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

The use of a liposomal paramagnetic agent with a T1‐relaxivity that increases markedly at temperatures above the phase transition temperature (Tm) 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 Tm of 57°C containing gadodiamide (GdDTPA‐BMA) were injected iv, and two additional ablations were performed. T1‐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 (ΔSI) 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 (ΔSI = 1.4%, P = 0.35). For ablations made after injection of the agent, a significant increase was found in the laser (ΔSI = 34.5%, SD = 11.9) and radiofrequency group (ΔSI = 21.6%, SD = 22.7). The persistent signal enhancement found in areas exposed to a temperature above the threshold temperature above Tm allows thermal monitoring of MRI guided thermal ablation. Magn Reson Med 52:1302–1309, 2004. © 2004 Wiley‐Liss, Inc.