Answer to Question #14581 Submitted to "Ask the Experts"

Does the exposure button for a panoramic x ray release radiation every time it is pushed? If no images are captured and the machine's gantry does not move was the patient exposed to radiation? The machine did turn on, but it did not take any pictures or rotate.

These are two separate questions, so let's address them in order.

Before getting to the answers though, here is a short review of the operation of a dental panoramic machine. There are four main components of a modern digital panoramic machine: (1) the x-ray generator, (2) the gantry which rotates the generator, (3) the receptor around the patient, and (4) the computer/monitor system to process and display the image. Of course, there are the accompanying electrical circuits and switches necessary for operation. The exposure switch that the operator activates is designed such that the exposure button must remain depressed to ensure continuous operation of the x-ray generator and gantry during the exposure. This is important. If a machine malfunction occurs or if the patient moves excessively or suffers a medical emergency, the operator is trained to release the exposure button which interrupts power to the x-ray generator and gantry, thereby stopping the exposure.

The answer to your first question, yes, radiation is produced by the x-ray generator when the exposure button is depressed. And, assuming the machine is in proper operating order, this will occur every time the button is depressed.

The second part of your question is a bit more difficult to answer. According to medical physicists at the Food and Drug Administration's (FDA) Center for Devices and Radiological Health, there is no federal requirement that panoramic machines contain an internal circuit or switch to automatically interrupt power to the x-ray generator if the gantry does not mechanically operate properly. This function is assumed to be under the control of the operator who is observing the patient and machine and would observe the machine malfunction upon depressing the exposure switch. If the gantry did not rotate, the operator would release the button, power to the x-ray generator would be interrupted and the exposure would stop.

More specifically, would the patient receive radiation if the machine did turn on but there were no images visible, and the machine gantry did not rotate? For this question, in conversations with the FDA, we made the assumption that the x-ray generator did operate for a very short period of time (the machine turned on) and that the operator recognized the machine malfunction (the gantry was not rotating) and then released the exposure button. Under these conditions, the patient did not receive "direct" radiation. If the patient had received "direct" radiation there would have been some type of image formation which would have been visible on the image. However, since the gantry never left the "starting" position, some radiation would have been emitted from the "slit" panoramic collimator. This radiation would be classified as "scatter" radiation to the patient since the patient was not in the direct beam of radiation. How much of a radiation dose would that have been? That's extremely difficult to calculate, but the patient dose would have been very minimal since there was zero direct dose. Here is an estimate—the radiation dose rate for an average panoramic is about 16 microSieverts per (μSv hr^-1). An estimate for a couple of seconds of scattered radiation in this scenario would be far less than 1 μSv hr^-1, or about the amount of background radiation the average person in the US receives in three hours, just by being alive.

Jeffery B. Price, DDS, MS