Chapter 2 x-ray imaging Flashcards
dominant source of scatter in x-ray imaging
compton
coherent is < 5 %
ratio of scatter x-rays to primary x-rays exiting a patient
5:1 in abdominal radiography
what does scatter do?
reduce contrast
-does not affect spatial resolution or mottle
say a lesion transmits half the x-rays of adjacent tissues. What is the contrast for a scatter to primary ratio of 0:1 and 1:1?
0:1 - contrast is 50%. 100 primary photons and 50 for lesion
1:1 contrast is 25 %. scatter does nothing. Get 200 photons for adjacent tissue and 150 for lesion
how is scatter radiation minimized?
scatter removal grids
what reduces/increases scatter
-scatter increases with increasing field size
-collimation reduces scatter. Also benefits patients as reduces dose to patient by restricting field size
-increases with increased patient thickness
-max patient thickness without a grid is 12 cm
-less scatter with high Z materials like bone (PE effect dominates)
-scater is less with lower tube voltage because PE will dominate over Compton
is scatter as concerning with small fields?
No, because most of the scattered photons will miss the image receptor
tube voltage, scatter, and grid required for extremity x-ray
55 kV, very low scatter, no grid
tube voltage, scatter, and grid required for skull x-ray
80 kV, moderate, grid
tube voltage, scatter, and grid required for chest x-ray
120 kV, high, grid
tube voltage, scatter, and grid required for abdomenx-ray
80 kV, very high, grid
what are anti-scatter grids made of?
parrallel bars of highly attenuating material like lead. strips are filled with low attenuating material like graphite or Al to keep strips aligned.
where are grids placed
between patient and image receptor
what is grid ratio
ratio of strip height H over gap between the strips D (i.e HD)
common grid ratios in radiography and fluoroscopy
~10
common line density in radiography and fluoroscopy
~ 45 lines/cm
are grids focused?
yes, and have diverging strips that must be used at a specified distance
what is a reciprocating grid?
grid moves during the treatment, rendering them invisible
what is the device that moves the grid?
Bucky
define primary transmission
% of primary (useful) radiation that passes though grid
-grids transmist about 70% of primary photons and absorb the rest
how much of the scattered photons do grids absorb?
absorb 90 %, transmit 10%
what is Bucky factor
-ratio of radiation incident on the grid to transmitted radiation
-increase in patient dose due to use of a grid
busky factor for abdominal radiography
5
define contrast improvement factor
ratio of constrast with a grid to contrast without a grid, usually >/= 2
artifacts with grid
grid cut-off.
-caused by improper alignment, wrong focal spot to film distance for the focused grid, inverted grids
depedence of primary transmission, scatter transmission, image contrast, and patient dose with grid ratio
primary transmission increases for smaller grid ratio
scatter transmission increases for smaller grid ratio
dose decreases with smaller grid ratio
contrast decreases with smaller grid ratio
when does geometric magnification occur?
when there is air gap between patient and image receptor
what happens to primary vs scattered photons when using an air gap?
primary photons will reach the image receptor and be geometrically magnified
scattered photons are less likely to reach the image receptor
usually with air gaps, do not require additional scattere removal
disadvantage of air gaps
-causes magnification which increases focal spot blurring- can require a smaller focal spot
where is magnification imaging typically used?
mammography and neuroradiology
-can improve lesion visibility of a microcalcification cluster or a small blood vessel
what does analog film consist of?
-thin emulsion supported by a Mylar base
-emulsion contains many small silver halide grains
briefly describe how film works
-x-rays sensitive the silver halide grains
-exposed films containing latent image are processed through developer solution then a fixer solution
-in the developer solution, sensitivzed silver halide grains are reduced chemically and form a speck of silver
-unexposed grains are removed in the fixer
-fixer also makes image permanent
what corresponds to light/dark regions when viewing a film?
-silver grains (where dose was delivered) appear dark because loight cannot prenetrate
what is film blackening
pattern represents the distribution of x-rays reaching the image receptor
describe film density
film density of 1 transmist 10% of visible light; density of 2 transmits 1%, density of 3 transmits 0.1%
optical density
-log(Itransmit/Iincident) or log (Iincident/Itransmit)
what is the target average film density in film radiography?
1.5
what is fog and base
level of blackening in absense of any radiation exposure
base is density of film base alone
-unexposed film has a base + fog level of 0.2 OD units
what is max film OD
3
-occurs when all grains in the film have been sensitized and reduced to silver grains during development
what is characteristic curve? screeen film
represents relation betwen radiation intensity incident on film (Kair) and resultant film OD
regions of screen film characteristic curve?
toe= low exposure region
shoulder = high exposure region
linear region in between toe and shoulder
-only in linear region do you get good contrast
what is the amount of film contrast proportional to?
gradient (steepness) of linear portion of characteritic curve
what is latitude?
range of radiation intensities that result in satisfactory contrast
what is contrast film and where is it used
has steep cruve, narrow latitude
used in mammo
where are wide latitude films used?
