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FRCR Part 1 Physics > Ultrasound (Erin) > Flashcards

Ultrasound (Erin) Flashcards

1
Q

Velocity of USS Waves in a medium equation: C = what?

A

_________________________
_/(Bulk Modulus/density)

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2
Q

C (velocity) = What? X What?

A

C (velocity) = frequency x wavelength

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3
Q

We set frequency of USS Waves but the velocity depends on the …?

A

Medium. The wavelength changes to compensate.

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4
Q

Diagnostic USS Waves travel at what frequencies in general?

A

2-15MHz

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5
Q

Humans can only hear what frequency range?

A

20-Hz - 20kHz

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6
Q

In soft tissue, Uss waves travel at what speed?

A

1540m/s

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7
Q

Pulse Echo: Time taken for USS wave to go there an back = what?

A

2xDepth / velocity

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8
Q

What relationship of frequency shift to Pulse Repetition Frequency causes Aliasing?

A

Aliasing occurs when PRF is less than half of the frequency shift.
ie Frequency shift > 1/2 PRF

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9
Q

State the Doppler shift equation

A

2 x frequency x velocity of blood x cos(theta)
—————————————————
velocity of sound

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10
Q

Thermal index refers to the power needed to increase temperature by how many degrees?

A

1 degree

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11
Q

Mechanical Index must be less than what number to avoid risk of cavitation?

A

0.7

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12
Q

True/false: mechanical bio effects are more likely to occur in the lung and intestines than soft tissues

A

True - risk of bruising and cavitations

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13
Q

There is (increased/decreased) likelihood of acoustic cavitation at lower frequencies

A

Increased

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14
Q

Thermal index should be kept below what number (generally)

A

1

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15
Q

Thermal index should be kept below what number for obstetrics?

A

0.7

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16
Q

Thermal index: temperature rise depends on…(4)

A

Frequency
Power
Duration of scan
Tissue type

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17
Q

Mechanical index should be kept below what level for neonates?

A

0.3

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18
Q

How safe are microbubble USS contrast agents?

A

Very safe. Can use in renal/liver failure, even in pts with allergies to iodine/gadolinium.

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19
Q

What is the effect of USS Harmonics on:
- lateral resolution
- axial resolution

A

Improves lateral resolution
Reduces axial resolution

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20
Q

USS Harmonics makes what structures appear clearer?

A

Cystic and fluid-filled structures

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21
Q

Acoustic impedance equation?

A

Z = p x c
Acoustic impedance = density of tissue x speed of sound

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22
Q

What is Snell’s law?

A

Angle of refraction at tissue boundaries.
sin(theta-1)/sin(theta-2) = c1/c2

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23
Q

The near field/Fresnel zone of the USS beam can be calculated as…?

A

Diameter^2 / 4x wavelength

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24
Q

What are the 3 methods of USS Beam focusing?

A
  1. Time-delays of firing waves from different transducer elements
  2. Curved crystals in transducer
  3. Lens to mould the beam
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25
Q

What is the speed of ultrasound in air?

A

300m/s

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26
Q

around x% of the signal is reflected at the kidney-perinephric fat boundary

A

1%

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27
Q

USS waves travel through Bone, Fat Muscle, liver and Air and in the following order from fastest to slowest:

A

Bone
Muscle
Liver/kidney
Fat
Air

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28
Q

True/false: The velocity of sound is independent of body temperature

A

False. Temp affects density.

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29
Q

T/F: Aliasing occurs if the Nyquist criteria are satisfied

A

False.

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30
Q

T/F: Aliasing occurs less at higher angles

A

True. higher angle = cos theta closer to 0 (between 0-90 degrees)

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31
Q

The wavelength of ultrasound in soft tissue is approximately…

A

0.1- 1.5 mm

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32
Q
A
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33
Q

Artefacts: Acoustic enhancement occurs when…

A

the sound travels through a structure which is heavily fluid-based.

