Towards standardization of x-ray beam filters in digital mammography and digital breast tomosynthesis: Monte Carlo simulations and analytical modelling

Abstract

In digital breast tomosynthesis and digital mammography, the x-ray beam filter material and thickness vary between systems. Replacing K-edge filters with Al was investigated with the intent to reduce exposure duration and to simplify system design. Tungsten target x-ray spectra were simulated with K-edge filters (50μm Rh; 50μm Ag) and Al filters of varying thickness. Monte Carlo simulations were conducted to quantify the x-ray scatter from various filters alone, scatter-to-primary ratio (SPR) with compressed breasts, and to determine the radiation dose to the breast. These data were used to analytically compute the signal-difference-to-noise ratio (SDNR) at unit (1 mGy) mean glandular dose (MGD) for W/Rh and W/Ag spectra. At SDNR matched between K-edge and Al filtered spectra, the reductions in exposure duration and MGD were quantified for three strategies: (i) fixed Al thickness and matched tube potential in kilovolts (kV); (ii) fixed Al thickness and varying the kV to match the half-value layer (HVL) between Al and K-edge filtered spectra; and, (iii) matched kV and varying the Al thickness to match the HVL between Al and K-edge filtered spectra. Monte Carlo simulations indicate that the SPR with and without the breast were not different between Al and K-edge filters. Modelling for fixed Al thickness (700μm) and kV matched to K-edge filtered spectra, identical SDNR was achieved with 37–57% reduction in exposure duration and with 2–20% reduction in MGD, depending on breast thickness. Modelling for fixed Al thickness (700μm) and HVL matched by increasing the kV over [0,4] range, identical SDNR was achieved with 62–65% decrease in exposure duration and with 2–24% reduction in MGD, depending on breast thickness. For kV and HVL matched to K-edge filtered spectra by varying Al filter thickness over [700,880]μm range, identical SDNR was achieved with 23–56% reduction in exposure duration and 2–20% reduction in MGD, depending on breast thickness. These simulations indicate that increased fluence with Al filter of fixed or variable thickness substantially decreases exposure duration while providing for similar image quality with moderate reduction in MGD.

Performance evaluation of digital breast tomosynthesis systems: physical methods and experimental data

“…This limits the utility of an increase in tube voltage in order to increase photon fluence. Shrestha et al (Shrestha et al 2017) also performed analytical calculations of SDNR to examine whether the change from K-edge to Al filtration had a detrimental effect on image quality. For all three schemes described earlier, use of Al filtration enabled the SDNR of the K-edge filtered case to be matched, but with reduced exposure duration and some reduction in mean glandular dose (MGD).…”

A prototype Multi-X-ray-source array (MXA) for digital breast tomosynthesis

“…Figure 5 shows standard bremsstrahlung spectra from a molybdenum anode. Monte Carlo simulations and analytical modelling (Shrestha et al 2017) have indicated that roughly 1 mm of aluminum, as used in these studies, provides good filtration for DBT when tungsten anodes are used. Longer term plans for this technology include cladding the molybdenum focal tracks with a tungsten/rhenium alloy, commonly employed outside of breast imaging applications for general purpose radiography and computed tomography.…”

Development of an additional filtration system by 3D printing for the implementation of new X-ray beam qualities used in diagnostic radiology.

“…Tests were carried out to verify the usability of the system in the routine, adding Ag (silver) and Cu (copper) filters, for the characterization of attenuated radiation beams of interest in diagnostic radiology [9,10].…”