ultrasound physics reflections & attenuation ‘97
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Ultrasound Physics
Reflections &
Attenuation
‘97
Perpendicular Incidence
Sound beam travels perpendicular to boundary between two media 90o
IncidentAngle
1
2Boundarybetweenmedia
Oblique Incidence
Sound beam travel not perpendicular to boundary
ObliqueIncidentAngle
(not equal to 90o)
1
2
Boundarybetweenmedia
Perpendicular Incidence
What happens to sound at boundary?reflected
sound returns toward source
transmitted sound continues in
same direction
1
2
Perpendicular Incidence
Fraction of intensity reflected depends on acoustic impedances of two media
1
2
Acoustic Impedance =Density X Speed of Sound
Intensity Reflection Coefficient (IRC)&Intensity Transmission Coefficient (ITC)
IRCFraction of sound intensity
reflected at interface<1
ITCFraction of sound intensity
transmitted through interface<1
Medium 1
Medium 2IRC + ITC = 1
IRC Equation
Z1 is acoustic impedance of medium #1
Z2 is acoustic impedance of medium #2
2 reflected intensity z2 - z1
IRC = ------------------------ = ----------
incident intensity z2 + z1
For perpendicular incidence
Medium 1
Medium 2
Reflections
Impedances equal no reflection
Impedances similar little reflected
Impedances very different virtually all reflected
2 reflected intensity z2 - z1
Fraction Reflected = ------------------------ = ----------
incident intensity z2 + z1
Why Use Gel?
Acoustic Impedance of air & soft tissue very different
Without gel virtually no sound penetrates skin
2 reflected intensity z2 - z1
IRC = ------------------------ = ----------
incident intensity z2 + z1
Acoustic Impedance
(rayls)
Air 400Soft Tissue 1,630,000
Fraction Reflected: 0.9995
Rayleigh Scattering
redirection of sound in many directionscaused by rough surface with respect to
wavelength of sound
Diffuse Scattering & Rough Surfaces
heterogeneous mediacellular tissueparticle suspension
blood, for example
ScatteringOccurs if
boundary not smoothRoughness related to frequency
frequency changes wavelength higher frequency shortens wavelength shorter wavelength “roughens” surface
Specular Reflections
Un-scattered soundoccurs with smooth
boundariessimilar to light reflection
from mirroropposite of scatter from
rough surfacewall is example of rough
surface
Backscatter
sound scattered back in the direction of source
Backscatter Comments
Caused byrough surfacesheterogeneous media
Depends on scatterer’ssizeroughnessshapeorientation
Depends on sound frequencyaffects wavelength
Backscatter Intensity
normally << than specular reflections
angle dependancespecular reflection very angle dependentbackscatter not angle dependent
echo reception not dependent on incident angle
increasing frequency effectively roughens surfacehigher frequency results in more backscatter
PZT is Most Common Piezoelectric Material
Lead Zirconate TitanateAdvantages
Efficient More electrical energy transferred to sound & vice-
versaHigh natural resonance frequencyRepeatable characteristics
Stable designDisadvantages
High acoustic impedance Can cause poor acoustic coupling Requires matching layer to compensate
Resonant FrequencyFrequency of Highest Sustained
IntensityTransducer’s “preferred” or resonantresonant
frequencyExamples
Guitar StringBell
Operating Frequency
Determined bypropagation speed of transducer
material typically 4-6 mm/sec
thickness of element
prop. speed of element (mm /sec)oper. freq. (MHz) = ------------------------------------------------ 2 X thickness (mm)
Pulse Mode Ultrasoundtransducer driven by short voltage
pulsesshort sound pulses producedLike plucking guitar string
Pulse repetition frequency same as frequency of applied voltage pulsesdetermined by the instrument (scanner)
Pulse Duration Review
typically 2-3 cycles per pulseTransducer tends to continue ringing
minimized by dampeningdampening transducer element
Pulse Duration = Period X Cycles / Pulse
Damping MaterialGoal:
reduce cycles / pulseMethod:
dampen out vibrations after voltage pulse
Constructionmixture of powder & plastic or
epoxyattached to near face of
