Answer to Question #5496 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
NCRP 147 "Structural Shielding Design for Medical X-Ray Imaging Facilities" states on page 21, item 2.3.6: ". . . although floor-to-floor height will range from 3 to 5 m, protective shielding need normally extend only to a height of 2.1 m above the floor, unless additional shielding is required in the ceiling . . ."
I wonder if this statement is absolutely valid for computed tomography (CT) equipment.
In my opinion, the above statement (the 2.1 m rule) is a good rule of thumb for other x-ray machines, but may be questionable for CTs. That's because the CTs generate more intense x-ray fields (higher kVp and mAs) than radiographic or fluoroscopic units—the CT gantry can be tilted, etc. Therefore, the radiation scattered above the 2.1 m height may be significantly higher in the case of CTs. As an example, in a new facility, the badges of the CT technicians recorded constantly 4 to 15 mrem/month. Their exposure was exclusively due to operating a 64-slice CT from the control room. The wall adjacent to a small CT scanner room was shielded with lead just to the 2.1 m height (no occupancy above the ceiling). Sure, the doses are still within the design goal. However, the technicians insisted that their doses should be decreased to reach the levels that they used to have at another facility (minimum recordings). Consequently, the facility installed lead up to the ceiling and the personal doses were reduced to minimum. All this seems to be a normal ALARA (as low as reasonably achievable) optimization process that should be approached from case to case (no generic formula such as the 2.1 m rule). However, the above NCRP 147 statement provides to the facilities and constructors an opportunity to argue that lead is necessary in the walls only up to the 2.1 m height (financial approach only!). Is this the purpose of the document in this respect? Should all the walls in diagnostic rooms be leaded just to the 2.1 m height (when no shielding is required in the ceiling)? If this is a case-by-case decision, how can a health physicist, like me, argue with a facility or constructor on this issue?
The first sentence in NCRP 147 states: "The purpose of radiation shielding is to limit radiation exposure to employees and members of the public to an acceptable level." The guide goes on to indicate that the goal of the report is to design facilities to meet "shielding design goals" and that "the shielding design goals will ensure that the respective annual values for E (effective dose) recommended in this Report for controlled and uncontrolled areas are not exceeded." The shielding design goal for controlled areas is 10 mrem/week, which is significantly above your reported dosimetry results of 4 to 15 mrem/month. Based on the criteria in NCRP 147, your facility has met the design goals and would be considered acceptable based on typical state limits for dosimetry and many facility ALARA trigger levels (typically set at 10 percent and 30 percent of the annual dose limits on a quarterly basis).
It is possible that there are cases where shielding would have to be
extended above the conventional 2.1 m height. Experience has shown our
group that many new CT installations need to have the shielding barrier
in the wall directly behind the gantry installed from floor to ceiling;
however, this must always be calculated on a case-by-case basis.
Benjamin R. Archer, PhD, personal communication.
Answer posted on 19 May 2006. The information and material posted on this website 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 Website. 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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