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Physics > Transducers > Flashcards

Transducers Flashcards

1
Q

Ultrasound transducers

A

Convert electric energy into ultrasound energy and vice versa

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2
Q

Piezoelectric element

A

A material/element when deformed by pressure produce a voltage

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3
Q

Reverse piezoelectric

A

The production of pressure when voltage deforms materials

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4
Q

Piezoelectric elements

A

Quartz
Synthetic crystals (ceramics)
- PZT
- Barium

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5
Q

How are piezoelectric elements made

A

Placed in strong magnetic field at high temp. This realigns molecular dipoles. Then cooled

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6
Q

Curie point

A

The temperature in which the magnetic properties of a solid can be changed

PZT curie point 350

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7
Q

What would happen if you brought the crystal back to the curie point but without the magnetic field ?

A

It would lose its piezoelectric properties

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8
Q

Transducer assembly

A 
Case
Damping
Element
3 matching layers
Gel
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9
Q

Natural frequency of piezoelectric element formula

A

Propagation speed of the element divided by 2x thickness (wavelength)

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10
Q

Propagation speed of PZT

A

4 mm/us

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11
Q

Thickness of element

A

0.2-1 mm

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12
Q

True or false:

Thinner elements have lower frequencies

A

False. Thinner elements have higher frequencies. Think of smaller bells

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13
Q

How many cycle US pulse does 1 cycle of alternating voltage create

A

2-3

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14
Q

Longer alternating voltage 5-30 is what

A

Doppler technique

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15
Q

What does Fvolt = Fo mean

A

One transducer can have more that one frequency based on the selected voltage

The transducer is driven at one of 2 or 3 selectable frequencies by voltage pulses with the selected frequency

Frequency must fall within the bandwidth of the transducer

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16
Q

Multi-hertz operation

A

2 or 3 frequencies in the same element

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17
Q

What does damping do

Good and bad

A 
Good:
- Decreases n (thus decreasing 
PD and SPL)
- Image resolution
- Bandwidth

Bad:

  • Decreases amplitude
  • Decreases sensitivity
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18
Q

Image resolution is inversely proportional to depth. If you need to go further, you must decrease:

A

PRF

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19
Q

If you don’t decrease PRF when going into deeper structures, what can happen?

A

Range ambiguity/echo misplacement

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20
Q

What are piezocomposites

A

Other materials added to Decrease z
Increase bandwidth
Increase sensitivity & resolution

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21
Q

Damping material reduces

A

Cycles per pulse - faster decay time

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22
Q

What are some unwanted things that happens with a damping material

A

Reduces amplitude - weaker sound out, weaker echo in

Decreases sensitivity (ability to detect weaker echoes)

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23
Q

Continuous wave ultrasound do not have damping material true or false

A 
True 
Not needed because pulses are not used. 
Higher efficiency 
Better sensitivity 
Worse resolution
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24
Q

The case/housing unit

A

Absorbs energy from sides of crystal

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25
Q

Matching layer

A

Reduces reflection of ultrasound at the transducer-element surface (reduce impedance)

Want more transmission, less reflection

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26
Q

How many matching layers are used and thickness of each

A

1-3 layers are used to reduce the large differences in impedance.

Thickness of each = 1/4 wavelength

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27
Q

Beam definition

A

The width of a pulse as it travels away from the transducer

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28
Q

Fresnel zone

A

Near zone

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29
Q

What does the near zone depend on

A
  1. Size of aperture

2. Operating frequency

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30
Q

Fraunhofer zone

A

Far zone

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31
Q

Beam width decreases with increasing distance from transducer

A

Near zone

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32
Q

Beam width increases with increasing distance from transducer

A

Far zone

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33
Q

Aperture

A

Element size/ width

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34
Q

What does beam width affect

A
  1. Resolution of signal at that depth

2. Intensity of the sound beam at that depth (intensity is not uniform within the beam)

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35
Q

At the focus, what is the size of the beam width?

A

Wb= 1/2 aperture size

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36
Q

At what point is the beam width the same as the element width

A

At double the NZL

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37
Q

If aperture increases x2 NZL

A

increases x4

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38
Q

If frequency increases by 4x NZL increases

A

4x

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39
Q

Focal length

A

The distance to the focus from the transducer

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40
Q

What do you do if you want a high frequency disk transducer to look at a superficial structure. Do you want a large or small footprint

A

Smaller because near zone length will be shorter, focus higher

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41
Q

What kind of resolution is better

A

Smaller - more fine details

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42
Q

Low level of disinfection for transducer example

A

Non-critical - contacts skin

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43
Q

When is a high level of disinfection of probe required

A

Semi critical- mucous membranes

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44
Q

When is sterilization of probe required

A

Critical- device enters tissue

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45
Q

High level disinfection methods

A

Cidex

Recert

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46
Q

What are NOT suitable disinfection methods

A

Bleach
Ammonia
Alcohol based solutions
Sprays

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47
Q

What are low level disinfection methods

A

5% hydrogen peroxide
CAVI wipes
Preempt

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48
Q

Invasive transducers

A

Transvag
Transrectal
Transesophageal
Catheter mounted

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49
Q

Why are invasive transducers good

A

Get much closer to the tissue

Can have high frequency without worrying about attenuation

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50
Q

Focus is only accomplished where?

