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Ultrasound physics book > Intro > Flashcards

Intro Flashcards

1
Q

How many shades of grey do most ultrasound systems have?

A

64

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2
Q

What is axial resolution?

A

Resolution along the length of the ultrasound beam

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3
Q

What does axial resolution depend on?

A

The pulse length
(this is equal to the number of cycles * wavelength)

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4
Q

What is the relationship between mechanical index (MI) and frequency ?

A

MI is inversely proportional to the square root of frequency

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5
Q

What are diffuse reflectors?

A

Reflectors that scatter the sound in all directions

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6
Q

What is gain?

A

The degree of amplification of the returning echo

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7
Q

What is PRF?

A

Pulse repetition frequency
- the speed at which echoes are transmitted and recieved

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8
Q

What is the piezoelectric effect?

A

The ability of a material to generate an electrical current in response to an applied pressure

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9
Q

What does acoustic impedance depend on?

A

How tightly or loosely packed a material’s particles are. i.e. a material with tightly packed particles will have a high acoustic impedance.
- acoustic impedance measures the resistance a material has to sound wave propagation through it

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10
Q

What is the equation for acoustic impedance? (Z)

A

Z = excess pressure / particle velocity

  • i.e. the higher the acoustic impedance, the faster sound will travel through the medium
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11
Q

What are the two regions of the ultrasound beam?

A
  1. A near field (cylindrical shape)
  2. A far field (cone shape)
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12
Q

How does moving a particle from the periphery to the centre of the ultrasound beam impact its appearance

A

It increases its brightness

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13
Q

What are side lobes?

A

Some of the energy from the transducer radiates at various angles to the transducer face - known as side lobes

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14
Q

How do side beams impact image quality?

A

15% of energy of the ultrasound beams will be in the side lobes
- will be mis-registered on the image

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15
Q

What are the two dimensions of the ultrasound beam?

A
  1. Beam width
  2. Slice thickness
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16
Q

How does beam width impact spatial resolution?

A

Decreased beam width increases spatial resolution

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17
Q

How many crystal elements does a transducer usually have?

A

128

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18
Q

What are the main components of the ultrasound transducer?

A
  • housing assembly
  • electrical connections
  • piezoelectric element
  • backing layer
  • acoustic lens
  • impedance matching layer
19
Q

What determines a transducers operating frequency?

A

The thickness of the piezoelectric element

20
Q

What is resonant frequency?

A

Frequency at which the transducer works at maximum efficiency
- occurs when thickness of the crystal corresponds to half a wavelength

21
Q

What is the purpose of the acoustic lens in a transducer?

A

Improves image (lateral) resolution by reducing beam width of the transducer

22
Q

What is axial resolution? And how can it be increased?

A

Resolution along the axis of the beam
- shorter pulse length gives better resolution

23
Q

What factors affect spatial pulse length and therefore axial resolution?

A

Frequency - lower frequency = lower pulse length
Transducer design - Increased damping makes pulse length shorter

24
Q

How does increasing the field of view impact axial resolution?

A

Increased field of view decreases axial resoluion

25
Q

What does lateral resolution depend on?

A

Beam width
- a narrow beam width gives a good spatial resolution

26
Q

What is contrast resolution?

A

The ability of the imaging system to differentiate between body tissues and display them as different shades of grey.

27
Q

What is temporal resolution and what is it determined by?

A

The ability of the imaging system to display events which occur at different times as separate images
- it is determined by frame rate

28
Q

What is the equation for frame rate?

A

Frame rate = PRF / lines per frame

29
Q

What is PRF dependent on?

A

Depth
- increased image depth results in lower PRF

30
Q

When can you use multiple focal zones?

A

If there is little patient movement

31
Q

How can temporal resolution be improved?

A

Reducing the sector angle and depth and selecting only a single focal zone

32
Q

What is attenuation?

A

The decreases in amplitude and intensity of a sound wave as it travels through the body

33
Q

What are the 5 main reasons why TGC is needed to amplify distant echoes?

A
  1. Absorption
  2. Reflection
  3. Scattering
  4. Refraction
  5. Divergence
34
Q

What is the main factor causing attenuation?

A

Absorption

35
Q

What is specular reflection?

A

Reflection of a smooth surface

36
Q

What is diffuse reflection?

A

Reflection in multiple directions off a rough surface

37
Q

When is scattering most likely to occur?

A

When an interface is equivalent to one wavelength in size

38
Q

How does increased frequency impact scattering?

A

Higher frequency causes more scattering

39
Q

What are the 3 main forms of artefact?

A
  1. Images that are not actually there
  2. Objects missing from image
  3. Misregistered structures
40
Q

How can you try to reduce reverberation artefacts?

A

Use more gel
Reduce gain
Move position of transducer

41
Q

What is acoustic shadowing?

A

An hypoechoic area behind an area of strongly attenuating tissue
- usually occurs at interfaces with large acoustic mis-matches

42
Q

What is acoustic enhancement?

A

A localized area of increased echo amplitude behind an area of low attenuation
- often behind fluid-filled structures e.g. cyst, bladder

43
Q

What is edge shadowing?

A

A combination of refraction and reflection occurring at the edge of rounded structures

44
Q

What is the equation for flow rate (Q)?

A

Q = change in Pressure / Resistance

Ultrasound physics book (1 decks)

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