Ultrasound Blocks for the Anterior Abdominal Wall

2. Ultrasound and Regional Anesthesia | 25
the wave (c) depends on the physical properties of the medium
(Figure 2.1).
Figure 2.1 – From left to right: Pressure wave and returning echoes.
Speed, wavelength and frequency correlations. Scattering effect.
Spatial compound imaging.
Ultrasounds are cyclic sound pressures with a frequency above
those which humans can hear. The human hearing limit is 20
kHz. Ultrasounds used in the medical setting have a frequency of
1 to 25 mHz. Sound waves are not ionising and are not harmful at
the energy levels used for diagnostic purposes. There is to date
no epidemiological evidence of harmful effects (Moore 2011).
Ultrasounds travel faster in dense bodies and slower in
compressible bodies. In soft tissue the speed of sound is 1540
m/s, in bone about 3400 m/s, and in air 330 m/s. In tissues where
the sound travels more slowly, the wavelength decreases.