1. Understand the physics and properties of
2. Explain the production of ultrasonic waves
3. Enumerate the Physiological effects of
4. Enumerate the therapeutic uses of ultrasound
5. Evaluate the Indications and contraindications
for applying ultrasound
6. Understand the precautions for applying
7. Select appropriate methods of ultrasound
application to produce desired therapeutic
8. Choose and use the appropriate treatment
parameters for the safe application of
9. Describe phonophoresis
10.Identify the molecules used for phonophoresis,
indications and contraindications for
• Sound is defined as the periodic
mechanical disturbance of an elastic
medium such as air.
• Sound requires a medium for its
transmission and cannot cross a vacuum.
• Ultrasound refers to mechanical
vibrations, which are essentially the same
as sound waves but of a higher frequency.
Such waves are beyond the range of
human hearing and can therefore be
• Vibration merges with sound at
frequencies around 20 Hz; vibration below
this frequency is often called infrasound
▫ Audible sound – 20 to 20000 Hz
▫ Ultrasound – Greater then 20000 Hz
▫ Infrasound – Less than 20 Hz
▫ Therapeutic ultrasound – 0.5 to 5 MHz
– 1 to 3 MHz
• Sonic waves are series of mechanical
compressions and rarefactions in the direction
of travel of the wave, hence they are called
• They can occur in solids, liquids, and gases and
are due to regular compression and separation
• Passage of these waves of compression
through matter is invisible because it is
the molecules that vibrate about their
average position as a result of the sonic
wave. It is energy that travels and not the
• As sound waves pass through any
material their energy is dissipated or
• All the energy is absorbed at once; sound
wave passes with almost no loss.
• The molecules of all matter are in constant
random motion; the amount of molecular
agitation is what is measured as heat – the
greater the molecular movement, the
greater the heat.
• Sound waves will pass more rapidly through
material in which the molecules are close
together, thus their velocity is higher in
solids and liquids than in gases
Other uses of ultrasound:-
• In industry low-frequency ultrasound is used
for cleaning and mixing processes since efficient
vibration of very small particles is achieved.
• It can also be used for cutting and engraving as
well as detecting cracks in metal such as
• The other major medical uses of ultrasound are
in body imaging (6-18 MHz) and dental drills /
descalers. These latter usually operate at
between 20 to 60 kHz.
Production of Therapeutic Ultrasound
• Piezo-electric effect: The production of a
small electro motive force(e.m.f.) across
certain substances on being subjected to
external pressure. Such substances are
known as piezo-electric substances
• Reverse piezo-electric effect: Production
of mechanical waves or vibrations due to the
application of e.m.f.
• Many types of crystal can be used but the most
favored are quartz, which occurs naturally, and
some synthetic ceramic materials such as barium
titanate and lead zirconate titanate (PZT).
• These crystals deform when subjected to a varying
potential difference – a piezo-electric effect
• In order to apply the electric charges, metal
electrodes must be fixed to the crystal.
• If a suitable metal plate is fixed to one surface of
the crystal while the opposite surface is in air, then
almost all the vibrational energy is transmitted
from the crystal to the plate and hence to any solid
or liquid to which it is applied.
• The other essential parts of a therapeutic ultrasound
generator are a circuit to produce oscillating voltages
to drive the transducer, which can turn the oscillator
on and off to give a pulsed output.
• A suitable circuit can maintain a constantly
oscillating electric charge to cause the
piezoelectric crystal to change shape at the same
• So drive the metal plate backwards and forwards
also at the same frequency in any medium with
which it is in contact.
• This amplitude is referred to as the intensity and
is the energy crossing unit area in unit time
perpendicular to the sonic beam. It is therefore
measured in watts per square centimeter.
• Current supplied to the oscillator circuit can be
automatically switched on and off to produce a
pulsed output, typically giving ratios 1:1 or 1:4.
