therapeutic ultrasound.pdf

7/23/2019 Therapeutic Ultrasound.pdf

1/58

TherapeuticUltrasound

lecture VIII

Dr. Amaal H.M.Ebrahim


2/58

Introduction

Therapeutic

ultrasound (US) is

one of the most

common physical

agents used in

rehabilitation.

Ultrasound is amechanical not

electric energy.


3/58

Introduction

Ultrasound is a form of a caustic 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


4/58


5/58

IntroductionParticles 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 causeoscillation 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.


6/58

Introduction Sound waves of audible range are within

frequency 20Hz to 20000Hz. Waves below

these frequencies are called infrasonic

waves.

The amplitude of a longitudinal wave is the

greatest distance which a particle moves from

its rest position.


7/58


8/58

Introduction The wave length of longitudinal wave is the

distance from the middle of one compression tothe middle of the next.

If the frequency is 1MHz and the velocity is

about 1500m.s in water and tissue, then thedistance from the compression to

another will be one millionth of 1500m, or 1.5

mm. At higher frequencies the wave length will be

less.


9/58

The Frequency

The frequency of longitudinal wave motion is the number of thecomplete waves which pass a fixed point in unit time.

Velocity = Frequency x Wave length.

Most medical applications employ frequencies between 1MHzand 15 MHz:

1- Physiotherapy equipment has frequency 0.75MHz,

0.87MHz, 1MHz, 1.5MHz, 3MHz.

2- Diagnostic equipment has frequency between 1MHz and10MHz.

3- Surgical equipment has frequency between 1MHz and5MHz.


10/58

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.


11/58

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 piezoelectriceffect. In 1917 Langevin discovered that by

vibrating a quartz crystal with a high

frequency alternating current ultrasoundcould be produced.

Production of US


12/58

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.


13/58

Production of US 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.

Crystal usually vibrates at a natural frequency which depends

largely on its thickness. The frequency of vibration remains

constant for a given generator, but the intensity, in terms of

amplitude can be varied.


14/58

he ultrasonic generator

The therapeutic ultrasonic valve generator produces a

high frequency AC from about 0.75MHz to 3MHz. the

resonant frequency of the current is at the same

natural frequency as thatof the crystal. The high frequency current is applied to

the crystal.

In front of the crystal lies the transducer head which is

made to vibrate mechanically by the acoustic

vibration energy of the crystal.


15/58


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 thebody through the coupling medium


16/58


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 betweenthe head and the treatment area.


17/58


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.cm). it is obtained by measuring the total output

of the applicator (power),

and then dividing it by the size of the radiatingsurface of the applicator (area). Large applicators are

preferable.


18/58


As air is not dense enough to transmit ultrasonicenergy, a transmission medium is required to allowthe 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 atransmission medium can damage its

transducer. Many units have sensors that can detectwhen there is insufficient coupling and automaticallyshut down the generator.


19/58


Once in the bodys 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 differentdensities, its energy attenuates.

Ultrasound passes through water-rich tissues and

preferentially heats those tissues

that have a high collagen content.


20/58

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.


21/58


ULTRASOUND

Reflection

If there is an acoustic

mismatch between the two

media a certain amount ofreflection will occur at the

interface between the media.

If the two media have the

same characteristic acousticimpedance there will be no

reflection


22/58


ULTRASOUNDBone 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 stressat the boundary.


23/58


ULTRASOUND

Bone periosteum interface

This is called a shear stress wave, and is rapidly absorbed at the

periosteum. The periosteum is a vascular, and no cooling effect

occurs, so it quickly heats up and causes a periosteal pain. Thepatient will soon complain of the heating sensation, because the

periosteum is temperature-sensitive


24/58


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. Excessiveheating 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.


25/58


ULTRASOUND

Transducer head-skin interface with an air pocket

If the metal of the ultrasound head and the tissue arenot 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


26/58


ULTRASOUND

Refraction

When the angle of the incidence is 15degree, refraction of a beam is 90 degreeand will run parallel to the interface.Refraction is deviation that meansultrasonic energy invades the tissue atone angle and continues at a differentangle (angle of refraction). Only for

angles of incidence of less than 15degree will any energy pass into thetissue refraction occurs particularlywhere tendon join bone and lead toconcentration of energy


