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25 July 2008

Answer to Question #6498 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Equipment

The following question was answered by an expert in the appropriate field:

During a recent dental x ray, the x-ray machine head was positioned very far from my head (compared to what I am used to). The film did not come out right, but this may be due to another problem. I am worried that my entire head may have received significant radiation, rather than just my mouth. How collimated is the output beam of a dental x-ray machine?

It is not possible to provide a definitive answer to your question. There are several possible reasons for the position of your dentist's x-ray machine:

He has changed techniques to a longer x-ray source-to-skin distance. This provides more geometrically accurate images. It also reduces the volume of your tissue exposed to the primary beam because the beam diameter at skin entry is the same as before and the beam is less divergent.
The film or digital image receptor was placed in a different location. If it was the outside of your face instead of your mouth, a larger beam was required to produce the image.

Recommended beam collimation for intraoral dental radiography (film or digital receptor placed inside the mouth) is a rectangular beam matched to the size of the image receptor. However, most U.S. dentists are using the current regulatory requirement of a round beam, diameter no greater than 7 cm.

In any event, the dose to your face (or part of it) was quite small. Modern dental image receptors are quite sensitive, requiring a dose at skin entry of about 1.5 milligray (150 millirad), down from 3-5 milligray 20 years ago. The current dose is not known to produce any ill effects.

S. Julian Gibbs, DDS, PhD

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