-shallow curves
-used in chest radiography to capture wide range of exposures
how much of the incident x-rays does a film along system absorb?
1 %
what are intensifying screeens for?
contain phosphor crystals to absorb 50 times more of the incident x-rays than radiographic film
how do intensifying screens work?
-each x-ray absorbed in screen produces hundreds of visible light photons
-screens convert x-ray pattern to light pattern
what do intensifying screens do for exposure times and patient doses?
-decrease exposure time and patient dose 50 X compared to film alone
what is spectral matching
some screens emit mainly blue light, others green light
-light output from the screen and the light sensitivty of the film must be matched
pros and cons of thick vs thin screens
-thick screens absorv more x-rays but increase image blur because the light diffuses more before striking the film
-thin screes absorb less x-rays but yield sharper images
where do white artifacts come from?
stuff stuck between the screen and film- light from the screen cannot reach the film
what happens if Kair is too low or too high?
too low = underexposed film
too high = overexposed
want Kair of about 3 uGy
explain automatic exposure control and how it affects patient dose
-AEC keeps Kair fixed
-higher kV = less skin dose because photons are more penetrating and therefore less photons are required to get appropriate Kair (therefore mAs is reduced)
-vice versa also true
how does filtration affect patient dose?
more filtration= higher energy photons = reduced mAs and dose when Kair is fixed
what is a scintllator?
emits light when exposed to radiation
scintillator absorbs a 35 kEv photon- how many light photons are produced?
1000
what are radiographis screens in mammo made from?
Gd2O2S (gadolinium oxysulfide)
2 compoments of flat panel detectors?
scintillator (CsI) coupled to array of light detectors (2000x2500)
why is CsI a commonly used scintillator?
k-edges are 36 kEv (Cs) and 33 keV (I)- good match for average x-ray energies of 40 keV (for 80 kVp) in radiography
are scintillators direct detectors?
no, indirect because x-rays are absorbed to produce light and this light is converted to charge
what are image intensifier input phosphors?
scintillators, usually made of CsI
do scintillators produce blur?
yes, because of spreading of light from x-rays
-can be manufactured into columns which restrict the amount of light diffusion and reduce detector blurring
how do photoconductors works?
-absorb x-rays which are converted into charge. A voltage collects the charge
-electronic signal is proportional to x-ray energy deposited
are flat panel detectors that use photoconductors direct detectors?
yes, because liberated charge is directly collected
what is most common photoconductor?
selenium
k-edge is 13 keV so poor x-ray absorber at higher energies (> 80 kV). Alternatives are PbI and HgI
-Se is good in mammo (average energy 20 keV)
pros and cons of thick vs thin scintillators
thick- high x-ray absorption, low patient dose, lots of blur
thin- low x-ray absorption, high patient dose, less blur
characteristic curve for a digital detector?
linear over wide range of intensities (no toe or shoulder)
-image brightness and contrast is independent of x-ray intensity, unlike screen-film
-doubling x-ray intensity doubles the signal detected
do photoconductors or scintillators have sharper images?
photoconductors- charge doesn’t spread out, thus less blur
explain photostimulable phosphors (computer radiography)
-made of barium fluorohalides (BaFBr)
-x-ray photons are absorbed in the phosphor- absorbed energy is stored and creates latent image
-plates are read out with red lasers to release the stored energy, that is emitted as blue-green light
-amount of blue-green light from each pixel represents number of x-rays reaching this pixel
-detected light pattern (5 million pixels, 2000x2500) is digitized and stored
-white light erases the photostimulable phosphor plates so they can be reused
pros and cons of thick vs thin photostimulable phosphors
thick is impractical because of unavoidable light scatter. The read out (red) light scatters into adjacent pixels and releases light, increasing blur (i.e. light from different pixels scatters into “wrong” pixels)
-thin absorbs fewer photons than scintillators- increases patient dose
is photostimulable phosphor as sharp as scintillator or photonconductor image?
No
briefly explain how gas detectors work (this is clearly for the non RO folks)
-ionization chamber
-incident x-rays liberate electrons in a gas (Compton, PE- then those electrons lose their energy with collisons with atoms, producing more electrons)
-total electron charge liberated is the signal- electrons are collected by positive anode
what would imaging gas detectors use for gas?
-not air
-high Z gases
-xenon (k-edge 35 keV)
why are gas detectors operated at high pressures?
-improve ability to absorb x-rays
are gas detectors used for clinical x-ray imaging systems?
rarely
what x-ray intensities can digital detectors handle? screen film?
