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Ultrasound Physics SPI - Registry Review COURSE |
TEST & LEARN QUIZ
MODULE 1 - BASICS OF SOUND
BASICS OF SOUND - VIDEO TUTORIAL
BASICS OF SOUND - COURSE OUTLINE
Ultrasound Physics: Basics of Sound - Ultrasound Facts
Diagnostic ultrasound utilizes an ultrasound unit/machine to send sound wave pulses into tissues and to obtain information based on the return echoes. Gray-scale imaging is produced by the return echoes. The brightness of the returning echo corresponds to its strength and anatomic location of the tissue interface encountered by the ultrasound pulse.
Ultrasound is sound with frequencies above the level of human hearing. NOTE: Frequencies are expressed in Hertz(Hz)
• Infrasound > 20 Hz
• Audible Sound = 20 Hz to 20,000 Hz
• Ultrasound > 20,000 Hz
Ultrasound Units to Know – Units Matter!
• M “Mega” = Million
• k “kilo” = Thousand
• c “centi” = HundredTH
• m “milli” = ThousandTH
• µ “micro” = MillionTH
NOTE: When calculating equations be sure your units complement each other.
• Meters (m) and seconds (sec)
• Millimeters (mm) and microseconds (µsec)
Ultrasound Physics: Basics of Sound - Transducers
Ultrasound Waves are produced by Piezoelectric transducers, which use polarized polycrystalline ferroelectric ceramic materials to convert pressure energy into mechanical energy and vice versa.
• The conversion of electrical energy into mechanical energy – transmission of the sound beam
• The conversion of mechanical energy into electrical energy – receiving the reflected beam information
• The piezoelectric effect was discovered by the Curie brothers in the 1800’s
• Natural piezoelectric crystals exist (i.e. quartz and tourmaline) but are typically too weak for diagnostic imaging
• Typically Lead Zirconate Titanate (PZT) is utilized for transducer crystal elements
• The Curie point (Tc) of PZT is more than 200°C
• Cannot be heated beyond Curie point or will lose its polarization (no autoclave)
Ultrasound Physics: Basics of Sound - Sound Waves
Sound is a wave. It is created by vibrations of a moving object. It is made up of high and low areas within the wave called compressions and rarefactions. A few things to know about sound waves:
• Sound Waves propagate (travel) and consist of many wave parameters and variables.
• Sound waves carry energy and information from one place to another, but they do not carry matter.
• Sound waves require a medium to travel through. Sound CANNOT travel through a vacuum or in outer space.
• Sound waves have natural occurring, measurable acoustic variables which affect and disturb the surrounding tissues.
• Sound waves travel in a straight line (in general)
• Sound waves are Longitudinal and Mechanical Waves
Sound Waves – Longitudinal and Mechanical Waves
Longitudinal Waves are waves that oscillate in the same path that the sound wave is moving. Particles move in the same direction as the wave. Longitudinal Waves are produced by the rapid back-and-forth vibration of an object and produce areas of compression and rarefaction throughout the wave.
Mechanical Waves are pressure waves that produce a regular mechanical vibration and travel through matter as a waveform.
• Compression - increase in pressure and/or density
• Rarefaction - decrease in pressure and/or density
Sound Waves - Propagation
As sound propagates (travels), the sound waves react to tissue interfaces in a number of ways. Those behaviors include the following:
• Through Transmission
• Attenuation
• Diffraction
Through Transmission
Through transmission refers to the continued propagation of sound relatively unchanged from one medium to another through a tissue boundary. The path of the sound beam continues forward in a straight line without change in speed or direction.
Attenuation
Acoustic Attenuation is the gradual loss of intensity and reduction of the force of sound as it propagates through a medium. Attenuation occurs through Reflection, Absorption and Scatter as the sound wave travels through a medium and encounters tissue boundaries and interfaces.
Attenuation ∙ Reflection
Reflection of sound is the return of echoes from a tissue boundary or interface. Reflection can occur in whole or in part. The strength of the returning echoes determine gray-scale imaging and the display of the ultrasound image.
Attenuation ∙ Absorption
Absorption refers to the process by which a medium takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing medium. The strength of the remaining propagating sound wave is decreased.
Attenuation ∙ Scatter
Scatter occurs when ultrasound waves encounter a medium with a nonhomogeneous surface. A small portion of the sound wave is scattered in random directions while most of the original wave continues to travel in its original path.
Refraction
Refraction is the change in direction of propagation of a sound wave. This occurs as a result of a difference in propagation speeds between two mediums at a tissue boundary or interface (i.e. Soft tissue to fluid).
Diffraction
Diffraction is the spreading of a wave, around the edge of an object.
Ultrasound Physics: Basics of Sound - Acoustic Variables
Acoustic variables specifically identify sound waves. When an acoustic variable changes rhythmically over time, this indicates that a sound wave is present.
