D.4 MEDICAL APPLICATIONS
Magnetic resonance imaging. MRI combines a strong static magnetic field, rapidly changing gradient fields, and pulsed RF energy. The RF frequency is set by the main magnetic-field strength and is approximately 64 MHz for a 1.5 T hydrogen-imaging system and 128 MHz for a 3 T system. Unlike an ordinary communications transmitter, an MRI scanner delivers short, carefully controlled RF pulse sequences through a body or local transmit coil. The RF amplifier may be capable of several kilowatts to tens of kilowatts of peak output, but the pulses have limited duration and duty cycle; absorbed energy is therefore controlled principally through patient-specific absorption limits, commonly expressed as specific absorption rate, rather than by quoting a simple continuous transmitter power. RF heating is especially important where implants, cables, monitoring leads, or other conductive objects are present, because induced currents can cause localized heating or burns. The principal MRI hazards also include the static field and switched gradients, so MRI should not be treated solely as an RF source.
Therapeutic diathermy. Medical diathermy intentionally warms tissue using electromagnetic energy. Shortwave diathermy commonly operates at 13.56, 27.12, or 40.68 MHz, with 27.12 MHz widely used, while microwave diathermy commonly operates near 915 MHz or 2.45 GHz. Depending on treatment mode and equipment type, delivered RF output is commonly of the order of 50–500 W. Continuous-wave operation produces sustained heating, whereas pulsed shortwave systems may have peak powers of hundreds of watts but much lower time-averaged outputs. The field is concentrated between capacitive electrodes or around inductive applicators, although significant stray fields may occur near cables and applicator heads. Output setting, applicator position, treatment duration, tissue perfusion, and the presence of metal implants all influence the thermal dose; generator power alone does not define patient or staff exposure.
Surgical diathermy and RF ablation. Electrosurgical generators use high-frequency current, typically in the hundreds-of-kilohertz to low-megahertz region, to cut tissue, coagulate blood vessels, or produce controlled thermal destruction. General surgical generators commonly have selectable outputs from a few watts to about 300–400 W, depending on the cutting or coagulation mode. The energy is conducted through an electrode in direct contact with tissue rather than intentionally radiated into free space, although currents and electric fields can occur around leads and instruments. RF ablation systems are usually lower powered and more tightly controlled: cardiac ablation commonly uses approximately 20–60 W, with one commercial cardiac generator rated at 60 W, while some tumor, vein, spinal, or gastrointestinal systems operate from several tens of watts to about 100–200 W. Power, application time, electrode size, contact quality, irrigation, tissue impedance, and temperature feedback determine the resulting lesion. Microwave ablation systems operating near 915 MHz or 2.45 GHz form a related but distinct category and commonly provide approximately 30–150 W.