USG TRANSDUCER- BASIC STRUCTURE & FUNCTION

SPEAKER: -DR.HARITHA RANIMODERATOR: -DR.KALYAN

ULTRASOUND TRANSDUCER- BASIC

STRUCTURE & FUNCTION.

What is ultrasound?

Ultrasound is the name given to high frequency sound waves, over 20k cycles per second(20kHz)

These are inaudible to humans, & can be used to scan tissues of the body

Sonic waves used in diagnostic imaging is b/w 1MHz to 20MHz.

Sound beam is similar to x ray beam as both waves transmit energy except that x rays pass through vacuum but ultrasound waves need medium to transmission.

Velocity of sound depends on nature of medium

The particles in sound beam oscillate back & forth but over a short distance only a few microns in liquids & even smaller in solids.

Transducer

- device that converts one form of energy into another form of energy

USG transducer: A part of ultrasound unit which

Converts electrical energy into ultrasonic energy (sound waves/ acoustic energy) during transmission into the tissues &

Coverts ultrasonic energy reflected back from tissues into electrical energy during reception.

This Conversion is accomplished through the piezoelectric effect

Thus a Transducer is both a transmitter & a receiver.

A transducer often called a SCANNER/PROBE & is connected to the ultrasound unit by a flexible cable.

Transducers are most expensive & fragile part of ultrasound unit ,and must be handled very carefully.

A large variety of sizes & shapes of transducers are available to perform specific functions;but they all have this basic design.

• Electronic:-– Linear Arrays– Curved Arrays– Phased Arrays • Mechanical:-– Oscillating– Rotating

Transducer Types:-

Mechanical scanners

In mechanical scanners, single element transducer or group of single element transducers are mechanically moved to form image.

There are 3 types, two of these use single transducer that is caused to oscillate, whereas 3rd uses two, three or four transducers mounted on a rotating wheel.

All produces an image with a sector format.

LINEAR ARRAY:-

Consists of number of small rectangular transducer elements arranged in a line, could be 64 to 200 transducers forming an assembly from 4 to 10 cm

Elements are arranged in linear fashion

Rectangular image format on monitor.

used for small parts like breast, thyroid , vascular imaging etc

CURVED ARRAY:-

Crystals are arranged in concentric rings

Large surface field of view

used for general abdominal , obstetric scansSmall ,high frequency curved arrays are used

for TVS, transrectal and pediatric imaging

PHASED/SECTOR ARRAY:-

The sector scan is obtained by Electronically moving the US Beam

These scans are fan shaped.

Electronic Focusing

Used in Gynaecological &Cardiological examination.

Expensive & no Side Resolution

BASIC STRUCTURE:-

Transducer consists of 1. Piezoelectric Crystal 2.Backing material 3.Acoustic insulator 4.Electrical connections 5.Tansducer housing

Piezoelectric Crystal element :-

It is the most important component. It is thin{approx.0.5mm}located near the face

of transducer The front & back surfaces of crystal are coated

with a thin conducting film to ensure good contact with the 2 electrodes that will supply the electric field used to strian the crystal.

When the voltage is applied, deformity of crystal occurs-strian.

Change in physical dimensions of material when electric field is applied is called PIEZOELECTRIC EFFECT

Crystal surfaces are plated with gold and silver electrodes.

Outside /Ground electrode is grounded to prevent electric shock to patient & its outside surface is coated with a water tight electrical insulator.

Inner/live electrode abuts against a thick backing block that absorbs sound waves transmitted back into the transducer.

PIEZOELECTRIC MATERIALS

Natural - QUARTZArtificial – most used in medical USG are

manmade called as ferroelectrics – Barium titanate – Lead zirconate(PZT)ADVANTAGE – they can be formed into

different shapes

Synthetic CrystalsMan-made crystals:

less expensivemore durablemore efficient in converting mechanical

energy to electrical energyoften combined with non-piezoelectric

polymer to create a material called piezo-composites

These composites have lower impedance, improved bandwidth, sensitivity & resolution.

Lead Zirconate Titanate (PZT)

- is the most common piezoelectric material found in diagnostic imaging transducers

Backing block:-

Dampens the vibrations between voltage spikes, so that it primes the transducer to generate multiple short pulses of sound, which is to detect returning echoes.

Made up of tungsten and rubber powder in epoxy resin

It accepts all waves that it receives and completely absorb the energy of the waves.

Dynamic damping - electronic means to suppress the ringing by applying a voltage of opposite polarity to the crystal after the excitation pulse

Acoustic insulator:-

Made up of rubber or corkIt prevents the sound from passing into the

housing

INSULATING COVER/Matching Layer (facing material)

Thin layer of aluminum powder in epoxy resin in front (facing) of the crystal

the impedance difference between the crystal & the skin

matching layer thickness = ¼ of crystal’s resonating frequency

ELECTRICAL CONNECTIONS/Wiring :-

Carries electrical pulse to the crystal

Transmits voltage from the receiving crystal back to the US unit

Each crystal requires electrical contact

PLASTIC HOUSING/Insulating Case:-

Strong Plastic casing around transducerProtects:

Sonographer & Pt. from electrical shocks

Keeps outside interference/electrical noise from entering

Protects the transducer’s components

FUNCTION:-

Transducer is the most important part of ultrasound unit& its function is based upon the pulse-echo principle occurring with ultrasound piezoelectric effect of crystals, ultrasound transducers convert:

– Electricity into sound = pulse– Sound into electricity = echo.The function of transducer is to transmit the

ultrasound pulses &recieves back the echoes during scanning.

Piezoelectric Effect

piezo in Greek for ‘to press’ & elektron in Greek for ‘amber’.

--Change in physical dimensions of material when electric field is applied and vice versa,this is called PIEZOELECTRIC EFFECT.

Charecteristics of piezoelectric material:-

Piezoelectric crystal material in the transducer, is made up of many dipoles arranged in specific geometric configuration

An electric dipole is a distorted molecule that appears to have a positive charge on one end &negative charge on the other.

The +ve & -ve ends are arranged so that an electric field will cause them to realign, thus changing the dimensions of crystal.

When the voltage is applied, deformity of crystal occurs

Piezoelectric Effect

No current flows through crystals, but the plating electrodes behave as capacitors & it is the voltage b/w them that produces the electric field

This causes the crystal to change in shape.If the voltage is applied in sudden burst or

pulse the crystal vibrates & generates sound waves which passes through the tissues.(pulse).

As sound pulse passes through the body,echoes reflect back towards the transducer from each tissue interface.

The echoes carry energy & they transmit their energy to the transducer,causing a physical compression of the crystal element.

This compression forces the dipoles to change their orientation,which induces a voltage b/w the electrodes.(REVERSE PIEZOELECTRIC EFFECT)

The voltage is amplified in the scanner& serves as the ultrasonic signal for display on a TV monitor.

FUNCTION:-Voltage applied crystal vibrate ultrasonic waves tissues reflected back ultrasonic waves electrical voltage Stored in SCAN CONVERTER Produces a DIGITAL SIGNAL displayed in TV monitor.

SELECTION OF TRANSDUCER :-

Superficial vessels and organs within 1to 3 cms depth and intra operative imaging -7.5 to 15 Hz

Deeper structures in abdomen and pelvis within 12 to 15cms – 2.25 to 3.5Hz

Transducer Frequencies• 2.5 MHz • Deep abdomen,OB/Gyn• 3.5MHz • General abdomen,OB/Gyn

• 5.0 MHz• Vascular, Breast, Gyn

• 7.5 MHz• Breast, Thyroid

• 10.0 MHz • Breast, Thyroid,Superficial veins, Superficial masses

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