Acoustic Variables include the following:
• Pressure
• Density
• Distance
• Temperature
Ultrasound Physics: Basics of Sound - Acoustic Parameters
Sound is described by distinct parameters which are common to ALL wave types. These parameters describe how sound waves are comprised and how they interact with surrounding tissue interfaces:
• Frequency
• Period
• Amplitude
• Intensity
• Power
• Wavelength
• Propagation Speed
Ultrasound Physics: Basics of Sound - Echogenicity
Gray-scale imaging is produced by the returning echoes received by the ultrasound transducer. The brightness of the returning echo corresponds to its strength and anatomic location of the tissue interface encountered by the ultrasound pulse. The description of the echo strength and how it appears on the ultrasound image is referred to as its echogenicity. Echogenicity descriptions include:
Echogenicity - Anechoic
Anechoic structures show up as black on 2D ultrasound. Tissues exhibiting anechoic characteristics on ultrasound are comprised of fluid. These can include blood vessels, heart chambers containing blood, fluid such as amniotic sac, ascites or pleural/pericardial effusions and other fluid collections (i.e. cysts).
Echogenicity - Hyperechoic
Hyperechoic structures show up on ultrasound as brighter areas of white and gray when compared to surrounding structures. Tissues exhibiting hyperechoic characteristics on ultrasound are typically comprised of dense tissue collections or calcium (i.e. fetal bone, renal stones, gallstones, valvular plaque, and stenosis).
Echogenicity - Hypoechoic
Hypoechoic structures show up on ultrasound as darker areas of gray when compared to surrounding structures. Tissues exhibiting hyperechoic characteristics on ultrasound are typically comprised of less dense soft tissue and organs or masses with a fluid component (i.e. Liver, Kidneys).
Echogenicity - Isoechoic
Isoechoic structures show up on ultrasound as the same or similar appearance to those of the surrounding tissue or structures.
Ultrasound Physics: Basics of Sound - Acoustic Artifacts
The ultrasound machine makes assumptions about the path of the ultrasound beam and about the return echoes. If there is any deviance from those assumptions, an acoustic artifact is created. Acoustic artifacts can affect how the returning echo appears on the ultrasound image. Acoustic Artifacts include missing information or improper brightness, size, shape and/or position on the ultrasound image.
Acoustic Artifacts include the following:
Attenuation Artifacts
Diagnostic ultrasound utilizes an ultrasound unit/machine to send sound wave pulses into tissues and to obtain information based on the return echoes. Gray-scale imaging is produced by the return echoes. The brightness of the returning echo corresponds to its strength and anatomic location of the tissue interface encountered by the ultrasound pulse.
Ultrasound is sound with frequencies above the level of human hearing. NOTE: Frequencies are expressed in Hertz(Hz)
• Infrasound > 20 Hz
• Audible Sound = 20 Hz to 20,000 Hz
• Ultrasound > 20,000 Hz
Ultrasound Units to Know – Units Matter!
• M “Mega” = Million
• k “kilo” = Thousand
• c “centi” = HundredTH
• m “milli” = ThousandTH
• µ “micro” = MillionTH
NOTE: When calculating equations be sure your units complement each other.
• Meters (m) and seconds (sec)
• Millimeters (mm) and microseconds (µsec)
Ultrasound Physics: Basics of Sound - Transducers
Ultrasound Waves are produced by Piezoelectric transducers, which use polarized polycrystalline ferroelectric ceramic materials to convert pressure energy into mechanical energy and vice versa.
• The conversion of electrical energy into mechanical energy – transmission of the sound beam
• The conversion of mechanical energy into electrical energy – receiving the reflected beam information
• The piezoelectric effect was discovered by the Curie brothers in the 1800’s
• Natural piezoelectric crystals exist (i.e. quartz and tourmaline) but are typically too weak for diagnostic imaging
• Typically Lead Zirconate Titanate (PZT) is utilized for transducer crystal elements
• The Curie point (Tc) of PZT is more than 200°C
• Cannot be heated beyond Curie point or will lose its polarization (no autoclave)
Ultrasound Physics: Basics of Sound - Sound Waves
Sound is a wave. It is created by vibrations of a moving object. It is made up of high and low areas within the wave called compressions and rarefactions. A few things to know about sound waves:
• Sound Waves propagate (travel) and consist of many wave parameters and variables.
• Sound waves carry energy and information from one place to another, but they do not carry matter.
• Sound waves require a medium to travel through. Sound CANNOT travel through a vacuum or in outer space.
• Sound waves have natural occurring, measurable acoustic variables which affect and disturb the surrounding tissues.
• Sound waves travel in a straight line (in general)
• Sound waves are Longitudinal and Mechanical Waves
Sound Waves – Longitudinal and Mechanical Waves
Longitudinal Waves are waves that oscillate in the same path that the sound wave is moving. Particles move in the same direction as the wave. Longitudinal Waves are produced by the rapid back-and-forth vibration of an object and produce areas of compression and rarefaction throughout the wave.
