Physics Of Usg
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Physics of USG
Sound is ME travelling thru matter as a wave producing alternating compression and rarefactions. Limited physical displacement of the material thru which it is transmitted.
Basics of acoustics
Changes in pressure with time is…….. Distance between points on the time pressure curve is…..wavelength Time taken to complete one cycle is…period Number of cycles in unit time is….frequenccy. Sound for diagnostic applications is between 2 to 15 MHz 50 to 60 MHz under investigation
Propagation of Sound
Brief bursts of sound/energy transmitted into the body. Travels either as Transverse/Longitudinal waves Velocity depends resistance of medium to compression. Resistance depends on density and elasticity Average velocity in body is 1540m/s
Distance Measurement
Propagation velocity is known and time interval between transmission and return of echo known then distance can be measured.
Interactions of beam with tissues Acoustic impedance Reflection Attenuation
Acoustic Impedance
Measure of resistance to sound passing thru medium Product of density of the medium propagating sound and the propgation velocity of the medium. Interfaces with large Z reflects everything With small Z allows some to pass. Specular reflectors….
Reflection
Specular reflectors - Interfaces are large and relatively smooth sound is reflected in the same way as a mirror reflects light.
Beam has to be 90 degrees.
Diffuse reflectors – if the interfaces are smaller echoes are scattered in all direction
Speck : tissue texture of solid organs due to constuctive and destructive interference of sound.
Refraction
Change in direction of propagation of sound wave. Happens due to change in tissue medium with different propogation velocity Imp as it causes misregistration of structures in an usg image.
Attenuation
As beam propagates energy is transferred to the medium as heat Pressure waves decrease in amplitude Energy eventually removed due to combined effects of absorption, scatter and reflection. Higher freq and lower freq
Instrumentation
Transmitter Transducer Receiver Display Storage
Transmitter
Precisely timed high amp voltage results in brief pulses of energy Rate of pulses emitted by the transducer – PRF PRF’s of 1 to 10 Khz are used.
Transducer
Device that converts electrical energy to mechanical energy….thus generates acoustic pulses. Also a receiver of reflected echoes…converts back to electrical energy
Piezoelectricity principle Changing the polarity of a voltage applied to transducer changes the thickness of transducer,expands and contracts as polarity changes Results in generation of mechanical pressure waves Also results in gen of small potentials across the transducer as it is struck by returning echoes. Bandwith – range of frequencies produced by the transducer.
A single brief voltage change causes transducer to vibrate at its preferential freq Pulse length – Number of cycles of sound in each pulse Shorter pulse length better axial resolution Damping applied Fresnel zone/near field (pressure amp varies greatly) Frauenhaufer zone/far field (field diverges depending on radius of transducer and amp remains steady) This divergence can be corrected by proper focusing
Receiver
Weak returning echoes are detected and amplified. Different tissues attenuate differently. Transducer allows for compensating for the differences in echo strength at different depths by selectively amplifying the echoes from deeper structures/ suppressing from sup structures. Compression of varies freq into a range that can be displayed Dynamic range - Ratio of highest to the lowest amp displayed
Image display
A mode M mode B mode
Black no signal ..white greatest signal..grey intermediate 256 shades of grey possible for each pixel. 15 to 60 frames/sec.
Steering
Mechanical Single piezoelect Fixed focus Better suited where access limited smaller area available for contact.
Electronic Multiple elements Variable focus Require larger contact.
Arrays
Multiple elements Linear Curved Phased – elements fired in diff direction in particular sequence by precise timing
Special imaging modes
Harmonic imaging Spatial componding 3d/4d USG
ARTIFACTS
Reverberation Refraction Side lobes
Reverberation artifacts
Signal reflects repeatedly between highly reflective surfaces Give solid appearance in cyst Change the angle to avoid Helpful in identifying clips
Refraction
Targets not along the beam are insonated Structures appear not in the actual volume of the probe
Side lobe artifacts Creates appearance of debris in fluid filled structures
Shadowing. Due to strong reflector or attenuater
Homework!!