A

Near zone

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51
Q

How can sound be focused? (3)

A
  1. Curved transducer elements
  2. Lens
  3. Phasing
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52
Q

The limit to which a beam can be narrowed depends on (3)

A
  1. Wavelength
  2. Aperture
  3. Focal length
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53
Q

Mechanical scanning

A

Mechanical transducer
Single element
Sector image
Historical, obsolete

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54
Q

Automatic/electronic scanning

A

Live scanning
Many frames per min, looks like a live scan
Requires arrays

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55
Q

Arrays

A

Rectangular crystals
Assembled in a row

Two main types:

  1. Linear (sequenced/phased)
  2. convex (sequenced/phased)
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56
Q

Linear array

A

Firing groups of elements at same time

Each group acts like a single element - produce a pulse and receive an echo

Thus each group produces a scan line

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57
Q

In linear sequence array what is the aperture

A

Width of a group (not element)

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58
Q

In linear sequence array, what would be the width of the image?

A

The length of array

Rectangular image

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59
Q

Linear image consists of

A

Parallel scan lines

Travelling in same vertical direction

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60
Q

LPF

A

(scan) Lines Per Frame

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61
Q

128 element array fired in groups of 4 = how many scan lines

A

125

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62
Q

Increasing scan lines increases

A

Density and image quality

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63
Q

What type of array produces a modified sector image

A

Convex array / curvilinear transducer

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64
Q

Phased array

A

Straight line of elements

Elements fired with a small time difference

65
Q

What is the time delay between firing of each element in phased array?

A

< 1 us time delay

66
Q

In phased array, a wave always travels _____ to its ______

A

A wave always travels perpendicular to its wavefront

67
Q

A new waveform creates by secondary wavelets happens according to what principle

A

Huygen’s principle

68
Q

Phasing (3)

A
  1. Steers beam
  2. Provides electronic control of location of focus
  3. Generates echoes from a specific location with several viewing angles
69
Q

In phased array, how do we move the focus closer to the transducer?

A

Increase the delay, which increases the curvature

70
Q

In phased array, how do we move the focus deeper/ further from the transducer?

A

Decrease curvature/ delay

71
Q

How do we achieve curvature in phased array

A

Fire outside crystals first, middle crystals last

72
Q

What increases resolution but there is a trade off with temporal resolution (reduces frame rate)

A

Multiple foci where multiple frames focused at a different depth

“Montage” of overlaid images

73
Q

Further applications for phasing (6)

A
  1. Spatial compounding
  2. Clears out cysts
  3. Sharpens boarders, smoother image
  4. Assess large areas through small windows
  5. Vector array
  6. Variable aperture
74
Q

Spatial compounding

A

Hit same object from different angles

More pulses per image

75
Q

Vector array

A

Linear array + phased array
- phasing applied to linear sequence array.

Delays are the same for each group
Beam sent out in angled direction = parallelogram shaped display

Used in Doppler US

76
Q

Phasing can be applied to each element group in a linear sequences array to

A
  1. Steer pulses in various directions

2. Initiate pulses at various starting points across the array

77
Q

Variable aperture

A

When only some elements of a phased array are used to generate a pulse

78
Q

In variable aperture, smaller groups produce ______

And larger groups produce ______

A

smaller groups - Short focal length

Larger groups - foci located at increasing depth

79
Q

Side lobes

A

Additional beams / artifact resulting from single element transducers

80
Q

Grating lobes

A

Additional beans resulting from arrays (multi-element structure)

They are weak beams but can hit a strong reflector and produce strong echoes (diaphragm, bone, gas)

Artifacts - look like ‘amniotic bands’

81
Q

3 things that fix grating lobes

A
  1. Apodization
  2. Subdicing
  3. THI
82
Q

How does Apodization reduce grating lobes?

A

Reduces amplitude of voltage to outside element. Weaker beam going out, weaker echo coming back. Less artifact

83
Q

How does Subdicing fix grating lobes?

