4. XR Image Detectors Flashcards
dynamic range on digital vs film
larger dynamic range: digital
narrow dynamic range: film
detector response and x-ray exposure curve: digital vs linear
curvilinear response: film
linear response: digital
importance of a larger dynamic range
correct moderately under/overexposed images ; benefit of digital
digital vs analog: dose
higher dose efficiency; less dose with digital since you can do postprocessing
- wider dynamic range allows for underexposure
- increase kVP and half mA to decrease dose
pixel density and spatial resolution
more pixels per unit area, better spatial resolution
determinant of spatial resolution in digital imaging
pixel size and spacing
pixel pitch
measurement from one pixel to another
automatic exposure control mechanism
when ionization chamber charged, exposure terminated
controls s in mAs
duration of exposure in AEC determinants
density and thickness of area of patient placed over ionization chamber
bit depth
number of bits is the number of shades of gray that can be displayed on a computer monitor
12 bit = 4096 shades
2 ^x
types of digital detectors
CR (storage phosphor)
DR (flat panel detectors) –direct/indirect
differece between direct an indirect detectors
indirect = scintillators
xray -> light -> charge
direct = photoconductors
xray -> charge
define phoshor
substance that emits light when exposed to radiation (luminescence)
storage phosphor (CR) how does it work? phosphor type?
barium fluorohalides
xray –> causes electron in phosphor to change to metastable state that can hold for several days (europium)
cassette goes to docking station, developing film )photostimulable luminescence)
how is the CR cassette read?
plate exposed to laser (red) that liberates electrons
energy released as blue-green light which shines on photodetector/photomultiplier and converts it to an electronic signal
cassette exposed to bright white light to remove ghost image
ghost image
leftover signal that can cause artifact
sampling pitch
distance between laser positions as plate is being read
may also mean distance between adjacent detector elements (DELs)
flat panel detectors (DR)
digital dector, faster than CR or conventional film development
indirect CR
xray activates CsI (thalium doped cesium iodide)
scintilators emit liht in response to xray ==> converted by photodiode into electrical charge –> transmitted by TFT (thin film transistor) to workstation for processing/display
lateral light dispersion
problem with indirect CR whic use phosphors
light diffuses laterally and can create issues with spatial resolution, WORSE with increasing thickness of crystals
which type of phosphor has more lateral dispersion?
gadolium oxysulfide has more lateral dispersion/light scatter than CsI
how ti fix lateral dispersion
columnar structure that forms a matrix
direct DR
- homogenous bias charge to photoconductor (amorphous selenium)
- xrays absorbed by selenium/releases electrons
- released electrons (electron hole pair) travel to surface of selenium and neutralized applied charge; no laeteral dispersion
- electron charge drawn in along field lines to the charge storage capacitor electrodes connected to TFT
- pattern of charges scanned and converted to digital storage
tell me mor aboue amorphous selenium
heat/humidity sensitive
when exposed to radiation, electrical conductivity altered in proportion to radiation intensity
fill factor
area of detector sensitive to xrays in relation to entire dector area
higher fill factor = more efficient detector
which system has higher fill factor?
DR system has fill factor of nearly 100%, less with CR
DQE – detective quantum efficiency
estimate of required exposure level to create an optimal image; efficiency measurement
high DQUE = ? dose
low; less radiation needed to maintain signal
DQUE is better for DR vs CR?
DR
DR DQE: 0.45
CR DQE: 0.25
standard of care for digital display?
3 megapixels
factors affecting spatial resolution of CR
laser spot size (smaller)
phosphor plate density/thickness (thinner better, less light spreading)
sampling frequency
imaging plate (smaller for body part)
increasing x rays (does not improve maximum spatial resolution)
factors affecting spatial resolution of DR
superior to CR
less lateral dispersion
small detector elements = better spatial resolution
CR vs DR
CR: cassette based, centralized system
DR; decentralized system, image can be acquired/adjusted/transferred to PACS
limitations to spatial resolution in CR
light scattering from laser light
size of pixels
limitations to spatial resolution in DR
size of detector “del”
spread of light photons to light conversion (indirect)
relationship with pixel size and fill factor
increasing fill factor with increasing pixel size
