25.7 Ultrasound + Needle

Question 1

a) Outline the basic principles of ultrasound signal and image generation. (6 marks)

Answer 1

1. > > The transducer contains a piezoelectric crystal.

This means that current applied
across it causes it to expand and
contract as the polarity of the
voltage changes.

2. > > Ultrasound wave emission is therefore*
   stimulated in the range of 2.5–15 MHz.
3. > > The ultrasound wave is reflected
   at interfaces between structures of
   different acoustic impedance.*
4. > > The transducer is an emitter and
   receiver all-in-one:

when the sound wave returns,
it causes squeezing
and stretching of the crystal,

which generates a voltage
change across the surface,

thus transducing sound waves back
into an electrical current,
resulting in image generation.

Question 2

b) How may physical factors influence the image quality of an ultrasound device? (6 marks)

Answer 2

1&raquo_space; Attenuation:

loss of energy of the ultrasound beam
as it interacts with the
tissues through which it passes.

This affects higher-frequency ultrasound
more than low.

Attenuation occurs due to:

i • Divergence of the ultrasound beam.

ii • Refraction:
when the ultrasound beam hits
an interface at an angle
that is not 90°,
the path of the ongoing beam deviates,
resulting in artefact.

iii • Scattering:
when the ultrasound beam hits an
object that is the same size
or smaller than its own wavelength.

iv • Absorption:
resulting in heat generation.

2 • Reflection:
ultrasound bounces back at the
interface of tissues with
different acoustic impedances.

Acoustic impedance is determined
by the product of tissue density and
the velocity of ultrasound within
it.

Reflection is the mechanism
by which an ultrasound image is
generated,

but if reflection occurs at more
superficial interfaces,
then that part of the beam does not progress to deeper structures,

where it would contribute to image generation.

Acoustic shadowing is the absence of image
of a tissue deep to a highly reflective
surface such as an interface with bone,
whereas post-cystic enhancement is the
enhanced signal received from structures
deep to fluid-filled spaces as
the ultrasound passes readily through the fluid.

3&raquo_space; Resolution:
the ability to distinguish between
two structural points.
Includes lateral, axial and temporal resolution.

Higher-frequency ultrasound waves
improve axial resolution and
focusing the ultrasound
beam improves lateral resolution

4&raquo_space; Anisotropy:
the image of tissues is dependent
on the angle to the ultrasound beam
at which they are viewed,

with better resolution when
the emitted and received ultrasound beams
follow the same trajectory
but in reverse.

The image quality of a
structure becomes poorer and
disappears altogether as the angle
between probe and skin becomes
more acute.

Question 3

c) Which two needling techniques are commonly used in ultrasound-guided nerve blocks and what are
the advantages and disadvantages of each? (8 marks)

Answer 3

Long axis, in-plane:
Advantages:

• Needle visualised along full length.
• Good visualisation of needle
tip near nerve.

Disadvantages:

• Difficult to keep full length of needle in view.
• Longer distance from skin to nerve,
increased potential for pain
(and possibly damage) as the
needle passes through more structures.

Short axis, out-of-plane:

Advantages:

> > Uses familiar entry points,
comparable to non-ultrasound-guided
nerve block techniques.

> > Shortest skin–nerve distance.

> > Less painful as the needle doesn’t
pass through muscle.

Disadvantages:
» Needle only seen as a bright
dot when in the ultrasound beam.

> > May be more difficult to visualise
the proximity of the needle tip to the nerve.

Question 4

2017 b

b) What patient factors (3 marks)

may influence the ultrasound
image quality?

Answer 4

Patient
» Obesity:
increased distance of fat results in
greater attenuation of
ultrasound beam.

> > Positioning:
optimum imaging for some techniques
requires specific
patient positioning which may not be feasible,
e.g. arm abduction for
axillary nerve block,
left lateral positioning during cardiac echo.

> > Ability to comply with the study:
patient will need to remain still to ensure
best possible image generation,
may not be feasible due to dementia,
tremor, delirium.

> > Previous surgical or traumatic disruption of the tissue to be imaged.

Question 5

2017 b

What Acoustic artefacts (4 marks)
may influence the ultrasound
image quality?

Answer 5

Acoustic artefacts:

1. > > Contact artefact:
   where the probe is not in contact with the skin
   (via ultrasound gel), the image will be lost.
2. > > Acoustic shadowing:
   much of the ultrasound beam is
   reflected back at
   interfaces between lesser and
   highly attenuating tissues.
   Tissues deep to these will therefore not be seen.
3. > > Post-cystic enhancement:
   ultrasound passes readily through fluid filled
   structures, resulting in enhancement
   of structures deep to them.
4. > > Lateral shadowing:
   when ultrasound beam hits the
   curved edges of a
   rounded structure,
   the beam is refracted
   and so does not bounce back to
   the ultrasound probe.

Imaging of these parts of
the structure is therefore lost.

5. > > Reverberation artefact:

reflection of the ultrasound beam
from a highly reflective interface
back to the probe,
back to the interface,
back to the probe,
resulting in multiple representations
of the same structure.

6. > > Insufficient resolution:
   use of an ultrasound wavelength
   that is greater than the size
   of the structures being imaged
   may result in failure of the
   image to demonstrate the
   separation of those structures.
7. > > Scattering:
   use of an ultrasound frequency
   of similar size or smaller
   than the structure being imaged
   will result in scattering of the reflected ultrasound beam rather than reflection of it back to the probe.
8. > > Refraction:
   when the ultrasound beam hits an
   interface at an angle that is
   not 90°, the path of the ongoing beam deviates resulting in artefact.
9. > > Anisotropy:
   the image of tissues is dependent
   on the angle to the
   ultrasound beam at which they are viewed,

with better resolution when
the emitted and received ultrasound
beams follow the same trajectory
but in reverse.

The image quality of a structure
becomes poorer and
disappears altogether as the
angle between the probe and the skin
becomes more acute.