Transcript RT 244 -12 Wk 14 Digital Artifacts & Imaging ERRORS

RT 244 -12 Wk 14
Digital Artifacts & Imaging ERRORS
The advantages of CR are its large dynamic range, digital
format, portability, and post-processing capability
But not PERFECT –Imaging Errors can still occur
See NOTES at bottom of slides for additional information
Some information obtained from: AR Online » Current Issue »
Artifacts and misadventures in digital radiography
By Charles E. Willis, PhD, DABR; Stephen K. Thompson, MS, DABR; S. Jeff Shepard, MS, DABR
Volume: 33 Number: 1 January 2004
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Artifacts
• Any irregularity on an image that is not caused
by proper shadowing of tissue by the primary
x-ray beam.
• Are undesirable optical densities or blemishes
on a radiograph.
• Can be very interesting at times. You become
the detective, what caused that?
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• CR artifacts require special attention.
• This is due to the fact that CR artifacts may be
produced from various components of the CR system
itself
• Artifacts may also be generated by the users who
are not aware of the proper imaging techniques or
selection of appropriate image processing protocols
• Since CR is also very sensitive to scattered radiation,
it is vital that anti-scattered grids be used as in
conventional radiography.
• Radiographers should be concerned since these may
generate unwanted artifacts that could not be
corrected by any image processing algorithm.
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Artifact Classification
Review of Film/Screen
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Artifact Classification
Digital – CR & DR
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OPTIMIZATION OF CR IMAGES
Quality of CR images
• Technologists are the key persons
• delivering good quality radiographs
• dose given to the patients.
• CR images can NOT always be
adjusted after exposing
• CRITICAL to CR/DR Imaging is
Technique, Positioning Collimation.
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Optimization of a CR image quality may be
achieved by optimizing the following
factors
• OBJECT ARTIFACTS:
–Positioning and collimation
• Exposure techniques
• Image processing selection
• Lifetime of the PSP
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CR Artifacts
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Positioning errors
Collimation errors
Backscatter radiation
LUT selection/histogram
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Positioning of Part/ Collimation
• 2 or more two projections on one IR
• Is not a good practice with CR technique,
• since double or multiple exposures on a single
PSP) can lead to a failure of the image
processing software to detect the image
boundary.
• Matching the positioning and collimation with
the image processing parameters is also
crucial.
• Image processing will eventually fail to process
since the input information is totally different.
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Errors in collimation can cause mistakes in detection
of the boundary, with a dramatic loss of image
contrast
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Positioning Errors
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Collimation Errors
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Loss of contrast due to
partitioning errors
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Alignment Error: Same technique, different
centering and collimation
S# 592
S# 664
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􀁹2 on 24 X 30
􀁹Technique adjusted
2 on 24 X 30
􀁹Same technique
􀁹Rescaling error.
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Single field per plate - better
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What is the problem?
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Placement of gonadal shields is no longer trivial,
but may adversely affect image quality.
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Software to compensate for metal in
patient
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Same pt – what improved this image?
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Acquiring good quality images
• Regardless of the acquisition technology, good radiographic
images can be produced only when certain fundamental
requirements are met.
• Appropriate radiographic technique must be used,
• proper tube potential (kVp),
• beam current (mAs),
• source-to-image distance (SID),
• collimation,
• alignment of the X-ray central ray,
• and positioning of detector and subject for the specific
anatomic projection.
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Exposure techniques
• CR may be operated at a different film speed,
and then optimizing the exposure technique
accordingly.
• Existing CR has a speed similar to medium
speed film-screen system (200 -400) while
spatial resolution is still generally inferior
• Proper selection of an image processing
algorithm specific to each type of x-ray
examination is thus important.
• The technical skills of radiographers definitely
play a crucial role in determining the quality of
the radiographic image.
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Exposure factor “creep"
• related to the wide exposure latitude of DR.
• noise in DR images exposed at one-fourth to onehalf of the appropriate level.
• artifacts are generally not apparent until the
exposure exceeds 10 times the appropriate level.
• Technologists avoid repeating an underexposed
study by routinely increasing the radiographic
technique.
• Thus, the potential for gross overexposure exists
in DR. Image optical density (OD), the usual
indicator of proper exposure, is arbitrary in DR.
• Managment of exposure factor in DR must rely on
the value of a derived exposure indicator
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Errors in the selection of the anatomic projection can
cause inappropriate processing
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Wrong Algorithm
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Over/Under Exposed
• Even though a CR image may be adjusted to improve
the image visibility in the cases of over- or underexposures, it would still be impossible for an image
processing to improve the visibility of clinical
features that were not available in the raw image.
