1. TherapeuTic
ulTrasound
lecTure Viii
dr. amal h.m. ibrahim

2. Objectives
• By the end of this lecture, you should be able to do
the following:
1. Define what is Ultrasound?
2. Describe what does it do?
3. List the frequency of US.
4. Describe the propagation of US
5. Give a brief description of US production.
6. Know the US generators.
7. Give a brief explanation of physical phenomenon of
US.

3. Introduction
 Therapeutic ultrasound (US) is one of the most
common physical agents used in rehabilitation.
 Ultrasound is a mechanical not electric
energy.
 Deep penetrating modality.
 Capable of producing changes through both
thermal and mechanical mechanisms.
 Uses acoustical energy.
 Depending on the frequency it can be used for
diagnostic imaging, tissue healing or tissue
destruction

4. Introduction
 Ultrasound is a form of acoustic vibration
propagated in the form of longitudinal waves
consisting of areas of compression and
rarefaction at frequencies too high and cannot
be heard by human ears.

5. Longitudinal vs. Transverse Waves
• Longitudinal waves
• molecular displacement is along
direction in which waves travel
(bungee cord)
– Compression – regions of high molecular density
(molecules in high pressure areas compress)
– Rarefraction – regions of low molecular density
(molecules in low pressure areas expand)
• Transverse waves – molecular displacement in
direction perpendicular to wave (guitar string)

8. Longitudinal Waves
In a longitudinal wave the particle displacement is parallel to the direction of
wave propagation. The animation below shows a one-dimensional longitudinal
plane wave propagating down a tube. The particles do not move down the tube
with the wave; they simply oscillate back and forth about their individual
equilibrium positions. Pick a single particle and watch its motion. The wave is
seen as the motion of the compressed region (ie, it is a pressure wave), which
moves from left to right.

9. Transverse Waves
In a transverse wave the particle displacement is perpendicular to the direction
of wave propagation. The animation below shows a one-dimensional
transverse plane wave propagating from left to right. The particles do not
move along with the wave; they simply oscillate up and down about their
individual equilibrium positions as the wave passes by. Pick a single particle
and watch its motion.

10. Water Waves
Water waves are an example of waves that involve a combination of both
longitudinal and transverse motions. As a wave travels through the waver, the
particles travel in clockwise circles. The radius of the circles decreases as the
depth into the water increases. The movie below shows a water wave
travelling from left to right in a region where the depth of the water is greater
than the wavelength of the waves. I have identified two particles in yellow to
show that each particle indeed travels in a clockwise circle as the wave
passes.

11. Introduction
 Particles of a material, when exposed
to a sound wave will oscillate about a
fixed point rather than move with the
wave itself.
 As the energy within the second wave
is passed to the material it will cause
oscillation of the particles of that
material.
 Clearly any increase in the molecular
vibration in the tissue can result in
heat generation, and ultrasound can be
used to produce thermal changes in
the tissues.

13. The Frequency
• Human ear can hear sound waves from
16 to 20,000 Hz
• Any sound above this is ultrasound
• Any sound below this range is
infrasound.

14. Frequency
• Frequency: number of times an event occurs
in 1 second; expressed in Hertz or pulses per
second
– Hertz: cycles per second
– Megahertz: 1,000,000 cycles per second
• In the U.S., we mainly use ultrasound frequencies of 1,
2 and 3 MHz
• 1 MHz = low frequency;
• 3 MHz = high frequency
• ↓ frequency = ↑ depth of penetration
• ↑ frequency = sound waves are absorbed in
more superficial tissues (3 MHz)

15. The Frequency
Most medical applications employ frequencies between 1MHz
and 15 MHz:
1- Physiotherapy equipment has frequency 0.75MHz,
0.87MHz, 1MHz, 1.5MHz, 3MHz.

Most frequently used are 1 MHz and 3 MHz
2- Diagnostic equipment has frequency between 1MHz and
10MHz.
3- Surgical equipment has frequency ý between 1MHz and
5MHz.

16. Seeing With Sound
http://www.3d-4d-ultrasounds.com/images/gallery/before-after.jpg
http://www.genesisobgynonline.com/ultra08.jpg
http://www.dximaging.com/images/ultras10.jpg
http://www.nsbri.org/NewsPublicOut/Photos/20051102b.jpg
Background Image: http://krohnert.org/gallery/d/6382-2/ultrasound-20-weeks.jpg

17. Velocity
• Velocity = Frequency x Wave length.
• The speed of sound wave is directly related to the
density (↑ velocity = ↑ density)
• Denser & more rigid materials have a higher
velocity of transmission
• At 1 MHz, sound travels through soft tissue @ 1540
m/sec and 4000 m/sec through compact bone

18. Propagation and speed of
ultrasound waves
• The human body processes a
characteristic resistance
against the propagation of
ultrasound. Each tissue in the
body has a characteristic
impedance (Z).
• It is directly proportional to
the velocity of propagation (V)
and the density (P) of the
tissue.

