Ultrasound Blocks for the Anterior Abdominal Wall

2. Ultrasound and Regional Anesthesia | 29
Since the speed of the wave in different tissues is known, the
time for the reflected wave to return back indicates the depth of
the tissue.
All this information is converted into a two-dimensional image
on the screen. This slice may be directed in any anatomical
plane: sagittal (or longitudinal), transverse (or axial), coronal (or
frontal), or some combination (oblique).
During an ultrasound-guided nerve block, the left side of the
screen should correspond to the left side of the transducer. An
indicator on the transducer is used to orient the user to the
orientation on the screen. By convention the indicator
corresponds to the left side of the screen as it is viewed frontally.
The transducer should be placed also in order to have the
indicator on the left side of the transducer.
Transducers
Since ultrasound examinations are best suited for
investigations of soft tissues, they are indicated for the
visualization of the abdominal wall.
Lower-frequency ultrasounds have better penetration and are
used for deeper organs, but have a lower resolution. The deeper
the structure, the lower the needed frequency.
Higher-frequency ultrasounds provide better resolution, but
with a low penetration. So high-frequency ultrasounds are useful
in the case of superficial tissues. Depending on the abdominal
wall thickness, typical transducers/probes used to visualize the
abdominal wall are linear ones from 10 to 20 mHz (Figure 2.2).
Linear compound array transducers allow better visualization
of structures poorly visualized by ultrasounds such as nerves.
For 0 to 3 cm of depth, linear >10 mHz transducers are necessary.
For 4 to 6 cm of depth, 6 to 10 mHz linear transducers are used.
Structures which are deeper than 6 cm need 2 to 6 convex
transducers. The transducer should be positioned perpendicular