A method for the reconstruction of diagnostic x-ray spectra from measurements of attenuation

Estimation of tube potential in mammography from transmission measurements

A computer program has been used to study the performance of a method for estimating the potential of mammographic x-ray tubes. The method uses measurements of the transmission of the x-ray beam through a series of filters. The transmissions are fitted to values calculated using tables of mammographic x-ray spectra, thus providing an estimate of tube potential. The performance of the method was studied using a Monte Carlo simulation of experimental errors. The number of measurements and the selection of filter material were both varied, and the precision and accuracy were used as performance indicators. It was found that, for a single filter material, the precision was improved by increasing the number of transmission data, but the accuracy was unaffected. The poorest accuracy and precision were obtained when K-edge filters, or filters with very high or very low atomic number were used. Using combinations of two filter materials, a high transmission (25%) K-edge filter in combination with a low transmission (3%) non-K-edge filter produced results comparable to the best results obtained from seven transmission measurements using a single filter material. By reducing the non-K-edge filter transmission to 1%, an accuracy of +/- 0.4% and a precision of +/- 1.4% were achieved at both 25 kV and 30 kV.

Reconstruction of megavoltage photon spectra by attenuation analysis

A simple analytical model for reconstructing photon spectra from the indirect measurement of transmission or depth-dose curves is presented, based on the Schiff expression for forward-directed bremsstrahlung differential in photon energy with added inherent filtration. Calculated transmission curves for 21 simulated and measured bremsstrahlung spectra with maximum photon energies in the range 4 to 30 MeV covering a broad range of filtration conditions were used to assess the model's ability to represent megavoltage spectra. Input data were reproduced to within 0.1 per cent in all cases, which is the same order of magnitude as the uncertainty involved in its calculation. Inclusion of a fourth parameter in the model is shown to allow the effective maximum photon energy present in the spectrum to be derived to an accuracy of approximately 3 per cent.

Monte Carlo calculation of conversion factors for the estimation of mean glandular breast dose

The IPSM report on the commissioning and routine testing of mammographic x-ray systems recommends that breast dose be specified as the mean dose to the glandular tissues within the breast and gives the size and compositions of a standard breast phantom for the comparison of doses. The dose to this standard breast phantom can be determined by measuring the incident air kerma to a Perspex phantom and applying appropriate multiplicative conversion factors. These conversion factors have been evaluated by Monte Carlo calculations for a wide range of mammographic x-ray spectra. Some factors are provided for a range of breast thicknesses to supplement existing tabulations. Results are also given for equivalent thicknesses of Perspex and breast tissue.

Enhanced risk from low-energy screen--film mammography X rays

Modern screen-film mammography with molybdenum-anode X rays results in tissue doses being delivered primarily by photons with an energy of less than 20 keV. Such photons interact with tissue predominantly through the photoelectric effect, producing low-energy electrons that have different patterns of energy deposition at the cellular level compared with those from higher-energy X rays. These differences result in low doses of typical molybdenum-based mammography X rays having an estimated radiobiological effectiveness of approximately 1.3 compared with 80 kVp or 250 kVp X rays, and approximately 2 compared with higher-energy gamma rays. Thus the risk from mammography could be higher, by such factors, than previously estimated. This would result in the optimal age for beginning mammographic screening, derived from risk-benefit ratios, being increased by at least 1-2 years and possibly by as many as 10 years.