Generation and observation of radio frequency thermal lesion ablation for interventional magnetic resonance imaging

RATIONALE AND OBJECTIVES. Recently, there has been increased interest in interventional magnetic resonance (MR) imaging and minimally invasive cancer therapy via radio frequency (RF) thermal ablation. In this work, we examined RF thermal lesion generation in phantoms and ex vivo bovine liver and correlated them with MR images under a variety of conditions, which begins our assessment of the role of MR imaging in this new method for cancer treatment. METHODS. Radio frequency lesions were created in gel phantoms and ex vivo bovine liver, using stationary (bovine liver) and variable speed (gel) moving electrodes to create lesions with shapes mimicking tumors. Ex vivo bovine liver lesions were made with the tissue held at room temperature (n = 4) and in a 37°C saline bath (n = 3) using a 16-gauge electrode (tip temperature: 70°C, 80°C, and 90°C; ablation time: 1-3 minute). Electrical impedance and RF power were plotted during ablation. After ablation, RF- induced lesions were imaged with a 0.2-tesla (T) MR system using a variety of pulse sequence. RESULTS. Complex shaped lesions were created successfully in phantoms. Averaged maximum ex vivo lesion volume made at 90°C ablation experiments holding the tissue temperature at 37°C and at room temperature were 1.58 ± 0.35 cm³ and 1.0 ± 0.26 cm³ respectively (confidence interval: 90%). The aspect ratios and RF power of the lesions decreased as ablations proceeded. Impedance dropped during the first 2 minutes of the ablation. Ex vivo lesions appeared as regions of low-signal amplitude in T2- weighted MR images. CONCLUSIONS. Phantom ablation experience may be useful and applicable in thermotherapy planning. Lesions made in ex vivo bovine liver held at 37°C via a saline hath are larger than those created at room temperature. Lesion shapes are ablation time dependent until thermal equilibrium is reached. Impedance reduction and lesion formation are related; 0.2-T MR systems can image RF energy-induced thermal lesions.