Search

Methods are presented for determining the thickness requirements for barriers against scatter and leakage radiation generated in a diagnostic x-ray facility. Equations are developed that express the shielded doses due to scatter and leakage with explicit dependence on the operating potential used in clinical settings. The 1972 scatter experiment of Trout and Kelley is revisited, with suggested values for the scatter fraction that are somewhat different from those used in Report Number 49 of the NCRP. The dose from leakage radiation due to operation at clinical kVp settings is seen to be orders of magnitude below that predicted by the model in NCRP49. The unshielded secondary radiation dose in air at unit distance and workload is presented as a function of operating potential and for clinical workload distributions. The net transmission of secondary radiations through common shielding materials is presented. Compared to that for the primary beam at the same potential, the transmission of secondary radiation is more penetrating due to the inclusion of leakage radiation that has been hardened by the tube housing. Sample shielding calculations illustrate the utility of this scheme.

Traditional methods of diagnostic x-ray shielding design assume that the raw primary beam impinges directly on the structural barrier with attenuation by the patient and by the hardware in the x-ray beam (cassette, cassette holder, x-ray table) being ignored. Moreover, primary barrier calculations are done assuming a single, conservatively high value of the operating potential, rather than the lower kVp values more representative of clinical usage. This results in extreme conservatism, which is no longer defensible in light of the lowered dose limit to a member of the general public of 1 mSv y-1, which is equal in magnitude to natural background radiation (excluding radon). A shielding model is developed that accounts for attenuation of the primary beam prior to impinging on the structural barrier and which uses a workload which is distributed over a range of clinically realistic operating potentials. The resulting model is as simple to use as that, assuming a single value of the kVp. Sample calculations are performed and the results compared to those from more traditional shielding calculation methods.

The intensity of scattered x rays about two clinically installed mammographic units was measured at 25, 30, and 35 kVp over a range of scattering angles. The scatter-to-primary ratio for backscattering is measured to be 4 to 7 times greater than that for 90 degrees scattering. The scatter fraction is the ratio of scatter dose at 1 m to primary dose at 1 m per primary x-ray field area. Averaged over various phantom and field sizes, the scatter fraction for mammography ranges from 0.01 x 10(-6) cm-2 to 0.2 x 10(-6) cm-2 at 0 degrees, approximately 0.25 x 10(-6) cm-2 at 90 degrees, to 1.7 x 10(-6) cm-2 at 163 degrees, with not more than 30% difference between the scatter intensity generated at 25 and 35 kVp. The differences in the scatter fraction measured off two mammography units from two manufacturers are less than 10% for scattering angles greater than 45 degrees. However, the intensity of forward scatter depends strongly on the construction of the image receptor assembly and x-ray tube collimator, with a sharp rise in the scatter fraction at approximately 25 degrees for the 18 x 24 cm2 image receptor apparently due to scatter off the collimator. The dose (normalized at 1 m per 100 patients) in directions toward adjacent walls, ceiling, and floor is conservatively calculated, providing a quick method for estimating the unshielded dose to an occupied area.

Recently published exposure transmission curves for broad diagnostic x-ray beams in lead, concrete, gypsum wallboard, steel, plate glass, and wood have been used to calculate the transmission in 5 kVp increments over the 25 to 35 kVp range for molybdenum-anode tubes and 50 to 150 kVp for tungsten-anode tubes. The data are fit to a three parameter model for ease in calculating the x-ray transmission with computers or calculators.

A general method for extending the principles and techniques of NCRP Report No. 49 of the National Council on Radiation Protection and Measurements has been developed. Equations are derived that determine the photon exposure through a radiation barrier taking into account the primary, scatter and leakage radiation from multiple radiation sources of various beam qualities. Applications of this include a general solution for medical radiation structural shielding and an analysis of commonly used diagnostic radiology shielding tables.

A Monte Carlo calculation has been performed to determine the transmission of broad constant-potential x-ray beams through Pb, concrete, gypsum wallboard, steel and plate glass. The EGS4 code system was used with a simple broad-beam geometric model to generate exposure transmission curves for published 70, 100, 120 and 140-kVcp x-ray spectra. These curves are compared to measured three-phase generated x-ray transmission data in the literature and found to be reasonable. For calculation ease the data are fit to an equation previously shown to describe such curves quite well. These calculated transmission data are then used to create three-phase shielding tables for Pb and concrete, as well as other materials not available in Report No. 49 of the NCRP.

Shielding requirements for mammography installations have been investigated. To apply the methodologies of NCRP Report No. 49, the scatter-to-incident ratio of a typical mammography beam was measured, and the broad beam transmission was calculated for several representative beam spectra. These calculations were found to compare favorably with published low kVp tungsten-targeted x-ray transmission through a variety of shielding materials. Radiation shielding tables were developed from the calculated transmissions through Pb, concrete, gypsum, steel, plate glass, and water, using a technique which eliminates the "add one HVL" rule. It is concluded that Mo-targeted x-ray beams operated at 35 kVp require half the shielding of W-targeted beams operated at 50 kVp, and that adequate, cost-effective shielding calculations will consider alternatives to Pb.

A unified approach to the problem of barrier thickness calculations for diagnostic x-ray rooms containing multiple x-ray tubes emitting primary, scatter, and leakage radiation is presented. Given the reasonable constraint that all x-ray tubes in a room operate at the same maximum potential (kVp), the required barrier thickness is determined completely and accurately from a single transcendental equation relating the Kux and BLx values calculated for each radiation source separately. A useful approximation to this is discussed and examples given.