Test 4 Flashcards
What is a digital image
any imaging process that creates an electronic image that can be viewed and manipulated on a computer
digital vs traditional
no more physical film just digital and can be manipulated after exposure
when was the first digital manipulation of angiography
1977
when did it become common to use digital manipulations
1980
when was the first computerization of CT and ultrasound
1970-1990
what helped change radiography to digital? what were the changes?
insurance requirements pushed
change: film to digital allowing film processing systems
what is concerned as digital radiography
computer radiography and direct capture radiography
what do cassette based systems use?
traditional type film
imaging plate stimulated by phosphors and storage
what uses cassette based systems
computed radiography
what is indirect digital
radiographers have to move imaging plate
is cassette based systems indirect digital? cassette-less?
yes
no –> direct digital
what is direct digital
detector and reader are permanent part of table or wall unit
why are detectors important for direct digital
small detectors = active matrix array = enhanced contrast resolution
where is the matrix visible in
IR and monitor
what forms a matrix
digital image acquisition
numerical values stored in the computer’s memory
cells in each row and column
what is a pixel
picture element –> each cell in the matrix
how do we improve digital image quality
larger matrix size
what does a large matrix size give better images
more smaller pixels within matrix = more contrast
what is the smallest element in matrix
pixel
how does a pixel work
each pixel assigned single number to represent brightness by location in matrix
what does a pixel correspond to
area of patient’s tissue
what is an analog image
captures or measures continuously changing signals
what does analog signals control (3)
level of brightness
shapes
colors
difference between analog and digital
analog:
- xray enters IR in analog form and converted into digital
- single sample exposure
digital:
- records as multiple numeric values and divides into several small elements to be process in many ways
- multiple sampling
how does an analog system convert to digital
xray energy –> light waves
how does a digital system produce an image
analog signals –> numbers
what controls digitizing data? how?
pixel values limits number values –> difference between white and black
why do we want a limited digitizing data set?
for pre-set pixel values –> contrast
what does a analog-to-digital converter (ADC) do
digitizes incoming analog data
Steps for digitizing an image
scanning
sampling
quantization
what is scanning
the field of the image divided into a matrix of small cells
how does scanning work
it detect activated cells (pixels) through detector to make up initial matrix
ex. collimator
what is sampling
detection and measurement of signal intensity coming into the system from each pixel location
what controls sampling? pro?
mAs controls each pixel = better contrast
what is quantization
leveling out brightness level for each pixel to nearest available gray level in preset dynamic level
why is quantization vital?
compares given data to move/fix to preset values
what is dynamic range
range of pixel values (shades of gray) available from computer system (hardware/software) to create final digitalized image
limitation of dynamic range? pro?
control: subset of bit depth of system
pro: allows image manipulation
what is a subset of dynamic range
grayscale (displayed image) –> contrast
what is pixel bit depth
- max range of pixel values that computer or hardware can store
- number of bits within a pixel
what is the number of gray tones within a pixel
2 to the power of bit depth
what does gray level determine
image contrast resolution
____ grayscale = ____ constrast = _____ pixelation
more
more
less
spatial resolution
distinguishes one dot between another dot
what determines the size of a pixel
size of matrix
____ pixel = ___ detail
smaller
greater
size of pixel is directly related _____
amount of spatial resolution or detail in image
which number of pixels in a matrix is better?
1024 x 1024 or 16 x 16
1024 x 1024
what is attenuation coefficient
% or ratio of original xray beam intensity absorbed by each different type of tissue area within body
what does attenuation coefficient create
3D projection onto 2D IR
voxel
volume of tissue to pixel
averaged by dexel
dexel function
rounds average attenuation coefficient to nearest preset value in dynamic range
imaging chain of events
- patient placed between Xray source and IR
- technique and geometric factors selected
- capture image and latent image is formed
how is a latent image formed?
an invisible image is created from the remnant beam altering the atomic structure of photostimulable phosphors
what are the types of digital radiography
digital radiography (DR)
computed radiography (CR)
types of DR conversions
direct
indirect
difference between DR and CR
DR: IR directly connected to digital processor electromagnetically
uses TFT
CR: uses PSP
what is a AMA? what does it contain?
