In physics, a shockwave as used in medicine is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary sound wave, a shockwave carries energy and can propagate through a medium but is characterized by an abrupt, nearly discontinuous change in pressure, and density of the medium.
This might sound a bit unfamiliar, but it describes a shockwave quite well as a short acoustic pulse of high amplitude (pressure) traveling through the soft tissue at the speed of sound. In medical application these pulses are repeated several times per second and several hundred up to a few thousand shockwaves are applied per therapy session.
To get the shockwave front to the region of interest, a metallic reflector can be used. With an reflector shaped like an half ellipsoid and the bubble generated in the first focus F1 of the ellipsoid, the shockwave front gets reflected and focused to the second focus F2. At F2 the shockwave has its highest intensity and pressure. The region of interest normally is placed around the F2. To transfer the shockwave from the reflector to the body, a silicone membrane is placed between the reflector and the body. With some ultrasound gel or water between the membrane and the body surface, the shockwaves can travel with only little losses into the tissue.
To generate a shockwave, an electric high voltage discharge between two electrode tips in water can be used. This discharge generates a gas bubble of partially ionized water which expands rapidly at a speed higher than the sound velocity. The expansion decelerates quickly and a shockwave front is released from the surface of the bubble at the very moment the expansion velocity drops below sound velocity.
