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Vascular 1 > Essentials of ultrasound physics > Flashcards

Essentials of ultrasound physics Flashcards

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1
Q

What is acoustic pressure and its units?

A

Quantification of the strength of the wave
- units = Pascals

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2
Q

What determines speed of sound through a material?

A

c = Sqrt(K / p)

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3
Q

What is the difference between characteristic impedance and acoustic impedance?

A

They are practically the same

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4
Q

What are the units for acoustic impedance?

A

Pressure per velocity per area
(or rayls)

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5
Q

What is acoustic impedance and its equation?

A

The acoustic pressure divided by the resultant particle velocity
Z = pc

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6
Q

What is Snell’s law?

A

The law for angle of refraction of an ultrasound beam

sin(theta2) / sin(theta1) = c2 / c1

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7
Q

What two conditions are required for refraction?

A
  1. Must be an incident angle between transducer and interface
  2. Must be 2 materials with different c
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8
Q

What is the difference between specular and diffuse reflection?

A

Diffuse = all directions
Specular = one direction

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9
Q

What are Rayleigh scatterers?

A

Scatterers that are much smaller than the wavelength of sound
- scatter sound in all directions

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10
Q

What is the 3dB rule?

A

Whenever intensity changes by 3dB this corresponds to a doubling of the intensity

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11
Q

What are the two sources of attenuation in the body?

A
  1. Reflection and scatter at interfaces
  2. Absorption (conversion to heat)
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12
Q

How is attenuation of ultrasound in tissue measured?

A

dB / cm

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13
Q

What is the purpose of the backing layer?

A

Allows shorter pulse duration

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14
Q

What is the equation for pulse duration and frequency?

A

PD = n / f
n = No. of cycles
f = frequency

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15
Q

What are both the near field length and the far field spread dependent on?

A

The transducer aperture

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16
Q

What is the interference of 2 waves 180 degrees apart?

A

Destructive - will cancel if they are the same sine wave

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17
Q

What are ultrasound transducer elements made of?

A

PZT (lead zirconate titanate)

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18
Q

How are PZT elements polarised?

A

They are heated above 365 degrees to allow particles to move.
A voltage is then applied across them and they are cooled with the voltage applied

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19
Q

What is the main determinant of the resonant frequency of a PZT crystal?

A

The thickness of the element

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20
Q

What is the purpose of backing material?

A

To provide damping needed for short pulses

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21
Q

What are Huygen outlets?

A

An ultrasound transducer with a collection of point sources that fire coherently to produce a beam

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22
Q

What is apodization?

A

Signal strength varies at different points on the transducer face - weaker at sides to reduce side lobe interference

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23
Q

Define focal zone?

A

The region over which the beam width is less than 2x the beam width at the focus

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24
Q

What are the two main advantages of using arrays?

