This document provides an overview of ultrasound physics, transducers, and transducer jelly. It discusses the characteristics of sound waves including their generation through mechanical vibration and their transmission through solids, liquids, and gases. The history of ultrasound and piezoelectricity is summarized. Key ultrasound concepts like wavelength, frequency, propagation velocity, amplitude, and absorption are defined. The components and function of ultrasound transducers including the piezoelectric crystal and backing block are described. Finally, the properties and ingredients of transducer jelly used to couple the transducer to the skin are outlined.

1. ULTRASOUND PHYSICS
TRANSDUCERS
TRANSDUCER JELLY
SEMINAR BY
DR NAVNI GARG

2. 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
vacuum.
• Sound must be generated mechanically by
vibrating body matter

3. HISTORY OF ULTRASOUND
• Piezoelectricity discovered by Pierre
and Jacques Curie in 1880 using
natural quartz
• SONAR was first used in 1940s war
time
• Diagnostic medical applications in
use since late 1950’s

4. • 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
rarefraction
• D: alternate movement of
the piston to right and left
establishes a longitudinal
wave in the medium

5. COMPRESSION WAVE

6. 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
compression. meter.)

7. FREQUENCY
• 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
/sec.).

8. WAVELENGTH
• 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.

10. Wavelength is one of the main factors
affecting axial resolution of an
ultrasound image
• 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
children.
• 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.

11. PROPAGATION VELOCITY
• The propagation velocity is the velocity at
which sound travels through a particular
medium
• 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
metres/second.

12. COMPRESSIBILITY
• 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
compressed.
• Solids > liquids > gases

13. DENSITY
• Dense materials have large molecules with
large inertia : difficult to move or stop once in
motion
• Propagation of sound requires rhythmic
starting and stopping of particles
• Density is inversely related to velocity

15. AMPLITUDE/INTENSITY
• 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
disturbance
• Determined by the length of oscillation of
particle
• Greater amplitude = more intense sound

16. • Sound intensity is measured in
decibel (dB).
• Ultrasonic intensities are expressed
in power / unit area (watts/cm2)

17. TRANSDUCER
• 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.

18. COMPOSITION OF TRANSDUCER

19. • 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.

20. • 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.

22. • 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

23. • 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
interface.

24. • 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
electrodes

25. • The voltage is amplified and serves as the
ultrasonic signal for display on television
monitor.
• Compression force and associated voltage are
responsible for the name piezoelectricity
which means “ pressure “ electricity.

26. • Naturally occurring materials possess
piezoelectric properties : Quartz
• Man made material ( ferroelectrics ) :
Barium titanate
lead zirconate titanate

27. • Curie Temperature :
is the temperature at which polarization is
lost.
• Heating the piezoelectric crystal above the
Curie temperature reduces it to a useless
piece of ceramic so transducer should never
be autoclaved.

28. Resonant frequency :
• The thickness of piezoelectric crystal
determines its natural frequency called its
resonant frequency.
• 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

29. 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 “.

30. • High Q : useful for doppler USG transducers
because it furnishes narrow range of sound
frequencies
• Low Q : useful for organ imaging because it
can furnish short ultrasound pulses and will
respond to a broad range of returning
frequencies

31. • 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.

32. 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

34. • 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.

35. • 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 .

37. • At a particular angle of incidence known as
the critical angle , total reflection occurs at the
skin

38. REFRACTION
• 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
incidence .
• 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.

39. ABSORPTION
• 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

40. TYPES OF ELECTRONIC SCANNING
LINEAR ARRAY PHASED ARRAY

42. 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 )

43. SECTOR TRANSDUCER
• Crystal arrangement : phased array
• Footprint size : small
• Operating frequency : 1-5 MHz
• Ultrasound beam shape : sector, almost
triangular
• Use : small acoustic windows ,mainly ECHO,
gynecological ultrasound, upper body
ultrasound

44. LINEAR TRANSDUCER
• Crystal arrangement : linear
• Footprint size: usually big ( small for hockey
transducers )
• Operating frequency : 3-12 MHz
• Ultrasound beam shape : rectangular
• Use : USG of superficial structures e.g.
obstetrics ultrasound , breast,thyroid,vascular
ultrasound

45. CONVEX TRANSDUCER
• Crystal arrangement : curvilinear
• Footprint size : big ( small for the micro convex
transducers )
• Operating frequency : 1-5 MHz
• Ultrasound beam shape
• Use : useful in all USG types except ECHO,
typically abdominal ,pelvic and lung ( micro
convex transducer )

47. 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
imaging
• 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

48. PROPERTIES
• Non allergenic
• Odourless
• Non staining
• Harmless
• Neutral ph
• Easily removable with tissue or towel

49. USG GEL INGREDIENTS
• Water
• 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
soluble.
• EDTA
• 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

50. Thank you