X-ray fluorescence (XRF) tomography is an emerging imaging technology with the potential for high spatial resolution molecular imaging. One of the key limitations is the background noise due to Compton scattering since it degrades the signal and limits the sensitivity. In this Letter, we present a linear focused anti-scatter grid that reduces the Compton scattering background. An anti-scatter grid was manufactured and evaluated both experimentally and theoretically with Monte Carlo simulations. The measurements showed a 31% increase in signal-to-background ratio, and simulations of an improved grid showed that this can easily be extended up to > 75%. Simulated tomographies using the improved grid show a large improvement in reconstruction quality. The anti-scatter grid will be important for in vivo XRF tomography since the background reduction allows for faster scan times, lower doses, and lower nanoparticle concentrations.

X-ray fluorescence (XRF) tomography from nanoparticles (NPs) shows promise for high-spatial-resolution molecular imaging in small-animals. Quantitative reconstruction algorithms aim to reconstruct the true distribution of NPs inside the small-animal, but so far there has been no feasible way to predict signal levels or evaluate the accuracy of reconstructions in realistic scenarios. Here we present a GPU-based computational model for small-animal XRF tomography. The unique combination of a highly accelerated Monte Carlo tool combined with an accurate small-animal phantom allows unprecedented realistic full-body simulations. We use this model to simulate our experimental system to evaluate the quantitative performance and accuracy of our reconstruction algorithms on large-scale organs as well as mm-sized tumors. Furthermore, we predict the detection limits for sub-mm tumors at realistic NP concentrations. The computational model will be a valuable tool for optimizing next-generation experimental arrangements and reconstruction algorithms.

X-ray fluorescence computed tomography (XFCT) with nanoparticles (NPs) as contrast agents shows potential for molecular biomedical imaging with higher spatial resolution than present methods. To date the technique has been demonstrated on phantoms and mice, however, parameters such as radiation dose, exposure times and sensitivity have not yet allowed for high-spatial-resolution in vivo longitudinal imaging, i.e., imaging of the same animal at different time points. Here we show in vivo XFCT with spatial resolution in the 200-400 mu m range in a proof-of-principle longitudinal study where mice are imaged five times each during an eight-week period following tail-vein injection of NPs. We rely on a 24 keV x-ray pencil-beam-based excitation of in-house-synthesized molybdenum oxide NPs (MoO2) to provide the high signal-to-background x-ray fluorescence detection necessary for XFCT imaging with low radiation dose and short exposure times. We quantify the uptake and clearance of NPs in vivo through imaging, and monitor animal well-being over the course of the study with support from histology and DNA stability analysis to assess the impact of x-ray exposure and NPs on animal welfare. We conclude that the presented imaging arrangement has potential for in vivo longitudinal studies, putting emphasis on designing biocompatible NPs as the future focus for active-targeting preclinical XFCT.

Nanoparticle (NP) based contrast agents detectable via different imaging modalities (multimodal properties) provide a promising strategy for noninvasive diagnostics. Core-shell NPs combining optical and X-ray fluorescence properties as bioimaging contrast agents are presented. NPs developed earlier for X-ray fluorescence computed tomography (XFCT), based on ceramic molybdenum oxide (MoO2) and metallic rhodium (Rh) and ruthenium (Ru), are coated with a silica (SiO2) shell, using ethanolamine as the catalyst. The SiO2 coating method introduced here is demonstrated to be applicable to both metallic and ceramic NPs. Furthermore, a fluorophore (Cy5.5 dye) was conjugated to the SiO2 layer, without altering the morphological and size characteristics of the hybrid NPs, rendering them with optical fluorescence properties. The improved biocompatibility of the SiO2 coated NPs without and with Cy5.5 is demonstrated in vitro by Real-Time Cell Analysis (RTCA) on a macrophage cell line (RAW 264.7). The multimodal characteristics of the core-shell NPs are confirmed with confocal microscopy, allowing the intracellular localization of these NPs in vitro to be tracked and studied. In situ XFCT successfully showed the possibility of in vivo multiplexed bioimaging for multitargeting studies with minimum radiation dose. Combined optical and X-ray fluorescence properties empower these NPs as effective macroscopic and microscopic imaging tools.

Propagation-based phase-contrast X-ray imaging is an emerging technique that can improve dose efficiency in clinical imaging. In silico tools are key to understanding the fundamental imaging mechanisms and develop new applications. Here, due to the coherent nature of the phase-contrast effects, tools based on wave propagation (WP) are preferred over Monte Carlo (MC) based methods. WP simulations require very high wave-front sampling which typically limits simulations to small idealized objects. Virtual anthropomorphic voxel-based phantoms are typically provided with a resolution lower than imposed sampling requirements and, thus, cannot be directly translated for use in WP simulations. In the present paper we propose a general strategy to enable the use ofthese phantoms for WP simulations. The strategy is basedon upsampling in the 3D domain followed by projection resulting in high-resolution maps of the projected thickness for each phantom material. These maps can then be efficiently used for simulations of Fresnel diffraction to generate in silico phase-contrast X-ray images. We demonstrate the strategy on an anthropomorphic breast phantom to simulate propagation-based phase-contrast mammography using a laboratory micro-focus X-ray source.

Mechanical ventilation of living animals is routinely used to achieve high-resolution pulmonary imaging, but this can damage the subject. Here, an alternative, free-breathing method enables X-ray tomography with 30 mu m resolution. Phase-contrast X-ray lung imaging has broken new ground in preclinical respiratory research by improving contrast at air/tissue interfaces. To minimize blur from respiratory motion, intubation and mechanical ventilation is commonly employed for end-inspiration gated imaging at synchrotrons and in the laboratory. Inevitably, the prospect of ventilation induced lung injury (VILI) renders mechanical ventilation a confounding factor in respiratory studies of animal models. Here we demonstrate proof-of-principle 3D imaging of the tracheobronchial tree in free-breathing mice without mechanical ventilation at radiation levels compatible with longitudinal studies. We use a prospective gating approach for end-expiration propagation-based phase-contrast X-ray imaging where the natural breathing of the mouse dictates the acquisition flow. We achieve intrapulmonary spatial resolution in the 30-mu m-range, sufficient for resolving terminal bronchioles in the 60-mu m-range distinguished from the surrounding lung parenchyma. These results should enable non-invasive longitudinal studies of native state murine airways for translational lung disease research in the laboratory.

Respiratory X-ray imaging with phase contrast leads to improved sensitivity, as demonstrated in animal models to date. The translation to humans is limited by currently available technology, leaving the future clinical impact of the technique an open question. Here we demonstrate phase-contrast chest radiography using a proof-of-principle in silico framework. Specifically, we apply our previously developed preprocessing strategy to state-of-the-art realistic virtual human torso phantoms, then generate virtual chest radiographs through wave-propagation simulations. From a blind reader study conducted with clinical radiologists, we predict that phase contrast edge-enhancement has negligible impact for pulmonary nodule detection (6-20 mm). However, edge-enhancement of bronchial walls can visualize small airways (< 2 mm) invisible in conventional radiography. Our results predict that phase-contrast chest radiography could play a future role in diagnosis of small-airway obstruction (e.g., in asthma or chronic obstructive pulmonary disease) thereby motivating the experimental development needed for clinical translation.