Chapter 7
Digital Radiographic Image Processing
and Manipulation
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Objectives
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Describe formation of an image histogram.
Discuss automatic rescaling.
Compare image latitude in digital imaging with
film/screen radiography.
List the functions of contrast enhancement
parameters.
State the Nyquist theorem.
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Objectives
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Describe the effects of improper algorithm
application.
Explain modulation transfer function.
Discuss the purpose and function of image
manipulation factors.
Describe the major factors in image management.
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Key Terms
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Archive query
Automatic rescaling
Contrast manipulation
Edge enhancement
High-pass filtering
Histogram
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Image annotation
Image orientation
Image sampling
Image stitching
Latitude
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Key Terms
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Look-up table
Low-pass filtering
Magnification
Manual send
Modulation transfer
function
Nyquist theorem
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Patient demographics
Shutter
Smoothing
Spatial frequency
resolution
Window and level
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Digital Radiographic Image Processing and
Manipulation
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In cassette-based and cassetteless systems, once
the x-ray photons have been converted into electrical
signals, these signals are available for processing
and manipulation.
The reader is used only for cassette-based systems,
but the processing parameters and image
manipulation controls are similar for both systems.
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Preprocessing
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Preprocessing takes place in the computer where the
algorithms determine the image histogram.
Postprocessing is done by the technologist through
various user functions.
Digital preprocessing methods are vendor-specific.
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CR Reader Functions
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The computed radiography (CR) imaging plate
records a wide range of x-ray exposures.
If the entire range of exposure were digitized, values
at extremely high and low ends of range would also
be digitized.
This would result in low-density resolution.
To avoid this, exposure data recognition processes
only the optimal density exposure range.
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CR Reader Functions
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Data recognition program searches for anatomy
recorded on the imaging plate as follows:
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Finding collimation edges
Eliminating scatter outside the collimation
Failure of the system to find the collimation edges
can result in incorrect data collection.
Images may be too bright or too dark.
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CR Reader Functions
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Data within collimation result in generation of a
graphic representation called a histogram.
Because information within the collimated area is
signal used for image data, the information is the
source for a vendor-specific exposure data indicator.
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CR Image Sampling
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Plate is scanned.
Image location and orientation is determined.
Size of the signal is determined.
Value is placed on each pixel.
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CR Image Sampling
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Histogram is generated that allows system to find useful
signal by locating the minimum (S1) and maximum (S2)
signal within the anatomic regions of interest in the
image.
Histogram identifies all densities on the imaging plate in
the form of a graph:
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X-axis is related to amount of exposure.
Y-axis displays the number of pixels for each exposure.
Graphic representation appears as a series of peaks and valleys
and has a pattern that varies for each body part.
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CR Image Sampling
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Low energy (kilovoltage peak) gives a wider
histogram.
High energy (kilovoltage peak) gives a narrow
histogram.
Histogram shows the distribution of pixel values for
any given exposure.
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CR Image Sampling
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For example:
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Pixels have a value of 1, 2, 3, and 4 for a specific exposure.
Histogram shows the frequency of each of those values and
actual number of values.
Histogram sets the minimum (S1) and maximum (S2)
“useful” pixel values.
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Histogram Analysis
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Analysis is complex.
Shape of the histogram stays fairly constant for each
part exposed (anatomy specific).
For example:
•
Shape of histogram for a chest radiograph on a large adult
patient looks different from a knee histogram generated from
a pediatric knee exam.
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Histogram Analysis
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It is important to choose the correct anatomic region
on the menu before exposing the patient.
Raw data used to form the histogram are compared
with a “normal” histogram of the same body part by
the computer.
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The Nyquist Theorem
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Theorem states that when sampling a signal, the
sampling frequency must be greater than twice the
bandwidth of the input signal so that the
reconstruction of the original image will be nearly
perfect.
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At least twice the number of pixels needed to form
the image must be sampled.
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If too few pixels are sampled, the result is a lack of
resolution.
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The Nyquist Theorem
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The number of conversions in CR—electron to light,
light to digital information, digital to analog signal—
results in loss of detail.
Some light is lost during the light-to-digital conversion
because of the spreading out of light photons.
Because there is a small distance between the
phosphor plate surface and the photosensitive diode
of the photomultiplier, some light spreads out there as
well, resulting in loss of information.
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The Nyquist Theorem
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The longer the electrons are stored, the more energy
they lose.
When laser stimulates electrons, some lower-energy
electrons escape the active layer.
If enough energy was lost, some lower-energy
electrons are not stimulated enough to escape and
information is lost.
All manufacturers suggest that imaging plates be
read as soon as possible to avoid this loss.
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The Nyquist Theorem
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Indirect and direct radiography lose less signal to
light spread than conventional radiography.
The Nyquist theorem is still applied to ensure that
sufficient signal is sampled.
