Figure 2. A graphic display of power (in watts) and tissue impedance (in ohms) during RFA of a malignant liver tumor. Using ultrasound guidance, the LeVeen needle electrode is placed into the tumor and the multiple array is deployed. Treatment is then initiated at 50 W of power and increased by 10 W at 1-minute intervals up to a maximum power of 90 W. At 8 minutes, RFA-induced coagulative necrosis of the tumor is occurring; the tissue impedance at this point rapidly rises to more than 200 ohms and the power output precipitously falls (roll-off). After waiting 20 seconds to allow heat in the tumor to dissipate, RF energy is again applied at approximately 75% (70 W) of the maximum power achieved, until tissue impedance again rises and power rolls off. This two-phase application of RF energy is performed in each area treated with the LeVeen multiple array needle electrode.