Lecture 4 Flashcards
what does the filter do for an Xray
it takes out the lower range of the images
how do you calculate the magnification of an image
M=image/object= a+b/b M>1
what is a
source to object
what is b
object to image
how do you calculate the object size from the image
object=image/magnification
how do we accomplish magnification in real life
do a size marker at the level of the object so then calibrate based off known object
film screen system what does it do and what are the light and dark areas
it sees optical light and optical density so darkness is high absorption, so bone?
what is the cassette
it is the light tight structure that supports the screen and film
what does the screen do
it gives off optical lights so when the X-ray hits it, it turns light and translates to the film
what does an intensifying screen do
screen converts X-ray to visible light and increases efficiency so increase intensifying. Close contact with fillm, cassette with compressible foam.
what is used to help improve light production
gadolinium oxysulfide, LaOBr, YTaO4 which are rare earth metasl
as the screen increases in thickness, what happens
increasing blur
what is radiographic film made of
plastic base coated with light sensitive emulsion consisting of silver halide help in water soluble gelatin.
what reaction does silver halide go through on the film
siler halide is exposed to light and a small number of silver ions reduced (gain an electron) and become metallic silver, and a latent image is formed and stored.
how do we process the film (basic
developer and fixer
what is the developer
when film contacts developing chemicals, it contains a reducing agent and the Ag in the latent image acts as a catalyst and causes the remaining silver ions in that grain to be reduced. A grain of Ag developed is a dark speck.
what is the fixer
it is the aqueous oxidizing solution that dissolves the remaining silver halide that not exposed to light and is then rinsed with water
with greater exposure what happens to the film-like lung
darker
with less exposure what happens to the film like heart
it is lighter
what is the characteristic curve called and look like
H&D curve. xaxis is exposure and yaxis is optical density
what is the toe of the characteristic curve
it is the area lowest on the curve where the low dose and some optical density
what is the shoulder of the characteristic curve
it is the highest dose, so it can represent saturation
what is the best dose range for the characteristic curve
it is the linear region
what is the base+fog
it is the base of the vurve
how do you calculate Optical density
OD=-log10(T) where T is the transmittance through the film,
what is T in formula
it is the I/I0
draw how the H&D curve changes as film speed increases and decreases
if the speed increases the dose decreases for the same OD. If the speed decreases, the dose increases for the same OD.
what is the latitude of the curve and what changes it
latitude is how wide the linear area is. if it is thinner than it is higher contrast so better for detail. If the lower contrast, wider range of doses so better if hard to tell what is happening
draw where the highest level of image contrast is on each of the images you drew
top of the linear portion
computed radiography- how does it work
photostimulatable phosphors (BaFBr) barium fluorohalide a fraxtion of the absorbed X-ray energy trapped in the PSP screen and amount of energy stored in phosphor is proportional to the entrance X-ray exposure. Stores a latent image and visible light is emitted upon stimulation with ease beam for readout
how is the computed radiography read
a laser stimulates the BaFBR and it creases an electronic signal and it creates and electrical signal. and trapped electrons are proportional
what does the curve look like for computed radiography
it is a line instead so it is linear and density/dose is limited because of the linearity
what is a flat panel thin film transister array detector
it is a pixelated flat panel and each pixel is light sensitive which generates a signal
what are the different digital radiography
direct and indirect
indirect digital radiography means what
X-rays reate visible light then electron charge and voltage signal and digital signal. The X-ray creates light through hitting a scintillator.
what is a scintillator usually made of
cesium iodide which is CsI
what are the common uses of indirect digital radiography
it is used for bones, lung, mammography, it is more expenstive, but it couples detection and readout, better read out. It is much quicker so can so fluoro. it is less versatile than CR
what is pixel fill factor
active area is limited by electrical gate lines and other electrical components. Fill factor is the percent of the defined pixel area that actively detects signal. Fill factor decreases as pixel size decreases
what happens to fill factor with smaller pixels
less usable area
what happens to efficiency as fill factor decreases
it decreases
what is direct digital radiography
xrays- electron charge- voltage signal- digital signal
what chemical does direct DR use
selenium with no scintillator needed
what is the fill factor of direct DR use
100% or close to it
what factors affect spatial resolution
focal spot size, detector size, magnification
geometric blurring
since the focal spot is not truly a point source, the edges of the image will have some blur called a penumbra. Draw it.