Mechanical Waves are pressure waves that produce a regular mechanical vibration and travel through matter as a waveform.
• Compression - increase in pressure and/or density
• Rarefaction - decrease in pressure and/or density
Sound Waves - Propagation
As sound propagates (travels), the sound waves react to tissue interfaces in a number of ways. Those behaviors include the following:
• Through Transmission
• Attenuation
- Reflection
- Absorption
- Scatter
• Diffraction
Through Transmission
Through transmission refers to the continued propagation of sound relatively unchanged from one medium to another through a tissue boundary. The path of the sound beam continues forward in a straight line without change in speed or direction.
Attenuation
Acoustic Attenuation is the gradual loss of intensity and reduction of the force of sound as it propagates through a medium. Attenuation occurs through Reflection, Absorption and Scatter as the sound wave travels through a medium and encounters tissue boundaries and interfaces.
Attenuation ∙ Reflection
Reflection of sound is the return of echoes from a tissue boundary or interface. Reflection can occur in whole or in part. The strength of the returning echoes determine gray-scale imaging and the display of the ultrasound image.
Attenuation ∙ Absorption
Absorption refers to the process by which a medium takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing medium. The strength of the remaining propagating sound wave is decreased.
Attenuation ∙ Scatter
Scatter occurs when ultrasound waves encounter a medium with a nonhomogeneous surface. A small portion of the sound wave is scattered in random directions while most of the original wave continues to travel in its original path.
Refraction
Refraction is the change in direction of propagation of a sound wave. This occurs as a result of a difference in propagation speeds between two mediums at a tissue boundary or interface (i.e. Soft tissue to fluid).
Diffraction
Diffraction is the spreading of a wave, around the edge of an object.
Ultrasound Physics: Basics of Sound - Acoustic Variables
Acoustic variables specifically identify sound waves. When an acoustic variable changes rhythmically over time, this indicates that a sound wave is present.
Acoustic Variables include the following:
• Pressure
• Density
• Distance
• Temperature
Ultrasound Physics: Basics of Sound - Acoustic Parameters
Sound is described by distinct parameters which are common to ALL wave types. These parameters describe how sound waves are comprised and how they interact with surrounding tissue interfaces:
• Frequency
• Period
• Amplitude
• Intensity
• Power
• Wavelength
• Propagation Speed
Ultrasound Physics: Basics of Sound - Echogenicity
Gray-scale imaging is produced by the returning echoes received by the ultrasound transducer. The brightness of the returning echo corresponds to its strength and anatomic location of the tissue interface encountered by the ultrasound pulse. The description of the echo strength and how it appears on the ultrasound image is referred to as its echogenicity. Echogenicity descriptions include:
- Anechoic
- Hyperechoic
- Hypoechoic
- Isoechoic
Echogenicity - Anechoic
Anechoic structures show up as black on 2D ultrasound. Tissues exhibiting anechoic characteristics on ultrasound are comprised of fluid. These can include blood vessels, heart chambers containing blood, fluid such as amniotic sac, ascites or pleural/pericardial effusions and other fluid collections (i.e. cysts).
Echogenicity - Hyperechoic
Hyperechoic structures show up on ultrasound as brighter areas of white and gray when compared to surrounding structures. Tissues exhibiting hyperechoic characteristics on ultrasound are typically comprised of dense tissue collections or calcium (i.e. fetal bone, renal stones, gallstones, valvular plaque, and stenosis).
Echogenicity - Hypoechoic
Hypoechoic structures show up on ultrasound as darker areas of gray when compared to surrounding structures. Tissues exhibiting hyperechoic characteristics on ultrasound are typically comprised of less dense soft tissue and organs or masses with a fluid component (i.e. Liver, Kidneys).
Echogenicity - Isoechoic
Isoechoic structures show up on ultrasound as the same or similar appearance to those of the surrounding tissue or structures.
Ultrasound Physics: Basics of Sound - Acoustic Artifacts
The ultrasound machine makes assumptions about the path of the ultrasound beam and about the return echoes. If there is any deviance from those assumptions, an acoustic artifact is created. Acoustic artifacts can affect how the returning echo appears on the ultrasound image. Acoustic Artifacts include missing information or improper brightness, size, shape and/or position on the ultrasound image.
Acoustic Artifacts include the following:
Attenuation Artifacts
- Shadowing
- Enhancement
- Edge Shadow
- Propagation Speed Error
- Reverberation
- Comet Tail
- Ring Down
- Mirror Image
- Aliasing
- Doppler Ghosting
- Range Ambiguity
- Multi-Path
- Side Lobe
MODULE 2 - ACOUSTIC VARIABLES & PARAMETERS
ACOUSTIC VARIABLES & PARAMETERS - VIDEO TUTORIAL
ACOUSTIC VARIABLES & PARAMETERS - COURSE OUTLINE
Acoustic Variables
Acoustic variables specifically identify sound waves. When an acoustic variable changes rhythmically over time, this indicates that a sound wave is present.
