Radiofrequency ablation has attracted increasing attention as a minimally invasive method of treating hepatic malignancies.1 Image guidance can play an important role in enhancing the safety and effectiveness of ablative therapy. Imaging should ensure that ablation is precise, enabling complete coagulation of tumor tissue within a safety margin. The applicator should be positioned and energy delivered without injuring critical structures.
The ideal targeting technique should delineate tumor tissue and the surrounding anatomy clearly. It should offer real-time imaging, as well as multiplanar and interactive capabilities. The guiding modality should visualize therapy effects during intervention so that target tissue coverage can be assessed and unintended thermal injury avoided.
Interventional MRI meets all these requirements.2 The modality provides excellent soft-tissue contrast, multiplanar capabilities, and good sensitivity to thermal effects throughout the ablation procedure.3,4 In fact MRI is the only imaging technique capable of monitoring acute thermal effects.
RF ablation uses alternating electric current oscillating at a high frequency to induce thermal injury in target tissue. The electric current causes frictional heat from ion agitation. Near-immediate tissue coagulation is induced at temperatures between 60°C and 100°C, the target temperature for RFA. At temperatures higher than 100°C or 110°C, the tissues vaporize and carbonize, reducing the effectiveness of applied energy.1
In monopolar RF ablation, the electric circuit is closed between an "active" electrode, which is placed within the target tissue, and grounding pads on the body surface. In bipolar RFA, both electrodes are placed within the target tissue and no grounding pads are required. The two electrodes can be located on the same applicator shaft or on different applicator shafts.5,6
The specifications for an RF device that is appropriate for MR-guided RFA include low receptiveness for mechanical movement or heating by the MR system, good visibility, and few artifacts on MR images. Magnetic susceptibility differences between the applicator and surrounding tissue can produce a signal void in the vicinity of the applicator, which may lead to an apparent widening on MRI. Magnetic field disturbance at the needle tip may additionally cause a blooming ball-shaped signal void. The center of this signal void still provides a good estimate of the actual location of the needle tip, however.7
Artifacts at the needle tip depend on sequence parameters, as well as the material properties and geometric shape of instruments.
