Optimization of a table-top x-ray fluorescence computed tomography (XFCT) system.

Pencil beam x-ray fluorescence computed tomography (XFCT) has typically used a single spectrometer and prohibitively long scan times. However, detecting backscattered fluorescent x-rays from gold nanoparticles (AuNPs) using multiple spectrometers greatly reduces image noise and scan time. The arrangement of eight spectrometers for combined K-shell and L-shell XFCT was investigated along with a variety of conditions. A 2.5 cm-diameter cylindrical water phantom containing 4 mm-diameter vials with 0.1%-2% AuNP concentrations by weight was modeled by TOPAS, a GEANT4-based Monte Carlo software. The phantom was irradiated to 30 mGy by a 0.5 mm Pb-filtered 120 kVp and 1 mm Al-filtered 30 kVp 1 mm2 x-ray pencil beam to yield respective Au K-shell and L-shell fluorescent x-rays, with 50 0.5 mm translation and 2-degree rotation steps. Eight CdTe and silicon drift detector (SDD) spectrometers were placed 2.25 cm away from the isocentre. The respective energy resolution was applied to the detected energy spectra and the spectra were corrected for detector response before extracting the fluorescence signal. Three CdTe and SDD spectrometer configurations (isotropic/backscattered grid/backscattered row arrangements), two CdTe crystal sizes (9 mm2 /25 mm2 ), two scanning techniques (moving/stationary spectrometers) and five vial-edge depths (0-4 mm) were considered in optimizing the contrast-to-noise ratio (CNR) for each XFCT image reconstructed with a maximum-likelihood expectation maximization (MLEM) algorithm. The isotropic spectrometer arrangement had AuNP detection sensitivities of 0.106% for K-shell and 0.132% for L-shell XFCT at 4 mm depth. Comparatively, the backscattered grid arrangement had the best AuNP sensitivity of 0.055% and 0.095%. The highest K-shell (0.044%) and L-shell (0.004%) AuNP sensitivities were found for vials at 0 mm depth. Using stationary spectrometers or the 9 mm2 CdTe crystal compromised the CNR. For the best-case arrangement, L-shell XFCT is superior at vial-edge depths less than 3.0 mm. This work demonstrated the importance of spectrometer arrangement and vial depth for improving AuNP sensitivity and will guide the design for our table-top XFCT system.