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34
Q

Artefact: Ring down artefact is caused by…?

A

resonance within gas bubbles

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35
Q

Artefact: Reverberation happens close to…

A

strongly reflective structures

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36
Q

Artefact: Comet tail artefact is…

A

a kind of reverberation artefact seen underneath small foci of highly reflective material such as the crystals found in the gallbladder wall

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37
Q

Full destructive USS wave interference only happens if…

A

two waves are of the same wavelength and opposite phase when they meet

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38
Q

Mechanical Index is calculated using which 2 figures?

A

peak rarefaction pressure
Square root of Frequency

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39
Q

What is an acceptable frame per second rate?

A

30 fps

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40
Q

How many lines per frame makes a good ultrasound image

A

100 lines per frame

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41
Q

What percentage of the gas/tissue interface is reflected?

A

99.9%

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42
Q

What percentage of the bone-soft tissue interface is reflected?

A

Around 40%

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43
Q

What percentage of the fat to muscle interface is reflected

A

1%

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44
Q

What percentage of the liver to muscle interface is reflected

A

0.01%

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45
Q

What is the acoustic impedence equation

A

Acoustic Impedence (Z) = density (ρ) x speed of sound in the tissue (v)

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46
Q

What unit is acoustic impedence measured in

A

Rayls

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47
Q

In doppler what happens to frequency if the blood is travelling towards the transducer

A

Increase in frequency

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48
Q

In doppler what happens to frequency if the blood is travelling away from the transducer

A

Decrease in frequency

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49
Q

How is velocity measured in doppler ultrasound

A

Measure of the change of frequency

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50
Q

What transducer angle does doppler work best/worst at?

A

best at 0 degree (theoretically can’t in actual practice)
Worst at 90 degrees

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51
Q

In Duplex Ultrasound what colour is given to higher frequencies (representing flow towards the transducer)

A

Red
(BART - blue away, red towards)

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52
Q

In Duplex Ultrasound what colour is given to lower frequencies (representing flow away from the transducer?

A

Blue
(BART - blue away, red towards)

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53
Q

Does duplex scanning produce quantitative date?

A

No

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54
Q

Define Thermal Index

A

The Thermal Index (TI) is a figure which compares the power being emitted to that which would cause a temperature rise of 1oC

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55
Q

At what level is the TI considered definitely safey

A

TI<0.5

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56
Q

At what TI should care be made to keep scanning time to a minimum

A

TI >0.7

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57
Q

What is the effect of a higher frequency on temperature rise in tissue

A

Higher effect

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58
Q

What is the effect of a lower frequency on temperature rise in tissue

A

Lower effect

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59
Q

How is MI calculated?

A

Calculated as the peak rarefaction (NEGATIVE) pressure divided by the square root of the frequency of ultrasound being used.

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60
Q

At what MI level does the risk of cavitation start?

A

MI >0.7

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61
Q

At what MI does the risk of Neonatal lung and intestinal damage occur?

A

MI >0.3

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62
Q

What is the risk with increasing mechanical index

A

Cavitation

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63
Q

At what TI is fetal scanning not recommended, no matter how brief?

A

TI >3

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64
Q

How is mechanical damage caused by ultrasound

A

At subcellular level, the acceleration of particles within cells can damage the cell wall

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65
Q

Alignment of subcellular structures is a mechanism of damage caused by USS T o r F?

A

True

Acoustic streaming – repeated passage of waves in one direction cause alignment of subcellular structures in the direction of the sound travel. This can have a negative effect on the cell membranes.

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66
Q

What does 1Hz stand for

A

1 Hz is 1 wavelength per second

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67
Q

How is acoustic impedence (Z) calculated?

A

Acoustic impedence (Z) = density (p) x speed of sound in that tissue (V)

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68
Q

How is the reflected portion calculated?

A

(Z1-Z2)2 / (Z1 + Z2)2

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69
Q

What is the unit of acoustic impedence Z?

