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RAD413 > ultrasound principle > Flashcards

ultrasound principle Flashcards

1
Q

what does ultrasound use for detection

A

soundwaves

  • high frequency (inaduible to human)
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2
Q

what frequency range are sound waves

A

about 20 kHz

ultrasound frequency begins at 20kHz

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3
Q

know that sound waves can be formed into a narrow bean and focussed

they can be reflected, refracted, differacted, scattered and absorbed

A
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4
Q

what type of wave is a sound wave and what structure/propergation of particle energy does it follow

A

mechnaical wave (not electromagnetic)

  • longitudinal wave
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5
Q

what requirement is needed to be met for a sound wave to be able to travel

A

requires medium to travel through

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6
Q

what is the units US is measured in

A

Megahertz

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7
Q

are sound waves ionising

A

no

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8
Q

define a longitudinal wave

A
  • particle motion parallel to the direction fo wave propagation
  • particle motion is pushed back and further in direction of travel resulting in bands of high and low pressure
  • pressure wave propagates forward
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9
Q

how do you describe the particle displacement in a longitudinal wave

A
  • rarefaction (spaced)
  • compressed (bunched)
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10
Q

why is a medium required for sound waves to be able to travel

A

needs a medium in order to compress and expand

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11
Q

areas of compression have higher pressure and areas of rarefracgion have lower pressure

A
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12
Q

know what even though wave propagates to the right, the individual particle is actually oscillating about a point

A
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13
Q

define frequency

A
  • number of cycles per second
  • Hz
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14
Q

what frequency sound wave is used in medical ultrasound

A

2 - 20Mhz or higher

at 2Mhz is where the ultrasound begins to be able to penetrate into the human body

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15
Q

define wavelength + units

A
  • distance between consecutive wave crests

i.e between compression to compression

  • meters
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16
Q

what is the wavelength used in medical imaging

A

0.1mm (for 15 Mhz)

0.5mm (for 3MHz)

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17
Q

what is the wave equation

A

c = f x wavelength what

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18
Q

is the speed of sound in tissue

A

similar to sound passing through sea water as tissue contains a lot of water

  • 1540 meters per second
  • higher in bone
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19
Q

what happens to the wavelength when you change from a 5MHz to 10Mhz probe

A

follow the wave equation so

  • double frequency
  • with same velocity
  • requires you to half the wavelenght
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20
Q

what is acoustic pressure

A

the amount of acoustic energy per time unit.

-Acoustic power shows the amplitude of the pulse pressure of the ultrasound beam.

  • sound wave pressure is the excess pressure, follows a sinusoidal curve and can be positive or negative
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21
Q

what machine control directly affects acoustic pressure

A

output

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22
Q

what effect is used to produce sound waves

A
  • piezoelectric effect
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23
Q

what material is used in the piezoelectric effect

A

lead zirconate titanate
(type of ceramic)

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24
Q

explain how the piezoelectric effect works

A
  • pressure is applied across crystal
  • generating a voltage (released by crystal) proportional to pressure applied
  • applying alternating voltage causes crystals to expand and contract with same frequency
  • releasing sound waves of a certain frequency
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25
Q

the piezoelectric effect in US can act as a transducer, what is this

A

can transmit and recieve

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26
Q

the transducer will have a natural vibrational frequency called the resonance frequency, what is htsi

A

the natural frequency where a medium vibrates at the highest amplitude

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27
Q

what is the resonance frequency of a transducer dependant on

A

crystal thickness (PZT place)

speed of sound in crystals

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28
Q

in piezoelectric effect, when would resonance occur

A

when crystal thickness = half wavelength

  • resulting in constructive interference between waves produced from each face
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29
Q

how does crystal thickness and frequency relate

A

the thinner the crystal, the higher the frequency

as the thinner the crystal, the smaller the wavelength which results in higher frequency

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30
Q

what is aimed for in order to get an efficient generation of US waves

A
  • want to match the electrical driving frequency to the natural resonance of the crystal to get efficient generation of US waves
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31
Q

a resonating transducer will generate continuous ultrasound waves, to create short pulses of US, what must be done

A

damping

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32
Q

what is damping

A

Decrease or decay of ultrasonic wave amplitude with respect to time or distance.

