Abstract
Purpose: To design an x-ray fluorescence CT (XFCT) imaging system for high-sensitivity molecular imaging of gold and platinum probes by means of minimizing Compton scatter contamination using Monte Carlo (MC) simulations. Methods: XFCT and conventional CT images of 5-cm diameter phantoms containing solutions of gold and platinum were simulated in a modified version of the EGSnrc/DOSXYZnrc MC code. In our MC model, fluorescence x-rays excited by heavily filtered 110 kVp x-ray pencil beam were detected by photon-counting detectors arranged in a number of geometries with the aim to vary the contribution of Compton scatter. XFCT images were reconstructed based on sinograms generated using the net number of fluorescence x-rays with simple back-projection. The extraction of the net number of fluorescence x-rays and XFCT image quality as a function of MC-estimated Compton scatter for a range of scattering angles was investigated. A number of detector arrangements (angular distributions and thicknesses) as well as their energy resolution were studied. Results: We found that the imaging sensitivity significantly increases when forward Compton scattered photons are excluded from data acquisition. For 2-cm wide detectors with 1 % energy resolution and a dose of ∼5 cGy, the imaging sensitivity for a full detector ring was 0.086% and 0.060% for gold and platinum, respectively, and it increased to 0.057% and 0.037% when x-rays scattered by less than 90° and 45° degrees were excluded from data acquisition. Moreover, we observed a decrease in sensitivity with decreasing the detector width and its energy resolution. Conclusions: XFCT data acquisition geometry should be carefully chosen for each combination of excitation x-ray spectrum, detector energy resolution, and imaging probe to minimize the contribution of Compton scatter in order to maximize XFCT imaging sensitivity. Based on our MC simulations, the design of an x-ray fluorescence CT scanner will be proposed.
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