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Answer to Question #3033 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
In evaluating x-ray units for diagnosis and therapy, it is noted that at the same kVp and with the same filters, the half-value layers measured were different depending on the geometry used for measurements. Half-value layer determinations were dependent on (a) the diameter of absorbents, (b) the focal-filter distance, and (c) the focal chamber distance. Why were variations in half-value layer as a function of distance observed?
A
The variations in measured half-value layer that you are seeing are probably due to contributions from scatter radiation. In general (1) if the filters are placed too close to the ion chamber, (2) if the ion chamber is placed too close to a scattering medium, such as a concrete floor or wall, or (3) if the field is set too large, additional scatter will enter the detector volume contributing to the measured exposure and making the measured half-value layer appear erroneously high. Also, if the cross-section of the x-ray field is larger than the filters at the point where the beam passes through the filter, the amount of scatter will vary from the amount seen through larger filters. There are a number of references that offer excellent discussion of filtration. The two that I used are The Physics of Radiology (4th ed.) by H.E. Johns and J.R. Cunningham (Chapter 8), available through Amazon.com, and Christensen’s Introduction to the Physics of Diagnostic Radiology (3rd ed.) by T.S. Curry III, J.E. Dowdey, and R.C. Murry, Jr. (Chapter 6), available through Medical Physics Publishing. There are a number of different recommended methods for measuring x-ray machine output and half-value layer. Johns and Cunningham recommend that inherent filters be placed close to the x-ray beam exit port and that added test filtration be positioned close to the end of any cone or collimator assembly. Added filters should be at least 5 cm by 5 cm. In addition, the ion chamber should be placed at least 50 cm from any of the filters or other scatter media, for example, walls, floors, etc., with the ion chamber centered in the direct beam, which should be collimated to 5 cm by 5 cm at this distance. It should be noted that some ion chambers may be larger than this field size, so the person performing testing should follow the ion chamber manufacturer’s recommended instructions for testing. Ken "Duke" Lovins, CHP
Answer posted on 16 October 2003. 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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