active matrix array
flat panel with thousands of individual dexels
what is the main component of all DR detectors
AMA
what is the size of one dexel
100 microns square or 1/10th of a pinhead
what material is the detection surface of a dexel made of in Direct DR
amorphous selenium
why is amorphous selenium good
high absorption efficiency for xrays
how does a AMA work in direct DR
converts remnant beam directly into electrical charges for computer to read
3 components of dexel
radiation-sensitive detector surface (a-Se)
thin-film transistor (TFT)
small capacitor
function of radiation-sensitive detector surface (a-Se)
semiconductor sensitive to xrays (direct) or light (indirect)
function of TFT
individual switch for each pixel to change states on/off quickly
function of capacitor
stores electrical charge
function of amorphous selenium
converts xray energy to electrical charge
what does ionization of selenium produce? what ionizes selenium?
electron hole pair (+)
dexel electrode (-)
xray ionizes
direction of electrical charge movement in direct DR
+ moves down toward dexel electrode and stored in capacitor
- moves up to be collected and drained off
types of wires in AMA in direct DR
gate lines
data lines
gate line function
reads out information on exposed DR detector by changing bias voltage from -5 to +10 volts
data line function
sudden change in gate line = electricity flow and charge stored up in capacitor flows to data line
what material is indirect detector dexel made of?
amorphous silicon
indirect vs direct AMA
indirect: has phosphorescent screen (aka scintillator) laid over AMA
photodiode (a-Si)
direct: photoconductor (a-Se)
function of phosphorescent screen
phosphor converts xray into light –> light goes to hit AMA
how does AMA work in indirect DR
remnant beam hits phosphor screen to fluoresce (visible light) when exposed
then same process as direct DR occurs
difference between DR and CR
DR uses TFT
CR uses PSP plate
function of amorphous silicon
high absorption efficiency of visible light
components of CR
cassette
processor (CR reader)
IR –> PSP plate
construction of cassette
material: plastic
memory chip in corner for patient/procedure information
components of PSP plate (7)
front protective layer
phosphor layer
reflective layer
electroconductive layer
polyester base layer
light-shielding layer
back protective layer
function of protective layer and material
function: protects PSP from mechanical damage
material: low absorbing carbon fiber
function of phosphor layer
active layer containing europium activated barium fluorohalide (phosphor)
function of reflective layer
reflects emitted light photons towards photomultiplier tube during scanning (NO INTERACTIONS OCCUR)
function of electroconductive layer
prevents static build up –> no artifacts on image
light shielding layer
prevents extra light from erasing later before its scanned
what material would stimulated phosphorescence occur
barium fluorochloride and barium fluorobromide
why use barium fluorochloride and barium fluorobromide
contains defects (metastable sites) in crystals to trap free electrons in, when ionized
ability of metastable sites
traps free electrons and store them until excitation occurs to release electrons
steps for CR processing
PSP removed from cassette by processor –> scanned by helium-neon red laser beam moving across plate to index down one row at a time
what occurs during scanning with red laser beam
metastable sites activated by electromagnetic energy to emit dim light –> image electronically amplified to be displayed
what is fluorescence and example
immediate emission of light by substance under some type of stimulation
ex. xray exposure of phosphor plate
what is phosphorescence and example
delated emission of stored energy in the form of light
ex. laser beam
what occurs in CR reader (processor)
PSP plate pulled by suction cups and rollers –> fast scan and slow scan
what occurs in fast scan
laser beam is deflected off rotating mirror to scan across PSP plate
what occurs in slow scan
rollers are used to direct/move PSP plate
what determines pixel size in DR and CR
DR –> IR size
CR –> defined during processing
fast scan controls?
pixel width
slow scan controls?
