How to identify tooth decay on dental x-rays.

How to identify tooth decay on dental x-rays.

How to identify tooth decay on dental x-rays.
The basic premise involved is this ...
When searching for a cavity on a tooth, a dentist looks for any sign on the radiograph that indicates or suggests that the density of any portion of the tooth's hard calcified tissues (enamel and/or dentin) is different than normal or expected.

How dental x-rays work.
When a dentist takes a radiograph, a tooth's hard mineralized tissues will block some of the radiation (x-rays) that have been aimed through them.

Due to this effect, those portions of the x-ray film (or digital image receptor, the more modern way to take an x-ray), that lie behind these heavily calcified tissues will be less exposed (have fewer x-rays hit them).

As a result, these protected portions of the picture will look lighter in color. (These areas of the film don't turn as dark because fewer x-rays were able to penetrate through the tooth to strike them.)
On x-rays, more calcified tooth structures appear lighter in color.
Animation of the progression of tooth decay (see below).

On an x-ray (see illustration):

The enamel outer covering of a tooth shows the lightest color. That's because it has a higher mineral content than the other parts.
In comparison, the dentin layer (a less calcified tissue) has a somewhat darker appearance on the film.
The outline of the hollow space inside a tooth (it's pulp chamber, the location where its nerve tissue resides), gives the darkest appearance.

That's because in that region much of the thickness of the tooth is comprised of non-calcified soft tissue, thus a higher percentage of x-rays are able to pass through that region and expose the film.

Why cavities show up on x-rays.
Since decay is an area of tooth demineralization (an area of reduced mineral content), or even possibly an outright hole (a space that would have no mineral content at all), those locations where it has formed will show as a darkened area on an x-ray.

That's because the decayed portion of the tooth is less "hard" (less dense or intact) and therefore the x-rays penetrate that portion of the tooth more easily and ultimately expose the film more so (making the corresponding area on the film look darker).

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When evaluating a radiograph:
The tip off to the dentist that a problem exists is that the dark area on the picture involves a location or aspect of the tooth that in health wouldn't be expected to be less dense (have reduced mineral content).
And characteristically, there are certain areas on a tooth that are more prone to decay formation than others (like right in the region of contact between two teeth). So if the dark spot corresponds with that type of location, it is especially telling.

Stages of cavity formation that show up on x-rays.
Our mockup of a dental x-ray illustrates some of the stages that the decay process goes through.
The characteristic look of advancing tooth decay on dental x-rays.

Frame A: This frame illustrates the earliest stage of tooth decay formation that shows up on a dental x-ray.
Notice how there's just ever so slightly a darkened area in the enamel portion of the tooth right exactly where it touches its neighbor (the contact point). That's the decay. (This person has not been flossing enough.)
Most dentists won't recommend placing a filling until the x-ray shows that the decay has completely penetrated the enamel layer of the tooth (as shown in Frame B). The lesion shown in Frame A may not progress further if this person starts to floss.

Frame B: Once the x-ray image shows that the decay has penetrated through the tooth's enamel layer and into dentin, a dentist will typically recommend placing a filling (see discussion below).


Real-life examples of the carious lesions illustrated in Frames A, B and C.

Frame C: As mentioned above, the dentin portion of a tooth is less mineralized (less "hard") than a tooth's enamel layer. And because of this, it will decay at a faster rate.
This phenomenon shows up on x-rays. Notice how in Frame C the lesion in the enamel layer has only slightly increased in size while the decay in the dentin portion of the tooth has advanced significantly.
It's especially important to tend to a lesion like this promptly, before it has a chance to spread enough that it significantly compromises the structural integrity of the tooth or damages its nerve.

What type of treatment does this x-ray show is needed?
When reading this image, a dentist would diagnose a need for the following work.

A bitewing dental x-ray.
This film shows two fully formed cavities and one that's just starting.
Circle A - The "notch" on the 2nd molar would be classified as a "watch" or incipient lesion.
The fact that some decay has already formed is worrisome. The dentist will need to keep a close eye on this area when future x-rays are taken so to make sure the lesion hasn't progressed further.
Treatment involving some type of preventive measure to deter further progress is indicated. (This might involve: topical fluoride applications, applications of amorphous and reactive calcium phosphate complexes, the use of synthetic hydroxyapatite in an acid paste to repair the defect.)
Circles B and C - The extent of the decay process in these areas has clearly penetrated the enamel layer of the tooth. And due to this fact, the underlying dentin aspect of the tooth is now clearly involved too.
Historically, these types of lesions were considered full-fledged cavities, and as such an indication for filling placement. Nowadays, techniques like resin infiltration might be used in an attempt to arrest the progress of these lesions instead. If so, future monitoring with x-rays will be needed.
FYI - The very white objects on this radiograph are existing metal (amalgam) dental fillings. They appear as bright white because the filling's metal is so dense that few, if any, x-rays have been able to penetrate it and expose the x-ray film/sensor.

"Bitewing" x-rays.
The film shown here is the same radiograph as above. In dental terminology it's referred to as a "bitewing" x-ray. This is the standard type of image that's used to look for cavities.

A bitewing dental x-ray showing 3 areas of decay.

The name "bitewing" comes from the fact that the patient bites on a tab (a single "wing") attached to the film or digital sensor. This holds it in place and still while the picture is taken.
Bitewings are the only type of routine intraoral dental x-ray that shows both the upper and lower teeth. ("Intraoral" means the film or sensor is placed inside your mouth when the image is taken.)
The intention is to show the crowns of the teeth (the portion of each tooth that extends past the gum line), as well as the crest of the jawbone that surrounds and holds them in place.
(In regard to cavity detection, when this is accomplished the picture shows the full extent of each tooth on which it's possible for decay to start.)
A primary purpose of taking bitewing x-rays is to detect cavity formation in the general area of the point of contact between two teeth.
(This is a location that can't be directly visualized by the dentist, and is often difficult or impossible for them to fully examine clinically. As such, taking bitewing x-rays may be the only method available to adequately evaluate this region.)
When taken, the goal of the technician is to "split the contacts" for the picture. This refers to getting everything lined up so the contact point between all adjacent teeth can be clearly seen.

In cases where the x-ray machine isn't lined up correctly, the teeth will look overlapped. That makes reading the x-ray much more difficult, if not impossible. For example, minute changes like those in circle A in our picture above might not be detected at all.

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