Because sample is preprocessed by the computer
immediately, signal loss is minimized but still occurs.
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Aliasing
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Spatial frequency is greater than the Nyquist
frequency.
Sampling occurs less than twice per cycle.
Information is lost.
Fluctuating signal is produced.
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Aliasing
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Wraparound image is
produced.
Image appears as two
superimposed images slightly
out of alignment.
Aliasing results in a moiré
effect.
Aliasing can be problematic
because of the same effect
occurring with grid errors.
It is important that the
technologist remembers to look
at both.
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Automatic Rescaling
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Exposure is greater than or less than what is needed
to produce an image.
Automatic rescaling occurs to display the pixels for
the area of interest.
Images are produced that have uniform density and
contrast regardless of the amount of exposure.
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Automatic Rescaling
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Problems occur with rescaling:
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When too little exposure is used, resulting in quantum mottle
When too much exposure is used, resulting in loss of
contrast and loss of distinct edges because of increased
scatter production
Rescaling is no substitute for appropriate technical
factors.
Danger exists of using higher than necessary
milliampere-second values to avoid quantum mottle.
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Look-Up Table
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The look-up table (LUT) is a reference histogram.
LUT is used as a cross-reference to transform the
raw information.
LUT is used to correct values.
LUT has a mapping function:
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All pixels are changed to a new gray value.
Image will have appropriate appearance in brightness
and contrast.
LUT is provided for every anatomic part.
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Look-Up Table
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LUT can be graphed as follows:
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Plotting the original values ranging from 0 to 255 on the
horizontal axis
Plotting new values, also ranging from 0 to 255 on the
vertical axis
Contrast can be increased or decreased by changing
the slope of this graph.
Brightness (density) can be increased or decreased
by moving the line up or down the y-axis.
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Latitude
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Latitude is the amount of error that still results in a
quality image.
Histograms show a wide range of exposure because
of automatic rescaling of the pixels.
Actual exposure latitude is slightly greater than that of
screen/film exposures.
In CR, if exposure is more than 50% below ideal
exposure, quantum mottle results.
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Latitude
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If exposure is more than 200% above ideal exposure,
contrast loss results.
Biggest difference between digital and film/screen
radiography lies in the ability to manipulate the
digitized pixel values, which results in what seems
like greater exposure latitude.
Proper kilovolt and milliampere-second values
prevent mottle and contrast loss.
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Enhanced Visualization Image Processing
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Kodak
Takes image diagnostic quality to a new level
Increases latitude while preserving contrast
Process decreases windowing and leveling
Virtually eliminates detail loss in dense tissues
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Modulation Transfer Function
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Modulation transfer function (MTF) is the ability of
a system to record available spatial frequencies.
Sum of the components in a recording system cannot
be greater than the system as a whole.
When the function of any component is compromised
because of interference, the overall quality of the
system is affected.
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Modulation Transfer Function
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MTF is a way to quantify the contribution of each
system component to the overall efficiency of the
entire system—e.g., ratio of the image to the object.
A perfect system would have an MTF of 1 or 100%.
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Modulation Transfer Function
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Digital detectors
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X-ray photon energy excites a phosphor.
Phosphor produces light.
Spreading out of the light will always occur.
Light spread reduces system efficiency.
The more light spread, the lower the MTF.
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Quality Control Workstation Functions
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Image processing parameters
Contrast manipulation
Spatial frequency resolution
Spatial frequency filtering
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Image Processing Parameters
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Digital systems have greater dynamic range than
film/screen imaging.
Initial digital image appears linear when graphed
because all shades of gray are visible.
Digitalization gives the image a wide latitude.
If all shades were left in the image, contrast would be
too low.
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Image Processing Parameters
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To avoid this, digital systems make use of various
contrast-enhancement parameters.
Names differ by vendor; Agfa uses MUSICA, Fuji
uses Gradation, and Kodak uses Tonescaling.
Purpose and effects are basically the same.
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Contrast Manipulation
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Contrast-enhancement parameters convert the digital
input data to an image with appropriate density and
contrast.
Image contrast is controlled by using a parameter
that changes the steepness of the exposure gradient.
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Contrast Manipulation
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Density can be varied at the toe and shoulder of the
curve, removing the extremely low and extremely
high density values using a different parameter.
Another parameter allows density to remain
unchanged while contrast is varied. These
parameters should be used to enhance the image
only; no amount of adjustment takes the place of
proper technical factor selection.
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Workstation Screen Showing Contrast
Manipulation Choices
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Spatial Frequency Resolution
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Sharpness control is referred to as spatial frequency
processing.
Sharpness is controlled in film/screen by various
factors such as focal spot size, screen and/or film
speed, and object image distance.
Digitized images can be further controlled for
sharpness.