pixel length
what prevents distortion in CR
equal frequency in fast/slow scan
what occurs during erasing process
after scanning –> PSP plate moved by rollers into eraser section
PSP plate exposed to intense white light –> removes remaining information
plate reloaded and ejected from machine
cons to PSP plate
very sensitive to background radiation –> prefogging
1mGy = noticeable fog
FIX: always erase before use
difference between scatter and background radiation
background: can change how image is processed
scatter: can be corrected during processing
what are characteristics for image quality? (5)
brightness
grayscale (contrast resolution)
noise
spatial resolution (sharpness)
distortion
what characteristics is classified as visibility
brightness
grayscale
what characteristics is classified as sharpness
resolution
distortion
what determines the overall quality of a radiographic image
visibility
sharpness
what is a photographic property? Geometric property?
P: visibility
G: sharpness and distortion
what is brightness
luminous intensity of the display monitor’s light emission
what does brightness measure
amount of light emission of a display monitor
if an image is too light….
excessive brightness to allow visualization of anatomic structures
if an image is too dark…
insufficient brightness and anatomic part cannot be seen well
is brightness and IR exposure related? why?
no brightness is a monitor control that can change lightness and darkness of an image on a display monitor
what controls brightness
window leveling and technique
what is brightness measured in? (unit)
candela
how does adjusting window level affect brightness
changes average gray level –> center gray shade on dynamic range
what occurs if window leveling is unchanged
average brightness level is uncahnged
how can we change brightness without changing window leveling
minimum change of mAs by 30%
which post processing method is preferred and why?
window leveling because window width has a narrower dynamic range which can cause misdiagnosis and potential legal issues
what does window level correspond to?
pixel value
increasing level = _______ image
decreasing level = ______ image
darker
brighter
what occurs during underexposure
exposure to IR is too low for anatomic area
what effect would underexposure cause
excessive quantum noise
what effect would overexposure cause
saturation –> super black and white
what does an exposure indicator provide
numeric value indicating level of radiation exposure to digital IR
when does dose creep occur? issue?
lack of attention to wide dynamic range
issue: overdosing patient
why is digital IR bad?
allows exposure errors to occur –> wide range of IR exposures = dose creep
what is contrast resolution
ability of digital system to display subtle changes in shades of gray
contrast resolution is directly related to what?
bit depth of pixels
high contrast resolution = ?
enhanced densities
what is grayscale also known as?
image contrast
how is grayscale represented?
percentage or ratio of differences between IR exposures
what does grayscale measure
differences of clear white through varying shades of gray to black
what does grayscale affect
visibility of detail on displayed digital image and differences between IR exposures
what does dynamic range describe
describes contrast concept displayed and is a characteristic of overall image system
what is dynamic range
the range of brightness of display monitor light emission
what does dynamic range represent? limited by?
number of shades of gray
Limited by computer system
what is actual dynamic range
max number of shades of gray represented by numeric range of each pixel or bit depth
what does actual dynamic range represent
capabilities of the overall system
can the actual dynamic range be less than bit depth
yes
what does bit depth represent?
hardware components
what is high contrast
difference between adjacent IR exposures that greatly affect contrast
how does high contrast affect images
less varying grays
high contrast is also known as?
short scale
what is low contrast?
differences between adjacent IR exposures are minimal
how does high contrast affect images
more various shades of gray
low contrast is also known as?
long scale
what is the primary method to adjust display contrast?
window width
does kVp and mAs affect grayscale?
no because image processing will correct
how does the system adjust the grayscale
Histogram (linear range algorithm) and look up table (LUT)
what does the look up table do? affect on image?
provides proper grayscale
consistent image
what is on the look up table
stored data to sub new values for each pixel during processing
look up table limitations?
needs correct histogram selection
cannot compensate exposure values outside normal range
what is window width
range of pixel values thats incorporated into display width
increasing window width = _______ contrast
lower
decreasing window width = _______ brightness
increase
what is subject contrast
range of difference in intensity of xray beam after being attenuated by subject
how does subject contrast occur
differential absorption
what is subject contrast dependent on
kVp
amount and type of irradiated material
what is the primary controller of subject contrast
kVp
if kVp adequate
low kvp = _____ subject contrast
high
what is the easiest way to improve contrast
use collimator
is contrast same throughout the whole body?
