Developing a Microbubble-based Contrast Agent for Synchrotron In-line Phase Contrast Imaging.

X-ray phase contrast imaging generates contrast from refraction of X-rays unlike absorption in conventional radiology. That enhances visualization of soft tissues, often at a reduced absorption radiation. Our goal is to develop a contrast agent for X-ray in-line phase contrast imaging based on ultrasound microbubbles (MBs), by assessing size, shell material, and concentration. We constructed MBs with two different shell materials: phospholipid and polyvinyl-alcohol (PVA). Polydisperse perfluorobutane-core lipid-MBs were size separated using centrifugation into five groups between 1 and 10 μm. We generated two size populations of air-core PVA-MBs, 2 and 3 μm and 3-4 μm, whose shells were either coated or integrated with iron oxide nanoparticles (SPIONs). Microbubbles were then embedded in agar at three concentrations: 5×107, 5×106 and 5×105 MBs/ml. In-line phase contrast imaging was performed at Canadian Light Source with filtered white beam micro computed tomography. Phase contrast intensity was measured by counting detectable MBs, and comparing the mean pixel values (MPV) in minimum and maximum intensity projections. We can detect lipid-MBs 6-10 μm, lipid-MBs 4-6 μm and PVA-MBs coated with SPIONs at every concentration. With MPV, both lipid-MBs 6-10 μm and 4-6 μm showed an increase in positive contrast at high concentration, whereas only lipid-MBs 6-10 μm showed a significance at moderate concentration. Substantial changes in negative contrast were also seen from two largest lipid-MBs at high concentration. These data indicate that lipid-MBs larger than 4 μm are candidates for in-line phase contrast imaging, and 5×106 MBs/ml may be the lowest concentration suitable for generating visible phase contrast in vivo.