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33
Q

what is the quality / Q factor of the transducer

A
  • the more ‘damped’ a resonator is, the ‘sharpness’ of the resonant frequency decreases
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34
Q

how does Q factor, bandwidth and frequency relate

A

low Q factor (less damping) = broad broadband width = large range of frequencies

high Q factor (more damping) = short bandwidth, more specific frequency

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35
Q

know that high Q factor produces a pure note and the doppler response to only that note

A
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36
Q

why is low Q factor more preferred

A

low Q factor = low spatial pulse time = high bandwidth = high frequency = low wavelength overall better image quality

(shorter time is more preferred as well)

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37
Q

what are the 2 operational modes in ultrasound

A
  • continuous
  • pulsed
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38
Q

where is the continuous operational mode used in US

A

therapy deivces
non imaging doppler mode

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39
Q

where is the pulsed operational mode used in US

A

imaging
imaging dopple

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40
Q

what are the 3 types of broad bandwidth probes

A
  • curvilinear
  • linear
  • sector
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41
Q

what is the frequency range for the 3 different types of probes

A

curvilinear = 1.5 - 6 MHz

linear = 4 - 11 MHz

sector probe = 1.8 - 5.2 MHz

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42
Q

know what the 3 different probes look like

A
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43
Q

find image of transducer and label their components and function

A
  • PZT crystal
  • front and rear electrodes to apply voltage
  • housing and RF shielding
  • backing material (damping)
  • matching layer
  • lens
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44
Q

what is the role of the front and rear electrode in a transducer

A
  • apply voltage to PZT plate
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45
Q

role of backing material in transducer and how does it work

A

damping
- absorbs sound energy

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46
Q

role of matching layer in transducer

A

maximises transmission of US

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47
Q

role of lens in transducer

A

beam shaping and focus

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48
Q

in practice, the transducer is sub-divided into what 4 elements

A
  1. acoustic lens
  2. acoustic matching layer
  3. piezoelectric element
  4. backing material
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49
Q

how many leads are there connecting to the rear electrodes

A

128

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50
Q

describe the field of the beam from an unfocussed beam

A
  • near to filed is parallel beam, far field it diverges
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51
Q

describe the field of the beam from a focussed beam

A

electronically controlled and uses focussing lend - divergence only occurs beyond the focal zone

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52
Q

what is another word used to describe the near and far field of beam

A

near = fresnel zone
far = Fraunhofer zone

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53
Q

divergent field is not useful for imaging

A
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54
Q

describe the pressure distribution in the near and far fild

A

near field = complex pressure distribution

far field = uniform pressure distribution

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55
Q

describe how the crystal elements of the PZT plate (in electronic array) can be fired to form a good beam shape or longer aperture

A
  • each crystal element can be fired independently but
  • fired in groups will achieve larger aperture/ good beam shape
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56
Q

know that the beam position can be moved along by width of single crystal so there is better line density

A
57
Q

what is the number range of crystal elements in the electronic array

A

64-200

58
Q

what is scan plane focussing

A
  • appropriate delays to the pulses transmitted from each transducer array element
  • so that the transmitted energy adds constructively a desired focal point to form a transmit beam.
59
Q

what is elevation plane focus

A

The transmitted beams are focused both in elevation plane and in scan plane by energizing different apertures of the transducer at different focal depths

  • fixed focus by lens
60
Q

overall, what is electronic focusin

A
  • adding time delay to firing of each of the piezoelectric elements
  • pulses interact constructively to form a pulse which converges at the focal point
61
Q

number of active crystals also altered to change beam characteristics

A
62
Q

what is beam steering

A

altering the angle of the ultrasound beam with respect to the transducer without moving the probe.

63
Q

define resolution

A

minimum spacing between 2 reflectors that allow them to be distinguished on display

64
Q

define axial resolution (resolution along beam axis) of US and what would its value be

A
  • ability to separate structures parallel to the ultrasound beam

-axial resolution is approx half the pulse elngth

e.g pulse is approx 3/4 wavelengths so axial resolution is around 1.5-2 wavelenghts

65
Q

why is a long pulse bad

A

echoes overlap and cannot be resolved

66
Q

what is lateral resolution

A
  • ability to distingue structures along the beam long axis (perpendicular to the beam)
67
Q

brightness profile of laterally spaced targets begins to merge when their spacing is less than the beamwidth

A

w

68
Q

what is the value at which structures in the axial or longitudinal can be resolved

A

if beam pulsing/ pacing is about half the beam width

69
Q

what is adpodisation

A

reducing side lobes in an ultrasound focusing system

(side lobes are an unwanted component of the US beam)

70
Q

describe the difference in the beam shape of a larger and smaller diameter transducer