Acoustic Variables include the following:
Acoustic Variables - Pressure
Pressure is described as the concentration of force in an area. It is expressed in units called pascals (Pa). As sound waves propagate through a medium the pressure in the wave will increase and decrease (compression and rarefaction) and can affect how the wave propagates and the surrounding tissues, as well as how the return echoes are displayed.
Acoustic Variables - Density
Density is described as the concentration of mass in a volume or concentration of matter. It is expressed in units of kg/cm3. As sound travels from through tissues with varying density the echoes received back to the receiver will display different image characteristics based on the strength of the reflector and this will affect the 2D ultrasound image displayed. NOTE: Density IS NOT the same as Stiffness
Acoustic Variables – Distance
Distance is described as a measure of particle motion. It is expressed in units of length such as centimeters, meters, inches, feet and miles.
As a sound wave propagates through a medium, the surrounding particles are set into motion. NOTE: Sound waves cause particle motion but DO NOT carry particles from one place to another. Sound waves only carry energy not particles or any other material.
Acoustic Variables - Temperature
Temperature is described as the measure of heat. It is expressed in units of degrees in Fahrenheit or Celsius. Temperature can be affected by the propagation of the sound wave. This occurs especially with increased beam power and intensity, the surrounding tissues can increase in temperature and cause tissue cavitation (microbubbles) to occur.
Acoustic Variables – Diagnostic Ultrasound
Sound waves are safe. There are no known biological effects of diagnostic sound waves. When used at therapeutic levels or with higher output power and beam intensity, the ultrasound beam can cause changes to the tissue due to these acoustic variables. Output power must be set to the ALARA standards which means “As Low As Reasonably Achievable”. This standard ensures that the acoustic variables remain within safe parameters reducing the likelihood of bio-effects from the sound beam and tissue cavitation.
Acoustic Parameters
Sound is described by distinct parameters which are common to ALL wave types. These parameters describe how sound waves are comprised and how they interact with surrounding tissue interfaces:
Acoustic Parameters - Frequency
Frequency, in diagnostic ultrasound is described as the number of cycles in the wave or the number of cycles of an acoustic variable that occur in one second.
Ultrasound is sound with frequencies above the level of human hearing.
Note: Frequencies are expressed in Hertz(Hz)
Frequency is inversely related to penetration/depth. Frequency is directly related to axial resolution (The ability to recognize two different objects at different depths along the axis of the ultrasound beam). This means that higher frequency improves image quality. Frequency is equal to the number of cycles in the pulse.
Equation:
F = #cycles/seconds
Acoustic Parameters - Period
Period is described as the time required to complete a single cycle or the time from the start of a cycle to the start of the next cycle.
Acoustic Parameters – Period vs Frequency
Period and Frequency are reciprocals of each other or inversely related.
Frequency abbreviated as f or F
Period abbreviated as p or T
Equations:
Acoustic Parameters - Amplitude
Amplitude is described as difference between the average value and the maximum value of an acoustic variable or the variation of an acoustic variable as it relates to the strength of the ultrasound beam.
Amplitude decreases as sound propagates through the body (attenuation).
Amplitude is expressed in units as they relate to the Acoustic Variables:
Acoustic Parameters - Power
Power is described as the rate at which work is performed, or the rate of energy transfer.
Acoustic Parameters - Intensity
Intensity is described as the concentration of energy in a sound beam.
Intensity depends upon both the power and the cross sectional area of the beam. Intensity is equal to the beam’s power divided by the beam's cross sectional area.
Equation:
Intensity (Watts/cm2) = Power (Watts) / Beam Area (cm2)
Acoustic Parameters – Amplitude, Power and Intensity
Acoustic Parameters – Wavelength
Wavelength is described as the length or distance of a single cycle.
Equation:
Abbreviated as "λ"
λ = c/F Wavelength (mm) = Propagation Speed mm/µsec
Frequency (MHz)
Wavelength and Frequency are reciprocals of each other and are inversely related.
Acoustic Parameters – Propagation Speed
Propagation Speed is described as the rate that sound travels through a medium.
The average Propagation Speed of all sound in soft tissue is:
Propagation Speed increases or decreases depending on the density and stiffness of the medium it travels through.
Propagation Speed Typical Values:
Propagation Speed of sound, regardless of the frequency, travels at the same speed through any specific medium. This means that sound with a frequency of 5MHz and sound with a frequency of 10MHz travel at the same propagation speed if they are traveling through the same medium.
