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Purpose:
To test the hypotheses that (1) Directly injected intraparenchymal saturated NaCl solution can be monitored in vivo by MR fluoroscopy; (2) MRI can guide and monitor the creation of thermal lesions within injected saline; and (3) the injection of hypertonic saline is associated with creation of larger thermal lesions.
Materials and Methods:
20 MR guided RF ablations were performed in paraspinal muscles of 5 pigs using a 2cm tip copper electrode (Radionics, MA) and monopolar technique. All procedures were entirely guided and monitored on a 0.2 T open MR scanner (Magnetom Open, Siemens, Germany). For 10 ablations, 3 ml of saturated NaCl solution was first injected at ~1 ml/min under interactive MR guidance using a PSIF sequence (TR/TE/FA/NSA: 15.2/7.4/45/3, 5.85s temporal resolution). The RF electrode was then positioned within the injected saline under MR fluoroscopy. Ablations were performed at 90°C for 10 min. RF current and impedance were recorded at 1 minute intervals. Post-ablation scans consisted of T2WI, STIR and CE T1WI. Thermal lesion shape and signal characteristics were recorded and compared in the 2 groups. Lesion volumes were statistically compared. Animals were immediately sacrificed and muscles were harvested for pathology.
Results:
Injected saline was clearly detected as a growing bright signal on PSIF sequence in all injections. Subsequent interactive electrode positioning within the saline was successful in all trials. All 10 saline augmented thermal lesions were clearly depicted on each of post-ablation scans. Although less regular than standard lesions, augmented lesions were distinguishable from background fluid. These lesions displayed the same MR signal characteristics observed in standard lesions: dark on all sequences with surrounding bright rims on T2, STIR and CE T1. Mean volume of saline enhanced lesions was significantly larger (11 ± 3.6 ml) than that of standard lesions (5.9 ± 1.3 ml) (p<0.001). Mean RF current was significantly higher with enhanced (0.8 ± 0.1 A) than with standard (0.3 ± 0.04 A) ablations (p < 0.001). Mean impedance was significantly lower with enhanced (68 ± 7 W) than with standard (92 ± 7 W) ablations (p<0.001).
Conclusion:
This investigation demonstrates the feasibility of in vivo interactive monitoring of hypertonic saline injection and illustrates the ability of MRI to monitor the evolution of thermal lesions within such altered biological environment. It also supports the hypothesis that creating high ionic concentration in tissues allows the deposition of higher RF current and thereby the creation of significantly larger thermal lesion.
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