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21 November 2009

Answer to Question #4041 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

Can radiation dose be reduced by increasing focus to film distance and increasing the mAs to give adequate density for a diagnostic x ray?

A

No. Initially, increasing the focus to film distance will decrease patient dose, but there will not be sufficient x rays reaching the image receptor to give an adequate image. As you suggested, the mAs would have to be increased to compensate for the greater distance (Curry et al. 1984). Due to the inverse square law, the net result would be to increase the mAs to get the same number of x rays to strike the image receptor to get the same image. Since the number of x rays at the receptor and, essentially, the patient, doesn’t change, the dose to the patient will be the same.

Ken "Duke" Lovins, CHP

Reference
Curry TS III, Dowdry JE, Murry RC Jr. Christensen’s introduction to the physics of diagnostic radiology. 3rd ed. Philadelphia, PA: Lea and Febiger; 1984: 159.

Answer posted on 11 November 2004. 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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