as focal spot size increases what happens
increased penumbra
as focal spot size decreases, what happens
decreased penumbra
what happens as magnification increases
the penumbra increases with increased magnification,because it gets more spread out
geometric magnification how do you minimize it
you place the image target closer to the image detector and use long source to image distance. long source to image is a large numerator. also the object and detector need to be close
how to calculate the focal spot size
Fd= Fs (b/a)=Fsx (m-1)
what is the optimum magnification for spatial resolution
the effect of source and detector size on spatial resolution is influenced by magnification. the source prefers lower magnification and the detector prefers higher magnification
what is the dominant source of noise
xray quantum noise
when is quantum X-ray noise most evident
under exposure results in images with increased appearance of quantum noise and mottle
what is SNR approximated to
square root of barN
in screen film, what does the brightness depend on
depends on exposure. over exposed is dark and underexposed is lighter. Brightness is dependent on exposure
increased radiation does what to SNR
increased radiation leads to increased photons, increased SD, and increased signal so increased SNR
in digital image receptors what is the brightness dependent on
controlled by image processing. The exposure matters, but the histogram adjusts it.
when the noise highest for digital image rectpros
it is underexposures
what generates scatter in the image sustem
it is from the human generating photons
what does scatter do to image quality
reduction in image contrast
what is the scatter to primary ratio
SPR=S/P
what are ways to reduce scatter
grids, air gap, collimator
what do grids do
place in front of the detector Anti-scatter because the have parallel things. It reduces aberrant signal while letting primary go through.
what are the septa of grids made from
lead
how do you calculate the grid ratio
H/W height of material/ grid opening width
what does a high ratio grid do
it removes more scatter photons, because very marry margin, but need better alignment of patient
what grid is used with fixed radiographic table
high ratio grid because decreased patient factors so need 10:1 or 16:1 grid
what grid is used for portable Xray
it is a grid of 6:1 or 8:1 because the are manually position so need lower grid ration
what happens if the grids are not foccused
it comes out bad
what does off center grid look like
gradient
in a lung what would off center look like
it could lead to a lung being brighter than the other
what happens to a grid if the focal distance is off
it is a blurry less clear imagee
what can happen with stationary grids in an image
can see the grid
what is the bucky grid
the grid is movable so it has motion and it blurs the grid lines
what is the bucky factor
the amount of primary photons removed from by a grid so need increased dose and have same exposure
what is the primary transmission factor
T- and ideal is 1 and it is dependent on KV
what is scatter transmission facotr
ideal is 0
what is CDF
reduction of contrast due to scatter
what is selectivity and what do you want it to be
want it towards infintity
what is the airgap and how does it affect scatter
scatter increases with decreased aigret. increased aigret better imaged because scatter misses the plate. This means the gap of air between patient and detector
what is collimation
reduces exposed volume of tissue and less scatter so better image contrast. Collimation means a smaller field is exposed so there is less scatter and reduces the exposed tissue to just whatever you want
what are the technique factors that can be optimized for imaging
KvP, MA, exposure time, focal spot size, SID, and grid use
what are the best parameters for heart radiography
long source to image disaffect to reduce geometric magnification of the heart, and high MA for short exposure time and large focal size for high MA. Geometric unsharpness is minimal due to long SID. High KV reduces visbility of bone and overall contrast
what is best for pediatric radiography
small focal spot for best spatial resolution, and high MA for shorter exposure time because motion. low MAs for reduced patient dose, so do not use grid because want to eliminate higher doses
what is automatic exposure control
photo timer used to regulate exposure level to the radiographic image receptor. Automatically shuts off X-ray when predetermined exposure level is reached. KVP, MA, and focal spot size are manually selected.
in AEC what is used to regulate exposure level
photo time used to regulate exposure level to the radiographic image receptor and it shuts off X-ray when a certain level is reached
what is controlled manually in AEC
kvp, coal spot, MA
is there just one photo timer or multiple
multiple want AEC over what you want optimized tissue wise
if the image is super bright when underexposed what system are they using
film screen
what is best Ma for an image
want the lowest it can be with best image quality. Too much leads to excess exopsure and clipping. Under exposed need to retake from noise
what is the exposure index
measure of detector response to incident radiation in a relevant image region. EI=K/100 K=radiation to the detector for a standard beam. Directly proportional to exposure of detector
what happens to the MAS if the EI is doubled
it doubles
what is the deviation index
it compares EI to a target EI so if you are shown a EI then you don’t need to factor in the target. SI is defined as body regions comapred.
what is EI most dependent on
positioning of the patient. Relative region so anatomy is included and dark is exclused. Collimation is very important then
what is dual energy radiography
it is z and rho so it gets rid of overlying so it can separate the things. it is dependent on the relationships
