Introduction
This topic covers the use of ultrasound (US) to treat tissue. The two main therapeutic actions are thermal, where the energy is absorbed and turned into heat, and mechanical, where the wave causes cavitation and vibration. The topic also covers sonophoresis, the use of ultrasound to drive drugs through the skin, and high-intensity focused ultrasound (HIFU), which destroys tissue at a chosen focal point.
Physical basis of therapeutic ultrasound
Therapeutic ultrasound uses frequencies from about 0.5 to 3 MHz for deep and superficial tissues, and frequencies in the kilohertz range for sonophoresis. The intensity of the wave is the power per unit area, and it falls as the wave travels through tissue according to I = I_0 e^{-\mu x}, where μ is the attenuation coefficient of the tissue and x is the depth. Attenuation is greatest in bone and gas-filled structures and least in fluid. Higher frequency means stronger attenuation and a shallower effect.
Mechanisms of action
- Thermal effect: part of the wave energy is absorbed by tissue and heats it. Heating increases the extensibility of collagen, reduces joint stiffness and muscle spasm, and improves local blood flow.
- Cavitation: the alternating pressure creates gas bubbles that grow and collapse. Stable cavitation makes bubbles oscillate without collapsing; inertial (transient) cavitation makes them collapse violently and can damage cells.
- Acoustic streaming: the wave pushes fluid in a steady flow near the bubbles and cell membranes, which increases the transport of substances across cell membranes.
- Radiation pressure: the wave exerts a small steady pressure on the tissue surface, which is used in some diagnostic and manipulation methods.