A

Rayls

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70
Q

What is the velocity of USS waves in air?

A

330 ms-1 (approx)

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71
Q

What is the velocity of USS waves in soft tissue?

A

Approve 1540 ms -1

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72
Q

What is the velocity of USS waves in bone?

A

4000ms-1/5080 ms-1

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73
Q

Are USS waves faster in air or bone?

A

Bone

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74
Q

Are USS waves faster in soft tissue or bone?

A

Bone

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75
Q

Are USS waves faster in soft tissue or in air?

A

Soft tissue

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76
Q

How is USS velocity calculated?

A

_________________________
C = _/(stiffness/density)

Speed = square root of (stiffness/density)

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77
Q

If the stiffness increases what happens to the speed of USS waves

A

IncreasesI

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78
Q

If the density increases what happens to the speed of the USS waves

A

Decreases

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79
Q

What is the effect of increased temperature on the speed of ultrasound waves

A

Increases

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80
Q

Describe constructive interference

A

Interaction that occurs when the two waves are exactly in step and their amplitudes add up

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81
Q

Describe destructive interference

A

Interaction that occurs when the two waves are out of phase
Results in reduction in intensity
If equal and exactly out of phase they completely cancel each other out

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82
Q

What is frequency of a sound wave?

A

How many times per second the compression phases passes ant single point in the medium

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83
Q

Define acoustic impedence

A

Resistance experienced by an ultrasound beam in the medium

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84
Q

What is the main crystal type used in ultrasound?

A

Lead Zirconate Titanate (PZT)

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85
Q

What is the curie temperature for PZT?

A

350 degrees C

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86
Q

What is the effect of tissue harmonic imaging on lateral resolution

A

improves lateral resolution

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87
Q

What is the effect of tissue harmonics on axial resolution

A

Reduces axial resolution

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88
Q

What is the effect of using tissue harmonic imaging on resolution when scanning patients with a large BMI?

A

Improved resolution

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89
Q

What is the effect of tissue harmonics on reverberation and side lobe artefacts

A

Reduces side lobe and reverberation artefacts

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90
Q

What is the effect of tissue harmonics on imaging tissue structures

A

Makes tissue structures seem clearer

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91
Q

What is the wavelength of ultrasound in soft tissue

A

Around 0.1-1.0mm

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92
Q

How are contrast agents given in ultrasound?

A

IV injectiona

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93
Q

What is the contrast used in ultrasound?

A

Microbubbles

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94
Q

Is iodine used in ultrasound for contrast

A

No

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95
Q

What is the wavelength of 1.5MHz ultrasound wave in soft tissue?

A

1.0mm

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96
Q

What is the wavelength of 15MHz ultrasound wave in soft tissue?

A

0.1mm

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97
Q

Does bone have a high Z or low Z (acoustic impedence)

A

High Z

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98
Q

Does PZT crystal have a low or high Z (acoustic impedence)

A

High Z

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99
Q

Does air have a high or low Z (acoustic impedence)?

A

Low

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100
Q

Does lung have a high or low Z (acoustic impedence?)

A

Low

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101
Q

What causes scattering in USS?

A

Structures smaller than the wavelength of the beam scatter US rasiation in all directions

102
Q

At what size of objects does scattering occur in ultrasound studies using 1.5MHz frequency?

A

If object <1mm

103
Q

At what size of objects does scattering occur in ultrasound studies using 15MHz frequency?

A

If object <0.1mm

104
Q

When does diffuse reflection occur?

A

If reflecting interface is rough or had undulations that are smaller than the wavelength of the radiation

105
Q

When does specular reflection occur?

A
  • the beam strikes a large smooth interface that is larger than the wavelength
  • the acoustic impedence of the tissues on either side of the interface are non equal
106
Q

What is the angle of reflection equal to?