A

small =
- narrow beam in near field (short)
- widely diverting in far field

large =
- wider beam in near field (longer)
- less divergent in far field

71
Q

describe the difference in beam shape of a higher and lower frequency transducer

A

low freq =
- shorter near field
- more divergence

high freq =
- longer near field
- less divergence

72
Q

what is speed of sound in terms of US

A

how fast the pressure waves travel away from the sound source

73
Q

what 2 components of the medium affect the speed of wave travel

A

stiffness
density

74
Q

how does stiffness and density correlate to wave speed

A

increase stiffness = increased speed

increase density = decrease in speed

75
Q

know that low masses can be accelerated quickly by stiff springs

a stiffer spring will transfer energy more efficient than a less stiff one

less dense masses will be more easily accelerated than heavier ones

A
76
Q

define acoustic impedance

A
  • It describes how much resistance an ultrasound beam encounters as it passes through a tissue
  • gear that the medium operates in to transmit the ultrasound energy
77
Q

what 2 things is acoustic impedance dependant on

A

stiffness and density

78
Q

what 4 things does acoustic impedance determine what happens at interface between 2 mediums

A

reflection
refraction
scattering
absorption

79
Q

how does acoustic impedance and reflection relate

A

the greater the mismatch in acoustic impedance, the greater the reflection

80
Q

what is snells law of refraction

A

a relationship between the path taken by a ray of light in crossing the boundary or surface of separation between two contacting substances and the refractive index of each.

= n1 sin 1 = n2 sin 2

81
Q

what is law of reflection

A

angle of incidence = angle of reflection

82
Q

when does scattering occur in ultrasound

A
  • when dimensions of the boundary are small compared with the wavelength
  • strongly dependant on freq
83
Q

what causes a speckle pattern to be seen on US

A

random interference pattern produced by scattering

84
Q

how does absorption occur in US

A
  • sound energy converted to heat energy by friction in medium
  • motion of particles cant keep up with frequency of wave, go out of step and lose energy
85
Q

how does sound frequency and absorption rate of particles in US relate

A

increasing sound frequency = increasing absorption

86
Q

other than friction/frequency speed what other thing affects absorption

A

higher viscosity of tissue leads to higher absoprtion

87
Q

what is the rule of thumb/ approx value of US beam penetration

A

100 wavelengths

88
Q

define attenuation in US

A

loss of energy from ultrasound beam

89
Q

what 3 things is attenuation dependent on

A

type of tissue
ultrasound freq
ultrasound beam shape

90
Q

what is the attenuation coefficient in soft tissue

A

0.7dB /cm/MHz

91
Q

how does attenuation and frequency link to penetration

A

In the low-frequency range, the attenuation coefficient increases approximately linearly with the increasing of frequency

  • resulting in poorer penetration
92
Q

how does scattering and frequency link to sensitivity

A

scattering increases with frequency which overall increases the sensitivity

93
Q

what is the pulse-echo technique and how does it work

A
  • uses a single transducer as both the transmitter and receiver
  • at tissue interface, some pulse is transmitted and some is reflected (echo)
  • echo signal detected by transducer
  • echoes returning first are from structures close to probe vice versa
94
Q

type of interface determines echo amplitudwe

A
95
Q

what equation is used to calculate depth (which is also the speed taken for echo return)

A

d = c x (t/2)

96
Q

describe the full process of forming an ultrasound image

A
  • pulse sent out by transducer
  • echo train detected at transcode = analogue signal
  • Gain and TGC applied
  • amplitude digitised and entered into memory location related to origin of echo
  • interpolation required to fill in the gas
97
Q

what is TGC

A

Time gain compensation (TGC)

  • additional feature that reduces impact of wave attenuation by tissues through increased intensity of the received signal in proportion to the depth.
98
Q

what is interpolation

A

a method of constructing new simulated data points to fill in the gaps.

99
Q

what is spatial.frequency compound imaging

A

images obtained using multiple beam angles or multiple frequency (and summed up)

100
Q

what is the benefit of compound imaging

A
  • random pattern change with true echos remaining the same
  • average echos from differenct directions smooth out speckle apperance
  • improved identification of boundaries
  • reduced effect of shadowing
101
Q

what is THI

A

tissue harmonic imaging

  • based on the nonlinear interaction of the emitted ultrasound with the tissues, resulting in higher frequencies that return to the probe.
  • waveform distorts creating harmonic
102
Q

how does distortion and depth relate

A

there increased distortion with increasing depth

103
Q

what type of range is tissue harmonic image more effective in

A

mid to far range

104
Q

what type of filtering is used in tissue harmonic imaging and why

A
  • narrow bandwidth to decrease spatial resolution
105
Q

what is the phase inversion technique used in THI

A
  • 2 pulses of opposite phases sent
  • returning signals are added
  • linear response canceled out (removing original frequency)
  • non linear response (harmonics) is left
106
Q

know that THI has reduced side lobes

A
107
Q

Describe the beam width in THI

A

narrower effective beam width

108
Q

when phase inversion is used in THI, what remains maintained

A

axial resolution maintained if phase inversion used

109
Q

describe the overall apperance of THI after all its factors accounted

A

images appear sharper, less noise, higher contrast resolution

110
Q

what are some assumptions made in echo mapping

A
  • US travels in straight line
  • speed of sound is constant
  • US beam is infinitely naoorw
  • echo amplitude relates directly to a structure
  • echoes received originate from most recently transmitted pulse
111
Q

know that echoes that appear oj image that do not correspond in location or intensity to actual interfaces in patient are artefacts

