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