CHARACTERISTICS OF SOUND
• A sound beam is similar to x-ray beam in that
both are waves transmitting energy but
important difference is that x-rays pass
through a vacuum where as sound require a
material medium ( solid , liquid , gas ) for
transmission, they will not pass through the
• Sound must be generated mechanically by
vibrating body matter
HISTORY OF ULTRASOUND
• Piezoelectricity discovered by Pierre
and Jacques Curie in 1880 using
• SONAR was first used in 1940s war
• Diagnostic medical applications in
use since late 1950’s
• A : uniform distribution of
molecules in a medium
• B: movement of the piston
to the right produces a
zone of compression
• C: withdrawl of the piston
to left produces a zone of
• D: alternate movement of
the piston to right and left
establishes a longitudinal
wave in the medium
PROPERTIES OF SOUND WAVE
• Ultrasound obeys the wave equation :
u = vλ
where v = frequency
( Hz , number of cycles / sec. )
u = velocity of sound
( meter / sec.)
λ = the wavelength
( which is distance between two successive
• Frequency refers to the number of
cycles of compressions and rarefactions
in a sound wave per second, with one
cycle per second being 1 hertz.
• Medically used ultrasound involves 1-10
MHz frequencies .(1 -10 million Hz
• The wavelength is the distance traveled by
sound in one cycle, or the distance between
two identical points in the wave cycle i.e. the
distance from a point of peak compression to
the next point of peak compression.
• It is inversely proportional to the frequency.
Wavelength is one of the main factors
affecting axial resolution of an
• Smaller wavelength
• Higher frequency
• Higher resolution
• Lesser penetration
• Therefore, higher frequency
probes (5 to 10 MHz)
provide better resolution
but can be applied only for
superficial structures and in
• Higher wavelength
• Lesser frequency
• Less resolution
• Deeper penetration
• Lower frequency probes (2
to 5MHz) provide better
penetration albeit lower
resolution and can be used
to image deeper structures.
• The propagation velocity is the velocity at
which sound travels through a particular
• Dependant on the compressibility and density
of the medium.
• The average velocity of sound in soft tissues
such as the chest wall and heart is 1540
• The velocity of sound is inversely related to
the compressibility of the conducting material.
That means less compressibility of material ,
the more rapidly transmits the sound.
• Sound waves move slowly in the gas because
the molecules are far apart and are easily
• Solids > liquids > gases
• Dense materials have large molecules with
large inertia : difficult to move or stop once in
• Propagation of sound requires rhythmic
starting and stopping of particles
• Density is inversely related to velocity
• It is a measure of the degree of change within
a medium, caused by the passage of a sound
wave and relates to the severity of the
• Determined by the length of oscillation of
• Greater amplitude = more intense sound
• Sound intensity is measured in
• Ultrasonic intensities are expressed
in power / unit area (watts/cm2)
• Transducer is the device which generates
ultrasound wave .
• Transducers are used to convert an electric
signal into ultrasonic energy that can be
transmitted into tissue , and to convert
ultrasonic energy reflected back from the
tissue into an electric signal.
• The most important component is a thin (0.5
mm) piezoelectric crystal element located near
the face of the transducer .
• The piezoelectric crystal consist of lead zirconate
titanate or PZT.
• The front and back faces of the crystal are coated
with a thin conducting film to ensure good
contact with the two electrodes that will supply
the electric field used to strain the crystal.
• Crystal is made up of numerous dipoles
arranged in a geometric pattern.
• Dipole is a polarized molecule, one end
positive and other end negative .
• The positive and negative ends arranged so
that an electric field will cause them to realign
thus changing the dimensions of the crystal.
• No current flows through the crystal
• Plating electrodes behave as capacitors
and it is the voltage between them that
produces the electric field which causes
change in crystal shape
• When the high frequency voltage pulse is
applied across the crystal , the crystal vibrates
like a cymbal that has been struck a sharp
blow and generates sound waves.
• The backing block must stop the crystal
vibration within a microsecond because the
transducer must be ready immediately to
receive reflected waves (echoes) from tissue
• As the sound pulse passes through the body
,echoes reflect back towards the transducer
from each tissue interface. These echoes carry
energy and they transmit their energy to the
transducer , causing a physical compression of
the crystal element . This compression forces
the tiny dipoles to change their orientation ,
which induces a voltage between the
• The voltage is amplified and serves as the
ultrasonic signal for display on television
• Compression force and associated voltage are
responsible for the name piezoelectricity
which means “ pressure “ electricity.
