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Physics 2 > Module 1 : Transducer Construction > Flashcards

Module 1 : Transducer Construction Flashcards

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1
Q

The Ultrasound Probe

A

Aka = transducer
Converts energy from one form to another
EXAMPLES - MICROPHONES, EAR, LIGHTBULB, OVENS, BATTERY, MOTOR
In ultrasound we convert electrical energy to mechanical energy then back to electrical

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2
Q

Transducer

A

Piezoelectric crystal or element

Signal conversion device

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3
Q

Crystal

A

Piezoelectric material

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4
Q

Element

A

Another name for piezoelectric electric crystal

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5
Q

Scan head

A

Another name for transducer

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6
Q

Probe

A

Another name for transducer

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7
Q

Transducer assembly

A

Another name for transducer including housing and internal circuitry

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8
Q

Parts of the transducer

A

Housing
Backing material
Crystal
Matching layer

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9
Q

Housing

A

Contains all prob components

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10
Q

Backing material

A

Mixture of metal and plastic or epoxy bonded to the back of the crystal

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11
Q

Crystal

A

Ceramic element that has piezoelectric properties

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12
Q

Matching layer

A

Used to reduce sound reflection from the skin and enhance sound transmission
WE SEE ONE MATCHING LAYER ON THE TRANSDUCER FACE

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13
Q

Tuning coil

A

Helps machine match the frequency of the probe

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14
Q

Electric shield

A

Keeps away outside interference from affecting image

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15
Q

Insulator ring

A

Stops radial mode from occurring

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16
Q

THE CRYSTAL - history

A

Piezoelectric principle discovered by curie brothers in 1880

Principle explains why some materials can convert electrical energy to mechanical and vice versa

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17
Q

THE CRYSTAL - materials

A

Natural materials - quartz, lithium sulphate, rochelle salt, tourmaline

Man made ceramics - lead zirconate, lead titanite, barium titanate, LEAD ZIRCONATE TITANATE (PZT- ceramic), *POLYVINYLIDENE FLUORIDE (PVFD- polymer)

*most currently used and achieves the best image

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18
Q

THE CRYSTAL- piezoelectric effect

A

The DIRECT piezoelectric effect occurs when mechanical pressure deforms the crystal which changes the orientation of the electric DIPOLE producing a small voltage

the reverse INDIRECT effect is the opposite when the electric voltage changes the orientation of the DIPOLES cause the crystal to expand and contract

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19
Q

THE CRYSTAL - dipoles

A

Dipoles are molecules within the crystal that have a positive charge at one end and a negative charge at the other end

It can be influenced by an electric or magnetic field

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20
Q

THE CRYSTAL -dipole orientation

A

Normally the dipoles are in random alignment which makes the crystal inefficient for vibration when an electrical current applied

If the dipoles are in alignment then the vibration of the crystal will be much improved

21
Q

THE CRYSTAL - vibration

A

When the crystal vibrates we much consider the different modes of vibration that may occur

Early probes the crystal was disc shaped and could vibrate in a THICKNESS MODE or a RADIAL MODE ( dont like radial it creates artifact NEED INSULATOR RING)

22
Q

THE CRYSTAL - modes

A

Modern probes the crystal is shaped differently and there are three modes of vibration

THICKNESS
LENGTH
WIDTH
thickness is the one we want

23
Q

THE CRYSTAL - synthetic

A

Synthetic material used in the production of the crystal so the a more pure product can be developed

Less imperfections if process is careful

24
Q

THE CRYSTAL - curie temperature

A

When a substance is heated beyond the curie temp the bonds weaken
If the substance is subjected to an electrical field when dipoles align accordingly
Substance then cooled and bonds strengthen
* curie temp for PZT is 350’

This is how we align properly to enhance the piezoelectric effect

25
Q

THE CRYSTAL - heat

A

Heat can polarize the dipoles then reheating could potentially depolarize as well

26
Q

THE CRYSTAL - frequencies

A

The crystal determines what frequencies the probe can emit

There are 4 different frequencies

  • RESONANT FREQUENCY
  • DRIVING FREQUENCY
  • OPERATING FREQUENCY
  • HARMONIC FREQUENCY
27
Q

THE CRYSTAL - resonant frequency

A

The frequency that the crystal likes to ring at

Determined by the MATERIAL and the THICKNESS

aka fundamental frequency

28
Q

THE CRYSTAL - driving frequency

A

determines by the AC voltage and sent to the crystal

If the voltage is altered then the crystal can be forced to ring at a different frequency then the fundamental