A
  1. Electronic beam steering
  2. Electronic focusing and beam forming
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25
How is electronic focusing achieved?
By exciting elements at different times - introducing delay
26
What determines slice thickness for linear, curved or phased array transducers?
The fixed focal length lens attached to the entire array
27
What is the duty factor?
The fraction of time the transducer actively transmits sound
28
What is demondulation?
When the receiver converts the amplified raw echo signal (pulses) into a single pulse spike
29
What is persistence?
Signals for each pixel location are combined with previous signals from the same location
30
What is the Doppler frequency?
fD = (2 v fT cos(theta)) / c
31
What is a possible explanation of ISB?
Differences in Cos(theta) due to the difference in angle from the middle and edges of the transducer
32
What are the 4 main pulsed doppler controls?
1. Range gate position 2. Gate or sample volume size 3. Pulse duration 4. Flow angle cursor
33
How is a digital signal converted to a spectral waveform?
Fast Fourier Transform
34
What can cause the filling of the spectral window?
1. Turbulence 2. Spectral broadening due to a wider range of doppler frequencies from the sample volume (wider range gate)
35
How do you calculate pulsatility index?
PI = (PSV - EDV) / average
36
How do you calculate resistive index?
RI = (max - min) / max
37
When does aliasing occur?
When the PRF < two times the maximum frequency
38
What is the Nyquist limit?
The condition when PRF = 2fD - it defines the minimum sample rate for a signal before aliasing will occur
39
What is the equation for maximum velocity detectable with pulsed Doppler?
Vmax = ( c * PRFmax) / 4f - as frequency is on the bottom, lower frequencies will allow a higher vmax to be detected
40
What does high PRF mode do?
Creates multiple range gates
41
What is PRF the same as?
Scale
42
What are colour packets?
The multiple pulse-echo signals that are required for colour flow imaging
43
What is colour hue?
The attribute of colour that allows it to be classified as red, yellow, green, blue or an intermediate - hue is associated with wavelength of light
44
What is a pure hue?
A completely saturated colour with only 1 wavelength associated with it
45
What causes a less saturated colour?
The presence of more white light - white light is composed of many wavelengths
46
What does the colour of power mode doppler depend on?
The strength of Doppler signal, number of reflecting moving targets
47
What are advantages of Power doppler?
1. It is less impacted by angular effects 2. It has more sensitivity than standard colour imaging 3. If aliasing is present it does not affect energy mode display
48
What are disadvantages of power doppler?
1. Image build up and image rates tend to be lower than colour 2. There are greater amounts of flash artefact
49
What is a specular reflector?
Reflection at a smooth surface
50
What are diffuse reflectors?
Reflectors that scatter sound in all directions
51
What are 3 main assumptions of an ultrasound scanner?
1. Reflectors giving rise to echoes lie along the beam axis 2. The speed of sound is constant 3. The echo strength indicates echogenicity
52
When can spectral mirroring occur?
1. When gain is too high 2. When Doppler beam approaches 90 degrees to the vessel axis
53
How can beam width artefacts occur?
1. Small objects are broadened - appear as lines 2. Smearing of echo information
54
In which direction is beam width widest?
Perpendicular to the image plane (in the slice thickness direction) has greater thickness than the image plane
55
What is acoustic power?
The rate at which energy is transmitted from the transducer to the medium being scanned
56
What is a hydrophone?
A device used to measure acoustic pressure and intensity
57
How does intensity relate to pressure?
Intensity is proportional to square of pressure
58
How do time average and pulse average intensities compare?
Pulse average are often much higher (1000x) than time average
59
What is thermal index?
TI is the ratio of acoustic power produced by the transducer to the power required to raise the temperature in tissue by 1 degree.
60
What does a TI value of 1 mean?
When the probe is stationary, it has the potential to raise tissue temperature by 1 degree
61
What is cavitation?
The activity of tiny gas bubbles in tissue in the presence of ultrasound waves - ultrasound can generate tiny bubbles from dissolved gas in fluid
62
What is mechanical index?
The likelihood of a transducer causing cavitation
63
What is MI proportional to?
The peak rarefactional pressure
64
What is MI inversely proportional to?
The sqrt (f) - if frequency increases, MI decreases
65
What are the advantages of ultrasound safety indices?
1. Standardization of output specification information between manufacturers 2. Presentation of output quantities relevant to potential bioeffects from ultrasound 3. Information is available for users to implement ALARA
66
When can MI and TI be underestimated?
When a large fraction of the path between transducer and isonified region contains fluid e.g. bladder or amniotic fluid.
67
What is the biological effect concerned with TI?
Heating, caused by absorption of energy from the ultrasound beam
68
What are the two types of cavitation?
1. Stable cavitation - creation of bubbles that oscillate with the ultrasound beam 2. Transient (or inertial) cavitation - oscillations grow so strong that bubbles collapse - producing very intense, localised effects
69
What are non-cavitation mechanical effects?
Bioeffects caused by particle displacements

Vascular 1 (5 decks)

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