ULTRASOUND PHYSICSTRANSDUCERS

TRANSDUCER JELLY

SEMINAR BY

DR NAVNI GARG

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

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

• 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

COMPRESSION WAVE

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.)

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.).

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.

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.

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.

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

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

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

• Sound intensity is measured in decibel (dB).• Ultrasonic intensities are expressed

in power / unit area (watts/cm2)

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.

COMPOSITION OF TRANSDUCER

• 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 interface.

• 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

• 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.

• Naturally occurring materials possess piezoelectric properties : Quartz

• Man made material ( ferroelectrics ) : Barium titanate lead zirconate titanate

• 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.

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

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 frequencies

• Low Q : useful for organ imaging because it can furnish short ultrasound pulses and will respond to a broad range of returning frequencies

• 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

• 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 skin

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.

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

TYPES OF ELECTRONIC SCANNING

LINEAR ARRAY PHASED ARRAY

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 )

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

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

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 )

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

PROPERTIES

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

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

Thank you