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RD1037 > digital fluoroscopy fluoro 2 > Flashcards

digital fluoroscopy fluoro 2 Flashcards

1
Q

problems with image intensifiers

A
  • big and bulky
  • prone to image un-sharpness, distortion and noise
  • vacuum tube can break down
  • limited image post-processing
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2
Q

what is the order of the image intensifier chain

A

x rays –> light –> electrons –> light

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3
Q

what goes between x-rays and light

A

input phosphor

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4
Q

what goes between light and electrons

A

photocathode

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5
Q

what goes between electrons and light

A

electron optics and output phosphor

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6
Q

what do image intensifiers provide that is difference to digital flat panel detectors

A

a smaller dynamic range of image densities than digital detectors

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7
Q

what type of response do image intensifiers give

A

a non-linear response to exposure and narrow dynamic range

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8
Q

what happens during image acquisition

A
  • x-ray beam attenuated through patient normally
  • x-ray photons incident to detector plate (same as DR)
  • sequence of static images taken during screening (at regular intervals)
  • images are post processed + can be played back as video
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9
Q

advantages of digital fluoroscopy

A
  • more compact equipment
  • less radiation dose
  • less noise
  • reduced artefacts
  • image magnification without increased dose/ loss of brightness + resolution
  • PACS + image distribution
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10
Q

image of image intensifier

A
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11
Q

image of flat panel detector

A
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12
Q

density of analogue images

A

continuous varying density

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13
Q

density of digital images

A

has discrete pixels with numerical density values

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14
Q

what is signal to noise ratio

A

the ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time

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15
Q

two ways digital output can be obtained

A
  • by taking output from image intensifier (old fashioned)
  • by using flat panel solid state detector (more modern)
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16
Q

what type of digital technology do many detectors in fluoro use

A

indirect

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17
Q

what absorbs x-rays and emits light in flat panel detectors

A
  • caesium iodide scintillator layer
  • gives more signal per XR dose received compared to image intensifiers
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18
Q

why are CsI in needle form

A

channels light downwards towards silicon diode

19
Q

how do flat panel detectors work

A
  • light strikes silicon photodiode which emits electrons
  • electrical charge collected in thin film transistor (TFT) array
  • signal passes through analogue to digital converter (ADC)
20
Q

what type of digital technology do SOME flat panel fluoro detectors use

A

direct

21
Q

direct digital technology

A
  • a photoconductor that absorbs x-rays
  • amorphous selenium, a-Se
  • XR converted into directly to electrical signal
  • photoconductor generates electrical charge
  • electrical charge collected in thin film transistor (TFT) array
  • signal passes through ADC
22
Q

what are flat panel direct detectors

A
  • detectors are direct imaging matrices using thin film sensors
  • detectors very similar to those used in plain radiographic imaging
23
Q

what does an ADC determine

A

the digital values allocated to each pixel

24
Q

what does ADCs consist of

A
  • integrated circuits containing ‘silicon chips’ that perform complex electronic processes
25
Q

what does high voltage (ON) and low voltage (OFF) represent

A

high voltage: digit 1
low voltage: digit 0

26
Q

the voltage at each point determines…

A

grey level

27
Q

how many grey levels available

A

256
(0=black)
(255=white)

28
Q

what is a look up table (LUT)

A
  • a mathematical computer processing
  • converts an input value to output value
  • i.e. converts charge on a transistor to density in image pixel
29
Q

features of flat panel fluorography

A
  • higher detective quantum efficiency (DQE) gives reduced dose
  • small pixels
  • large dynamic range
  • magnified images without the loss of bright + resolution
  • no image distortion
30
Q

what does detective quantum efficiency (DQE) measure

A

efficiency of an imaging systems transfer of XR signal to image signal
- and of incoming signal to noise ratio to outgoing signal to noise ratio

31
Q

what does a high DQE mean

A

gives better ability to detect fine details in image
- high contrast and resolution

32
Q

‘perfect imaging system’

A
  • a DQE of 100%
  • not achievable as noise always added
33
Q

the greater the level of bit depth …

A

the greater the contrast resolution and smoother the image

34
Q

contrast resolution

A

ability to demonstrate subtle differences in XR attenuation

35
Q

spatial resolution

A

ability to discriminate and detect small adjacent objects (fine details)

36
Q

describe sampling of a video signal

A

conversion of analogue signal to digital signal via periodic measurement (over time)

37
Q

what does a low sample rate result in

A

coarse digital signal

38
Q

what would happen in sampling rate doubled

A
  • pixel size would be halved
  • matrix size doubled
  • spatial resolution improved by factor of 2
  • BUT requires more memory
39
Q

the larger the sampling matrix …

A

the greater the spatial resolution

40
Q

features of a digital spot image

A
  • high resolution
  • short exposure at high mA = freezing movement
  • video system becomes inoperable and camera writes it in computer memory
41
Q

how can greyscale numbers be processed and manipulated before reading out an image from memory

A
  • windowing
  • noise reduction (averaging or smoothing
  • edge enhancement
  • data shifting
  • quantification
42
Q

what is data subtraction aniography

A

if two images are almost the same theyre subtracted from each other so resulting image only shows whats different
- technique regularly used to show contrast flowing through blood vessels

43
Q

how to produce a DSA image

A
  • sequence of pulsed fluoroscopic images of relevant body part is recorded as contrast injected into vessel
  • pre-contrast image then substracted from post contrast image to give DSA
44
Q

how to average/ reduce noise

A
  • FRAME ADDITION
  • image of same subject are added together + averaged
  • useful signals added
  • patient must be immobile

RD1037 (8 decks)

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