Initial study of quasimonochromatic 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 a fully 3D tomography application is the expected improved ability to separate tissues with very small differences in attenuation coefficients while maintaining dose level at or below existing dual view mammography. X-ray spectra were simulated for a range of tungsten tube potentials, K-edge filter materials, filter thicknesses, and a 12 cm uncompressed breast. The detector was modeled as a digital flat-panel CsI(Tl) detector. Figures of merit computed included: ratio of mean beam energy postbreast to prebreast; ratio of measured lesion contrasts with and without filtering; and exposure efficiency (SNR²/exposure). Initial experiments were performed with a commercially available X-ray tube/generator system, Ce 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 optimal FOMs. Initial experimental measurements with Ce foil filters corroborate simulation results. Simulations and initial experiments with one type of filter show that this approach can improve image quality while reducing exposure, and implemented in our multimodality X-ray CT/SPECT computed mammotomography system.