unit 4 review Flashcards
why would an image be too dark
to many photons hitting IR, too dense
how to fix an image that is too dark
window level adjustment
why would an image appear too light
not enough photons hitting the IR, underexposed
how to fix an image that is too light
can’t , repeat image
controlling factor affecting density / IR exposure
mAs
influencing factor affecting density / IR exposure
kVp
more kVp =
more scatter
mAs must be changed from ____% to see a visible change in density
25-35
mAs & density are
directly proportional; BUT just because we double mAs does not mean density is doubled
in digital what are the graphic representations of the IR exposure
histograms
the width of the histogram
displays the range of exposures
the 2 ways kVp alters intensity
a change in kV alters the intensity of the beam when all other factors remain the same & affects the production of scatter
kVp being altered affects both
quality & quantity (scatter)
with a large focal spot
size bloom may occur , increases IR exposure
__% density variation from anode to cathode
45
as distance increases IR exposure
decreases
density / IR exposure decreases when filtration is
increased
compensating filters require an ____ in technique (mAs)
increase
beam restriction (collimation) reduces
the total # of photons, reducing IR exposure & density
air ( radiolucent contrast)
increases density/ IR exposure
barium / iodine (radiopaque contrast)
decreases density/ IR exposure
the more efficient the grid
the less density IR /exposure
high grid ratios will
decrease density / IR exposure
wide dynamic range “margin of error”
— higher latitude over film screen
DQE - detective quantum efficiency is
how efficiently a system converts the incoming x ray into an image - determines the latitude
contrast is
the difference in adjacent densities/ IR exposure
dynamic range is the concept of
contrast as it is DISPLAYED on a monitor
what is the digital processing function on the display that controls the range of brightness / contrast
window width
contrast / dynamic range / window width all describe the
grayscale bit depth
image contrast is
the amount of contrast acquired from the IR
subject contrast
results from the anatomical part being radiographed ( NOT talking about IR)
high contrast
black & white , fewer shades of gray (think extremities)
low contrast
more shades of grays , long scale
the # of useful visible density or shades of gray
scale of contrast
short scale is
maximal differences (black & white)
high contrast
long scale is
minimal differences between densities but maximal total number of densities
- low contrast
the number of different shades of gray that can be stored and displayed by a computer system - monitor function
gray scale
physical contrast
densities/ IR exposures that can be recorded by the IR
visible contrast
total range of densities and IR exposure that can be perceived by the human eye
term used to describe the ability of an IR to distinguish between objects having similar contrast
contrast resolution
increased contrast, low kVp, short (narrow) dynamic range / window width
high contrast
decreased contrast , high kVp, large wide dynamic range
low contrast
kVp only affects
pre exposure
kVp is controlled by
LUT
Subject contrast is dependent on
kVp & tissue type/ thickness
digital image receptor contrasts primary controller is
window width manipulations
asses the range and quality of IR exposures
histogram
will produce higher contrast because low energy photons will be absorbed by thicker parts — will not reach IR - more contrast
low kVp
Will be able to penetrate more equally and reach IR, thus causing a more uniform image - less contrast
high kVp
as body part thickness increases x-ray absorption
increases
overall thicker part =
decreased subject contrast
overall thinner part =
increased subject contrast
mAs affecting contrast; if IR exposure is too high (overexposed) or to low ( underexposed)
contrast will be decreased
influencing factors affecting contrast
mAs, focal spot size, anode heel effect, distance, filtration, beam restriction, anatomical part, grid
SID - since IR exposure is affected by distance
so is contrast
beam restriction
Reduces scatter therefore increases contrast
images are improved with a
Larger matrix size with a larger number of pixels
The square picture elements making the matrix - individual boxes
pixel
