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Ultrasound Physics > Image Optimization and Quality > Flashcards

Image Optimization and Quality Flashcards

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1
Q

what are the three types of resolution

A

Detail
Contrast
Temporal

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2
Q

what determines contrast resolution

A
  • Scan converter
  • # bits/pixels
  • post processing curves
  • monitor settings
  • Ambient light
  • Acoustic impedance mismatch
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3
Q

What is contrast resolution

A

Ability to distinguish structures based on variations of brightness

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4
Q

Restriction for contrast resolution

A

Number of bits per pixel in the display

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5
Q

Two limiting factors of contrast

A

Dynamic range of human eye and non-linear compression necessary o map the enormous signal dynamic range to the smaller dynamic range

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6
Q

more bits and pixels is useful for what

A

Color 9can distinguish more shades of colour then gray)

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7
Q

Low contrast resolution

A

ability to discern features with subtle differences in signal level and depends on the magnitude of the signal differences as well as the size of the object

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8
Q

Displayed contrast translates to what

A

Pixel calculates to brightness levels on the monitor via gray scale mapping

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9
Q

Temporal resolution

A

Ability to distinguish dynamics or changes over time

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10
Q

what is the principal limit to temporal resolution

A

Frame rate

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11
Q

every factor that lowers frame rate lowers what

A

Temporal resolution

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12
Q

what factors can lower frame rate

A 
Depth of imaging
Sector size
Multiple foci
Persistence 
Number of acoustic lines
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13
Q

monitor display rate can degrade what

A

Temporal resolution

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14
Q

Techniques that _____can affect temporal resolution

A

Average

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15
Q

what are two examples of averaging techniques that can affect temporal resolution

A

Compound Imaging

Persistence

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16
Q

How to improve frame rate

A
  • Narroe the imaging sector
  • Decreasing depth which decrease PRP
  • Decreasing line density (fewer lines)
  • Turning of multi-focus which decreases the number of pulses needed per line
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17
Q

detail resolution

A

lateral, axial and elevational

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18
Q

what is detail resolution dependent on

A

Transducer characteristics

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19
Q

lateral resolution

A

Resolve two structure side-by-side (lateral dimension)

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20
Q

lateral resolution is the…..

A

minimum reflector separation in the direction perpendicular o the beam direction that can produce two separate echoes when the beam is scanned across the reflectors

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21
Q

Lateral resolution is determined by

A

Beam width and scan line density

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22
Q

lateral is resolution is best where

A

Focal zone of the beam

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23
Q

best lateral resolution is where

A

Narrowest beam laterally

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24
Q

elevational resolution

A

Determined by the beam width in the elevational plane

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25
Q

elevation resolution changes with….

A

Depth

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26
Q

where is elevational resolution best

A

Narrowest point of the beam

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27
Q

Elevational resolution=

A

Elevation beamwidth

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28
Q

Axial resolution

A

minimum reflector separation along the direction of sound travel

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29
Q

what is axial resolution determined by

A

Spatial pulse length

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30
Q

what do broadband transducers do?

A

Operate at high frequencies and produce short duration pulses

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31
Q

to improve axial resolution

A

Must reduce the SPL

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32
Q

Axial resolution equation

A

SPL/2

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33
Q

Signal

A

Any phenomenon desired to be measured

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34
Q

Noise

A

Any unwanted signals

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35
Q

Noise floor

A

The amplitude level below which no signals are visible because of the presence of noise

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36
Q

Signal to noise ratio

A

The amplitude of the signal divided by the amplitude of the noise

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37
Q

SNR determines what

A

Sign quality

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38
Q

higher SNR

A

Better imaging situation and excluding artifact, more trustworthy data

39
Q

Overall desired situation (SNR)

A

High signal

low noise

40
Q

Apparent versus true SNR

A

True SNR does not chnage with gains while the apparent SNR does
-Increasing amplification will increase noise and signal by the same amount

41
Q

If gain is too low

A

Can make signal appear weak even if there is good SNR

42
Q

Noise: electronic noises

A

The electronics used to amplify the tiny returning echoes add random signals
This added energy is created by random excitations of electrons within the electronics
The amplitude of these signals are small, and are seen as white speckle on the image or in the Spectrum
In colour, electronic noise shows up as random colour pixels

43
Q

Noise: electrical interfernce

A

It is possible for transducers or ultrasound machines to receive energy emanating from other electrical devices
This energy often shows up as a bright ‘flashlight’ down the middle of an image or as a bright horizontal or zigzagging lines in spectral Doppler referred to as Doppler tones

44
Q

Noise: clutter

A

Large returning echoes from structures that obliterate weaker signals
Examples include large specular reflections in imaging and valves in Doppler

45
Q

Noise: Haze

A

There are many types of haze. It can created by poor transducer to skin contact or by beam aberration from tissue characteristics
All cardiovascular sonographers have seen haze associated with imaging through the lungs

46
Q

Preprocessing

A

Signal conditioning that occurs in real time and cannot be removed from an image once acquired
Some examples are receiver gain, receive focusing, and receiver compression

47
Q

all receiver functions are

A

Preprocessing

48
Q

Post porcessing

A

Any processing which can be changed after the data is acquired such as data compression, colorization and reject. Post processing can be performed on frozen data as well as live imaging

