Monte Carlo Modeling of An Experimental Benchtop L-shell X-ray Fluorescence Imaging/CT System Adopting Two Silicone Drift Detectors.

x-ray fluorescence (XRF) photons induced from gold nanoparticles (GNPs) after being irradiated by an x-ray beam allow for highly sensitive XRF imaging/computed tomography (XFCT) of biological samples containing GNPs at low concentrations on the order of parts-per-million (ppm). The primary goal of this Monte Carlo (MC) study was to investigate the feasibility of upgrading an existing experimental benchtop XRF/XFCT imaging setup adopting a single silicon drift detector (SDD), developed based on the aforementioned concept (often known as XFCT), by deploying another SDD within the same setup. Specifically, an MC model of the original single SDD XFCT setup, along with a filtered 62 kVp polychromatic x-ray beam, was developed using the Geant4 MC toolkit, and added with a second SDD at an equidistance of 11 cm from the isocenter on the opposite side. Besides the aforementioned feasibility study, this MC model was used to investigate other parameters that may affect the system performance but cannot be easily tested experimentally (e.g., use of monoenergetic photon beams). Upon completion of MC studies, a new commercial SDD was experimentally characterized for optimal performance and deployed to an existing single SDD XFCT setup. The MC results demonstrated the feasibility of highly sensitive detection of gold XRF photons and XRF/XFCT imaging of GNP-containing phantoms using the modeled dual SDD setup. According to the MC results, the GNP detection limit achievable from the dual SDD setup can be in the range of 5 to 10 ppm with a filtered 62 kVp beam and slightly improved (down to ~5 ppm) with the monoenergetic x-ray beams (15 and 20 keV). The experimental results showed the new SDD performed optimally under the current experimental conditions at the manufacturer suggested bias voltage (-135 V). The experimentally measured detection limit from both old and new SDDs was 10 ppm, consistent with the MC results. In addition to the results summarized above, some additional benefits/characteristics from a dual SDD setup (e.g., 2-fold reduction in the sample scan time at a given dose or 2-fold improvement in the image resolution at a given scan time) are immediately discernable.