usg transducer- basic structure & function
TRANSCRIPT
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
THANK U