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Answer to Question #6673 Submitted to "Ask the Experts"Category: Medical and Dental Equipment/Shielding — Equipment The following question was answered by an expert in the appropriate field: Q
I work in a cardiac catheterization laboratory. Do you
get more scatter radiation with the patient farther from the x-ray tube
or closer to the x-ray tube?
A
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. References Turner JE. Atoms, radiation, and radiation protection. 2nd ed. New York: Wiley & Sons; 1995: 269. Wagner LK, Archer BR. Minimizing risks from fluoroscopic x-rays: bioeffects, instrumentation, and examination. 3rd ed. The Woodlands, Texas: Partners in Radiation Management; 2000: 28-29.
Answer posted on 14 August 2007. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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