• A meter is often included which measures the
electrical oscillations applied to the crystal but
not the vibration of the crystal
BOUNDARIES BETWEEN MEDIUM
• Sonic waves involve vibratory motion of molecules so
that there is a characteristic velocity of wave
progression for each particular medium.
• It depends on the density and elasticity of the medium
and together these specify the acoustic impedance of
• Acoustic impedance = density of medium x velocity of
• Some of the energy is reflected back. The amount of
the energy reflected is proportional to the difference in
acoustic impedance between the two media.
– Water / Glass – 63% of energy is reflected
– Water / Soft tissue – 0.2% of energy is reflected
• Refraction also occurs with sonic waves due to the
difference in acoustic impedance.
• The beam of sonic energy that passes through the
second medium does not continue in a straight
line but changes direction at the boundary
because of the different velocities in the two
• If the acoustic impedances are closely matched
little refraction will occur.
Absorption of Sonic waves
• Kinetic energy is converted to heat energy as
it passes through the material.
• The energy will decrease exponentially with
distance from the source because a fixed
proportion of it is absorbed at each unit
distance so that the remaining amount will
become a smaller and smaller percentage of
the initial energy
• The conversion of sonic energy to heat is due
to increased molecular motion
• Half value depth: depth of tissue at which
the US intensity is half its initial intensity
• Absorption of sonic energy is greatest in
tissues with largest amounts of structural
protein and lowest water content.
• Blood – least protein content and least
• Bone - greatest protein content and greatest
Attenuation of Ultrasound in the Tissues:
• The loss of energy from the ultrasound beam in
the tissues is called attenuation and depends on
both absorption and scattering
• Absorption accounts for some 60 – 80% of the
energy lost from the beam. The scattered energy
may also be absorbed other than in the region to
which the ultrasound beam is applied.
• Scattering is caused by reflections and
refractions, which occur at interfaces throughout
the tissues. This is particularly apparent where
there is a large difference in acoustic impedance.
ultrasound unit that
contains the crystal
• Power: amount of
acoustic energy per
unit time (watts)
• Intensity: power per
unit area of the
• Spatial average
intensity of the US output
over the area of the
• Spatial peak intensity:
Peak intensity of the
ultrasound output over the
area of the transducer. The
intensity is usually great in
the centre of the beam and
lowest at the edges of the
• Beam non-uniformity
ratio (BNR): Ratio
between peak intensity
and average intensity in
the beam. The lower the
BNR the more uniform
• With BNR 5:1, when the
spatial average intensity
the spatial peak
intensity would be
• Duty cycle:
proportion of the total
treatment time that the
US is on. This can be
expressed in percentage
or a ration
• 20% or 1:5 duty cycle,
is on for 20% of the time
and off for the 80% of
• Spatial average
intensity: spatial average
intensity of the US during
the on time
• Clinically US displays SATP
intensity and duty cycle
• Spatial average
intensity: The spatial
average intensity of the US
averaged over both the on
time and the off time
• SATP X duty cycle = SATA
• SATA is frequently used in
research and non clinical
• Frequency: number of
rarefraction cycles per
unit of time, usually
expressed in cycles per
• Increasing the
frequency of US causes a
decrease in its depth of
concentration of the US
energy in the superficial
• Effective radiating
area (ERA): The area
of the transducer from
which the US energy
radiates. Since the
crystal doesn’t vibrate
uniformly , the ERA is
always smaller than the
area of the treatment
• Some waves cancel out, others reinforce so that the
net result is a very irregular pattern of the sonic
waves in the region close to the transducer face,
called the near field or Fresnel zone.
• Beyond this, the far field or Fraunhofer zone,
the sonic field spreads out somewhat and becomes
much more regular because of the differing path
lengths from points on the transducer.
• The length of the near field depends directly on
the square of the radius of the transducer face
and inversely proportional to the wavelength of
the sonic waves.
• Length of Fresnel zone = r2
• For practical purposes therapeutic ultrasound
utilizes the near field. The relatively more energy
on average is carried in the central part of the
cross-section of the beam.
