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US Physics > Week 2: Transducer design > Flashcards

Week 2: Transducer design Flashcards

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1
Q

What is a transducer?

A

device converting non-electrical into electrical signals

i.e. electrical transmitted pulses into US pulses & US echo to electrical signals

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2
Q

in the piezoelectric effect; materials expand or …. when a postive (lead, ….. & titanium) or a …. (O2 ions) voltage is applied

A

1) contract
2) Zirconium
3) negative

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3
Q

When is a drop produced in relation to the probe and material?

A

When material is stretched or compressed by external force

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4
Q

Describe Dipolar molecules

A

+ve charge one end, -ve at the other

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5
Q

What does +ve poles move towards, and what does this do to alignment?

A

moves towards -ve electrodes. If voltages change across element so does alignment = effecting thickness

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6
Q

What is the most common material used for manufacturing medical imaging PZT probes?

A

synthetic polycrystalline (crystalline grains)/ ceramic material (lead, zirconate, titanate)

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7
Q

Why is this PZT plate/probe used (crystalline grains)?

A
  • high sensitivity

- cope w/ large acoustic powers

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8
Q

What is an advantage of crystalline grains PZT?

A
  • can be formed to a curve or flat shape

- elements fired to make rigid (original powder form)

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9
Q

After firing, +ve charged lead, zirconium and titanium ions and -ve charged O2 ions are …… so each …. contains +ve net and -ve net charge. They are fixed…. but equal distance apart.

A

1) arranged
2) net
3) opposite

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10
Q

What are PZT divided into and polarised (orientated in the same direction)?

A

regional numbers (AKA domain)

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11
Q

How is PZT polarised to make a uniform direction? (image 3.3a - no net polarisation)

A
  • materials heated >200 degrees C, applying strong electrical field across it = poling
  • Makes the dipoles (in the domain) switch to direction closest to direction of poling electric field
  • when cooled ‘frozen’ to form near orientation –> parallel to poling field.
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12
Q

what are the advantages of modified PZT?

A
  1. increased BW

2. Increased sensitivity

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13
Q

With modified PZT, there are narrow spaces/channels filled with polymer (image 3.34). What does this help with?

A

the columns can vibrate more efficiently, reduced density of polymers resulting in reduced overall density and reduced Z

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14
Q

Why is the thickness of the plate chosen to be 1/2 of λ at the centre f of transmit pulse?

A

makes it expand and contract strongly when alternating voltages at that f are applied = 1/2 Resonance

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15
Q

Why does 1/2 resonance occur (producing a large amplitude)?

A

reverberating wave traveling back and forth across the plate will travel exactly 1 λ when arriving back at starting position.

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16
Q

What are PDVF (plastic polyvinyldifluroide) used in and its properties?

A

used in: membrane hydrophones

  • wide BW
  • short impulse response
  • greater focusing
  • increased sensitivity
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17
Q

What elements are single crystal transducers doped with?

A

either 1) lead, magnesium and niobium (PZT-PT)

2) lead, zinc and niobium (PZN-PT)

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18
Q

What does single crystal transducer do that ceramic material PZT transducer can’t?

A

during polarisation of ceramic PZT, structure limits alignment of domains
Single crystal - has no grain boundaries can be poled to give perfect alignment of dipoles

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19
Q

What advantages does single crystal transducer have?

A
  • wider BW
  • improved axial resolution
    • -> used for multi-f and harmonic imaging
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20
Q

mirocrmachined US transducers (MUT’s) have better …., wider …. providing shorter US pulses, this improving ….. …..

A

1) Z
2) BW
3) axial res

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21
Q

What does this image show and what is required?

A

capacitive micro-machined transducer (CMUT)

steady bias voltage between electrodes

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22
Q

What can happen if increasing the bias voltage in CMUT?

A

increased beyond a limit (collapse voltage limit) causes membrane to contact with the base = electrical breakdown or damage

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23
Q

When is the > efficiency obtained?

A

when membrane is close to base (collapse voltage). Solved with placing small structure beneath membrane

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24
Q

What is this image?

A

peizoelectric micro-machined US transducer (PMUT)

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25
Q

How does PMUT (2-22MHz) function in T and R?

A 
T = voltage applied leads to vibration of membrane
R = deflection of membrane due to incoming pressure waves = lateral changes in dimension, which produce electrical signal at electrodes
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26
Q

What can CMUT probes be used for?

A
  • HI

- high f small-scale (intravascular imaging)

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27
Q

What does ‘reasonce’ mean?

A
  • element vibrates strongly at f, thickness = 1/2 a λ
  • f where material/structure vibrates naturally

image HERE –> 1/2 max transducer output -3dB BW

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28
Q

generated waves will be partially ….. from front and back faces and …… within the element

A

1) reflected

2) vibrate

29
Q

in piezoelectric element, thickness governs ….. ….

A

resonant f

30
Q

What is this image showing (Hugyens principle)?