19. Production of uS
• The brothers, Pierre and Jacques Curie,
discovered that when a quartz crystal is
stressed, a potential difference is produced
across its faces. This is called piezoelectric
effect. In 1917 Langevin discovered that by
vibrating a quartz crystal with a high
frequency alternating current ultrasound
could be produced.

20. Production of uS
• Piezoelectric effect: is the ability of some materials
(notably crystals and certain ceramics) to generate an
electric potential in response to applied mechanical
stress. When crystals are deformed (compressed), they
produce small electric charges. The electropiezo effect
(reverse of piezoelectric effect) occurs when an electrical
current passed through a crystal causes the crystal
expand or contract.
• Using the electropiezo effect, ultrasound units produce
high frequency waves by passing an alternating current
through a piezoelectric crystal. The higher the current's
frequency, the higher the frequency of the ultrasonic
output.

21. the ultraSonic
generator
The transducer or treatment head is a crystal
inserted between two electrodes. The crystal
translates the electrical oscillations directly into
mechanical vibrations which pass through a metal
cap into the body through the coupling medium

22. the ultraSonic
generator
•
•
•
•
•
•
For therapeutic purposes the applicator should have
a radiating surface which is slightly smaller than the
total applicator surface.
This makes it easier to maintain full contact between
the head and the treatment area.
When the machine is switched on the frequency
energy applied to the crystal is increased to the
required level.
The average ultrasonic intensity is expressed in watts
per square centimeter (w.cm²).
It is obtained by measuring the total output of the
applicator (power), and then dividing it by the size of
the radiating surface of the applicator (area).
Large applicators are preferable.

23. The ulTrasonic
generaTor
• As air is not dense enough to transmit
ultrasonic energy, a transmission
medium is required to allow the energy
to pass from the sound head to the
tissues.
• Sound waves cannot exit transducer
when no medium is present. Operating
the ultrasound unit when its head is not
in contact with a transmission medium
can damage its transducer.
• Many units have sensors that can detect
when there is insufficient coupling and
automatically shut down the generator.

24. The ulTrasonic
generaTor
• Once in the body’s tissues, the sound waves cause
the molecules to vibrate, creating various thermal
and mechanical effects. As the ultrasonic energy
passes through the tissue layers and meets
different densities, its energy attenuates.
• Ultrasound passes through water-rich tissues and
preferentially heats those tissues that have a high
collagen content.

25. soMe PhYsical PhenoMena oF
ulTrasounD
•
Reflection
When ultrasound passes from
one medium to another it is
important to know the acoustic
impedance (Z) of each medium.
• Reflection – occurs when the
wave can’t pass through the next
density
• If there is an acoustic mismatch
between the two media a certain
amount of reflection will occur at
the interface between the media.
• If the two media have the same
characteristic acoustic impedance
there will be no reflection

26. soMe PhYsical PhenoMena oF
ulTrasounD
Bone periosteum interface
•
•
As periosteum and bone tissue have different acoustic impedance,
about 70% of the energy is reflected,and the balance (30%) is absorbed
by the bone. The total load on the periosteum is equal to the total
incident power plus the reflected power. This causes shear waves to
occur around the periosteum.
The particles of both media oscillate at right angles to the direction of
propagation and, as the wavelength is different in each medium, the
particles move in different directions and cause a shear stress at the
boundary.

27. soMe PhYsical PhenoMena oF
ulTrasounD
Bone periosteum interface
• This is called a shear stress wave, and is rapidly absorbed at the
periosteum. The periosteum is avascular, and no cooling effect
occurs, so it quickly heats up and causes a periosteal pain. The
patient will soon complain of the heating sensation, because the
periosteum is temperature-sensitive

28. soMe PhYsical PhenoMena oF
ulTrasounD
Tissue-air interface
• Reflection also occurs at tissue-air interface.
• Here air acts as a reflector, and the ultrasound beam is
reflected back to the surface of the tissue area being treated.
Excessive heating will occur, causing a heating pain in the
skin.
• This can occur if ultrasound is given to a thin area such as the
palm of the hand.

