Radiofrequency energy is characterized by a frequency between 3 Hz and 300 GHz. Wave frequency is the main characteristic defining energy – tissue interaction since it characterizes both penetration length (which affects the power distribution) and absorption rate (which affects the heating velocity). At frequencies below the visible light simply by increasing the frequency, the absorption rate of water increases. Due to this absorption, the intensity of the electromagnetic field steeply decreases during crossing electromagnetic tissues. Penetration length is defined as the distance traveled before the intensity of the field decreases to one third of the initial value. Absorption rate and penetration length are inversely proportional: a high absorption rate causes a short penetration and vice versa.

In clinical applications radiofrequency energy is applied as a continuous sinusoidal waveform at a frequency between 400 and 500 kHz. The radiofrequency current flows into tissues through the electrode’s active tip (which is uninsulated) and causes ions such as sodium, chloride, and potassium to oscillate at a frequency between 400 and 500 kHz. This rapid ionic movement results in friction and subsequent heating with coagulation necrosis as end result. The produced heat is primarily generated in the tissue around the electrode’s active tip and is conducted both inward (to the cannula lumen) and outward (creating concentric tissue lesions of lower temperature as the distance increases).

Clinical indications for application of radiofrequency energy include cutting and cauterization in surgical operations, treatment of chronic venous insufficiency, renal denervation, cardiac arrhythmias and Barrett’s esophagus, neoplasm destruction (for both benign and malignant cases), and neurolysis for pain management. Radiofrequency energy in clinical applications work through a closed circuit: the radiofrequency electrode acts as the circuit’s cathode, and applied ground pads act as the anode with energy being conducted from the generator to the electrode through the tissues to the grounding pads (Fig. 2.2). The electrode’s small sectional area results in very high energy flux, while the large cross-sectional area of ground pads disperses and minimizes the energy flux. The electromagnetic field induces ohmic dissipation of the ionic currents resulting in heating. The dissipated power density governs the generated heat amount. This relation is proportional to the square power of the current and to tissue properties (impedance).

Radiofrequency ablation depends upon current intensity, lesion duration, distance from the active tip, as well as electrical and thermal tissue conductivity for increasing the temperature locally. Around the radiofrequency electrode, the heating pattern can be subdivided in three distinct areas (Fig. 2.3):

The properties of the active heating actually set the ablation footprint: the geometry of the necrosis is determined by the power’s distribution, while the duration of the ablation is determined by the active heating velocity. Tissue impedance measures the tissue resistance “opposed” to the radiofrequency current circulation and is governed by the tissue hydratation.

Temperature increase is lethal for mammalian tissue. Exposure to 50 °C for 1 h results in cell death; exposure to 60 °C results in immediate cell death [3]. At 100 °C death is instantaneous; evaporation (with micro-bubbles formation) and charring occur immediately. The desiccated tissue acts as insulation preventing further energy conduction and thus limiting tissue destruction. In order for achieving large ablation volumes and extensive tissue destruction, time is equally important to tissue conductivity. The objective is to heat the tissue in temperatures between 50 °C and <100 °C for 4–6 min without causing charring and tissue vaporization [3].

Apart from charring and tissue vaporization, “heat sink” effect limits the effectiveness of radiofrequency ablation. When the target lies close to a vessel with diameter > 3 mm, blood flow “cools down” the tissue during ablation preventing production of lethal temperatures. The potential result is viable residual tumor tissue near the vessel wall. Successful ablation is governed by coagulation necrosis extent which depends upon deposited energy; the latter is governed by local tissue interactivity minus heat lost. This has been expressed by Goldberg and Depuy in the form of the following equation:

[4].