• The irregularity of the near field can be ‘ironed
out’ to some extent by continuous movement of the
treatment head during the therapy.
• Shear waves can be formed which transmit
energy along the periosteal surface at right
angles to the ultrasound beam.
• Due to the fact that this reflection is quite
large (almost 25%) and that sonic energy is
absorbed almost immediately in bone, there
is marked heating at the bone surface.
• This is considered to account for the
periosteal pain that can arise with excessive
doses of therapeutic ultrasound.
Heating in the tissues due to the Ultrasound:-
• The important factor for heating in the tissue due to
ultrasound is the rate of tissue heating, which is,
influenced both by the blood flow, which constantly
carries heat away, and by heat conduction.
• In highly vascular tissues such as muscle it is likely
that heat would be rapidly dissipated preventing
any large temperature rise; on the other hand, less
vascular tissue, such as dense connective tissue in
the form of tendon or ligament, may experience a
relatively greater temperature rise.
Moving the transducer head during the
treatment is important because of following
• To smooth out the irregularities of the near field
• It reduces the irregularities of absorption that might
occur due to reflection at interfaces, standing waves,
refraction, and differences in tissue thermal conduction
or blood flow
• It also reduces shear wave formation and thereby
reduces chances of periosteal pain
• Thus resulting heating pattern is likely to be much more
evenly distributed. It has been estimated that for an
output of 1 W/cm2 there is a temperature rise of
0.8°C/min if vascular cooling effects are ignored
• The effect is not the same because with pulsed
treatment there is time for heat to be dissipated by
conduction in the tissues and in the circulating
blood. Therefore, higher intensities can be safely
used in a pulsed treatment because the average
heating is reduced.
• Ultrasound application can increase rates of ion
diffusion across cell membranes; this could be due
to increased particle movement on either side of
the membrane and possibly, increased motion of
the phospholipids and proteins that form the
Physical & Physiological effects:
• As oscillation or sonic energy is passed through
the body tissue, it causes transfer of heat energy
in the body tissues. If this energy is not dissipated
by normal physiological response, then there is
local rise in temperature, which accounts for
• If heat dissipation equals heat generation there is
no net rise in temperature and any effects are
said to be non-thermal.
• Using low intensities or pulsing the output
achieves non-thermal effects.
• The advantage of using ultrasound to achieve
heating is due to the preferential heating of
collagen tissue and to the effective penetration
of this energy to deeply placed structures.
• Heating fibrous tissue structures such as joint
capsules, ligaments, tendons, and scar tissue
may cause a temporary increase in their
extensibility, and hence a decrease in joint
• Mild heating can also have the effect of reducing
pain and muscle spasm and promoting healing
Non thermal effects:-
• Cavitation is the formation of tiny gas bubbles
in the tissues as a result of ultrasound vibration.
These bubbles, generally of a micron (10-6
• These can be of two types, namely stable
cavitation or transient(non-stable) cavitation.
• Stable cavitation occurs when the bubbles
oscillate to and fro within the ultrasound
pressure waves but remain intact.
• Transient (or collapse) cavitation occurs when
the volume of the bubble changes rapidly and
then collapses causing high pressure and
temperature changes and resulting in gross
damage to tissues.
• Stable cavitation associated with acoustic
streaming, is considered to have therapeutic
value but the transient cavitation, which is only
likely to occur at high intensities, can be
In practice the danger of tissue damage due to
cavitation is minimized by the following
• Using space-averaged intensities below 4W/cm2
• Using a pulsed source of ultrasound
• Moving the treatment head during insonation
• Acoustic streaming is a steady circulatory flow due
to radiation torque.
• Additionally, as a result of either type of cavitation
there is a localized, unidirectional fluid movement
around the vibrating bubble.
• These very small fluid movements also occur
around cells, tissue fibres, and other boundaries,
which is known as microstreaming.
