Ultrasound/Catalog of methods in biophysics

Ultrasound has frequencies higher than 20,000 Hz, they are inaudible to humans. However, some animals can also hear higher frequencies, e.g. a dog up to 35 kHz, a cat up to 50 kHz, a bat up to 98 kHz.

Biological effects

Biological effects are caused by the absorption of sound energy in the tissue.

Absorption of ultrasound

• in liquids and solids it is less compared to absorption in gases
• depends on the frequency, or wavelength

Biological effects - in medicine

• increasing membrane permeability
• violation of the conductivity of nerve fibers - dampening effect on the transmission of impulses
• tissue pH change
• analgesic and spasmolytic effect - pain relief through complex direct and indirect mechanisms
• softening of fibrous tissue altered by disease processes
• improving trophic by increasing local blood circulation and increasing metabolism

Diagnostic use of ultrasound

Ultrasound penetrates living tissue well, and with appropriately chosen intensities, ultrasound imaging is risk-free. Due to the inhomogeneity of the tissues, the ultrasound is partially reflected in them at the acoustic interfaces, which makes them visible.

Therapeutic use of ultrasound

• to treat a lower frequency than in diagnosis (about 800 to 1000 kHz)
• the intensity depends on the therapeutic intention
• rehabilitation medicine
• utilization due to the above mentioned biological effects
• dentistry - so-called low-frequency ultrasound (20-30 kHz) is used to remove tartar

Shock waves

A shock wave differs from an ultrasonic wave in that it propagates alone through the environment as a single massive pressure wave, and in its slightly different time course.

• takes about one microsecond
• high positive instantaneous sound pressure values
• its negative pressure half-wave is relatively shallow

The shock waves of lithotriptors generated by an electric spark have a peak value of positive acoustic pressure of around 100 MPa. The amplitude of the negative pressure half-wave is about ten times smaller, but still sufficient to cause the expansion and subsequent cavitational collapse of the gas microbubbles present (especially those formed on the surface of the stone to be disturbed in the body).

Effect and use of shock waves

• acts:

• directly - by mechanical stress
• indirectly - through cavitation

• extrapolar shock wave (LERV) lithotripsy

• the essence is the application of shock waves
• several hundreds to thousands of shock waves focused by an ellipsoidal mirror crush the gallbladder or kidney stone into sand, which can leave through natural channels
• relatively little damage to the surrounding soft tissues.

Links

Source

• KYMPLOVÁ, Jaroslava. https://portal.lf1.cuni.cz [online]. [cit. 2012-09-20]. <https://portal.lf1.cuni.cz/clanek-793-katalog-metod-v-biofyzice>.