A

Subdicing of each element into a group of smaller crystals reduces inter-element interactions

ONLY for grating lobe artifacts

84
Q

How does THI fix grating/side lobes?

A

Grating/ side lobes are too weak to produce harmonic signals

85
Q

What are the three aspects of imaging resolution? Which are attributed to the transducer and which are attributed to the instrument?

A
  1. Detail/spatial (transducer)
  2. Contrast (instrument)
  3. Temporal (instrument)
86
Q

What are the three types of resolution?

A

Axial
Lateral
Elevational

87
Q

Minimum reflector separation along scan line to produce separate echoes

A

Axial resolution

88
Q

Minimum reflector separation perpendicular/across scan line to produce separate echoes

A

Lateral resolution

89
Q

Minimum reflector separation perpendicular to scan plane to produce separate echoes

A

Elevational resolution

90
Q

All that THI fixes

A
  1. Side lobes
  2. Grating lobes
  3. Partial volume artifact (section thickness artifact)
91
Q

How do you improve LR

A
  1. Decrease beam width

2. Focusing

92
Q

Lateral resolution is equal to

A

Beam width

93
Q

What is the lateral resolution at the focus?

A

1/2 the aperture

94
Q

What is partial volume artifact / section thickness

A

Filling in of anechoic structures. Shows echoes from outside the intended scan plane

95
Q

How can you reduce the axial resolution

A

Decrease SPL via:

  • Decreasing wavelength (increase frequency)
  • Decrease n (damping layer)
96
Q

True or false

Axial resolution is 3mm
Structures 2 mm apart are seen as two structures

A

False , seen as one structure. If structures are 3mm apart or more it will be seen as two structures

97
Q

Contrast resolution

A

Being able to separate two shades of gray

98
Q

Being able to separate echoes in time

A

Temporal resolution - dependent on instrument. Lag = poor TR

99
Q

Diagnostic ultrasound uses frequency range

A

2 to 20 MHz

100
Q

Lower range of ultrasound necessary when (2)

A
  1. Obese (depth)

2. Transcranial (High attenuation)

101
Q

Higher range of ultrasound appropriate when imaging (3)

A

Breast
Thyroid
Peds

102
Q

Frequencies up to 50 MHz may be used for:

A

Intravascular
Ophthalmologic
Dermatologic

103
Q

Volumetric imaging used for

A

Obstetrics

Breast

104
Q

What is 4D imaging

A

3D imaging plus time

105
Q

Delaying the echoes coming to the transducer allows for (3$

A
  1. Steering
  2. Focusing
  3. Aperture
106
Q

Concentric rings of piezoelectric material - what is it called and why don’t we use it anymore

A

Annular arrays

Not used anymore because we can do the same with phasing, electronic focusing

107
Q

What type of voltage is the transducer driven by

A

Alternating current

108
Q

Other names for natural vibrational frequency (3)

A

Natural frequency
Operating frequency
Resonance frequency

109
Q

What determines the operating frequency of an element

A

The thickness of the piezoelectric crystal determines its natural frequency

110
Q

Transducer bean scanned by sequencing (3)

A
  1. Linear array
  2. Convex array
  3. Vector array

NOT phased array

111
Q

Transducer beam scanned by phasing (2)

A
  1. Phased array

2. Vector array

112
Q

Transducer beans focused by phasing

A
  1. Linear array
  2. Convex array
  3. Phased array
  4. Vector array
113
Q

Where is the damping material found?

A

Attached to the rear face of the transducer

114
Q

Why is the matching layer required

A

Transducer elements are 20x the impedance of the tissues by itself, this would create a large reflection at the skin and very little waves would be transmitted into the body

115
Q

Where is the matching layer located

A

At the surface of the transducer

116
Q

What is the beam profile

A

Beam profile describes the shape of the main beam also referred as hourglass shaped.

Sound beam narrows from the aperture to the transition (focal) point then diverges through the far zone

117
Q

What is self-focusing effect or natural focus seen in the beam profile?

A

Self focusing effect is the natural narrowing of the sound beam in a non-focused single element transducer

118
Q

What size is the width of the beam at 2x the near zone length

A

The size of the aperture

119
Q

Why is focusing of the beam required

A

Focusing improves resolution and increases intensity of the sound beam

120
Q

What are the methods of focusing?

A

Mechanical - lens or curved element

Electronic - phasing

121
Q

Two methods of real time scanning

A

Mechanical

Electronic/automatic

122
Q

Major difference between mechanical and electronic transducers

A

Mechanical has a moving part to steer the beam, electronic transducer does not

123
Q

Two modes of activation to produce a beam

A
  1. Sequencing

2. Phasing

124
Q

How do sequenced arrays work?

