X-Rays

Are dental x-rays dangerous?

Some people do not want diagnostic x-rays because they have heard that the radiation is dangerous. In fact, they pose very little danger. There are currently two units used to measure the exposure of biological organisms to radiation.

These units are measures of equivalent dose. Equivalent dose units are used to compare radiation doses on different body parts on an equivalent basis because radiation does not affect different parts in the same way. Equivalent dosing units make possible the comparison of radiographs of different types and sizes in different parts of the body. They also allow comparison with exposure from natural background radiation. They allow for a more meaningful comparison between radiation sources that expose the entire body (such as natural background radiation) and those that only expose a portion of the body (such as dental vs. medical radiographs).

The first, oldest and still probably the most frequently used unit of equivalent dose in the US is called a rem. A second unit, used outside of the US is the sievert. 1 sievert = 100 rems. A rem is a large unit, (And a sievert is an even larger unit), so exposure to medical radiation is generally measured in millirems (mREM) and millisieverts (mSV).

Other units you may hear about are measures of radiation called rads and grays. These are units of absorbed dose, and are generally applied to non biological bodies. They do not take into account the differing effects of radiation on different tissues in the body. This type of measurement does not concern us in the study of dental x-rays.

The average dental x-ray delivers about 1 mREM per exposure. Thus a full mouth series of dental x rays (18 intraoral films) delivers about 18 mREM. (Note: These figures are based on the use of E-speed film. Kodak InSight, an F-Speed dental film, lets you reduce radiation exposure by up to 60 percent as compared to Kodak Ultra-Speed dental film, a D-speed product.) A panorex film delivers about 2 mREM. By comparison, the average person in the US is exposed to about 360 mREM per year just from naturally occurring background sources. By this measure, it would take approximately 20 full series of dental radiographs to equal the background radiation that the average citizen is exposed to on a yearly basis. Note that most dentists take a new full series every three to five years on average.

For a more in-depth (and more accurate) discussion of this subject, please see this page in my Course in Dental Radiography.

The average person in the US is exposed to about 360 mREM per year just from background sources, but the actual amount of background radiation received by any given person varies quite a bit depending upon that individual's lifestyle choices. Background radiation comes from outer space, the earth, natural materials (including natural foods), and even other people. For example, flying cross country exposes a person to about 3-5 mREM over and above the normal radiation he receives from outer space while simply walking outdoors for the same length of time. Cooking with natural gas exposes us to about an additional 10 mREM per year because of the naturally occurring radon gas the cooking gas contains. Living in a brick building adds an additional 10 mREM per year over and above the radiation you would receive from living in a wooden structure. Simply sleeping next to another person exposes each bed partner to an extra 2 mREM per year.

The Washington State Department of Health has set the maximum safe occupational whole body radiation exposure to 5000 mREM (5 rem)per year. The same limit holds true for other states as well (ex. New york -- see section 16.6). Finally, 5000 mREM is the federal total effective, whole body, yearly occupational dose limit. By this reckoning, it would take about 278 full mouth series of dental x-rays (18 films per survey) over the course of a year to equal one years maximum safe occupational radiation level. It would take 2500 panorex films or about 5000 individual intraoral x-rays to get to this limit. The 5000 mREM yearly limit applies to persons who are routinely exposed to ionizing radiation in the course of their jobs. This is not to suggest that a member of the general public should routinely expect to be exposed to 5000 mREM per year of diagnostic x-rays, but it is an indication that the benefits of routine yearly diagnostic x-rays far, far outweigh the dangers posed by the radiation.

Dental x-rays are aimed in a tight beam at a small spot on the face. The only structures that receive the full dose of x-radiation are the tissues in the direct line of fire. The rest of the body receives only the radiation that is scattered off of the structures in the line of fire. (Much less radiation scatters from an object in an x-ray beam than from an object in a beam of ordinary light due to the difference in the nature of the respective radiation sources. Click here for a better understanding of scatter radiation.) Furthermore, the tissues at which dental x-rays are aimed are much less prone to injury from x-radiation than are tissues in other parts of the body, such as the intestinal lining or reproductive organs and other constantly reproducing tissues. The newest unit of measurement, the milisievert was designed to take this factor into account.

The use of digital radiography further reduces the exposure to about one third of the values in the chart below. This would mean that it would take 50 full series of x-rays (taken with a digital sensor) to equal the amount of radiation the average citizen picks up from naturally occurring background sources each year---that means 950 intraoral films:

The table below is adapted and updated from the website of the American Dental Association which in turn took its information from Frederiksen NL. X-Rays: What is the Risk? Texas Dental Journal. 1995;112(2):68-72, It is quite helpful in comparing the amount of radiation received from dental x-rays to other medical and natural sources. As you can see, by this more realistic measure, it would take 20 full series of x rays to equal the amount of radiation the average citizen picks up from background sources each year:

Note also that radiation to the gastrointestinal (GI) tract is MUCH more damaging than radiation to the chest. This is due to the increased vulnerability of the lining of the intestine because the cells there are constantly reproducing and being replaced while the cells in the lungs are less frequently replaced.

