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PHYS855: Medical Imaging > Ultrasound > Flashcards

Ultrasound Flashcards

(71 cards)

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1
Q

What is ultrasound?

A

A sound wave at a frequency above the threshold of human hearing (ultrasonic), used to caputure images.

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2
Q

What is the frequency of ultrasound?

A

> 20 kHz

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3
Q

What type of waveform is sound?

A

A compression/longitudinal waveform

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4
Q

True or false: sound propagates through a vacuum.

A

FALSE: it can only propagate through air or another medium.

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5
Q

How are sound waves formed?

A

They are generated by the vibration of the matter (particles) that they are travelling through.

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6
Q

The propagation of sound waves depends on the ______ and _____________ of the medium they are travelling through.

A

Density
Compressibility

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7
Q

Define frequency (f)

A

The number of cycles of a continuous wave occuring in 1 second, where 1 Hz is one cycle in one second.

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8
Q

Define time period (T)

A

The reciprocal of frequency. The time taken for one cycle of a continuous wave.

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9
Q

Give the equation for time period

A

T = time period
f = frequency

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10
Q

Define wavelength

A

The distance travelled by a wave in one period.

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11
Q

Give the equation for wavelength

A

λ = wavelength
c = speed (of light)
f = frequency

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12
Q

The speed of sound is a ________ within any specific medium.

A

Constant

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13
Q

What happens to frequency, speed, and wavelength as a sound wave travels from one medium to another?

A
  • Frequency stays constant
  • Speed may change
  • Wavelength may change
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14
Q

What is the typical wavelength range of ultrasound used in clinical applications?

A

0.1 - 0.5 mm

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15
Q

What is the typical frequency range of ultrasound used in clinical applications?

A

15 - 3 MHz

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16
Q

The speed of sound in a material is determined by its _______ and _____________.

A

Density
Compressibility

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17
Q

Give the equation for the speed of sound in a material

A

c = speed of sound
K_s = Bulk modulus
ρ = density

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18
Q

Define acoustic pressure (p)

A

The contribution to pressure at a chosen point, solely due to the passage of a sound wave (it is excess to any existing pressure).

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19
Q

Define acoustic impedance (Z)

A

A physical property of tissue that describes how much resistance an ultrasound beam encounters as it passes through tissue.

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20
Q

What does acoustic impedance depend on?

A
  • Density of the tissue
  • Speed of the sound wave
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21
Q

As the density of tissue ________, acoustic impedance increases.

A

Increases

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22
Q

What happens if the difference between the acoustic impedance of two tissue types is large?

A

Most of the sound waves are reflected (the higher the difference, the higher the level of reflection)

