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8/12/2019 Good One UT -Physics
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George DavidAssociate Professor
Ultrasound Physics
04:Scanner
97
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Pulse Mode Ultrasound
transducer driven by short
voltage pulsesshort sound pulses produced
Like plucking guitar string
Pulse repetition frequency sameas frequency of applied voltagepulsesdetermined by the instrument (scanner)
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Pulse Duration Review
typically 2-3 cycles per pulse
Transducer tends to continue
ringingminimized by dampening transducer element
Pulse Duration = Period X Cycles / Pulse
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Damping Material
Goal:reduce cycles / pulse
Method:dampen out vibrations after voltage pulse
Constructionmixture of powder & plastic or epoxy
attached to near face of piezoelectric
element (away from patient)DampingMaterial
Piezoelectric
Element
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Disadvantages ofDamping
reduces beam intensity
produces less pure frequency (tone)
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George DavidAssociate Professor
Bandwidth
Damping shortens pulsesthe shorter the pulse, the higher the range of
frequencies
Range of frequencies producedcalled bandwidth
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Bandwidth
range of frequencies present inan ultrasound pulse
Frequency
Intensity
Ideal
Frequency
Intensity
Actual
Bandwidth
OperatingFrequency
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operating frequency
Quality Factor = -----------------------------bandwidth
Quality Factor (Q)
Unitless
Quantitative Measureof Spectral Purity
Frequency
Intensity
Actual
Bandwidth
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Which has a Higher Quality Factor?
Frequency
Intensity
A
Frequency
Intensity
B
operating frequency
Quality Factor = -----------------------------bandwidth
Same Operating Frequency!
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George DavidAssociate Professor
Damping
More damping results in shorter pulses
more frequencies
higher bandwidth
lower quality factor
lower intensity
Rule of thumb for short pulses (2 - 3 cycles)
quality factor ~ number of cycles per pulse
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George DavidAssociate Professor
An Aside about Reflections
Echoes occur atinterfaces between
2 media of differentacousticimpedancesspeed of sound X density
Medium 1
Medium 2
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Intensity Reflection Coefficient (IRC)&
Intensity Transmission Coefficient(ITC)
IRCFraction of sound intensity
reflected at interface
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IRC Equation
Z1 is acoustic impedance of medium #1
Z2 is acoustic impedance of medium #2
2reflected intensity z2- z1
IRC = ------------------------ = ----------
incident intensity z2+ z1
For perpendicular incidence
Medium 1
Medium 2
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George DavidAssociate Professor
Reflections
Impedances equal no reflection
Impedances similar little reflected
Impedances very different virtually all reflected
2reflected intensity z2- z1
Fraction Reflected = ------------------------ = ----------
incident intensity z2+ z1
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Why Use Gel?
Acoustic Impedance of air & soft tissue verydifferent
Without gel virtually no sound penetrates skin
2reflected intensity z2- z1
IRC = ------------------------ = ----------
incident intensity z2+ z1
AcousticImpedance
(rayls)
Air 400
Soft Tissue 1,630,000
Fraction Reflected: 0.9995
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Transducer Matching Layer
Transducer element has differentacoustic impedance than skin
Matching layer reduces reflections at
surface of piezoelectric elementIncreases sound energy transmitted into body
Transducerskin interface
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Transducer Matching Layer
placed on face of transducer impedance between that of
transducer & tissue
reduces reflections at surface ofpiezoelectric elementCreates several small transitions in acoustic impedance
rather than one large one
reflected intensity z2- z1
IRC = ------------------------ = ----------
incident intensity z2+ z1( )
2 MatchingLayer
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Transducer Arrays
Virtually all commercialtransducers are arraysMultiple small elements in single housing
Allows sound beam to beelectronicallyFocused
Steered
Shaped
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George DavidAssociate Professor
Electronic Scanning
Transducer ArraysMultiple small transducers
Activated in groups
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George DavidAssociate Professor
Electrical Scanning
Performed with transducerarraysmultiple elementsinside transducer
assembly arranged in either a line (linear array)
concentric circles (annular array)
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George DavidAssociate Professor
Linear Array Scanning
Two techniques for activating groupsof linear transducersSwitched Arrays
activate all elements in group at same time
Phased Arrays Activate group elements at slightly different times
impose timing delays between activations of elements ingroup
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Linear Switched Arrays
Elements energized as
groupsgroup acts like one large
transducer
Groups moved up &down through elementssame effect as manually
translating
very fast scanning possible(several times per second)
results in real time image
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Linear Switched Arrays
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Linear Phased Array
Groups of elements
energizedsame as with switched arrays
voltage pulse applied to
all elements of a groupBUT
elements not all pulsed at
same time
1
2
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Linear Phased Array
timing variations allow
beam to beshaped
steered
focused
Above arrows indicatetiming variations.By activating bottomelement first & top last,beam directed upward
Beam steered upward
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Linear Phased Array
Above arrows indicatetiming variations.By activating topelement first & bottomlast, beam directeddownward
Beam steered downward
By changing timing variations between pulses,beam can be scanned from top to bottom
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Linear Phased Array
Above arrows indicatetiming variations.By activating top &bottom elementsearlier than centerones, beam is focused
Beam is focused
Focus
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Linear Phased Array
Focus
Focal point can be moved toward oraway from transducer by altering timing
variations between outer elements ¢er
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Linear Phased Array
Focus
Multiple focal zones accomplished bychanging timing variations between pulses
Multiple pulses requiredslows frame rate
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Listening Mode
Listening direction can besteered & focused similarly tobeam generationappropriate timing variations applied to
echoes received by various elements of agroup
Dynamic Focusinglistening focus depth can be changed
electronically between pulses by applyingtiming variations as above
2