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Spatial Frequency Resolution
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Controls are available for the following:
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Structure to be enhanced
Degree of enhancement for each density to reduce image
graininess
How much edge enhancement is applied
If improper algorithms are applied, image formation is
affected.
It is possible to degrade image information if
algorithms are improperly applied.
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Spatial Frequency Filtering
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Edge enhancement
Smoothing
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Edge Enhancement
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When the signal is obtained, averaging of the signal
occurs to shorten processing time and storage.
The more pixels involved in the averaging, the
smoother the image appears.
Signal strength of one pixel is averaged with the
strength of adjacent pixels or neighborhood pixels.
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Edge Enhancement
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Edge enhancement occurs when fewer pixels in the
neighborhood are included in the signal average.
The smaller the neighborhood, the greater the
enhancement.
When frequencies of areas of interest are known,
they can be amplified and other frequencies can be
suppressed.
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Edge Enhancement
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Amplification, also known as high-pass filtering, results in
an increase of contrast and edge enhancement.
Suppression of frequencies, also known as masking, can
result in loss of small details.
This technique is useful for enhancing large structures such
as organs and soft tissues but can be noisy.
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Smoothing
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Smoothing is another type of spatial frequency
filtering.
Smoothing is also known as low-pass filtering.
Smoothing results from averaging of the frequency of
each pixel with surrounding pixel values to remove
high-frequency noise.
Result is a reduction of noise and contrast.
Low-pass filtering is useful for viewing small
structures such as fine bone tissues.
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Basic Functions of the
Processing System
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Image manipulation
Image management
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Image Manipulation
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Window and level
Background removal or shutter
Image orientation
Image stitching
Image annotation
Magnification
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Window and Level
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Window and level are the most common controls for
brightness and contrast.
Window controls how light or dark the image is.
Level controls the ratio of black to white, or contrast.
User is able to manipulate quickly through use of the
mouse.
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Window and Level
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One direction, vertical or horizontal, controls
brightness, and the other direction, contrast.
To control density and contrast further, contrast
enhancement parameters are used.
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Background Removal or Shutter
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Unexposed borders around the collimation edges
allow excess light to enter the eye.
Effect is known as veil glare.
Glare causes oversensitization of a chemical within
the eye called rhodopsin.
This results in temporary white light blindness.
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Background Removal or Shutter
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Eye recovers quickly enough so that viewer
recognizes only that the light is very bright.
Glare is a great distraction that interferes with image
reception by the eye.
In film/screen radiography, black cardboard glare
masks or special automatic collimation view boxes
were used to lessen the effects of veil glare, but no
techniques were entirely successful or convenient.
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Background Removal or Shutter
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In CR, automatic shuttering is used to blacken out the
white collimation borders.
This eliminates veil glare.
Shuttering is a viewing technique only.
Shuttering should never be used to mask poor
collimation practices.
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Background Removal or Shutter
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Removal of the white unexposed borders results in
an overall smaller number of pixels.
This reduces the amount of information to be stored.
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Image Orientation
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Image reader scans and reads the image from the
leading edge of the imaging plate to the opposite
end.
Image is displayed exactly as it was read.
Different vendors mark the cassettes in different
ways.
Cassette must be oriented so that the image is
processed to display as expected.
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Image Orientation
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Fuji uses a tape-type orientation marker.
Kodak uses a sticker.
Some exams require unusual orientation of the
cassette.
Reader must be informed of the orientation of the
anatomy with respect to the reader.
In digital radiography, the position of the part should
correspond with the marked top and sides of the
imaging plate.
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Image Stitching
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Stitching is used for anatomy or areas of interest too
large to fit on one cassette.
Multiple images can be “stitched” together.
Sometimes, special cassette holders are used and
positioned vertically, corresponding to foot to hip or
entire spine radiography.
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Image Stitching
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Images are processed in computer programs that
nearly seamlessly join the anatomy.
Computer displays one single image.
Process eliminates the need for large (36-inch)
cassettes previously used in film/screen radiography.
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Image Annotation
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Information other than standard identification must be
added to the image.
In screen/film radiography, additional information is
marked by the following:
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Time and date stickers
Grease pencils
Permanent markers
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Image Annotation
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Annotation function allows
selection of preset terms and/or
manual text input.
Annotation can be useful when
such additional information is
necessary.
Annotations overlay the image as
bitmap images.
Annotations may not transfer to
picture archival and
communication system (PACS).
Input of annotation for
identification of the patient’s left
or right side should never be
used as a substitute for
technologist’s anatomy markers.
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Magnification
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Two basic types of magnification techniques are
standard with digital systems:
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One type functions as a magnifying glass:
• A box is placed over a small segment of anatomy on the main
image.
• Box shows a magnified version of the underlying anatomy.
• The size of the magnified area and the amount of magnification
can be made larger or smaller.
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Magnification
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Other technique is “zoom.”
Zoom allows magnification of the entire image.