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23
Q

Give the equation that relates sound wave velocity and acoustic pressure

A

z = characteristic acoustic impedance
p = acoustic pressure
v = velocity

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24
Q

Give the equation for characteristic acoustic impedance

A

z = characteristic acoustic impedance
ρ = material density
c = speed of sound

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25
Does acoustic impedance depend on wave frequency?
NO
26
What are the 4 things that can happen to a sound wave as it passes through a boundary?
1) Attenuation 2) Scattering 3) Reflection 4) Refraction
27
When a sound wave meets an interference where z changes, a _________ wave is produced with carries with it a fraction of the _____ of the original wave.
Reflected Power
28
What does the strength of a reflected wave depend on?
Differences in the acoustic impedance of the two materials.
29
Give the equation for the reflected fraction of a sound wave
Z_1 = initial material Z_2 = boundary material
30
State the 3 components of an ultrasound scanner
1) Transducer 2) Mineral oil gel 3) Monitor
31
Why is mineral oil gel used in ultrasound imaging?
It is applied between the skin and the transducer to overcome the very high reflection coefficient of air.
32
What is the purpose of the transducer (probe) in ultrasound scanning?
It acts as a loudspeaker and microphone: it generates the sound waves and receives the reflected signal.
33
What is an ultrasound transducer made of?
300 piezoelectric crystals arranged in a row that emit and receive an ultrasound beam in rapid succession.
34
How do transducers produce and receive ultrasound signals?
1) When an electric current is applied across a piezoelectric crystal it rapidly changes shape 2) These vibrations cause outward travelling sound waves which cause a change in pressure 3) Waves penetrate into the body and reflect from the tissue 4) When the reflected wave strikes the crystal it applies pressure and generates a voltage
35
How do transducers prevent back-reflections of sound waves?
They contain an absorbing substance that eliminates back reflections from the probe itself.
36
How do transducers focus emitted waves?
They contain an acoustic lens.
37
Why to transducers come in different shapes/sizes?
To govern the field of view.
38
In order to generate an image, which wave characteristics must an ultrasound scanner determine?
- How long it took each echo to be received (earlier echoes are from nearer reflecting structures) - How strong each echo was (as this relates to the difference in acoustic impedance between reflecting structures)
39
What determines the penetration depth of ultrasound waves?
The frequency of emitted sound waves
40
What determines the resolution of an ultrasound image?
The frequency of emitted sound waves
41
Describe how the brightness of each pixel in an ultrasound image is selected.
The ultrasound system detects the amplitude and time taken to receive each sound wave and assigns this to a greyscale brightness value.
42
Define refraction
The change in direction of a wave due to a change in propagation speed across a boundary (caused by differences in density or elasticity of the material).
43
Give the equation for Snell's law of refraction
44
Why does refraction cause ultrasound artefacts?
Because it is assumed that all pulsed waves and returning echoes travel along a straight line.
45
Why does scattering occur in ultrasound imaging?
It occurs when an ultrasound wave strikes a small/rough structure with a difference acoustic impedance to the surrounding tissue and a wavelength less than that of the incident ultrasound wave.
46
What is a diffuse reflector?
A structure within the body that causes scattering of ultrasound waves
47
What is the result of scattering in ultrasound imaging?
Multiple echoes propagate from diffuse reflector structures and interact with one another to cause contructive and destructive interference. This increases the noise of the image.
48
Define attenuation
The loss of energy (of a wave) with distance travelled.
49
What are the 2 main causes of attenuation?
- Scattering - Tissue absorption of the incident beam
50
Intensity loss per unit distance (_____) is the ___________ __________.
dB/cm Attenuation coefficient
51
The attenuation coefficient is direction proportional to and increases with ________. It also depends on the _______ that the wave is travelling in.
Frequency Medium
52
State the 4 main uses of ultrasound
1) Medical imaging 2) Physiotherapy 3) Lithotripsy 4) Cleaning
53
Diagnostic ultrasound involves ___ power levels and ____ frequencies to image internally.
Low High
54
Which internal structures are typically imaged with a 7-18 MHz ultrasound beam?
- Neonatal brains - Muscles - Tendons
55
Which internal structures are typically imaged with a 1-6 MHz ultrasound beam? Why?
- Liver - Kidneys These frequencies are deeper penetrating
56
How is ultrasound used in physiotherapy?
Physiotherapists use therapeutic ultrasound with high power levels and low frequencies to heat up tissue and encourage healing of an injured part of the body. No image is produced in this technique.
57
What is lithotripsy?
A technique that uses focused ultrasonic sound waves to break large stones in the kidneys and liver into smaller stones that can pass through the urinary system.
58
What frequency of ultrasound is used for lithotripsy?
23-25 kHz
59
How is ultrasound used for cleaning?
Ultrasound agitates cleaning fluid, producing cavitation bubbles to clean medical equipment.
60
Describe how A-mode ultrasound works
This is the simplest of all ultrasound modes. A single transducer scans a line through the body with returning echoes plotted as a function of depth.
60
What are the 3 main ultrasound modes for imaging?
A-mode (amplitude) B-mode (brightness) M-mode (motion)
61
Describe how B-mode ultrasound works
A linear array of transducers simultaneously scan a plane through the body that can be viewed as a 2D image.
62
Describe how M-mode ultrasound works
A rapid sequence of B-mode scans are delivered, producing a sequence of images to display a range of motion.
63
Which tissues are not suitable for ultrasound scanning?
- High density materials (e.g. bone) - Gas-filled areas (e.g. lungs)
64
Which tissues/processes are well suited for ultrasound scanning?
- Imaging soft tissues and fluid filled spaces - Imaging bodily functions (e.g. breathing/urination/movement) - Imaging organ function (e.g. kidneys/heart/blood vessels) - Guiding fine needles - Guiding tissue biopsy
65
What is the doppler effect in ultrasound?
The change in frequency (or wavelength) of a wave as perceived by an observer due to relative motion between the source of the wave and the observer. If a particle or tissue interface is moving towards the source of an ultrasound wave, the reflected wave has a frequency that is higher than that of the incident wave, conversely if it is moving away from the source then the frequency is reduced.
66
Where is the doppler effect used in ultrasound?
To give informtion about blood flow due to echoes scattered off blood cells.
67
What is doppler shift?
The difference in frequency between the frequency of a transmitted wave and the frequency received back at the probe.
68
The magnitude of doppler shift is _________ ____________ to the speed of the particle.
Directly proportional
69
Give the equation for doppler shift
f_D = doppler shift v = velocity of target θ = angle between ultrasound beam and target f_T = frequency of transmitted wave c = speed of sound
70
What are the pros and cons of ultrasound? (state 5 of each)
+ Non-invasive + Portable + Inexpensive + Non-ionising and no known damage to tissue + Fast images are generated in real time - Limited field of view - Required a trained operator - Difficulty in imaging structures obscured by bone - Difficulty in imaging gas filled structures - Increase depth requires a lower frequency and lower resolution

PHYS855: Medical Imaging (5 decks)

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