Reduced acquisition time for L-shell x-ray fluorescence computed tomography using polycapillary x-ray optics.

Abstract

X-ray fluorescence computed tomography (XFCT) is an emerging molecular imaging modality for preclinical and clinical applications with high atomic number contrast agents. XFCT allows detection of molecular biomarkers at tissue depths of 4-9mm at L-shell energies and several centimeters for K-shell energies, while maintaining high spatial resolution. This is typically not possible for other molecular imaging modalities. The purpose of this study is to demonstrate XFCT imaging with reduced acquisition times. To accomplish this, x-ray focusing polycapillary optics are utilized to simultaneously increase x-ray fluence rate and spatial resolution in L-shell XFCT imaging. A prototype imaging system using a polycapillary focusing optic was demonstrated. The optic, which was custom-designed for this prototype, provided a high fluence rate with a focal spot size of 2.6mm at a source to isocenter distance of 3cm with a ten times higher fluence rate compared to standard collimation. The study evaluates three different phantoms to explore different trade-offs and limitations of L-shell XFCT imaging. A low-contrast gold phantom and a high-contrast gold phantom, each with three target regions with gold concentrations of 60, 80, and 100 μgml for low contrast and 200, 600, and 1000μgml for high contrast, and a mouse-sized water phantom with gold concentrations between 300 and 500μgml were imaged. X-ray fluorescence photons were measured using a silicon drift detector (SDD) with an energy resolution of 180eV FWHM at an x-ray energy of 11keV. Images were reconstructed with an iterative image reconstruction algorithm and analyzed for contrast to noise ratio (CNR) and signal to noise ratio (SNR). The XFCT data acquisition could be reduced from 17h to under 1h. The polycapillary x-ray optic increases the x-ray fluence rate and lowers the amount of background scatter which leads to reduced imaging time and improved sensitivity. The quantitative analysis of the reconstructed images validates that concentrations of 60μgml of gold can be visualized with L-shell XFCT imaging. For a mouse-sized phantom, a concentration of 300μgml gold was detected within a 66min measurement. With a high fluence rate pencil beam from a polycapillary x-ray source, a reduction in signal integration time is achieved. It is presented that subtle amounts of contrast agents can be detected with L-shell XFCT within biologically relevant time frames. Our basic measurements show that the polycapillary x-ray source technology is appropriate to realize preclinical L-shell XFCT imaging. The integration of more SDDs into the system will lower the dose and increase the sensitivity.