4. XR Image Detectors

Question 1

dynamic range on digital vs film

Answer 1

larger dynamic range: digital

narrow dynamic range: film

Question 2

detector response and x-ray exposure curve: digital vs linear

Answer 2

curvilinear response: film

linear response: digital

Question 3

importance of a larger dynamic range

Answer 3

correct moderately under/overexposed images ; benefit of digital

Question 4

digital vs analog: dose

Answer 4

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

Question 5

pixel density and spatial resolution

Answer 5

more pixels per unit area, better spatial resolution

Question 6

determinant of spatial resolution in digital imaging

Answer 6

pixel size and spacing

Question 7

pixel pitch

Answer 7

measurement from one pixel to another

Question 8

automatic exposure control mechanism

Answer 8

when ionization chamber charged, exposure terminated

controls s in mAs

Question 9

duration of exposure in AEC determinants

Answer 9

density and thickness of area of patient placed over ionization chamber

Question 10

bit depth

Answer 10

number of bits is the number of shades of gray that can be displayed on a computer monitor

12 bit = 4096 shades
2 ^x

Question 11

types of digital detectors

Answer 11

CR (storage phosphor)

DR (flat panel detectors) –direct/indirect

Question 12

differece between direct an indirect detectors

Answer 12

indirect = scintillators
xray -> light -> charge

direct = photoconductors
xray -> charge

Question 13

define phoshor

Answer 13

substance that emits light when exposed to radiation (luminescence)

Question 14

storage phosphor (CR) 
how does it work? phosphor type?

Answer 14

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)

Question 15

how is the CR cassette read?

Answer 15

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

Question 16

ghost image

Answer 16

leftover signal that can cause artifact

Question 17

sampling pitch

Answer 17

distance between laser positions as plate is being read

may also mean distance between adjacent detector elements (DELs)

Question 18

flat panel detectors (DR)

Answer 18

digital dector, faster than CR or conventional film development

Question 19

indirect CR

Answer 19

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

Question 20

lateral light dispersion

Answer 20

problem with indirect CR whic use phosphors

light diffuses laterally and can create issues with spatial resolution, WORSE with increasing thickness of crystals

Question 21

which type of phosphor has more lateral dispersion?

Answer 21

gadolium oxysulfide has more lateral dispersion/light scatter than CsI

Question 22

how ti fix lateral dispersion

Answer 22

columnar structure that forms a matrix

Question 23

direct DR

Answer 23

1. homogenous bias charge to photoconductor (amorphous selenium)
2. xrays absorbed by selenium/releases electrons
3. released electrons (electron hole pair) travel to surface of selenium and neutralized applied charge; no laeteral dispersion
4. electron charge drawn in along field lines to the charge storage capacitor electrodes connected to TFT
5. pattern of charges scanned and converted to digital storage

Question 24

tell me mor aboue amorphous selenium

Answer 24

heat/humidity sensitive

when exposed to radiation, electrical conductivity altered in proportion to radiation intensity

Question 25

fill factor

Answer 25

area of detector sensitive to xrays in relation to entire dector area

higher fill factor = more efficient detector

Question 26

which system has higher fill factor?

Answer 26

DR system has fill factor of nearly 100%, less with CR

Question 27

DQE – detective quantum efficiency

Answer 27

estimate of required exposure level to create an optimal image; efficiency measurement

Question 28

high DQUE = ? dose

Answer 28

low; less radiation needed to maintain signal

Question 29

DQUE is better for DR vs CR?

Answer 29

DR
DR DQE: 0.45
CR DQE: 0.25

Question 30

standard of care for digital display?

Answer 30

3 megapixels

Question 31

factors affecting spatial resolution of CR

Answer 31

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)

Question 32

factors affecting spatial resolution of DR

Answer 32

superior to CR
less lateral dispersion
small detector elements = better spatial resolution

Question 33

CR vs DR

Answer 33

CR: cassette based, centralized system

DR; decentralized system, image can be acquired/adjusted/transferred to PACS

Question 34

limitations to spatial resolution in CR

Answer 34

light scattering from laser light

size of pixels

Question 35

limitations to spatial resolution in DR

Answer 35

size of detector “del”

spread of light photons to light conversion (indirect)

Question 36

relationship with pixel size and fill factor

Answer 36

increasing fill factor with increasing pixel size