Tungsten anode tubes with K-edge filters for mammography

A number of authors have calculated the optimum X-ray energies for mammography using, as a criterion, the maximum signal to noise ratio (SNR) per unit dose to the breast, or conversely the minimum exposure for constant SNR. Filters having absorption edges at appropriate energy positions have been used to modify the shape of tungsten anode spectra to bring them close to the calculated optimum. The suitability of such spectra for practical use has been assessed by comparing both the film image quality and the incident breast dose obtained using a K-edge filtered tungsten anode tube with that obtained using a molybdenum anode tube. Image quality has been assessed both by using a "random" phantom and by comparing mammograms of women attending a screening clinic where one breast was radiographed using a filtered tungsten anode tube and the other using a standard molybdenum anode unit. Relative breast doses were estimated from both ionisation chamber measurements with a phantom and thermoluminescent dosimetry measurements on the breast. The film image quality assessment indicated that the filtered tungsten anode tube gave results which were not significantly different from those obtained with a molybdenum anode tube for a tissue thickness of about 4 cm and which were better for larger breast thicknesses. The dose could be reduced to between one-half and one-third with the filtered tungsten anode tube.