Radiofrequency energy has a wide range of medical applications, including noninvasive treatment of wrinkles and body contouring. This technology works by differential heating of skin and soft tissue layers causing dermal remodeling or adipolysis, ultimately leading to observable effects. This article reviews the physics of radiofrequency as applied clinically.
Key points
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Radiofrequency energy has multiple medical applications, including many noninvasive aesthetic uses.
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Nonablative technology, through capacitive coupling, can deliver therapeutic energy to dermal structures without damaging the overlying epidermis.
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Radiofrequency technology includes multiple indications for skin tightening, body contouring, or promotion of healing, with low morbidity and a limited side effect profile.
Introduction
Radiofrequency (RF) energy is used frequently in a variety of consumer technologies, such as radio and television broadcasting, telecommunication, microwave ovens, and radar systems. RF energy has long been used for medical applications, dating back to the 1920s as a means of electrocautery. At present, RF technologies extend to a multitude of clinical uses, ranging from tumor ablation to MRI to an assortment of noninvasive aesthetic practices.
The first monopolar RF device (Thermage, Hayward, CA) was approved by the US Food and Drug Administration (FDA) for use on periorbital tissues in 2002. Given its safety profile, FDA approval was quickly expanded to full facial use in 2004 and then body contouring in 2006. Since this initial RF device, delivery devices have continued to advance with a growing range of indications. As consumer demand for noninvasive aesthetic procedures increases, there will continue to be an expanding market for this evolving technology. This article reviews the background physics of RF as applied clinically for minimally invasive aesthetic treatment purposes.
Introduction
Radiofrequency (RF) energy is used frequently in a variety of consumer technologies, such as radio and television broadcasting, telecommunication, microwave ovens, and radar systems. RF energy has long been used for medical applications, dating back to the 1920s as a means of electrocautery. At present, RF technologies extend to a multitude of clinical uses, ranging from tumor ablation to MRI to an assortment of noninvasive aesthetic practices.
The first monopolar RF device (Thermage, Hayward, CA) was approved by the US Food and Drug Administration (FDA) for use on periorbital tissues in 2002. Given its safety profile, FDA approval was quickly expanded to full facial use in 2004 and then body contouring in 2006. Since this initial RF device, delivery devices have continued to advance with a growing range of indications. As consumer demand for noninvasive aesthetic procedures increases, there will continue to be an expanding market for this evolving technology. This article reviews the background physics of RF as applied clinically for minimally invasive aesthetic treatment purposes.
Biophysics
A basic grasp of physics is necessary to understand RF technologies and their potential applications. By definition, RF includes any electromagnetic wave frequencies within the range of 3 kHz to 300 GHz. An RF field is composed of both electrical and magnetic components. These components are typically measured in volts per meter (V/m) for an electric field and amperes per meter (A/m) for a magnetic field. RF waves are further characterized by frequency and wavelength ( Table 1 ).
| Term | Definition | Units |
|---|---|---|
| Electric field | Electromagnetic force per unit charge | V/m |
| Magnetic field | Magnetic effect of electric currents and magnetic materials | A/m |
| Wavelength | Distance covered by 1 complete cycle of an electromagnetic wave | Distance (m) |
| Frequency | Number of occurrences of a repeating event per unit of time | Hz |
| Impedance | Resistance to current within a circuit when voltage is applied | Ohms |
| Radiation | Energy emitted as waves or particles | V |
| Specific absorption rate | Amount of radiation absorbed | W/kg |
When RF is applied by an alternating current, an electric field is generated throughout the skin. Rapid shifts in polarity within this electric field then cause alterations in particle orientation. All tissues have some intrinsic resistance (impedance) to these oscillating electrons and this causes heat to be generated as the electric current is converted to thermal energy. This relationship can be described using Ohm’s law ( Fig. 1 ).
Ultimately, this heat generated by variable resistance within biological tissue results in the observed clinical effects. These thermal effects from RF energy are similar to the way a microwave heats food. Because different biological tissues have variable levels of impedance, a specific frequency of applied RF has divergent effects in different tissue types. In particular, the eyes and testes are more sensitive to RF energy. Successful transfer of RF energy depends on both the size and depth of the tissue being treated ( Table 2 ).
| Frequency | RF Designation | Effects |
|---|---|---|
| 3–30 Hz | Extremely low frequency | Causes tingling sensation, often imperceptible |
| 3–30 kHz | Very low frequency | Low level communications |
| 30–300 kHz | Low frequency | Long-distance communications |
| 30–300 GHz | Extremely high frequency | Alters cell growth and proliferation, stimulates repair |
| 0.3–3 THz | Terahertz radiation | Medical imaging, spectroscopy |
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