Image can be enlarged enough that only parts of it are
visible on the screen.
Those parts can be seen through mouse navigation.
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Image Management
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Patient demographics input
Manual send
Archive query
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Patient Demographics Input
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Proper identification of the patient is even more
critical.
Retrieval can be nearly impossible if image is not
properly and accurately identified.
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Patient Demographics Input
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Demographic information about the patient includes
the following:
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Name
Health care facility
Patient identification number
Date of birth
Exam date
Other pertinent information
Input or linked via barcode label scans, before the start of
the exam and before the processing phase
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Patient Demographics Input
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Occasionally, errors are made and demographic
information must be altered.
If technologist performing the exam is absolutely positive
that image is of the correct patient, then demographic
information can be altered at the processing stage.
This function should be tracked and changes should be
linked to the technologist altering the information to
ensure accuracy and accountability.
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Patient Demographics Input
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Problems occur if the patient name is entered
differently from visit to visit or exam to exam.
For example:
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Patient’s name is Jane A. Doe and is entered that way.
Name must be entered that way for every other exam.
If name is entered as Jane Doe, then system will save it as a
different patient.
Merging of files can be difficult.
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Patient Demographics Input
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Problems:
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Several versions of the name are given.
Suppose the patient gives a middle name on one visit but
has multiple exams under his or her first name.
Retrieval of previous files will be difficult.
The right images must be placed in the correct data files.
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Manual Send
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Because the quality control workstation is networked
to the PACS, it also has the capability to send images
to local network workstations.
The manual send function allows the quality control
technologist to select one or more local computers to
receive images.
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Archive Query
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PACS archive can be queried for historical images.
Function allows retrieval of images from the PAC
system based on the following:
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Date of exam
Patient name or number
Exam number
Pathologic condition
Anatomic area
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Archive Query
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Example:
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Technologist could query PACS to retrieve all chest
radiographs for a particular date or range of dates.
Technologist could query retrieval of all of a patient’s images.
Multiple combinations of query fields are possible:
• Can generate general retrieval
• Specific recovery of images
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Summary
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Recognition of exposure data involves processing
only the optimal density exposure range and
generates a graphic representation or histogram of
the optimal densities.
The plate is scanned, and the image location and
orientation are determined. A value is place on each
pixel, and the histogram is generated displaying the
minimum and maximum diagnostic signal.
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Summary
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The histogram is anatomic region specific and
remains fairly constant from patient to patient.
Automatic rescaling allows pixel display for the area
of interest, regardless of the amount of exposure
unless the exposure is too low or too high. In those
cases, quantum mottle or contrast loss occurs.
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Summary
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There is no substitute for proper kilovoltage peak and
milliampere-second settings. Images cannot be
created from nothing; that is, insufficient photons,
insufficient penetration, or overpenetration will result
in loss of diagnostic information that cannot be
manufactured by manipulation of the image
parameters.
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Exposure latitude is slightly greater with digital
imaging than that of film/screen imaging because of
the wide range of exposures recorded with digital
systems.
Contrast-enhancement parameters allow
enhancement of the image by controlling the
steepness of the exposure gradient, density variance,
and contrast amount.
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Spatial frequency resolution is controlled by focal
spot, object image distance, and computer
algorithms.
The Nyquist theorem is applied to digital images to
ensure that sufficient signal sampling occurs so that
maximum resolution is achieved.
MTF refers to the contribution of all system
components to total resolution. The closer the MTF
value is to 1, the better the resolution.
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Edge enhancement is accomplished by limiting the
number of pixels in a neighborhood of the matrix.
Known area of interest frequencies can be amplified
or high-pass filtered to increase contrast and edge
enhancement.
Suppression of frequencies of lesser importance,
known as masking, can cause small detail loss.
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Low-pass filtering or smoothing is the result of pixel
averaging to remove high-frequency noise. Contrast
and noise are decreased, allowing small structures to
be seen.
Window and level parameters control pixel brightness
and contrast.
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Shuttering is a process that removes or replaces the
background in order to block distracting light
surrounding a digital image. This does not take the
place of proper collimation and can be removed to
show proper collimation.
Digital imaging cassettes are marked for orientation
to the top and right sides. This ensures that images
are displayed correctly.
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Image stitching is a computer program process that
allows multiple images to be joined when the
anatomy is too large for one exposure. The result is a
nearly seamless, single image.
Magnification techniques are available with digital
systems that allow small area enlargement or whole
image enlargement.
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Proper patient demographic input is the responsibility
of the technologist performing the exam. Any
alterations of patient demographics should be
avoided unless absolute identification is possible.
The manual send function allows images to be sent
to one or more networked computers.
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Historical study of patient exams can be
accomplished through the archive query function.
Retrieval of radiographic studies can be specific as to
patient name, date, and exam or broad such as date
ranges and combinations of anatomic areas.
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