Read usg physics Read artefacts..article in radiographics
Sound is ME travelling thru matter as a wave producing alternating compression and rarefactions. Limited physical displacement of the material thru which it is transmitted.
Basics of acoustics
Changes in pressure with time is…….. Distance between points on the time pressure curve is…..wavelength Time taken to complete one cycle is…period Number of cycles in unit time is….frequenccy. Sound for diagnostic applications is between 2 to 15 MHz 50 to 60 MHz under investigation
Propagation of Sound
Brief bursts of sound/energy transmitted into the body. Travels either as Transverse/Longitudinal waves Velocity depends resistance of medium to compression. Resistance depends on density and elasticity Average velocity in body is 1540m/s
Distance Measurement
Propagation velocity is known and time interval between transmission and return of echo known then distance can be measured.
Interactions of beam with tissues Acoustic impedance Reflection Attenuation
Acoustic Impedance
Measure of resistance to sound passing thru medium Product of density of the medium propagating sound and the propgation velocity of the medium. Interfaces with large Z reflects everything With small Z allows some to pass. Specular reflectors….
Reflection
Specular reflectors - Interfaces are large and relatively smooth sound is reflected in the same way as a mirror reflects light.
Beam has to be 90 degrees.
Diffuse reflectors – if the interfaces are smaller echoes are scattered in all direction
Speck : tissue texture of solid organs due to constuctive and destructive interference of sound.
Refraction
Change in direction of propagation of sound wave. Happens due to change in tissue medium with different propogation velocity Imp as it causes misregistration of structures in an usg image.
Attenuation
As beam propagates energy is transferred to the medium as heat Pressure waves decrease in amplitude Energy eventually removed due to combined effects of absorption, scatter and reflection. Higher freq and lower freq
Instrumentation
Transmitter Transducer Receiver Display Storage
Transmitter
Precisely timed high amp voltage results in brief pulses of energy Rate of pulses emitted by the transducer – PRF PRF’s of 1 to 10 Khz are used.
Transducer
Device that converts electrical energy to mechanical energy….thus generates acoustic pulses. Also a receiver of reflected echoes…converts back to electrical energy
Piezoelectricity principle Changing the polarity of a voltage applied to transducer changes the thickness of transducer,expands and contracts as polarity changes Results in generation of mechanical pressure waves Also results in gen of small potentials across the transducer as it is struck by returning echoes. Bandwith – range of frequencies produced by the transducer.
A single brief voltage change causes transducer to vibrate at its preferential freq Pulse length – Number of cycles of sound in each pulse Shorter pulse length better axial resolution Damping applied Fresnel zone/near field (pressure amp varies greatly) Frauenhaufer zone/far field (field diverges depending on radius of transducer and amp remains steady) This divergence can be corrected by proper focusing
Receiver
Weak returning echoes are detected and amplified. Different tissues attenuate differently. Transducer allows for compensating for the differences in echo strength at different depths by selectively amplifying the echoes from deeper structures/ suppressing from sup structures. Compression of varies freq into a range that can be displayed Dynamic range - Ratio of highest to the lowest amp displayed
Image display
A mode M mode B mode
Black no signal ..white greatest signal..grey intermediate 256 shades of grey possible for each pixel. 15 to 60 frames/sec.
Steering
Mechanical Single piezoelect Fixed focus Better suited where access limited smaller area available for contact.
Electronic Multiple elements Variable focus Require larger contact.
Arrays
Multiple elements Linear Curved Phased – elements fired in diff direction in particular sequence by precise timing
Special imaging modes
Harmonic imaging Spatial componding 3d/4d USG
ARTIFACTS
Reverberation Refraction Side lobes
Reverberation artifacts
Signal reflects repeatedly between highly reflective surfaces Give solid appearance in cyst Change the angle to avoid Helpful in identifying clips
Refraction
Targets not along the beam are insonated Structures appear not in the actual volume of the probe
Side lobe artifacts Creates appearance of debris in fluid filled structures
Shadowing. Due to strong reflector or attenuater
Homework!!
Read usg physics Read artefacts..article in radiographics
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