Answer to Question #14752 Submitted to "Ask the Experts"
Category: Medical and Dental Patient Issues — Dental
The following question was answered by an expert in the appropriate field:
When I go to the dentist, the dentist or technologist usually uses a ring to aim it so the tip (end) of the x-ray machine is at least 5 cm away from my cheek. During a recent root canal treatment (the tooth was in the middle) the endodontist couldn't use a ring to aim and placed the port right against my cheek (lightly touching).
My questions are: 1. Is that standard practice? Is that safe? 2. What is the radiation dose in that case?
The overall radiation doses in question are extremely low, but millions of dental radiographs are exposed each year and all aspects of radiation safety are important.
The aiming ring is called a positioning indicating device (PID). An important factor to know is how long the PID is, and exactly where the actual focal spot is located. The focal spot is where the x rays are generated. There is a small mark on the external surface of the x-ray head that denotes the position of the focal spot. From this point to the end of the PID, the measurement should be at least 20 cm and ideally at least 30 cm, per the latest guidelines from NCRP Report #177.
Another point is that the end of the PID should either just barely touch the patient's skin or be almost touching the patient's skin. The reason for this relates to projection geometry, which in this case is like what happens to a flashlight beam at night which begins to spread out to each side as it travels farther from the light source. The same thing happens to x rays when they leave the focal spot. The collimator keeps the x rays focused, but as soon as the x-ray photons exit the collimator (round or rectangular) they begin spreading out; therefore, the open end of the collimator should be as close as reasonably possible to the object of interest, i.e., the teeth.
To answer the first part of your question about standard practice: Yes, it is standard practice to use a collimator, aka, PID, but the PID should be close to the cheek, or almost touching it. And, yes, this technique is safe.
Regarding the second part of your question regarding calculating dosage, standard dosimetry testing is accomplished with the PIDs positioned in the manner described above. And the farther away the PID/x-ray source is from the cheek/teeth, the less radiation dose received by the patient. This is not an arithmetic relationship, but rather is an inverse squared relationship such that if the distance were to double, the radiation dose would decrease by the square of distance, or in this case the dose would be one-fourth the original dose, not one-half.
The drawback of the approach of moving the x-ray source farther from the cheek is that there is more tissue being irradiated, but with fewer x-ray photons. The question then arises as to whether the operator increases the "normal" technique factors of kilovolt peak (kVp) and millliamperes (mA). Therefore, specifically addressing your question regarding whether the dosage from a radiograph exposed with the PID that is 5 cm or more from your cheek will be providing more or less radiation than a normal radiograph is difficult to answer in your specific case. If the kVp and mA were normal factors that remained unchanged then, the dosage will be less. But, if the operator increased the kVp to compensate for the loss of x-ray photons due to the increased distances, then the radiation dose would likely be slightly increased, although by only a small amount. There are not any studies documenting the amounts of increased radiation dosage in this case, but the amount is likely in the 10 to 15% range, or 0.001 millisieverts (mSv) to 0.0011 mSv, which is still a small dose.
The doses of dental radiographs are in the very low dose category, but we dentists do expose a lot of radiographs on an annual basis and this issue illustrates the importance of practicing good radiation safety.
Jeffery B. Price, DDS, MS, MAGD, FICD