Microbubbles as novel pressure-sensitive MR contrast agents

Magnetic resonance imaging contrast agents that are sensitive to pressure would be useful for evaluating cardiovascular function. One such potential contrast agent conslsts of gas‐filtod liposome microbubbles. The magnetic susceptibility of the microbubbles locally perturb the statte magnetic field, which influences the transverse‐relaxation properties of the surrounding medium. Changes in the pressure atter the bubble dimensions, which affects the magnetic field perturbations and, hence, the transverserelaxation. The effect of these microbubbles on the T2 relaxation times of a water‐based medium was measured for liposomes filled with different gases—nitrogen, argon, air, oxygen, xenon, neon, perflu‐oropentane, perfluorobutane, and sulfur hexafluoride. The air‐filled, perfluoropentane‐fllled and the oxygen‐filted liposomes demonstrated the largest effect on transverse‐relaxation. The influence of pressure on both gradient‐echo and spin‐echo signal intenstties for air‐filled microbubbles was also evaluated. Pressure‐induced changes in signal intensity were consistently observed for both the spin‐echo and gradient‐echo pulses sequences.