MICROBUBBLES AS NOVEL PRESSURE-SENSITIVE MR CONTRAST AGENTS

Magnetic resonance imaging contrast agents that are sensitive to press ure would be useful for evaluating cardiovascular function, One such p otential contrast agent consists of gas-filled liposome microbubbles. The magnetic susceptibility of the microbubbles locally perturb the st atic magnetic field, which influences the transverse-relaxation proper ties of the surrounding medium. Changes in the pressure alter the bubb le dimensions, which affects the magnetic field perturbations and, hen ce, the transverse-relaxation. The effect of these microbubbles on the T-2 relaxation times of a water-based medium was measured for liposom es filled with different gases-nitrogen, argon, air, oxygen, xenon, ne on, perfluoropentane, perfluorobutane, and sulfur hexafluoride. The ai r-filled, perfluoropentane-filled and the oxygen-filled liposomes demo nstrated the largest effect on transverse-relaxation. The influence of pressure on both gradient-echo and spin-echo signal intensities for a ir-filled microbubbles was also evaluated, Pressure-induced changes in signal intensity were consistently observed for both the spin-echo an d gradient-echo pulses sequences.