no because of difference densities
what do radiologist want on images
uniform contrast
can mAs compensate for inadequate kVp
no use 15% rule
factors that affect final displayed image contrast/grayscale
look up table
kVp
mAs
how can final displayed image contrast/grayscale be altered
window width
what is the final displayed image contrast/grayscale mainly affected by
look up table
collimation affects?
contrast
patient dose
negatives of noise
interferes with formation of image
no useful diagnostic information
what type of noise is there? (4)
anatomic
radiographic
equipment
quantum (mottle)
what is signal to noise ratio
the strength of radiation exposure compared with amount of noise apparent in digital image
why is SNR important
it shows how much noise can be tolerated in an image
how to improve image quality with SNR? con?
increase SNR (higher signal) –> less noise
CON: increases patient dose
how do we get quantum noise?
little xray photons reaching IR to reach latent image
what other factors can cause mottle? (3)
materials such as:
IR
electrical current
computer algorithms
ways to lower noise to best capabilities
why do we do this?
set appropriate target exposure value (IE#)
routine monitoring
avoid exposure creep
what is considered appropriate exposure value
image production with acceptable noise level without excessive or unnecessary exposure to the patient
optimal image = ______
may not be the best image
spatial resolution is also known as?
definition
sharpness
recorded detail
spatial resolution in relation to a system
ability for system to show small details of an object
how do we know that we have good spatial resolution
dots are very distinguishable between each other
what does spatial resolution control
detail or sharpness of structural lines
what is considered as a geometric property
spatial resolution
distortion
what is spatial resolution
the degree of geometric sharpness or accuracy of structural lines actually recorded in a image
what is spatial frequency
digital imaging recorded detail based on frequency of wavelength
what is high spatial frequency?
Pros?
high frequency with shorter wavelength
signal pair of lines are closer together
high resolution
better for smaller objects
what is low spatial frequency?
Pros?
low frequency long wavelength
signal pair of lines are further apart
low resolution
better for larger objects
what determines sharpness?
matrix size
pixel size
grayscale bit depth
how is sharpness measured? (4)
point spread function (PSF)
line spread function (LSF)
modulation transfer function (MTF)
system noise
what is sharpness
characteristic of final displayed digital image
what is MTF
modulation transfer function
trueness or fidelity of an image
what does MFT measure
accuracy of image compared to original object (scale 0-1)
percentage of object contrast that is recorded
if MFT is 0 ….
no image –> no signal
if MFT is 1….
perfect exactness
if spatial frequency increases = MFT _____
decreases
what does detective quantum efficiency (DQE) measure?
efficiency of IR in converting xray exposure it receives to quality radiographic image
if we have 1.0 DQE what does that mean?
no information lost during conversion –> 100% DQE
higher DQE = _____
decrease radiation exposure/patient dose
factors affecting spatial resolution in order
eliminate motion
reduce OID –> affected side close to IR
reduce focal spot size –> penumbra
increase SID –> use 46” instead of 40”
what is distortion? types?
misrepresentation of size or shape of the structures examined
TYPES: size and shape
how can we tell if something is distorted?
by understanding normal radiographic anatomy
what property is distortion? what does it affect?
geometric
affects image quality
what are factors that affect size distortion
magnification –> SID and OID
post processing –> resizing images
to control magnification we want…. SID/OID
maximize SID
minimize OID –> further away = increase mag
_____ magnification = _____ spatial resolution
decrease
increase
what is the main effector of magnification
SID –> source-IR
longer SID = ____ magnification
shorter SID = ____ magnification
decreases
increases
given spine or chest exam what would we use for SID
large SID whenever possible
objects _____ to IR = _____ magnification
closer
decreases
magnification factor?
M = SID/SOD
what does magnification radiography do? how?
enhances visualization of small structures by purposely increasing OID while keeping SID constant
HOW: use magnification factor
when would magnification radiography be uses
interventional radiography
mammography
Pros and cons of magnification radiography? Fix for con?
