ON THE PRIMARY BARRIER IN DIAGNOSTIC-X-RAY SHIELDING

The traditional approach to shielding design for diagnostic x-ray facilities has been to designate as primary barriers the floor and those walls on which the useful beam may impinge, and to ignore the attenuation provided by the patient, grid, cassette, cassette holder, and x-ray table in computing the required thickness of these barriers. The degree of attenuation provided by the aforementioned materials has been measured on three-phase x-ray equipment including a variety of modem x-ray tables, grids, and cassettes. The primary beam is shown to be attenuated by more than two orders of magnitude at 100 kVp by the x-ray tables tested prior to impinging on the floor (ignoring patient attenuation). If patient attenuation is included, the attenuation is more than three orders of magnitude. Transmission factors as well as lead and concrete equivalencies for the various attenuating materials have been determined and included in tabular form. Example calculations for a heavy work load show that only a modest amount of concrete is required in the floor as a primary barrier if attenuation by these materials is taken into consideration (less than 2.5 cm at 80 kVp and less than 4 cm at 100 kVp) and the required secondary barrier may be thicker than the primary barrier. As verification, the actual exposure just below the table bucky was measured over a period of 3 weeks in two radiographic rooms using film dosimetry, and was found to be less than 0.015 mSv week-1 (1.5 mR/week) for a heavy work load of 24 patients per day, thus no primary beam shielding would have been required in the floor for these x-ray rooms. © 1994, American Association of Physicists in Medicine. All rights reserved.