Lasers in Surgery and Medicine 45:326–338 (2013) Effect of Fibrous Septa in Radiofrequency Heating of Cutaneous and Subcutaneous Tissues: Computational Study Joel N. Jimenez Lozano, PhD, Paulino Vacas-Jacques, PhD, R. Rox Anderson, MD, Walfre Franco, PhD* Wellman Center for Photomedicine, Harvard Medical School, Massachusetts General Hospital, Boston, Massachusetts, 02114 Background and Objectives: Radiofrequency (RF) energy exposure is a popular non-invasive method for generating heat within cutaneous and subcutaneous tissues. Subcutaneous fat consists of fine collagen fibrous septa meshed with clusters of adipocytes having distinct structural, electrical and thermal properties that affect the distribution and deposition of RF energy. The objectives of this work are to (i) determine the electric and thermal effects of the fibrous septa in the RF heating; (ii) investigate the RF heating of individual fat lobules enclosed by fibrous septa; and, (iii) discuss the clinical implications. Methods and Results: We used the finite element method to model the two-dimensional, time-dependent, electro- thermal response of a three-layer tissue (skin, subcutane- ous fat, and muscle). We considered two different config- urations of subcutaneous fat tissue: a homogenous layer of fat only and a honeycomb-like layer of fat with septa. Architecture of the fibrous septa was anatomically accurate, constructed from sagittal images from human micro-MRI. For a large electrode applied to the skin surface, results show that the absorbed electric power density is greater in some septa than in the surrounding fat lobules, favoring the flux of electric current density. Fibers aligned parallel to the electric field have higher electric flux and, consequently, absorb more power. Heat transfer from the septa occurs over time during and after RF energy delivery. There is a greater temperature rise in fat with fibrous septa. Conclusions: The presence of septa affects the local distribution of the static electric field, facilitates the flux of electric current and enhances the bulk electric power absorption of the subcutaneous fat layer. Fibrous septa aligned with the local electric field have higher absorbed power density than septa oriented perpendicular to the electric field. Individual fat lobules gain heat instantly by local power absorption and, eventually, by diffusion from the surrounding septa. Lasers Surg. Med. 45:326–338, 2013. ß 2013 Wiley Periodicals, Inc. Key words: skin; fat; hypodermis; cellulite; electrother- molysis; heat transfer; hyperthermia INTRODUCTION Radiofrequency (RF) energy is a form of electromagnetic (EM) radiation. When EM radiation interacts with matter, it can be absorbed, transferring the energy to the medium. The absorption process is divided into certain categories that correspond to modes of molecular energy storage. These categories include thermal, vibrational, rotational, and electronic modes [1]. The thermal mode of energy storage is due to the translational movement modes, in which atoms move horizontally and vertically about their lattice points in a medium, usually referred to as heat. The amount of energy that a material will absorb from radiation depends on the operating frequency, intensity of beam, and the duration of exposure. The most important of these parameters is the frequency. RF is in the range of hundreds of kilohertz (kHz) to a few megahertz (MHz). RF energy exposure is a well-established method for generat- ing heat, which can be used to cut or induce metabolic processes in the body target. Therapies using EM sources, like RF, are known as thermal therapies. Thermotherapy, or thermal therapy, encompasses all therapeutic treat- ments based on the transfer of thermal energy into or out of the body. In clinical situations, the most important objective of thermal therapy is to achieve an efficient treatment outcome without damaging normal tissues. In general, thermal therapy is categorized into three different modalities: diathermia, heating up to 418C; hyperthermia, temperatures in the 41–458C range; and necrosis or ablation at high temperatures, above 458C [1]. RF devices for medical applications are monopolar, bipolar, or multipolar. Monopolar systems deliver current through a single contact point (electrode), usually with a large, distant grounding pad (return) for the current flow to complete the electrical circuit. Bipolar devices circulate electrical current between two electrodes, and multipolar devices between a larger number of electrodes, in which no grounding pad is necessary. RF energy is commonly used Conflict of interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.  Correspondence to: Walfre Franco PhD, Massachusetts Gen- eral Hospital, Wellman Center for Photomedicine, Harvard Medical School, 50 Blossom St, Boston, MA 02114. E-mail: wfranco@partners.org Accepted 23 April 2013 Published online 3 June 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/lsm.22146 ß 2013 Wiley Periodicals, Inc.