piezoelectric element (away from patient)
DampingMaterial
PiezoelectricElement
Disadvantages of Damping
reduces beam intensityproduces less pure frequency (tone)
Bandwidth
Damping shortens pulsesthe shorter the pulse, the higher the range
of frequencies Range of frequencies produced called
bandwidthbandwidth
Bandwidthrange of frequencies present in an
ultrasound pulse
Frequency
Intensity
Ideal
Frequency
Intensity
Actual
Bandwidth
OperatingFrequency
operating frequencyQuality Factor = ----------------------------- bandwidth
Quality Factor (“Q”)
UnitlessQuantitative Measure of
“Spectral Purity”
Frequency
Intensity
Actual
Bandwidth
Damping
More damping results inshorter pulsesmore frequencieshigher bandwidth lower quality factor lower intensity
Rule of thumb for short pulses (2 - 3 cycles)
quality factor ~ number of cycles per pulse
Transducer Matching LayerTransducer element has different acoustic
impedance than skinMatching layer reduces reflections at surface
of piezoelectric elementIncreases sound energy transmitted into body
Transducer – skin interface
Transducer Matching Layerplaced on face of transducerimpedance between that of
transducer & tissuereduces reflections at surface of
piezoelectric elementCreates several small transitions in acoustic
impedance rather than one large one
reflected intensity z2 - z1
IRC = ------------------------ = ----------
incident intensity z2 + z1
( )2 Matching
Layer
Transducer ArraysVirtually all commercial transducers
are arraysMultiple small elements in single
housingAllows sound beam to be electronically
FocusedSteeredShaped
Electronic Scanning
Transducer ArraysMultiple small transducersActivated in groups
Electrical ScanningPerformed with transducer arraysarrays
multiple elements inside transducer assembly arranged in either a line (linear array)
concentric circles (annular array)
Curvilinear Array Linear Array
Linear Array Scanning
Two techniques for activating groups of linear transducers Switched ArraysSwitched Arrays
activate all elements in group at same time Phased ArraysPhased Arrays
Activate group elements at slightly different times impose timing delays between activations of elements in
group
Linear Switched ArraysElements energized as
groupsgroup acts like one large
transducerGroups moved up & down
through elementssame effect as manually
translatingvery fast scanning possible
(several times per second) results in real time image
Linear Switched Arrays
Linear Phased ArrayGroups of elements energized
same as with switched arrays
voltage pulse applied to all elements of a group
BUTelements not all pulsed at
same time
1
2
Linear Phased Arraytiming variations allow beam
to be shapedsteeredfocused
Above arrows indicate timing variations.By activating bottom element first & top last, beam directed upward
Beam steered upward
Linear Phased Array
Above arrows indicate timing variations.By activating top element first & bottom last, beam directed downward
Beam steered downward
By changing timing variations between pulses, beam can be scanned from top to bottom
Linear Phased Array
Above arrows indicate timing variations.By activating top & bottom elements earlier than center ones, beam is focused
Beam is focused
Focus
Linear Phased ArrayFocus
Focal point can be moved toward or away from transducer by altering timing variations between outer elements & center
Linear Phased ArrayFocus
Multiple focal zones accomplished by changing timing variations between pulses•Multiple pulses required•slows frame rate
Listening ModeListening direction can be steered
& focused similarly to beam generationappropriate timing variations
applied to echoes received by various elements of a group
Dynamic Focusinglistening focus depth can be
changed electronically between pulses by applying timing variations as above
2
1.5 Transducer~3 elements in elevation directionAll 3 elements can be combined for thick slice1 element can be selected for thin slice
Elevation
Direction
1.5 & 2D TransducersMultiple elements in 2 directionsCan be steered & focused anywhere in 3D
volume
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