Grad class notes III Flashcards
what does image intensifier in fluoro consist of?
-input layer (xrays- electrons)-
-electron optics (electrons- phosphors)
-output layer (electrons-light)
photocathode in fluoro
-releases electrons upon absorption of light photons
vaccuum window in fluoro
-curved to withstand air pressure
-Al
substrate in fluoro
-thin enough to allow xrays to pass
-strong enough to support input phosphor
inout phosphor in fluoro
-CsI
-converts xray to visible light
anode in fluoro
-Al coating on output phosphor
-removes electrons once they deposit energy
output phosphor in fluoro
-ZnCd doped with silver
-converts electron Ek to light
output windown in fluoro
-allows light from output phosphor to propagate out of vaccuum
veiling glare in fluoro
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
“order” in fluoro
-input window
-substrate
-input phosphor
-photocathode
-lenses
-anode
-output phosphor
-output window
fluoro amplification factor
ratio of input FOV to output FOV
-output image is smaller than input image
fluoro conversion factor
-divide output bu input
fluoro brightness gain
-product of electronic and minification gain
-electronic gain ~ 50
-minification gain decreases with FOV
fluoro magnification mode
-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)
contrast ratio in fluoro
-measure of veiling glare
-max contrast between white and black that can be achieved by II for a fixed exposure rate
quantum detection efficiency in fluoro
-fraction of incident xrays at input window to those that are absorbed in input phosphor
S distortion in fluoro
spatial warping caused by interaction of electron optics with magnetic field
f stop (optical distributor)
-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
Kell effect
shape of object changes eye’s peception of resolution
photo-spot camera in fluoro
-use optic photons
-75-100 uR/image entrance exposure
-take directly from II (with lens)
-full resolution of II
spot-film devices
-separate xray system, with anti-scatter grid, and AEC
-does not involve II
-600 uR/image entrance dose
cine camera
-captures rapid motion (heart)
-synchronizes shutter and xray tube pulses
-10-15 uR/frame at entrance
modes of operation in fluoro
-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
frame averaging
-reduce temporal resolution to increase SNR
-can produce lag
-can reduce dose proportional to number of frames averaged
last frame hold
xray exposure is stopped but last image is displayed
road mapping
last frame hold but software enhanced
-selected image can be overlaid on real-time images or subtracted
automatic brightness control
-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
does video, spot, and cine, realize full resolution of II?
Video no
spot and cine yes
what is temporal resolution determined by?
lag (ie fraction of image data from one frame being combined with another frame)
-lag increases SNR
RAP meter
-roentgen area product meters
-sensitive to exposure rate and FOV of beam
-unaware of source-patient distance, magnification mode, changes in kVp
flat panel detector in fluoro
-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
properties of flat panel detectors vs II
-larger dynamic range
-typically better SNR but worse resolution
-less dose
-at lower FOV, have narrow beam geometry- larger entrance dose)
can you use CR cassette in fluoro?
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
CR reading
-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
exposure required for CR vs screen-film for same image quality
CR requires 2X as much exposure
CCD
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
reading CCD
ready each column by applying voltage levels to pixel boundaries, go on pixel by pixel basis
do CCDs require screen?
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
indirect vs direct flat panel detectors
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)
TFT fill factor
light sensitive area/area of detector element
TFT capacitor
-stores charged produced during exposure
TFT transistor
allows charge to be read off of capacitor
resolution in TFT
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
MTF of CR vs screen/film vs TFT
TFT > SF > CR
resolution of hard vs soft copy
hard copy= higher resolution, but is fixed
dark noise
non-zero gray levels in absence of xray exposure
flat field
-correct for pixel-pixel variation with map
-hot pixels are treated as dead
-cold pixels have an exposure dependent gain
what is convolution used for in image processing?
-enhance edges, reduce noise