• Microstreaming exerts stress on the cell
membrane and thus may increase membrane
• This may alter the rate of ion diffusion causing
therapeutically useful changes, which includes
increased secretion from mast cells, increased
calcium uptake, and production of
• All these effects could account for the
acceleration of repair following ultrasound
• Standing waves are due to reflected waves
being superimposed on the incident waves.
• The result is a set of standing or stationary
waves with peaks of high pressure (antinodes).
• Gas bubbles collect at the antinodes, and cells
collect at the nodes.
• This pressure pattern causes stasis of cells in
• The endothelium of the blood vessels exposed to
standing waves can also be damaged leading to
• There is also the possibility of marked local
heating where the amplitude of the combined
waves is high.
• If transducer head is moved during the
treatment, then standing waves are unlikely to
• The micromassage effect of ultrasound occurs
at a cellular level where the cells are alternately
compressed and then pulled further apart.
• The waves of compression and rarefaction may
produce a form of micromassage, which could
• Ultrasound has been found to be effective at
reducing recent traumatic oedema and chronic
• Stable cavitation and acoustic streaming
increases calcium ion diffusion across the cell
membrane, which works as a cellular
‘secondary messenger’, and thereby increases
the production and release of wound-healing
• These include the release of histamine from
mast cells and growth factors released from
• In this way, ultrasound has the potential to
accelerate normal resolution of inflammation
providing that the inflammatory stimulus is
• This acceleration could also be due to the gentle
agitation of the tissue fluid, which may increase
the rate of phagocytosis and movement of
particles and cells.
• Thus, ultrasound has a pro-inflammatory, not
an anti-inflammatory action.
Proliferative (Granulation) stage:-
• This begins approximately 3 days after injury
and is the stage at which the connective tissue
framework is laid down by fibroblasts for the
new blood vessels.
• During repair, fibroblasts may be stimulated to
produce more collagen; ultrasound can
promote collagen synthesis by increasing cell
membrane permeability, which allows the entry
of calcium ions, which control cellular activity.
• Not only is more collagen formed but it is also
of greater tensile strength after ultrasound
• Ultrasound encourages the growth of new
capillaries in chronic ischaemic tissue and the
same could happen during repair of soft tissues
• The enhanced release of growth factors from
macrophages following exposure to therapeutic
ultrasound may cause proliferation of
• It has been suggested that ultrasound treatment
given during the first 2 weeks after injury
accelerates bony union, but, if given to an
unstable fracture during the phase of cartilage
formation, it may result in the proliferation of
the cartilage and consequently delay of bony
• This stage last months or years until the new
tissue is as near in structure as possible to the
• Ultrasound is considered to improve the
extensibility of mature collagen such as is found
in scar tissue, which occur by promoting the
reorientation of the fibres (remodelling), which
leads to greater elasticity without loss of
• Ultrasound promotes healing of varicose ulcers
and pressure sores (decubital ulcer).
• Varicose Ulcer: Ulcer (circumscribed depressed
lesion on the skin or mucous membrane of any
internal organ following sloughing of necrotic
inflammation) in the leg associated with
varicose veins is known as varicose ulcer.
• Pressure Sore: A bed sore; a decubital ulcer
appearing on dependent sites usually on
lumbosacral region, most commonly in bed-
ridden elderly persons is known as pressure
• Ultrasound is used in herpes zoster, low
backache, prolapsed intervertebral disc (PIV)
and many other conditions.
• Herpes Zoster: Shingles (band-like involvement
of neurocutaneous tissues) caused by
• It involves posterior root ganglia and presents
with severe continuous pain in the distribution
of the affected nerve
• Prolapsed Intervertebral Disc: Abnormal
descent of intervertebral disc between the
vertebra is known as prolapsed intervertebral
Acute tissue injury:-
• Ultrasound is used in soft tissue and sport
injuries, in occupational injuries and post-natal
injuries. It is used for perineal post-natal pain,
for painful shoulders and for both neurogenic &
• Ultrasound improves quality of scar tissue and
excessive fibrous tissue. It is used in conditions like
Dupuytren’s contracture and plantar fasciitis.
