principle of usg imaging, construction of transducers
TRANSCRIPT
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Principle of USG imaging, construction of transducers and USG controls
DR. DEV LAKHERA
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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
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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
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• Longitudinal waves travel with alternate compression and rarefaction.
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• 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.
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Propagation of sound (velocity)
Depends on:
Density
Resistance to compression
1540 m/sec
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• 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
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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
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Frequency
Higher the freq Lower the penetration and Higher the resolution
Low the freq higher the penetration and lower the resolution
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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
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Transducer
• Device that converts one form of energy into another
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Components
• Piezoelectric crystal
• Electrodes with conducting material
• Backing block
• Coaxial cable
• Casing
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• Backing block
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Piezoelectric crystals
• Piezoelectric effect- certain materials on application of electric energy change their physical dimensions
• Naturally occurring: Quartz
• PZT- Lead zirconate titanate
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• Dipoles –geometric pattern
• Plating electrodes
• Voltage applied in a pulse causes this crystal to vibrate
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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
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Characteristics of a USG Beam
• Fresnel zone- Determined by radius of transducer
• Fraunhofer zone (divergent part)
• Fresnel zone- Increases with frequency and diameter
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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)
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Phase array transducer
Linear transducer
Convex transducer- C60
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Interactions between sound and matter
Reflection
Refraction
Attenuation
Scattering
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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
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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
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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
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• Tissue - air interface – 99.9 % beam is reflected
• Coupling agent is needed
• Ultrasound gel
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Angle of incidence
• Higher the angle of incidence lesser is the reflection
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Specular reflectorDiaphragm
Wall of urine-filled bladder
Endometrial stripe
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Echogenicity (caused by Reflection)
Anechoic Hypo-Echoic Hyper-Echoic
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Scattering
• Redirection of sound in several directions
• Caused by interaction with small reflector or rough surface
• Only portion of sound wave returns to transducer
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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
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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
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• The deeper the wave travels in the body, the weaker it becomes
• The amplitude of the wave decreases with increasing depth
Attenuation
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Ultrasonic display
• Electronic representation of data
• A – mode
• M – mode
• 2D B mode
•
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Amplitude Modulation (A- mode)
• line through the body with the echoes plotted on screen as a function of depth
• Stronger echoes produce larger spinkes
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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
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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
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USG Imaging Controls
• TGC- Time gain compensator
• Near gain
• Far gain
• Intensity
• Coarse gain
• Reject
• Delay
• Enhancement
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Time gain compensator
TGC adjusts the degree of amplification of
echoes
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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
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• Delay – Regulates the depth at which the TGC begins to augment the weaker signal
• Q-scan - automatically optimises key imaging parameters
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• General mode
• Resolution mode (high frequency setting)
• Penetration mode
• (low frequency setting)
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• 2D – sets to default B-mode
• Depth
• Zoom
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• 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
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Tissue Harmonic Imaging-
Selectively removes phase
aberrations generated by
variation of velocity between
interfaces
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Speckle reduction filter
• ultrasound speckle image degradation and loss of contrast.
• Interaction of generated acoustic fields grainy appearance
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THANK YOU
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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
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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
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Types of Resolution
• Spatial Resolution• also called Detail Resolution
• the combination of AXIAL and LATERAL resolution
• some customers may use this term
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Types of Resolution• Contrast Resolution
• the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects
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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