Answer to Question #4105 Submitted to "Ask the Experts"

What are the factors involved in determining the radiation protection features of a room suitable for barium enema investigations? The only thing I know so far is appropriate lead shielding for things like the doors, etc.

Barium enema studies have two parts, fluoroscopy and radiography. A typical radiographic and fluoroscopic (R&F) room has an under-the-couch screening system with two x-ray tubes. One tube is mounted in the tilting table for under-the-couch fluoroscopy with the x-ray receptor above the table. The other tube is mounted on the ceiling and used for radiography with a table Bucky as well as a vertical Bucky. For the radiographic images, the system is operated remotely with the operator located behind a shielded screen. Typically, various other examinations may also be performed in this room. 

Determination of shielding in the walls, floor, ceiling, and doors of an R&F room depends in part upon the weekly workload (mA-min/week) and distribution of operating potentials (kVp) with the workload, since not all procedures are performed at the same kVp. The weekly workload is the time integral of current, mA-min. The magnitude of the workload determines the radiation dose generated by the tube. The kVp distribution of the workload determines the dose as well as the transmission of the beam through the shielded barrier, for example, higher kVp values will require more shielding.

In addition to the operation parameters of the equipment, shielding requirements will depend on the dose being deposited in adjacent areas. The dose in any occupied location will be the sum of the doses from the primary x-ray beam, scattered radiation, and leakage radiation. The scattered radiation is the radiation that scatters from the patient when the primary x-ray beam is incident on the patient. The leakage radiation is the radiation that leaks through the x-ray source head, but not through the portal through which the primary beam emanates. During the fluoroscopy part of the exam, the image intensifier is well shielded and significantly attenuates the primary beam. Thus, only the scattered and leakage radiation needs to be considered in room shielding. For radiography exams, exposures can be made with the tube pointed down to the table Bucky or film cassette holder, sideways to the upright or wall-mounted Bucky, cross table for film cassette exposures, etc. The primary beam as well as the scattered radiation and leakage radiation should be considered for radiographic procedures for each direction the tube will be pointed.

Finally, the occupancy of areas around the x-ray room and distances from the x-ray source(s) to these areas of interest are needed to evaluate the expected doses to both occupational radiation workers and nonoccupational workers or the public at these locations. The above information together with the dose limits can be used to determine the transmission of the various barriers. The thickness of shielding required can then be determined.

The National Council of Radiation Protection and Measurements (NCRP) has issued a new document, NCRP Report 147, "Structural Shielding Design for Medical X-Ray Imaging Facilities (2004)," which will replace Report 49, "Structural Shielding Design and Evaluation for Medical Use of X-rays and Gamma Rays up to 10 MeV." A good summary on diagnostic x-ray shielding can be found on a website by Robert L. Dixon, Wake Forest University. Another helpful reference is "Radiation Shielding for Diagnostic X-Rays: Report of a Joint BIR/IPEM Working Party," edited by D.G. Sutton and J.R. Williams and published by the British Institute of Radiology, London, England.

With regard to regulatory requirements, each state has special requirements for the use of x rays in the healing arts, including fluoroscopic and radiographic installations. Information on specific state radiation protection programs requirements can be found at the Conference of Radiation Control Program Directors website.

Nisy E. Ipe, PhD, CHP