A
112
Q

what are the 2 categories of US artefacts

A

location (path or speed)
attenuation

113
Q

what is a side lobe artefact/ what is seen on image

A
  • high reflective object (black eclipse) adjacent to beam position
  • can reflect secondary lobes, falsely displaying those echoes at beam position
  • low energy off-axis beams immediately adjacent to primary beam
114
Q

what is a reverberation artefact / what does it look like on image

A

generated when echo from strongly reflecting interface parallel to transducer is partially reflected from transducer face back to interface

(partial echo reflected back from transducer to interface)

  • generates second echo, which is displayed at twice depth of interface
  • often see lines or comet tail
115
Q

how can we reduce the reverberation artefact

A

using tissue harmonic imaging

decreasing gain

changing AOI

using multiple windows

116
Q

where is reverberation artefact most commonly found scannign

A

liquid filled areas e.g bladder

117
Q

what are some structures that can be seen due to reverberation on image

A
  • comet tail
  • ring down (looks like a long extended version of comet tail)
118
Q

what is a mirror artefact

A

occur when the transmitted pulse and returning echo reflect off of a highly reflective interface (an acoustic mirror) and change direction before returning to the transducer

119
Q

what is the affect of a mirror artefact on an image / what do u see

A

structure images to be deeper than it actually is

  • see a curved bright line
120
Q

what will happen to an image if there is refraction

A

if beam is refracted at angled surface, the display places object where it its expected not at the true location

121
Q

why is it important to localise an AOI in more than one plane

A

to aid in location of objects true position as refraction can change where object is found on imaging

122
Q

what can be seen on image as a result of refraction and what is this

A

ghosting

  • object may appear in duplicate or triplicate due to refraction
123
Q

compared to the true object, what does the duplicated structures look like in ghosting artefact

A

looks grey

124
Q

refraction can also cause edge shadowing, what is this

A

edge shadowing (defocusing) at edge of structure with different speed of sound than surrounding medium

  • refraction greatest at edges, resulting in shadowing due to decreased beam intensity to deeper tissue
125
Q

what does edge shadowing artefact look like on image

A
  • extend shadow from edge of object
126
Q

how does shadowing relate to frequency

A

shadowing increases with increased frequency

(and THI)

127
Q

what can be done to reduce shadowing

A

reduce the echo strength distal to highly attenuating or reflective object

128
Q

what is a boundary distortion / what causes it

A
  • speed of sound of object is less than assumed during calculation of depth
  • longer time for transmitted and returning echoes = mis-registration of target

boundary distortion artefact occurring deep to a structure with a lower speed of sound than surround tissue

129
Q

what is beam width artefacrt

A
  • US beam assumed to be linear but actual beam narrows at focal point then widens distal to focal one
130
Q

what do you usually see on beam width artefact

A

echos within structure

  • small grey echo within black structure
131
Q

what is the doppler effect

A
  • difference in frequency of the transmitted and received wave
  • moving blood causes change in US frequency and can detect and calculate velocity

(racing car sound analogy)

132
Q

change in frequency = doppler effect

A
133
Q

how does Doppler effect relate to velocity between transmitter and recieveer

A

Doppler effect is PROPORTIONAL to relative velocity between transmitter and receiver

134
Q

what 3 ways is doppler information displayed graphically

A
  • using spectral doppler
  • colour doppler (directional) (flow)
  • power doppler (non directional) (volume)
135
Q

what is a duplex scan

A

image + doppler

  • allows angle of incidence to be measure and velocity to be estimated
136
Q

what is power doppler

A

displays amplitude of doppler signal rather than the detected frequency

  • relates to blood volume and tissue perfusion
137
Q

what are some adv and dis of power doppler

A

adv
- not angle dependant
- no aliasing
- better vessel wall definition

dis
- no velocity info
- non directional
- low temporal resolution

138
Q

longer pulse gives more accurate measurement

A
139
Q
A

RAD413 (24 decks)

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