Initial study of quasi-monochromatic X-ray beam performance for X-ray computed mammotomography

We evaluate the feasibility, benefits, and operating parameters of a quasimonochromatic beam for a newly developed x-ray cone beam computed mammotomography application. The value of a near monochromatic x-ray source for fully 3D dedicated mammotomography is the expected improved ability to separate tissues with very small differences in attenuation coefficients while maintaining dose levels at or below that of existing dual view mammography. In previous studies, simulations for a range of tungsten tube potentials, K-edge filter materials, filter thicknesses, and a 12 cm uncompressed breast, with a digital flat-panel CsI(Tl) detector model, indicated that thick, rare earth filter materials may provide optimized image quality. Figures of merit computed included: lesion contrast under different filtering conditions; ratio of measured lesion contrast with and without filtering; and exposure efficiency (SNR²/exposure). Initial experiments are performed with a custom built x-ray mammotomography system, cerium foil filters, and plastic breast and lesion tissue-equivalent slabs. Simulation results showed that tube potentials of 50-70 kVp with filters of Z=57-63 yielded quasimonochromatic x-ray spectra with improved FOMs. Initial experimental measurements corroborate simulation results in that, relative trends and rank order of contrast ratios and exposure efficiency were in agreement. These studies show that this approach can be implemented practically with simple hardware and yield improved exposure efficiency versus the unfiltered or minimally filtered case.