Answer to Question #6673 Submitted to "Ask the Experts"

Location of the patient in relation to the x-ray tube does not, in and of itself, determine the amount of scatter radiation produced within the patient. Scatter radiation is directly related to the output (x-ray intensity) of the primary x-ray beam and is about 0.1 percent of the primary beam if measured 90 degrees from the primary beam at 1 meter away (i.e., if the x-ray tube is below the patient, at 1 meter straight out from the side of the patient).

For any given patient, the x-ray beam output is determined by the distance from the x-ray tube to the image intensifier. As the image intensifier distance from the x-ray tube increases, the x-ray output has to increase to cause enough x-rays to reach the image intensifier to produce an image. If the patient is kept as close to the image intensifier as possible, then the patient is farther away from the x-ray tube also, meaning that even though the x-ray output is going up, there is a corresponding decrease in patient dose because of the inverse square law. On the other hand, if the image intensifier (and the patient) are moved closer to the x-ray tube, the x-ray output decreases (because there are enough x-rays striking the image intensifier) while patient dose increases. Because these effects roughly cancel each other out, the amount of scatter doesn't change much (assuming the patient is always close to the image intensifier and the x-ray field size is constant). The reason to move the patient farther away from the x-ray tube is to reduce patient exposure.

Ken "Duke" Lovins, CHP

References

Turner JE. Atoms, radiation, and radiation protection. 2^nd ed. New York: Wiley & Sons; 1995: 269.