• Naturally occurring materials possess
piezoelectric properties : Quartz
• Man made material ( ferroelectrics ) :
lead zirconate titanate
• Curie Temperature :
is the temperature at which polarization is
• Heating the piezoelectric crystal above the
Curie temperature reduces it to a useless
piece of ceramic so transducer should never
Resonant frequency :
• The thickness of piezoelectric crystal
determines its natural frequency called its
• The crystal is designed so that its thickness is
equal to exactly half the wavelength of the
ultrasound to be produced by the transducers.
• Thickness = wavelength/2
Transducer Q Factor :
• Two characteristics :- purity of sound & the length
of time that the sound persists.
• A high Q transducer produces a nearly pure
sound made up of narrow range of frequencies.
• A low Q transducer produces whole spectrum of
sound covering wider range of frequencies.
• The interval between initiation of the wave and
complete cessation of vibration is called the “ ring
down time “.
• High Q : useful for doppler USG transducers
because it furnishes narrow range of sound
• Low Q : useful for organ imaging because it
can furnish short ultrasound pulses and will
respond to a broad range of returning
• The Q factor can be controlled by altering the
characteristic of the damping block.
• Damping block consist of powered rubber and
tungsten blended with an epoxy resin.
• Ratio of tungsten to resin is chosen to satisfy
the impedance requirements
• Rubber is added to increase the attenuation of
sound in the backing block.
RECEPTION OF ULTRASOUND
• 1. Reflection :
• Both ultrasound and light obey the law of
reflection , the angle of incidence and the
angle of reflection are equal.
• The factor that determines the percent of the
incident beam undergoing reflection is a
property , peculiar to various tissues , called
• Acoustic impendence Z = p u rayls
• where p is density , u is velocity
of sound in cm/sec.
• The velocity of sound in all soft tissue is
virtually same 1540 m/sec.
• So , Z α p. example air and bone.
• As sound waves pass from one tissue to
another , the amount of reflection is
determined by the difference in the
impedances of the two tissues .
• At a particular angle of incidence known as
the critical angle , total reflection occurs at the
• This occurs when an ultrasound beam passes,
at an angle other than 90 degrees, from one
tissue into another with change in velocity.
• It increase with the increasing angle of
• It passes deeper into the body where it gives
rise to artifacts.
• If angle of incidence is less than 3 degrees,
very little refraction seen.
• Due to friction among molecules in their back –
forth movement , reduction in intensity of the
ultrasound beam occurs as it traverse matter.
Friction results in degradation of part of
molecules kinetic energy to heat.
• The greater the frequency , the greater the
attenuation coefficient. This means high
frequency beam shows less penetration than a
low frequency beam.
• Attenuation in soft tissue is 1 dB/cm/MHz
TYPES OF TRANSDUCERS
The ultrasound transducers differ in
construction according to
• Piezoelectric crystal arrangement
• Aperture ( footprint )
• Operating frequency ( which is directly related
to the penetration depth )
• Crystal arrangement : phased array
• Footprint size : small
• Operating frequency : 1-5 MHz
• Ultrasound beam shape : sector, almost
• Use : small acoustic windows ,mainly ECHO,
gynecological ultrasound, upper body
• Crystal arrangement : linear
• Footprint size: usually big ( small for hockey
• Operating frequency : 3-12 MHz
• Ultrasound beam shape : rectangular
• Use : USG of superficial structures e.g.
obstetrics ultrasound , breast,thyroid,vascular
• Crystal arrangement : curvilinear
• Footprint size : big ( small for the micro convex
• Operating frequency : 1-5 MHz
• Ultrasound beam shape
• Use : useful in all USG types except ECHO,
typically abdominal ,pelvic and lung ( micro
convex transducer )
TRANSDUCER JELLY/COUPLING AGENT
• Air and other gases impede sound waves
• At tissue-air interface, more than 99.9% of the
beam is reflected so none is available for further
• Jelly acts as a special aqueous conductive
medium for the sound waves
• Prevents the formation of bubbles between the
transducer and the patient’s skin
• Acts as a lubricant
USG GEL INGREDIENTS
• Carbomer : synthetic high molecular weight polymer of
acrylic acid cross linked with allyl sucrose and
containing 50-68% of carboxylic acid groups.
Neutralized with alkali hydroxide to make it water
• Propylene glycol : organic oil compound that doesnot
irritate the skin and helps retain moisture
• Glycerine and trolamine : neutral colorless gel that
absorbs moisture from air
• Colorant : occasionally used, usually blue color