A

The angle of incidence

107
Q

How does refraction occur

A

If the speeds of sound of the tissues on either side of the interface are non equal

108
Q

How does the intensity of any ultrasound beam change with increasing depth in tissue

A

Intensity of any ultrasound beam decreases exponentially with depth in tissue

109
Q

What 3 factors cause attenuation of an ultrasound beam

A

Absorption
Scattering
Reflection

110
Q

What unit are ultrasound attenuation co-efficients quoted in?

111
Q

What is the pulse repetition frequency

A

Number of pulses emitted each second

112
Q

What information does the time taken for the pulse to return provide?

A

Information on depth

113
Q

How is the depth of an echo calculated

A

Return time x wave speed /2

114
Q

How does TGC work

A

Echoes arriving later are from a greater depth and have suffered greater attenuation
In TCG pulses are amplified according to when they arrive at the transucer
All echoes from the same boundary will have the same brightness regardless of depth

115
Q

What is the crystal thickness in ultrasound probes dependent on?

A

1/2 wavelength of emitted US waves

116
Q

Do high frequency probes have thin or thick crystals?

A

Thin crystals
0.1mm at 7.5MHz

117
Q

Do low frequency probes have thin or thick crystals?

A

Thick crystals
0.5mm at 1.5MHz

118
Q

What are the 3 main types of ultrasound probes

A

Linear
Curvillinear
Phased array

119
Q

What are linear array transducers useful for?

A

Scanning superficial structures i.e neck, breast, scrotum

120
Q

What shape is the field of view produced by a linear array transducer

A

rectangular

121
Q

Which has a higher FOV - linear or curvilinear transducers?

A

Curvilinear

122
Q

Do curvilinear transducers operate at high or lower frequencies

A

Low frequencies

123
Q

What are curvilinear transducers used for?

A

Abdominal, obstetric etc

124
Q

In B mode what 3 factors does lateral resolution depends on?

A

Beam width
Number of scan lines per frame
Frequency of US radiation

125
Q

How does beam width affect lateral resolution

A

Best resolution in the focal zone
(Narrowest zone)

126
Q

Define lateral resolution

A

Ability to separate two structures side by side at the same depth

127
Q

Define axial (depth) resolution

A

Ability to separate two interfaces along the same scan line

128
Q

How does number of scan lines affect lateral resolution

A

More scan lines provide better lateral resolution
(but each frame takes longer to acquire)

129
Q

How does frequency of US radiation affect lateral resolution

A

Resolution is better at higher frequency

130
Q

What is the typical lateral resolution at 3MHz

131
Q

What is the typical lateral resolution at 10MHz

132
Q

What is axial resolution determined by? (In B mode)

A

Spatial pulse length

133
Q

How is spatial pulse length calculated

A

Wavelength x number of waves per pulse

134
Q

What is spatial pulse length?

A

Finite length of an ultrasound pulse

135
Q

How is axial resolution calculated

136
Q

How does axial resolution change with frequency

A

Improves with increasing frequency

137
Q

Is axial resolution depth dependent?

138
Q

In absorption:
Energy is transferred to the material it is travelling through as …..

139
Q

Name 2 disadvantages of ultrasound scanning

A

Can’t image through bone or gas
Very dependent on operator skill

140
Q

Describe the advantages of ultrasound imaging

A

No radiation exposure
Non-invasive
Fast
Inexpensive
Real time imaging
Can measure velocity e.g of blood
Cross sectional imaging

141
Q

How is the thickness of PZT crystals calculated

A

1/2 desired wavelength

142
Q

How does a higher Q value affect vibration?

A

Vibrated for a long time (have a light dampening effect) and pulse persists for a long time

143
Q

How does a lower Q value affect vibration?

A

Dampen the vibration quickly and the pulse lasts for a shorter time

144
Q

How many crystal elements does a phased transducer normally contain

A

64-128 crystals

145
Q

What is the doppler shift?