A
  • row of sources, spherical waves from each interface forming a series of wavefronts
  • waves propagate out
  • parallel to surface forming plane waves
  • non-parallel wave parts interfere destructively and cancel each other out
31
Q

Linear arrays have …. elements and a λ ~ ….. width and have wide …..

A

1) 128
2) 1.3
3) Field of view (FOV)

32
Q

Linear arrays allow…? and separate elements …?

A

a) elements to be narrow for wide of angles in scan plane

b) to prevent cross talk

33
Q

Why is a matching layer on a transducer needed?

A

if PZT directly contacted w/ patient ~80% wave power is reflected back = poor sensitivity

IMAGE HERE

34
Q

what does the intensity of reflected wave depend on when talking about matching layer?

A

Z of tissue

35
Q

Z of matching layer between impedance of PZT …. and impedance of tissue (t) should be equal to what equation?

A

1) element

√ (Zpzt x Zt)

36
Q

What should 100% transmission through matching layer that only occurs at 1 f, thickness be in relation to λ?

A

‘thickness equal to 1/4 of λ’

IMAGE HERE

37
Q

While matching layer improves sensitivity at centre f, it also acts as…?

A
  • f filter
  • reducing BW
    e. g. -3dB transducer with 1/4 λ, around 60% centre f is achieved.
38
Q

How can the reduced BW be improved?

A
  • larger BW transducer, also improved axial res.
39
Q

what can multiple matching layers do?

A
  • changes in Z from PZT to skin - effective over different f

- improves BW

40
Q

How can composite PZT help with matching layers?

A
  • close narrow spaced channels & filled w/ polymer = reduced Z than PZT alone = reduced matching problem
41
Q

Why is a backing layer required?

A

prevents excess vibration

42
Q

what does reduced vibration do w/ backing layer properties?

A
  • shorter US pulses generated

- improve axial res

43
Q

How can a backing layer help with long pulses?

A
  • “dampens”/reduced ringing in materials with high Z characteristics and ability to absorb US
44
Q

why is a (increased f = shorter λ & shorter pulse) short pulse often desirable in US imaging?

A

increases axial res

45
Q

What is pulse length (PL) dependent on?

A

f

46
Q

What is this imaging showing?

IMAGE HERE

A

1) T pulse sent out, spacial PL (SPL) with 2 objects = 1/2 SPL apart
2) pulse coming into 1st boundary, some US is reflected back (# sent back = Z mismatch properties)
3) T pulse reaches 2nd boundary causing more US reflection, w/ R pulse going back t transducer
4) R pulse looks like 1 long pulse (2 reflect pulses sent back) = appears as just 1

47
Q

What does thin element cause in relation to PL & f?

A

greater resolution, poor penetration

48
Q

What does thick element cause in relation to PL & f?

A

greater penetration, poor res

49
Q

In wider beams (i.e. BW), what can this effect?

A

lateral res - 2 objects cannot be separately identified

50
Q

In narrow beams (i.e. BW), what does this mean with lateral res?

A

2 objects can be separately identified

51
Q

Single element transducer beam shape dependent on what?

A

diameter of element & λ of pulse

52
Q

How can the lens on the transducer help when imaging?

A

focus the beam

53
Q

in Array transducer, several element are used to form the beam what does this affect?
- beam maybe focused

A

beam shape

54
Q

What is not the same in array transducers?

A

T and R beam shape

55
Q

In broadband transducers (wide BW = produce short pulses), -3dB range of f over which….

A

output is at leas 1/2 of the maximum

IMAGE here

56
Q

whats the advantage of broadband transducers?

A
  • used in HI - decreased f signal 1st T and harmonic is R
  • pulses can be broken down down to different component f
  • able to T and R over wide range of f
57
Q

What can broadband transducers also be used in?

A
  • multi-f imaging - transducer operate a different centre f, user select f to optimise penetration and res
  • f compounding - different f to create different depths of image
58
Q

What does HI require?

A

a fundamental f

59
Q

what is HI due to?

A

non-linear propagation (changes in pulse spectrum)

60
Q

if a fundamental f (F0) is sent out/T, what is R?

A

Echoes returned at harmonic f, 2f0, 3f0 etc

61
Q

When HI is forming an image, which pulse does it this with?

A

2f0

62
Q

What are the advantages of HI?

A
  • narrow beam = suppresses artefacts (side lobes)

- reduced acoustic noise i.e. reverberations

63
Q

What is assumed with non-linear propogation?

A
  • C is a fixed speed (1540ms)

- a linear relationship w/ amplitude of wave at source & amplitude elsewhere in the beam

64
Q

High pressures >1MPa result in what?

A

picture breaks down and non-linear propogation is noticeable

65
Q

speed of wave travels related to….., which either …. or ….. local speed

A

1) local particle velocity
2) increases
3) decreases

66
Q

What does non-linear propogration result in?

A

some energy in the pulses being transferred from fundamental f to it’s harmonics

67
Q

What happens to the medium at high pressure amplitudes?

A

becomes compressed increasing stiffness (K) and C

68
Q

What can the increased K and C result in?

A

compression catches up with rarefaction resulting in a shock front and increase the pressure

US Physics (6 decks)

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