Equation:
Abbreviated as “c”
c= F x λ Speed (mm/µs) = Frequency (Hz) x Wavelength (mm)
F= c/λ Frequency(Hz) = Propagation Speed (mm/µs) /Wavelength(mm)
NOTE: When calculating equations be sure your units complement each other.
Acoustic variables specifically identify sound waves. When an acoustic variable changes rhythmically over time, this indicates that a sound wave is present.
Acoustic Variables include the following:
- Pressure
- Density
- Distance
- Temperature
Acoustic Variables - Pressure
Pressure is described as the concentration of force in an area. It is expressed in units called pascals (Pa). As sound waves propagate through a medium the pressure in the wave will increase and decrease (compression and rarefaction) and can affect how the wave propagates and the surrounding tissues, as well as how the return echoes are displayed.
Acoustic Variables - Density
Density is described as the concentration of mass in a volume or concentration of matter. It is expressed in units of kg/cm3. As sound travels from through tissues with varying density the echoes received back to the receiver will display different image characteristics based on the strength of the reflector and this will affect the 2D ultrasound image displayed. NOTE: Density IS NOT the same as Stiffness
Acoustic Variables – Distance
Distance is described as a measure of particle motion. It is expressed in units of length such as centimeters, meters, inches, feet and miles.
As a sound wave propagates through a medium, the surrounding particles are set into motion. NOTE: Sound waves cause particle motion but DO NOT carry particles from one place to another. Sound waves only carry energy not particles or any other material.
Acoustic Variables - Temperature
Temperature is described as the measure of heat. It is expressed in units of degrees in Fahrenheit or Celsius. Temperature can be affected by the propagation of the sound wave. This occurs especially with increased beam power and intensity, the surrounding tissues can increase in temperature and cause tissue cavitation (microbubbles) to occur.
Acoustic Variables – Diagnostic Ultrasound
Sound waves are safe. There are no known biological effects of diagnostic sound waves. When used at therapeutic levels or with higher output power and beam intensity, the ultrasound beam can cause changes to the tissue due to these acoustic variables. Output power must be set to the ALARA standards which means “As Low As Reasonably Achievable”. This standard ensures that the acoustic variables remain within safe parameters reducing the likelihood of bio-effects from the sound beam and tissue cavitation.
Acoustic Parameters
Sound is described by distinct parameters which are common to ALL wave types. These parameters describe how sound waves are comprised and how they interact with surrounding tissue interfaces:
- Frequency
- Period
- Amplitude
- Intensity
- Power
- Wavelength
- Propagation Speed
Acoustic Parameters - Frequency
Frequency, in diagnostic ultrasound is described as the number of cycles in the wave or the number of cycles of an acoustic variable that occur in one second.
- Determined by the SOUND SOURCE
- CANNOT be adjusted by the sonographer
- Expressed in units of cycles: per second, Hertz (Hz)
Ultrasound is sound with frequencies above the level of human hearing.
Note: Frequencies are expressed in Hertz(Hz)
- Infrasound > 20 Hz
- Audible Sound = 20 Hz to 20,000 Hz
- Ultrasound > 20,000 Hz
Frequency is inversely related to penetration/depth. Frequency is directly related to axial resolution (The ability to recognize two different objects at different depths along the axis of the ultrasound beam). This means that higher frequency improves image quality. Frequency is equal to the number of cycles in the pulse.
Equation:
F = #cycles/seconds
Acoustic Parameters - Period
Period is described as the time required to complete a single cycle or the time from the start of a cycle to the start of the next cycle.
- Determined by the SOUND SOURCE
- CANNOT be adjusted by the sonographer
- Expressed in units of time: microseconds (µsec), seconds (sec), hours (hrs)
Acoustic Parameters – Period vs Frequency
Period and Frequency are reciprocals of each other or inversely related.
Frequency abbreviated as f or F
Period abbreviated as p or T
Equations:
- F x T = 1 Frequency (Hz) x Period (sec) = 1
- T = 1/ F Period (sec) = 1 / Frequency (Hz)
- F= 1/ P Frequency (Hz) = 1 / Period (sec)
Acoustic Parameters - Amplitude
Amplitude is described as difference between the average value and the maximum value of an acoustic variable or the variation of an acoustic variable as it relates to the strength of the ultrasound beam.
- Determined by the SOUND SOURCE
- CAN be adjusted by the sonographer
- Expressed in units as they relate to the Acoustic Variables.
Amplitude decreases as sound propagates through the body (attenuation).
Amplitude is expressed in units as they relate to the Acoustic Variables:
- Pressure: Pascals (Pa)
- Density: grams/cm3
- Particle motion: cm, inches, units of distance
- Decibels: dB
Acoustic Parameters - Power
Power is described as the rate at which work is performed, or the rate of energy transfer.
- Determined by the SOUND SOURCE
- CAN be adjusted by the sonographer
- Expressed in units of Watts
Acoustic Parameters - Intensity
Intensity is described as the concentration of energy in a sound beam.