A

A sequenced array applies voltage pulses to a group of elements in succession to form scan lines

125
Q

How does a phased transducer work?

A

A phased array applies voltage pulses to all the elements with short time delays to steer the beam

126
Q

How is the beam directed to the right?

A

Voltage is applied with time delays in rapid progression from left to right directing the beam to the right, based on Huygen’s principle

127
Q

What is vector array, why is it useful

A

Vector array is a linear phased array, converts a linear rectangular format into a sector-like FOV

128
Q

How is electronic focusing achieved

A

By using a curved pattern of phased delays, an increase or decrease in the curvature or the delay pattern moves the focus shallower or deeper, respectively

129
Q

What is the limit to multiple foci?

A

Temporal resolution reduces

130
Q

What is variable aperture, focusing?

A

To focus deeper, you need a bigger aperture

For a shallow focused you need a smaller aperture

Therefore the transducer fires only the elements needed depending on the distance of the focus

For closer natural focusing, smaller groups of elements are fired

131
Q

What is dynamic aperture

A

As the depth changes, the aperture changes to maintain a constant focal width

132
Q

Huygens principe

A

Huygens principe is that every point on a wavefront can be considered a source for secondary wavelets. These wavelets combine to form a wavefront that heads in a direction perpendicular to the combined wavelets

133
Q

What is spatial compounding

A

The use of phasing to strike objects from multiple angles.

Images are compounded to produce an average image which will reduce artifacts and sharpen boarders

134
Q

What is the limit for spatial compounding

A

Temporal resolution

135
Q

Difference between grating lobes and side lobes

A

Grating lobes involve multi element transducers side lobes are for disk transducers

136
Q

What is resolution

A

The ability to distinguish echoes in terms of space (detail) time (temporal) and strength (contrast)

137
Q

What is axial resolution

A

Ability to separate interfaces that line along the beam axis (one on top of each other)

138
Q

How can the operator improve the axial resolution?

A

Reducing SPL either with wavelength or number of cycles in a pulse.
Increase frequency

139
Q

How is the transducer built to improve axial resolution

A

Damping layers decrease number of cycles per pulse

140
Q

What is lateral resolution?

A

Lateral resolution is the ability to separate interfaces that lie perpendicular to the beam

141
Q

How can the operator improve lateral resolution?

A

The more narrow the beam, the better the lateral resolution

So we can reduce the beam diabetes by applying the focus to the area of interest (1/2 the aperture)

142
Q

When you decrease beam width and improve lateral resolution, what other parameters are affected

A

Increases intensity due to focusing

Decreases ability to penetrate due to higher frequency

143
Q

Lateral resolution formula

A

LR = Wb

144
Q

What is elevational resolution?

A

Elevational resolution is the ability to separate interfaces that lie perpendicular to the beam axis (one in front of the other)

145
Q

How can the operator improve elevational resolution

A

higher frequency

THI to lower beam

146
Q

Which type of focusing cannot be used with a single element transducer?

A

Only arrays may be electronically focused

Single element transducers are fixed focused by mechanical means which include crystal shaping or the use of an acoustic lens or mirror.

147
Q

Temporal resolution

A

Ability to separate closely spaced events in time

148
Q

What determines frame rate? (2)

A

Image depth

Lines per frame

149
Q

Divergence in the far field is determined by (2)

A

Crystal diameter/thickness (also aperture)

Frequency

150
Q

What will increase the near zone length (2)

A

Large crystal diameter/aperture

High frequency

151
Q

What will decrease beam divergence in the far field (2)

A

Large crystal diameter/aperture

High frequency

152
Q

Synonyms for axial resolution (3)

A

Range
Depth
Radial

153
Q

Which transducer is also referred to as a sector or vector transducer

A

Linear phased array

154
Q

Which transducer produces a pie-shaped image?

A

Phased array

155
Q

At the face of a single, unfocused transducer, the beam diameter is ??? to the element diameter

A

At the face, beam diameter=element diameter

156
Q

At the distance of 1 NZL in an unfocused transducer, the beam diameter is equal to _____ of the diameter of the element

A

Beam diameter is one half of the diameter of the element at 1 NZL of an unfocused, single element transducer

157
Q

At 2 NZL of an unfocused transducer, the beam diameter is ??? to the element diameter

A

At a distance of two near zone lengths, the beam diameter equals the element diameter

158
Q

Which transducer is best for imaging deep structures in the abdo

A

Curved sequenced array

159
Q

Frame rate equation

A

frame rate = PRF/lines per frame

Physics (9 decks)

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