A

Change in frequency between the emitted and detected ultrasound wave

146
Q

How is TI calculated

A

TI = W / Wdeg

where W is the acoustic power of the beam at a specified point, and Wdeg is the power required for a maximum temperature rise of 1°C anywhere in the beam

147
Q

What is the restriction on exposure time for Thermal index (soft tissue) <0.7

A

No restriction
Generally considered safe TI<0.7

148
Q

How is frequency related to wavelength?

A

Inversely proportional to one another

149
Q

What is the speed of ultrasound in lung

150
Q

What is acoustic impedence a measure of?

A

How a tissue resists the passage of the US wave

151
Q

In ultrasound what does Z stand for

A

acoustic impedence

152
Q

What percentage of the lung-soft tissue boundary is reflected back?

A

Around 50%

153
Q

Why does refraction occur

A

Due to difference in ultrasound speed between two tissues

154
Q

How does attenuation of an ultrasound beam alter with US frequency?

A

Attenuation increases with US frequency

155
Q

What is the soft tissue attenuation coefficient at 1MHz and 5MHz

A

1MHz = 0.8 dB/cm= (17% reduction in intensity per cm)
If they quote between 0.5-1dB/cm in exam say true!!!!!!!

5MHz = 4dB/cm = (60% reduction in intensity per cm)

156
Q

What is the water/blood attenuation coefficient at 1MHz and 5MHz

A

1MHz = 0.15 dB/cm= (3% reduction in intensity per cm)
5MHz = 0.45dB/cm = (10% reduction in intensity per cm)

157
Q

What is the bone attenuation coefficient at 1MHz

A

1MHz = 15 dB/cm (97% reduction in intensity per cm)

158
Q

What is the lung attenuation coefficient?

A

> 30dB/cm/MHz

159
Q

How does the Piezoelectric effect result in ultrasound production?

A

Alternating voltage applied to the end faces of the transducer crystal causes it to vibrate

160
Q

How does the Piezoelectric effect result in Ultrasound detection?

A

Waves arriving at the crystal face cause it to vibrate and generate an alternating voltage

161
Q

How does the near field alter with higher frequencies?

A

Near field is narrower at higher frequencies

162
Q

How does the pulse echo determine the depth of a reflecting structure?

A

If velocity of pulse in tissue is known (or assumed)
Depth can be determined by

Depth of echo = return time x wave speed/2

163
Q

How many wavelengths is a typical ultrasound pulse?

A

Length is typically 2-3 wavelengths

164
Q

What is the typical duration of an ultrasound pulse

A

1 microsecond (us)

165
Q

What is the typical Pulse Repetition Frequency?

166
Q

What does the vertical axis in A mode graph represent?

A

Amplitude of each echo

167
Q

What does the horizontal axis of A mode graph represent?

168
Q

Which specialty often uses A Mode scanning?

A

Opthalmology

169
Q

What is image brightness proportional to in B mode?

A

Strength of echo

170
Q

Which probe has better lateral resolution? Curvilinear or Linear

171
Q

What does the horizontal axis on an A mode graph represent?

172
Q

Phased array transducer: how are the signals fired?

A

All segments are fired at the same time

173
Q

Continuous wave Doppler - how does it work?

A

Uses 2 separate transducers - 1 transmits and 1 receives
There is no depth information though
Often used in cardiac scanners or fetal heart scans

174
Q

What is power Doppler? Advantage?

A

Displays magnitude of blood flow, not direction
No aliasing, good for finding low flow rates

175
Q

How are segment fired in phased array transducers?

A

Fired simultaneously

176
Q

What is spectral Doppler? Why do we use it?

A

Detailed flow information is displayed as a spectrum
Allows velocity calculation analysis

177
Q

What 3 factors does lateral resolution depend on?

A

Beam width
Number of scan lines per frame
Frequency of US radiation

178
Q

What is the effect of more scan lines per frame on lateral resolution

A

More scan lines provide better resolution

179
Q

Is lateral resolution depth dependent?

