Jimenez Lozano Joel N, Vacas-Jacques Paulino, Anderson R Rox, Franco Walfre
Wellman Center for Photomedicine, Harvard Medical School, Massachusetts General Hospital, Boston, MA 02114, USA.
Lasers Surg Med. 2013 Jul;45(5):326-38. doi: 10.1002/lsm.22146. Epub 2013 Jun 3.
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.
We used the finite element method to model the two-dimensional, time-dependent, electro-thermal response of a three-layer tissue (skin, subcutaneous fat, and muscle). We considered two different configurations 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.
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.
射频（RF）能量暴露是一种在皮肤和皮下组织内产生热量的常用非侵入性方法。皮下脂肪由精细的胶原纤维间隔组成，这些间隔与成群的脂肪细胞交织在一起，脂肪细胞具有独特的结构、电学和热学特性，会影响射频能量的分布和沉积。本研究的目的是：（i）确定纤维间隔在射频加热中的电学和热学效应；（ii）研究被纤维间隔包围的单个脂肪小叶的射频加热情况；（iii）讨论其临床意义。
我们使用有限元方法对三层组织（皮肤、皮下脂肪和肌肉）的二维、随时间变化的电热响应进行建模。我们考虑了皮下脂肪组织的两种不同结构：仅为均匀的脂肪层和带有间隔的蜂窝状脂肪层。纤维间隔的结构在解剖学上是准确的，由人类微观磁共振成像（micro-MRI）的矢状图像构建而成。对于应用于皮肤表面的大电极，结果表明，某些间隔中的吸收电功率密度高于周围的脂肪小叶，有利于电流密度的通量。与电场平行排列的纤维具有更高的电通量，因此吸收更多的功率。在射频能量传递期间及之后，间隔会随着时间发生热传递。有纤维间隔的脂肪中温度升高幅度更大。
间隔的存在会影响静电场的局部分布，促进电流通量，并增强皮下脂肪层的整体电功率吸收。与局部电场对齐的纤维间隔比垂直于电场方向的间隔具有更高的吸收功率密度。单个脂肪小叶通过局部功率吸收以及最终通过从周围间隔扩散而立即获得热量。