- Determined by the SOUND SOURCE
- CAN be adjusted by the sonographer
- Expressed in units of Watts/cm2
Intensity depends upon both the power and the cross sectional area of the beam. Intensity is equal to the beam’s power divided by the beam's cross sectional area.
Equation:
Intensity (Watts/cm2) = Power (Watts) / Beam Area (cm2)
Acoustic Parameters – Amplitude, Power and Intensity
- Amplitude, Power and Intensity are directly related to each other.
- All three of these parameters decrease in strength as sound propagates through the body (attenuation).
Acoustic Parameters – Wavelength
Wavelength is described as the length or distance of a single cycle.
- Determined by BOTH the MEDIUM and SOUND SOURCE
- CANNOT be adjusted by the sonographer
- Expressed in units of distance or length:
- meters (m), centimeters(cm), millimeters (mm)
- Wavelength abbreviated as λ
- Wavelength is usually equal to half the thickness of the PZT crystal
- Typical Values: 0.1 – 0.8 mm
Equation:
Abbreviated as "λ"
λ = c/F Wavelength (mm) = Propagation Speed mm/µsec
Frequency (MHz)
Wavelength and Frequency are reciprocals of each other and are inversely related.
- As frequency is increased, wavelength is shorter.
- As frequency is decreased, wavelength is longer.
Acoustic Parameters – Propagation Speed
Propagation Speed is described as the rate that sound travels through a medium.
- Determined by the MEDIUM
- CANNOT be adjusted by the sonographer
- Expressed in units of distance over time: m/sec, mm/µsec
The average Propagation Speed of all sound in soft tissue is:
- 1540 m/s
- 1.54 mm/µsec
Propagation Speed increases or decreases depending on the density and stiffness of the medium it travels through.
- Density is related to the weight of the medium and as this increases, propagation speed decreases.
- Increased Density = Decreased Speed
- Stiffness is related to the compressibility of the medium, as this increases, propagation speed also increases. (i.e. Bone is very stiff and has a high propagation speed).
- Increased Stiffness = Increased Speed
Propagation Speed Typical Values:
- Air: 330 m/s
- Lung: 300 m/s - 1200 m/s
- Fat: 1450 m/s
- Fluid: 1480 m/s
- Soft Tissue: 1540 m/s
- Bone: 2000 – 4000 m/s
Propagation Speed of sound, regardless of the frequency, travels at the same speed through any specific medium. This means that sound with a frequency of 5MHz and sound with a frequency of 10MHz travel at the same propagation speed if they are traveling through the same medium.
Equation:
Abbreviated as “c”
c= F x λ Speed (mm/µs) = Frequency (Hz) x Wavelength (mm)
F= c/λ Frequency(Hz) = Propagation Speed (mm/µs) /Wavelength(mm)
NOTE: When calculating equations be sure your units complement each other.
- Meters (m) and seconds (sec)
- Millimeters (mm) and microseconds (µsec)
MODULE 3 - ACOUSTIC ARTIFACTS
ACOUSTIC ARTIFACTS - VIDEO TUTORIAL
ACOUSTIC ARTIFACTS - COURSE OUTLINE
Ultrasound Physics: Acoustic Artifacts
The ultrasound machine makes assumptions about the path of the ultrasound beam and about the return echoes. If there is any deviance from those assumptions, an acoustic artifact is created. Acoustic artifacts can affect how the returning echo appears on the ultrasound image. Acoustic Artifacts include missing information or improper brightness, size, shape and/or position on the ultrasound image.
Machine Assumptions:
Acoustic Artifacts include the following:
Attenuation Artifacts
Acoustic Artifacts – Shadowing
Shadowing occurs when a structure has a high level of attenuation. Structures deep to it will not be seen because there will not be a sufficient amount of returning echoes due to the highly attenuating structure.
Acoustic Artifacts – Enhancement
Enhancement is the opposite of shadowing. It occurs when a structure has a lower than normal level of attenuation. Structures deep to it will appear hyperechoic because there will be a greater than normal amount of returning echoes due to the decreased attenuation of the structure.
Acoustic Artifacts – Edge Shadow
Edge shadowing occurs when imaging a round structure. There is a small amount of refraction at the edges. When this occurs, the sound beam does not reflect and is not recorded. A shadow is shown on the displayed ultrasound image at the edges of the round structure.
Acoustic Artifacts – Propagation Speed Error
An ultrasound machine assumes that the medium the sound wave is traveling through is soft tissue, with a propagation speed of 1540 m/s. If the sound beam travels through material that has a different propagation speed, there will be a time/distance error. Reflectors will be displayed at incorrect depths.
Acoustic Artifacts – Reverberation
Reverberation is usually created from sound bouncing between two strong reflectors. There will be multiple, equally spaced echoes in a line displaying deep to the reflectors that lie parallel to the sound beam.
