George David
Associate Professor of Radiology
Medical College of Georgia
Quick Review of Technology
Computed Radiography (CR)
 Re-usable metal imaging plates replace
film & cassette
CR Exposure
Reading Imaging Plate
 Reader scans plate
with laser light
using rotating
mirror
 Plate pulled
through scanner
by rollers
 Light given off by
plate measured by
PM tube &
recorded by
computer
CR Operation
 after read-out, plate erased using a bright light
 plate can be erased virtually without limit
 Plate life defined not by erasure cycles but by
physical wear
 Very large
CR Latitude
 Plate responds to
decades of input
exposure
 Computer scale inputs
exposure to viewable
densities
CR Very Sensitive to Scatter
Digital Radiography (DR)
 Digital bucky incorporated into
equipment
 Direct digital output
 High Lattitude
Raw Data
Imageimage as read from
 Unprocessed
receptor
 Not a readable diagnostic image
 Requires computer processing
before presentation as finished
radiograph
Enhancing Raw Image (Image
Segmentation)
*
Identify collimated image border
Separate raw radiation from anatomy
Apply appropriate tone-scale to image
1.
2.
3.


Create look-up table (LUT)
Maps pixel value to display gray shade
This process is
specific to a
particular body
part and
projection
Image Segmentation
 Establish location of collimated
border
• Define anatomic region
• Produce look-up table based
on histogram from
anatomical area
• Body part & projection specific
Gross Overexposure
 Properly
exposed
• 8X
increase
in mAs
Gross Underexposure
 Properly
exposed
• mAs
reduced
by ~ 100
Artifacts in Digital Radiography
Artifact Categories
 Non-digital-related technical errors
 Look-up table / image processing errors
 Exposure artifacts
 CR artifacts
 DR artifacts
 Interference artifacts
 Display artifacts
Radiography Artifacts
Digital or Not
 Mis-positioning
 Motion
 Incorrect patient ID
 Double exposures
 Grid cut-off
http://radiographics.rsnajnl
s.org/cgi/reprint/11/2/307?
maxtoshow=&HITS=10&hit
s=10&RESULTFORMAT=&
fulltext=grid+artifacts&ando
rexactfulltext=and&searchi
d=1&FIRSTINDEX=0&sort
spec=relevance&resourcet
ype=HWCIT
No grid
102 cm SID
.33R ESE
Grid
122 cm SID
2.2 R ESE
References
 “Digital Radiography Artifacts”
from Veterinary Radiology & Ultrasound
 Wm Tod Drost, David J. Reese, William J. Hornof
 “Artifacts in Digital Radiography” from Veterinary
Radiology & Ultrasound
 David A. Jimenez, Lauara J. Armbrust, Robert T.
O’Brien, David S. Biller
 Radiographics
 http://www.slideshare.net/ricksw78/digital-
radiography
Look-up Table / Image Processing
Errors
•Diagonal collimation caused
algorithm to improperly
recognize collimated area
•Gonadal shields can fool
algorithm
•2 exposures on one plate can
fool algorithm
http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow
=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searc
hid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=H
WCIT
Chest incorrectly
specified by
technologist to CR
reader as lumbar
spine
http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HIT
S=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRS
TINDEX=10&sortspec=relevance&resourcetype=HWCIT
Uberschwinger Artifact
 Causes appearance of fine
black line halo
 Result of implementation
of edge enhancement
 Can be mistaken for
evidence of loosening of
orthpedic device
 Can mimic pneumothorax
More Uberschwinger
Clipping
 Occurs because of applying LUT during pre-
processing
 Kodak recommends technologists do not window/level
on qc workstation.
 Raw data: 12-14 bit image
 LUT during pre-processing: Only 10-12 bits saved
 Information may be lost
Low Exposure Results in Image
Noise
 Increasing mAs
Exposure Artifact
 VERY high exposures can result in saturation
 Maximum detector response reached
 No response to increased dose
 Uniformly dark
 Cannot be windowed/leveled
CR
Artifacts
 Fading
 Light leak
 Physical damage to imaging plates
 Cracks, scuffs, scratches
 Contamination
 Dust / dirt
 Dirt in reader
 Highly sensitive to scatter radiation
 Upside-down insertion into bucky
CR Artifact: Fading
 CR latent image consists of excited electrons stuck in
high energy state orbits
 Over time if image not read then electrons fall back to
ground energy state
 Several days required before significant fading occurs
CR Artifact: Light Leak in Cassette
 CR plates erased by exposure to bright light
 Light leaks in cassette can cause partial premature
erasure of plate
•On imaging plate
during reading
•In reader
•Can get on plate
•Can be blocked in
light guide of reader
•White
line in
direction of plate
movement
•CR readers require
periodic preventive
maintenance & cleaning
http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow
=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searc
hid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=H
WCIT
Dirt on CR plate. Right image
repeated on screen-film
http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=1
0&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX
=10&sortspec=relevance&resourcetype=HWCIT
What are white
lines?
Cracks on image plate due to mechanical wear
http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&
HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=
1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCIT
http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow
=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searc
hid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=H
Transport problems in CR reader
Rotating
Mirror
PMTube
Light
Laser
Analog/ Digital
Computer
Beam scanned
across film
Plate
Travel
CR Plates are Very Susceptible to
Scatter
 Do not keep plates in
room during other
exposures
 Do not store where
likely to receive scatter
 Erase plate before use
if it has been sitting a
long time
DR Artifacts
 Dead detector elements
 Software correction to a
point…
 R/F interference
 Detector shielded to prevent
this
 Periodic pattern
DR Artifacts: Spatial Variations in
Background Signal & Gain
 Addressed by acquiring flood and building calibration
mask
 Similar to uniformity correction in nuclear medicine
 Artifact results if attenuator in beam during
calibration, then removed
 Contrast
 Tape
Tape artifact seen after tape
removed
Interferance
CR Grid Interference
 103 lines / inch grids have same frequency as
CR laser scanner. This can cause “Moire”
pattern artifact
 Align grid lines perpendicular to scan
orientation whenever possible
 Reduces chances of artifacts caused by laser scanner.
Moire effect because of
interference between scan
frequency of matrix and
spiral
Display
 Poor calibration
 Dead pixels
 Backlight issues
The End
?