What do cavities look like on dental x-rays? -

How dentists read bitewing radiographs. Signs they look for. | Decay stages- How advancing cavities look on x-rays. | How dentists determine when a cavity has advanced enough that a filling is needed.

The scenario.

Possibly you've been at your dentist's office for a check up. X-rays have been taken and you've just been told that you have a cavity.

Your dentist shows you the pictures of your teeth, so you'll be fully informed. But the problem is, you don't really know what you're looking at or what to look for.

Interpreting dental x-rays takes a little skill.

We can't make you an expert but this page outlines how dentists read x-rays.

It describes:  1) What they look for,  2) Why what they see occurs  and gives  3) Real life examples of cavities on bitewing x-rays (the standard type of film used to identify them).

How to identify tooth decay on dental x-rays.

When searching for cavities, a dentist looks for any sign on the radiograph (x-ray) that indicates that there's been a change in the density of any tooth's enamel and/or dentin portions.

Animated illustration identifying the portions of a tooth made up of enamel and dentin.

Dentin and Enamel: The calcified tissues of a tooth.

How x-rays work.

When a radiograph is taken, the tooth's mineral content (calcium, etc...) will block some of the radiation (x-rays) that's been aimed through it.

That means that areas of the film (or electronic sensor) that lie behind heavily calcified tissues, such as enamel and dentin, will be less exposed (have fewer x-rays hit them). As a result, these portions of the picture will look lighter in color.

Animated illustration showing how the progression of tooth decay looks on a dental x-ray.

The characteristic progression of tooth decay (explained below).

On an x-ray (see illustration), the enamel layer covering the teeth has the lightest color. That's because it has a higher mineral content than other parts of the tooth. In comparison, the dentin layer (a less calcified tissue) has a slightly darker appearance on the film.

cavities using:

Why cavities show up on x-rays.

Since tooth decay is an area of tooth demineralization (an area of reduced mineral content), or even possibly an outright hole, those locations where it has formed will appear 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).

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.

Animated illustration showing progressing tooth decay on a 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 recommend placing a filling.
A bite-wing x-ray showing examples of cavities on 2 teeth.

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.


  • Frame D: Frame D illustrates one worst-case scenario situation.

    If a developing cavity is left untreated, it can advance all the way to the tooth's nerve tissue. If it does, not only must the decay be removed and the damaged tooth structure repaired but additionally the tooth will require root canal treatment.

    Clearly the early treatment of tooth decay (preferably at the point shown in Frame B) is a case where a stitch in time saves nine.


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.

A bite-wing x-ray showing cavities on 2 teeth and an incipient lesion on a third.

This film shows two fully formed cavities and one that's just starting.

  • Circles B and C - The extent of the decay has clearly penetrated the enamel layer of the tooth. These are full-fledged cavities and fillings need to be placed.
  • Circle A - This would be termed a "watch" or incipient lesion. The fact that some decay has formed is worrisome. The dentist will keep a close eye on this area when future x-rays are taken to make sure it hasn't progressed further.
  • FYI - The very white objects on this radiograph are existing metal (amalgam) dental fillings.
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"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 bite-wing x-ray showing areas of decay on 3 teeth.

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

  • The name "bitewing" comes from the fact that the patient bites on a tab attached to the film (or electronic sensor) to hold it in place while the picture is taken.
  • This is the only type of intraoral dental x-ray that's commonly taken that shows both the upper and lower teeth. ("Intraoral" means the film or sensor is placed inside your mouth when the image is taken.)
  • When this type of x-ray is taken, the goal of the technician is to "split the contacts." This means they try to get everything lined up so the contact point between adjacent teeth can be clearly seen.

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



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