Acoustic Artifacts - Ring Down vs. Comet Tail
Ring Down and Comet Tail artifacts are types of reverberation artifacts and are often misunderstood and thought to be the same. The primary difference between the two artifacts is the display of discretely separate echoes.
Acoustic Artifacts – Comet Tail
Comet tail artifacts involve two closely spaced reflective interfaces, which generate closely spaced separate and discrete echoes. Comet tail artifacts also show a decrease in the amplitude of the echoes due to attenuation, which results in the width of echoes being increasingly diminished. The result is an artifact caused by the principle of reverberation, but with a conical or triangular shape. This artifact occurs primarily when the ultrasound beam comes into contact with metal objects and small calcific/crystalline highly reflective objects.
Acoustic Artifacts – Ring Down
Ring Down artifacts are similar but they are fundamentally different from Comet Tail artifacts. Ring Down artifacts do not display discretely separate echoes due to the continuous emission of sound. Ring Down artifacts are primarily seen with gas or air interfaces (rather than metal or calcifications, as in the case of comet tail artifacts). This produces the artifact and is shown as a long line or series of bands extending parallel below the level of the gas. Air is the primary cause of this artifact, and therefore this is seen as "dirty shadows" in the ultrasound image. Ring Down artifacts can be helpful in diagnosis of emphysematous cholecystitis or pyelonephritis. Ring Down artifacts can also be used to differentiate basic fluid collections from abscesses.
Acoustic Artifacts – Mirror Image
Mirror Image artifacts occur when there is a strong reflector in the path of the beam. A great example is the diaphragm. Sound is reflected at an oblique angle and then returns to the machine.
Acoustic Artifacts – Aliasing
Aliasing is created when the Pulse Repetition Frequency (PRF) is not high enough and the Nyquist limit is exceeded. The color and/or Doppler will “wrap” around and create an aliased effect on the waveform or color image.
Doppler Equation:
Nyquist Limit = 1/2PRF or PRF/2
Acoustic Artifacts – Doppler Ghosting
Doppler systems convert frequency shifts into a spectrum of colors. Frequency shifts arise from movement. A Doppler ultrasound unit measures the large frequency shifts which are the movement of red blood cells. In addition to the frequency shift from the moving red blood cells, blood vessel walls also pulsate. This wall pulsation causes its own Doppler shift, in addition to the Doppler shift created by the movement of the red blood cells. The machine creates a color map that “bleeds” into the surrounding tissue or in pulsed Doppler can create a “wall thump” at the baseline. This is known as Doppler ghosting.
Acoustic Artifacts – Range Ambiguity
Range Ambiguity is created when an echo reflection from a deep reflector arrives after another pulse is already created. When the Pulse Repetition Frequency (PRF) is too high the machine assumes that the echo came from the second pulse and displays the echo too shallow and in the wrong location on the image.
Acoustic Artifacts – Multi Path
Multi-path artifact is due to refraction at a strong reflector and occurs when the angle of incidence is acute. This results in a reflector positioned incorrectly on the display because the path that the echo returned on is longer that the path the initial beam took to get to the reflector. This causes a poor image, rather than a specific misplaced echo.
Acoustic Artifacts – Side Lobes
Side lobe artifacts are caused by low energy “side lobes” of the main ultrasound beam. When an echo from a side lobe beam becomes strong enough and returns to the receiver, it is “assigned” to the main beam and displayed at a false location. Side-lobe artifacts are usually seen in hypoechoic or echo-free structures and appear as bright and rounded lines.
The ultrasound machine makes assumptions about the path of the ultrasound beam and about the return echoes. If there is any deviance from those assumptions, an acoustic artifact is created. Acoustic artifacts can affect how the returning echo appears on the ultrasound image. Acoustic Artifacts include missing information or improper brightness, size, shape and/or position on the ultrasound image.
Machine Assumptions:
- Sound traveling at 1540m/s
- Sound traveling in a straight line
- Sound attenuation at 0.5dB/cm/MHz
Acoustic Artifacts include the following:
Attenuation Artifacts
- Shadowing
- Enhancement
- Edge Shadow
- Propagation Speed Error
- Reverberation
- Comet Tail
- Ring Down
- Mirror Image
- Aliasing
- Doppler Ghosting
- Range Ambiguity
- Multi-Path
- Side Lobe
Acoustic Artifacts – Shadowing
Shadowing occurs when a structure has a high level of attenuation. Structures deep to it will not be seen because there will not be a sufficient amount of returning echoes due to the highly attenuating structure.
Acoustic Artifacts – Enhancement
Enhancement is the opposite of shadowing. It occurs when a structure has a lower than normal level of attenuation. Structures deep to it will appear hyperechoic because there will be a greater than normal amount of returning echoes due to the decreased attenuation of the structure.