29. soMe PhYsical PhenoMena oF
ulTrasounD
Transducer head-skin interface with an air
pocket
• If the metal of the ultrasound head and the tissue
are not completely intact with another and there is
a small air pocket, reflection in the transducer head
will cause excessive heating of the head and the
skin lead to danger of burn

30. soMe PhYsical PhenoMena oF
ulTrasounD
Refraction
When the angle of the incidence is 15
degree, refraction of a beam is 90
degree and will run parallel to the
interface. Refraction is deviation that
means ultrasonic energy invades the
tissue at one angle and continues at a
different angle (angle of refraction).
Only for angles of incidence of less than 15
degree will any energy pass into the
tissue refraction occurs particularly
where tendon join bone and lead to
concentration of energy

31. Transmission of Ultrasound
• For ultrasound to be an effective
agent, the US wave must be
transmitted from the unit to the
tissue via a conducting medium.
• The most common transmission
mediums include US gel, mineral oil,
lotions, gel pads, and water.
Ultrasound gels and pads appear to
be the best conducting mediums.

32. Transmission of Ultrasound
. Ultrasound treatments conducted when the target tissue is
immersed in water are often used for areas with irregular
surfaces where it is difficult to maintain contact on the
treatment area. However, immersed ultrasound is not as
effective as ultrasound applied directly to the tissue via gel

33. Transmission of Ultrasound
•
•
Another consideration with ultrasound is the type of tissue being treated,
or what tissue the US must travel through to reach the target. Ultrasound
energy is absorbed at different rates by different tissues and this is related
to both the water and protein content of the tissue.
Skin and adipose tissue absorb less acoustic energy than muscle, tendon,
and ligament. Nerve tissue and bone absorb the greatest amount of US
energy. Therefore, you should consider not only the depth of the tissue to
be treated, but also the type of tissue when determining treatment
parameters

34. Attenuation
• Decrease in a wave’s intensity resulting from
absorption, reflection, & refraction
– ↑ as the frequency of US is ↑ because of molecular
friction the waves must overcome in order to pass
through tissues
•
US penetrates through tissue high in water content & is
absorbed in dense tissues high in protein
•
•
•
↑ Absorption = ↑ Frequency (3 MHz) , and
↑ Penetration = ↓ Absorption (1 MHz) , so
↑ Penetration = ↓ Frequency + ↓ Absorption (1 MHz)
•
•
Tissues ↑ water content = low absorption rate (fat)
Tissues ↑ protein content = high absorption rate
(peripheral nerve, bone)
– Muscle is in between both

35. Transmission of Ultrasound
• Attenuation: is the progressive loss of the acoustic power as
ultrasonic travels through a medium. The amount of
attenuation varies from tissue to tissue. Attenuation is linear
and inversely proportional to the frequency. If the frequency
is changed from 1MHz to 3MHz the attenuation in muscle
changes from 2% to 6% per millimeter.
• For a frequency of 3MHz, attenuation is 50% at 25 millimeter,
while at 0.75MHz the attenuation is 50% at 90 millimeter

36. Transmission of Ultrasound
• Absorption of ultrasound: it is generally accepted that absorption of
ultrasound energy takes place at molecular level, the proteins are the
major absorbers. The protein in nerve is sensitive to ultrasound.
Muscles absorb twice as much as fat. The viscosity of the medium
opposes the particle motion, and so absorption of energy occurs.
Absorption of ultrasonic energy depends on the following
• 1- Acoustic impedance of the tissue.
• 2- Propagation velocity of sound.
• 3- Density of tissue.
• 4- Frequency of ultrasound

37. Transmission of Ultrasound
5- Protein content.
6- Fat and water content.
7- Angle of incidence of acoustic energy.
8- Viscosity of fluid.
9- Reflection.
10- Refraction.
11- Shear waves.

38. Pulsed Ultrasound
• Most ultrasound generator gives pulsed ultrasound
of 2 ms (2 thousands of second) pulses. The ratio of
pulse time to the off time is variable from 1:1 to 1:4
modes, or as the duty cycle, which is the ratio of the
pulse time to the total time of pulse and pulse
interval represented as a percentage

39. Pulsed Ultrasound
• If pulsed ultrasound is applied at a mark :
space ratio of 1:1 the amount of introduced
energy is one-half of that introduced by
continuous ultrasound applied for the same
period of time and the same intensity.

40. Pulsed Ultrasound
• So the therapist can apply pulsed
ultrasound with the same intensity but
with double time of treatment or
double the intensity with the same
time of application to produce the
same amount of ultrasound energy.

41. Pulsed Ultrasound
•Yet the effect is not the same as
continuous ultrasound because with
pulsed ultrasound there is time for the
heat to be dissipated by conduction in
the tissues and in the circulating blood.
The pulsed ultrasound is safety because
the average heating is reduced.

42. Effect of Pulsed Ultrasound
1- Increase rates of ion diffusion across cell
membrane due to increase particle
movement on the either side of the
membrane.
2- Increase motion of the phospholipids and
proteins that form the membrane.

43. Questions??????