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RD1037 > image manipulation and compression > Flashcards

image manipulation and compression Flashcards

1
Q

what is a quantum sink

A

a material/ substance that is very effective at absorbing x-rays

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2
Q

what happens when an XR photon interacts with quantum sink

A

transfers energy to the sink
- absorption allows us to create detailed images of internal structures of body

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3
Q

example of contrast agents/ contrast media

A

quantum sinks

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4
Q

quanta

A

refers to discrete packets/ units in which energy is quantized
- quanta = ‘how much’

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5
Q

what does an image histogram represent

A
  • graphical representation of distribution of pixel intensities in image
  • visual summary of tonal range and contrast of image
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6
Q

what does the x-axis of histogram represent

A

pixel intensities

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7
Q

what does the y-axis of histogram represent

A

frequency/ number of pixels at each intensity level

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8
Q

how do you construct an image histogram

A
  • convert image to greyscale
  • count pixel intensities
  • plot the histogram
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9
Q

value of 0 represents

A

black (in 8-bit greyscale image)

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10
Q

value of 255 represents

A

white ( in 8-bit greyscale image)

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11
Q

what does a high peak in histogram represent

A

peaks in histogram indicate intensity levels that are more dominant in image
= significant features/ objects

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12
Q

low/ flat regions in histogram

A

indicate low frequency/ absence of pixels at certain intensity levels
= uniform background/ low contrast

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13
Q

saturated regions

A
  • shows spikes at extreme ends (0 or 255) - could suggest image full black/ white
    = underexposed/ overexposed areas
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14
Q

where does histogram have to be placed for conventional radiography

A

within latitude of the film

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15
Q

where does histogram have to be placed for digital radiography

A
  • placed adjacent to image display
  • provides real time information about distribution of pixel intensities in image
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16
Q

how do we know if detector has received sufficient exposure?

A

typically done by evaluating image’s overall brightness + contrast
- consider histogram of pixel intensities

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17
Q

what are signals

A
  • amount of transmitted beam we receive at image receptor –> converted into our image
  • quanta of photons reaching detector
18
Q

how can the amount of signal the receptor receives be changed

A

by changing exposure factors

19
Q

which type of DR detector is more prone to loss of signal

A
  • iDR
  • more prone to light scattering = loss of spatial resolution
  • light attenuation - conversion of light through scintillator material = loss
  • QDE = converting XR quanta to light (QDE) can affect overall signal:noise
20
Q

what are exposure indices

A
  • numerical values to quantify exposure received by digital detector
21
Q

EI

A

value that represents radiation dose received by detector during image acquisition
- allows for monitoring and maintaining consistent image quality

22
Q

DI

A
  • deviation index compares actual exposure level of acquired image to reference exposure level
  • tells us if there is a deviation between two values
    • DI = overexposure
  • -ve DI = underexposure
23
Q

SNR

A
  • signal to noise ratio
  • measure of quality + clarity of image
  • high SNR = better image quality
24
Q

what is a look up table (LUT)

A
  • data structure commonly used in computer science + digital systems to map input values to corresponding output values
25
Q

two main components of LUT

A
  • input values
  • output values
  • when specific input value provided to LUT –> returns with corresponding output value associated with the input value
26
Q

essential what does LUT provide

A

fast and efficient way to retrieve output values without having to perform complex calculations

27
Q

what happens as a result of LUT to a graph

A

curve applied to linear lined graph

28
Q

post-processing image manipulations

A
  • windowing
  • edge enhancement
  • shuttering
  • image orientation
  • markers
  • LUT changes
29
Q

what does post processing generally do to image quality

A

degrades

30
Q

how does windowing affect image quality

A
  • affects contrast of image
  • alters brightness
  • does not affect density
31
Q

steep/ shallow line on curve

A

contrast

32
Q

position of curve to pixel data

A

brightness

33
Q

what is image compression

A
  • minimising file size of image file without degrading quality of image to unnaceptable level
34
Q

what are bitmaps

A
  • common type of digital representation
  • composed of individual grid pixels where each pixel represents a specific colour
  • generally large files
35
Q

RLE

A

run length encoding

36
Q

what is RLE

A
  • compression technique used to reduce size of data
    eg
    AAAABCCD
    = 4AB2CD
37
Q

LZW compression

A

lemel ziv welch compression

38
Q

what is a LZW compression

A

replaces frequently occuring patterns with shorter codes = reduces size of data
- lossy

39
Q

two types of compression

A
  • lossy
  • lossless
39
Q

RAID

A

redundant array of inexpensive disks

40
Q

pros and cons of RAID

A
  • disk failure redundancy
  • massive data space
  • expensive
  • can still fail

RD1037 (8 decks)

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