PRO: no grid necessary
CON: increased patient dose and decrease in image detail/spatial resolution
FIX: use small focal spot to reduce loss of image detail
what is shape distortion
misrepresentation by unequal magnification of actual shape of structure examined
how does shape distortion occur? how is the degree of distortion determined?
object plane and image plane are not parallel
Determined: object’s angle of incline and lateral position from central axis
how to reduce shape distortion?
making body part and IR parallel with CR perpendicular
factors that affect shape distortion
object thickness
object position
object shape
types of shape distortion and affect
elongation –> longer than it really is
foreshortening –> shorter than it really is
thick object = ____ OID = ______
increased
increased distortion
how do we get elongation effect
tube angle or IR is improperly aligned
how do we get foreshortening effect
body part is improperly aligned
what is spatial distortion
misrepresentation in image of the actual spatial relationships among objects
how do we get spatial distortion
when object positioned shifted laterally from CR
how do we fix spatial distortion
by getting 2 or more projections to get more 3D examination
do we ever want distortion?
Yes –> controlled distortion
removes superimposition by tube angulation
what does conventional film measure
how much chemical changes occurred within exposure through ionization
conventional vs digital processing
conventional: development, fixing, washing and drying
digital: pre/post processing
what does digital film measure
how much electrical charge is built up within exposure
digital imaging is recorded as what?
pixel values
preprocessing steps (4)
Field uniformity corrections
Noise and del drop-out corrections
Image and histogram analysis
Rescaling (processing)
what is preprocessing
all computer operations that compensate for flaws in image acquisition
what is preprocessing also known as
basic image acquisition
postprocessing steps (3)
gradation processing (LUTs)
detail processing
preparation for display
what is postprocessing
customized refinements specific to radiographic procedure
what is postprocessing also known as
specific anatomical procedure
displayed image steps (2)
operator adjustments
application of special features
what is segmentation? pre or post processing?
when 2 or more images are taken on the same PSP plate and the computer but segment/separate the images out
PREprocessing
what is segmentation failure
computer’s inability to segment or separate individual exposure areas
does segmentation failure occur on all systems?
No –> DR systems sent directly to computer after each exposure
what is correcting for dexel dropout? pre or post processing?
computer scans for dead pixels through noise reduction software (kernel)
PREprocessing
what are dead pixels
dexel areas that did not receive enough data due to electronic failure
what is interpolation
noise reduction software (kernel) reads and averages pixel values surrounding dead pixel and inserts the relative number into dead spot
correcting for mottle types? pre or post processing?
quantum mottle (random)
electronic mottle (periodic)
PREprocessing
what is quantum mottle? How is it fixed?
occurs from xray beam being randomly distributed (Poisson distribution)
FIXED: kernel
what is electronic mottle? How is it fixed?
occurs in consistent size and intervals (forms pattern)
FIX: filtering algorithms (frequency processing)
can we avoid noise? which type of mottle is the worst?
no it is unavoidable
electronic mottle –> severe mottle is impossible to distinguish
what is field uniformity? pre or post processing?
electronic amplification (computer software) used to compensate for areas that are outside the range of uniformity
PRE processing
why do we need field uniformity?
all digital system have flaws in receptor system –> uneven distribution of background density of image
what helps field uniformity
anode heel effect partially compensates
what does histogram display? pre or post processing?
bar graph representing brightness value of each pixel
PREprocessing
how do you read a histogram?
left to right –> light to dark
histogram shape displays each body part
how to make a histogram
count is made up of all pixels sharing the same pixel value (density/brightness) in dynamic range
what is histogram analysis? pre or post processing?
elimination of extreme data that skews the rescaling of image so that it is not too light or too dark –> identifies useful pixels
PREprocessing
how does histogram analysis occur?
computer compares actual histogram from exposed image to expected histogram for that specific procedure
what is exposure field recognition? what type of preprocessing is it apart of?
