Microbubbles as a novel contrast agent for brain MRI

Gas-filled microbubbles have the potential to become a unique MR contrast agent due to their magnetic susceptibility effect, biocompatibility and localized manipulation via ultrasound cavitation. In this study, two types of microbubbles, custom-made albumin-coated microbubbles (A-MB) and a commercially available lipid-based clinical ultrasound contrast agent (SonoVue), were investigated with in vivo dynamic brain MRI in Sprague-Dawley rats at 7 T. Microbubble suspensions (A-MB: 0.2 mL of approximately 4% volume fraction; SonoVue: 0.2 mL of approximately 3.5% volume fraction) were injected intravenously. Transverse relaxation rate enhancements (DeltaR(2)(*)) of 2.49+/-1.00 s(-1) for A-MB and 2.41+/-1.18 s(-1) for SonoVue were observed in the brain (N=5). Brain DeltaR(2)(*) maps were computed, yielding results similar to the cerebral blood volume maps obtained with a common MR blood pool contrast agent. Microbubble suspension DeltaR(2)(*) was measured for different volume fractions. These results indicate that gas-filled microbubbles can serve as an intravascular contrast agent for brain MRI at high field. Such capability has the potential to lead to real-time MRI guidance in various microbubble-based drug delivery and therapeutic applications in the central nervous system.