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9/30/13
1
Basic Ultrasound Physics
Speaker has no disclosures to make
Kirk Spencer MD
The Physics of Echo • Sound
– Audible range 20Khz – Medical ultrasound Megahertz range – Advantages of imaging with ultrasound
• Directed as a beam • Tomographic • Reflected from small objects • Non-ionizing
– Disadvantages • Propagates poorly through air • Penetration poor (attenuation)
The Physics of Echo λ = wavelength = v/f v= velocity f = frequency cycle
• Velocity of sound α density and temperature 1,540 m/sec soft tissue
• Frequency 3.5 MHz (1-7 MHz) • λ = (1,540 m/sec)/(3.5 MHz) = 0.44 mm
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Scattering – echoes originating from
relatively small, weakly reflective, irregularly shaped objects
– object size > λ/4 – not angle dependant – inefficient
The Physics of Echo
Reflection – echoes originating from
relatively large, regularly shaped objects with smooth surfaces
– objects large α wavelength – angle dependant – valve, endocardium,
pericardium
The Physics of Echo
Is there pericardial calcification? The Physics of Echo
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Resolution: • Lateral resolution - the ability to
resolve objects side by side • Structures must be separated by
more than the width of the ultrasound beam to be distinguished as separate – Transducer size (larger
better) – Frequency (higher better) – Focusing – Gain (lower better)
The Physics of Echo
Beam width
The Physics of Echo
Low gain
Medium gain
High gain
Resolution: • Axial Resolution - Axial resolution
is the ability to resolve objects that lie along the path of the ultrasound beam
• Related to frequency of transducer and pulse duration
• In practical terms, axial resolution is roughly twice the wavelength
The Physics of Echo
Frequency Wavelength 2.2 MHz 0.68 mm 3.5 MHz 0.43 mm 5.0 MHz 0.3 mm
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Axial resolution vs penetration • ↑ frequency leads to better
resolution • Penetration ∝ wavelength (1/
frequency) – More scattering (more reflection by
smaller scatterers) – More attenuation
The Physics of Echo
Reso
lutio
n
Pene
tratio
n
Attenuation – Loss of ultrasound energy as it passes
through tissue (scattering and absorption) half-power (cm)
• Water 380 • Blood 15 • Soft tissue 1-5 • Muscle 0.6-1 • Bone 0.2-0.6 • Air 0.08
The Physics of Echo
The Physics of Echo Resolution vs penetration • ↑ frequency leads to better resolution • Penetration ∝ wavelength (1/frequency)
Use the highest frequency probe that gives you an adequate image
2.5 Mhz - 3.5 MHz
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Permanently-polarized material such as quartz (SiO2) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect.
The Physics of Echo
• Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezoelectric effect
• This shape deformation creates ultrasound waveforms
The Physics of Echo
Scanning – Mechanical transducers
• Rotating multiple elements, or a single element and set of acoustic mirrors to generate the sweeping beam for 2D imaging
– Electronic / array transducers • Have the ability to be steered by
sequentially stimulating each element. This feature creates the sector scan by rapidly steering the beam from left to right to give the two dimensional cross sectional image.
The Physics of Echo
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Electronic / array transducers Linear array • Sequential firing or array elements
moves beam linearly • Require large acoustic window • Creates a linear or rectangular
shaped scan plane
The Physics of Echo
Electronic / array transducers Phased array • Phased control of array firing
controls beam direction (and thus scan line)
• Creates a sector or pie shaped scan plane.
The Physics of Echo
Imaging – Electrical stimulate piezoelectric
crystal which sends ultrasound pulse
– Transducer then “listens” for
returning ultrasound signals – Transducer “listens” 99 percent
of time, which increases sensitivity
The Physics of Echo
1-2 µsec
0.4 µsec
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Modes:
• A Mode - amplitude mode. Where the signals are displayed as spikes that are dependent on the amplitude of the returning sound energy.
• B Mode - brightness mode. Where the signals are displayed as various points whose brightness depends on the amplitude of the returning sound energy.
The Physics of Echo
Modes:
• M Mode - motion mode. The application of B-mode and a strip chart recorder allows visualization of the structures as a function of depth and time.
The Physics of Echo
Modes: • 2D Mode - 2 dimensional
mode. The spatially oriented B-mode where structures are seen as a function of depth and width. The beam is rapidly swept back and forth to create a cross section of the imaged structures.
The Physics of Echo
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The Physics of Echo
Shadowing The loss of information
behind an object because the sound energy was reflected back by the object such that no signal passes beyond it
Bone, metal valve, air
The Physics of Echo Artifacts - sidelobe Ultrasound reflections off
real objects, but from the ultrasound beam sidelobes, not the central beam
Occurs because ultrasound beam has width to it
Worse when gain is high
Is there a catheter in RA?
The Physics of Echo
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The Physics of Echo Artifacts - reverberations • Multipath artifacts • Sound bounces back
and forth between two interfaces. This prolongs the time of flight, producing an artifact deep to the interface.
Artifacts - reverberation • Results from ultrasound strikes a target composed of
several highly reflective interfaces • Appear as relatively parallel irregular bright lines
extending from the structure
Artifacts - reverberation • Appears as a linear
brightness in the direction of the sound beam and deep to a strong reflector
• Results from multiple back and forth reflections
• Appear as relatively parallel irregular bright lines extending from the structure
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The Physics of Echo
• Harmonics • Mechanical Index • Compression • Focus • Transducer frequency • Gain • Grayscale / power Doppler
• Depth • Triggering • Frame rate • PRF • Packet size • Post-processing
The Physics of Echo Overall gain • Increases the intensity of received echoes • Makes image brighter
The Physics of Echo
Depth • Use the least depth that fits the structure of
interest on the screen
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Nonlinear distortion of ultrasound
Frequency
Am
plitu
de
Frequency
Am
plitu
de Fundamental
Fundamental
Harmonics
The Physics of Echo
The Physics of Echo
Harmonic imaging Lateral resolution
– Smaller harmonic beam width
Clutter reduction – Sidelobe levels decrease
with increasing harmonic number
Near field artifact reduction
Am
plitu
de
Frequency MHz
Fund Harm
On-Axis Reflectors
Off-Axis Reflectors
The Physics of Echo Doppler Effect • Christian Johann Doppler 1842 • If a source of sound is stationary, the wavelength
and frequency of sound emanating from the source are constant
• If a source of sound is moving toward you , it’s wavelength is decreasing (frequency increasing)
• If a source of sound is moving away from you , it’s wavelength is increasing (frequency decreasing )
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The Physics of Echo
The Physics of Echo
Dependence on angle between scatter and incident
ultrasound beam
The Physics of Echo Doppler
echocardiography Continuous wave
– Separate transmit and receive transducer
– Continuously receiving – No maximal velocity
limit – Range is ambiguous
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The Physics of Echo Doppler echocardiography Continuous wave
The Physics of Echo Doppler echocardiography Pulsed wave • Range gated Doppler • Inability to detect high
frequency Doppler shifts • Inability to detect high
velocities
The Physics of Echo
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The Physics of Echo
Doppler echocardiography Color Doppler • Multiple pulsed Doppler
samples along each scan line
The Physics of Echo Doppler echocardiography Color Doppler • Velocities colored coded • Blue- away, Red - toward