Re: pyetje-pergjigje ne mjeksi
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
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.
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 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
X-rays and Intraoral Pictures
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 radiographs
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 Sjögren'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 recei ve 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.
Dental radiography
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Dental radiographs, commonly referred to as X-ray films, or informally, X-rays, are pictures of the teeth, bones, and surrounding soft tissues to screen for and help identify problems with the teeth, mouth, and jaw. X-ray pictures can show cavities, cancerous or benign masses, hidden dental structures (such as wisdom teeth), and bone loss that cannot be seen during a visual examination. Dental X-rays may also be done as follow-up after dental treatments.
A radiographic image is formed by a controlled burst of X-ray radiation which penetrates oral structures at different levels, depending on varying anatomical densities, before striking the film or sensor. Teeth appear lighter because less radiation penetrates them to reach the film. Dental caries, tooth decay, infections and other changes in the bone density, and the periodontal ligament, appear darker because X-rays readily penetrate these less dense structures. Dental restorations (fillings, crowns) may appear lighter or darker, depending on the density of the material.
The dosage of X-ray radiation received by a dental patient is typically small, equivalent to a few days' worth of background radiation environmental radiation exposure, or similar to the dose received during a cross-country airplane flight. Incidental exposure is further reduced by the use of a lead shield, lead apron, sometimes with a lead thyroid collar. Technician exposure is reduced by stepping out of the room, or behind adequate shielding material, when the X-ray source is activated.
Once photographic film has been exposed to X-ray radiation, it needs to be developed, traditionally using a process where the film is exposed to a series of chemicals in a dark room, as the films are sensitive to normal light. This can be a time-consuming process, and incorrect exposures or mistakes in the development process can necessitate retakes, exposing the patient to additional radiation. Digital x-rays, which replace the film with an electronic sensor, address some of these issues, and are becoming widely used in dentistry as the technology evolves. They may require less radiation and are processed much quicker than conventional radiographic films, often instantly viewable on a computer. However digital sensors are extremely costly and have historically had poor resolution, though this is much improved in modern sensors.
This preoperative photo of tooth #3, (A), reveals no clinically apparent decay other than a small spot within the central fossa. In fact, decay could not be detected with an explorer. Radiographic evaluation, (B), however, revealed an extensive region of demineralization within the dentin (arrows) of the mesial half of the tooth. When a bur was used to remove the occlusal enamel overlying the decay, (C), a large hollow was found within the crown and it was discovered that a hole in the side of the tooth large enough to allow the tip of the explorer to pass was contiguous with this hollow. After all of the decay had been removed, (D), the pulp chamber had been exposed and most of the mesial half of the crown was either missing or poorly supported.
It is possible for both tooth decay and periodontal disease to be missed during a clinical exam, and radiographic evaluation of the dental and periodontal tissues is a critical segment of the comprehensive oral examination. The photographic montage at right depicts a situation in which extensive decay had been overlooked by a number of dentists prior to radiographic evaluation of the area.
how Much Radiation Do You Get From Dental X-Rays?
By Steve D. Rima, CHP
Just the mention of the word “radiation” conjures up an unpleasant image for most people. We associate it with bombs, cancer, and all manner of other bad things. But do you know that there are many beneficial uses of radiation? One type of radiation, x-rays, are used extensively in the medical and dental professions to diagnose and treat a wide variety of conditions.
Just how much radiation do you get from a dental x-ray and how harmful is it? First, let’s talk about what an x-ray is. X-rays are energy in the form of waves, identical to visible light. In fact, the only difference between light and x-rays is that light doesn’t have enough energy to go through your body and x-rays do. Both can make an image on photographic film, so both types of energy are used to make pictures; light makes photographs of the “outside” of objects, x-rays make pictures of the “inside” of objects, including your body.
A unit called a “rem” is used to measure radiation. A rem is a large unit, much like a mile is a large unit of length, so we usually use a millirem (mrem) instead, much as you would measure in inches instead of miles for most purposes. (It takes 1000 mrem to equal one rem.)
Advances in x-ray equipment, especially film technology, allow your dentist to get a good x-ray image using much less radiation than was previously required. A typical dental x-ray image exposes you to only about 2 or 3 mrem. The National Council on Radiation Protection (NCRP) says that the average resident of the U.S. receives about 360 mrem every year from background sources. This comes from outer space, radioactive materials in the earth, and small amounts of radioactive material in most foods we consume.
Some typical sources that may expose you to radiation also include smoke detectors (less than 1 mrem per year), living in a brick house instead of a wood one (about 10 mrem per year due to radioactive materials in the masonry), cooking with natural gas (about 10 mrem per year from radon gas in the natural gas supply), reading a book for 3 hours per day (about 1 mrem per year due to small amounts of radioactive materials in the wood used to make the paper), and even from flying in an airplane (about 5 mrem for one cross-country flight because of the increased altitude.) In fact, you receive about 2 mrem per year from sleeping next to someone! This is because all of us have very small amounts of naturally occurring radioactive materials in our bodies.
Obviously, you probably would not refuse to fly on an airplane, live in a brick house, read books, live without smoke detectors, or sleep with your spouse because of the small amount of radiation you receive from these activities. Since your dentist gains valuable information from x-rays to aid you in keeping healthy teeth, it is also not in your best interest to refuse dental x-rays because of the very small amount of radiation you receive from them.
Steven D. Rima is a Board Certified Health Physicist with over 20 years of experience in radiation safety, including teaching medical and dental professionals for state licensure to take medical and dental x-rays.
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