DR projection technology Flashcards
where is the tube in fixed radiography system
generally ceiling mounted on rails
fixed radiography system components (5)
ceiling mounted tube
generator cabinet
operators console
often two buckies
screen behind where exposures carried out
what is in the bucky
grid
mechanism to ensure grid moves
AEC sensors
casette or DR detector
purpose of AEC
correct amount of radiation to image receptor
taking out guesswork
what two things does the AEC need to be able to cope with
varying body thickness and tube voltage
where is the AEC located
behind grid
in front of image receptor
excl. mammo
top to bottom:
xrays
patient
grid
AEC detector
image receptor
what does the AEC consist of
ionisation monitor chambers
3-5 sensors
how does AEC work
monitors air kerma
terminates exposure when predetermined limit reached
requirement for AEC
transparancy to x rays
major components of mobile radiography system
arm for positioning of tube
high voltage generator and batteries integrated
xray tube assembly
collimator assembly
mobile radiography system components
integrated generator
tube on adjustable arm
integrated console
generally no grid or AEC
no screen
where are mobile systems used
in wards
mammo system: what kind of detector
where is mounted on arm
what does the gantry do
what is possible
digital detector
tube and detector
gantry rotates
advanced imaging techniques
intra-oral dental:
size of mA
what kind of anode
what receptors are used
where is exposure switch
low mA
stationary anode (small tube)
film and digital
on a long cable for rad protection
panoramic dental unit:
1. what is it for
2. anode type
3. what do the tube and detector do
4. what about the receptor
- looking at dentition and the jaw
- stationary
- rotate about patient
- translates
fixed fluoro:
1. what is mounted
2. what are they mounted on
3. how is it operated
4. where are exposures carried out
5. where are the screens
- x ray tube and image receptor
- c arm sometimes ceiling
- console and sometimes pedestal with controls
- in the room
- ceiling mounted
e.g. of fluoro imaging
barium swallow
mobile fluoro:
1. where is generator
2. where is image display
3. where are x ray tube and receptor mounted
4. what doesnt it have
- integrated
- on seperate cart
- often on c arm
- screen
what does c arm allow
large degree of movement (of x ray tube and image receptor)
4 types of image receptors
glass plates
film
screen film systems
digital radiography detectors
and computed radiography
advantages of analogue imaging (3)
- proven technology
- high res
- affordable
e.g. of analogue radiography
screen film
cons of analogue radiography (4)
- contrast-latitude compromise
- limited dyn range
- no post processing
- film processing stability
pros of digital imaging (5)
- contrast performance
- larger dyn range
- dose efficency
- image processing
- integration with pacs
cons of digital imaging (2)
- lower res
- start up costs
how does screen film radiography work
what happens when photons reach screen
what is light recorded on
how to get image
what is a challenge
- photons captured by fluorescent screens and give off light
- light recorded on double emulsion film
- chemically process the film
very non-linear response so needs precise exposure
order of screen film components
x rays
screen
film
screen
latent image in film
visible light produced in screen
how does digital radiography work
1. how are photons captured (2)
2. what happens to the signal
3. what happens to image
advantage
- by fluorescent screens or digitally
- digitals and assigned to pixel
- processed and displayed on variable image display
linear response over wide dyn range
3 main types of digital radiographic receptors
- indirect
- direct
- CR
indirect digital receptor
order of mechanisms
material
xrays
light
charge
signal
amorphous silicon flat panel
direct digital receptor
order of mechanisms
material
xrays
curent
signal
amorphous selenium flat panel
CR digital receptor
order of mechanisms
xrays
delayed light
signal
direct and indirect digital detectors electronics array
1. consists of
2. what do they do
3. where is the array normally deposited
4. how is image created
- thin film transistors
- each transistor forms a pixel
- on a glass base
- signal from pixels transferred to computer
CR
1. where is the imaging plate housed
2. what is a laser used for
3. what is emitted and from what
4. what happens to the light
5. last step to produce image
- in a cassette
- read out of the imaging plate
- blue light from imaging plate
- collected by light gate and amplified by PMT
- digitise the signal
4 steps of CR image acquisition and readout
- erase
- expose
- readout out
- display
how to erase CR imaging plate
shine with intense white light
two types of receptors for fluoro
image intensifier
flat panel detector
x ray image intensifier (used in fluoro)
1. what is x ray incident on
2. what is produced
3. what is the light incident on and what is then produced
4. how is flux gained
5. how is minification gained
5. what is light detected by and what for
- input phosphor
- light produced
- photocathode - electrons
- electrons accelerated across vacuum , gain energy and incident on output phosphor
- input phosphor area large and mapped to small area of output
- camera to display image
what is flat panel detector similar to
radiographic detector
components of flat panel detector (3)
Csl phosphor
TFT
photodiode array
fps of readout in flat panel detector
up to 30
order of events in flat panel detector
x rays
into light
into charge
read out
creates digital image
two main types of radiographic image receptors
digital and computed radiography
two main types of fluoro image receptors
image intensifer and flat panel
3 main things which determine whether you can see an object in an image
contrast (from bg)
noise in image
object size
what does PE effect provide and how
contrast by removing photons from beam
PE effect: why is there bone-soft tissue contrast
rapid increase of probability of photons removed with beam with Z (proportional to Z^3)
what is k-edge used for
to optimise x ray capture in image receptor
and as contrast agents
contrast
if I1=intensity through whole patient
I2=intensity through object
what is contrast defined as
decrease in intensity of imaging the patient with and without the object
i.e. I2-I1 / I1
formula for image contrast
I2-I1 / I1 = 1 - exp(x(mu1-mu2))
what does ‘subject’ or radiation contrast depend on
thickness of object
difference in mu between object and background
how to attenuation coefficients vary with energy
fall
how does the difference in attentuation coeff change at higher energy
becomes smaller
which energy (low or high) maximises contrast and why
low - greater diff between mu
how does compton scatter affect contrast
reduces if deflected photons are captured by detector
how does prob. of compton scatter vary over diagnostic energy range
flat
how does compton scatter vary with Z
independent
when is compton scatter the dominant interaction
at higher energies and in thicker sections
where is the anti scatter grid
between patient and receptor
what does anti scatter grid improve in image
contrast
what is noise
any extraneous info that obscures the desired signal
3 types of noise in DR
quantum
electronic
fixed pattern
noise:
- what is the no. of x ray photons in a small area governed by
- how does quantum noise arise
- what describes quantum noise
- e.g. of a measure of noise
- random processes
- fluctuations in the no. of x ray photons in an area
- Poisson stats
- st dev
noise:
- what is it proportional to
- what is signal proportional to
- what SNR is proportional to
- the square of the no. detected photons
- no. photons
- sqrt (N)
what noise dominates over clinical range
quantum
when can electronic noise be significant
at low doses
what is fixed pattern noise
pixel to pixel variation
why would fixed pattern noise be low for digital radiography
because of flat fielding
what detector characteristics affect spatial res
pixel size
spread of signal
electron focusing in image intensifier
what two other factors (not detector characteristics) affect spatial res
focal spot size
magnification
what is spatial res often quantified in
lp/mm
