**16.1 Tissue temperature measurements**

Non uniform treatment or over-heating the treatment area may result in the risk of unwanted thermal damage to the skin during the treatment. To avoid or minimize this risk of a skin burn, real time thermal measurements are necessary. There are two basic methods of skin temperature measurements:


Advantages of IR thermometers is the speed of measurements and that they do not need to be built into the device thus are independent of the treatment. The obvious weakness of this method is collecting IR radiation from relatively large area which depends on distance from the measured area. You are also relying on a third party that is not linked in time of space to the thermal treatment being performed. Most importantly, you are not measuring the internal thermal profile.

A typical IR thermometer measures area which depends on distance between skin and thermometer and it varies from 1cm<sup>2</sup> to a few square inches at large distance from the patient. It allows you to monitor average skin temperature in treatment area but does not protect from appearance of small hot spots that lead to the full thickness skin burns.

The thermistors or thermocouples are extremely miniature and can be embedded into the electro-surgical instrument. Limitation of such contact measurements is response time which depends on heat transfer from the tissue to the sensor. However, special design allows to reduce response time to sub-second range.

Ideally, the user should know the temperature inside the body where energy is utilized for the fat coagulation and FSN tightening, and temperature on the skin surface above the treatment zone to ensure skin safety.

In addition, during the procedure sophisticated mechanisms monitor the tissue temperature together with its dynamic characteristics as the speed of temperature rise, allowing precautional measures before the critical temperature is reached.

Temperature monitoring for EBD is important not only for safety but also for treatment efficacy. Collagen contraction occurs in relatively narrow range of temperatures from 50 °C to 80 °C and overheating may result in denaturation of collagenous tissue and uncontrolled scar formation.

RFAL technology has maximum thermal safety measurements including:


#### **16.2 Monitoring of delivered energy**

Most types of energy cannot be monitored directly but rather electrical supply to the energy source is monitored. RF energy has unique properties resulting from continuity Eq. (4) allowing to measure RF voltage and RF current flowing through the tissue and get in real time all information about energy deposition in the tissue. Measurement of electrical RF parameters is not difficult engineering project and it can be performed every micro-second that allows to control the RF energy delivery even for very short pulses.

Measurements of RF current (*I*) and RF voltage (*V*) allows to calculate RF power (*P*) and RF impedance (*R*) using Ohm's law

$$R = \frac{V}{I} \tag{18}$$

and Joule's law

$$P = \mathbf{V} \, I \tag{19}$$

The RF energy can be calculated as integral of RF power measurement over the time:

$$E = \int\_{\mathcal{O}}^{t} \mathbf{P} \, dt \tag{20}$$

RFAL and Morpheus8 technologies of InMode Ltd. utilize all these measurements to control the treatment safety and efficacy.

#### **16.3 Impedance sensing and control of RF output**

Measurements of tissue impedance should be considered separately because of importance of this parameter for different aspects of treatment. The most obvious use of the impedance measurements is indication of contact between electrodes and treated tissue. Contact measurements are important to avoid poor coupling of the RF device with patient and avoid arcing. Contact monitoring has become a common feature for most RF-based devices.

Referring to **Figure 9** one can see that coagulation, dehydration of tissue and eschar formation result in impedance increase. Monitoring of tissue impedance can be used to limit heating process and avoid undesired treatment effect.

Another use of impedance monitoring is to control the lower limit, which may indicate that the distance is too small between electrodes. In RFAL technology it is used to reduce the risk of the cannula coming too close to the skin surface.

#### **16.4 Safety features of the RF devices**

All above mentioned measurements of RF parameters worth nothing if its not used for enhanced treatment safety helping physician to optimize the procedure.

The BodyTite device from InMode Ltd. uses RFAL technology, combines the maximal number of safety features, and should be used as the gold standard for safety features for RF devices.

Performing liposuction, the physician should be concentrated on safe manipulation with the minimally invasive accessory. Safety features related to the thermal component of the treatment should be implemented in automatic or in a very intuitive way not disturbing physician attention.

*The Basic Science of Radiofrequency-Based Devices DOI: http://dx.doi.org/10.5772/intechopen.96652*

The skin impedance for each patient is different and may vary for the different treatment zones, amount of tumescent applied or treatment depth. RF energy is adjusted by the device automatically to provide the required optimal energy to the patient.

Tissue impedance is monitored constantly by the BodyTite and the device automatically cuts RF energy if some of the limits are exceeded.

The user may set desired temperature cut-off limits for skin and internal electrode. The device applies full power when the temperature is significantly below the threshold and starts to reduce power automatically as treatment approaches the required target temperature. This scheme allows to avoid thermal overshooting and maintains desired heat profile. RF energy delivery is accompanied by an audible signal which speeds up as the cut-off temperature is approached, similar to modern car approaching wall while parking. RF power is switched on and off automatically to maintain the desired temperature as the user scans the treatment area with the cannula.

If the cannula accidentally comes too close to the dermis, the tissue volume between the electrodes is reduced and the applied RF power heats the tissue extremely fast. To address this issue, a temperature surge protection is implemented in BodyTite device. When the temperature sensor measures a temperature increase as too fast, the device automatically shuts RF energy and produces an audible sound to attract the physician's attention.

## **17. Summary**

RF based medical devices are a common tool for plastic surgeons, used during most surgical procedures. RFAL and RF fractional technologies have become important modalities for about 20% of plastic surgeons, for enhancing liposuction results or by its own for patients for whom reduction of adipose compound is not a main esthetic goal. Over the last 100 years extensive knowledge has been acquired about RF technology and RF-tissue interaction. The information in this chapter can help a potential buyer of new equipment make a rational choice, based on goals of treatment and physics of the RF device in question. Even more importantly, expanding the physician's understanding of his or her devices already in use can maximize treatment outcomes and minimize unwanted side effects and complications.