Dental radiographs exposure: Single intraoral (d-speed film) Bitewings (4 films-D-Speed) Bitewings (4 digital radiographs) Full-mouth series (about 19 films) Full-mouth series (19 taken digitally) Panorex (panoramic jaw film)	(mSV) .0095 0.038 0.013 0.180 0.060 0.019	(mREM) .95 3.8 1.3 18 6 1.9
Medical radiographs exposure: Lower GI series Upper GI series Chest	4.060 2.440 0.080	406 244 80
Average radiation from outer space In Denver, CO (per year)	0.510	51
Average radiation in the U.S. from Natural sources (per year)	3.000	360

Adapted from Frederiksen NL. X-Rays: What is the Risk? Texas Dental Journal. 1995;112(2):68-72

A good reference for persons looking for the relative dosage from other sources of medical diagnostic and treatment procedures should consult the website of the Health Physics Society

The American Nuclear Society also offers an excellent web page that allows you to calculate your own exposure to ionizing radiation.

This site offers a course in dental radiology

This course is designed for radiology technicians, dentists, hygienists and dental assistants, and covers all aspects of taking intraoral x-rays. The most intriguing part of the course for most trained professionals is an emphasis on shadow casting and how to use this knowledge to advantage while taking those difficult intraoral films.

What about danger to the x-ray technician? (scatter radiation)

The x-radiation figures mentioned above pertain to the patient who is in the direct line of fire from the x-ray tube. The radiation received by the person taking the x-ray comes exclusively from scatter, which is most easily understood by thinking about a flashlight aimed at a wall in a completely darkened room. The spot on the wall where the flashlight is aimed is the brightest because it is in the direct line of fire, however, the rest of the room is also dimly illuminated by the light that scatters off the wall. This scatter is what concerns us since nothing but the patient's face and jaws is directly in the line of fire of the beam. The flashlight analogy is inexact since x-ray beams are better collimated (they form a tighter beam), and much less x-radiation is scattered from the target than light from the wall because of the nature of the x-radiation itself. But the analogy still helps you to understand the concept of scatter versus direct illumination. Furthermore, the strength of the radiation (or light) hitting any unit area falls off geometrically depending on the distance from the source of scatter. Think of the flashlight analogy again. In a very large, dark room the area of the wall two feet from the bright spot is much brighter than an area 20 feet away. The "brightness" of the scatter illumination falls off as the square of the distance. A person standing 6 feet away from the target receives one ninth (1/9) as much scatter radiation as a person standing two feet away from the target (6 feet is 3 times further away than 2 feet, and 3 squared is 9). A person standing 10 feet away (5 times further away) from the target receives one twenty-fifth (1/25).

Dental radiography using film

A majority of dental offices still use intra oral film to take their x-rays. There are three standard film speeds. D speed film is the slowest, E speed is midrange and F speed is the fastest. Each jump in speed has two consequences. First, Each succeeding speed film requires less radiation to expose than the one before. Thus, switching from D to E speed produces a 30-40% reduction in exposure. Switching from E to F speed produces a 20-25% reduction in exposure, and switching from D to F-speed film produces a 60% reduction in exposure. Second, the faster the film, the larger the grain size (the size of the silver nitrate particles on the surface of the film), and thus the lower the film resolution. While lowering the patient exposure to x-rays is obviously a good thing, the lower resolution (the amount of clarity) the less diagnostic information is available to make the diagnosis. Therefore, about 70% of offices using film still use D speed film, 21% use E speed and only 9% use F speed.

Digital X-rays

In digital radiography, a sensor replaces the film normally used for traditional radiographs. The sensor plugs into the USB port on an ordinary computer. The most common type of Intraoral sensors are solid-state electronic devices called “charged-coupled devices” (CCD). A CCD is composed of millions of light sensitive silicon cells arranged in a rectangular array on the face of the sensor. Each cell on the face of the sensor will eventually result in one pixel (picture element) in the final image.

The x-ray photons falling upon each cell create an analog (continuous) electrical voltage. The level of the voltage produced depends on the number of photons reaching the cell, and this in turn depends on the density of the structures (teeth and bone) between the x-ray source and the CCD. The voltage level for each pixel is converted to digital data (numbers between 0 and 65,536) by a relatively simple device called an "analog to digital converter". Each value is interpreted by the computer as a shade of gray. Zero corresponds to pure white, and 65,536 corresponds to pure black with intermediate values corresponding to varying shades of gray. In this way, the image is converted to millions of tiny digital picture elements (pixels) which are reassembled by the computer into a coherent image.

