Optimizing beam quality for X-ray computed mammotomography

The purpose of this study was to determine the optimum beam quality in terms of kVp and filtration for a tungsten-target x-ray source for a newly developed cone beam computed mammotomography application. The optimized beam is expected to yield enhanced image quality along with a low dose, equal to or less than that of dual view x-ray mammography. X-ray spectra were computer generated for a range of tube potentials, filter materials, and filter and breast thicknesses. The uncompressed breast was modeled from 8, 12, and 16 cm thick tissue, and breast lesions were modeled as a 0.5 cm thick striated muscle mass and a 0.02 cm thick microcalcification. The detector was modeled as a digital flat-panel detector with a 0.06 cm thick CsI x-ray absorption layer. Figures of merit computed included the ratio of the mean beam energy post-breast to pre-breast as an index of beam hardening, ratio of lesion contrasts with and without filtering, and SNR2/exposure. Tube potentials between 50 and 70 kVp provided continuous spectra that, when filtered with Z-filters between 56 and 62 yielded quasi-monochromatic x-ray spectra with optimal SNR2/exposure and contrast while providing minimum beam hardening. Figures of merit improve with increasing filter thickness, but diminishing returns are seen beyond the 500th value attenuation layer. While uncompressed breast thickness affected absolute values of the measurement criteria, they had little effect on beam hardening and contrast ratio and did not alter the optimal operating range. Filter thickness near the 500th value layer is expected to be well within the operating range for an intended commercial x-ray tube, indicating that these highly attenuating filters can provide superior performance for mammotomography.

Duke Scholars

Author Ehsan Samei Radiology