Developing a Microbubble-Based Contrast Agent for Synchrotron Multiple-Image Radiography

Multiple-image radiography (MIR) is an analyzer-based synchrotron X-ray imaging approach capable of dissociating absorption, refraction, and scattering components of X-ray interaction with the material. It generates additional image contrast mechanisms (besides absorption), especially in the case of soft tissues, while minimizing absorbed radiation dose. Our goal is to develop a contrast agent for MIR using ultrasound microbubbles by carrying out a systematic assessment of size, shell material, and concentration.

Microbubbles were synthesized with two different shell materials: phospholipid and polyvinyl-alcohol. Polydisperse perfluorobutane-filled lipid microbubbles were divided into five size groups using centrifugation. Two distributions of air-filled polymer microbubbles were generated: 2-3 µm and 3-4 µm. A subset of polymer microbubbles 3-4 µm had iron oxide nanoparticles incorporated into their shell or coated on their surface. Microbubbles were immobilized in agar with different concentrations: 5 × 10⁷, 5 × 10⁶, and 5 × 10⁵ MBs/ml. MIR was conducted on the BioMedical Imaging and Therapy beamline at the Canadian Light Source. Three images were generated: Gaussian amplitude, refraction, and ultra-small-angle X-ray scattering (USAXS). The contrast signal was quantified by measuring mean pixel values and comparing them with agar.

No difference was detected in absorption or refraction images of all tested microbubbles. Using USAXS, a significant signal increase was observed with lipid microbubbles 6-10 µm at the highest concentration (p = 0.02), but no signal was observed at lower concentrations.

These data indicate that lipid microbubbles 6-10 µm are candidates as contrast agents for MIR, specifically for USAXS. A minimum concentration of 5 × 10⁷ microbubbles (lipid-shell 6-10 µm) per milliliter was needed to generate a detectable signal.