Answer to Question #4322 Submitted to "Ask the Experts"

I have some questions regarding panoramic and cephalometric dental x rays.

1. I had a cephalometric x ray before removal of my wisdom teeth 10 years ago. Given that the patient's brain is being irradiated in this exam, what is the value of having a cephalometric x ray done for the extraction of wisdom teeth? It seems to me that the area of interest is in the mouth/jaws of the patient, not the entire skull.
2. I recently had a panoramic dental x ray done. The machine used was a Yoshida 10CSU. Does the patient's head/neck region receive a huge amount of radiation from this procedure since the axis of rotation is in head/neck area?
3. I've read on some Web sites that the amount of radiation that I was exposed to is equal to the amount of radiation one receives from a cross-country flight or from a few days' worth of natural background radiation. Does this mean that a panorex will expose the organs (thyroid, brain, skin, and bone marrow) to the same amount of radiation  as a few days' worth of natural background radiation or a cross-country flight?
4. Do digital panoramic machines use rare earth/image intensifying screens?
5. Do digital panoramic machines use less radiation than film-based panoramic machines? If so, how much less?
6. How much radiation does the brain, thyroid, skin, and bone marrow each receive in each of these procedures (in mrems)?
7. Why is ESE (Entrance Skin Exposure) no longer being used?
8. Can you explain what effective dose is? All the definitions that I've seen on the Web for effective dose seem very confusing.

Answers:
1. I am very surprised to hear that you had a cephalometric radiograph done for wisdom teeth. Most dentists/oral surgeons prefer a panoramic radiograph so they can judge the relationships of the third molars or wisdom teeth to the adjacent teeth and nerve canal. I have never heard of anyone using a cephalometric radiograph for that purpose. Since it was so long ago, perhaps you might not be remembering the examination correctly.

2. The axis of rotation in a modern panoramic x-ray machine is constantly moving, unlike the original model that had a single rotation point. Therefore, no single spot is receiving a "huge" amount of radiation. In fact, a panoramic radiograph is one of the lowest dose dental examination techniques available today.

3. In order to compare exposure to a limited portion of the body, such as from a dental panoramic radiograph with other examinations or to exposure to the entire body as from background radiation, the exposure is commonly expressed in terms of effective dose. The dose to each tissue exposed is measured and then weighted by the type of radiation and then by a factor that accounts for the tissue's sensitivity to radiation. These weighted doses are then summed to arrive at the effective dose. This unit allows us to compare the radiation risk of the limited body exposure to the risk of a total body exposure. When we do that for a panoramic radiograph, the numbers you cited from the Web sites are typical. While certain organs receive more than that value, it is the risk to the entire body that is calculated for an effective dose. Unless someone has a special concern about a specific organ, the effective dose appears to be a useful and valuable quantity.

4. No. Standard film-based panoramic machines use rare-earth intensifying screens to reduce the amount of radiation required. Depending on the speed of the system, a dose reduction of up to 1/60 can occur compared with direct exposure film. Digital panoramic machines use an electronic detector which is either a CCD, i.e., a charge-coupled device which similar to what is in a digital camera, or a photo-stimulated storage phosphor.

5. This is highly dependent on the individual machine. There is a paper that will be published soon in Dentomaxillofacial Radiology (DMFR) that summarizes the doses for film-based panoramic machines and measures the doses for several digital machines (Gijbels et al. 2005). The article will probably be in the May or July issue of DMFR.

Typically, the effective dose for film-based panoramic examinations range from 3.9 to 10 µSv. The effective dose for digital panoramic exams range from 2.5 to 6.2 µSv. As you can see, there is overlap between film and digital. The average background radiation is 3 mSv/year or about 8 µSv/day.

6. From the same paper quoted above, the organ doses from digital panoramic radiography are: brain: 10.1-85.7 µGy; thyroid: 10.4-52.2 µGy; skin: 0.1-4.1 µGy; bone marrow: 4.6-12.1 µGy. To convert to mrem, or more actually to mrad, divide each dose in µGy by 10.

7. The problem with ESE is that there are many things not accounted for in this unit. For example, the size of the beam does not factor into the ESE, but does in calculating the effective dose as larger beams irradiate more tissue. The ESE also does not account for the radiation sensitivity of the tissue exposed. It is convenient to measure but doesn't really tell us much about doses to the patient.

8. As I explained one for of your earlier questions, it is a complex calculation, but it allows us to calculate a "weighted average" of exposure over the body. This makes it easier to compare the risks of various types of radiation exposures.

Sharon L. Brooks, DDS, MS
Diplomate, American Board of Oral and Maxillofacial Radiology

Reference
Gijbels F, Jacobs R, Debaveye D, Verlinden S, Bogaerts R, Sanderink G. Dosimetry of digital panoramic imaging. Part I: Patient exposure. Dentomaxillofacial Radiology, in print; 2005.