Therapeutic Ultrasound in Physiotherapy

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THERAPEUTIC
ULTRASOUND
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SOUND
 Periodic mechanical disturbance of an elastic medium
such as air.
 Sound is a vibration that typically propagates as an
audible wave of pressure, through a transmission medium
such as a gas, liquid or solid.
 Humans hear sound when the frequency lies between about
20 Hz and 20 kHz.
 Sound waves above 20 kHz are known as ultrasound.
 Sound waves below 20 Hz are known as infrasound.
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Light Waves and Sound Waves
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Sound Waves vs Light Waves
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Sound waves Light waves
Velocity in air Approximately 1,100 feet per second Approximately 186,000 miles per
second
Form A form of wave motion A form of wave motion
Wave composition Longitudinal Transvers
Transmitting medium All substances Empty space and all substances except
opaque materials
Relation of transmitting
medium
The denser the medium, the greater
the speed
The denser the medium, the slower the
speed
Sensations produced Hearing Seeing
Variations in sensations
produced
A low frequency causes a low note; a
high frequency, a high note
A low frequency causes red light; a
high frequency, violet light

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Characteristics of Sound Waves
1. Wavelength: the distance over which the wave's shape repeats.
2. Frequency/Pitch: Number of oscillations.
3. Amplitude/Loudness: is a measure of its change over a single
period.
4. Quality/Timbre: Timbre is the quality of sound which allows us to
distinguish between different sound sources producing sound at the
same pitch and loudness.
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Properties of Sound waves
• Transmission
• Reflection
• Refraction
• Diffraction
• Absorption
• Scattering
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Transmission
 Sound can only be transmitted by a material medium such as particles in
the air.
 Sound travels at different speeds in different materials.
1. In air..........................340 metres /second.
2. In water.....................1500 metres /second.
3. In steel.......................6000 metres /second.
9https://slideplayer.com/slide/226942/

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Reflection
 Change in direction of a wave front
at an interface between two
different media.
 The law of reflection states that the
incident ray, the reflected ray, and
the normal to the surface of the
mirror all lie in the same plane.
 Furthermore, the angle of
reflection is equal to the angle of
incidence.
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https://www.physicsclassroom.com/mmedia/optics/lr.cfm

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Refraction
 Change in the direction of
acoustic waves when it passes
from one medium to another
where their speed is different.
 Snell’s law
 Ratio of sine of angle of
incidence to the sine of angle of
refraction remains constant in
same media.
 Known as the refractive index of
the two media.
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https://www.acs.psu.edu/drussell/demos/refract/refract.html
http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/refrac.html#c2

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Diffraction
 Direction change of acoustic
waves as they pass through
obstacles or apertures.
 Because of diffraction of sound
waves one can easily hear
around corners and walls, and
through open windows and
doors.
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https://web2.ph.utexas.edu/~coker2/index.files/diff.htm

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Absorption
 Acoustic absorption refers to the process by which a material takes
in sound energy.
 The absorbed energy can transformed into heat and transmitted
through the absorbing body.
 The fraction of sound absorbed is governed by the acoustic
impedances of both media and is a function of frequency and the
incident angle.
 Sound Absorption Coefficient is the fraction of sound
energy absorbed by a material.
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Absorption
 Absorption coefficients are
tissue and frequency specific.
 They are highest for Tissues
with highest collagen content
and
 Increase in proportion to the
ultrasound frequency
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Absorption coefficients in decibels/cm

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Scattering
 Scattering is a general physical
process where some forms
of radiation, such as light
or sound, are forced to deviate
from a straight trajectory by one
or more paths.
 This can be due to localized
non-uniformities in the medium
through which they pass.
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https://science.nasa.gov/ems/03_behaviors

