Principle of USG imaging, construction
of transducers and USG controls
DR. DEV LAKHERA

Topics
• Properties of sound wave
• Propagation of sound wave
• Transducer components
• Workings of a transducer
• Interaction between sound and matter
• Ultrasonic image display
• USG controls

Sound as a wave
• Mechanical
• Require a medium for transport
• Normal auditory frequency – 20Hz-20 KHz
• Ultrasonic - > 20 KHz
• Diagnostic imaging – 1 MHz – 20 MHz

• Longitudinal waves travel with
alternate compression and
rarefaction.

• Wavelength (λ) – bet two
compression bands
• Time (T) to complete a single
cycle is called the period.
• frequency (f ) -number of
complete cycles in a unit of
time.

Propagation of sound (velocity)
Depends on:
 Density
 Resistance to compression
1540 m/sec

• Sound travels slowest in gases, intermediate in liquids and fastest
through solids
• Body tissues behave as liquids (avg 1540 m/sec)
• V = Freq x Wavelength (velocity constant in a medium)
• Freq – inc  wavelength - dec

Propagation of sound (intensity)
Depends on:
 Amplitude of oscillation
Db
Defines the Brightness of the image
Irrespective of the Freq the Amp remains
constant
The Higher the Amp the brighter the image
and the lower the more darker the images
Returning Waves

Frequency
Higher the freq Lower the penetration and
Higher the resolution
Low the freq higher the penetration and
lower the resolution

Formation of USG image
1. Electrical Energy
converted to Sound
waves
2. The Sound waves
are reflected by
tissues
3. Reflected Sound waves are
converted to electrical signals
and later to Image

Transducer
• Device that converts one
form of energy into another

Components
• Piezoelectric crystal
• Electrodes with conducting material
• Backing block
• Coaxial cable
• Casing

• Backing block

Piezoelectric crystals
• Piezoelectric effect- certain materials on
application of electric energy change their
physical dimensions
• Naturally occurring: Quartz
• PZT- Lead zirconate titanate

• Dipoles –geometric pattern
• Plating electrodes
• Voltage applied in a pulse
causes this crystal to vibrate

Receive
• Echoes reflect back and from each tissue interface and cause physical
compression of crystal element
• Dipole change their orientation
• Causes generation of voltage  received and displayed

Characteristics of a USG Beam
• Fresnel zone- Determined by radius of
transducer
• Fraunhofer zone (divergent part)
• Fresnel zone- Increases with frequency
and diameter

Advantage of high frequency beams
• Superior superficial resolution , longer frensel zone
• Tissue absorption increases with increasing frequency so low
frequency beam required to penetrate thick parts
• Larger transducers however reduce side to side resolution.(now
reduced due to focused transducers)

Phase array transducer
Linear transducer
Convex transducer- C60

Interactions between sound and matter
Reflection
Refraction
Attenuation
Scattering

REFLECTION
Images are produced by the reflected portion of beam
Percentage of reflected beam depends upon
1.Tissue’s acoustic impedance
2.Beam’s angle of incidence

Acoustic Impedance
• How much resistance an ultrasound beam encounters as it passes
through a tissue.
• Acoustic impedance depends on:
the density of the tissue (d, in kg/m3)
the speed of the sound wave (c, in m/s)
• Amplitude of returning echo is proportional to the difference in
acoustic impedance between the two tissues

Acoustic Impedance
• Two regions of very different
acoustic impedances, the
beam is reflected or absorbed
• Acoustic impedance of tissue
is constant (speed of
transmission is constant)
Examples of impedance
for bodily tissues (in
kg/(m2s)):
•air 0.0004 × 106
•lung 0.18 × 106
•fat 1.34 × 106
•water 1.48 × 106
•kidney 1.63 × 106
•blood 1.65 × 106
•liver 1.65 × 106
•muscle 1.71 × 106
•bone 7.8 × 106

• Tissue - air interface – 99.9 % beam is reflected
• Coupling agent is needed
• Ultrasound gel

Angle of incidence
• Higher the angle of incidence lesser is the
reflection

Specular reflector
Diaphragm
Wall of urine-filled bladder
Endometrial stripe

Echogenicity (caused by Reflection)
Anechoic Hypo-Echoic Hyper-Echoic

Scattering
• Redirection of sound in several directions
• Caused by interaction with small reflector or rough surface
• Only portion of sound wave returns to transducer

Refraction
• Sound passes from one medium to other
at an angle change in velocity but
frequency is constant so there is a change
in wavelength.
• Causes a change in direction
• spatial distortion

Absorption
• Due to frictional forces opposing the movement of particles in a
medium
• Utrasonic energy  Thermal energy
• Depends on 1) frequency
2) viscosity of the medium
3) relaxation time of the medium

• The deeper the wave travels in the body, the weaker it becomes
• The amplitude of the wave decreases with increasing depth
Attenuation

Ultrasonic display
• Electronic representation of
data
• A – mode
• M – mode
• 2D B mode
•

Amplitude Modulation (A- mode)
• line through the body with the echoes
plotted on screen as a function of depth
• Stronger echoes produce larger spinkes

Motion mode (M- Mode)
• pulses are emitted in quick succession
• organ boundaries that produce
reflections move relative to the probe
• commonly in cardiac and fetal cardiac
imaging

B-mode or 2D mode: (Brightness mode)
• Most used imaging mode
• Produces a picture of a slice of tissue
• Brightness depends upon the amplitude or
intensity of the echo

USG Imaging Controls
• TGC- Time gain compensator
• Near gain
• Far gain
• Intensity
• Coarse gain
• Reject
• Delay
• Enhancement

Time gain compensator
TGC adjusts the degree of amplification of
echoes

Amplitude
• Intensity control- Increases the potential difference between transducer
• Coarse gain – Increases the height of all echoes proportionately
• Reject control- It helps remove echoes below
a minimum amplitude

• Delay – Regulates the depth at which the TGC begins to augment the
weaker signal
• Q-scan - automatically optimises key imaging parameters

• General mode
• Resolution mode (high
frequency setting)
• Penetration mode
• (low frequency setting)

• 2D – sets to default B-mode
• Depth
• Zoom

• Power doppler
Uses amplitude of Doppler signal to detect
moving matter
• Pulse wave doppler
Emits USG in pulses
Lower velocity
• Continuous wave doppler
Transducer emits and receives continuously
High velocity
Color flow
Type of power doppler emits pulses

Tissue Harmonic Imaging-
Selectively removes phase
aberrations generated by
variation of velocity between
interfaces

Speckle reduction filter
• ultrasound speckle image degradation and loss of contrast.
• Interaction of generated acoustic fields grainy appearance

THANK YOU

Types of Resolution
• Axial Resolution
• specifies how close together two objects can be along the
axis of the beam, yet still be detected as two separate
objects
• frequency (wavelength) affects axial resolution

Types of Resolution
• Lateral Resolution
• the ability to resolve two adjacent objects that are
perpendicular to the beam axis as separate objects
• beamwidth affects lateral resolution

Types of Resolution
• Spatial Resolution
• also called Detail Resolution
• the combination of AXIAL and LATERAL resolution
• some customers may use this term

Types of Resolution
• Contrast Resolution
• the ability to resolve two adjacent objects of similar
intensity/reflective properties as separate objects

Types of Resolution
• Temporal Resolution
• the ability to accurately locate the position of moving
structures at particular instants in time
• also known as frame rate
• VERY IMPORTANT IN CARDIOLOGY