Digital Radiography Technique Chapter 14
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46 Terms
How is Spatial Resolution described?
described as the size of an object that can be viewed (how small) and by the quantity “spatial frequency”
Smaller pixel size = ?
better spatial resolution
How is spatial resolution measured?
measured in line pain test tool
Spatial frequency refers to
line pair (lp/mm)
What is a line pair?
the line and an interspace of same width
higher spatial resolution = ?
better spatial resolution (more lp)
x-ray spatial resolution is determined by
focal spot size
What is Digital imaging spatial resolution limited to?
pixel size
What is Modulation Transfer Function (MTF)
helps quantify the overall imaging performance of a system interms of resolution and contrast
(Essentially, it measures how effectively a lens or imaging system can reproduce the fine details (high spatial frequencies) and overall contrast of an object in the resulting image - google)
MTF - Ability of an imaging system to
show objects of different sizes on an object
what objects are more difficult to image?
small
an imaging system that produces an image exactly as the object appears has an MTF of?
one (1); lower than 1 is worse
the 10% MTF is identified as the
system spatial resolution
what does DR have?
has higher MTF than screen film at low spatial frequencies due to expanded dynamic range and higher DQE
DQE
Detective Quantum Efficiency
What is DQE?
The probability that an x-ray will interact with an image receptor
thickness of the capture layer
anatomic composition
What is DQE related to?
absorption coefficient and spatial frequency (x-ray energy dependent, you want to match k-edge of capture element)
Increase in DQE = ?
less x-rays needed to produce image = lower patient dose
What does Digital imaging have better?
contrast resolution than screen film
screen film
30 shades of gray
digital range
256 - 65,536 shades of gray
how is the dynamic range of a digital system identified?
bit capacity of each pixel
8 bits =?
256
16 bits. =?
65,536
what entails postprocessing?
window width (contrast) . window leveling (brightness)
Why is post processing valuable?
valuable for imaging soft tissue (breast)
What is more important that spatial resolution?
Contrast Resolution
SNR
Signal-to Noise Ratio
SNR: Signal
the image forming x-rays that represent anatomy
Noise Limits
contrast resolution
increase mAs = ? SNR = ? PT DOSE?
INCREASE SNR = INCREASE pt dose
what happens to SNR when you use more x-rays
increase
What also increases SNR?
DSA
DSA
Digital Subtraction Angiography
What do some techs not do regarding patient dose?
they do not adjust technical factors
what is dose creep?
Dose creep is the gradual acceptance over time by radiographers of the use of higher radiographic exposures,
What do digital images have higher?
DQE
When should exposures not be repeated?
for brightness or contrast concerns
What can DR systems not compensate for?
noise caused by quantum mottle
overexposed images
do not have to be repeated
Does image contrast change with dose?
no
should digital images be repeated for exposure factors?
no
what does kVp and mAs control in SF imaging
optical density
increasing kVp and reducing mAs do what in digital?
reduce pt dose; contrast and spatial remain constant
if technique is very low
low SNR, too much noise (graininess), difficult to interpret soft tissue detail