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• Example of artifacts in CR an image with loss of contrast
as a result of improper selection of image processing;
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the same image now
shows acceptable
image quality as a
result of proper
selection of image
processing.
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Too many X-rays are a disservice to the patient and may also
produce poor images (Figure 3).
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Whether from underexposure or misalignment of a scatter reduction
grid, too few X-rays produce noisy images
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Different exam parameters
in different rooms
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Double exposure is a classic operator error
• that constitutes approximately 2% of all rejected
images.
• The consequence of double exposure can be either a
single repeated examination, when an inanimate
object is involved (Figure 11), or two repeated
examinations when two patients are involved (Figure
12).
• In DR, double exposures can also be caused by power
interruptions and communications errors, as well as
by inadequate erasure secondary to overexposure or
erasure mechanism failure.
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Double exposure (Figure 11),
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Double exposure # 12
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#8
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Edge Enhancement
• The secondary function of image processing is to
customize contrast in the region of interest
• This type of image processing includes modifying the
image to enhance the contrast and sharpness of
some features while compromising the contrast and
sharpness of others,
• as well as modifying the image to make it appear
more like a conventional film.
• This secondary image processing is applied in a
manner that is usually specific to the anatomic
projection.
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standard image
edge sharpening
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• Halo effect with Edge enhancement
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Post Processing
• An auxiliary purpose of image processing is to
improve the usability of the digital image.
• This includes imprinting demographic overlays,
adding annotations, applying borders and shadow
masks, flipping and rotating, increasing
magnification, conjoining images for special
examinations like scoliosis, and modifying the
sequence of views.
• This processing may require a separate QC
workstation
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IR Artifact - Digital
• Pixel Failure = CR & DR plates should last for
thousands of exposures. Interpolation can be
used to fix defects in a small area.
• Ghosting artifacts = exposure to
environmental radiation or incomplete
erasure.
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background radiation
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Image Receptor Artifacts
• Debris on image
receptor in DR can
be confused with
foreign bodies
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Image Receptor Artifacts
Line caused
from dirt
collected
in a CR
Reader
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imaging plate was not fully erased before the chest
examination was performed
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Dirt on screens
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#9
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Lifetime of the PSP
• One of the major advantages of CR is that the
phosphor plate is reusable.
• However, there are a number of factors that may
affect the lifetime of an imaging plate.
• The plates are subjected to normal wear and tear
from scratches, scuffs, cracks, and contamination
with dust and dirt, which may interfere with the
production of a good image.
• The establishment of a well-organized quality control
program will play an important role in assessing the
clinical quality of the imaging plate. This may easily
be carried out by artifact assessment and uniformity
evaluation across the plate.
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Software Artifacts
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Processing Errors
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Digital Radiography Image Sampling
• Image sampling, the plate is scanned, and the
image’s location and its orientation are
determined. The size of the signal is then
determined, and a value is placed on each
pixel. A histogram is generated from the image
data.
• The 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
• States that when sampling a signal (such as the
conversion from an analog to a digital image), 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.
• At least twice the number of pixels needed to form
the image must be sampled. If too few pixels are
sampled, the result will be a lack of resolution.
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Aliasing & Grid errors = Moiré
• Spatial frequency is greater than the Nyquist
frequency a wraparound image is produced.
moiré effect
• Stationary Grids: grid lines and the scanning
laser are parallel moiré effect
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Moiré effect
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Software Artifacts
• Image Compression – Used with teleradiology.
Compression techniques “lossless” or “lossy”
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Image Compression
• Lossless compression: image can be
reconstructed to be exactly the same as the
original image. Compressed 10% or 50%.
• Lossy compression: image is compressed
100:1. Used only when fine detail is not
required. Not useful for medical imaging.
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Image Compression
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• Wider dynamic range means that technologists
have to pay attention to exposure indicator
values, instead of brightness and contrast.
Without this attention, patient dose will escalate.
If exposure indicator logs are available, they need
to be evaluated. If they aren't, this will need to be
done manually.
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Misuses of image processing include
• compensating for inappropriate radiographic technique,
• compensating for poor calibration of acquisition and display
devices,
• and surreptitious deletion of nondiagnostic images.
• Image processing to recover nondiagnostic images to prevent
re-exposure should be a last resort, not a routine activity.
• Routine reprocessing indicates a problem with automatic
image processing or technical practice. Access to imageprocessing software is essential to develop and maintain
appropriate processing parameters
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