Acoustic Artifacts – Edge Shadow
Edge shadowing occurs when imaging a round structure. There is a small amount of refraction at the edges. When this occurs, the sound beam does not reflect and is not recorded. A shadow is shown on the displayed ultrasound image at the edges of the round structure.
Acoustic Artifacts – Propagation Speed Error
An ultrasound machine assumes that the medium the sound wave is traveling through is soft tissue, with a propagation speed of 1540 m/s. If the sound beam travels through material that has a different propagation speed, there will be a time/distance error. Reflectors will be displayed at incorrect depths.
- If the Propagation speed is greater than 1540 m/s then the resulting echo will be placed more superficial than the real reflector.
- If the Propagation speed is less than 1540 m/s then the resulting echo will be placed deeper than the real reflector.
Acoustic Artifacts – Reverberation
Reverberation is usually created from sound bouncing between two strong reflectors. There will be multiple, equally spaced echoes in a line displaying deep to the reflectors that lie parallel to the sound beam.
Acoustic Artifacts - Ring Down vs. Comet Tail
Ring Down and Comet Tail artifacts are types of reverberation artifacts and are often misunderstood and thought to be the same. The primary difference between the two artifacts is the display of discretely separate echoes.
Acoustic Artifacts – Comet Tail
Comet tail artifacts involve two closely spaced reflective interfaces, which generate closely spaced separate and discrete echoes. Comet tail artifacts also show a decrease in the amplitude of the echoes due to attenuation, which results in the width of echoes being increasingly diminished. The result is an artifact caused by the principle of reverberation, but with a conical or triangular shape. This artifact occurs primarily when the ultrasound beam comes into contact with metal objects and small calcific/crystalline highly reflective objects.
Acoustic Artifacts – Ring Down
Ring Down artifacts are similar but they are fundamentally different from Comet Tail artifacts. Ring Down artifacts do not display discretely separate echoes due to the continuous emission of sound. Ring Down artifacts are primarily seen with gas or air interfaces (rather than metal or calcifications, as in the case of comet tail artifacts). This produces the artifact and is shown as a long line or series of bands extending parallel below the level of the gas. Air is the primary cause of this artifact, and therefore this is seen as "dirty shadows" in the ultrasound image. Ring Down artifacts can be helpful in diagnosis of emphysematous cholecystitis or pyelonephritis. Ring Down artifacts can also be used to differentiate basic fluid collections from abscesses.
Acoustic Artifacts – Mirror Image
Mirror Image artifacts occur when there is a strong reflector in the path of the beam. A great example is the diaphragm. Sound is reflected at an oblique angle and then returns to the machine.
Acoustic Artifacts – Aliasing
Aliasing is created when the Pulse Repetition Frequency (PRF) is not high enough and the Nyquist limit is exceeded. The color and/or Doppler will “wrap” around and create an aliased effect on the waveform or color image.
Doppler Equation:
Nyquist Limit = 1/2PRF or PRF/2
Acoustic Artifacts – Doppler Ghosting
Doppler systems convert frequency shifts into a spectrum of colors. Frequency shifts arise from movement. A Doppler ultrasound unit measures the large frequency shifts which are the movement of red blood cells. In addition to the frequency shift from the moving red blood cells, blood vessel walls also pulsate. This wall pulsation causes its own Doppler shift, in addition to the Doppler shift created by the movement of the red blood cells. The machine creates a color map that “bleeds” into the surrounding tissue or in pulsed Doppler can create a “wall thump” at the baseline. This is known as Doppler ghosting.
Acoustic Artifacts – Range Ambiguity
Range Ambiguity is created when an echo reflection from a deep reflector arrives after another pulse is already created. When the Pulse Repetition Frequency (PRF) is too high the machine assumes that the echo came from the second pulse and displays the echo too shallow and in the wrong location on the image.
Acoustic Artifacts – Multi Path
Multi-path artifact is due to refraction at a strong reflector and occurs when the angle of incidence is acute. This results in a reflector positioned incorrectly on the display because the path that the echo returned on is longer that the path the initial beam took to get to the reflector. This causes a poor image, rather than a specific misplaced echo.
Acoustic Artifacts – Side Lobes
Side lobe artifacts are caused by low energy “side lobes” of the main ultrasound beam. When an echo from a side lobe beam becomes strong enough and returns to the receiver, it is “assigned” to the main beam and displayed at a false location. Side-lobe artifacts are usually seen in hypoechoic or echo-free structures and appear as bright and rounded lines.
MODULE 4 - ULTRASOUND SYSTEMS
MODULE 5 - TRANSDUCERS
MODULE 6 - BEAM DYNAMICS
MODULE 7 - DECIBELS & ATTENUATION
MODULE 8 - DOPPLER & HEMODYNAMICS
MODULE 9 - QUALITY ASSURANCE
MODULE 10 - EQUATIONS
iHeartEcho is a Division of All About Ultrasound, Inc.