identifies field as “false” densities to not overcompensate –> keeps collimation tight
histogram analysis
what types of histogram analysis is there? how do we choose which type to use?
type 1, type 2, type 3
select procedure from computer menu that auto assigns which type to be used
what does type 1 analyze? ex?
analyzes two lobe histograms with tail spike in background densities
ex. chest
what does type 2 analyze? ex?
analyzes single lobe histograms
ex. AB or extremities
what does type 3 analyze? ex?
analyzes three lobes with some metallic metal present
ex. contrast exam
cons in histogram analysis (5)
segmentation errors
fail to match histogram to actual image taken
patient with prosthetic or lead appears in collimated area
pre-fogging of IR from background radiation
LARGE amount of scatter radiation
what is normalizing an image
overall brightness of image and degree of grayscale/contrast are manipulated mathematically until it has a normal appearance to a conventional radiograph
what is a rescaled image that has undergone post processing
normalized image
why do we want to have a normalized image
rescaling gives ideal level of brightness and balanced grayscale regardless of technique
what is the primary goal of RAD technique
give enough signal to reach IR so computer can successfully process
will digital processing always produce a diagnostic image?
almost always –> fails only under extreme/unusual circumstances
what are Q-values
standardized labels assigned with preset pixel values that represent certain brightness levels for pixels
processed data
how can rescaling be done?
electronically or with software (most common)
what are S-values
data that has not been rescaled
how does the computer rescale images
computer program gets S-values and reassigns them as Q-values –> places into permanent LUT
is there anything that can affect the rescaling process? why?
no –> regardless of incoming pixel values –> output pixel values are always adjusted to same output Q values set by permanent LUT
why is rescaling important? limitations?
align image brightness levels perfecting
Limitations: can only align overall image grayscale partially because of min/max Q values
how is an anatomical LUT set?
when tech selects radiographic procedure from menu on xray console
what are the 3 adjustments made to a histogram before displaying image?
histogram analysis –> selects useful pixel values of interest
rescaling to fit average histogram for body part and fixes small exposure errors
LUT –> gives image correct amount of brightness and contrast
types of digital processing domains (3)
spatial location
shade (intensity)
size (frequency)
where does an image begin and end in?
spatial location domain
what are the subdivisions of spatial domain (3)
point processing operations
area processing operations
global operations
what does spatial domain deal with
pixels acted upon depending on their location in the matrix
what operations does spatial domain include? (5)
magnification
translation (flipping)
inversion (flopping)
image subtraction
all kernel operations
what does point processing do? ex?
performs a specific algorithm on each individual pixel in sequence —> pixel by pixel
ex. image subtraction
what is image subtraction? ex?
values in each specific pixel is subtracted from the value in the corresponding pixel from another image by comparing non-contrast and contrasted images
ex. angiography –> mask image subtracted from contrast image
what does area processing do? ex?
area/local processing operations use mathematical function on subsection of image
ex. magnification
how do you magnify an image? what occurs?
select portion of image and zoom
value of each single pixel spreads out across 4 hardware pixels
what does global operations do?
all image reorientations across matrix –> inversion, flipping, or translating
what is translation?
switches corresponding columns’ pixel values except for the middle column
what is shade/intensity domain deal with
operates on pixel values –> brightness or darkness
what does size/frequency domain deal with?
the number of pixels per row in matrix
what operations are in shade/intensity domain? (3)
windowing
construction of original histogram
histogram analysis
low frequency are for…? wave?
large objects/details
long waves
high frequency are for…? wave?
small details
short waves
does size/frequency domain alter?
structures or objects within images
how does size/frequency domain occur
identify object –> sort and group by size into binds
detail processing operations (4)
smoothing
edge-enhancement
background suppression
local contrast of only fine details
purpose of gradation processing? pre or post processing?
edit final image brightness and contrast based on anatomy and predominant pathologies displayed
post-processing
what does a gradient curve describe
brightness and grayscale relating to IR
curves superimpose over the histogram
high gradient = ____ contrast = _____ curve
high
steep
low gradient = ____ contrast = _____ curve
low
steadier incline
body of each curve represents?