49
Q

Dynamic range

A

refers to the ratio of the maximum to the minimum of any quantity
Largest to the smallest echoes from a patient
Largest to the smallest signal the human eye can detect

50
Q

signal dynamic range is larger/smaller then display dynamic range

A

Larger

51
Q

The display dynamic range

A

Exceeds the visual dynamic range of the human eye

52
Q

compression

A

Is the general term for any technique which maps a larger dynamic range into a smaller dynamic range
Due to the limit of the human eye, reflected signals must be compressed to map the enormous signal dynamic into significantly smaller dynamic range

53
Q

there are a ______amount of compression schemes

A

infinite

54
Q

Compression is known as

A

lossy compression scheme

55
Q

because the dynamic range of signals returning from the body are so much greater then dynamic range of the human eye

A

Non-linear (log) compression must be used

56
Q

dynamic range of the human eye is less than 36dB

A

equivalent of 65 shades of grey

57
Q

Potential issue with ompression

A

Inability to distinguish mass from tissue

58
Q

back end compression

A

Under user control

59
Q

why is some compression done in the back end (under user control)

A

to help eliminate the loss of information associated with compression

60
Q

Tissue colorization

A

dynamic rnage of the eye is extended

intended to improve visualization when significant dynamic range must be preserved

61
Q

colour maps help make a distinction for what

A

low-level echoes

62
Q

what is the role of the scan converter

A

Converting A-Mode lines to B-Mode lines
Organizing the successive lines of data into a formatted image
Must keep track of which lines of data should be presented at what location on the screen

63
Q

Averaging based techniques

A

improvement in SNR

64
Q

Averaging techniques use what

A

Constructive and destructive interference

65
Q

Signal coherence

A

signal does not change from one instance to another (but noise does)

66
Q

Using averaging techniques……

A

Two signals will be added together while noise will be less as it is not a constant signal

67
Q

Spatial compound imaging

A

Multiple images are created over time then averaged together to create one image
These multiple images are formed at various angles and then average together to form a frame

68
Q

Spatial compound imaging uses what

A

Various angles

69
Q

what does a varying steered angle between frames result in

A

Many specular reflector based artifacts are reduced/eliminated by compound imaging

70
Q

what are two benefits of spatial compound imaging

A

Improved SNR

Reduction in specular reflectors

71
Q

frame rate and temporal resolution: spatial compound imaging

A

uses 3-9 images

  • buffering technique to limit degradation
  • Any process that uses averaging creates a degradation in temporal resolution, short duration events have the potential to be averaged out
72
Q

Image persistance

A

Like compound imaging, the goal of persistence is to reduce noise and improve SNR
Uses an algorithm

73
Q

what are differences between image persistence and spatial compound imaging

A

The angle is not changed between frames

A weighted average is applied to each frame so that newer frames “count” for more than older frames

74
Q

Spatial averaging

A

looks at small local regions within the same frame

75
Q

how does spatial averaging reduce noise

A

By adjusting pixels according to an algorithm that looks at the values of the nearest neighbors

76
Q

spatial averaging gives image what look

A

Smooth

77
Q

Frequency compounding/Averaging

A

The reduction in speckle artifact and noise by dividing reflected signals into sub bands of limited frequencies, making their own images, then combining those images to make a composite image

78
Q

frequency compounding/averaging helps to reduce what

A

Speckle artifact

79
Q

what is adaptive frame averaging designed to do

A

Increase the SNR

80
Q

what does adaptive frame averaging allow for

A

short duration events to be evident without the penalty for lag for the fast moving structures

81
Q

Adaptive Processing

A

Adjust system parameters to optimize the image based on algorithms

82
Q

Adaptive processing is known as

A

Auto-optimization

83
Q

Confocal imaging

A

Each transmit focal zone is formed with a different center frequency

84
Q

What does confocal imaging take advantage of

A

broad transmit bandwidth is split into multiple frequency ranges

85
Q

what does Confocal imaging do

A

Improves SNR (lateral resolution)

86
Q

Confocal imaging does what

A

Multiple small transmit bandwidths with different center frequencies are used to collect signals along each line of sight

87
Q

Multiple transmit foci improves what

A

Lateral reoslution

88
Q

however, multiple transmit foci reduces what

A

temporal resolution because there are multiple foci and therefore the frame rate is slower

89
Q

what is banding noise

A

Due to imperfect fusing of the multiple transmitted lines

Common with multiple foci

90
Q

Panoramic Imaging

A

extend the field of view by acquiring lines of data while the sonographer scans the transducer across the patient. Correlation techniques are used to decrease the effect of movement on the quality of the image. (Think of it like stitching the image together)

91
Q

Zoom

A

Technique to accomdate the desire to visulize regions of images in a larger format

92
Q

what are the two common approaches to zoomed image

A 
Acoustic zoom (write)
Non-Acoustic (read)
93
Q

Non acoustic

A

Simply magnifies existing pizels without increasing line desnity

94
Q

Acoustic zoom

A

retransmits acoustic lines potentially using different line density to achieve better rolution

Ultrasound Physics (10 decks)

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