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Attenuation
 Attenuation is the result of
absorption, reflection, and
refraction
 Absorption accounting for about
one-half of attenuation.
 Attenuation coefficients are tissue
and Frequency specific.
 They are higher for tissues with a
higher collagen content and
 increase in proportion to the
frequency of the ultrasound
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THERAPEUTIC ULTRASOUND
 Frequency - Typically 1 or 3 MHz
 Wavelength - @ 1MHz would be 1.5mm and @ 3 MHz would be 0.5 mm.
 velocity of ultrasound - Sound waves can travel more rapidly in a denser
medium. The velocity varies from
 331 m/sec in air
 1450 m/sec in fat,
 1570 m/sec in blood
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Transducer (sound head)
 Consist of a quarts crystal that
converts electrical energy into
sound.
 Synthetic plumbium zirconium
titanate (PZT), and
 Barium titanate.
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Production
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Schematic representation of ultrasound production. (Ward A. 2006)

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Ultrasound Machine & Coupling Agent
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Effective Radiating Area (ERA)
 area of the sound head that
produces ultrasonic waves;
 expressed in square centimeters
(cm2)
 Always lesser area than actual
size of sound head
 Large diameter heads – column
beam
 Small diameter heads – more
divergent beam
 Low frequency (1 MHz) – diverge
more than 3 MHz
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Beam Nonuniformity Ratio
 Spatial Peak Intensity: The peak
intensity of the ultrasound output over
the area of the transducer.
 Spatial Average Intensity: The
average intensity of the ultrasound
output over the area of the transducer.
 Beam Nonuniformity Ratio (BNR) :
The ratio of the spatial peak intensity
to the spatial average intensity .
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Beam Nonuniformity Ratio
 The smaller the BNR is, the
more uniform the ultrasonic
being radiated are.
 A BNR ≤ 5 is good.
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(Minato Medical Science Co.2018)
Uniformity of ultrasonic beam radiated
from the probe having various BNR.

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Types of Ultrasound Beams
 Continuous Wave - no interruption of beam.
• best for maximum heat buildup
 Pulsed Wave - intermittent “on-off” beam
modulation
• builds up less heat in tissues.
• used for post acute injuries
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Pulsed Wave
Mark Space ratio
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 Spatial Average Temporal Peak
(SATP) Intensity: The spatial
average intensity of the ultrasound
during the on time of the pulse.
 Spatial Average Temporal Average
(SATA) Intensity: The spatial
average intensity of the Ultrasound
averaged over both the on time and
the off time of the pulse.
 SATP x duty cycle = SATA
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Near Field/Far Field
 The near field, also known as the
Fresnel zone is the convergent
region and
 the far field, also known as the
Fraunhofer zone, is the divergent
region
 Length of near field = r2 /λ
 r = Radius of transducer2
 λ= Wavelength of ultrasound
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Acoustic Impedance
 It is a measure of the resistance of particles of medium to
mechanical vibrations
 This resistance increases in proportion to the density of
medium and
 velocity of ultrasound in the medium
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Half value depth
 This is the tissue depth at which
50% of the ultrasound delivered
at the surface has been
absorbed.
The average 1/2 value depth of
 3 MHz at 2.5 cm and
 1 MHz at 4.0 cm
1 MHz 3 MHz
Muscle 9.0 mm 3.0 mm
Fat 50.0 mm 16.5 mm
Tendon 6.2 mm 2.0 mm
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• Typical half value depths for therapeutic
ultrasound at different tissues.
• As the thickness of each of these layers
varies in an individual patient, average half
value depths are employed for each
frequency.

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Half value depth
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Quantity of US
(fraction of beam being
Further propagated)
1.0
.5
.25
.125
1st Half Value 2nd Half Value 3rd Half Value 4th Half Value
The quantity of the ultrasound beam
decreases as the depth of the medium
(tissue) increases.
Tissue depth

