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 reflectorDiaphragm

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

Directional color coding

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