American Society of Regional Anesthesia and Pain Medicine August 2016 - 27

the scan head moves back and forth as it scans in two planes
simultaneously. The transducer operates at high frequencies and
produces high-resolution images. Matrix array probes scan and
generate images simultaneously in multiple planes. The images
are generated electronically, so the scanning head is not required
to move. These probes have smaller footprints and are smaller
in size, making them ergonomically superior.2 Compared with
mechanically steered probes, matrix array probes produce images
with a high display frame rate during live scanning. Unfortunately,
they operate at lower frequencies and therefore sacrifice image
resolution.

Figure 4: Coronal view (bird's eye view) in a volumetric scan showing
the axillary artery and radial nerve.

Volumetric ultrasound scanning provides a view that can be
accessed either in real time (4D) or after image acquisition and
processing (3D). Three-dimensional and 4D ultrasound can be used
to obtain a coronal view along the z-axis. The coronal view presents
the anatomy as a bird's eye view of an area of interest that would
not otherwise be possible to get using 2D technology (Figure 4).
With 2D ultrasound, a scan along the path of a nerve to assess
the spread of the LA can be done either during block performance
or at the end. This type of evaluation is only qualitative in nature;
a quantitative evaluation of the spread of LA would require a
volumetric scan.

tip is positioned either outside or below the complex fascial sheath
surrounding it.8

A wide array of studies describing new applications and advantages
of 3D and 4D scanning has been published. The anatomy of
the sciatic nerve and its relation to surrounding structures has
been studied using 3D imaging.3,4 Volumetric 3D ultrasound
imaging has been used to
study the anatomy relevant
to thoracic paravertebral
blocks. The anatomical
information obtained from
3D images is more detailed
than that typically seen
on 2D ultrasound images
and provides better spatial
awareness of the paravertebral
space.5

Ensuring circumferential spread of LA around the target nerve
may be essential to achieve superior blockade9; however, it has
been shown that circumferential spread is not always necessary to
obtain an adequate sensory-motor block.3,10 Smaller volumes of LA
can be used in nerve blocks
performed under ultrasound
guidance; as a result, the
risk of depositing LA inside
muscles or nerves with
subsequent LA-induced
muscle necrosis or nerve
lesions can be reduced.
Furthermore, reduction of
the injected volume of LA
may help reduce systemic toxicity associated with larger volumes.11

Three-dimensional multiplanar imaging has the potential to improve
the accuracy of needle placement and has already been used to
guide a number of interventional radiologic procedures in various
specialties.6 Four-dimensional ultrasound has been successfully
used to perform central venous cannulation and has the potential to
reduce errors in identifying vessels and preventing overshoot of the
needle.2

More research is needed to better understand how the spread
of LA affects the quality of blocks of various nerves in different
locations. Four-dimensional imaging can be used to visualize the
spread pattern of LA during injection and subsequently the effective
volume and dose (EV95, ED95), making it a useful tool to enhance our
understanding of the correlation between the spread pattern and
volume of LA and block quality.

Three-dimensional ultrasound has been used to confirm the
placement of a sciatic perineural catheter at the popliteal fossa and
the spread pattern of LA around the nerve.7 It has also been used to
analyze the spread of LA around the sciatic nerve when the needle

Using 3D and 4D ultrasound requires a higher level of knowledge,
training, and technical skills than what is currently needed
for 2D ultrasound. Three-dimensional anatomy, the spatial
relationship of the structures being imaged, needle trajectory,

"The new technology can help us enhance
our understanding of neural and perineural
anatomy, shed more light on perineural
catheter placement techniques..."

American Society of Regional Anesthesia and Pain Medicine
2016

27
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Table of Contents for the Digital Edition of American Society of Regional Anesthesia and Pain Medicine August 2016

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http://www.brightcopy.net/allen/asra/18-3
http://www.brightcopy.net/allen/asra/18-2
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http://www.brightcopy.net/allen/asra/15-4
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https://www.nxtbook.com/allen/asra/15-1
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