MR imaging was performed with an open-magnet 0.23T system (Panorama®, Philips Medical Systems, The Netherlands). RF ablation was done with standard RF generator (Radionics®, Tyco Healthcare, Zürich, Switzerland) which was placed outside the magnetic shielding. Application of RF energy was performed within the magnetic field with a special MR-compatible 17G probe (Celon AG, Berlin, Germany). The probe length was 15 cm and the length of the active tip was 3 cm. Probe placement was done with an interactive device using an optical 3D tracking device (iPath 200®, Philips Medical Systems, The Netherlands) (Fig 1). This system also allows free selection of the image plane with respect to the probe path. The impedance-controlled, cooled-tip mode was used with standard parameters (15 minutes / 100-135 W) for energy deposition within tissue.

MR compatibility of the RF probe and connecting devices, imaging properties of the probe and macroscopic effects of RF delivery were first tested in ex vivo pig liver. T1W images were obtained before and in regular intervals of 2 minutes after RF application (Fig. 2).

Six patients, aged from 36 to 70 years, underwent MR-guided RF ablation with the above described probe and technique. The clinical indications for treatment were as follows (table 1). Four patients presented liver metastases from different origins and two patients presented lesions of hepatocellular carcinoma (HCC) in cirrhotic livers. In all cases, the tumor nodules had been diagnosed with MR imaging and they were not adequately visible with US, either due to poor lesion contrast or due to changes induced by previous RF treatment. Five lesions were situated just under the diaphragm, thus rendering the angle for probe placement impractical for CT. All patients gave informed consent to undergo this treatment. RF treatment was done under general anesthesia in all instances. Eleven tumor nodules (eight liver metastases measuring 12-24 mm in maximal diameter and three HCC lesions measuring 17-24 mm) were treated in 8 sessions. Preliminary imaging of the entire liver was done with T2W sequence respiratory triggering and a T1W breathhold sequence (Fig. 3). A ring-shaped coil was placed over the liver region to be treated. Probe placement was done with the use of the iPath 200® device (Fig. 4A) and with intermittent MR monitoring with contiguous T1W slices of 5 mm thickness during breathhold intervals (Fig. 4B). Once adequate probe position had been achieved, RF application was done. Following RF, “hot withdrawal” of the probe was performed at maximal power output, in order to coagulate the needle track and prevent tumor seeding. Then, MR images were obtained immediately after procedure with T1-W and T2-W images. Follow-up was documented at 1, 3, 6, 9 and 12 months. In metastases, we used contrast-enhanced MR imaging with Mn-DPDP and in HCC lesions, we used either dynamic CT or dynamic Gadolinium-enhanced MRI at 1.5T.