Answer to Question #10565 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
There is no specific time period recommended between radiographs. A panoramic x-ray is a very low dose dental radiograph. Published radiation doses in terms of effective dose range from about 9 µSv to about 26 µSv (White and Pharoah 2009). To put this in perspective, the average background radiation dose in the United States is about 8 µSv per day. Background radiation includes things like cosmic radiation, radon, naturally occurring radioactive materials in soil, building materials, etc. The typical panoramic radiograph has a radiation risk equivalent to approximately one to three days of natural background radiation. There is no reason to delay having the repeat radiograph taken if it is needed for planning the orthodontic treatment.
It is very reasonable, however, to ask the person taking the radiograph what went wrong the first time and what steps are being taken to correct the problem. Sometimes the issue is with something the patient did, for example, moving while the scan was taking place or not holding the tongue against the roof of the mouth. Not holding the tongue up can cause a dark air shadow to cross the roots of the teeth, making them hard to interpret. Probably the most common cause for a retake is incorrect positioning of the patient in the machine. Even though the machine looks pretty simple, the procedure is very dependent on correct patient positioning. Most machines have lights that shine on the patient's face, which can guide the operator. With film-based panoramic radiographs, another source of error was problems in the developing of the film. That is not an issue with modern digital equipment.
Sharon L. Brooks, DDS, MS
White SC, Pharoah MJ. Oral radiology principles and interpretation, 6th ed. St. Louis: Mosby; 2009.