• Dupuytren’s contracture: Thickening and
contracture of palmar fascia, typically affects the
ring finger and may involve years later
incompletely little finger is called Dupuytren’s
• Plantar fasciitis: Tenderness under the heel from
plantar fibromatosis or tear of plantar fascia is
called plantar fasciitis.
• Ultrasound therapy in the first 2 weeks after
bony injury can increase bony union, but,
given to an unstable fracture during the
phase of cartilage proliferation, it may
result in the proliferation of cartilage and
therefore decrease bony union. Ultrasound
has also been used in the early diagnosis of
Chronic Indurated Oedema:
• The mechanical effect of ultrasound has an
effect on chronic oedema and helps in its
treatment. It also breaks down adhesions
formed between adjacent structures.
• Tumors – it might encourage neoplastic
growth and provoke metastases or over
precancerous tissue should be avoided
• Pregnant Uterus – avoid applying
ultrasound over a pregnant uterus, probable
risk to the rapidly dividing and differentiating
cells of the embryo and fetus
• Epiphyseal plates – avoid giving ultrasound
over epiphyseal plates as growth of the bone is
Spread of Infection - Bacterial or viral
infection could be spread by ultrasound,
presumably by facilitating microorganism
movement across membranes and through the
tissues. The low-grade infections of venous
ulcers, or similar, would seem to be safe to
Tuberculosis - Due to the possible risk of
reactivating encapsulated lesions tuberculous
regions should not be treated.
• Circumstances in which hemorrhage might
provoke should not be treated. For example, where
bleeding is still occurring or has only recently been
controlled, such as an enlarging haemarthrosis
or haematoma or uncontrollable haemophilia.
• Severely ischaemic tissues should be avoided
because of the poor heat transfer and possible
greater risk of arterial thrombosis due to stasis and
• Treatment over recent venous thrombosis might
extend the thrombus or disrupt its attachment to
the vein wall forming an embolus. Areas of
atherosclerosis are best avoided for the same
• Haemarthrosis: Bleeding into the joint
usually from an injury, which results in a
swelling of the joint, is known as
• Haematoma: A collection of blood inside the
body, caused by bleeding from an injured vessel
is called haematoma.
• Haemophilia: An inherited coagulation defect
characterized by a permanent tendency to
hemorrhages due to a defect in the coagulation
of blood is known as haemophilia.
• Atherosclerosis: A condition caused by
intramural deposition of Low Density
Lipoprotein (LDL), secondary to exposure of
smooth muscles to lipid, resulting in platelet
induced smooth muscle proliferation, formation
of fibrotic plaques and calcification is known as
Radiotherapy - Areas that have received
radiotherapy in the last few months should not
be treated because of the risk of encouraging
Nervous System - Where nerve tissue is
exposed, e.g. over a spina bifida or after a
laminectomy, ultrasound should be avoided.
Treatment over the cervical ganglia or vagus
nerve might be dangerous in cardiac disease.
Specialized Tissue - The fluid-filled eye offers
exceptionally good ultrasound transmission
and retinal damage could occur. Treatment
over the gonads is not recommended.
Implants - Smaller and superficial implants,
like metal bone-fixing pins subcutaneously
placed; as a precaution, low doses should be
used in these circumstances.
• Treatment over implanted cardiac pacemakers
should not be given because the sonic vibration
may interfere with the pacemaker’s stimulating
Anaesthetic areas - High doses should not be
given over anaesthetic areas.
Dangers of Ultrasound:
• There are very less evidences of dangers of
ultrasound but it may occur in some
– Burns could occur if the heat generated exceeded
the physiological ability to dissipate it.
– Tissue destruction would result from transient
– Blood cell stasis and endothelial damage may
occur if there is standing wave formation.
• These dangers would be more likely with
high-intensity continuous output with a
stationary head or over bony prominences