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Imaging Review CCPM > Grad class notes III > Flashcards

Grad class notes III Flashcards

1
Q

what does image intensifier in fluoro consist of?

A

-input layer (xrays- electrons)-
-electron optics (electrons- phosphors)
-output layer (electrons-light)

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2
Q

photocathode in fluoro

A

-releases electrons upon absorption of light photons

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3
Q

vaccuum window in fluoro

A

-curved to withstand air pressure
-Al

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4
Q

substrate in fluoro

A

-thin enough to allow xrays to pass
-strong enough to support input phosphor

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5
Q

inout phosphor in fluoro

A

-CsI
-converts xray to visible light

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6
Q

anode in fluoro

A

-Al coating on output phosphor
-removes electrons once they deposit energy

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7
Q

output phosphor in fluoro

A

-ZnCd doped with silver
-converts electron Ek to light

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8
Q

output windown in fluoro

A

-allows light from output phosphor to propagate out of vaccuum

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9
Q

veiling glare in fluoro

A

some light emitted by phosphor is reflected at the output window
-reduced by making output window a piece of glass and coating sides with light absorber

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10
Q

“order” in fluoro

A

-input window
-substrate
-input phosphor
-photocathode
-lenses
-anode
-output phosphor
-output window

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11
Q

fluoro amplification factor

A

ratio of input FOV to output FOV
-output image is smaller than input image

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12
Q

fluoro conversion factor

A

-divide output bu input

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13
Q

fluoro brightness gain

A

-product of electronic and minification gain
-electronic gain ~ 50
-minification gain decreases with FOV

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14
Q

fluoro magnification mode

A

-can reduce operated FOV
-electronic lens adjusts the focusing of the smaller FOV on the full FOV of the output phosphor
-xray source is collimated to same FOV as image intensifier
-ABC compensates for reduction in energy reaching output hosphor by increasing xray exposure rate (maintains brightess but increases dose)

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15
Q

contrast ratio in fluoro

A

-measure of veiling glare
-max contrast between white and black that can be achieved by II for a fixed exposure rate

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16
Q

quantum detection efficiency in fluoro

A

-fraction of incident xrays at input window to those that are absorbed in input phosphor

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17
Q

S distortion in fluoro

A

spatial warping caused by interaction of electron optics with magnetic field

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18
Q

f stop (optical distributor)

A

-larger f stop = higher dose, higher SNR (less light per quanta= get more quanta)
-f stop changes amt of light per each quanta
-change in 1 fstop reduces light by 2 X

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19
Q

Kell effect

A

shape of object changes eye’s peception of resolution

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20
Q

photo-spot camera in fluoro

A

-use optic photons
-75-100 uR/image entrance exposure
-take directly from II (with lens)
-full resolution of II

21
Q

spot-film devices

A

-separate xray system, with anti-scatter grid, and AEC
-does not involve II
-600 uR/image entrance dose

22
Q

cine camera

A

-captures rapid motion (heart)
-synchronizes shutter and xray tube pulses
-10-15 uR/frame at entrance

23
Q

modes of operation in fluoro

A

-continuous fluoro (10 R/min entrance)
-high dose rate (20 R/min entrance)
-variable frame rate pulsed fluoro- lower temporal resolution when not needed to reduce dose
-frame averaging
-last frame hold
-road mapping

24
Q

frame averaging

A

-reduce temporal resolution to increase SNR
-can produce lag
-can reduce dose proportional to number of frames averaged

25
Q

last frame hold

A

xray exposure is stopped but last image is displayed

26
Q

road mapping

A

last frame hold but software enhanced
-selected image can be overlaid on real-time images or subtracted

27
Q

automatic brightness control

A

-adjusts xray exposure to keep brightness at constant level
-can increase kVp (contrast and dose decrease)
-can increase mAs (contrast stays same but dose increases at patient entrance)
-can increase electronic gain at expense of SNR

28
Q

does video, spot, and cine, realize full resolution of II?

A

Video no
spot and cine yes

29
Q

what is temporal resolution determined by?

A

lag (ie fraction of image data from one frame being combined with another frame)
-lag increases SNR

30
Q

RAP meter

A

-roentgen area product meters
-sensitive to exposure rate and FOV of beam
-unaware of source-patient distance, magnification mode, changes in kVp

31
Q

flat panel detector in fluoro

A

-replaced II
-each pixel is photodiode that converts light into electrons
-first, CsI converts xrays to visible light
-better quantum efficiency than II
-can combine 4 pixels into 1, reducing resolution but increasing SNR
-magnification possible but at expense of higher dose

32
Q

properties of flat panel detectors vs II

A

-larger dynamic range
-typically better SNR but worse resolution
-less dose
-at lower FOV, have narrow beam geometry- larger entrance dose)

33
Q

can you use CR cassette in fluoro?

A

yes
-made of BaFBr/BaFI and doped with Eu
-xray causes Eu2+ to go to Eu3+
-excited electrons become mobile and some get trapped in F centers
-# of electrons trapped/area is proportional to xray energy absorbed

34
Q

CR reading

A

-red light stimulates emission of trapped energy
-releases visible light, which is collected by light guide and directed to PMT
-PMT signal is digitized and stored
-plate can be exposed many times but must first be exposed to bright light

-red laser is too low energy to promote electrons from atoms to conduction band, except those in F centers
-electron de-excites by returning to Eu2+, releasing blue-green visible light
-light pipe attenuates the red laser light but not the blue-green light

35
Q

exposure required for CR vs screen-film for same image quality

A

CR requires 2X as much exposure

36
Q

CCD

A

charged coupled device
-visible light falls on photosensitor pixel, electrons are liberated and build up in the pixel
-voltage on each side of pixel so electrons liberated in a pixel stay there

37
Q

reading CCD

A

ready each column by applying voltage levels to pixel boundaries, go on pixel by pixel basis

38
Q

do CCDs require screen?

A

yes, to covnert xray energy into visible light
-lenses focus light from screen onto CCD
-minification factor: loss of light between screen and CCD
-2nd quantum sink- too few light photons reach the CCD; therefore image quality is not proportional to the patient dose used to create the image

39
Q

indirect vs direct flat panel detectors

A

indirect: CsI converts xray to optical light, which interacts with TFT elements
direct: photoconductor converts xray energy into electrons, which are directed to TFT elements by electric field (electrons don’t diffuse like light photons will)

40
Q

TFT fill factor

A

light sensitive area/area of detector element

41
Q

TFT capacitor

A

-stores charged produced during exposure

42
Q

TFT transistor

A

allows charge to be read off of capacitor

43
Q

resolution in TFT

A

resolution= pixel size
-however since size of electronic portion is fixed, smaller TFT = smaller fill factor (therefore get lower contrast)
-direct TFTs can retain contrast resolution with small fill factors by designing the applied E filed to focus electrons on sensitive area of the pixel

44
Q

MTF of CR vs screen/film vs TFT

A

TFT > SF > CR

45
Q

resolution of hard vs soft copy

A

hard copy= higher resolution, but is fixed

46
Q

dark noise

A

non-zero gray levels in absence of xray exposure

47
Q

flat field

A

-correct for pixel-pixel variation with map
-hot pixels are treated as dead
-cold pixels have an exposure dependent gain

48
Q

what is convolution used for in image processing?

A

-enhance edges, reduce noise

Imaging Review CCPM (11 decks)

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