Development of a novel MR‐conditional microwave needle for MR‐guided interventional microwave ablation at 1.5T

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Abstract
Purpose To develop an MR-conditional microwave needle that generates a spherical ablation zone and clear MRI visibility for MR-guided microwave ablation. Methods An MR-conditional microwave needle consisting of zirconia tip and TA18 titanium alloy tube was investigated. The numerical model was created to optimize the needle's geometry and analyze its performance. A geometrically optimized needle was produced using non-magnetic materials based on the electromagnetics simulation results. The needle's mechanical properties were tested per the Chinese pharmaceutical industry standard YY0899-2013. The MRI visibility performance and ablation characteristics of the needle was tested both in vitro (phantom) and in vivo (rabbit) at 1.5T. The RF-induced heating was evaluated in ex vivo porcine liver. Results The needle's mechanical properties met the specified requirements. The needle susceptibility artifact was clearly visible both in vitro and in vivo. The needle artifact diameter (A) was small in in vivo (A(shaft:) 4.96 +/- 0.18 mm for T1W-FLASH, 3.13 +/- 0.05 mm for T2-weighted fast spin-echo (T2W-FSE); A(tip:) 2.31 +/- 0.09 mm for T1W-FLASH, 2.29 +/- 0.08 mm for T2W-FSE; tip location error [TLE](:) -0.94 +/- 0.07 mm for T1W-FLASH, -1.10 +/- 0.09 mm for T2W-FSE). Ablation zones generated by the needle were nearly spherical with an elliptical aspect ratio ranging from 0.79 to 0.90 at 30 W, 50 W for 3, 5, 10 min duration ex vivo ablations and 0.86 at 30 W for 10 min duration in vivo ablations. Conclusion The designed MR-conditional microwave needle offers excellent mechanical properties, reliable MRI visibility, insignificant RF-induced heating, and a sufficiently spherical ablation zone. Further clinical development of MR-guided microwave ablation appears warranted.

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