Simulation study of a quasi-monochromatic beam for x-ray computed mammotomography

The purpose of this simulation study was to evaluate the feasibility, benefits, and potential operating parameters of a quasi-monochromatic beam from a tungsten-target x-ray source yielding projection images. The application is intended for newly developed cone beam computed mammotomography (CmT) of an uncompressed breast. The value of a near monochromatic x-ray source for a fully 3D CmT application is the expected improved ability to separate tissues with very small differences in attenuation coefficients. The quasi-monochromatic beam is expected to yield enhanced tomographic image quality along with a low dose, equal to or less than that of dual view x-ray mammography. X-ray spectra were generated with a validated projection x-ray simulation tool (XSpect) for a range of tungsten tube potentials (40-100 kVp), filter materials (Z=51-65), and filter thicknesses (10th to 1000th value layer determined at 60 kVp). The breast was modeled from ICRU-44 breast tissue specifications, and a breast lesion was modeled as a 0.5 cm thick mass. The detector was modeled as a digital flat-panel detector with a 0.06 cm thick CsI x-ray absorption layer. Computed figures of merit (FOMs) included the ratio of mean beam energy post-breast to pre-breast and the ratio of lesion contrasts for edge-located and center-located lesions as indices of breast beam hardening, and SNR2/exposure and SNR2/dose as indices of exposure and dose efficiencies. The impact of optimization of these FOMs on lesion contrast is also examined. For all simulated filter materials at each given attenuation thickness [10th, 100th, 500th, 1000th value layers (VLs)], the mean and standard deviation of the pre-breast spectral full-width at tenth-maximum (FWTM) were 16.1 +/- 2.4, 10.3 +/- 2.2, 7.3 +/- 1.4, and 6.5 +/- 1.5 keV, respectively. The change in beam width at the tenth maximum from pre-breast to post-breast spectra ranged from 4.7 to 1.1 keV, for the thinnest and thickest filters, respectively. The higher Z filters (Z=57-63) produced a quasi-monochromatic beam that allowed the widest tube potential operating range (50-70 kVp) while maintaining minimal beam hardening and maximal SNR2/exposure and SNR2/dose, and providing a contrast greater than that obtained in the unfiltered case. Figures of merit improved with increasing filter thickness, with diminishing returns beyond the 500th value layer attenuation level. Operating parameters required to produce optimal spectra, while keeping exposures equal to that of dual view mammography, are within the capability of the commercial x-ray tube proposed for our experimental study, indicating that use of these highly attenuating filters is viable. Additional simulations comparing Mo/Mo, Mo/Rh, and W/Rh target/filter combinations indicate that they exhibit significantly lower SNR2/exposure than the present approach, precluding them from being used for computed mammotomography, while maintaining dose limitations and obtaining sufficient SNR. Beam hardening was also much higher in the existing techniques (17%-42%) than for our technique (2%). Simulations demonstrate that this quasi-monochromatic x-ray technique may enhance tissue separation for a newly developed cone beam computed mammotomography application for an uncompressed breast.