Therapeutic ultrasound uses high-frequency sound waves to treat musculoskeletal conditions. These sound waves create pressure when applied to the skin. A key factor influencing ultrasound’s effects is the duty cycle, which determines how the energy is delivered. The duty cycle refers to the proportion of time the ultrasound beam actively transmits energy during a treatment cycle.
Understanding Duty Cycle in Ultrasound
Duty cycle refers to the ratio of the ultrasound beam’s “on-time” to the total time of one pulse cycle, which includes both the “on-time” (pulse duration) and the “off-time” (rest period). This ratio is expressed as a percentage. For example, a 50% duty cycle means the ultrasound transmits energy for half the treatment time and is off for the other half.
Varying the duty cycle directly controls the total acoustic energy delivered to tissues. A higher duty cycle means more continuous energy delivery, while a lower duty cycle introduces more intermittent rest periods.
Pulsed vs. Continuous Ultrasound
The duty cycle differentiates continuous wave (CW) ultrasound from pulsed wave (PW) ultrasound. Continuous ultrasound operates at a 100% duty cycle, emitting sound waves without interruption throughout treatment. This constant emission results in a sustained transfer of acoustic energy, leading to a primary thermal effect, which is an increase in tissue temperature.
Pulsed ultrasound, in contrast, uses a duty cycle less than 100%, ranging from 20% to 50%. This intermittent delivery includes “off” periods that allow tissues to cool, minimizing significant temperature increases. As a result, pulsed ultrasound primarily emphasizes non-thermal, mechanical effects on tissues, rather than heat generation.
Therapeutic Applications and Effects
The choice of duty cycle directly influences the physiological effects and therapeutic applications. Continuous ultrasound, with its 100% duty cycle, is primarily used for thermal effects in deep tissues, reaching depths of up to 5 centimeters. This deep heating can increase blood flow, relax muscles, reduce joint stiffness, and decrease chronic pain and inflammation. Conditions like chronic inflammation, muscle spasms, and joint contractures often benefit from continuous ultrasound, helping prepare tissues for stretching or other interventions.
Pulsed ultrasound, with lower duty cycles (e.g., 20%, 33%, 50%), focuses on non-thermal, mechanical effects, making it suitable for acute injuries where heating is not desired. These mechanical effects include stable cavitation (vibration of microscopic gas bubbles) and acoustic streaming (movement of fluids along cell membranes). Such mechanical actions can:
- Increase cell membrane permeability
- Stimulate fibroblastic activity
- Enhance intracellular calcium levels
- Promote protein synthesis
All of these contribute to tissue healing and repair. Pulsed ultrasound is commonly used to reduce inflammation, decrease swelling, and promote healing in acute injuries like sprains, strains, and tendinitis.
Safety Considerations
Therapeutic ultrasound is a safe procedure when administered by trained professionals who understand its parameters and contraindications. Proper dosage and technique are important to avoid adverse effects. The ultrasound probe should remain in constant motion on the skin to prevent localized overheating and potential superficial burns.
There are specific situations where therapeutic ultrasound should be avoided. Contraindications include application over cancerous lesions, areas with active bleeding or infection, and over the uterus during pregnancy due to unknown effects on the fetus. It is also not recommended over pacemakers, metal implants (due to potential for rapid heating), eyes, reproductive organs, or growth plates in children. Individuals with impaired sensation or circulation in the treatment area should also avoid ultrasound, as they may not be able to detect overheating or have reduced ability to cool the tissue.