Experimental application of thermosensitive paramagnetic liposomes for monitoring magnetic resonance imaging guided thermal ablation

The use of a liposomal paramagnetic agent with a T-1-relaxivity that increases markedly at temperatures above the phase transition temperature (T-m) of the liposomal membrane was evaluated during magnetic resonance imaging (MRI) guided hyperthermia ablation. A neodymium-yttrium aluminum garnet (Nd-YAG) laser unit and a radiofrequency ablation system were used for tissue ablation in eight rabbit livers in vivo. One ablation was made in each animal prior to administration of the liposomal agent. Liposomes with a T-m of 57degreesC containing gadodiamide (GdDTPA-BMA) were injected iv, and two additional ablations were performed. T-1-weighted scans were performed in heated tissue, after tissue temperature had normalized, and 15-20 min after normalization of tissue temperature. Increase in signal intensity (DeltaSI) for ablations prior to injection of the agent was 13.0% (SD = 5.7) for the laser group and 9.1% (SD = 7.9) for the radiofrequency group. Signal intensity after administration of the agent unrelated to heating was not statistically significant (DeltaSI = 1.4%, P = 0.35). For ablations made after injection of the agent, a significant increase was found in the laser (DeltaSI = 34.5%, SID = 11.9) and radiofrequency group (DeltaSI = 21.6%, SID = 22.7). The persistent signal enhancement found in areas exposed to a temperature above the threshold temperature above T-m allows thermal monitoring of MRI guided thermal ablation. (C) 2004 Wiley-Liss, Inc.