principles of ultrasound & basic knowledge for anaesthesiologists

1. Basics of ultrasound uses in
regional anaesthesia
By
Radhwan Hazim Alkhashab
Consultant anaesthesia & ICU
2022

2. Why ultrasound ?
● Ultrasound (US) use has rapidly entered the field of acute pain medicine
and regional anesthesia and interventional pain medicine over the last
decade.
● Compared to the use of fluoroscopy-guided procedures that can only
visualize bony tissue, US additionally allows the visualization of soft
tissues. US equipment is also more portable and less expensive. Moreover,
even regular use of US does not place patients and practitioners at risk of
harmful radiation exposure

3. Introduction
● Ultrasound waves are generated by piezoelectric crystals in the handheld
probe. Application of an electrical current to the probe causes cyclical
deformation of the crystals, which leads to generation of ultrasound
waves.
● The current is then converted to mechanical (ultrasound) energy and
transmitted to the tissues at very high (megahertz) frequencies. The
ultrasound energy produced then travels through the tissues.

4. Basic physical principles
● Ultrasound is sound waves with frequencies higher than the upper audible limit
of human hearing .
● Echolocation, also called bio sonar, is a biological sonar used by several animal
species. Echolocating animals emit calls out to the environment and listen to the
echoes of those calls that return from various objects near them. Echolocation is
used for navigation and hunting in various environments.
● Bat echolocation range in frequency from 14,000 to well over 100,000 Hz,
mostly beyond the range of the human ear (typical human hearing range is
considered to be from 20 Hz to 20,000 Hz)

5. The ultrasound probe acts as both a transmitter and receiver. Using the piezoelectric
crystals in the probe convert the mechanical energy of the returning echoes into an
electrical current, which is processed by the machine to produce a two-dimensional
grayscale image that is seen on the screen. The image on the screen can range from
black to white.

6. Tissues echogenicity
Echogenicity of the tissue refers to the ability to reflect or transmit US waves in the
context of surrounding tissues.The greater the energy from the returning echoes from
an area, the whiter the image will appear. So we have:
1) Hyperechoic areas have a great amount of energy from returning echoes and are
seen as white.
2) Hypoechoic areas have less energy from returning echoes and are seen as gray.
3) Anechoic areas without returning echoes are seen as black.

7. US image of popliteal area. 1) Sciatic nerve (hyperechoic). 2) Adipose tissue
(hypoechoic); 3) Muscles (note the striations and hyperechoic fascial lines on muscle
surfaces); 4) Vein (anechoic – partially collapsed under pressure to US transducer); 5)
Popliteal artery (anechoic – pulsating); 6) Bone (hyperechoic rim with hypoechoic
shadow below it.

8. Acoustic impedance
Acoustic impedance is the resistance to the passage of ultrasound waves, the greater
the acoustic impedance, the more resistant that tissue is to the passage of
ultrasound waves. The difference in acoustic impedance between various types of
soft tissue, such as blood, muscle, and fat, are very small and result in hypoechoic
images.

9. Fade of ultrasound waves
Three things can happen to ultrasound waves as they travel through tissue –
reflection, attenuation, and refraction.

10. 1.Reflection
The generation of ultrasound images is dependent on the
energy of the echoes that return to the probe. The angle of
incidence is an important factor in determining the amount
of reflection that occurs. The more perpendicular an object
is to the path of the ultrasound waves, the more reflection
that will occur and the more parallel an object is to the path
of the ultrasound waves, the less reflection that will occur .

11. 2. Attenuation
Attenuation is the loss of mechanical energy of ultrasound waves as they travel
through tissue. About 75% of attenuation is caused by conversion to heat, which
is called absorption.

12. 3. Refraction
When the acoustic impedance between tissue interfaces is small, the ultrasound
wave’s direction is changed slightly at the tissue interface, rather than being
reflected
directly back to the probe.
Refracted waves may not return to the probe in order
to be processed into an image. Therefore, refraction
may contribute to image degradation.

13. Ultrasound transducers
Ultrasound transducers, or probes, can be categorized based on their frequency range,
low frequency vs. high frequency, and the shape of the probe, curved vs. linear. Linear
probes are high-frequency probes with short wave length & have less tissue
penetration but good near-field image resolution. Curved probes are low-frequency
probes with long wave length & have greater tissue penetration; however, resolution is
compromised.