CCD's used in dental imaging are essentially the same as the CCD's used in digital cameras. In your home camera, the CCD contains color filter arrays for each pixel so the image can be reassembled in color. Since dental radiographs are monochrome (shades of gray), the dental CCD does not contain these filters

While digital radiography is a newer technology than the film it replaces, it must be stressed that the image obtained on a digital x-ray is not necessarily any better than one taken using standard x-ray film. Digital technology does, however, require substantially less radiation than film. Digital radiography requires only about a quarter as much exposure time as D speed film, and a little more than half the time as the newest F-speed films, while delivering about the same resolution as D speed film.

The largest benefit of digital x-rays is the ability to computer-enhance the images, making them larger, clearer, or higher contrast at will. This can be helpful, particularly for dentists with less experience in reading traditional film, but it is rarely essential in making a correct diagnosis. Larger, sharper images are helpful in patient education and in helping patients to accept a treatment plan. There is no darkroom developing of the images, and the sensor can be moved about in the mouth more quickly than films, which must be exchanged for new ones for each shot. Thus digital radiography cuts down on the time it takes to expose and process a series of intraoral films. For these reasons, digital radiography is gaining increasing acceptance in dental offices throughout the US and Europe.

The major dental series

There are three major types of dental x-ray surveys: the initial full mouth series, the yearly bite wing series, and the Panoramic x-ray film.

The Full Mouth Series (FMX)

This is an example of the full mouth series we take in our office. It consists of 4 bite wing films which are taken at an angle specifically to look for decay, and 14 periapical films which are taken from other angles to show the tips of the roots and the supporting bone. Not all full series look exactly like this one, but they all use some combination of bite wing and periapical x-rays to show a complete survey of the teeth and bones. We take a full mouth series on everyone over the age of 25 at the initial oral examination, and retake it again every 3 to 5 years.

Notice that each tooth is seen in multiple films. This redundancy is important because it gives us lots of information we would not otherwise have. Each x-ray is shot from at least a slightly different angle and the difference in angulation can reveal many different aspects of the tooth in question. X-rays are not ordinary 2 dimensional pictures. They are actually 2 dimensional shadows of 3 dimensional objects. As you know, shadows may be longer or shorter than the object which casts them depending on the angle of the light source and the screen upon which they are projected. They may also be distorted in other ways as well. The shadow of your hand may show all 5 fingers spread out if you hold it palm forward facing the light source with the screen directly behind the back of the hand. On the other hand, the fingers will not be visible at all if the hand is turned so that the thumb is facing the light source and the little finger is facing the screen. This happens with x-rays also, except that the objects which cast the shadow appear translucent on the film, and it is actually possible to see several objects superimposed over each other. This is what gives x-rays their 3 dimensional quality, and this is why it is very helpful to have several views, taken from different angles, of any given tooth.

The bitewing series

A bitewing series consists of either 2 or 4 films taken of the back teeth (although some offices take them on front teeth as well), with the patient biting down so the films contain images of both the top and bottom teeth. A bitewing series is the minimum set of x-rays that most offices take to document the internal structure of the teeth and gums. In our office, we take 2 on children under the age of 12, and 4 on everyone older, supplemented by the other periapical films associated with a full series of x rays if the patient is over the age of 25.

The difference between bitewing and periapical films

In a bitewing film, all three elements, the teeth, the film, and the x-ray beam are optimized to give the most undistorted shadows possible. (The film and teeth are parallel, and the beam is aimed directly at both; at a 90 degree angle.) Thus bitewing films afford the most accurate representation of the true shape of the teeth and associated structures such as decay, fillings, shape of nerves and bone levels. (To see how the big cavity in the lower tooth was filled, click here.)

A periapical film like the one on the right is shot from an angle in which the three elements are not necessarily aligned parallel. Some distortion is introduced on purpose to be sure that the shadow of the entire tooth or teeth in question falls on the film. This is done because in many instances, the space available in the mouth, or the curvature of the roof of the mouth will not permit parallel placement of the film. This patient had an abscess and was in pain when the film was shot. (To see how this situation is treated, click here.)

The Panoramic Film (Panorex)

As you can see from the image above, the Panorex is a large, single x-ray film that shows the entire bony structure of the teeth and face. It takes a much wider area than any intra oral film showing structures outside of their range including the sinuses, and the Temperomandibular Joints. It shows many pathological structures such as bony tumors and cysts, as well as the position of the wisdom teeth. They are quick and easy to take, and cost a little more than a full series of intraoral films. In addition to medical and dental uses, panoramic films are especially good for forensic (legal) purposes in the identification of otherwise unrecognizable bodies after plane crashes or other mishaps.