27/58

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 bethe best conducting mediums.
http://images.google.com/imgres?imgurl=http://www.rehabmart.com/resizeimage_send.asp%3Fpath%3D/imagesfromrd/ME-1852.jpg%26width%3D200%26height%3D240&imgrefurl=http://www.rehabmart.com/category/Gels_and_Lotions.htm&usg=__4LE-SE07Q6YRASZEIwyqMUkvFxA=&h=240&w=200&sz=6&hl=en&start=100&um=1&tbnid=GYAxdlCo9cCMnM:&tbnh=110&tbnw=92&prev=/images%3Fq%3Dtherapeutic%2BUltrasound%26start%3D90%26ndsp%3D18%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:*:IE-SearchBox%26rlz%3D1I7GTKR%26sa%3DNhttp://images.google.com/imgres?imgurl=http://www.rehabmart.com/resizeimage_send.asp%3Fpath%3D/imagesfromrd/ME-1852.jpg%26width%3D200%26height%3D240&imgrefurl=http://www.rehabmart.com/category/Gels_and_Lotions.htm&usg=__4LE-SE07Q6YRASZEIwyqMUkvFxA=&h=240&w=200&sz=6&hl=en&start=100&um=1&tbnid=GYAxdlCo9cCMnM:&tbnh=110&tbnw=92&prev=/images%3Fq%3Dtherapeutic%2BUltrasound%26start%3D90%26ndsp%3D18%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:*:IE-SearchBox%26rlz%3D1I7GTKR%26sa%3DN


28/58


. Ultrasound treatments conducted when the target

tissue is immersed in water are often used for areas

with irregular surfaces where it is difficult tomaintain contact on the treatment area. However,

immersed ultrasound is not as effective as

ultrasound applied directly to the tissue via gel


29/58


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 waterand protein content of the tissue.


30/58


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, butalso the type of tissue when determining treatment

parameters


31/58


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 frequencyis 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% at90 millimeter


32/58


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 ofultrasonic energy depends on the following

1- Acoustic impedance of the tissue.

2- Propagation velocity of sound.

3- Density of tissue.

4- Frequency of ultrasound


33/58


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.


34/58

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 dutycycle, which is the ratio of the pulse time to the total

time of pulse and pulse interval represented as a

percentage


35/58

Pulsed Ultrasound

If pulsed ultrasound is applied at a mark

: space ratio of 1:1 the amount of

introduced energy is one-half of thatintroduced by continuous ultrasound

applied for the same period of time and

the same intensity.


36/58

Pulsed Ultrasound

So the therapist can apply pulsed ultrasound withthe same intensity but with double time oftreatment or double the intensity with the sametime of application to produce the same amount of

ultrasound energy. Yet the effect is not the same as continuous

ultrasound because with pulsed ultrasound there istime for the heat to be dissipated by conduction in

the tissues and in the circulating blood. The pulsedultrasound is safety because the average heating isreduced.


37/58

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.


38/58

Beam Characteristics

Beam Profile

Ultrasound generatorsproduce an irregularly-shapedbeam. This is caused by thewaves originating from manyindividual points on the face ofthe transducer. As this energytravels away from the

transducer, it merges with otherwaves forming areas of peakintensity known as hot spots.


39/58

Beam Characteristics

Beam Non Uniformity Ratio

The output intensity presented on the ultrasound

unit's meter represents the average output within the

beam's near field in total watts or W/cm2


40/58


41/58

Spatial Average Intensity

Ultrasonic output

may be measured

either in terms of

total watts (W) or

watts per square

centimeter

(W/cm2), thespatial average

intensity (SAI).


42/58

Spatial Average Intensity

In the examplesabove, 10 watts

being passed

through an effective

radiating area of 10cm2 results in a SAI

of 1.0 W/cm2.

Passing 10 watts

through a 5 cm2

ERA doubles the

SAI to 2.0 w/cm


43/58

Beam Divergence

An ultrasound beam tends

to diverge as it leaves the

sound head. The degree ofdivergence is dependent on

the size of the sound head

and the output frequency.
http://www.google.com/imgres?imgurl=http://www.diamondathletic.com/img/prod/43151cc0ce835.jpg&imgrefurl=http://www.diamondathletic.com/product%3Bcat,39%3Bitem,341%3BTherapeutic-Modalities-Ultrasound-Head---Intelect-Legend&usg=__hoJs_6ISxw03Ulcs5FmVeQQ0cEY=&h=289&w=300&sz=11&hl=ar&start=35&um=1&itbs=1&tbnid=gz4RJMns4Xm7jM:&tbnh=112&tbnw=116&prev=/images%3Fq%3Dultrasound%2Bhead%26start%3D20%26um%3D1%26hl%3Dar%26safe%3Dactive%26sa%3DN%26rls%3Dcom.microsoft:ar-sa:IE-SearchBox%26rlz%3D1I7GGLL_en%26ndsp%3D20%26tbs%3Disch:1


44/58

Beam Divergence

Ultrasound having a frequency of 1 MHz

produces a more divergent beam than 3

MHz ultrasound, which produces a

collimating beam.