180
Q

At what depth (area/zone) is the lateral resolution the best?

A

Focal distance

181
Q

What is axial resolution in B mode scans determined by?

A

Spatial pulse length

182
Q

What is spatial pulse length?

A

Finite length of ultrasound pulse

183
Q

How is spatial pulse length calculated?

A

Wavelength x no of waves per pulse

184
Q

How is axial resolution calculated?

185
Q

How does axial resolution change with increasing frequency?

A

Axial resolution improves with increasing frequency

186
Q

Refraction Artefact: what is this?

A

Position of the object is incorrectly placed because the pulses are supposed to travel straight but this artefact occurs when the beam is obliquely incident on a tissue boundary with different speeds of sound (causing refraction)

187
Q

Is axial resolution depth dependent?

188
Q

How does axial resolution vary with depth?

A

Axial resolution is not depth dependent

189
Q

What is the typical axial resolution at 3MHz?

190
Q

What is the typical axial resolution at 10MHs

191
Q

What is the effect on doppler frequency obtained is the beam is more aligned in the flow direction?

A

Higher doppler frequency

192
Q

How many transducers does CW doppler require?

A

2 transducers
1 transmits continuously
1 receives continuously

193
Q

Why is there no depth information in CW dopppler?

A

Because pulse-echo principle is not used

194
Q

How does doppler signal arise in continuous wave doppler?

A

Arises from where the beams overlap

195
Q

How is CW doppler detected?

A

Audible sound due to the Doppler shift being in the audible sound frequency range

196
Q

What are the advantages of CW doppler? (3)

A

Cheap
Easy to use
Sensitive to flow

197
Q

What are the disadvantages of CW doppler (3)?

A

Can’t measure velocity
Usually combines arterial and venous signal
Can’t determine depth

198
Q

What are the 3 types of pulse wave doppler?

A

Colour
Power
Spectral

199
Q

What type of scanning mode does PW combine with?

200
Q

What is the maximum detectable frequency shift equation for PW doppler?

201
Q

When does aliasing occur in PW doppler?

A

When freq shift >1/2 PRF

202
Q

What 2 things does colour flow doppler tell use about blood flow?

A

Direction and magnitude of blood flow

203
Q

What 1 thing does power doppler tell us about blood flow?

A

Magnitude of blood flow

204
Q

Does power doppler tell us about the direction of blood flow

205
Q

Do aliasing artifacts occur with power doppler?

206
Q

What is power doppler useful for?

A

Detecting low flow rates

207
Q

What does spectral doppler display?

A
  • A display of the spectrum of Doppler frequencies (flow velocities) on the vertical axis vs time on the horizontal axis.
  • Examines detailed flow information from one site only. Detailed analysis of distribution of flow
208
Q

What is the effect of a smaller SPL on axial resolution

A

Improved axial resolution

209
Q

What is the effect of a low Q value (i.e greater dampening on axial resolution)

A

Improved axial resolution
Shorter pulse

210
Q

What is the attenuation in dB/cm of water, blood, tissue, bone and lung

A

water = 0.0022
blood = 0.18
Average tissues = 0.7
Bone = 15
Lung = 40

211
Q

What is more attenuating?
Water or bone

212
Q

What is more attenuating bone or lung?

213
Q

What is the effect of increasing probe frequency or increasing transducer size on far field divergence?

A

Reduced far field divergence

214
Q

Describe vibration in materials with a low Q value

A

Dampen the vibration quickly
Pulse lasts for a shorter time

214
Q

How is MI calcualted

A

Peak rarefraction/ (square root of) ultrasound frequency

214
Q

How thick is the transducer matching layer (in terms of wavelength)

A

1/4 wavelength thick

215
Q

Describe vibration in materials with a high Q value

A

Vibrate for a long time (light dampening effect)

216
Q

What causes a longer PRF?