Panoramic films differ from the others in that they are entirely extraoral, which means that the film remains outside of the mouth while the machine shoots the beam through other structures from the outside. It fits into a broad category of medical x-rays called tomographs. A tomograph is a computer assisted method of focusing x-rays on a particular slice of tissue and showing that slice on the film as if there were no other structures outside of that slice. It has a number of real advantages over the intraoral variety of film discussed above. Since it is entirely extraoral, it works quite well for gaggers who could not otherwise tolerate the placement of films inside their mouths. The patient stands in front of the machine (pictured on the right), and the x-ray tube swivels around behind his head. Another advantage of the panoramic film is that it takes very little radiation to expose it. The amount of radiation needed to expose a panoramic x-ray film is about the same as the radiation needed to expose two intraoral films (periapical or bitewing). The reason for this is that the film cassette contains an intensifying screen which fluoresces upon exposure to x-rays and exposes the film with visible light as well as x-rays.

For much more on how a panoramic unit actually creates its image, click on the image of the machine above, or click here.

The film on the right is a panoramic view of a child under the age of 12. You can see the adult teeth that are forming underneath the baby teeth. You can also see the adult second molars which are the 4 half formed teeth toward the outside of the film. The fact that the second molars are not yet erupted is the reason a dentist or anthropologist can tell that this child is under the age of 12. For a better understanding of this film click here.

These films have one major disadvantage. The panoramic film is a lower resolution picture than the intraoral films. This means that the individual structures which appear on them (such as the teeth and bone) are somewhat fuzzy, and structures like caries (tooth decay) and bony trabeculation (the spongelike bone inside the marrow spaces) are imaged without the fine detail seen on intraoral films. They are not considered sufficient for the diagnosis of decay, and must be accompanied by a set of bitewing x-rays if they are to be used as an aid for full diagnostic purposes. The combination of a set of bitewings and a panoramic film is particularly useful for those patients who are to be referred for orthodontic consult, and for extraction of wisdom teeth. We use the bitewing/panorex combination frequently instead of a full series of intraoral films on patients between the ages of 13 and 30.

The CAT scan

A tomograph is a two-dimensional image of a slice or section through a three-dimensional object. An example of a primitive tomograph is the panoramic x-ray defined above. While the panoramic x-ray utilizes a photographic film or an electronic array of charged-coupled devices (CCD) to make an image directly from a fan shaped beam of x-rays which sweeps around the jaw, Computerized Tomography (CT) uses fan shaped, or cone shaped beams of x-rays that scan each point in an object from multiple angles to create an array of data points. The detector array is the same width as the fan shaped beam. The detector and the x-ray source are mounted on opposite sides of a gantry which moves around the subject. This arrangement allows objects within plane of the beam to be scanned from numerous angles as the gantry rotates around them. The detector takes numerous "snapshots" called "views." About 1,000 views are taken in one rotation. Each profile is analyzed by computer software, and the full set of profiles from each rotation is compiled into a two dimensional image representing a "slice" through the subject in the same plane as the beam. For much more on the CT scan and the theory behind it, see this page on my course on dental radiography, or click on the icon above.

Cone Beam CT scanning

Cone beam computerized tomography has been available in the United states since 2001. The cone beam CT scanner (CBCT) does not image slices. Instead its cone shaped beam scans a complete volume at once. By rotating the beam around the subject and creating a very large array of data points, the area of interest is observed from a large number of different angles. The cone beam scans both the maxilla and mandible at one time, and requires about 2-8 times the amount of radiation used in a panoramic radiograph. This is still quite low when compared with the dose supplied by the CT scanner. The data is captured by a two dimensional array and creates between 150 and 600 high resolution images (also called "views"). These two dimensional views are then combined to form a coherent three dimensional image of the bony structures in the field of view. (Click the icon to read much more on the cone beam and the theory behind it) Unlike the CT scanner, the cone beam is generally tuned to make images of hard tissues (bone and teeth), which is the reason that the radiation exposure to the patient is so low. CT scans expose the patient to much more ionizing radiation because they are generally tuned to get images of soft tissue. This means that while a cone beam uses a low intensity, high energy x-ray beam, the CT scanner uses a high intensity low energy beam which is more efficiently absorbed and scattered by biological tissues than the higher energy beams used in cone beam technology.

Cone beam machines are quite expensive, and the clinician who takes or orders one has a heavy legal liability, so cone beam scans are likely to be fairly expensive

This site offers a course in dental radiology