range of exposures
how to read gradient curve graph
average brightness level –> left to right
contrast level –> curve slope
windowing controls?
brightness and contrast (grayscale)
when is gradation processing used? using?
on every image before displayed
uses LUT –> look up table
window width controls?
window level controls?
grayscale
brightness
when does data clipping occur
when either bit depth of hardware/dynamic range of the system are too limited to allow for windowing adjustments
why is data clipping bad? fix?
limits radiologist’s ability to window the image = misdiagnosis
FIX: do not save before sending to PACS
why does data clipping occur?
pixel values too large for dynamic range or computer system –> pixel values/data lost because values too dark for system to handle
what is dynamic range compression (DRC)
removal of darkest and lightest extremes of pixel values from grayscale
why is DRC better than data clipping?
allows windowing
saves computer space by eliminating unneeded data that human eye cannot identify
adjusts grayscale to be within dynamic range without removing data
what soft tissue adjustment to technique is used? why?
film screen –> 20% kVp and no mAs change
lightens image
less penetration to tissue –> increased differential absorption
lightens bones
how does the computer adjust to soft tissue techniques? (digital imaging)
rescales and adjust images based on LUT
uses dynamic range compression to equalize tissue/contrast and control dynamic range
what does detail processing do
selects structures in images based on size and singled out for contrast enhancement or suppression
how does detail processing affect very small details?
contrast increased so it can stand out more
how does detail processing affect mid-size structures
image suppressed = contrast decreased
moves structure into background
how does detail processing work? (4 steps)
digital algorithms fix fine details of image separately
image sorted by size of object and into own file/bin
alters contrast/brightness of each individual object
separates local contrast from global contrast so fine details are more visible but whole image has about the same amount of contrast
pixel waveform represents?
alternating densities
pixel waveform
peaks =
dips =
black (darkest) pixels
white (brightest) pixels
zero point of each individual wave corresponds to?
transition border between each pair of pixels
wavelength of each pulse represents?
pixel size
smaller pixels = ____ frequency
higher
how to read frequency on display monitor
left to right
number of up/down cycles = distance
each cycle = _____
each pulse = _____
2 pulses
1 pixel
given 300 cycles
how many pixels are there?
600 pixels
frequency of objects correlate to?
number of pixels that occupy in each row
large objects = ______ wavelengths = ____ frequency
large
low
why do large objects have low frequency
fewer photons to fit across the screen
small objects = ____ wavelengths = _____ frequency
short
high
waveform interpretation
amplitude =
wavelength =
tall waves =
wide waves =
gray level/pixel value
number of pixels in object across the row (laterally)
darker object
larger object with more pixels occupying row
tall skinny spike in wave =
dark and small object
what is fourier transformer
mathematical process that allows frequency processing to separate structures according to size
how does fourier transformer work?
it breaks up complex waveforms into component waves (long, medium and short) based on pulses with different wavelengths
what does wavelengths represent in fourier transformation
different sizes of objects or structures in one row in image
what is multiscale processing?
Decomposition of original image into 8+ frequency layers to perform various operations on selected individual layers and recomposing the image
how does multiscale processing determine splitting the layers
repeatedly splits into high frequency component and low frequency components and so on until there are 8+ levels
how does multiscale processing end?
completed image laters are put back together using inverse fourier transformation = final image
what is filtering in multiscale processing
selected layer is left out when reconstructing final image
when is band-pass filtering used
noise reduction –> electronic mottle
what does low pass filtering do
an algorithm passes through low frequency layers
most common filter? what does it do? con?
smoothing function
removes noise from image
losses some fine detail
what does high pass filtering do? aka?
passes through highest frequency layers
edge enhancement
what does background suppression do
eliminates very lowest frequencies
how does image domains transition?
spatial –> intensity –> spatial –> frequency –> spatial = final image
or
spatial to frequency or intensity then spatial whatever is needed
why is the intensity domain important for image transition
histogram analysis and gradation processing
why does it go back to spatial domain to go to frequency domain?
