ON THE PRIMARY BARRIER IN DIAGNOSTIC-X-RAY SHIELDING

The traditional approach to shielding design for diagnostic x-ray faci lities has been to designate as primary barriers the floor and those w alls on which the useful beam may impinge, and to ignore the attentuat ion provided by the patient, grid, cassette, cassette holder, and x-ra y table in computing the required thickness of these barriers. The deg ree of attenuation provided by the aforementioned materials has been m easured on three-phrase x-ray equipment including a variety of modern x-ray tables, grids, and cassettes. The primary beam is shown to be at tenuated by more than two orders of magnitude at 100 kVp by the x-ray tables tested prior to impinging on the floor (ignoring patient attent uation). If patient attenuation is included, the attenuation is more t han three orders of magnitude. Transmission factors as well as lead an d concrete equivalencies for the various attenuation materials have be en determined and included in tabular form. Example calculations for a heavy work load show that only a modest amount of concrete is require d 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 tha n the primary barrier. As verification, the actual exposure just below the table bucky was measured over a period of 3 weeks in tow radiogra phic rooms using film dosimetry, and was found to be less than 0.015 m Sv 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.