Ultrasound Blocks for the Anterior Abdominal Wall

2. Ultrasound and Regional Anesthesia | 27
Early ultrasound devices used a single crystal to create a one
dimension image, called a-mode image. Modern machines
generate a b-mode or two-dimensional or gray-scale image
created by 128 or more crystals. Each crystal receives a pulse
that produces a scan line used to create an image on the screen.
This image is renewed several times each second to produce a
real-time image. Additional modes, including high resolution
real time gray scale imaging, Doppler mode, color-flow Doppler
mode, color-velocity Doppler and tissue harmonic modes are
now commonly available.
Imaging
Depending on the medium’s physical properties and the
contact with different interfaces into the medium, the energy of
the wave is dissipated, attenuated and reflected. At the interface
where one tissue borders another tissue, the wave is refracted
and reflected back as an echo. The reflection depends on the
tissue density and thus on the speed of the wave. So, as the
waves penetrate tissues, they detect where soft tissue meets air,
or soft tissue meets bone, or where bone meets air. Instead, some
structures will completely absorb the sound waves. Thus, echoic
tissues are those tissues that reflect the wave whereas anechoic
tissues do not reflect the wave.
Ultrasounds penetrate well through fluids that are anechoic
and appear as black on the monitor. Fluids allow ultrasounds to
pass through more or less attenuated until they encounter the
surface of denser structures. Bone or air are poorly penetrated
by ultrasounds and generate a kind of “sound-shadow”.
The transverse appearance of nerves is round or oval and
hypo-echoic (Figure 2.3). They may appear as honeycomb
structures containing hyper-echoic points or septa inside them.
Nerves are surrounded by a hyper-echoic border that
corresponds to connective tissue. Tendons have a similar
appearance. On the longitudinal scan, tendons disappear while