The picture on the right is an old sad story.  It's one that we see in our dental office at least a dozen times a year, and it never has a happy ending.  This patient had been a regular in our office for three years, getting exams and cleanings every year.   Then he vanished for another three years and finally turned up with a toothache.   We took an x-ray and saw the decay you can see here.  Believe it or not, even at this stage, the decay was not clinically visible (i.e. you could not see it without an x-ray).  In this case, the decay has reached the nerve, and due to its advanced state, as well as the periodontal disease also visible on the film, it was necessary to extract the tooth.  It takes a LONG time for decay to develop to this extent.   Why didn't we see it at his last two or three regular exams?

In this case, the decay formed between the teeth where it never became visible to the examining dentist or the hygienist.  But it is certainly visible on the x-ray.  So why didn't we take x-rays as a routine part of our yearly dental examination?  Because the patient would not let us!  When he came in at each cleaning visit, he declined the routine x-rays because he wanted to save a few dollars.  He saved, a little,  but in the end he lost the tooth.  Even if he had allowed us to take a simple set of x-rays when he first came in six years earlier, we would have seen the decay beginning to form and could have filled the tooth preventing the entire painful emergency scenario. 

In the case below I have sharpened both images to show what can happen in the real world.  The x ray on the left shows minor decay under the filling denoted by the dark area just under the bright filling (see the yellow arrow).  The unsharpened image is quite vague and neither the hygienist nor the doctor noticed.  Ordinarily, this type of problem would have been picked up on the next set of bitewing x-rays scheduled one year hence because, presumably, over the course of a year, the decay would have worsened and become more visible on the x-ray.  Unfortunately, the patient declined routine x-rays for two years in a row.  He finally presented with pain, and a new x-ray revealed a massive cavity under the filling.  The nerve was already inflamed (ie. painful) due to the proximity of the decay, and the patient eventually had the tooth extracted because he couldn't afford the cost of a root canal, post and core and a crown.  Had we taken routine yearly bitewing x-rays, the tooth could probably have been saved with a simple filling.

The x-ray below on the left shows decay under the filling that was invisible when viewed looking at the tooth in the mouth.  The one on the right shows what happened after the patient ignored it for five years.  The filling had fallen out of the tooth when the decay expanded into a wide gaping hole.  

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 methods of measuring exposure to radiation.  

The first, oldest and most frequently used unit of measure is called a rem.  A rem is a large unit, so exposure to medical radiation is generally measured in millirems (mrem).  (It takes a thousand millirems to make a rem.)  The average dental x-ray delivers about 2 mrem.  Thus a full mouth series of  dental x rays (18 intraoral films) delivers about 36 mrem.  A panorex film delivers about 4 mrem.  By comparison, the average person in the US is exposed to about 360 mrem per year just from background sources.  By this measure, it would take approximately 10 full series of dental radiographs to equal the background radiation that the average citizen is exposed to on a yearly basis.  Note that we take a new full series every three to five years on average.  The Washington State Department of Health has set the maximum safe occupational whole body radiation exposure to 5000 mrem per year.  By this reckoning, it would take over 138 full mouth series of dental x-rays over the course of a year to equal one years maximum safe occupational radiation level.  It would take 1,250 panorex films to get to this limit. 

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 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 second, newer measure of radiation is the millisievert (mSV) which is a unit of measure that allows 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 and medical radiographs).

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 table below is lifted from the website of the American Dental Association and 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 not. 

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 3 dimensional shadows.  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 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 difference between bitewing and periapical films

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.)