation, the type used in dental X-rays, the effects of radiation exposures are cumulative. Every little bit counts.

“Most importantly, don’t discount the risk,” says Linda Otis, D.D.S., M.S., associate professor of oral medicine and director of oral and maxillofacial radiology at the University of Pennsylvania School of Dental Medicine. “Cancers associated with radiation can take 20 years to show up. After many studies, looking at populations that had been irradiated and comparing them to populations that hadn’t, there was a small but statistically significant increase in certain types of cancers in those populations.”

Of course, Dr. Otis points out, many of the populations studied were exposed to much higher doses of radiation than is currently used for dental radiography.

On the other hand, there’s no reason to worry unnecessarily. Dentistry is making great advances in lowering the radiation dose administered. For example, in the 1930s and 1940s, the irradiating cone was huge, covering most of the head and neck. Now, the diameter of the cone is very small, less than 3 inches, so radiation is only pointed at the small spot where the X-ray film is located. Many of the critical organs that might show radiation effects, such as the breasts and gonads, are not threatened during a dental exam because they are not in the path of radiation.

How You Can Increase Safety

Most of the work to reduce the risk of X-rays is already being done. For example, frequent innovations have decreased the amount of X-ray scatter to eliminate unnecessary exposure. Plus, by law, all X-ray machines in the U.S. are checked for accuracy and safety every two years and some states require more frequent checks. You can ask your dentist for a license that certifies that the machine being used has passed inspection.

Patients can and should talk with their dentists about how often they need X-rays and why. It used to be that X-rays were given on a pre-determined schedule. Current guidelines, however, require that X-rays be given only when needed for clinical diagnosis. Of course, the number and frequency of X-rays needed varies from person to person. Children often need more frequent X-rays because their teeth are prone to cavities. People with a great number of dental fillings and other restorations also need more X-rays because decay can easily occur beneath existing fillings and do a great deal of damage before it can be detected. And people with periodontal disease will need X-rays more often to monitor the course of their disease.

Dr. Otis recommends talking with your dentist before your next set of X-rays. “Ask, “how will these X-rays influence the treatment you’re going to give me? Is there something in your clinical exam that has determined that I need this X-ray?” As a consumer, an answer I wouldn’t like would be: “It doesn’t matter, we need these X-rays to find things that we can’t see,’” Dr. Otis says. “A better answer would be, ‘I noticed that your mouth is very dry, and we know that cavities happen more frequently in people whose mouths are dry,’ …or, ‘You have some areas that I’m concerned about and I really need a radiograph to help me decide if there’s a cavity there or not.’”

Watch for developments in X-ray film, as well. Most dentists currently use D-speed film. But two newer types, E-speed and F-speed, require less radiation to develop the same quality picture. E-speed requires about half the radiation as D-speed and F-speed requires approximately one-third the radiation of D-speed. Ask your dentist what speed film is currently used. If it’s D-speed, ask if there is any way to get one of the lower radiation films instead.

“A smart consumer wants to minimize their overall dose of radiation,” Dr. Otis says. “Although you cannot, and should not, avoid X-rays altogether, it makes sense to use them with discretion, only when we need them.”

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