15. Ultrasound machine controls
Knobology

16. Depth
The depth of tissue imaged can be
adjusted on the machine and relates to the
type of probe being used. Low-frequency
probes will be able to image deeper tissue
depths than high-frequency probes.
With a linear array probe, as the depth is
increased, the image on the screen will
appear narrower and structures will appear
smaller

17. Gain
Ultrasound probes transmit ultrasound
waves 1% of the time and spend the
remaining 99% of the time listening for
the returning echoes. Increasing the gain,
increases signal amplification of the
returning ultrasound waves, in this way
the gain function can be used to
compensate for loss of energy due to
tissue attenuation

18. Time gain compensation
Time gain compensation (TGC) allows selective control of gain at different depths . Ultrasound
waves returning from deeper structures have undergone greater attenuation. To compensate
for the loss of signal intensity, TGC allows for stepwise increase in gain to compensate for
greater attenuation of ultrasound waves returning from deeper structures. Time gain
compensation controls should be moved to the right in a stepwise fashion to “amplify” the
returning signal from the deeper structures

19. Color-flow Doppler
Color-flow Doppler allows for detection of flow within
vascular structures. Moving objects, such as red blood
cells (RBCs), affect returning ultrasound waves
differently than stationary objects. Color-flow Doppler
can differentiate between RBCs moving away from the
probe and RBCs moving towards the probe.
Red blood cells moving towards the probe will return
ultrasound waves at a higher frequency and are
displayed as red, RBCs moving away from the probe will
return ultrasound waves at a lower frequency and are
displayed as blue

20. Tissue appearance under ultrasound
Computer generated two-dimensional images seen on the ultrasound machine range from
white to black. Strongly reflected waves, such as those from boundaries of tissues with
great differences in acoustic impedance (bone/soft tissue), will have a white or hyperechoic
appearance. Examples of hyperechoic appearance would be bone, diaphragm, or a block
needle.
Ultrasound waves from those returning from deeper regions that have undergone extensive
attenuation have a gray or hypoechoic appearance. Examples of hypoechoeic appearance
would be soft tissue, such as muscle, solid organs, and fat.
When waves are not reflected and travel unimpeded, the structure will have a black, or
anechoeic appearance. Large blood vessels have an anaechoic appearance because the
ultrasound waves travel through blood without being reflected.

22. Positioning
The operator should be well comfortable to avoid
backache of improper probe positioning , the
machine better positioned in front of operater
Scanning

23. Orientation marker
Ultrasound probes have a mark that
corresponds to a mark on the ultrasound
machine’s screen. This orientation marker is
placed to the right of the patient when the
probe is a transverse plane to the patient’s
body, and placed cephalad when the probe is in
a longitudinal plane to the patient’s body.

24. Handling the ultrasound probe and proper movement is essential to
obtaining optimal ultrasound images. Learn the Essential Movements:
•Sliding •Tilting •Rotating •Rocking •Compression
Probe handling

25. Transverse scan
During a transverse scan, the ultrasound probe is placed in a perpendicular plane to the
target being imaged . The image on the screen is a cross-sectional view of the nerve or
blood vessel. During a transverse scan, nerves and vessels appear round.

26. Longitudinal scan
During a longitudinal scan, the probe is placed in the same plane as the target being imaged.
The ultrasound beam travels along the long axis of the nerve or blood vessel. In a
longitudinal scan, blood vessels and nerves appear as linear structures

27. In plane (IP)
The needle is inserted in the same plane as the
ultrasound beam. The goal is for the path of the
needle to be entirely within the beam of the
ultrasound. The more parallel the needle is to the
probe ,the easier the needle will be to visualize.
Needle insertion

28. Out of plane (OOP)
The needle is perpendicular to the beam of the ultrasound. The needle is seen as a
small hyperechoic dot on the screen. Finding the needle tip in an OOP approach can be
challenging for the beginner.

29. Local anesthetic injection
Local anesthetic injected under ultrasound
appears as an expanding hypoechoic region.
Injection of local anesthetic should be slow in
order to avoid high injection pressures, which
may lead to nerve damage. Monitoring local
anesthetic spread is very important. For example,
it is important to gently aspirate prior to injection
of local anesthetic and after each needle
movement, looking for blood return in the syringe

30. Regardless, if the spread of local anesthetic cannot be visualized while the
needle is in view, be alert to the possibility of an intravascular injection. Local
anesthetic injected in a large vessel will give a hazy/smoky appearance.
Complete coverage of a nerve or nerve plexus may require a single injection or
it may require multiple injections.

31. Hydrolocation
Hydrolocation is the technique of using small injections of local anesthetic (0.5 to 1 ml)
in order to visualize the tip of the needle. An area of expanding hypoechogenicity
caused by injecting a small amount of local anesthetic can be helpful in confirming
needle-tip position.