-digital detectors handle x-ray intensities 100 X lower and 100 x higher than screen - film
-screen-film requires Kair of 3 uGy (or else underexposed or overexposed_
dynamic range of digital radiographic systems vs screen-film
10,000:1 vs 40:1 for screen film
what happens if a digital detector is exposed to too little radiation?
-noisy image
-won’t be “light”
-when digital detector signal is low, electronic noise becomes important and may degrade image quality
what happens to digital detector is exposed to too much radiation?
-image quality rarely affected but patient gets more dose
what does ADC do?
-analog to digital converter
can also have digital to analog converters
what is a bit and a byte
computers store data in bits (can be 0 or 1)
byte is 8 bits
what is required to display digital data?
digital to analog converter
-computer graphics board is example of DAC
what is kilobyte
1024 bytes (2^10 bytes)
how much data do medical images contain?
megabytes (1024 kilobytes)
storage requirement for radiology department is on order of..
terrabyte (1024 GB)
what is baud rate?
rate of information transfer in bits/second
how long do computers take to transmit a chest x-ray (10 MB)?
ethernet- 10 Mbps - 1 minute
fast ethernet - 100 MBps - < 10 s
Gigabit ethernet- 1,000MBps - < 1 s
what is matrix size
of pixels in a given dimension
what are pixels?
individual picture elements in a 2D image
what is information content of an image?
number of pixels * number of bytes/pixel
explain how pixels are coded using 1 or 2 byte
1 byte = 8 bits, 2 bytes = 16 bits
-256 (2^6) shades of grey can be coded for 8 bits
-adding a bit doubles number of shades of grey; removing a bit halves number of shades of grey
how much memory would a 1024x1024 matrix use?
1 MB for 1 byte coding, 2 MB for 2 byte coding
typical matrix size, byte per pixel, and nominal image size (MB) for nuc med
128x128, 1, 1/64
typical matrix size, byte per pixel, and nominal image size (MB) for MRI
256x256,2, 1/8
typical matrix size, byte per pixel, and nominal image size (MB) for digital ultrasound
512x512,1,1/4
typical matrix size, byte per pixel, and nominal image size (MB) for ncomputed tomography
512x512,2,1/2
typical matrix size, byte per pixel, and nominal image size (MB) for TV digital photospot/DSA
1024x1024,2,2
typical matrix size, byte per pixel, and nominal image size (MB) for digital readiography
2560x2048,2,10
typical matrix size, byte per pixel, and nominal image size (MB) for digital mammo
4096x6144,2,50
what is soft copy display?
present images on flat panel monitors
typical luminence of monitors
300 cd/m^2
where are 2 MP vs 3 MP pixel monitors used?
3 MP- digital display radiographs
2MP- MRI, CT
2MP color- NM, ultraosund, PET/CT
who developed test patterns to evaluate monitor performance?
what is the test?
society of motion picture and television engineers
-intensities range from 0% to 100% in 10% increments
-display is good when 5% square in 0% background (dark) and 95% square in 100% background (white) can both be visualized
what are individual pixels assigned in digital imaging?
-location
-gray scale value by group of bits
what offers identical grayscale image display on different monitors?
DICOM grayscale standard display function
what does lookup table do? ie LUT
-straight line that maps pixel values to image intensities
what does window level define?
center (midpoint) of window width and therefore overall brightness
what does window width display?
range of gray scale values displayed, resultant image contrast
-all pixels with values below this range register as black and all those above as white
what happens to image contrast within the window range as the window narrows?
-increases
what does image inversion do?
make dark regions appear white and vice versa
what is unsharp masking?
-subtract a smoothed version from original and add it to replicate original
-improves visibility of tubes, lines cathethers
-increases noise and can introduce artifacts
what does histogram equalization do?
eliminates while and black pixels that contribute little diagnostic info and expands remaining data to full display range
what is energy subtraction?
-sutract projection radiographs obtained at 2 energies
-ex: 2 radiographs obtained at high and low kV can be subtracted to eliminate bone-like structures and improve depiction of lung and soft-tissue
-or produce bone-only image, to help see calcified and non-calcified lung nodules
are grids tyically used for bedside chest radiographs?
no, difficult to align
explain grid cutoff
when grids are not aligned corrently, the resultant images exhibit grid cutoff artifacts showing light regions where no x-rays reached the image receptor
what is gray scale display function?
relates pixel value to monitor brightness
-provides identical grayscale image display on different monitors
typical bucky factor in adult abdo radiography
5
films with steep gradient result in images with improved…?
contrast
screen films absorb how many more xrays than film alone?
50X more
film will absorb 1 %. Film-screen will absorb 50%
increasing film quality does what to patient entrance Kair?
reduces it
since fluence is more penetrating, less is required at entrance to obtain what is required at the film
which has best resolution?
photoconductor
scintillator
photostimulable phosphor
photoconductor because charge produced is not dispersed during the detection process
-charge from scintillators is dispersed
what types of images require the least storage?
MRI