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Standing Wave
 When reflected, ultrasound meets further oncoming waves, a
standing wave (hot spot) may be created, which has potential
adverse effects upon tissue.
Such effects can be minimized by ensuring that
 the apparatus delivers a uniform wave,
 using pulsed waves and
 moving the transducer during treatment
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LAB ACTIVITIES
1. Orientation to the Ultrasound Equipment
 Select an ultrasound unit and record the information described below.
1. Manufacturer:
2. Last Inspection Date or Manufacture Date:
3. Available Frequencies:
4. Available Transducer Sizes:
5. Effective Radiating Areas:
6. Available Duty Cycles:
7. Beam Nonuniformity Ratios:
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LAB ACTIVITIES
2. Locate each of the following components, describe them,
and inspect them;
1. for wear.
2. Generator:
3. Coaxial Cable:
4. Transducer:
5. Timer:
6. Intensity Control:
7. Duty Cycle Control:
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LAB ACTIVITIES
Testing the Transducer for Acoustical Output
 Prior to any treatment it is sensible to check that there is
an output from the machine.
 This can be done by 2 Methods.
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LAB ACTIVITIES
Testing the Transducer for Acoustical Output: Method 1:
 A suitable container filled with
water and a metal plate kept at an
angle at one end.
 Place the treatment head just
below the water surface and direct
the beam to a metal plate
 Observe for disturbance (ripples) at
the surface of water.
 The apparatus should be on and
off with the treatment head below
the water.
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Singh J, 2012

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LAB ACTIVITIES
Testing the Transducer for Acoustical Output: Method 2:
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Ultrasound
transducer
is wrapped with
cellophane tape.
Make a complete
circle with the tape
capable of
holding tap water.
Tap water
is added to
the top of the
transducer.
Carefully holding the
transducer upright, increase
the intensity and look for
water ripples
Behrens B J, 2006

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Physiological Effects
 Therapeutic ultrasound may induce clinically significant
responses in cells, tissues, and organs through both;
1. Thermal and
2. Nonthermal biophysical effects.
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Thermal Effects
 The clinical effects of ultrasound heating of tissues are the
following:
1. An increase in the extensibility of collagen fibres found in tendons
and joint capsules.
2. Decrease in joint stiffness.
3. Reduction of muscle spasm.
4. Modulation of pain.
5. Increased blood flow.
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Factors Affecting Temperature Increase
 Absorption coefficient of the
tissue: This increase with
1. increased collagen content and
2. in proportion to the ultrasound
frequency.
 Thus higher temperatures are
achieved in tissues with high
collagen content and with the
application of higher-frequency
ultrasound.
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Temperature distribution for 1
and 3 MHz ultrasound at the
same intensity.
Cameron MH. Physical Agents in Rehabilitation: From Research to Practice. 2nd ed. Philadelphia: W.B. Saunders; 1999: 185-217

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Non-thermal effects
 Cavitation
 Micro streaming
 Acoustic streaming
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Cavitation
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Cavitation caused by Ultrasound. If the pressure is below a critical value, the bubble undergoes sustained
oscillations – stable cavitation/inertial cavitation. If the inertial forces are above the cavitation threshold,
then the collapse with the bubble implosion occurs. (Turánek J et al 2015)
 Cavitation is the formation, growth, and pulsation of gas filled bubbles caused by ultrasound.
 Classified into Stable or Unstable.
 Stable cavitation the bubbles oscillate rapidly but do not burst.
 Unstable cavitation is the growth of violent large excursions of the bubbles over many cycles
and then suddenly implode which causing large, brief, local pressure and temperature
increase.