29
Q

THE CRYSTAL - operating frequency

A

One that you us to scan

30
Q

THE CRYSTAL - 2nd harmonic frequency

A

Two times the resonant or operating frequency

31
Q

THE CRYSTAL - thickness

A

Frequency of the crystal, relates to the propagation speed of sound and the thickness of the crystal

THICKER CRYSTAL = LOWER FREQUENCY

thickness of the crystal which determines the resonance frequency is 1/2 wavelength

32
Q

THE CRYSTAL - equation thing

A

Doubling the thickness will half the frequency and vice versa

WHEN CALCULATING THICKNESS YO HAVE TO USE THE SPEED OF SOUND IN THE CRYSTAL AS THE CONSTANT

33
Q

THE BACKING MATERIAL

A

Aka dampening block
Very important
Made of epoxy reusing and metal powder (tungsten)

34
Q

THE BACKING MATERIAL - purpose

A

Reduce the spatial pulse length to improve axial resolution

BUT

also reduced the amplitude reducing sensitivity and decreasing penetration

Increase dampening material = shorter pulse

ALSO

absorbs sound so that reflections don’t occur from behind the crystal
- Z value must be comparable to the element

35
Q

THE BACKING MATERIAL - short pulses

A

short pulse is ideal for resolution but in Doppler you need longer pulses

36
Q

THE BACKING MATERIAL - DYNAMIC DAMPENING

A

And electronic means to suppress the ringing of the crystal

sending a second pulse after the first pulse that will result in complete destructive interference to stop the ringing

Uses Huygens to our advantage

Dynamic used more in 2D scanning not in Doppler

37
Q

The matching layer- function

A
  • impedence mismatch between the crystal and skin is very large and without matching layer sound would return to the probe before entering the patient
38
Q

The matching layer - mechanics

A
  • matching layer has a z value between the crystal and the skin to reduce amount of reflection
  • potential problem is the reflections that can occur between them and crystal
  • to solve suit matching layer to 1/4 wavelengths, which creates destructive interference between matching layer
39
Q

Matching layer - bandwidth

A
  • more than one matching layer is used since there are multiple frequencies
  • matching layers accommodate multiple frequencies improving the transmission and reception of wide bandwidth frequencies
  • gel also matching layer
40
Q

Probe excitation

A
  • we excite the probe with a voltage that helps determine the operating frequency
  • older technology used the SPIKE voltage method
  • newer t chnology uses BURST voltage method
41
Q

Spike voltage

A
  • uses direct current to vibrate crystal
  • current from pulse hits the crystal where one spike is equal to one pulse
  • driving frequency always equals resonant frequency
  • also called SAW TOOTH VOLTAGE
42
Q

Burst voltage

A
  • uses alternating current to vibrate the crystal
  • current from the pulse hits the crystal where one voltage burst is equal to one pulse
  • driving frequency of voltage determines the resonant frequency
  • looks like a sine wave = frequency of burst voltage determines frequency of probe
43
Q

Frequency bandwidth

A
  • range of frequencies produced by the pulse
  • crystal stimulated it will ring at resonant frequency with other small frequencies produced
  • when crystal is dampened to shorten length of probe then more frequencies are emitted
44
Q

Frequency bandwidth - determining factors

A
  • crystal thickness and material determine most efficient frequency to ring at
  • the dampening determines size of bandwidth
  • THE SHORTER THE PULSE THE WIDER THE BANDWIDTH
    + short pulse better resolution
  • wider bandwidth more options for driving frequency
45
Q

Frequency bandwidth - usable bandwidth

A
  • number of driving frequencies limited by size of bandwidth and attenuation
  • any frequencies that have an amplitude of less than half of resonant frequency are too weak
  • 6dB is usable bandwidth where it is equal to one half the amplitude of 1/4 intensity of resonant frequency
46
Q

Frequency bandwidth - influential factors

A
  • damping material effects bandwidth just like it effects SPL
  • increase dampening then shorten pulse then increase range of frequencies emitted
  • BUT sensitivity reduced
47
Q

Fractional bandwidth

A
  • way to express bandwidth
  • bandwidth/frequency
  • probe with FB over 80% is broad band probe
48
Q

Quality factor

A
  • another way to describe bandwidth
  • reciprocal of fractional bandwidth
  • frequency-/bandwidth
  • desirable to have low Q for 2D and get higher with different Doppler’s
49
Q

Bandwidth summary

A
  • to optimize 2D use more dampening to shorten pulse
  • this will increase resolution by reducing SPL and will increase bandwidth
  • increase in bandwidth = lower Q factor
  • modes requiring more sensitivity will benefit from narrower bandwidth and higher Q

Physics 2 (8 decks)

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