George David Associate Professor Ultrasound Physics 04: Scanner ‘97

George DavidAssociate Professor

Ultrasound PhysicsUltrasound Physics

04:Scanner

‘97

Resonant FrequencyResonant Frequency

• Frequency of Highest Sustained Intensity

• Transducer’s “preferred” or resonantresonant frequency

• ExamplesGuitar StringBell

Pulse Mode UltrasoundPulse Mode Ultrasound• transducer 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 ReviewPulse Duration Review

• typically 2-3 cycles per pulse

• Transducer tends to continue ringingminimized by dampeningdampening transducer element

Pulse Duration = Period X Cycles / Pulse

Damping MaterialDamping Material• Goal:

reduce cycles / pulse

• Method: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

Disadvantages of Damping

• reduces beam intensity

• produces less pure frequency (tone)


BandwidthBandwidth

• Damping shortens pulsesthe shorter the pulse, the higher the range of

frequencies

• Range of frequencies produced called bandwidthbandwidth

BandwidthBandwidth

• range of frequencies present in an ultrasound pulse

Frequency

Intensity

Ideal

Frequency

Intensity

Actual

Bandwidth

OperatingFrequency

operating frequencyQuality Factor = -----------------------------

bandwidth

Quality Factor (“Q”)Quality Factor (“Q”)

• Unitless• Quantitative Measure of

“Spectral Purity”

Frequency

Intensity

Actual

Bandwidth

Which has a Higher Quality Factor?Which has a Higher Quality Factor?

Frequency

Intensity

A

Frequency

Intensity

B

operating frequencyQuality Factor = -----------------------------

bandwidth

Same Operating Frequency!


DampingDamping

• 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


An Aside about ReflectionsAn Aside about Reflections

• Echoes occur at interfaces between 2 media of different acoustic impedancesspeed of sound X density

Medium 1

Medium 2

Intensity Reflection Coefficient (IRC)&

Intensity Transmission Coefficient (ITC)

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 EquationIRC 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


ReflectionsReflections

• Impedances equal no reflection

• Impedances similar little reflected

• Impedances very different virtually all reflected


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


Why Use Gel?Why Use Gel?

• Acoustic Impedance of air & soft tissue very different• Without gel virtually no sound penetrates skin


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


Acoustic Impedance

(rayls)

Air 400Soft Tissue 1,630,000

Fraction Reflected: 0.9995

Transducer Matching LayerTransducer Matching Layer• Transducer element has different

acoustic impedance than skin

• Matching layer reduces reflections at surface of piezoelectric elementIncreases sound energy transmitted into body

Transducer – skin interface

Transducer Matching LayerTransducer Matching Layer

• placed on face of transducer• impedance between that of transducer

& tissue• reduces reflections at surface of

piezoelectric element Creates several small transitions in acoustic impedance

rather than one large one

reflected intensity z2 - z1

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


( )2 Matching

Layer

Transducer ArraysTransducer Arrays

• Virtually all commercial transducers are arraysMultiple small elements in single housing

• Allows sound beam to be electronicallyFocusedSteeredShaped


Electronic ScanningElectronic Scanning

• Transducer ArraysMultiple small transducersActivated in groups


Electrical ScanningElectrical Scanning

• Performed with transducer arraysarraysmultiple elements inside transducer

assembly arranged in either» a line (linear array)

» concentric circles (annular array)


Linear Array ScanningLinear 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 ArraysLinear Switched Arrays• Elements energized as

groupsgroup acts like one large

transducer

• Groups moved up & down through elementssame effect as manually

translatingvery fast scanning possible

(several times per second)» results in real time image

Linear Switched ArraysLinear Switched Arrays

Linear Phased ArrayLinear Phased Array• Groups of elements

energizedsame as with switched arrays

• voltage pulse applied to all elements of a group

BUT• elements not all pulsed at

same time

1

2

Linear Phased ArrayLinear Phased Array• timing 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 ArrayLinear 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


Above arrows indicate timing variations.By activating top & bottom elements earlier than center ones, beam is focused

Beam is focused

Focus


Focus

Focal point can be moved toward or away from transducer by altering timing variations between outer elements & center


Focus

Multiple focal zones accomplished by changing timing variations between pulses•Multiple pulses required•slows frame rate

Listening ModeListening Mode

• Listening 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

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Documents

George David Associate Professor Ultrasound Physics 04: Scanner ‘97