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Streaming
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Non-thermal effects
Possible therapeutic benefits of non-thermal effects
 difficult to make distinction from thermal benefits
 Increased capillary density & cell permeability
 Increased fibroblastic activity and associated collagen
production
 Increased cortisol production around nerve bundles reduce
inflammation
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44. Output Frequency
Duration
Duty Cycle
Output Intensity
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 Power: The amount of acoustic energy per unit time. This is
usually expressed in Watts.
 Intensity: The power per unit area of the sound head. This is
usually expressed in Watts/centimeter2.
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Output Frequency
 Determines the treatment depth
 1 MHz Output
 Deep (5 cm) tissues
○ Rotator cuff, vastus intermedius, gastroc
 3 MHz Output
 Superficial (up to 2.5 cm deep) tissues
○ Patellar tendon, MCL, brachialis
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Treatment Duration
 Depends on:
 Size of the treatment area
 Output intensity
 Therapeutic goals
 Vigorous heating
 1 MHz output
○ 8 to 10 minutes
 3 MHz output
○ 3 to 4 minutes
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48. Direct Coupling
Immersion Method
Pad/Bladder Method
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Coupling Methods
 Ultrasonic energy cannot pass through the air
 A coupling medium is required
 Medium should be water-based
 Coupling method should confirm to the body area
 The body area should be clean and relatively hair-free
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Direct Coupling
 Gel or Creams
 Only use approved coupling
agents
 Apply liberally to area
 Remove air bubbles by
passing sound head over area
(before power is increased)
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Direct Coupling
 Move the sound head s-l-o-w-
l-y
 4 cm/sec
 Moving the head faster
decreases heating
 If the patient describes
discomfort, decrease the
output intensity
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Coupling Ability of Various Media
Substance Transmission
 Saran Wrap 98
 Lidex gel, fluocinonide (.05%) 97
 Thera-Gesic 97
 Mineral oil 97
 US Transmission gel 96
 US Transmission lotion 90
 Chempad-L 68
 Hydrocortisone powder (1%) 29
 Hydrocortisone powder (10%) 7
 Eucerin cream 0
 Myoflex 0
 White petrolatum gel 0
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Immersion Technique
 Used to treat irregularly
shaped areas
 The limb is immersed in a tub
of degassed water
 Transducer is held appx. 1cm
from the body part
 Avoid the formation of air
bubbles
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Pad (Bladder) Method
 A mass of conductive gel
 Commercial pads
 Self-made bladders
 Conforms to the treatment
area
 Commercial pads help limit
the size of the treatment area
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Contraindications
 Acute injuries (100% duty cycle)
 Ischemic areas
 Areas of impaired circulation
including arterial disease
 Over areas of deep vein
thrombosis
 Anesthetic areas
 Over cancerous tumors
 Over sites of active infection or
sepsis
 Over the spinal cord or large nerve
plexus in high doses
 Exposed metal that penetrates the
skin (e.g., external fixation devices)
 Areas around the eyes, heart, skull,
or genitals
 Over the thorax in the presence of
an implanted pacemaker
 Pregnancy when used over the
pelvic or lumbar areas
 Over a fracture site before healing
is complete
 Stress fracture sites or sites of
osteoporosis
 Over the pelvic or lumbar area in
menstruating female patients
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Precautions
 Symptoms may increase after the initial treatments.
 Use caution when applying ultrasound around the spinal cord,
especially after laminectomy.
 The use of ultrasound over metal implants is not contraindicated
 Keep the sound head moving
 Use caution when applying ultrasound over epiphyseal plates of
growing bone
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PHONOPHORESIS
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PHONOPHORESIS
 It is the movement of drugs through skin into the
subcutaneous tissues under the influence of ultrasound
 Also known as sonophoresis or ultrasonophoresis
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Pathways of drug penetration
1. Through stratum corneum
2. Transfollicular
3. Through sweat gland
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Advantages
 Avoid risk and inconvenience of IV therapy
 Bypass liver in terms of elimination
 Less chance of overdose or underdose
 Allow easy termination
 Permit both local and systemic treatment effects
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Effectiveness
 Depends upon
 Anatomical area treated
 Hydration of the skin
 Health or pathological condition of the skin
 State of cutaneous and systemic metabolism
 Patient’s age
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Methods of application
 Adequate quantity of drug rubbed into the skin over the target
area
 Same gel mixed with standard ultrasound gel placed over
transducer head as coupling medium
 US is then applied as a direct contact method
 Standard intensity is 1 to 2 w/cm²
 Standard duration is 5 to 10 minutes
 Lower ultrasonic frequencies and pulsing lead to deeper
penetration
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Phonophoretic agents
Drug Indication
Reactions/
contraindications
Hydrocortison Anti inflammatory Skin rashes
Lidocaine/xyclocaine Acute pain
Methyle salicylate Chronic painfull MS disorders Sensitivity to aspirin
Zinc oxide/siloderm Open wounds Allergy to metals
Iodine
Adhesion, calcification,
adhessive capsulitis
Allergic to sea food
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