Likewise, sound heads having a smaller

ERA produce a more divergent beamthan heads having a larger ERA.

O P (C i d


45/58

Output Parameters (Continuous and

pulsed US) Duty Cycle

Ultrasound can be delivered to the body using

continuous (thermal) or pulsed (non-thermal)

modes. The duty cycle is the relationship betweenthe pulse length (on time) and the pulse interval (off

time)

O P (C i d


46/58

Output Parameters (Continuous and

pulsed US) Duty Cycle

Duty cycle is calculated by:

Duty cycle = Pulse length / (Pulse length + Pulse

interval) X100 Higher duty cycles primarily produce thermal

effects within the body. Lower duty cycles result in

predominantly mechanical (non-thermal) effects


47/58

Physiological effects of ultrasound

The sonic energy that absorbed from ultrasound

causes oscillation of particles about their mean

position. This sonic energy is converted into heat

energy which is proportional to the ultrasoundintensity and causing thermal effects in the tissues.


48/58

Thermal Effects

The thermal effect increases tissue temperature

which must be maintained between 40 and 45C for

at least 5 minutes.

Heating fibrous tissue structures such as jointcapsules, ligaments, tendons and scar tissue can

cause a temporary increase in their extensibility and

hence a decrease in joint stiffness.

Mild heating can reducing pain and muscle spasmand promoting healing processes.


49/58

Non-thermal Effects

a-Cavitation

Is a very small gas bubble filled

voids within the tissues and

body fluids. There are twotypes, first, stable cavitations

which are tine bubbles

oscillate to and fro within theultrasound pressure waves but

remain intact.


50/58

Non-thermal Effects

a-Cavitation Second, unstable cavitations

(transient or collapsed) areformation of tiny bubbles with

changing volume at high intensities. The collapsed

bubbles causing high pressureand

temperature changes areresulting in gross damage totissue.


51/58

Non-thermal Effects

b- Acoustic streaming

As a result of both types of cavitations, thereis a very small localized unidirectional fluid

movement around the vibrating bubbles andcells called acoustic streaming. These acousticstreaming is affecting the permeability of cellmembrane leading to alter the rate of

diffusion and permeability of ions across cellmembrane.


52/58

Non-thermal Effects

b-Acoustic streaming

For example, calcium which is a second

messenger may result in the stimulationof healing processes. Also sodium may

alter the electrical activity of nerves

leading to relief pain.


53/58

Non-thermal Effects

c-Standing waves

The reflected waves of ultrasound causestationary waves with peaks of high pressure

(antinodes) and another with no pressure(nodes). This pressure pattern causes stasis ofcells in blood stream exposing the epitheliumof the blood vessels to damage, (thrombosis

formation).


54/58

Non-thermal Effects

d-Micromassage

Micromassage is a mechanical effect

cause molecules to vibrate, possiblyenhance tissue inter change and affect

tissue mobility.

It can used to reduce edema.


55/58

Non-thermal Effects

e-Role of US in Tissue Repair

US plays an important role in enhancement of

tissue repair and reduces of inflammation as

follow :

1- During acute stage (inflammation)

a-US produces gentle agitation of the tissue

fluid causing stimulation of histamine andother growth factors from mast cells.


56/58

Non-thermal Effects

e-Role of US in tissue repair

1- During acute stage (inflammation)

b- US increases calcium ion diffusion across thecell membrane causing degranulation.

c- US increases the rate of phagocytosis and themovement of particles and cells.

d- US has a pro-inflammatory action not ananti- inflammatory action.


57/58

Non-thermal Effects

e-Role of US in tissue repair 2- Granulation stage:

Start approximately three days after injury.

a- Connective tissue framework is laid down by

flbroblasts.

b- US promote collagen synthesis (due to increase cellmembrane permeability.

c- US allow the entry of calcium ions for more control of

cellular activity.

d- US encourage the growth of new capillaries in chronicischemic tissues.


58/58

Non-thermal Effects

e-Role of US in tissue repair

3- Remodeling stage

This stage takes months or years.

a- US can improve the extensibility of mature

collagen (scar tissue).

b- This is believed to occur by promoting the

reorientation of the fibers leading to more

elasticity without loss of strength.

therapeutic ultrasound.pdf

Documents