Good One UT -Physics

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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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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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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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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Reflections

Impedances equal no reflection

Impedances similar little reflected

Impedances very different virtually all reflected


Fraction Reflected = ------------------------ = ----------



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Why Use Gel?

Acoustic Impedance of air & soft tissue verydifferent

Without gel virtually no sound penetrates skin


IRC = ------------------------ = ----------


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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Electronic Scanning

Transducer ArraysMultiple small transducers

Activated in groups


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Electrical Scanning

Performed with transducerarraysmultiple elementsinside transducer

assembly arranged in either a line (linear array)

concentric circles (annular array)


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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 &center


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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

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