A

Deeper structures being imaged (the longer it takes to go and come back, the longer the listen phase of the pulse has to be)
More lines per frame

217
Q

How is PRF calculated

A

Frame rate x line per frame

218
Q

How is distance by beam calculated?

A

time x velocity x 0.5

219
Q

Hows is depth of view calculated

A

0.5 x sound velocity/ PRF

220
Q

How is near field calculated (as per radiology cafe)

A

D (squared) / (4 x wavelength)

D - diameter of transducer

221
Q

What is the effect of a larger diameter of transducer on near field distance

A

Increased near field distance

222
Q

What is the difference between phased and linear array

A

Phased array all segments are fired simultaneously

223
Q

What are phased arrays useful for imaging?

A

Cardiac imaging

224
Q

If the reflectors are separated by a distance >SPL/2 will they be resolved?

225
Q

If reflectors are separated by a distance <SPL/2 will they be resolved

A

No
The returning echoes will overlap and will appear as a single structure in the image

226
Q

Does CW doppler use high Q or low Q?

A

High Q
With no backing block
To produce a precise narrow frequency bandwith with high output

227
Q

What is the effect of focussing on lateral resolution

A

improved lateral resolution

228
Q

What is the effect of smaller aperture on lateral resolution

A

Improved lateral resolution

229
Q

What is the effect of increasing ultrasound intensity on echo amplitude at depth?

A

Increase echo amplitude at depth

230
Q

What size are the microbubbles for contrast used in USS?

A

1-10 micrometres

231
Q

Are microbubbles used as contrast in US destroyed by low or high ultrasound intensities?

232
Q

How is doppler frequency shift related to the cosine of the angle of incidence

A

Doppler frequency shift is DIRECTLY PROPORTIONAL TO THE COSINE OF THE ANGLE OF INCIDENCE

233
Q

How is doppler shift frequency related to the frequency of the incidental beam?

A

Directly proportional

234
Q

What is the difference between linear array and phased array

A

Linear array = small group adjacent elements are simultaneously activated to create a scan line. Sequential activation of adjacent groups of crystal created a series of scan lines across the transducer surface

Phased array =all segments are fired simultaneous

235
Q

How is near field calculated

A

d (squared) / 4 x wavelength

236
Q

What is the effect of a longer diameter transducer on near field distance

237
Q

What is the effect of increased frequency on near field distance

238
Q

What is the effect of a wider diameter transducer on far field divergence

A

Reduced far field divergence

239
Q

What is the effect of increased frequency on far field divergence

A

Reduced far field divergence

240
Q

What is the effect of Low Q on side lobes

A

less side lobes

241
Q

In what 2 ways can physical focusing be achieved

A
  1. Curved PZT element (greater curvature)
  2. Plastic or silicone acoustic lens
242
Q

How does electronic focussing work

A

Outermost pair is energised first then each adjacent pair in succession ending with the centre

243
Q

How does steering work

A

If small delays are introduced in rapid sequence along the transducer the pulses reinforce at an angles from the transducer face and destructively interfere in all other directions
Producing a steered plane wave

244
Q

Low Q means ______ dampening leading to a _______ pulse

A

Heavy dampening
Shorter pulse

245
Q

High Q means __________- dampening leading to a _________pulse

A

Lighter dampening
Longer pulse

246
Q

What is the effect of Low Q/Shorter pulse on bandwidth?

A

Larger bandwidth

247
Q

What is the effect of High Q/Longer pulse on bandwidth

A

Smaller bandwidth

248
Q

What two factors can increase axial resolution

A

Higher frequency (shorter wavelength)
Low Q (greater dampening, shorter pulse)

Overall want a smaller SPL

249
Q

Does lateral resolution = beam width

250
Q

Typical wavelength of ultrasound in abdominal scanning (in soft tissue) is 0.5mm T or F

FRCR Part 1 Physics (20 decks)

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