frequency detail processing needs spatial domain in order to put reconstructed pixels back into the spatial matrix
example of submatrix?
kernel
what is a submatrix
a small matrix passed over a larger matrix of the whole image –> changes all of the pixel values mathematically
how does kernel move?
left to right along row and moves down to repeat
what is equivalent to frequency detail processing?
spatial detail processing
what does spatial detail processing affect? (4)
smoothing
noise reduction
edge enhancement
background suppression
what is a pro about spatial detail processing?
uses kernels to affect image –> no need to transition between domains
What is the imaging cycle in order?
exposure –> histogram –> permanent LUT (rescaling) –> anatomical LUT (gradation) –> default detail processing –> displayed image
can you alter a procedure algorithm? How?
yes by selecting an alternate histogram or altering window settings (brightness and contrast)
why would we select an alternate histogram? Con?
gives wider grayscale
CON: histogram errors, image storage issues, record-keeping and legal liability
when should we select an alternate histogram?
when approved by radiologist
window leveling corresponds with …..?
brightness
window level is also known as?
brightness
center
density
S number
increase brightness = _____ window level
decrease
window width corresponds with ….?
grayscale
does window width alter brightness or density?
no
wider window = _____ grayscale
longer
increase contrast = ______ window width
decrease
what does smoothing remove? con? overall effect?
removes highest frequency layers
CON: removes some shades of grey/fine details
Effect: edges are softened
how does smoothing occur?
noise reduced by mathematical interpolation –> averages pixel values of white/black specks (corrects for dead dexels)
what type of postprocessing feature do you use to correct for moderate amounts of mottle? Can it be used for severe mottle?
smoothing
no –> extreme underexposure cannot be corrected
what is the best method to alternate procedure algorithms?
alter windowing settings
what type of postprocessing feature do you use to better visualize small details? How?
edge enhancement –> increases contrast
con of edge enhancement
increases noise and possible loss in detail
what type of postprocessing feature do you use to better visualize specific tissues (ex. fat pads)?
background suppression
how does background suppression occur?
Algorithms reduce contrast of larger mid-frequency and low-frequency structures
targeted area brightness correction effect? how?
percentage of tissue equalization by correcting brightness of specific portions of the image
what type of postprocessing feature do you use to allow more diagnostic area in an image
targeted area brightness correction
what type of postprocessing feature do you use to visualize bones as black?
image reversal
how does image reversal occur?
all pixel values are swapped –> high = low and low = high
does image reversal create new information?
no improvement –> negative image turns to positive image
color of bone in negative image and positive image
neg: white
positive: black
what type of postprocessing feature do you use to fix vale glare?
dark masking
what does dark masking do? when to use?
image is cropped so that the white border (from suppression error) turns into black border
WHEN: before sending image to PACS
what type of postprocessing feature do you use to correct an upside down image? how?
image orientation
HOW: rotating or flipping image
method types of magnification
zoom technique: magnifies entire image
Magnifying glass: magnifying box placed over anatomy
con of magnification
creates pixelation
what type of postprocessing feature do you use for elongated anatomy (ex. long bone or spinal imaging)
image stitching
how does image stitching occur?
computer software uses grid to superimpose, crop, and combine multiple images to one image
what type of postprocessing feature do you use to identify lesion or fracture locations
dual-energy subtraction
how does dual-energy subtraction affect image
separates image to tissue or bone only image
how does the dual-energy subtraction occur?
high energy (kVp) and low energy (kVp) obtained and computer compares images to identify xray absorption drop = soft tissue areas –> allows image reconstruction
what type of postprocessing feature do you use to remove grid lines? when does this occur?
grid line suppression
WHEN: using stationary grids –> portables
how does the computer remove grid lines
frequency processing identifies lines as large structures with low frequency
collimating pros for computer system?
reduces failure of system to find collimation edges
reduces incorrect data collection
reduces extremes in image brightness/darkness
pros of proper anatomy centering
ensure appropriate densities can be located
reduces extremes in image brightness/darkness
