ABSTRACT
Since the 1940s, diathermy has been used in rehabilitation medicine to relieve pain from sprains and strains. To accomplish therapy, radiofrequency (RF), microwave (MW), and ultrasound (US) energy have been used for deep tissue heating to increase blood flow and collagen tissue extensibility as well as to decrease joint stiffness and muscle spasm [1]. Since the mid-1970s, moderate temperature hyperthermia (40–45°C for 30–60 min) has been applied in combination with ionizing radiation and/or chemotherapy to treat cancer [2–7]. Due to widely varying requirements for controllably heating tissue in the head, thorax, pelvis, and extremities, equipment for heating tumors located near the surface or deep in the body continues to evolve to this day [8–10]. External heating systems generally rely on deposition of electromagnetic (EM) energy via electric fields radiated or capacitively coupled into the body, magnetic fields inductively coupled into the body, or ultrasound acoustic pressure fields conducted into the body. In the 1980s, there was extensive development of miniature implantable heat sources based on resistively or capacitively coupled RF currents, circular or linear polarization MW antennas, optical fiber mounted laser-illuminated diffuser crystals, or various hot source techniques all designed to produce moderate temperature rise in tumor when implanted in a closely spaced array of sources [11]. Over the next decade, these same implanted heat sources were adapted to apply higher powers to achieve complete tissue necrosis, or thermal ablation, at tissue temperatures between 50°C and 100°C for clinical applications like treating cardiac arrhythmias [12–14], and soon thereafter also for malignant tumors [15–17]. Diathermy, hyperthermia, and ablation all use the same EM energy deposition fundamentals to achieve different tissue temperature profiles that address unique clinical conditions and diseases. Other medical applications using the propagating characteristics of EM fields, such as radiofrequency telemetry to couple sound signals to implanted hearing devices [18,19], electroporation [20], microwave radiometry for temperature monitoring [21–24], active microwave imaging [25,26], and low frequency RF for wound healing, nerve regeneration, or nonthermal wave propagation applications are not discussed further in this review.
