Radiology/digital panoramic x-rays
QUESTION: I recently went to a dental office to get a digital panoramic x-ray of my teeth. I was offered a lead apron, which I wore. A protective thyroid collar wasn't offered, but when I requested one, the office readily complied. It was strapped on to my neck. I was told to rest my chin and to bite on a piece that had a plastic sleeve on it, which I did. I was then told to close my eyes, which I also did. As the x-ray was being taken, the machine emitted a message announcing that a successful image has been taken. But a few seconds later on the computer I was shocked to see an image of my shoulder and bones in my upper back. How did this happen? Wasn't this machine suppose to take a panoramic image of my teeth? And how could this machine record an image of my shoulder and bones in my back since I was wearing a lead thyroid collar and a lead apron on my chest? Please explain.
I'm also curious to know why I was told to keep my eyes closed. Do these diagnostic machines cause damage to the eyes? If so, why aren't safeguards in place to protect the eyes? The increased risk of cataracts would surely warrant more protective measures than just keeping your eyes closed.
Thank you very much!
ANSWER: Hello, Mike.
A panoramic x-ray of your teeth begins with the x-ray tube on one side of your head and the film or digital detector on the opposite side. During the x-ray exposure, the device rotates 180˚ around your head from ear to ear. The x-ray beam passes through your body onto the detector to create a flat picture of whatever lies in the focal plane.
This technique of moving the x-ray tube and receptor in opposite directions during exposure is called "tomography". The anatomy at the pivot point of the movement remains sharp while structures above and below the focal plane are blurred. In this case, your teeth & jaws should have been in the focal plane, so other anatomy would be impossible to see clearly.
How do you know the image that you saw on the computer was an image of your shoulder and upper back? If the technician did not repeat the x-ray, it's highly likely that the image was of the correct anatomy - your upper & lower jaw. Could you have mistaken images of your teeth for the bones in your upper back? The curve of the mandible for your shoulder?
On the other hand, I suppose you could clearly see the bones of your neck if the x-ray technician set the machine to rotate on the wrong side of your head, putting the focal plane at the back of your head instead of your teeth. But the x-ray would have been repeated if that were the case.
As for closing your eyes, I believe that's more about helping you hold still than anything else. Your eyelids will not protect your eyes from radiation. When imaging your head there is no way to shield your eyes without obscuring the anatomy of interest.
Hope this helps,
Santa Barbara Extremity MRI
---------- FOLLOW-UP ----------
QUESTION: Hi Delia, thank you for your description of how the panoramic machine should work.
The technician who took my panoramic x-ray showed me on the computer screen what was taken and she herself told me that the image we were both viewing were bones in my shoulder and upper back. Anyone could have easily seen that they were bones in the back and not bones in the mouth. She then pressed the cancel button, I presumed that erased the image from being saved on her machine. The technician did indeed take another image, but prior to doing so, she removed the thyroid collar from my neck, claiming the thyroid collar was at fault. She took the image, and a desired result was achieved.
If she had known that the thyroid shield interferes with the machine, why couldn't she have told me? Only one x-ray would have been taken, and I wouldn't have been subjected to unecessary radiation.
Do thyroid collars interfere with these diagnostic machines? Weren't they made to be worn during x-rays? Do you think my technician was at fault? For panoramic x-rays, is it usual for technicians to have bad results the first time around, and thus have to retake x-rays?
With that first x-ray, since I was wearing a lead thyroid shield and a lead apron covering the front and back of my body, how was the machine able to capture an image of the bones in my shoulder and upper back? Wouldn't the lead prevent the x-rays beams from ever reaching those bones?
Do you think I was given a defective or non-lead apron to wear?
You wrote, "When imaging your head there is no way to shield your eyes without obscuring the anatomy of interest." Such logic supports the fact that thyroid shields interfere with (or as you put it, obscure) dental x-rays. As the thyroid shield being worn very close to where the teeth are, would trigger a detrimental effect on the diagnostic machine, perhaps similar to the effect of protective eyewear worn during such an x-ray. Do you agree?
I don't think the thyroid collar was the problem. It sounds like the technician set up the machine 180 degrees opposite of how it should have been. In other words, the film/detector rotated around the back of head instead of around the face.
Operator error. It happens to all of us at one time or another. We X-ray Technologists and x-ray technicians try to make our first shot perfect, but it doesn't always work out that way.
Lead shielding prevents scatter radiation from reaching tissue outside the area of interest. It is possible for the lead to crack if the apron is not handled with care. Each facility is responsible for maintaining the integrity of the lead in their shielding. I have no way of knowing if your dentist's employees take appropriate care of their lead shielding.
Hope this helps,
Santa Barbara Extremity MRI