Purpose: To test whether (1) the combination of cool tip RF electrode with direct intraparenchymal saturated saline injection results in larger thermal lesions than those produced by either of the 2 techniques alone; (2) the resultant tissue necrosis is complete; and (3) the process can be guided and monitored by MRI.
Methods & Materials:33 MR guided RF-ITAs were performed in the thigh muscles of 6 pigs using a 1cm tip, 17G RF electrode (Radionics, MA) for 10 min. Randomizing ablation order and locations, experiments included 11 ablations using cool tip electrode to ablate normal tissue, 11 wet ablations using non-cooled electrode to ablate tissues preinjected with saturated NaCl solution, 9 combined cool-wet ablations, and 2 standard non-cooled ablations in normal tissue. Electrode tip temperature was kept at 90°C during non-cooled ablations and at 10-20°C during cool and cool-wet ablations. Post-ablation scans consisted of T2WI, STIR and CE T1WI. Animals were immediately sacrificed to harvest muscles for pathology. The last 6 specimens were TTC stained.
Results:Mean thermal lesion volume in cool-wet group (11.7 ± 6.2 ml) was significantly larger than that in cool (5.8 ± 2.1 ml, p= 0.03), wet (5.3 ± 2 ml, p= 0.02), or standard group (2.2 ± 0.8 ml, p<0.01). Cool-wet ablations were associated with significantly higher RF current and lower impedance. Mean current was 1.4 ± 0.4 A for cool-wet, 0.7 ± 0.1 A for cool (p<0.001), 0.5 ± 0.1 A for wet (p<0.001), and 0.3 ± 0.04 A for standard group (p<0.001). Mean impedance was 55 ± 14 W for cool-wet, 75 ± 6 W for cool (p<0.001), 53 ± 10 W for wet (p=0.3), and 69 ± 5 W for standard group (p<0.001). Cool ablations were associated with significantly higher RF current (p<0.001) but insignificantly larger mean lesion volume (p=0.6) than wet ablations. TTC staining showed complete tissue necrosis in all groups. All procedures were successfully guided by MR fluoroscopy. Standard and cool ablations were uniformly oval while wet and cool-wet ablations showed irregular yet definable outlines. All lesions displayed dark signal surrounded by bright rim on T2WI, STIR and CE T1WI.
Conclusion:Intraparenchymal saturated saline injection has a synergistic effect on tissue necrosis volume when combined with cool tip RF technology. This study reports the ability of the hybrid technique to achieve up to 21ml volume and 4.5cm diameter of tissue necrosis through a single application with a 1cm tip electrode length. It also highlights the feasibility of guiding and monitoring the entire procedure within the MR suite in a manner similar to that currently practiced with cool tip RF ablations.
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| CE-T1WI of thermal lesions created by standard (A), cool (B), wet (C), and cool-wet (D) techniques. The significantly larger size of lesions produced by cool-wet ablation is obvious. Also note the rather irregular outlines of lesions in C and D. The image scale is identical in parts A-D. |
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